Carbon sequestration in soils of cool temperate regions (introductory and editorial)

The cool temperate climate, dominance of perennial land use, and relatively large proportion of peat and organically rich soils, make the northern European regions to have a large potential of soil organic carbon (SOC) sequestration. However, with predicted global warming soils in these areas may become sources of atmospheric CO₂. Quantitative and reliable assessment methods and understanding of the spatial variability of SOC pools are required to make accurate mean estimate of available C and integrate variability into predictive models of SOC reserves and sequestration potential. Advanced analytical methods such as near-infrared spectroscopy and carbon isotope techniques can be used to estimate retention time and C turnover rates in soils. The rehabilitation of peat lands has shown a potential for SOC sequestration ranging from 25 to 45 gCm⁻² yr⁻¹ in Scandinavian countries. The potential of SOC sequestration in agricultural and forestry ecosystems depends on the land use and management practice adopted. Furthermore, the proven land management practices (e.g. conservation tillage, reduced soil erosion, restoring wetlands and degraded lands) coupled with improved cultivation practices (e.g. judicious fertilizer use, crop rotations and cover crops) can make the soil of this region as C sink.     

Conservation tillage for carbon sequestration

World soils represent the largest terrestrial pool of organic carbon (C), about 1550 Pg compared with about 700 Pg in the atmosphere and 600 Pg in land biota. Agricultural activities (e.g., deforestation, burning, plowing, intensive grazing) contribute considerably to the atmospheric pool. Expansion of agriculture may have contributed substantially to the atmospheric carbon pool. However, the exact magnitude of carbon fluxes from soil to the atmosphere and from land biota to the soil are not known. An important objective of the sustainable management of soil resources is to increase soil organic carbon (SOC) pool by increasing passive or non-labile fraction. Soil surface management, soil water conservation and management, and soil fertility regulation are all important aspects of carbon sequestration in soil. Conservation tillage, a generic term implying all tillage methods that reduce runoff and soil erosion in comparison with plow-based tillage, is known to increase SOC content of the surface layer. Principal mechanisms of carbon sequestration with conservation tillage are increase in micro-aggregation and deep placement of SOC in the sub-soil horizons. Other useful agricultural practices associated with conservation tillage are those that increase biomass production (e.g., soil fertility enhancement, improved crops and species, cover crops and fallowing, improved pastures and deep-rooted crops). It is also relevant to adopt soil and crop management systems that accentuate humification and increase the passive fraction of SOC. Because of the importance of C sequestration, soil quality should be evaluated in terms of its SOC content. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Soil C balances in Swedish agricultural soils 1990–2004, with preliminary projections

Swedish agricultural land comprises about 3 Mha and its topsoil contains about 270 Mt C (0–25 cm depth). Based on daily climate data, annual yield data and a soil database, we calculate the topsoil C dynamics for Swedish agricultural land 1990–2004, using a soil C balance model, ICBM. Losses from high C (organic) soils are calculated from subsidence, which in turn is calculated from soil properties, cropping system and weather conditions. We also present scenarios and projections into the future. Mineral soils are close to balance in all of the eight agricultural regions investigated. Average soil C mass roughly increases from South to North, since the lower yields and thus C inputs in Northern regions are more than balanced by the higher decomposition rates due to warmer climate in the South. The higher proportion of grass leys in the North also contributes to higher C mass. High C soils (>7% C, corresponding to 12% soil organic matter content) lose 2–6 t C ha⁻¹ year⁻¹, depending on weather and cropping system, and total annual loss from Swedish agricultural high-C soils is about 1 Mt year⁻¹. This loss is discussed in the context of plant production and remedial actions. Projections into the future, assuming that a temperature increase leading to increased decomposition rates also will lead to higher yields, indicate a potential to at least maintain soil C mass in Swedish agricultural mineral soils. Growing crops with residues more resistant towards decomposition would be an efficient way to increase soil C mass. See also .     

