4462 poudel weeds
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Agriculture, Ecosystems and Environment 90 (2002) 125137
Comparison of soil N availability and leaching potential, cropyields and weeds in organic, low-input and conventional
farming systems in northern California
D.D. Poudel a,, W.R. Horwath b, W.T. Lanini c, S.R. Temple d, A.H.C. van Bruggen e
a Department of Renewable Resources, University of Louisiana at Lafayette, P.O. Box 44650, Lafayette, LA 70504, USA
b Department of Land, Air, and Water Resources, University of California, Davis, CA 95616, USAc Department of Vegetable Crops, University of California, Davis, CA 95616, USA
d Department of Agronomy and Range Science, University of California, Davis, CA 95616, USAe Plant Sciences, Biological Farming Systems, Wageningen University, Marijkeweg 22, 6709 PG Wageningen, The Netherlands
Received 4 May 2000; received in revised form 28 November 2000; accepted 5 January 2001
Abstract
Increasing dependence on off-farm inputs including, fertilizers, pesticides and energy for food and fiber production in the
United States and elsewhere is of questionable sustainability resulting in environmental degradation and human health risks.
The organic (no synthetic fertilizer or pesticide use), and low-input (reduced amount of synthetic fertilizer and pesticide use),
farming systems are considered to be an alternative to conventional farming systems, to enhance agricultural sustainability
and environmental quality. Soil N availability and leaching potential, crop yields and weeds are important factors related toagricultural sustainability and environmental quality, yet information on long-term farming system effects on these factors,
especially in the organic and low-input farming systems is limited. Four farming systems: organic, low-input, conventional
(synthetic fertilizer and pesticides applied at recommended rates) 4-year rotation (conv-4) and a conventional 2-year rotation
(conv-2) were evaluated for soil mineral N, potentially mineralizable N (PMN), crop yields and weed biomass in irrigated
processing tomatoes (Lycopersicon esculentum L.) and corn (Zea mays L.) from 1994 to 1998 in Californias Sacramento
Valley. Soil mineral N levels during the cropping season varied by crop, farming system, and the amount and source of N
fertilization. The organic and low-input systems showed 112 and 36% greater PMN pools than the conventional systems,
respectively. However, N mineralization rates of the conventional systems were 100% greater than in the organic and 28%
greater than in the low-input system. Average tomato fruit yield for the 5-year period (19941998) was 71.0 Mg ha1 and
average corn grain yield was 11.6 Mg ha1 and both were not significantly different among farming systems. The organic
system had a greater aboveground weed biomass at harvest compared to other systems. The lower potential risk of N leaching
from lower N mineralization rates in the organic and low-input farming systems appear to improve agricultural sustainabilityand environmental quality while maintaining similar crop yields. 2002 Elsevier Science B.V. All rights reserved.
Keywords: Farming systems; Soil mineral N; Plant tissue N; Weeds; Processing tomato; Corn; California
Corresponding author. Tel.: +1-337-482-6163;
fax: +1-337-482-5395.
E-mail address: [email protected] (D.D. Poudel).
1. Introduction
Conventional farming systems and management
practices have been shown to produce high crop
yields, however, the sustainable soil fertility and
0167-8809/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.
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environmental quality of these production systems is
questionable. Conventional farming systems are often
associated with problems such as nitrate leaching and
groundwater pollution (Foster et al., 1986; Black-mer, 1987), degradation of soil structure (Jordahl and
Karlen, 1993), decreased surface infiltration of water
(Logsdon et al., 1993), and pesticide contamination.
In addition, these farming systems are also associ-
ated with decreased levels of total soil N (Drinkwater
et al., 1998; Wander et al., 1994) and total soil C
(Wander et al., 1994) over time. The answers to
problems associated with conventional practices are
alternative cropping systems that increase soil C and
N and leading to less N to escape soils.
The central Valley in the northern California pro-
duces several agricultural crops including processing
tomatoes (Lycopersicon esculentum L.), corn (Zea
mays L.), beans (Phaseolus vulgaris L.), safflower
(Carthamus tinctorius L.), wheat (Triticum aestivum
L.), cotton (Gossypium hirsutum), rice (Oryza sativa
L.) and sunflower (Helianthus annuus L.). The high
production capacity of this region is attributed to in-
tensive irrigation practices, agrochemical inputs and
intensive tillage. Because of the above mentioned
problems of environmental degradation and public
health risk associated with the conventional farm-
ing systems there is a growing interest in alternativefarming systems in the central Valley of California
and elsewhere. Therefore, alternative farming systems
including organic (no synthetic fertilizer and pesti-
cide use) and low-input (reduced amount of synthetic
fertilizer and pesticide use) farming systems, are be-
ing explored as ways to improve overall soil health,
agricultural sustainability, and environmental quality.
