California Grasslands

ECOLOGY AN D MANAG E M E NT

Edited by

MARK R. STROMBERG

JEFFREY D. CORBIN

CARLA M. D’ANTONIO

U N I V E R S I T Y O F C A L I F O R N I A P R E S SBerkeley Los Angeles London

Grassland soils are recognized for their capacity to sequester,or store, soil carbon (C), which is due to their high primaryproductivity, the accumulation of above- and belowgroundlitter and plant rhizodeposits, and the stability of by-productsproduced by soil biological processes during decomposition(Amundson 2001; Conant et al. 2001; von Lützow et al. 2006).By storing soil C, grasslands provide key ecosystem servicessuch as mitigating greenhouse gas emissions, enhancingnutrient cycling and retention, and promoting plant produc-tivity (Woods 1989; Burke et al. 1989; Bauer and Black 1994;Sparling et al. 2006). Organic C inputs to soil largely consistof plant materials. Soil biota mediate the plant-soil transfers,which retain C and other nutrients in soil and build soilorganic matter (SOM). The compositions of soil microbialand faunal communities, and their interactions, are increas-ingly recognized as important for the stabilization of soil Cfrom plant and microbial residues (Guggenberger et al. 1999;Denef et al. 2001). Changes in microbial community compo-sition are often linked to changes in nutrient transformationsin soil (Carney et al. 2004; Wardle et al. 2004), even thoughdirect relationships between specific taxa, e.g., identity andnumbers, and process rates have only rarely been identified(Okano et al. 2004; Wardle et al. 2006; Six et al. 2006).

Human activities have profound effects on soil biologicalprocesses and soil C sequestration, mainly through conver-sion of grasslands to cultivated agriculture, and vice versa.When grasslands are tilled for crop production, a large pro-portion of the soil C is lost since C in previously protectedsoil microsites becomes available for microbial utilization(Lal 2002). When tillage ceases, soil C is slowly sequesteredas SOM in old fields and restored grasslands (Amundson2001; McLauchlan et al. 2006), but rates can be increased bychanges in plant species composition (Dijkstra et al. 2006),higher primary production of plants (Ogle et al. 2005), andfertilization and irrigation (Conant et al. 2001). Grazing, incontrast, has not been shown have a consistent effect on soil

C, at least in the in the semiarid and Mediterranean climatesof the western United States (Reeder et al. 2004; Martenset al. 2005); associated changes in soil bulk density, plantcommunity composition, and stocking rates contribute tothe difficulty in assessing grazing effects on soil C.

In California, grassland soils have experienced a wide vari-ety of changes in human activities during the past century.Large anthropogenic impacts on net primary productivity(NPP) and biodiversity, and introduction of non-native specieshave occurred throughout California, particularly in the low-land areas that once supported native grassland and savannavegetation (Huenneke 1989; Williams et al. 2005). Replace-ment of grasslands by intensive agriculture and urbanizationhas resulted in higher and lower NPP, respectively, and greaternumbers of non-native species. Present-day grasslands are nowmainly devoid of native perennial species in much of the state,and are dominated by a fairly consistent group of non-nativegrasses (Peterson and Soreng, Chapter 2; D’Antonio et al.,Chapter 6). This pattern exists regardless of some aspects ofpast land use history (e.g., grazed vs. not grazed) (Strombergand Griffin 1996) and soil type (USDA-NRCS STATSGO Data-base; USDA-NRCS Soil Survey Staff 1999).

The objectives of this chapter are to describe the array ofsoil conditions that support grassland in California and tosynthesize the state of our knowledge of how land use historyin California has influenced soil biology and soil organic Ccycling. Grasslands today occur across a broad range ofedaphic, topographic, climatologic, and land use conditions,yet relatively little is known about how these different factorsaffect the soil processes involved in soil organic C storage. AsCalifornia begins to develop policies to sequester C tomitigate the impacts of global climate change, the need tounderstand land use effects on soil biology and SOM dynam-ics becomes more pressing. In fact, land use changes willprobably shift dramatically during the next 100 years ashigher temperatures alter the distribution of agricultural and

N I N E

Soil Biology and Carbon Sequestration in Grasslands

LOU ISE E. JACKSON, MARTI N POTTHOFF,

KE R R I L . STE E NWE RTH, ANTHONY T. O’G E E N,

MAR K R. STROM B E RG, KATE M. SCOW

1 0 7

grassland ecosystems in California (Hayhoe et al. 2004; Shawand Dukes, Chapter 19).

The chapter begins with a description of California soilsthat support grasslands and considers some of the soil factorsthat influence the potential vs. actual distribution of grass-lands in the state. Next, some scenarios of past land usechange are explained for grasslands in California, whichhighlight the difficulty in understanding the legacy of pastmanagement history on plant and soil communities. Then,some examples of relationships between soil organic Ccycling and the composition of microbial and plant com-munities are described after conversions from grassland tocultivated agriculture, and vice versa. Recent research thatutilizes agricultural practices to restore native perennial grass-land from annual grassland is also included. MontereyCounty in the Central Coast region of California is wheremost of the relevant research has been conducted. The chap-ter concludes with some implications of future land usechange on grassland soils and management.

Grassland Soils in California

California annual grassland and savanna in California occuron soils that constitute five of the 12 soil taxonomic orders(USDA-NRCS STATSGO Database; USDA-NRCS Soil SurveyStaff 1999). Hence the template upon which grasslands existin the state is diverse. The geographic extent of these soils—known as Mollisols, Inceptisols, Entisols, Alfisols, andVertisols—are shown in Figure 9.1a. The remaining sevensoil orders either possess environmental factors that do notsupport grasslands or do not occupy significant spatial extentwithin the state. A particular sub-grouping of soils in theGreat Valley are soils with duripans, i.e., hardpans or hori-zons cemented by silica, which are common on old terracesalong the eastern margin of the Great Valley and commonlysupport the unique vernal pool-grassland (Figure 9.1b).