Factors affecting organic carbon dynamics in soils of Nepal/Himalayan region – a review and analysis

We reviewed the factors and processes relevant to C (Carbon) stocks and dynamics in the soils of Hindu Kush-Himalayan region (HKH) in general, and Nepal in particular. Included in this paper are reviews of land use change, soil types, erosion, soil fertility status, land management and other pertinent information in relation to the SOC (Soil Organic Carbon) stock, dynamics and sequestration. Watershed degradation in the HKH region appears to be a serious problem affecting the SOC pool, which may be primarily attributed to deforestation, land use changes, forest degradation, soil erosion and fertility decline. Soils under degraded forest and grazing land and red soils were reported to have less than 1% SOC; however, well managed forests have considerably higher organic matter (SOC = 4%) levels than those cleared for cultivation. Our estimates show that both the soil and SOC losses are site specific, being as high as 256 kg C ha^–1 y^–1. Estimated net CO₂ losses from the erosion displaced SOC varied between <1 and 42 kg C ha^–1 y^–1 depending on initial SOC content and soil erosion rates in the specific sites. The land cover changes in the past 18 years in the two Nepalese watersheds, Mardi and Fewa, may have resulted in net loss of SOC stock (29% losses for Mardi and 7% losses for Fewa) compared to land cover in the base year (1978). The processes contributing to C pool, fluxes and sequestration are inadequately studied, and particularly in the HKH region, there is a lack of data on several essential aspects needed for estimating soil C fluxes and C sequestration potential. Systematic soil survey and long term experiments are needed on dominant soil types and land use systems of the HKH region for developing the database on soil fertility and SOC relationships to site specific management practices. Future research should focus upon generating data on spatial and temporal variation, depth distribution, quantification of various pools, and transport/translocation of SOC, as well as the establishment of soil/SOC databases, in relation to specific land use and management practices.     

The effect of reduced tillage agriculture on carbon dynamics in silt loam soils

Reduced tillage (RT) agriculture is an effective measure to reduce soil loss from soils susceptible to erosion in the short-term and is claimed to increase the soil organic carbon (SOC) stock. The change in distribution and total SOC stock in the 0–60 cm layer, the stratification of microbial biomass carbon (MB-C) content in the 0–40 cm layer and the carbon (C) mineralization in the upper 0–5 cm layer in silt loam soils in Western Europe with different periods of RT agriculture were evaluated. Ten fields at seven locations, representing the important RT types and maintained for a different number of years, and eight fields under conventional tillage (CT) agriculture with similar soil type and crop rotation were selected. RT agriculture resulted in a higher stratification of SOC in the soil profile than CT agriculture. However, the total SOC stock in the 0–60 cm layer was not changed, even after 20 of years RT agriculture. The MB-C was significantly higher in the 0–10 cm layer under RT agriculture, even after only 5 years, compared to CT agriculture. The higher SOC and MB-C content in the upper 0–5 cm layer of RT fields resulted in a higher C mineralization rate in undisturbed soil in the laboratory. Simulating ploughing by disturbing the soil resulted in inconsistent changes (both lower and higher) of C mineralization rates. A crop rotation with root crops, with heavy soil disturbance every 2 or 3 years at harvest, possibly limited the anticipated positive effect of RT agriculture in our research.     

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.     

Long-term impacts of pasture irrigation with treated sewage effluent on nutrient status of a sandy soil in Zimbabwe

