Soil carbon dynamics as influenced by tillage and crop residue management in loamy sand and sandy loam soils under smallholder farmers’ conditions in Malawi

Conservation agriculture (CA) characterised by minimal soil disturbance, permanent soil surface cover by dead or living plants and crop rotations is one way of achieving higher soil organic carbon (C) in agricultural fields. Sandy loam and loamy soil samples from zero tillage (ZT) and conventional tillage (CT) plots were taken from farmers’ fields during the dry season in August 2006. Soil organic carbon (SOC) and soil organic nitrogen (SON), microbial biomass carbon (MB-C) and microbial biomass nitrogen (MB-N), C mineralization and SOC distribution in particle size fractions in 0–20 cm layer were evaluated. Forty eight farmers’ fields were randomly sampled at four different locations in Central and Northern Malawi, representing ZT plots maintained for a different number of years, and ten fields under CT with similar soil type and crop grown were selected. SOC and SON in ZT fields were 44 and 41 % (4 years ZT) and 75 and 77 % (5 years ZT) higher, respectively, than CT plots. MB-C and MB-N in ZT fields were 16 and 44 % (4 years ZT) and 20 and 38 % (5 years ZT) higher, respectively, than CT plots. However, MB-C and MB-N in ZT fields were 27 and 25 % (2 years ZT) and 17 and 9 % (3 years ZT) lower than in CT plots. The proportion of the total organic C as microbial biomass C was relatively higher under CT than ZT treatments. The higher SOC and MB-C content in the ZT fields resulted in 10, 62, 57 % higher C mineralization rate in ZT plots of 3, 4 and 5 years of loamy sand soils and 35 % higher C mineralization rate in ZT plot of 2 years than CT of sandy loam soils in undisturbed soils in the laboratory. Simulating plough from the undisturbed soils that were used for C mineralization experiment resulted in linear curves indicating that all organic C was already depleted during the first incubation period. The relative distribution of soil organic matter (SOM) in silt and clay size fractions was strongly correlated (r = 0.907 and P ≤ 0.01) with silt percentages. Easily degradable carbon pool (C_A,f) was correlated (r = 0.867 and P ≤ 0.05) with organic carbon in sand size fraction. In developing viable conservation agriculture practices to optimize SOC content and long-term sustainability of maize production systems, priority should be given to the maintenance of C inputs, crop rotations and associations and also to reduced soil disturbance by tillage.     

Effects of tillage systems on soil organic carbon dynamics, structural stability and crop yields in irrigated wheat (Triticum aestivum L.)–cotton (Gossypium hirsutum L.) rotation in semi-arid Zimbabwe

In southern Africa, tillage research has focused on rainfed smallholder cropping systems, while literature on high-input irrigated cropping systems is limited. We evaluated the effects of conventional (CT), minimum (MT) and no-till (NT) tillage systems on soil organic carbon (SOC), bulk density, water-stable aggregates (WSA), mean weighted diameter (MWD) and crop yields in an irrigated wheat–cotton rotation. Soil data were monitored in the first and final year, while yields were monitored seasonally. Average bulk densities (1.5–1.7 Mg m⁻³) were similar among tillage systems, but often exceeded the critical limit (1.60 Mg m⁻³) for optimum root growth. Conversion from CT to MT and NT failed to ameliorate the high bulk densities associated with the alluvial soil. SOC (g kg⁻¹) at 0–15 cm was higher (P < 0.05) under MT (3.9–5.8) and NT (4.2–5.6) than CT (2.9–3.3). Corresponding horizon SOC stocks (Mg C ha⁻¹) for the tillage treatments were; 9.3–13.9 (MT), 9.3–13.5 (NT) and 7.3–7.7 (CT). In the final year, significant (P < 0.05) tillage effects on SOC stocks were also observed at 15–30 cm. Cumulative SOC stocks (Mg C ha⁻¹) in the 0–60 cm profile were higher (P < 0.05) under MT (32.8–39.9) and NT (32.9–41.6) than CT (27.8–30.9). On average, MT and NT sequestered between 0.55 and 0.78 Mg C ha⁻¹ year⁻¹ at 0–30 cm depth, but a net decline (0.13 Mg C ha⁻¹ year⁻¹) was observed under CT. At 0–30 cm, MT and NT had higher (P < 0.05) MWD (0.19–0.23 mm) and WSA (2.3–3.5%) than CT (MWD: 0.1–0.12 mm, WSA: ≈1.0%). Both MWD and WSA were significantly (P < 0.05) correlated to SOC. Seasonal yields showed significant (P < 0.05) tillage effects, but 6-year mean yields (t ha⁻¹) were similar (CT: 4.49, MT: 4.33, NT: 4.32 for wheat; CT: 3.30, MT: 2.82, NT: 2.83 for cotton). Overall, MT and NT improved soil structural stability and carbon sequestration, while impacts on crop productivity were limited. Therefore, MT and NT are more sustainable tillage systems for the semi-arid regions than conventional tillage. S. Chakanetsa—Deceased.     

