Carbon sequestration is any increase in soil organic carbon due to any land management changes which

increases soil carbon storage and mitigates climate change, only if those land management practices result in the

additional net transfer of carbon from the atmosphere to the soil. We discussed some essential elements of soil

organic carbon that could result in soil carbon sequestration with land use changes and soil management.

Afforestation and change in arable land to forest or grassland have an immediate effect on carbon sequestration

by incorporating carbon dioxide in plant biomass. There are many agricultural field practices like no-tillage

practices, cover crops, reducing soil disruption, improved crop rotation and application of soil organic

amendments indicated effectiveness towards carbon sequestration in soil.

Department of Botany, University of Delhi, Delhi - 110007Email: urvashitomar106@gmail.com

Urvashi Tomar* and Ratul Baishya

The term “Soil C sequestration” means replacement of atmospheric CO to fixed forms such as soil 2

organic/inorganic carbon by plants (Lal 2004). Batjes (1996) estimated that the global stock of soil organic carbon ranges from 684-724 pg to a depth of 30 cm and 1462-1548 pg to a depth of 1m. This represents that in the 0-30 cm layer the quantity of SOC is two times that of atmospheric CO and three times as to above-ground 2

vegetation (Powlson et al. 2011). Nowadays, “sequestration” has become a common term which suggests a contribution to mitigating climate change. But it can only be considered if there is a net additional transfer of C from the atmosphere to soil/vegetation, due to changes in land management practice resulting in a decrease in atmospheric CO concentration (Pawlson et 2

al. 2011). If a change in land management practices results in an increase in C content in soil then only that increase can be considered as “sequestration”, which lead to storage of additional C in soil and contributes to climate change mitigation (Powlson et al. 2011).

Sequestering additional C within biosphere may be achieved by changing land management practices which may ultimately contribute to mitigation of climate change (Pawlson et al. 2011). Even if there is an increase in soil organic C or it is maintained in soil without net additional C transfer from atmosphere to the soil, it is beneficial for soil quality and functioning (Pawlson et al. 2011). Increase in the SOC content affect the soil physical properties positively, resulting in increase in stable aggregates, decrease in run-off risk, erosion, increase in rate of water infiltration and water retention

specially in case of agricultural soils (Angers & Carter 1996; Sydney & Vazquez 2005; Johnston et al. 2009; Powlson et al. 2011). In non-agricultural soils, increase in soil SOC content results in an increase in vegetation cover and there is an improvement in their functioning within the environment, for example, increased water infiltrations rate and decreased erosion results in sediments transfer to surface water (Pawlson et al. 2011).

Carbon sink can be described as any SOC stock which is increasing over time i.e. C is moving into the stock due to environmental factors or any management practices. Whereas, if SOC is declining i.e, moving from SOC stock to the atmosphere then it is considered as the source (Powlson et al. 2011). There are many examples from different parts of the world showing SOC acting as source or sink. For example, in an experiment by Sun et al. (2010) in cropland of China, estimates of SOC were

CARBON STOCKS AND CARBON SINKS

The term “Carbon stocks” implies stock of C in soil/vegetation present at a given time. This value does not provide any information about the trends, as to whether the stock is increasing or decreasing (Powlson et al. 2011). In order to obtain such information the calculation of stock should be done at present and a later date then the two values should be compared. For example, Smith et al. (1997) estimated 22.95 pg the stock of organic C in the European soils, considering this amount as the starting point for estimation of the potential of carbon sequestration can be done (Pawlson et al. 2011).