The majority of California’s grasslands have xeric soil mois-ture and thermic soil temperature regimes (USDA-NRCSSTATSGO Database; Reever-Morghan et al., Chapter 7). Thexeric soil moisture regime is typical of Mediterranean cli-mates with warm dry summers and cool moist winters.Potential evapotranspiration is greatest during the dry sea-son, and thus soil profiles are rapidly depleted of moisture inthe summer months and are recharged in the winter, whichcreates soil conditions well suited for grassland and savannaecosystems. High mean annual soil temperatures (i.e., a ther-mic temperature regime) in many of California’s grasslandsoils appears to correspond with lower organic C contentsrelative to cooler grassland soils in other parts of the UnitedStates. Elevated temperatures increase biogeochemical reac-tion rates, hence processes such as organic matter turnoverare accelerated (Carlisle et al. 2006).

Mollisols are true grassland soils that accumulate highlydecomposed SOM into the mineral topsoil (Buol et al. 1997).This soil order is characterized by a thick (generally �25 cm),very dark brown to black surface horizon that has high base

saturation (Mg2�, Ca2�, K�, Na�) and organic C content of0.6% or more, resulting in loose, friable soils that have highcation exchange capacity and high fertility. Most Mollisols ofCalifornia occur in areas where plant available water is highor in water collection areas such as concave slopes, floodplains, valley floors and basins. Mollisols are also present inlandscapes with high clay content and/or calcium concen-tration, which can stabilize and protect SOM from microbialdecomposition. Mollisols have very high native fertility, andas a result, a vast majority of arable Mollisol soilscapes, whichare typically deep soils on relatively flat ground that oncesupported grassland, have been converted to agriculture.Some Mollisol soilscapes continue to support grasslands,such as small coastal prairies, stream terraces of narrowcanyons and valleys, and steeply sloping hillsides of theCoast Range and Sierra Nevada foothill region as well as the

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F IG U R E 9.1a. General soil map depicting the spatial extent ofdominant soil orders of major grassland regions in California. Imagefrom USDA-NRCS STATSGO Database.

shallow soils found on undulating topography of volcanicterrain (e.g., the Mehrten formation, along the eastern mar-gin of Great Valley).

Entisols are mineral soils that lack evidence of soil devel-opment. These soils often consist of a thin topsoil horizonoverlaying parent material. The dominant soil-formingprocess is the addition of small amounts of soil organic C toparent material. Grasslands on Entisols occur on geomorphi-cally active landscape positions such as recent alluvial fans,flood plains, or very steep slopes where the rate of soil erosionexceeds soil formation. Some Entisols, such as the widelymapped Yolo soil series, have high SOM content, but themechanism of accumulation is much different from that inMollisols. These soils display an irregular accumulation ofSOM with depth because they are associated with deposi-tional environments, such as flood plains, where SOM-rich

layers are sequentially buried by recurring sediment duringflood events. These expansive grassland soilscapes of the GreatValley and adjacent Coast Range Valleys have been convertedto agriculture and urban land. Entisols found on flood plainsof narrow valleys of the Coast Range still support grasslands.

Inceptisols are also young soils, but display greater pedo-genic development than Entisols (e.g., coloration due to therelease of iron from primary minerals in the subsoil) anddevelopment of soil structure or slight increases in clay con-tent relative to the parent material. In California, grasslands onInceptisols are commonly found in steep terrains of the coastaland Sierra foothills and on younger stream terraces. In theselocations, soil organic C content is usually less than 0.6% inthe topsoil. The Auburn soil series is an example of a widelydistributed Inceptisol found in the lower reaches of the SierraNevada Foothill region, where annual grasslands are common.

Alfisols are moderately weathered soils that display evidenceof translocation of clay minerals. The degree of leaching,however, is not great enough to remove excessive amounts ofbase cations. The topsoil of Alfisols typically contains less than0.6% organic C. The clay-rich subsoil horizons tend to bedominated by mixed clay mineralogy of vermiculite, smectite,and kaolinite, and as a result, the cation exchange capacity ishigh. Compared to Mollisols, SOM is low in Alfisols. Grass-lands on Alfisols are found on older terraces along the marginsof the Great Valley and on saddles and ridge tops of hills andmountains of the Sierra Nevada foothills and Coast Ranges.Most of the grasslands that remain along the margins of theCentral Valley consist of Alfisol soilscapes (Figures 9.1a and9.2). Grasslands tend to remain in these areas because they areprotected as vernal pool landscapes. The soils in this regioncontain clay pans and hardpans that restrict the downwardpercolation of water (Figure 9.1b). Alfisols are the most com-mon soil order found in vernal pool landscapes of California(Smith and Verrill 1998).

Vertisols are soils rich in smectite clay minerals that displayshrink-swell properties associated with wetting and dryingconditions. When dry, Vertisols have open cracks extendingfrom the soil surface into the subsoil. The permeability of Ver-tisols can be high when dry and cracks are open but decreasessignificantly when the soil is wet and cracks have swelled shut.Vertisols commonly support grassland plant communitiesbecause the shrink-swell nature of these soils destroys woodyroots, thus limiting the establishment of shrubs and trees.There is a wide range in soil organic C content in Vertisols, andbecause of the stabilizing effects of clay, SOM in topsoil isoften high if grasses are a component of the plant community.Vertisols are most commonly found in basin floor environ-ments where fine, clay-rich, alluvial parent materials havebeen deposited. Vertisols can also form from consolidatedparent materials that rapidly weather to clay, such as certainfine-grained sedimentary rocks or basalt bedrock. In the past,steeper slopes were cultivated for dry-farmed grains; thus,many present-day grassland soils have had a history oftillage. This is particularly the case across the western rim ofthe Great Valley on old uplifted fan remnants that support

S O I L B I O L O G Y A N D C A R B O N S E Q U E S T R AT I O N 1 0 9

F IG U R E 9.1b. Soils with Duripans (hardpans, horizons cemented bysilica) are common on old terraces along the eastern margin of theGreat Valley. These soils often support vernal pool landscapes. Imagefrom USDA-NRCS STATSGO Database.