Soil and vegetation carbon pools in a mountainous watershed of Nepal

Assessment of carbon stocks in vegetation and soil is a basic step in evaluating the carbon sequestration potential of an ecosystem. We collected soil (core and composite) samples from 0–10, 10–20, 20–40, and 40–70 cm depths, or down to the bed rock, in the soil profile of four types of forest (managed dense Shorea (DS), degraded forest (DF), pine mixed (PS), and Schima–Castanopsis (SC) forest) and two types of cultivated land (irrigated low land (Khet) and rain-fed upland (Bari)) in the Pokhare Khola watershed of Nepal. In addition to other essential properties, soil bulk density and carbon concentration were assessed. Fine roots were also collected from each sampling site. The biomass of standing trees and shrubs was estimated by using allometric relationships after measuring their diameter and height, while the biomass of grasses was estimated by a direct measurement of grass from a defined area. The carbon stocks in all forest vegetation (trees, shrubs, and ground grass) and in the soil profiles under different land uses were estimated. The vegetation carbon pool was largest in DS forest (219 ± 34 Mg ha⁻¹) and least in SC forest (36 ± 5 Mg ha⁻¹), while its order among forest types was DS > DF > PS > SC. The soil organic carbon (SOC) pool was largest in Bari land (15.7 ± 1.5 kg C m⁻²) and least in PS forest (6.2 ± 0.5 kg C m⁻²) but the overall order among land uses was Bari > DF > Khet > SC > DS > PS. The total SOC stock in the whole watershed was 59 815 Mg, of which 36, 32, and 32% were in the 0–20, 20–40, and >40 cm soil depths, respectively. In the surface layer (0–10 cm), SOC stock was highest in Bari (36%) followed by DS (31%), and least was in PS forest (3%). This distribution pattern can primarily be assigned to SOC concentration and area covered by these land uses.     

Erratum to: Long-term tillage,straw and N rate effects on quantity and quality of organic C and N in a Gray Luvisol soil

Long-term use of soil, crop residue and fertilizer management practices may affect some soil properties, but the magnitude of change depends on soil type and climatic conditions. Two field experiments with barley, wheat, or canola in a rotation on Gray Luvisol (Typic Cryoboralf) loam at Breton and Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, were conducted to determine the effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem] and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha⁻¹ in S_Ret, and 0 kg N ha⁻¹ in S_Rem plots) on pH, extractable P, ammonium-N and nitrate–N in the 0–7.5, 7.5–15, 15–30 and 30–40 cm or 0–15, 15–30, 30–60, 60–90 and 90–120 cm soil layers. The effects of tillage, crop residue management and N fertilization on these chemical properties were usually similar for both contrasting soil types. There was no effect of tillage and residue management on soil pH, but application of N fertilizer reduced pH significantly (by up to 0.5 units) in the top 15 cm soil layers. Extractable P in the 0–15 cm soil layer was higher or tended to be higher under ZT than CT, or with S_Ret than S_Rem in many cases, but it decreased significantly with N application (by 18.5 kg P ha⁻¹ in Gray Luvisol soil and 20.5 kg P ha⁻¹ in Black Chernozem soil in 2007). Residual nitrate–N (though quite low in the Gray Luvisol soil in 1998) increased with application of N (by 17.8 kg N ha⁻¹ in the 0–120 cm layer in Gray Luvisol soil and 23.8 kg N ha⁻¹ in 0–90 cm layer in Black Chernozem soil in 2007) and also indicated some downward movement in the soil profile up to 90 cm depth. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, elimination of tillage and retention of straw increased but N fertilization decreased extractable P in the surface soil. Application of N fertilizer reduced pH in the surface soil, and showed accumulation and downward leaching of nitrate–N in the soil profile.     