Land Use Changes and Soil Carbon Sequestration in Mitigation

of Climate Change

The Botanica 67: 87-93. 2017

THE BOTANICA 87

88 THE BOTANICA

SOIL ORGANIC CARBON POOL

Soil organic carbon comprises of plants, animals and microbial residues at different stages of decompositions. There are many organic compounds present in soil which are associated with inorganic soil particles (Post et al. 2007). The interactions between biological, chemical and physical processes in soil determine the turnover rates of soil organic compounds. The balance between C input by litterfall and rhizodeposition and release of C due to decomposition determines the C pool. The chemical quality of the C compounds (labile or stable C), site conditions (climate), and soil properties (clay content, soil moisture, pH, nutrient status) determines the turnover of SOC (Jandl et al. 2007). Physical fractionation techniques are often used to segregate different fractions of the soil organic carbon. Although there may be a continuum in soil organic carbon compounds with respect to decomposability and rate of turnover. These physically determined fractions contain integral structural and functional properties of soil organic carbon (Christensen 1996). The role of soil minerals and soil structure in SOC turnover are majorly emphasized in physical fractionation techniques, which relates directly to SOC dynamics. The light fraction organic carbon (LF-OC) is not associated with mineral matter, therefore it is free and includes particulate plant and animal residues which are undergoing decomposition (Spycher et al. 1983). Only some of this material like charcoal is biologically resistant (Skjemsted et al. 1990). This LF-OC can be present as intra-aggregate particulate carbon when part of it is physically stabilized in macroaggregates (Cambaedella and Elliot 1992, 1993). In case of boreal and tundra ecosystem, thick surface accumulation of LF-OC can be found due to a persistent low temperature which results in slow decomposition rates (Post et al. 2000). In forest and permanent grasslands despite high decomposition rates, the SOC accumulation of LF-OC is high as there is a significant return of plant litter (Post et al. 2000). LF-OC fractions are highly decomposable and due to changes in litter inputs can show seasonal fluctuations and special variations (Boone 1994). The amount of macroaggregate formation controls turnover rate of such LF-OC which has great impact of cropping and tillage in such ecosystems (Beare et al. 1994;

calculated in 1980 then again in 2000 when agricultural intensification was at peak in the country, they found that because of improved varieties and increased application of fertilizer together results in increase in crop yield and during this period of 20 years the stock of SOC increased by 437 Tg, thus cropland had acted as C sink (Powlson et al. 2011).

Transformation of SOC is mainly by bacterial action through which it is stabilized in clay or silt organomineral complexes as heavy fraction organic carbon (HF-OC) and form majority of SOC (Post et al. 2000). SOC is mostly associated with <5µm mineral particles and with the additions of simple substrates new SOC are found to be associated with mineral particle sizes of different range (Post et al. 2000). Silt-SOC is comparatively more stable then clay-SOC due to more rapid loss rates of clay-sized organomineral complexes besides of higher accumulation rate (Christensen 1996). Ecosystem types and land uses practices have great influence on amount, decomposability and placement of above-ground and below-ground inputs, for example in agricultural soils the surface layer consist of mechanically mixed aboveground and most roots, whereas in permanently vegetated soils, aboveground inputs are left on the surface to decompose and root and root exudates enters the soil directly (Post et al. 2000). Differences in moisture, temperature conditions, the degree of contact with mineral soil and exposure to soil organisms affects decomposition.

STABILIZATION OF SOIL ORGANIC MATTER

Biederbeck et al. 1994; Bremer et al. 1994)

The process of C stabilization and the process of accumulation are different from each other. Site factors inhibiting soil respiration, such as excess soil moisture or low temperatures determine the accumulation of C. It is necessary to identify sites where soil properties are conducive to C sequestration in order to increase stable soil C pool. Sites which are abundant in reactive surfaces of clay minerals and oxides, where C can form complexes with a low turnover rate results in stabilization of C (Jandl et al. 2007). Intact stabilization is when adsorption of organic matter at the mineral surface takes place with an intimate bond (Torn et al. 1997; Torn et al. 2002; Hagedorn et al. 2003; Jandl et al. 2007). C sequestration capacity is affected by the processes that affect the aggregation of the soil. The inherent recalcitrance of the molecules, bonding at oxide and clay mineral surfaces, or simply the inaccessibility of SOM for potential microbial grazers results in stabilization of SOC (Sollins et al. 1996; Six et al. 2002a, b; Jandl et al. 2007). The surface accumulation of SOC is directly related to the C input. Different clay minerals have gradual differences, for example, the bonding of SOM to smectite is tighter than to kaolinite and its turnover time is twice as long (Wattel-Koekkoek et al. 2003). The condensation reaction on the surface forms stable bondings (Keil et al. 1994; Kennedy et al. 2002).

The soil data (150 years) from Russian grasslands, showed that the abundance of amorphous minerals was the only most important factor determining the size of the soil C pool. The physical protection of C upon adsorption to the surface is the decisive factor. Even when marked differences in land use and climate occur, the C pool does not change once the C is stabilized. This can be seen in a comparison of recent data with archived soil material from the Russian steppe which shows minimal changes over a century. The recalcitrant C stock remained unchanged despite cultivation and global warming (Torn et al. 2002; Jandl et al. 2007). Soil properties play a dominant role in stabilization of soil C than site productivity (Jandl et al. 2007). In 13C tracer experiments, it was shown that the net accumulation of new tree-derived C can be greater in loamy soils with a low productivity than infertile sandy soils with a high productivity (Hagedorn et al. 2003).