Mollisols, Alfisols, and Vertisols. This may not be the case forthose Alfisols that were not farmed along the eastern rim ofthe Valley, where hardpans are present, unless ripped by deepcultivation or destroyed by dynamite.

Grasslands and Land Use Change

Humans have lived in California grasslands since the LatePleistocene Period (Anderson, Chapter 5). Native Americansundoubtedly burned grasslands to maintain certain types ofplant and animal species, but the extent, periodicity, and fullecological implications of these activities are largely unknown(Erlandson 1994; Keeley 2002; Anderson 2005). Europeanland use began in the 1760s with the establishment ofSpanish missions, ranches, and livestock grazing. Agriculturalproduction increased in response to human populationgrowth, particularly during the Gold Rush era and theconstruction of the transcontinental railroad that occurred inthe late 1880s. Some agricultural lands have periodically beenabandoned, and these have largely converted on their own toexotic annual grasslands. Much of California’s grassland todayhas been created from other types of vegetation (Huenneke1989). One example is from the Tulare Lake basin, where graz-ing lands today were originally Atriplex scrub, oak woodlands,and chaparral, while the original grasslands of the basin’svalley floor were converted to intensive agriculture. Typeconversions from chaparral and coastal sage scrub increasedthe extent of upland soils under grassland (McBride andHeady 1968; Minnich and Dezzani 1998; Cione et al. 2002).

Even during the period after European settlement, there arefew long-term accounts of land clearing (e.g., shrublands andriparian corridors that now are grasslands), arrival of invasivespecies, or management practices such as burning or pasttillage, that would allow for evaluation of the impacts of landuse change on grassland ecology and soils. Some of the fewexceptions are records from the University of California cam-puses and field stations (McBride and Heady 1968; Longcoreand Rich 1998; Stromberg and Griffin 1996); these recordsemphasize that grasslands represent one phase of dynamicvegetation changes that occurred in many parts of California.

Current grasslands form a distinct band around the crop-lands of the Great Valley, surrounded by a concentric ring ofoak savanna at higher elevation, which intermixes with hardand soft chaparral vegetation types (Keeler-Wolf et al.,Chapter 3) (Figure 9.2). Much of the area of California thatwas originally grassland, especially the valley floors of theGreat Valley, now supports field and row crops. Orchardsand vineyards occupy better-drained soils along the edge ofthe Great Valley. Near the coast, vineyard development is stillexpanding into grasslands, oak savannas, and oak wood-lands; one of the habitats that is most threatened by vine-yards in Sonoma County is oak woodland composed ofOregon, valley, black, blue and/or coast live oak (Quercusgarryana, Q. lobata, Q. kelloggii, Q. douglasii, and Q. agrifolia)with diverse understory of perennial grassland, toyon andmanzanita (Heaton and Meerenlander 2000). Urbanization

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has caused major losses of grassland near San Francisco andLos Angeles. By the end of the twenty-first century, projec-tions indicate that �300,000 hectares of current grazinglands will be lost to urbanization (Landis and Reilly 2003).Unfortunately, California lacks detailed records of how landuse has changed in relation to original vegetation, except ina few places where local historical records can be piecedtogether to construct a larger-scale pattern.

The Great Valley has witnessed the greatest extent of landconversion to agriculture, affecting 82% of its total landarea, according to the 1997 USDA-NRI program (USDA-NRCS Natural Resource Inventory 2000). Agricultural landencompasses approximately 52% in the Central Coast regionand 54% of the land area in the Sierra foothill region,although much less of this land is cultivated compared tothe Great Valley. Since the 1980s, current trends show reduc-tion in grasslands, mostly because of urbanization, but alsobecause of agricultural conversion such as for grapes in Mon-terey County and other counties in the Sierran Foothills(California Department of Conservation 2006). For two rep-resentative counties over a 10-year period from 1988 to1998, losses of grazed grassland and agricultural acreage,respectively, were 1.1% and 0.4% for Monterey County and1.8% and 5.5% for Sacramento County, while urban landincreased 0.5% and 4.5% for the two counties, respectively.

F IG U R E 9.2. Land cover map derived from the USGS GAP AnalysisProgram.

Urbanization is expected to continue throughout the state,more as a result of population growth rather than of lowfarm income (Kuminoff and Sumner 2001; Jantz et al.,Chapter 23).

In the Central Coast region, which is the context forsome of the few studies on soil biology and soil C cyclingacross land use types in California (see following para-graphs), land use has changed dramatically during the past150 years. In Monterey County, approx. 450,000 hectaresconsist of grazed land (i.e., grasslands and oak savanna),and approximately 110,000 hectares consist of irrigatedcropland that most likely was originally in these vegetationtypes (California Department of Conservation 2006). Sevenof the soil orders occur in this Monterey County, and, as inthe rest of the state, grasslands are found on soils of thesame orders as previously described (USDA-SCS, 1978). Asdescribed in previous chapters in this book (Schiffman,Chapter 4; Anderson, Chapter 5), the native vegetation inthis area before European settlement was perennial grass-land and oak savanna dominated by blue, interior, andcoast live oaks (Q. douglasii, Q. wizlizenii, and Q. agrifolia).Annual grasses from the Mediterranean Basin becamedominant during the mid-1800s when they were able tooutcompete the native perennial grasses after overgrazingand drought (Burcham 1957; Jackson 1985). Relict standsof native perennial bunchgrasses still exist in the CentralCoast region on sites that were never tilled (Stromberg andGriffin, 1996).