Tillage and residue management effects on soil carbon and CO<Subscript>2</Subscript> emission in a wheat–corn double-cropping system

The mitigation of CO₂ emission into the atmosphere is important and any information on how to implement adjustments to agricultural practices and improve soil organic matter (SOM) stock would be helpful. We studied the effect of tillage and residue management on soil carbon sequestration and CO₂ emissions in loam soil cropped in a winter wheat–corn rotation in northern China. There were five treatments: mouldboard ploughing, rotary tillage and no-tillage with chopped residues (MC, RC and NC), additional no-tillage with whole residue (NW) and mouldboard ploughing without residue (CK). After 5 years of each tillage system, MC and RC had higher annual CO₂ efflux from soil. The CO₂ effluxes were correlated with the ratio of dissolved organic carbon to soil microbial biomass (DOC/MBC) among treatments. This effect may be due to less immobilization of soil carbon by microorganisms under long-time intensive tillage. Although both MBC and DOC showed seasonal variability, when averaged across the sampling period only MBC discriminated between treatments. After 5 years of tillage, all treatments except CK increased SOM (0.16–0.99 Mg C ha⁻¹ year⁻¹) at 0–30 cm depth and NC was the greatest, resulting from historical SOM depletion and large C return from recent residues. Despite the lowest CO₂ flux being from the NW treatment, lower input residue from decreased biomass may have lowered C sequestration. To improve soil C sequestration in rotations, the input of residue and the CO₂ emission should be balanced by adopting appropriate tillage and residue management.     

Soil organic carbon sequestration in tropical areas. General considerations and analysis of some edaphic determinants for Lesser Antilles soils

Some general notions on soil organic carbon (SOC) sequestration and the difficulties to evaluate this process globally are presented. Problems of time- and space- scales are emphasized. SOC erosion, which is generally difficult to evaluate in relation to land use changes, is discussed in detail. Different aspects of SOC sequestration on the Lesser Antilles are presented for a wide range of soil types. Comparisons between soils revealed that the SOC stocks in the Lesser Antilles are highly dependent upon the mineralogy: higher stocks for allophanic (ALL) soils than for low activity clay (LAC) and high activity clay (HAC) soils. But in terms of potential of SOC sequestration (pSeq-SOC, differences between permanent vegetation and continuous cultivation situations), there are no differences between ALL and LAC soils (22.9 and 23.3 tC. ha⁻¹, respectively). On the other hand, the potentials of SOC sequestration were higher for HAC soils (30.8 – 59.4 tC. ha⁻¹, with the higher levels in the less Mg- and Na-affected Vertisol). Sheet erosion is a serious problem for Vertisol with high Mg and Na on exchange complex, causing high dispersability of fine elements. Thus, the lower SOC levels in these soils may be partly due to erosion losses. Laboratory incubations have shown that 37 – 53% of the protected SOC in these soils was located in aggregates larger than 0.2 mm. The effect of agricultural practices on SOC sequestration was studied for the Vertisols. Intensification of pastures led to higher plant productivity and higher organic matter restitutions and SOC sequestration. The gain was 53.5 and 25.4 tC. ha⁻¹ for the low and high-Mg Vertisol, respectively (0–20 cm layer). SOC sequestration with pastures also depends upon the plot history with lower mean annual increase in SOC for the initially eroded (1.0 gC . kg⁻¹ soil . yr⁻¹) than for the non-degraded (1.5 gC . kg⁻¹ soil . yr⁻¹) Vertisol. Loss of SOC in a pasture-market gardening rotation was 22.2 tC . ha⁻¹ with deep (30–40 cm) and 10.7 tC . ha⁻¹ with surface (10–15 cm) tillage. It was unclear whether the differences in SOC losses were due to mineralization and/or to erosion. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil organic carbon changes and distribution in cultivated and restored grassland soils in Saskatchewan

The impacts of grassland restoration on amounts, forms and distribution of soil organic carbon (SOC) were examined in paired cultivated and restored grassland catenae of the Missouri Coteau region in south-central Saskatchewan, Canada. Total SOC (0–15 cm depth) and light fraction organic carbon (LFOC) (0–7.5 cm) contents were determined in paired catenae in upland areas, and in the surface (0–15 cm) and at depth (>15 cm) in the wetland fringe areas. Mass of SOC was higher in the restored grassland catenae than in the cultivated equivalents. In both the cultivated and restored grassland catenae at the three sites, footslope positions consistently had a higher mass of SOC. However, the shoulder positions showed the greatest response in soil C sequestration to grass seed-down, with a 1.4–2.9 Mg ha⁻¹ year⁻¹ SOC increase apparent over an approximately eight-year period. The mass of LFOC and the proportion of SOC comprised of LFOC was also higher in the restored grassland, reflective of higher recent C inputs. Rates of C sequestration in the Missouri Coteau based on SOC differences in the paired comparisons were estimated to be 0.3–2.9 Mg C ha⁻¹ year⁻¹, depending upon site and slope position. In the wetland fringe region of the landscape, the three sites also had higher surface or subsurface SOC in the grassland restoration. In general, SOC changes at depth (below 15 cm) in the restored grasslands appeared to be less consistent than changes in SOC in the surface 0–15 cm soil. In conclusion, the findings suggest that a switch to permanent cover on these soils will significantly increase C sequestered in the soil.     