Several researchers have indicated considerable ef-
fects of crop rotation and management practices on
soil N availability (Kamimura et al., 1994; Varvel,
1994; Kolberg et al., 1999; Wienhold and Halvorson,1999), crop yields (Turner et al., 1972; Peterson and
Varvel, 1989; Omay et al., 1998), and weed pressure
(Young et al., 1994; Poudel et al., 1998; Daugovish
et al., 1999) in different crop production environments.
Studies on long-term effects of farming systems and
management practices on soil N availability, crop
yields and weeds, especially in an irrigated Mediter-
ranean row-crop production environment are limited.
An understanding of the effect of farming systems
and management practices on soil N availability, crop
yields, and weeds is important in designing manage-
ment strategies that will both increase agricultural
productivity and minimize the risk of environmental
pollution. The specific objectives of this 5-year study(19941998) were: to measure soil mineral N levels
during a cropping season, to assess N mineralization
rate, to assess crop response to weed pressure, and to
measure crop yields for organic, low-input, and con-
ventional farming systems in irrigated field row-crop
production systems in a Mediterranean environment.
2. Materials and methods
2.1. Site description and field experiment
This research was done as a part of a long-term
study called the Sustainable Agriculture Farming
Systems (SAFS) project initiated in 1989 at the
Agronomy Farm of the University of California at
Davis. The SAFS project consists of 11.3 ha of re-
search plots, and the location (3832N, 12147W,
18 m elevation) is characterized by a Mediterranean
climate with most rainfall occurring during the win-
ter months (DecemberMarch), and relatively little
rain during the growing season. Total annual rain-
fall is typically 400500 mm and daytime tempera-ture averages 2334 C during the growing season
(MarchOctober). The soil at the research site is clas-
sified partially as Reiff loam (coarse-loamy, mixed,
nonacid, thermic Mollic Xerofluvents) and partially
as Yolo silt loam (fine-silty, mixed, nonacid, thermic
Mollic Xerofluvents). On average, soil at the SAFS
site contains 350 g kg1 sand, 460 g kg1 silt, and
190gkg1 clay at 030 cm depth. By FAO classifica-
tion system, the soil at the SAFS site is classified as
Mollic Fluvisol.
At the SAFS project, the conventional, organic, andlow-input farming systems with 4-year rotations are
compared with a conventional farming system with a
2-year rotation (Table 1). The 4-year rotations include
processing tomato, safflower, corn, and bean which
follows winter wheat in the conventional 4-year sys-
tem (conv-4), and a bi-culture of oat (Avena sativa L.)
and vetch (Vicia spp.) in organic and low-input sys-
tems. The oatvetch mix is either harvested for seed,
cut as hay, or incorporated as green manure. The
conventional 2-year system (conv-2) is a tomato and
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Table 1
Description of farming system treatments in the SAFS project, University of California, Davis
Farming system Description
Organic 4-Year, five crop rotation including processing tomato, safflower, corn and oats/vetch followed by beans in the4th year; winter legume cover crops grown in between tomato and safflower; safflower and corn; and bean and
tomato; composted animal manure and organic supplements are used; no pesticide application
Low-input 4-Year, five crop rotation including processing tomato, safflower, corn and oats/vetch followed by beans in the
4th year; winter legume cover crops grown in between tomato and safflower; safflower and corn; and bean and
tomato; synthetic fertilizer use is reduced by about one-half the recommended rate; some pesticides used, but
60% less compared to conventional system
Conventional 4-year 4-Year, five crop rotation including processing tomato, safflower, corn and wheat followed by beans in the 4th
year; synthetic fertilizer and pesticide use is based on conventionally recommended rates
Conventional 2-year 2-Year, two crop rotation including processing tomato and wheat; synthetic fertilizer and pesticide use is based
on conventionally recommended rates
wheat rotation. More complete descriptions of the re-
search plots and management practices are reported by
other workers (Scow et al., 1994; Temple et al., 1994).
The organic system is managed according to the
regulations of California Certified Organic Farmers
(CCOF), which do not allow the use of synthetic chem-
ical pesticides or fertilizers. Legume cover crops are
grown in between tomato and safflower; safflower and
corn; and bean and tomato in organic and low-input
farming systems. The cover crop is mowed and incor-
porated about 3 weeks prior to planting of the maincrops. Composted manure is broadcast and incorpo-
rated before planting crops in the organic system. The
low-input system includes reduced fertilizer and pes-
ticide applications in addition to legume cover crops
and mechanical cultivation for weed management.