In the Salinas Valley, one of the largest valleys in the Cen-tral Coast region, dryland farming of grains (barley, wheat,oats, and millet) began in the late 1700s (Anderson 1989a).In lowland areas of the Salinas Valley, crop diversificationincreased at the turn of the century with construction of irri-gation systems. Since the 1920s, most of the land has beencommitted to intensive, high-input production of vegeta-bles with water coming from dams in the coastal mountainsand groundwater pumping. Levels of SOM in agriculturalfields have decreased by half since the area was dry-farmedat the turn of the century (Lapham and Heileman 1901;Carpenter and Cosby 1925; USDA-SCS, 1978; Wyland et al.1996). In the upland areas of the Salinas Valley, dry farmingof grains continued over considerable acreage (Anderson1989b; 1989c) until the 1990s, when the federal govern-ment’s Conservation Reserve Program (CRP) offered cashincentives to remove hilly land from production in order toreduce erosion, and many farmers participated. Approxi-mately 128,600 acres of farmland are currently enrolled inCRP in the Central Valley, Coast Range, and Coastal Valleys(USDA-NRCS Natural Resource Inventory 2000).

In the Carmel Valley, which is at higher elevation andcloser to the coast than the Salinas Valley, most of the open,reasonably level grassland was in cultivation by the late1800s (Stromberg and Griffin 1996). Here small homesteadfarms were prevalent around the turn of the nineteenth cen-tury, but many were abandoned by 1920, and much of thecultivated land reverted to grazed annual grassland. Some

land remained in cultivation for longer periods, and at pres-ent, some irrigated and nonirrigated farmlands still occur inthe Carmel Valley.

Ecological Responses to Land Use Change Along a Disturbance Gradient

PLANT COM M U N ITI E S I N MONTE R EY COU NTY G RASS LAN DS

In the upland grasslands of the coastal Santa Lucia mountainrange in Monterey County, historical land uses changed thecomposition of grassland plant species dramatically, andsome of these changes have persisted for many decades.Interviewing landowners and consulting aerial photographsand historical land ownership records provided the manage-ment histories on 80 grassland stands in the Carmel Valley(Stromberg and Griffin 1996). Cultivation for crops of smallgrains from 1860 to the 1930s eliminated the dominant,native perennial grasses (e.g., N. pulchra and Poa secunda).Historical grain fields can still be traced by the near linearedges of native bunch grasses in adjacent areas that were nottilled. These perennial grasses have not recolonized such oldfields, even in instances where they have been protected asungrazed natural areas for 70 years. Comparisons of the sameold fields through time to more recently abandoned fieldsindicate that these abandoned cultivated lands have a typi-cal composition of introduced annual plant species that,once altered by tillage, remain relatively constant.

In areas where relict stands exist, Nassella pulchra can persistin undisturbed patches for more than 100 years (Hamiltonet al. 2002). Native perennial grasses in the interior or uplandsites may potentially be superior competitors once they areestablished but may not dominate because of past overgraz-ing or other prior disturbance, low dispersal abilities fromremnant stands, and their relative rarity at present across thelandscape. In coastal sites that are closer to the Pacific Ocean,and thus receive more moisture or fog year-round, nativeperennial grasses can slowly become superior competitorswith annual grasses (Corbin and D’Antonio 2004b).

Gophers create a specific type of disturbance in the grass-lands of the Central Coast region, creating mounds of dis-turbed soil that vary spatially and annually spatial variation(Hobbs and Mooney 1995). Gopher tillage favors annuals atthe expense of native perennials because gophers depositmounds of soil on grassland surfaces. In some cases, thisdeposition can cover up to 30% of grassland surface area(Stromberg and Griffin 1996). Gophers preferentially feed inareas with more annual grasses and their foraging increasesannual grasses (Seabloom and Richards 2003). Gopherdisturbance decreases dramatically on finer-textured soils onsteeper slopes where tillage for small grains was rare. Morediverse, native grassland communities persist on theseuntilled and undisturbed sites (Stromberg and Griffin 1996).Data is not available on changes in soil C after gopherdisturbance, but effects may be similar to those after tillage(see following paragraphs).

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High levels of grazing by livestock and the associated soildisturbance can shift the species composition of grasslandstowards specific annual exotic species in California (Bossardet al. 2000). Release from grazing can likewise lead to high cov-erage of ruderal species with low forage value. Grazed standsof annual grassland in the Carmel Valley support fewer speciesthan ungrazed stands, but only on old field sites that were cul-tivated at some point in the past (Stromberg and Griffin 1996).Old field sites are lower in clay and silt; higher sand contentundoubtedly made these soils easier to cultivate. Total N washigher in the surface soil on the grazed, previously cultivatedsites, suggesting that grazing on these sandy, disturbed soilsmay have stimulated N turnover, cycling, and retention. Soilphosphorus, which is known to be heavily extracted by cattlegrazing and removed from the landscape in cattle bones (Jonesand Woodmansee 1979), was lower on the grazed, cultivatedsites. Other studies have also shown higher soil N and sulfurand lower phosphorus on grazed than on ungrazed annualgrasslands (Vaughn et al. 1986). Soil C was not measured in theStromberg and Griffin (1996) study in Carmel Valley, but else-where in California, light cattle grazing has been shown tohave little effect on soil C (Dahlgren et al. 1997).