Changes in soil nutrient content and enzymatic activity under conventional and zero-tillage practices in an Indian sandy clay loam soil

For 3 years we studied the impact of different tillage practices on biological activity, major nutrient transformation potential in a sandy clay loam soil and crop yield in a Himalayan subtemperate region. Field agroecosystems with a rotation of two grain crops per year (lentil-finger millet) received four different tillage practices: zero–zero (ZZ), conventional–conventional (CC), zero–conventional (ZC), and conventional–zero (ZC) tillage. Most of the chemical parameters were influenced by the type of tillage practice. ZZ increased the soil organic carbon (SOC) content in the upper soil layer from 6.8 to 7.5 mg g⁻¹ soil. Similarly available N was increased by 6.1% in ZZ over CC. Under zero tillage soil generally had higher P and K content than under other tillage practices. Soil carbohydrate content was also increased from 3.1 to 4.9 mg g⁻¹ and dehydrogenase activity was also increased significantly under continuous zero-tillage practice. Alkaline phosphatase, protease, and cellulase were most sensitive to changes due to tillage management. Alkaline phosphatase and protease activity was greater (by 9.3–48.1%) in the zero-tillage system over conventional practice. In contrast, cellulase activity was more (by 31.3–74.6%) in conventional practice than other management practices. We suggest that, by understanding the effects of tillage on soil biological properties, soil quality and agricultural sustainability of subtemperate hill agro-ecosystems may be improved.     

Soil carbon stocks under present and future climate with specific reference to European ecoregions

World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO₂ ever since settled agriculture began about 10–13 millennia ago. The amount of CO₂-C emitted into the atmosphere is estimated at 136 ± 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 ± 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30–50% over 50–100 years in temperate regions, and 50–75% over 20–50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4–6°C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called “CO₂ fertilization effect”, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO₂ in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO₂ if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO₂ fertilization effect. The magnitude of CO₂ fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70–190 Tg C yr⁻¹. This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.     

Nitrogen and residue management effects on agronomic productivity and nitrogen use efficiency in rice–wheat system in Indian Punjab

Development of a sustainable and environment friendly crop production system depends on identifying effective strategies for the management of tillage and postharvest crop residues. Three-year (2004–2007) field study was initiated on two soil types to evaluate the effect of straw management (burning, incorporation and surface mulch) and tillage (conventional tillage and zero tillage) before sowing wheat and four nitrogen rates (0, 90, 120 and 150 kg N ha⁻¹) on crop yields, N use efficiency, and soil fertility in the northwestern India. Effect of tillage and straw management on nitrogen transformation in soils was investigated in a laboratory incubation study. In sandy loam, grain yield of wheat with straw mulch-zero-till (ZT) was 7% higher compared to when residues were burnt-ZT but it was similar to straw burnt-conventional till (CT), averaged across 3 years. In silt loam, grain yield of wheat with straw mulch-ZT was 4.4% higher compared to straw incorporated-CT, but it was similar to straw burnt-CT. Response to N application was generally observed up to 150 kg N ha⁻¹ except in 2004–2005 on sandy loam where N response was observed up to 120 kg N ha⁻¹, irrespective of straw and tillage treatments. In sandy loam, RE was lower (49%) for straw burnt-ZT than in other treatments (54–56%). In silt loam, RE was higher in straw mulch-ZT compared with straw incorporation-CT (65 vs. 58%). In sandy loam, AE was higher in straw burnt-CT and straw mulch-ZT compared with the other treatments (19.2 vs. 16.9 kg grain kg⁻¹ N applied). In silt loam, AE was lower in straw incorporation-CT than in other treatments (16.0 vs. 17.6 kg grain kg⁻¹ N applied). Rice yield and N uptake were not influenced by straw and tillage management treatments applied to the preceding wheat. Recycling of rice residue (incorporation and surface mulch) compared with straw burning increased soil organic carbon and the availability of soil P and K. There was more carbon sequestration in rice straw mulch with zero tillage (25%) than in straw incorporation with conventional tillage (17%). Soil N mineralization at 45 days after incubation was 15–25% higher in straw retention plots compared with on straw burnt plots.     