The conventional systems are managed with practices
typical of the surrounding area, which include the use
of synthetic fertilizers and pesticides. Synthetic fertil-
izers are banded. Each farming system has four repli-
cations consisting of all possible crop rotation entry
points, thus resulting in a total of 56 plots, each mea-suring 68 m18 m. These farming system treatments
are laid out in a randomized block split plot design.
A single row of tomato cv. Brigade was direct
seeded (0.4 kg ha1) in the two conventional system
1520 cm high raised beds with seed centered on
the beds. Bed tops were 1 m wide and beds center
to center were 1.52 m apart. In low-input and or-
ganic systems, tomatoes were transplanted (about
20,000 plants ha1) several weeks after conventional
plot seeding. The delay allowed sufficient time for de-
composition of cover crops, and larger tomato plants
minimized early weed competition in these systems.
Pioneer 3162 corn was planted in the center of a
76.2 cm wide bed (about 70,000 seeds ha1). Both the
tomato and corn were machine harvested.
2.2. Soil analysis
In order to assess soil N availability, five compos-
ite soil samples (015 cm depth) were collected ev-
ery 23 weeks during the cropping season in tomatoand corn plots from 1994 to 1998. Composite soil
samples (015 and 1530 cm) were collected from
all 1998 tomato plots on 4 November 1998 to de-
termine soil mineral N levels after crop harvest in
the organic, low-input, and conventional farming sys-
tems. Soil samples were mixed thoroughly and sieved
through a 2 mm mesh screen. A subsample, 510 g of
soil, was placed in 40 ml 2 M KCl in the field, trans-
ported to the laboratory on ice, and extracted for ni-
trate and ammonium (Bremner and Keeney, 1966) on
the same day. These soil extracts were analyzed forNO3-N, and NH4-N with a diffusion-conductivity an-
alyzer (Carlson, 1978).
In order to study the effects of farming systems on
PMN, the 1997 and 1998 tomato plots were selected
for detailed soil sampling and analysis. After tomato
harvest on 26 July 1997, a composite soil sample
(015 cm depth) from 30 separate soil cores from each
organic, low-input, and conventional tomato plot was
collected on 22 September 1997. Samples were stored
at 4 C until processed. The mineralization of soil
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128 D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137
mineral N was determined over a 160-day laboratory
aerobic incubation (Bonde and Rosswall, 1987). The
mineralized NO3-N was extracted with 2.0 M KCl and
then analyzed on a continuous flow analyzer (LachetInstruments, Milwakee, WI) every 1015 days from
0 to 80 days and every 2025 days during reminder
of the long-term incubation. Potential N mineraliza-
tion data were fit to the following single compartment
exponential model to determine PMN and N kinetics
(Stanford and Smith, 1972):
Nm = N0(1 ekt) (1)
where Nm is the amount of N mineralized at time t,
N0 the initial amount of PMN, and kthe rate constant.
A 7-day anaerobic laboratory incubation (Waring
and Bremner, 1964) was conducted to determine PMNon composite soil samples (015 cm) collected four
times during the 1998 cropping season. A composite
soil sample was obtained from 30 cores per plot. Am-
monium N was extracted with 2.0 M KCl and then
analyzed as above.
To determine soil mineral N levels after crop har-
vest, composite soil samples (015 and 1530 cm)
were collected from all 1998 tomato plots on 4
November 1998.
2.3. Plant analysis
In tomato, petiole sampling was done at first bloom
(60% of plants with flowers) crop stage by walk-
ing through each plot in a U-shaped pattern, and
stripping the fourth leaf from the top of 20 tomato
plants, pulling off all leaflets so that only the petiole
remained, and placing petiole and leaflets in separate
bags. Tissue sampling in corn was done at eight-leaf
(V-8) crop stage. The V-8 stage of corn was gener-
ally around the 8th week after planting. At V-8 stage,
10 plants per plot were cut at soil level for nutrienttesting. The following three leaves, the first unrolled
leaf from the top, and the two leaves below that, were
snipped off where they attached to the stalk and were
saved in a bag. Remaining leaves were cut off and
discarded, and the stalk placed in a separate bag.