SOI L M ICROB IAL COM M U N ITI E S AN D ASSOCIATE D SOI L

CHARACTE R I STICS I N G RASS LAN D AN D AG R ICU LTU RAL SOI LS

OF CAR M E L AN D SALI NAS VALLEYS, MONTE R EY COU NTY

Cultivation of grasslands decreases soil quality and causesthe loss of soil C (Woods 1989; Burke et al. 1989; Lal 2002).Land use history and the associated changes in soil charac-teristics, such as shifts in soil C quality and quantity, pH, soilstructure, and soil macrofaunal communities, may create a

past-history or legacy effect: a long-term and strong effect onsubsequent development of plant and microbial communi-ties and nutrient cycling. Understanding the effects of pastland use history may be crucial when attempting to restorenative grassland ecosystems after disturbance. Specific soilprocesses and C storage may be important for the establish-ment and survival of both the unique microbial and plantcommunities that compose stable ecosystems that resemblerelict ecosystems. For most types of restoration activities,however, there is no information on the impact on the com-position of the soil microbial community, its activity andability to provide a specific suite of ecosystem services, andwhether this soil microbial community is appropriate for thedesired trajectory of the restored plant community.

To determine the relationships between microbial commu-nity composition, soil characteristics, and land use history,nine land use types were identified in a survey of 42 sites in theCarmel and Salinas Valleys (Figure 9.3) in early spring imme-diately after rainfall events so that environmental conditionswere as similar as possible (Steenwerth et al. 2003). All siteswere on sandy loam Mollisol soils of similar granitic origin.Grassland and cultivated soils differed markedly in soil C andN. Soil C and N content in the surface 0–6 cm layer decreasedin several categories of land use (cultivation and irrigation). TheC and N values, for the various categories are, in decreasingorder, annual grasslands had relatively high levels (25 mg cm�3

C and 2.4 mg cm�3 N), perennial grasslands were somewhatlower (25 and 2.3 mg cm�3), then irrigated agriculture (16 and1.5 mg cm�3), and lowest was non-irrigated agriculture (14 and1.4 mg cm-3). The perennial grasslands included old fields andrelict, never-tilled stands, but no clear differences emerged in

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F IG U R E 9.3. Sites in the Carmel and Salinas Valleys in Monterey County, California, that were surveyed for their respective landuse histories. See Figure 9.4 for results of PLFA analysis. Symbols indicate land use type as follows: � � Perennial grasslands, i.e.relict and old field grasslands with native or non-native perennial grasses; � � Cultivated sites, i.e., under vegetables, hayproduction, fallow, or agricultural fields for production of seed of native perennial grasses to be used in restoration projects;� � Annual grasslands, either grazed or not grazed. Image reproduced from Steenwerth et al. 2003.

their soil C and N content. Statistical differences were foundbetween grassland and cultivated soils, but neither betweenannual and perennial grasslands nor between nonirrigated andirrigated agriculture. However, as in perennial grasslands fromthe Central Plains region of the United States (Hook et al.1991), perennial grasslands in this region displayed greater soilheterogeneity in soil C distribution compared to annual grass-lands. For example, in a relict perennial grassland, the surfacerooting zone of N. pulchra (0–12 cm) had higher labile C pools(i.e., soil microbial biomass C and dissolved organic C) thanzones between bunchgrasses that were dominated by intro-duced annual grasses (Steenwerth et al. 2006).

A technique called phospholipid ester-linked fatty acid(PLFA) analysis was used to describe soil microbial communi-ties across this range of 42 sites. PLFA analysis uses cell mem-brane lipids as biomarkers to generate a profile or fingerprintof the microbial community. PLFA are quickly degraded in thesoil environment and thus represent the living soil microbialcommunity. The diverse set of PLFA from each soil sample, orPLFA profile, can then be analyzed by multivariate statistics, asemiquantitative approach that generates fingerprints of themicrobial community (ter Braak 1987). Different microbialgroups (e.g., types of bacteria or fungi) are characterized bydifferent PLFA markers. The total concentration of PLFA is ameasure of viable microbial biomass (Zelles 1997).

Soil microbial community composition was very consistentfor a given land use type, indicating that vegetation, past legacyeffects, and the associated environmental factors select for aspecific association of microorganisms (Steenwerth et al. 2003).For the survey of sites described above, nine land use typescould be clearly identified by their microbial communitiesusing multivariate statistics on the PLFA data (Figure 9.4). Adescription of the major differences in microbial markers isgiven in the caption of Figure 9.4. Briefly, clusters of points indi-cate similarity in the microbial communities from the 42 sites.Soil microbial communities differ markedly between grasslandsand cultivated fields. In particular, microbial communities fromannual grasslands cluster tightly together and thus appear to beunaffected by the time since last disturbance because tillagehad occurred between 2 and 70 years prior to sampling. In con-trast, microbial communities of perennial grasslands are moredifferent from one another than those in annual grasslands.There is greater dispersion among each type of perennial grass-land. Also, microbial communities of the relict native perennialgrasslands and perennial grassland old fields are segregated onopposite ends of the x-axis of the biplot.

Relict perennial grasslands are the one group for whichPLFA profiles differed substantially among sites (Figure 9.4).Although the same grass species (N. pulchra) is present, micro-bial communities differ between sites, suggesting differenti-ation due to local variation in environmental conditions.This might be attributed to legacy effects from the distinctland use histories of each site or to the arrival of differentbiota through time (Steenwerth et al. 2006).

These differences in PLFA profiles among land use typesare related to specific soil characteristics such as readily

available C, as indicated by total PLFA, or microbial biomass,exchangeable Mg2�, moisture, pH, and small differences inclay content (Figure 9.4). Despite the decrease in soil C andN with increasing soil disturbance in cultivated vs. grasslandsoils, soil microbial communities were more responsive toland use history than to total soil C and N content.