Measurement of Net Global Warming Potential in Three Agroecosystems

When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – needs to be considered. Storage of atmospheric CO₂ into stable organic carbon pools in the soil can sequester CO₂ while common crop production practices can produce CO₂, generate N₂O, and decrease the soil sink for atmospheric CH₄. The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO₂, i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO₂ equivalents ha⁻¹ y⁻¹. Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N₂O emission rates decline or increase with time after conversion to NT.     

Effects of 15 years of manure and inorganic fertilizers on soil organic carbon fractions in a wheat-maize system in the North China Plain

Soil organic carbon (SOC) and its labile fractions are strong determinants of chemical, physical, and biological properties, and soil quality. Thus, a 15-year experiment was established to assess how diverse soil fertility management treatments for winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) cropping system affect SOC and total N (TN) concentrations in the North China Plain. The field experiment included three treatments: (1) unfertilized control (CK); (2) inorganic fertilizers (INF); and (3) farmyard manure (FYM). Concentrations of SOC, TN, and different labile SOC fractions were evaluated to 1-m depth. In comparison with INF and CK, FYM significantly increased SOC and TN concentrations in the 0–30 cm depth, and also those of dissolved organic C (DOC), microbial biomass C (MBC), hot-water extractable C (HWC), permanganate oxidizable C (KMnO₄–C), and particulate organic C (POC) in the 0–20 cm depth. Despite the higher crop yields over CK, application of INF neither increased the SOC nor the labile C fractions, suggesting that by itself INF is not a significant factor affecting SOC sequestration. Yet, POC (18.0–45.8% of SOC) and HWC (2.0–2.8%) were the most sensitive fractions affected by applications of FYM. Significantly positive correlations were observed between SOC and labile organic C fractions in the 0–20 cm depth. The data support the conclusion that, wherever feasible and practical, application of FYM is important to soil C sequestration and improving soil quality under a wheat/maize system in the North China Plain.     

Quantifying the effect of soil organic matter on indigenous soil N supply and wheat productivity in semiarid sub-tropical India

A field experiment was conducted on a loamy sand soil for six years to quantify the effect of soil organic matter on indigenous soil N supply and productivity of irrigated wheat in semiarid sub-tropical India. The experiment was conducted by applying different combinations of fertilizer N (0–180 kg N ha⁻¹), P (0–39 kg P ha⁻¹) and K (0–60 kg K ha⁻¹) to wheat each year. For the data pooled over years, fertilizer N together with soil organic carbon (SOC) and their interaction accounted for 75% variation in wheat yield. The amount of fertilizer N required to attain a yield goal decreased as the SOC concentration increased indicating enhanced indigenous soil N supply with an increase in SOC concentration. Besides SOC concentration, the soil N supply also depended on yield goal. For a yield goal of 4 tons ha⁻¹, each ton of SOC in the 15 cm plough layer contributed 4.75 kg N ha⁻¹ towards indigenous soil N supply. An increase in the soil N supply with increase in SOC resulted in enhanced wheat productivity. The contribution of 1 ton SOC ha⁻¹ to wheat productivity ranged from 15 to 33 kg ha⁻¹ across SOC concentration ranging from 3 to 9 g kg^-1 soil. The wheat productivity per ton of organic carbon declined curvilinearly as the native SOC concentration increased. The change in wheat productivity with SOC concentration shows that the effect of additional C sequestration on wheat productivity will depend on the existing SOC concentration, being higher in low SOC soils. Therefore, it will be more beneficial to sequester C in soils with low SOC than with relatively greater SOC concentration. In situations where the availability of organic resources for recycling is limited, their application may be preferred in soils with low SOC concentration. The results show that an increase in C sequestration will result in enhanced wheat productivity but the increase will depend on the amount of fertilizer applied and the existing fertility level of the soil.     