In organic and low-input systems, aboveground
biomass samples for cover crops were obtained before
their incorporation into the soil. Weeds were sepa-
rated from the fresh cover crop samples. Cover crop,
weed, tomato petiole and corn tissue samples were
dried at 6065 C, ground to pass through 40-mesh
screen of a Wiley Mill, and submitted to the Univer-
sity of Californias Division of Agriculture and Natu-
ral Resources (UC DANR) Analytical Laboratory foranalyses. Tomato petiole and corn stalk samples were
analyzed for NO3-N (Carlson et al., 1990), and cover
crop and weeds aboveground biomass samples were
analyzed for total-N using combustion gas analysis
(Pella, 1990a,b).
2.4. Statistical analysis
Farming system effects on crop yields were ana-
lyzed by two-way analyses of variance (SAS, 1994).
Tissue N, crop yield, soil mineral N level and weed
biomass in different systems were compared byStudentNewmanKeuls (SNK) multiple range test.
The N mineralization data from 160-day aerobic in-
cubation were fit to Eq. (1) by nonlinear regression
using Marquardt method in SAS (SAS, 1994).
3. Results and discussion
3.1. N inputs
The amount and form of N application to toma-toes and corn varied by the farming systems
(Table 2). In addition to 1.3 kgN ha1 as starter
fertilizer, the organic tomatoes received, on aver-
age, 308 kgN ha1 (190kgNha1 as composted
manures and 118 kgN ha1 as aboveground cover
crop residue), while the low-input and conventional
tomatoes received 198 kg N ha1 (95kgNha1 as
synthetic fertilizers and 103 kg N ha1 as above-
ground cover crop residue) and 165.3 kg N ha1 (syn-
thetic fertilizers), respectively, during 19941998.
Average N application rates (19941998) for theorganic, low-input and conventional corn were
443kgNha1 (292kgNha1 as composted ma-
nures and 151 kg N ha1 as aboveground cover crop
residue), 260 kg N ha1 (106kgNha1 as synthetic
fertilizers and 154 kg N ha1 as aboveground cover
crop residue), and 231 kg N ha1 (synthetic fertiliz-
ers), respectively.
The type, quality, and amount of N additions
and pest control are major management factors that
differentiate the organic farming systems from the
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Table 2
Amount of N applied in tomato and corn crops at SAFS project, Davis (19941998)
Organic (kg ha1) Low-input (kg ha1) Conv-4 fertilizer
(kg ha
1
)
Conv-2 fertilizer
(kg ha
1
)Manurea Amendmentsb Cover cropc Fertilizerd Cover crop
Tomato
1994 123.2 2.8 110 100.8 111 168.0 168.0
1995 108.6 1.0 81 100.8 67 179.2 179.2
1996 308.0 0.4 115 78.4 136 147.8 147.8
1997 226.2 1.0 148 95.2 80 165.7 165.7
1998 181.4 1.0 136 100.8 120 165.7 165.7
Corn
1994 245.3 239 96.1 264 230.7 NAe
1995 175.8 86 141.1 85 230.7 NA
1996 371.8 191 96.3 174 230.7 NA
1997 315.8 116 96.3 121 230.7 NA
1998 350.6 124 97.0 128 231.4 NAa Composted poultry manure applied before planting.b Fish powder, sea weed applied at the time of planting.c N in aboveground biomass. Contribution from weeds is included.d Synthetic fertilizer applied at the time of planting (small amount as starter fertilizer) and 45 weeks after planting.e Not applicable.
conventional system. In conventional system, nutrient
imbalance in the soil and pesticide use often impact
agricultural sustainability and environmental quality.
The balance of N, P, and K in the soil and pesticide
use were identified as indicators of agricultural sus-tainability in Costa Rica (Jansen et al., 1995), while
nutrient replenishment (annual amounts of N, P, and
K) was identified as the most important factor related
to the sustainability of conventional vegetable farm-
ing systems in Mindanao, Philippines (Poudel et al.,
1998). At SAFS, the differences in the amount and
source of N in the organic, low-input and conven-
tional farming systems are expected to impact soil
mineral N levels during the cropping season. Several
researchers (Aulakh et al., 1991; Wander et al., 1994;
Franzluebbers et al., 1995) have reported the effects
of type, amount, and form of N application on soilmineral N levels in other parts of the USA.
3.2. Soil mineral N levels
Soil mineral N levels (015 cm depth) during a crop-
ping season differed by crops, amount and sources of
N application, and farming systems (Tables 2 and 3).