INCREASE IN SOC THROUGH LAND MANAGEMENT CHANGES

Arable soils usually contain lesser SOC content when compared to equivalent grassland or forest soil hence, the conversion of such lands into forest may lead to SOC accumulation (Powlson et al. 2011). Poulton et al. (2003) experimented on two temperate regions' accumulation of SOC after changing land use pattern i.e. from arable to woodland. For centuries, the areas of land at both the sides were used as arable cropping which was later abandoned about 130 years ago, these were gradually converted to deciduous woodland. At one site, i.e. Broadhalk because the soil was treated with calcium carbonate earlier, soil pH remained neutral or alkaline and the soil depth of 0-69 cm (taking account of bulk density changes over the period of reversion), organic C

−1 was found to increase by 64 t C ha in mineral soil within 120 years. At the other site, Geescroft, soil was found to be acidic and at the same site depth, organic C was found

−1to increase by 44 t C ha in mineral soil plus litter layer over 118 years. At both the sites, the above ground tree biomass plus roots were calculated as 342 and 191 t C at Broadbalk and Geescroft, respectively (Powlson et al. 2011). A study by Goulding and Poulton (2005) also

−1estimated an increase of approximately 18 t ha SOC in topsoil when in the same region arable land was converted to permanent grassland over a 35-year period. Johnston et al. (2009) reported an increase in SOC after conversion from arable cropping to permanent grass and reaching a maximum after about 150 years in the temperate climate of southern UK.

1. Arable Land Conversion to Grassland or Forest

Accumulation of carbon in soil plus vegetation is a result of photosynthesis by the newly established vegetation as a result of a change in land use pattern i.e. from arable to grassland or forest. Some part of newly formed photosynthate is transferred to soil with the help of roots and litterfall. Some of its parts returned to the atmosphere through rapid decomposition; but a fraction of it becomes a component of SOM after stabilization in the soil (Pawlson et al. 2011). This SOM which is newly formed, a part of it is transformed into stable fractions (HF-OC) which remain in the soil for decades (Coleman and Jenkinson 1996). This transfer of C from the atmosphere to stabilized forms in the biosphere can be considered as genuine C transfer which would have occurred when soil remained under cropping. Such conversion of land use pattern can, therefore, contribute to climate change through C sequestration (Powlson et al. 2011). Until a new equilibrium C content in soil is attained, such sequestration of C will continue. The study by Poulson et al. (2003) shows that even if tree biomass was increasing, the SOC content had approached a new equilibrium value after approximately 160 years. Hence, from this study, it can be concluded that after land use change annual rate of C sequestration is much higher during early years which then decreases gradually (Poulton et al. 2003; Johnston et al. 2009; Powlson et al. 2011). Therefore, it is essential that the total potential for C sequestration should not be estimated on the basis of extrapolation of initial rapid increase in SOC (Powlson et al. 2011). Land use changes which follow SOC sequestration that are as described above are considered beneficial for climate change but still, there are some complications which must also be considered.

Firstly, land use changes from agriculture results in an alternation of fluxes in other GHG (Green House Gases). It is expected that due to such land management changes decrease in N O emission would take place 2

because of termination of manure inputs and N fertilizers but there are many exceptions to this. In some regions where N compounds are deposited in a substantial amount, the input of N in forest/grassland may be higher than uptake of N by plants thereby, N is available in a surplus quantity which may lead to increase in N O 2

emission (Powlson et al. 2011). Goulding et al. (1998) measured the N O emission rates of both woodland and 2

arable part for the Broadbalk experiment at Rothamsted Research, UK. In small areas of woodland at Rothamsted, the rate of N O emission was measured up 2

−1 −1to 1.6 kg N O ha year which is found to be 2

approximately equal when compared with the arable −1land of Broadbalk which is receiving 288 kg N ha

THE BOTANICA 89

Thirdly, indirect impacts on food production should also be considered when such arable land is converted to forest or grassland i.e. due to this if there is the removal of natural forest elsewhere in the world, then the benefits to climate change are neglected (Powlson et al. 2011). Searchinger et al. (2008) introduced this indirect land use change concept with respect to change in land use to biofuels from food crops but it can be