Soil organic carbon dynamics of improved fallow-maize rotation systems under conventional and no-tillage in Central Zimbabwe

Fallowing increases soil organic carbon (SOC) during the fallowing phase. However, this benefit is lost quickly during the cropping phase. The objective of this study was to evaluate SOC dynamics of an improved fallow-maize rotation under no-tillage (NT) and conventional tillage (CT) from time of fallow termination, through the next two cropping seasons. The treatments studied were improved fallows of Acacia angustissima (A. angustissima) and Sesbania sesban (S. sesban), natural fallow and continuous maize. Our hypothesis is that fallowing maintained higher SOC and lower soil bulk densities through the cropping phase when compared with continuous maize system and that NT maintained higher SOC when compared with CT. Soil organic carbon was significantly greater under fallows than under continuous maize from fallow termination to the end of the second cropping season. Soil organic carbon for the 0–5 cm depths was 11.0, 10.0, 9.4 and 6.6 g kg⁻¹ for A. angustissima, S. sesban, natural fallow and continuous maize, respectively at fallow termination. After two cropping seasons SOC for the same depth was 8.0, 7.0, 6.1, 5.9 g kg⁻¹ under CT and 9.1, 9.0, 8.0, 6.0 g kg⁻¹ under NT for A. angustissima, S. sesban, natural fallow and continuous maize, respectively. Total SOC stocks were also higher under fallows when compared with continuous maize at fallow termination and after two cropping seasons. Soil bulk densities were lower under fallows when compared with continuous maize during the period of study. We concluded that fallows maintained greater SOC and NT sequestered more SOC than CT. Acacia angustissima was the better tree legume fallow for SOC sequestration when compared with S. sesban or natural fallow because it maintained higher SOC and lower bulk densities after two seasons of maize cropping.     

Long-term tillage,straw management and N fertilization effects on quantity and quality of organic C and N in a Black Chernozem soil

Soil, crop and fertilizer management practices may affect the amount and quality of organic C and N in soil. A long-term field experiment (growing barley, wheat, or canola) was conducted on a Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, to determine the influence of 19 (1980 to 1998) or 27 years (1980 to 2006) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem]and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha⁻¹ in S_Ret and 0 kg N ha⁻¹ in S_Rem plots) on total organic C (TOC) and N (TON), and light fraction organic C (LFOC) and N (LFON) in the 0–7.5 and 7.5–15 cm or 0–5, 5–10 and 10–15 cm soil layers. The mass of TOC and TON in soil was usually higher in S_Ret than in S_Rem treatment (by 3.44 Mg C ha⁻¹ for TOC and 0.248 Mg N ha⁻¹ for TON after 27 years), but there was little effect of tillage and N fertilization on these parameters. The mass of LFOC and LFON in soil tended to increase with S_Ret (by 285 kg C ha⁻¹ for LFOC and 12.6 kg N ha⁻¹ for LFON with annual rate of 100 kg N ha⁻¹ for 27 years)_, increased with N fertilizer application (by 517 kg C ha⁻¹ for LFOC and 36.0 kg N ha⁻¹ for LFON after 27 years), but was usually higher under CT than ZT (by 451 kg C ha⁻¹ for LFOC and 25.3 kg N ha⁻¹ for LFON after 27 years). Correlations between soil organic C or N fractions were highly significant in most cases. Linear regressions between crop residue C input and soil organic C or N were significant in most cases. The effects of tillage, straw management and N fertilizer on soil were more pronounced for LFOC and LFON than TOC and TON, and also in the surface layers than in the deeper layers. Tillage and straw management had little or no effect on C:N ratios, but the C:N ratios in light organic fractions significantly decreased with increasing N rate (from 20.06 at zero-N to 18.91 at 100 kg N ha⁻¹). Compared to the 1979 results, in treatments that did not receive N fertilizer (CTS_Rem0, CTS_Ret0, ZTS_Rem0 and ZTS_Ret0), CTS_Rem0 resulted in a net decrease in TOC concentration (by 1.9 g C kg⁻¹) in the 0–15 cm soil layer in 2007 (after 27 years), with little or no change in the CTS_Ret0 and ZTS_Rem0 treatments, while there was a net increase in TOC concentration (by 1.2 g C kg⁻¹) in the ZTS_Ret0 treatment. Straw retention and N fertilizer application at 50 and 100 kg N ha⁻¹ rates showed a net positive effect on TOC concentration under both ZT (ZTS_Ret50 by 2.3 g C kg⁻¹ and ZTS_Ret100 by 3.1 g C kg⁻¹) and CT (CTS_Ret50 by 3.5 g C kg⁻¹ and CTS_Ret100 by 1.6 g C kg⁻¹) treatments in 2007 compared to 1979 data. In conclusion, the findings suggest that retention of straw, application of N fertilizer and elimination of tillage would improve soil quality, and this might increase the potential for N supplying power of the soil and sustainability of crop productivity.     