Although statistically not significant, average surface
soil mineral N levels for the organic and low-input
tomato systems were lower during all the cropping
seasons compared to the conventional farming systems
(Table 3). In contrast, the organic and low-input corn
systems showed consistently higher surface soil min-
eral N levels during a cropping season compared to the
Table 3
Average soil mineral N (NO3-N+NH4-N) levels (015 cm depth)
during the cropping season for tomato and corn crops in the
organic, low-input and conventional systems at SAFS project,
Davis (19941998) (n = 5)a
Organic
(mg kg1)
Low-input
(mg kg1)
Conv-4
(mg kg1)
Conv-2
(mg kg1)
Tomato
1994 14.2 a 20.9 a 28.5 a 30.1 a
1995 19.3 a 24.1 a 31.0 a 25.7 a
1996 20.9 a 17 a 30.4 a 26.3 a
1997 27.6 a 24.3 a 36.2 a 39.5 a
1998 30.9 a 26.8 a 41.9 a 39.6 a
Corn
1994 42.8 a 28.5 a 14.3 a NAb
1995 23.9 a 20.9 a 17.9 a NA
1996 33.2 a 16.9 a 15.4 a NA
1997 61.9 a 42.1 ab 26.6 b NA
1998 37.9 a 20.8 b 17.4 b NA
a Different letters within a row are significantly different at
0.05 probability level by SNK.b Not applicable.
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conventional system. This suggests that soil mineral N
levels vary by the amount of N application (Table 2),
and crop N uptake pattern. Tomato has the highest N
absorption rate between 40 and 50 days after emer-gence (Dumas, 1990; Tesi and Giustiniani, 1987) with
a rate of 36 kgN ha1 per day (Cavero et al., 1997).
Corn takes up to 75% of its total N after tasseling, the
onset of the reproductive stage (Friedrich et al., 1979;
Mills and McElhannon, 1982). Tasseling generally oc-
curs during the 8th week after planting. Differences in
soil mineral N levels between tomato and corn crops
and among the organic, low-input and conventional
farming systems during a cropping season suggest that
appropriate crop specific N management strategies are
necessary to minimize environmental pollution from
these farming systems.
3.3. Potential N mineralization
The potential N mineralization assay by both
160-day aerobic incubation and 7-day anaerobic incu-
Fig. 1. Potentially mineralizable N assayed by 7-day anaerobic incubation during the 1998 cropping season. Urea (46% N) was sidedressed
on 18 May in low-input and on 20 May after soil sampling in conventional farming systems. Different letters above a column in a group
are significantly different at 0.05 probability as determined by SNK test (n = 4).
Table 4
Average amount of potentially mineralization N and N turnover
rate assayed by 160-day aerobic incubation for organic, low-input
and conventional farming systems at SAFS project, Davisa (n =
4) (different letters within a column are significantly different at0.05 probability by SNK)
Treatment Potentially mineralizable N
(mg NO3-Nkg1)
Turnover rate (rate
constant (k) per day)
Organic 100.5 a 0.0063 b
Low-input 64.3 b 0.0098 ab
Conv-4 46.8 b 0.0137 a
Conv-2 47.8 b 0.0113 ab
a Soil sampled on 22 September 1997; tomato harvested on 26
July 1997.
bation showed greater pools of PMN in the two covercrop-based farming systems (organic and low-input)
(Table 4, Fig. 1). Potentially mineralizable N has
been used as an index of N availability by many
workers (El-Haris et al., 1983; Drinkwater et al.,
1995; Franzluebbers et al., 1995; Vanotti et al., 1995;
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Barrios et al., 1998; Needelman et al., 1999). Results
from 160-day incubation assay showed 112 and 56%
greater PMN pool in the organic system compared to
the conventional and low-input systems, respectively.The in-season PMN levels were also consistently
higher in the organic system, except for the 20 May
spike in the low-input system (Fig. 1), attributable to
the sidedressing of 90 kg N ha1 as urea 2 days be-
fore sampling. Notwithstanding larger PMN pools in
cover crop-based farming systems (Table 4, Fig. 1),
N mineralization rate in the conventional system was
100% greater than in the organic and 28% greater
than in the low-input system (Table 4). The lower
turnover rate in the cover crop-based farming systems
is attributed to the differences in the quality of SOM
which is linked to the chemical stabilization and phys-
ical protection of its labile pool (Ladd et al., 1985;
Drinkwater et al., 1998). Wander et al. (1994) com-
pared biologically active SOM pools for three crop
rotations (organic with cattle manure and cover crops,
cash-grain-based organic cover crops, and a conven-
tional cash-grain-based rotation with mineral fertil-
izer). They found a higher total C and N, particulate
SOM, and reduced water dispersible organic C in
cover-cropped soil and suggested that the SOM pool
of the cover-cropped treatment was more stable than
the SOM in other treatments. The lower N miner-alization rate in the organic and low-input farming
systems corresponds with a greater accumulation of
N in these systems (Clark et al., 1998) and a reduced
risk for N leaching and groundwater pollution. In fact,
soil mineral N levels after crop harvest in the organic
and low-input systems were remarkably lower com-
pared to those in conventional plots at SAFS in 1998
(Table 5) which apparently suggests a reduced risk of
N leaching and groundwater contamination in alter-
native farming systems. Kamimura et al. (1994) also
found increased total soil N values for organic paddytreatments in their long-term field experiment in Japan.