−1year . When larger area of Woodland at Rothamsted, Knott Wood was compared then it was found less but found to be higher than the arable land which is receiving

−1 −1144 kg N ha year . Deposition of N in these regions is −1 −1

estimated about 40 kg N ha year which is much higher than annual uptake of N by trees. In areas of Europe, North America and East Asia deposition of N is

−1 −1estimated as 20–60 kg N ha year . Hence, emission of N in these areas is also expected to be significant (Galloway et al. 2008). C sequestration benefit of afforestation can be partly counteracted due to the large production of N O (Powlson et al. 2011). According to 2

the conclusion made by Dalal and Allen (2008) tropical forest could act as C sink but due to the emission of N O 2

which is connected possibly by high N mineralization than its deposition, the contribution towards climate change become negligible (Powlson et al. 2011).

Secondly, although it is expected that there would normally be an increase in SOC after conversion of arable land to woodland, Calhoun Forest site in South Carolina, USA can be considered as an interesting exception to this (Powlson et al. 2011). The site was cotton field previous and some 50 years ago, loblolly pine was planted. In this site, SOC has decreased in the soil below 35 cm besides C accumulation in tree biomass and surface soil (Richter et al. 1999; de Richter et al. 2007). Such decrease can be because of higher transpiration rate of tree then cotton crops. Thus, results in drying of deeper horizons thereby, increasing the rate of SOC decomposition due to anaerobic conditions in the sub-soil (Powlson et al. 2011). Another example of such condition is demonstrated by Bashkin and Binkley (1998) in a wet tropical forest where sugarcane fields were converted to fast-growing eucalyptus trees. After 10-13 years, soil C increased under eucalyptus by 1150 g

−2 −2C m in the top 10 cm soil but decreased by 1010 g C m 13

in the 10-55 cm layer. When C concentrations were examined Bashkin and Binkley (1998) were able to conclude that input rates of eucalyptus carbon into this deeper layer were smaller as compared to the previously grown sugarcane inputs (Post et al. 2000). However, how common these situations are likely to be on a global basis is not known (Powlson et al. 2011).

applied for land removal from grassland regeneration, afforestation or other non-food uses. As the demand for food is increasing (Royal Society 2009), removal of agriculture lands indirectly creates pressure for land clearing elsewhere for food production. Hence, areas that are least suitable for food productions should be used for such land use changes which increases terrestrial C stock (Powlson et al. 2011). Productive use land previously abandoned from agriculture can also be used for such purpose (Powlson et al. 2011). Some of these areas can be, former arable land of Eastern Europe which are abounded due to social and economic pressure after 1990 and also degraded areas in South America (Powlson et al. 2011). Also, the C released due to re-use in these areas would be less than C released from areas cleared for such changes. Increasing the areas of grass, shrubs, and trees can be another approach with the help of increasing the width of field boundaries. In Great Britain, preliminary estimates by Falloon et al. (2004) suggests that C sequestration is possible through this strategy. (Powlson et al. 2011). The annual accumulation of C in soil and vegetation was found to be in the range of 0.1–1.2% of annual UK emission of CO and it is 2

dependent on the width (2–20 m) of field margin and also the type of vegetation. If the estimated decrease in N O 2

emissions were considered then this offset was twice of this amount (Powlson et al. 2011).

2. Field Practices Enhancing Carbon Sequestration

Increasing demand for food does not allow us to convert food land to forest or grassland hence, it has become mandatory to include practices which enhance carbon sequestration in the crop fields also. If soil disturbance is minimized there is decreased SOC decomposition rate. Such changes also results in decrease of carbon transfer to the atmosphere from soil thereby, resulting in genuine climate change mitigation (Powlson et al. 2011) but there are many other factors that control such processes. The actual accumulation of C in soil, when specific management practices or cropping is considered more relevant besides; measuring emissions which provide a dynamic of C cycle for a particular period in time (Antille et al. 2015). Blanco-Canqui and Lal (2007), measured C storage to 600 mm depth in the profile and highlighted that it is important to assess the dynamics of sequestration to sufficient depth in order to monitor all changes in C. They compared no-tillage cropping over plow tillage in 13 major land resource areas of eastern U.S. They found that over this depth profile, there is no significant difference between no-tillage and plow tillage with respect to SOC sequestration (considering difference in bulk density) and these results were in accord with 14 out of 16 other