Soil organic carbon stocks in soil aggregates under different land use systems in Nepal

We investigated the soil organic carbon (SOC) associated with various aggregate size fractions in soil profiles under different land uses. Bulk soil samples were collected from incremental soil depths (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) from sites with the four dominant land use types [forest, grazing land, irrigated rice in level terraces (Khet) and upland maize-millet in sloping terraces (Bari)] of the Mardi watershed (area 144 km²), Nepal. The bulk soil was separated into five aggregate size fractions and the associated SOC contents were determined. Soil physical parameters necessary for estimating the soil SOC stock such as bulk density, stone and gravel content, and SOC content, were also measured for each soil depth. The SOC stock (mean ± SE, kg C m^–2) in the topsoil (0–10 cm) was higher in grazing land soil (3.4 ± 0.1) compared to forest soil (1.4 ± 0.2) and cultivated soil [Bari (2.0 ± 0.2) and Khet (1.2 ± 0.2)]. Forest and grazing lands had similar SOC contents, but the higher content of gravel and stone in forest soil resulted in a lower estimate of the SOC stock per unit area. The total SOC stock in the soil profile (to 1 m depth) over the entire watershed was estimated to be 721470 TC (tonnes of carbon). Its distribution was 52, 30, 11 and 7% in forestland, Bari, grazing land and Khet, respectively. The estimated depth wise distribution of SOC stock for 1 m soil depth in the entire watershed was 28, 22, 28 and 22% in the 0–10, 10–20, 20–40, and > 40 cm soil depths, respectively. There was a net loss of SOC stock (0–40 cm soil depth) of 29%, due to internal trading of land uses in the period from 1978 to 1996. Macro aggregates (> 1 mm) were found to be the dominant size in Bari and grazing land, whereas in forest and Khet soil micro aggregates (< 1 mm) dominated. Micro aggregates of size < 0.25 mm had a higher SOC concentration than aggregates of 0.25–0.5 mm, regardless of the depth or land uses and they may therefore contribute to soil C sequestration.     

Effects of agronomic practices on the soil carbon storage potential in arable farming in Austria

According to the Kyoto-Protocol for carbon dioxide mitigation the direct human induced sequestration potential of carbon in agricultural soils may in the future be included for calculating net changes in greenhouse gas emissions. Therefore we used long-term experiments on arable land in Austria differing strongly in climate and soil conditions to explore the effects of agronomic practices on changes in soil organic carbon content. Optimal mineral N fertilizer input increased the carbon stocks on an average to 2.1 t ha⁻¹compared with no N fertilization in a 36 years period. Additional farm yard manure application (10 t ha⁻¹ y⁻¹) enhanced carbon storage to about 5.6 t ha⁻¹ after 21 years. Site-specific influences must be considered. Losses of 2.4 t carbon per ha were caused by additional irrigation of sugar beet and maize in a rotation with cereals in a 21 years period. The incorporation of all crop residues resulted in an increase of 3.4 t ha⁻¹ organic carbon in topsoil after 17 years. In the uppermost soil layer (0–10 cm) minimum and reduced tillage treatment enhanced carbon stocks to about 4.7 t ha⁻¹ and 3.2 t ha⁻¹ compared to conventional soil management within a decade. Based on these results, only a limited soil carbon sequestration potential can be inferred: Manuring and incorporation of crop residues are well-proven practices on arable land and therefore no additional human induced carbon sequestration might be achieved. The adoption of minimum tillage on Phaeozems, Chernozems and Kastanozems could, roughly calculated, result in a supplementary carbon storage of about 0.6% of the entire present annual carbon dioxide emission in Austria. However, the storage of carbon in topsoil means only a mid-term sequestration. By changing practices in short-terms, these amounts of carbon might be a source of additional carbon dioxide in the future. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fertilization effects on yield sustainability and soil properties under irrigated wheat–soybean rotation of an Indian Himalayan upper valley

To date, the sustainability of wheat (Triticum aestivum)–soybean (Glycine max) cropping systems has not been well assessed, especially under Indian Himalayas. Research was conducted in 1995–1996 to 2004 at Hawalbagh, India to study the effects of fertilization on yield sustainability of irrigated wheat–soybean system and on selected soil properties. The mean wheat yield under NPK + FYM (farmyard manure) treated plots was ~27% higher than NPK (2.4 Mg ha⁻¹). The residual effect of NPK + FYM caused ~14% increase in soybean yield over NPK (2.18 Mg ha⁻¹). Sustainable yield index values of wheat and the wheat–soybean system were greater with annual fertilizer N or NPK plots 10 Mg ha⁻¹ FYM than NPK alone. However, benefit:cost ratio of fertilization, agronomic efficiency and partial factor productivity of applied nutrients were higher with NPK + FYM than NPK, if FYM nutrients were not considered. Soils under NPK + FYM contained higher soil organic C (SOC), total soil N, total P and Olsen-P by ~10, 42, 52 and 71%, respectively, in the 0–30 cm soil layers, compared with NPK. Non-exchangeable K decreased with time under all treatments except NPK. Total SOC in the 0–30 cm soil layer increased in all fertilized plots. Application of NPK + FYM also improved selected soil physical properties over NPK. The NPK + FYM application had better soil productivity than NPK but was not as economical as NPK if farmers had to purchase manure.     