Table 5
Average soil mineral N (NO3-N + NH4-N) levels after 1998 tomato harvest in the organic, low-input and conventional systems at SAFS
project, Davis (n = 4)a
Depth (cm) Organic (mg kg1) Low-input (mg kg1) Conv-4 (mg kg1) Conv-2 (mg kg1)
015 20.19 b 17.17 b 44.10 a 37.59 a
1530 7.11 a 5.86 a 15.30 a 9.84 a
a Different letters within a row are significantly different at 0.05 probability level by SNK.
3.4. Plant tissue N
Effects of the amount of N application and soil
mineral N levels on plant tissues N concentration intomatoes and corn (Table 6) crops were observed in
the organic, low-input and conventional farming sys-
tems. Plant tissue N results indicated that both the
organic tomato and corn crops had N limitation prob-
lem especially during the initial years of this study,
when N application rates in this system were rela-
tively small (Table 2). Petiole NO3-N concentrations at
first bloom crop stage was low in organic tomatoes in
1994 and 1995 (Table 6), which increased remarkably
following threefold increase in the amount of com-
post N application in 1996 in this system (Table 2).
Compost application rates in organic tomatoes de-
clined in 1997 and 1998 but were higher than that
of the 1994 and 1995. Increased amounts of com-
post N application in organic tomatoes was reflected
in higher levels of petiole NO3-N concentrations in
this system in 19961998 (Table 6). Organic corn tis-
sue N also showed a similar relationship with N input.
In 1995, when composted manure application in or-
ganic corn was reduced by 28% compared to the 1994
level (Table 2), stalk NO3-N concentration in the or-
ganic corn were lower compared to the low-input and
conv-4 systems (Table 6). However, when compostedmanure application in organic corn was doubled in
1996 and subsequent years compared to 1995, stalk
NO3-N concentrations between organic, low-input and
conv-4 systems did not differ. These results indicate
that the organic system required a large amount of
N input to meet crop demand and indicated differ-
ences in N mineralization potential between the or-
ganic, low-input and conventional farming systems.
Conventional systems received large amounts of in-
organic N through sidedressing (Table 2) at a time of
high plant demand, while organic systems more likelyobtained smaller amounts of N over a long period of
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Table 6
Average petiole NO3-N for tomato at the first bloom crop stage and stalk NO 3-N for corn at V-8 crop stage at SAFS project, Davis
(19941998) (n = 4)a
Organic(mg NO3-Nkg1) Low-input(mg NO3-Nkg
1) Conv-4(mg NO3-Nkg1) Conv-2(mg NO3-Nkg
1)
Tomato
1994 4267 b 1373 c 14275 a 14850 a
1995 1023 b 8580 a 7920 a 7770 a
1996 7893 b 8840 b 12350 a 11350 a
1997 10875 b 13000 a 8373 c 10575 b
1998 15200 a 13550 ab 9993 bc 7209 c
Corn
1994 7318 a 5808 a 6790 a NAb
1995 682 c 4250 b 7523 a NA
1996 8218 a 7943 a 6793 a NA
1997 4097 a 5038 a 5145 a NA
1998 5213 a 4968 a 5580 a NAa Different letters within a row are significantly different at 0.05 probability by SNK.b Not applicable.
time, from the slow release of N from compost and
cover crops. According to Westerman et al. (1988), po-
tentially about 50% of total available N in composted
broiler and turkey litter is available within a few weeks
after application, while potentially about 60% is avail-
able within 810 months. Several processes such as
mineralization, immobilization, and nitrification affect
the forms and amount of soil N available to crop plants(Reddy et al., 1979; Tisdale et al., 1993), and factors
such as air and soil temperature, transpiration, and soil
water potential affect N uptake and translocation (van
Keulen and van Heemst, 1982). Based on results from
this study, it can be safely stated that a close monitor-
ing of the N status of the plants is necessary especially
in the organic and low-input systems to improve agri-
cultural productivity and environmental quality of an
agroecosystem.