90 THE BOTANICA

In Summary, the global organic C stock in soil is in the range 684-724 Pg up to 30 cm depth and 1462-1548 Pg up to 1m depth (Batjes 1996). This soil organic C quantity is estimated to be about twice the amount of atmospheric C in the form carbon dioxide and thrice the global above-ground vegetation (Powlson et al. 2011). Considering changes in the land management practices

studies which assessed tilled and NT systems (e.g., Franzluebbers et al. 2012). Although, in some other publication, for example, Blanco- Canqui and Lal (2004) they have stated that “any cultivation practice that reduces the disruption of aggregates will enhance SOC sequestration.” Blanco-Canqui and Lal (2008), addressed it to some extent and stated “SOC under No-Tillage may be more stable with less turnover time and less seasonal changes than that under plowing.” But, the absence of reported differences between no-tillage and plow tillage is still unexplained (Antille et al. 2015). In Bronick and Lal (2005) it was clearly indicated that “the effectiveness of SOC in forming stable aggregates is related to its rate of decomposition” and “any practices that minimize soil disruption enhance aggregation and structural development.” Denef et al. (2004), suggested more SOC stable aggregated formation i.e. 90% of extra SOC is formed to a 200mm depth within micro-aggregates under No-tillage cropping when compared to moldboard plow-based system. Therefore, the intensity, type of tillage, and conditions under is largely responsible for the exposure previously protected SOC and formation of new aggregates (Antille et al. 2015). Smith and Faloon (2005) have suggested many options, which including reduced tillage and no-tillage, soil application of organic amendments, improved crop rotations, and intensification, among other measures. Lavelle et al. (1996) suggested that crop residues and manure can be used to enhance soil biodiversity thereby, enhances SOC concentration and soil quality. Nambiar (1994) reported an increase in mean weight diameter of soil aggregated in response to addition of manures for 14 years and the soils of Palampur showed an increase in mean weight diameter from 1.78 mm to 3.93 mm.

Komatsuzaki and Ohta (2007) recognized benefits of cover crops in sustainable soil management as scavengers of residual soil N (Antille et al. 2015) and have potential to sequester C. These cover crops enhances the quality of residue input, biodiversity and SOC pool (Lal 2004). Smith (2004) highlights that “soil C sequestration could meet at most about one-third of the current yearly increase in atmospheric CO , but the 2

duration of the effect would be limited, with significant impacts lasting between 20 and 50 years.”(Antille et al. 2015). Batjes NH 1996. Total carbon and nitrogen in

the soils of the world. Eur J Soil Sci 47: 151–163.

which are considered to enhance soil C sequestration, these practices should be critically examined as for whether there is a genuine transfer of C from the atmosphere to stable form in the biosphere is taking place or not. Also other issues like alteration in the fluxes of greenhouse gases which arises when arable land is converted to forest or grassland as discussed in the study by Goulding et al. (1998), Galloway et al. (2008) should be taken into account and strategy should mainly focus on reducing the disturbance due to afforestation. Re-vegetation in degraded land or land of limited agricultural value can be considered as a strategy for soil C sequestration but when only indirect land use change is avoided and minimum impact on food production. For example, Preger et al. (2010) reported an increase in

-1 SOC up to 9-15 t C ha range in the Highveld of South Africa where degraded cropland in which productivity is already very low was converted into pasture within 30 years (Powlson et al. 2011). In field practices, non-tillage cropping may not be sufficient to maintain and restore soil structural conditions in the presence of random traffic causing structural degradation, which thereby may not offer an opportunity for improvements in soil quality in the long run. But it can be reversed with the application of precision agricultural technologies such as no-tillage cropping with controlled traffic farming (Antille et al. 2015).

Antille DL, WCT Chamen, JL Tullberg, and R Lal 2015. The potential of controlled traffic farming to mitigate greenhouse gas emissions and enhance carbon sequestration in arable land: A critical review. In: American Society of Agricultural and Biological Engineers 58: 707-731.

Beare MH, ML Cabrera, PF Hendrix, and DC Coleman 1994. Aggregates-protected and unprotected organic matter pools in conventional and no-tillage soils. In: Soil Science Society of American Journals 58: 787-795.