R E S PON S E OF TH E SOI L M ICROB IAL

COM M U N ITY TO DI STU R BANCE

Microbial communities can acclimate to soil disturbancesassociated with land use. For example, short-term microbial

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‘Vegetable’‘Hayfield’‘PerGrass Ag’‘PerGrass’‘PerGrass+GRAZ’‘PerGrass Oldfield’‘AnnGrass+GRAZ’‘AnnGrass’‘Fallow’

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Total N

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Total PLFA

pH

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% Moisture

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Time since tillage

Grazed

F IG U R E 9.4. Ordination plot (Canonical Correlation Analysis) of thePLFA profiles from 42 sites of different land use history in the Carmeland Salinas Valleys in Monterey County plotted in Figure 9.3. The sitesare classified by the nine land use types shown in the legend. Themultivariate analysis used 32 PLFA that were in common among allsites. Vectors are the soil characteristics in the 0–6 cm layer that weresignificantly associated with the distribution of the PLFA in the biplot.Axis 1 and 2 represent 31 and 15%, respectively, of the variation in thedata. Along axis 1 in the biplot, native perennial grasslands that areeither livestock-grazed or not grazed (i.e., “PerGrass�GRAZ” and“PerGrass”) separate from the perennial grasslands that were plantedafter abandonment of cultivation (“PerGrass Oldfield”). Highermicrobial biomass (i.e., total PLFA) tends to be higher in the perennialgrassland oldfields, but exchangeable Mg2�, clay, and moisture (albeit ��0.1 MPa at all sites) are slightly lower. Annual grasslands (“AnnGrass”)cluster tightly between these two types of perennial grasslands, andthere is no effect of time since abandonment of cultivation (8 to �50years) on the PLFA profiles. Along axis 1, the perennial grasslands aredifferentiated mainly by markers for fungi, other eukaryotes, and gram-positive bacteria, which are associated with perennial grassland“oldfields,” whereas eubacterial anaerobes and gram-negative bacteriaare associated with relict perennial grasslands. Along axis 2, grassland andcultivated sites separate. The most intensively managed sites cluster at thetop of the axis, where a marker for sulfate-reducing bacteria andactinomycetes increases, and a fungal marker decreases (data onmarkers not shown). Image reproduced from Steenwerth et al. 2003.

responses to simulated tillage are much greater in annualgrassland than in intensively-managed agricultural soils(Calderón et al. 2000). When tillage is simulated by sievingsoil, grassland soil immediately shows decreases in microbialbiomass C, total PLFA, and ammonium; an increase innitrate; and decreases in PLFA markers for fungi andmicroeukaryotes. By comparison, microbial communitiesand activity in a soil under vegetable production, which istilled frequently, are more resistant and recover more quicklyafter the tillage-induced disturbance; that is, they are moreresilient to disturbance. Over the long term, changes in soilmicrobial community composition may be increasinglyaltered with repeated tillage due to lower bulk density, higherporosity, and higher temperatures, such that higher rates ofC mineralization continue to decrease SOM (Dao 1998;Jackson et al. 2003). Abandonment or restoration of cultivatedsoils with grassland vegetation, however, results in a changein microbial composition and an increase in soil C and N(Steenwerth et al. 2003). Nonetheless, the soil microbialcommunity does not necessarily return to a “pre-cultivationcomposition,” suggesting that the above- and belowgroundlinkages may still be affected by land use history despiterecovery in soil C and N (Hooper et al. 2000).

Rewetting of dry soils by rainfall or irrigation is anotherform of disturbance for soil microbes (Kieft et al. 1987). Thesewet-dry cycles commonly occur in Mediterranean-type grass-lands and significantly affect soil microbial communities andnutrient dynamics (Fierer et al. 2003b; Steenwerth et al. 2005).Wetting of dry grassland soil causes a dynamic responseexpressed as immediate and sustained increases (compared tocultivated soils) in microbial respiration and efflux of carbondioxide due to relatively greater soil C availability in grass-lands than in cultivated soils. Despite this greater functionalresponse in grasslands, soil microbial community composi-tion in perennial and annual grasslands has greater diversityand shows less change after soil rewetting compared to culti-vated soil. The resistance of the soil microbial communitycomposition of an annual grassland to wetting and dryingappears to parallel the relative stability of the annual grasslandplant community, suggesting a link between above- andbelowground community stability (Fierer and Schimel 2002;Fierer et al. 2003b; Steenwerth et al. 2003, 2005).

Differences in microbial responses to the disturbanceregime (e.g., tillage vs. periodic wetting and drying of soil), areimportant in a larger land use context. The magnitude ofthese short-term responses will become increasingly impor-tant in efforts to quantify CO2 emissions and C storage acrosslandscapes in California and to predict the effects of alterna-tive management practices in grasslands and cultivated fields.

Soil Biological Activity and Litter Decomposition in Grassland Soils

Primary production of plants depends on decompositionprocesses, where fauna cause litter fragmentation, and soilmicrobes mineralize organic compounds that in turn provide

nutrients that become available to plants. Factors such as lit-ter quality, climate and soil physical properties, e.g., bulkdensity and texture, can influence rates of litter decomposi-tion (Swift et al. 1979; Berg, 1986; Moore et al. 2004). Knowl-edge of the soil food web in California grasslands is verylimited but is essential to assessing ecosystem functions andnutrient cycling, such as the effects of prior cultivation ongrassland biodiversity and C storage in the soil.