3.5. Crop yields
There were significant farming system and year
interaction effects (ANOVA, p < 0.01) on tomato
fruit and corn grain yields during this study duration.
Tomato fruit yields were significantly different among
the farming systems for three (1994, 1995 and 1998)
out of five study years (19941998) (Fig. 2), while
corn yields showed system effects only for 1994 and
1995 (Fig. 3). Organic tomato yield was lower by 36%
in 1994, 4% in 1995, and 7% in 1997 compared to
conv-2 tomato. Organic tomato had a greater yield by
18% in 1996 and 13% in 1998 than conv-2 tomato. In
1998, tomato fruit yield was lowest in the conv-4 sys-
tem followed by the conv-2 system, as conventional
tomato yield, on average, dropped by 37%, while the
two cover-cropped, transplanted systems dropped by
17% from that of the previous year. The large decrease
for conventional systems was partially the result ofhaving to replant. The initial planting had emergence
problems due to an unusually wet spring, followed by
extreme heat in the summer. Increased organic tomato
fruit yield in recent years are attributed to increased
rates of composted manure application, which appar-
ently have prevented potential N deficiency problems
(Cavero et al., 1997). Although low-input corn grain
yields, on average, were 10% greater than conv-4 corn
yields, yield differences between low-input and conv-4
systems appears to be declining. Low-input corn grain
yields were 27% greater in 1994, 9% in 1995, and15% in 1996 compared to the conventional system,
while they were 1% lower in 1997 and 2% in 1998.
The 1998 low-input corn grain yield was 14% lower
than organic. These results indicate that the manage-
ment practices used in alternative farming system have
potential for producing comparable yields to conven-
tional farming systems. Similar results are reported
by other workers in Pennsylvania (Drinkwater et al.,
1998), in North Carolina (King and Buchanan, 1993)
and in California (Drinkwater et al., 1995; McGuire
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D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137 133
Fig. 2. Tomato fruit yields in the organic, low-input and conventional farming systems at SAFS Project, 19941998. Different letters above
a column in a group are significantly different at 0.05 probability as determined by SNK test (n = 4).
Fig. 3. Corn grain yields in the organic, low-input and conventional farming systems at SAFS Project, 19941998. Different letters above
a column in a group are significantly different at 0.05 probability as determined by SNK test (n = 4).
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134 D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137
et al., 1998). Comparable organic rice yields to con-
ventional farming systems are reported by Kamimura
et al. (1994) in Japan.
Although the organic, low-input and conventionalfarming systems at SAFS showed comparable crop
yields, they have differences in economic viability.
Poudel et al. (2001) reported lower cumulative net re-
turns for the organic system than for the low-input and
conventional farming systems over the last 11 years
at SAFS. Conventional farming system with 2-year
crop rotation was the most profitable system due to
growing tomatoes every other year, while organic sys-
tem with premium prices performed better than conv-4
and low-input system, a net loss occurred with the or-
ganic system without premium prices. Higher produc-
tion costs in the organic system were due to additional
expenses in compost use, planting, cover crop man-
agement, and pest control (Clark et al., 1999). This
suggests that large acreage in organic production will
pull the prices down and eventually loss of farm prof-
itability. It means even if the organic systems are eco-
logically and agronomically sound, economically their
widespread adoption is questionable.
3.6. Weeds
There were significant farming system effects onaboveground weed biomass in tomato in 1996 and
Table 7
Average aboveground weed biomass at harvest in the organic, low-input and conventional farming systems at SAFS project, Davis
(19941998) (n = 4)a
Organic
(dry weight kg ha1)
Low-input
(dry weight kg ha1)
Conv-4
(dry weight kg ha1)
Conv-2
(dry weight kg ha1)
Tomato
1994 150 a 157 a 173 a 141 a
1995 621 a 595 a 366 a 273 a
1996 1550 a 1690 a 467 b 204 b1997 264 a 124 a 278 a 74 a
1998 350 a 67 a 140 a 4 a
Corn
1994 69 a 49 a 79 a NAb
1995 2364 a 998 b 1572 ab NA
1996 454 a 311 a 461 a NA
1997 2354 a 1509 a 599 a NA
1998 c NA
a Different letters within a row are significantly different at 0.05 probability by SNK.b Not applicable.c Data not available.
in corn in 1995 (Table 7). However, percent weed
cover monitored during the cropping seasons showed
more weed pressure in the organic and low-input sys-
tems than conventional systems both in tomato andcorn during all the study years (data not shown). The
greater weed pressure in the organic and low-input
systems was reflected in the cost of weed manage-
ment reported by Clark et al. (1999) in the organic,
low-input and conventional tomato systems at the
SAFS Project. They found the organic or the low-input
tomato systems cost $571 ha1 for weed management
while the conventional systems cost $420 ha1 during
19931996.