Angers DA and MR Carter 1996. Aggregation and organic matter storage in cool, humid agricultural soils. Structure and Organic Matter Storage in Agricultural Soils (eds MR Carter & BA Stewart), pp. 193–211. CRC Press, Boca Raton, FL.

Bashkin MA and D Binkley1998. Changes in soil carbon following afforestation in Hawaii. Ecology 79: 828–833.

REFERENCES

THE BOTANICA 91

Cambardella CA and ET Elliot 1993. Carbon and nitrogen distribution in aggregates from cultivated and native grassland soils. Soil Sci Soc Amer J 57: 1071-1076.

Blanco-Canqui H and R Lal2004. Mechanisms of carbon sequestration in soil aggregates. Crit Rev Plant Sci 23: 481-504.

Dalal RC and DE Allen 2008. Greenhouse gas fluxes from natural ecosystems. Australian J Bot 56: 396–407.

Christensen BT 1996. Matching measurable soil organic matter fractions with conceptual pools in simulation models of carbon turnover: Revision of model structure. In: Evaluation of Soil Organic Matter Models (eds Powlson DS et al.), NATO ASI Series 1, Vol 38. Springer, Berlin.

Blanco-Canqui H and R Lal 2007. Regional assessment of soil compaction and structural properties under no-tillage farming. SSSA J 71: 1770-1778.

Bremer E, HH Janzen, and AM Johnston 1994. Sensitivity of total, light fraction and mineralizable organic matter on management practices in a Lethbridge soil. Can J Soil Sci 74: 131-138.

Biederbeck VO, HH Janzen, CA Campbell, and RP Zentner1994. Labile soil organic matter as influenced by cropping practices in an arid environment. Soil Biol Biochem 26: 1647-1656.

Bronick CJ and R Lal 2005. Soil structure and management: A review. Geoderma, 124, 3-22.

Cambardella CA and ET Elliot 1992. Particulate soil organic-matter changes across a grassland cultivation sequence. Soil Sci Soc Amer J 56: 777-783.

Coleman K and DS Jenkinson 1996. RothC-26.3: a model for the turnover of carbon in soil. Evaluation of Soil Organic Matter Models Using Existing Long-Term Datasets, NATO ASI Series I, Volume 38 (eds D.S. Powlson, P. Smith & P.J.U. Smith), 237–246. Springer- Verlag, Heidelberg.

de Richter DB, M Hofmockel, MA Callaham, DS Powlson, and P Smith 2007. Long-term soil experiments: keys to managing earth's rapidly changing ecosystems. Soil Science Society of America Journal 71: 266–279.

Denef K, J Six, R Merckx, and K Paustian 2004. Carbon sequestration in microaggregates of no-tillage soils with different clay mineralogy. SSSA J. 68: 1935-1944. Powlson DS, AP Whitmore, and KWT Goulding

Falloon P, DS Powlson, and P Smith 2004. Managing field margins for biodiversity and carbon sequestration. Soil Use & Management 20, 240–247.

Hagedorn F, D Spinnler D, M Bundt, P Blaser, and R Siegwolf 2003. The input and fate of new C in two forest soils under elevated CO . Global Change Biology 2

9: 862–872.

Galloway JN, AR Townsend, JW Erisman, M Bekunda, ZC Cai, JR Freney et al. 2008. Transformations of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320: 889–892.

Keil RG, DB Muntlucon, FG Prahl, and JI Hedges 1994. Sorptive preservation of labile organic matter in marine sediments. Nature 370: 549–552.

Goulding KWT, NJ Bailey, NJ Bradbury, P Hargreaves, MT Howe, DV Murphy et al. 1998. Nitrogen deposition and its contribution to nitrogen cycling and associated soil processes. In: New Phytol 139: 49–58.

Goulding KWT and PR Poulton2005. The missing link. In Geoscientist 15: 4–7.

Jandl R, M Lindner, L Vesterdal, B Bauwens, R Baritz, F Hagedorn, DW Johnson, K Minkkinen, and KA Byrne 2007. How strongly forest management influence soil carbon sequestration. Geoderma 137: 253-268.

Johnston AE, PR Poulton, and K Coleman 2009. Soil organic matter: its importance in sustainable agriculture and carbon dioxide fluxes. Advances in Agronomy 101: 1–57.