There is tremendous production of litter in annual grass-lands each year. Yet, although all plants in annual grasslanddie each year, only 30 to 70% of the biomass of each year’sannual litter disappears in the subsequent year, possiblybecause of low-temperature constraints on microbial activityduring the moist winter, low-moisture constraints during thewarm spring and summer, or high proportions of recalcitrantmaterials in the litter itself (Jackson et al. 1988; Dukes andHungate 2002). Much of the remaining litter appears to dis-appear during the following year, but studies have not docu-mented this two-year pattern. Litter quality is differentbetween annual, non-native grasses and native perennialgrasses, with higher C:N ratio in the latter, so that decompo-sition rates are potentially slower (Eviner and Firestone,

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F IG U R E 9.5. Dry weight losses from litterbags in relict and restoredperennial grassland and in annual grassland at Hastings Reserve,Carmel Valley, where adjacent ecosystems are all present on the samegranitic soil type. The abbreviations “nb,” “bb,” and “tb” indicatelitterbag placements near a bunch on the soil surface, betweenbunches on the soil surface, or suspended on top of a bunch (15 cmabove the surface), respectively. Litter was from the grasses present ineach ecosystem, except that annual grassland litter was placed on thetilled bare soil. Tilled bare soil indicates a plot supporting no plants.n � 5, statistical comparison of means using the Tukey HSD Test.

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F IG U R E 9.6. Mesofauna feeding activity in arelict perennial bunchgrass plot as indicatedby the bait lamina test at Hastings Reserve,Carmel Valley. Activity is expressed as theratio of baits taken to total number of baitsprovided after 70 days of bait exposition insoil. Number of strips tested � 14; error barsshow standard deviation.

Chapter 8). Also, the architecture of native perennial grassesslows down decomposition, because senescing leaves remainupright and do not directly touch the soil (Dukes and Hungate2002), so that the contribution of perennial grass litter to soilC may occur at slower rates than for annual grass litter.

In a litter bag study, grass litter from each type of grasslandecosystem was placed on the soil surface or suspended abovethe top of a bunch; litter decomposition was most rapid onthe soil surface in annual grassland, with little differencebetween perennial grass litter decomposition from relict andfrom restored perennial grasslands (Figure 9.5). Suspendingthe litter above the perennial bunchgrass did not decrease thedecomposition rate, suggesting the importance of biota livingin the leaf mass, or phyllosphere, for the decomposition of theperennial grass litter (Osono et al. 2002). Strong microbial col-onization of plant litter can occur without any contact to soil(Flessa et al. 2002; Potthoff et al. 2005a). The slowest decom-position rates occurred when annual grassland litter wasplaced on the surface of tilled, bare soil that had had no plantsfor several years. This may be related to much lower soilmicrobial biomass (Potthoff et al. 2005b), other soil biota, orperhaps to lower populations of phyllosphere colonizing

biota due to the long-term absence of plants in this treat-ment. The fate of litter C is an important repercussion ofthese different decomposition patterns, because soil C storageis undoubtedly reduced when decomposition is dominated byphyllosphere organisms, without contact with soil.

In a study of invertebrate activity, the bait lamina test(Torne 1990; Larink 1993) was used to estimate and com-pare feeding activity of mesofauna in soils, since ultimatelysuch differences can contribute to C stabilization and dis-tribution in the soil profile. Bait is provided in little holeson a PVC strip buried in soil, and activity is estimated fromthe ratio of baits taken by mesofauna to the total numberof baits provided. After 70 days in the spring, slightly moreactivity occurred in the annual grassland (36% of bait used)vs. 28% in the relict and restored perennial grasslands. Thisactivity is far lower than what is typical of mesic temperateforest or farmland soils in Europe (Larink 1993). Feedingactivity of mesofauna decreased gradually with depthbetween 0 and 8 cm in the relict perennial grassland,though the decrease was less pronounced when furtheraway from the bunchgrass (Figure 9.6). Depth distributionof feeding was more even for annual grassland and restored

perennial grassland. All sites, however, had similar activityin the 0–5 cm depth, despite lower litter accumulation onthe soil surface in the relict grassland, which was deter-mined by sampling with a point-frame in height incrementsabove the soil surface (Figure 9.7). More research on soilinvertebrates that inhabit the mineral soil and rarely cometo the surface (endogeic) vs. those living in the top soil andduff layer on the soil surface (epigeic) in California grass-land is needed to explore the importance and functionallinks of these animals on decomposition and nutrient sup-ply. They may be important for explaining the rates anddistribution of soil C sequestration.

Restoration of Native Perennial California Grasslands andImpact on Soil Biology

Restoring native perennial bunchgrasses in California’s non-native annual grassland is known to be difficult (White 1967;Heady 1977; Stromberg et al., Chapter 21). One of the fewsuccessful methods is to use tillage and herbicide for two tothree years to reduce populations of non-native annualsbefore seeding with native perennials (Stromberg andKephart, 1996; Chapter 21; DiTomaso et al., Chapter 22).This, however, may also reduce native annuals that are in theseedbank. Also, tillage clearly has a strong impact on soilbiology and biochemistry by reducing soil microbial bio-mass, C and N availability, and microbial activity in the sur-face soil, and changing soil physical characteristics (Doran1980; Woods 1989; Aslam et al. 1999).

Monitoring changes in vegetation, soil biology, and soilcharacteristics gives a more complete evaluation of the suc-cess of the restoration process, compared to plant samplingalone. A comparison of vegetation and soil profiles in relictand restored sites would give an accurate appraisal of thesuccess of restoration, but little is known about the soil biol-ogy and profile characteristics of the relict grasslands, whichare limited in extent, and thus destructive sampling is notdesirable.