Intensity cultivation induced effects on soil organic carbon dynamic in the western cotton area of Burkina Faso

The soil organic carbon (SOC) dynamic is a key element of soil fertility in savannah ecosystems that form the key agricultural lands in sub-Saharan Africa. In the western part of Burkina Faso, the land use is mostly linked to cotton-based cropping systems. Use of mechanization, pesticides, and herbicides has induced modifications of the traditional shifting cultivation and increased the need for sustainable soil fertility management. The SOC dynamic was assessed based on a large typology of land cultivation intensity at Bondoukui. Thus, 102 farm plots were sampled at a soil depth of 0–15 cm, considering field–fallow successions, the cultivation phase duration, tillage intensity, and soil texture. Physical fractionation of SOC was carried out by separating the following particle size classes: 2,000–200, 200–50, 50–20, and 0–20 μm. The results exhibited an increase in SOC stock, and a lower depletion rate with increase in clay content. After a long-term fallow period, the land cultivation led to an annual loss of 31.5 g m⁻² (2%) of its organic carbon during the first 20 years. The different fractions of SOC content were affected by this depletion depending on cultivation intensity. The coarse SOC fraction (2,000–200 μm) was the most depleted. The ploughing-in of organic matter (manure, crop residues) and the low frequency of the tillage system produced low soil carbon loss compared with annual ploughing. Human-induced disturbances (wildfire, overgrazing, fuel wood collection, decreasing fallow duration, increasing crop duration) in savannah land did not permit the SOC levels to reach those of the shifting cultivation system.     

Increasing organic C and N in soil under bromegrass with long-term N fertilization

Two field experiments were conducted on bromegrass (Bromus inermis Leyss.) on a thin Black Chernozem (Typic Boroll) at Crossfield, Alberta, Canada to determine the long-term effects of N fertilization on changes in concentration and mass of organic C and N in soil. In both experiments, bromegrass was harvested for hay each year. In the experiment where ammonium nitrate (AN) was applied annually at 0 to 336 kg N/ha for 27 consecutive years from 1968 to 1994, the concentration of total C in the 0–5 cm soil layer increased from 50.33 g/kg in the zero-N treatment to 61.64 g/kg with 56 kg N/ha and to 64.15 g/kg with the 112 kg N/ha rate. Total C in soil also increased in the 5–10, 10–15 and 15–30 cm layers but to a lesser extent. The mass of total C in the 0–30 cm soil layer was increased by 18.46 Mg/ha with 56 kg N/ha and by 23.38 Mg/ha with the 112 kg N/ha rate as compared to the zero-N treatment. Total N in soil followed a similar trend as total C. In the experiment which received four N sources [ammonium nitrate (AN), urea, calcium nitrate (CN) and ammonium sulphate (AS)] applied annually at 168 and 336 kg N/ha for 15 years from 1979 to 1993, the total C in soil was greater where N fertilizer was applied, but the increase in total C varied with N source. The concentration of total C in soil in the 0–5 cm layer tended to be greater with AN and AS than with CN, with the smallest increase from urea. The mass of total C in soil (average of four N sources) at the 168 kg N/ha rate was increased by 18.98 Mg/ha in 0–30 cm and by 43.48 Mg/ha in the 0–60 cm layer as compared to the check treatment. The concentration of total C in soil also increased in the deeper layers to a depth of 60 cm, but the increases were much smaller than in the 0–5 cm layer. The changes in total N in soil followed a similar pattern as total C. In conclusion, long-term annual additions of fertilizer N to bromegrass resulted in a marked increase in total C and N in soil and the increases were influenced by both rate and source of N fertilizer. The implications of these results are that grasslands can be managed to lessen the increase in atmospheric CO₂ concentration, while also improving fertility (N-supplying capacity) and tilth of soil. This revised version was published online in August 2006 with corrections to the Cover Date.