Kennedy MJ, DR Pevear, and RJ Hill 2002. Mineral surface control of organic carbon in black shale. Science 295: 657–660.

Komatsuzaki M and H Ohta2007. Soil management practices for sustainable agro ecosystems. Sustainability Sci. 2(1): 103-120.

Lal R (2004a). Soil carbon sequestration to mitigate climate change. Geoderma 123: 1-22.

Post W and K Kwon 2000. Soil carbon sequestration and land-use change: processes and potential. Global Change Biology 6: 317–328.

Poulton PR, E Pye, PR Hargreaves, and DS Jenkinson 2003. Accumulation of carbon and nitrogen by old arable land reverting to woodland. Global Change Biology 9: 942–955.

92 THE BOTANICA

2011. Soil carbon sequestration to mitigate climate change: A critical re-examination to identify the true and the false. Eur J. Soil Sci 62: 42-55.

Royal Society 2009. Reaping the Benefits: Science and the Sustainable Intensification of Global Agriculture. Royal Society Policy Report 11/09, Royal Society, London.

Powlson DS, AB Riche, K Coleman, MJ Glendining and AP Whitmore 2008. Carbon sequestration in European soils through straw incorporation: limitations and alternatives. Waste Management 28: 741–746.

Preger AC, R K¨osters, CC Du Preez, S Brodowski and W Amelung 2010. Carbon sequestration in secondary pasture soils: a chronosequence study in the South African Highveld. Eur J Soil Sci 61: 551–563.

Richter DD, D Markewitz, SA Trumbore, and CG Wells 1999. Rapid accumulation and turnover of soil carbon in a re-establishing forest. Nature 400: 56–58.

Searchinger T, R Heimlich, RA Houghton, F Dong, A Elobeid, J Fabiosa, et al. 2008. Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change. Science Express 319: 1238-1240.

Skjemsted WH, RE LeFeuvre, and RE Prebble 1990. Turnover of soil organic matter under pasture as

13determined by C natural abundance. Australian Journal of Soil Research 28: 267-276.

Smith KA and F Conen 2004. Impacts of land management on fluxes of trace greenhouse gases. Soil

Six J, P Callewaert, S Lenders, SD Gryze, SJ Morris, EG Gregorich, EA Paul, K Paustian, 2002a. Measuring and understanding carbon storage in afforested soils by physical fractionation. Soil Sci Soc Amer J 66: 1981–1987.

Six J, RT Conant, EA Paul, and K Paustian, 2002b. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant and Soil 241: 155–176.

Wattel-Koekkoek E, P Buurman, J van der Plicht, E Wattel, and N van Breemen 2003. Mean residence time of soil organic matter associated with kaolinite and smectite. Eur J Soil Sci 54: 269–278.

Smith P, DS Powlson, MJ Glendining, and JU Smith 1997. Potential for carbon sequestration in European soils: preliminary estimates for five scenarios using results from long-term experiments. Global Change Biology 3: 67–79.

Torn MS, SE Trumbore, OA Chadwick, PM Vitousek, and DM Hendricks 1997. Mineral control of soil organic carbon storage and turnover. Nature 389: 170–173.

Sollins P, P Homann, and B Caldwell 1996. Stabilization and destabilization of soil organic matter: mechanisms and controls. Geoderma 74: 65–105.

Torn MS, AG Lapenis, A Timofeev, ML Fischer, BV Babikov, and JW Harden 2002. Organic carbon and carbon isotopes in modern and 100-yearold- soil archives of the Russian steppe. Global Change Biology 8: 941–953.

Spycher G, P Sollins, and S Rose 1983. Carbon and nitrogen in the light fraction of a forest soil: Verticle distribution and seasonal patterns. Soil Science 135: 79-87.

Snyder VA and MA Vazquez 2005. Structure. Encyclopedia of Soils in the Environment, Volume 4 (eds D. Hillell, J.L. Hatfield, D.S. Powlson, C. Rozenweig, K.M. Scow, M.J. Singer et al.), pp.54–68. Elsevier, Oxford.

Sun W, Y Huang, W Zhang and Y Yu 2010. Carbon sequestration and its potential in agricultural soils of China. Global Biogeochemical Cycles 24: 1302–1307.

Smith P and P Falloon 2005. Carbon sequestration in European croplands. SEB Exp Bio Series 47-55.

Use Mgmt. 20: 255- 263.

93THE BOTANICA