On perennial bunchgrass restoration sites created fromannual grassland in Monterey County, at the University ofCalifornia’s Hastings Natural History Reserve, sandy loamsoil was tilled and treated with herbicide after a two-yearperiod of tillage to remove annuals. Then native perennialbunchgrasses (primarily N. pulchra) were direct-seeded withno further management for the next four years (Stromberget al. 2002; Potthoff et al. 2005b). Restored perennial grass-land in the third year after planting native perennial bunch-grasses was compared during peak seasonal activity in springwith annual grassland and with plots maintained with tillageand herbicide for six years. Plant species richness in therestored perennial grassland and annual grassland were sim-ilar. Of the total biomass, a greater proportion was nativespecies in the restored perennial grassland: 82% surroundingthe bunchgrass and 32% between the bunchgrasses, vs. 14%in the annual grassland (Potthoff et al. 2005b). The numberof native annual forb species was similar in both grasslands,

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FIGURE9.7.Litterheightandabundance(%)usingapoint frameingrasslandsat theHastingsReserve,CarmelValley, inOctober2002.Thebarsshowdifferentnumbersofhits for interceptionof litter ingrasslands: (a)relictperennialgrassland, (b) restoredperennialgrassland,and(c)annualgrassland.Thetotalnumberofpinsappliedvertically intheplotswas100,andwerearrangedevenlyacrossa10mlinetransectacrosseachplot.

indicating that the period of tillage and herbicide did nothave long-lasting effects (Stromberg et al. 2002; Potthoff et al.2005b).

Soil C pools in the annual and restored grasslands weresimilar at this time. In the uppermost layer (0–15 cm depth),total C, microbial biomass C, respiration, and CO2 effluxfrom the soil surface were higher in the grasslands comparedto a treatment that had been continuously tilled for thesix year period (Potthoff et al. 2005b). Data for microbialbiomass C are shown in Figure 9.8. At deeper depths, no dif-ferences occurred for any of these variables, except that theCO2 in the soil atmosphere was lower in the tilled soil, prob-ably because of the lack of C inputs from roots.

Roots of perennial bunchgrasses were less abundant at thesurface (0–15 cm) and slightly greater in the 15–30 cm layerthan in the annual grassland (Figure 9.8) (Potthoff et al.2005b). Surprisingly, the perennial and annual grasslandshad generally similar root distribution below 30 cm depth,even though N. pulchra can have a more prolific root systemat depth, as shown in other studies (Sampson and McCarty1930; Dyer and Rice 1999).

At four years after planting, the microbial communitycomposition of the restored perennial grassland was similarto that of the long-term annual grassland, except for minordifferences in the surface layer (Potthoff et al., 2006). PLFAfingerprinting indicated much stronger differences betweenthe tilled fallow treatment vs. grasslands in the surface layer(0–15 cm). Microbial communities in lower soil layersshowed little effect of management practices. Fungi wereassociated with the presence of plants and/or litter since thetotal amount and the relative proportion of fungal markersbecame reduced in the tilled bare fallow and in lower layersof the grassland treatments. Thus, microbial communitiesare clearly resilient to the grassland restoration process (i.e.,they return to the annual grassland “fingerprint” despitetwo years of tillage and herbicide), but do not show muchresponse in the early stages after restoration to the change inplant species composition that occurs after planting nativebunchgrasses.

Reintroduction of native perennial bunchgrasses intoannual grassland clearly is possible using agricultural prac-tices in the coastal regions of California (Stromberg et al.2002; Seabloom et al. 2003b; Corbin and D’Antonio2004a, b; Potthoff et al. 2005b) as well as where climate iswarmer and drier in the Great Valley (Brown and Bugg2001). Yet restoration of other biota of relict perennial grass-lands appears much less feasible because of the complexityof the soil environment and its biota as described here, andbecause of the composition of native forbs, whose abun-dance varies with the management methods used in therestored grassland (Brown and Bugg 2001). For example,native vs. non-native forbs in serpentine grasslands inCalifornia can alter soil microbial communities (Batten et al.2006a). In terms of ecosystem restoration, however, the resultsof the Monterey County land use survey and the perennialgrassland restoration experiments indicated little success in

restoring the microbial community of relict grasslands afterannual grasslands or tillage have occurred. Even though theplant community begins to resemble relict stands after a rel-atively short time, there appear to be legacy effects that pre-vent establishment and survival of the relict microbialcommunity. This may also be true for soil invertebrates.

Based on the research described here, the ecosystem servicesrelated to soil C processes that are provided by restored peren-nial grassland on decomposed granite soils in MontereyCounty appear to be fairly similar to those of annual grassland,at least within a decade of the initiation of the restorationprocess. Yet these may change through time. Restoration ofnative perennial grasslands also provides existence value tosociety (i.e., the benefit people receive from knowing that an

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F IG U R E 9.8. Comparison of long-term annual grassland (last tilled72 years before) with restored perennial grassland that was seededfour years prior after two years of tillage, and plots tilled for 6 years after66 years as annual grassland at Hastings Reserve, Carmel Valley. Sampleswere taken in April 2002. (a). Tilled plots have significantly lower soilmicrobial biomass at 0–15 cm and a significant treatment � depth(TxD) interaction, indicating lower values with depth in the tilledplots. (b) For root biomass, a significant TxD interaction shows lessbiomass with depth in the annual grassland, but no other significanttreatment effects (redrawn from Potthoff et al. 2005).

environmental resource exists), since there is widespreadcuriosity about the biota that were dominant before Europeansettlement. Thus, a current challenge is to facilitate the processof restoration to increase both biodiversity of native speciesand associated ecosystem services. Litter management, seedingof native legumes and other native forbs, and soil amend-ments are possibilities. A toolbox of management practices,and an understanding of potential site-specific interactions(e.g., grazing pressure, soil type, microenvironment, and plantspecies composition), would facilitate greater establishment ofrestored native grasslands on marginal lands, in response to

agricultural policies that favor soil conservation and poten-tially enhance C sequestration and nutrient retention. Even-tually this understanding could be employed to mitigate andadapt to climate change. This will require better informationon the impact of land use history on soil biology and soil Csequestration in relation to plant species composition. As thistype of information becomes available, it will also be possibleto scale up to landscape-level predictions of C sequestration bygrasslands across different soil types and management regimes,and to assess the tradeoffs involved in land use change fromgrasslands to other different types of ecosystems.

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