Several researchers reported effects of crop rota-
tion on weed pressure. Young et al. (1994) observed
increased winter annual grass weed populations over
time in monoculture wheat (winter wheat and spring
wheat) compared to a 3-year rotation of winter wheat,
spring barley ( Hordeum vulgare L.) and spring dry
pea (Pisum sativum L.) in the Pacific Northwest. Sim-
ilarly, Daugovish et al. (1999) reported the superiority
of 3-year rotations to 2-year rotations in controlling
winter annual grass weeds in Nebraska. We did not
see a difference in weed biomass between the con-
ventional 2-year and the conventional 4-year rotations
in our study. Although a shorter rotation might be
expected to have more weeds, hand weeding in thetomato crop every other year, prevented the buildup
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D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137 135
of weed seed in this system, unlike the other studies
where hand weeding was not used.
More weeds were observed in the organic and
low-input systems where herbicides were either notused or used less frequently. This is in agreement
with Barberi et al. (1998), who observed almost four
times more weed seed in an organic corn system than
in a conventionally managed system.
In this study, crop yields and weed biomass at har-
vest varied. The highest organic tomato yield occurred
in 1996 (Fig. 2) when weed biomass at harvest was
also highest (Table 7). Most of the weed growth was
in the furrow and edges of the bed, which did not im-
pact tomato growth. High rates of compost were ap-
plied in 1996, and weed growth was much greater in
the later part of the growing season, as a result of this
added N. Miyama (1999) observed more late season
weed growth in tomatoes following early weed-free
conditions, when applied N or compost was increased.
In contrast, corn yields in 1995 were lowest when
aboveground weed biomass at harvest was greatest.
Hand weeding is not used in corn, and thus weeds
that emerge early compete with the crop for the full
season. Although not statistically significant, organic
corn yields in 1997 were lower compared to low-input
and conventional treatments when organic corn had a
higher aboveground weed biomass at harvest. Theseresults suggest that more understanding of the ecol-
ogy of weeds and interrelationship between crops and
weed pressure and N application is needed to improve
agricultural productivity of the organic and low-input
farming systems while enhancing environmental qual-
ity of an agroecosystem.
4. Conclusions
Soil N availability and N leaching potential duringa cropping season varied by crop, farming system,
and the amount and source of N application. Cover
crop-based farming systems appear to have a larger
PMN pool, but a slower, more continuous release of
mineral N throughout the growing season, while con-
ventional systems supplied mineral N in pulses as a
result of fertilizer management. Late-season soil min-
eral N levels apparently depended on the amount of
N application and crop removal; and were generally
high both in tomatoes (conventional systems) and in
corn (organic and low-input systems) in the later part
of this study. Therefore, post-harvest N management
strategies appeared necessary to conserve N for future
crop use while minimizing the risk of N leaching andgroundwater pollution in field crop production sys-
tems. Crop response to N application rates and weed
pressure varied by crop. Tomato responded to the
amount of N application, while corn responded more
to weed competition. Tomato has a higher crop value
than corn and therefore hand weeding supplemented
cultivation. This reduction in weed competition re-
sulted in fertility being a more important variable
for yield. Organic and low-input systems generally
had higher amounts of aboveground weed biomass
at harvest indicating that these farming systems had
more weed competition. Application of appropriate
amount of N and development of N management plan
considering crops grown, management practices used,
weed consideration, and N mineralization potential
appears to be a very important factor to improve crop
yield and profitability while minimizing the risk of N
leaching and groundwater pollution.
Acknowledgements
We acknowledge the work of Oscar Somasco, MaryKirk Wyland, Diana Friedman, and Sean Clark, former
research managers on the SAFS Project, and William
Cruickshank, Don Stewart, and Peter Brostrom, SAFS
crop production managers. We are thankful to the stu-
dents, faculty, and staff members from the University
of California at Davis who contributed to research
efforts at the SAFS project during this study. Finan-
cial and/or technical support for the SAFS project
has been provided by several agencies including WR
SARE of the USDA, University of California Sus-
tainable Agriculture and Education Program (UC
SAREP) and University of California Division of
Agriculture and Natural Resources (UC DANR).
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