Declining freshwater resources and the need to safely dispose wastewater have led to a rapid increase in wastewater reuse in developing countries. However, empirical evidence on the effects of effluent-irrigation on soil fertility is limited. The study investigated the nutrient status of a sandy soil after 26 years of effluent irrigation. Soil samples from effluent-irrigated and non-irrigated sites were analysed for pH, electrical conductivity (EC), soil organic carbon (SOC), total and plant available forms of N and P, exchangeable bases and trace metals. Analysis of effluent quality showed that, besides Cr and Cd, all measured parameters were within acceptable limits for wastewater irrigation. Our results revealed that effluent-irrigation significantly (P < 0.05) enriched the soil with essential nutrients for plant growth, which are commonly deficient in most soils of Zimbabwe. Effluent-irrigated soils had significantly (P < 0.05) higher pH, EC, SOC, total and available N and P and, exchangeable Ca and Mg at 0–30 cm-depth. However, apart from Cr accumulation, effluent irrigation significantly (P < 0.05) depleted Zn, Cu and Cd probably due to plant uptake and enhanced mobility under acidic soil pH. Cr accumulation and depletion and mobility of Zn, Cu and Cd in effluent-irrigated soils could threaten the sustainability of the practice. We recommend a review of the current management practices based on crop water requirements, effluent quality and environmental considerations.     

Carbon resources of residue and manure in Japanese farmland soils

This study calculated the carbon (C) input to farmland soils in Japan in an effort to investigate the potential increase in soil C of farmland soils by proper application of crop residues (straw and root) and manure. The calculation was based on inventory and activity data obtained from statistics, literature sources and inquiry reports for the year 2005. The total C resources from crop residues and manure in Japan were 6.1 Tg C year⁻¹ and 2.3 Tg C year⁻¹, of which 4.9 Tg C year⁻¹ and 1.9 Tg C year⁻¹, respectively, were applied to farmland soil. The average C application rate was 1.7 ± 1.6 Mg C ha farmland⁻¹ year⁻¹ and the proportion of manure was 23 ± 26%. One scenario that improved the allocation of manure and crop residue input to farmland soil increased the average C input to farmland soil to 1.8 ± 1.3 Mg C ha farmland⁻¹ year⁻¹. This agricultural C flow represented only a small percentage of the global warming potential of the whole of Japan. Thus, management of C resources in the agricultural sector should focus on the sustainable use of soil rather than the C sequestration potential of soil. To improve the C flow for areas with high C input, the transportation of manure to neighboring municipalities failed to reduce the excessive amount of manure since those areas are concentrated in only a few regions. Other measures were required to reduce environmental problems due to the over-supply of manure to farmland soils. For areas with low C input, the introduction of green manure, changes in cultivation methods, and land use type itself must be considered in relation to the individual C requirements specific to land use, soil type and climate conditions.     

Factors affecting the agronomic effectiveness of phosphate rock for direct application

Phosphorus (P) is critically needed to improve soil fertility for sustainable crop production in large areas of developing countries. In recent years, phosphate rock (PR) for direct application has been tested in tropical acid soils as a potential alternative to conventional water-soluble P fertilizers like single superphosphate (SSP) and triple superphosphate (TSP). Some developing countries have PR deposits which, if used to supplement other imported P fertilizers, would allow a saving of much needed foreign exchange. Solubility of P fertilizers is not the only criterion in selection of the most suitable P fertilizer. This paper discusses the results of experiments to compare the relative agronomic effectiveness (RAE) of various PR sources with respect to SSP or TSP as influenced by four important factors: PR sources, soil properties, management practices, and crop species. Under certain conditions, PRs can be agronomically effective.     

Using the DSSAT-CERES-Maize model to simulate crop yield and nitrogen cycling in fields under long-term continuous maize production

Simulation models, such as the DSSAT (Decision Support System for Agrotechnology Transfer) Crop System Models are often used to characterize, develop and assess field crop production practices. In this study, one of the DSSAT Cropping System Model, CERES-Maize, was employed to characterize maize (Zea mays) yield and nitrogen dynamics in a 50-year maize production study at Woodslee, Ontario, Canada (42°13′N, 82°44′W). The treatments selected for this study included continuous corn/maize with fertilization (CC-F) and continuous corn/maize without fertilization (CC-NF) treatments. Sequential model simulations of long-term maize yield (1959–2008), near-surface (0–30 cm) soil mineral nitrogen (N) content (2000), and soil nitrate loss (1998–2000) were compared to measured values. The model did not provide accurate predictions of annual maize yields, but the overall agreement was as good as other researchers have obtained. In the CC-F treatment, near-surface soil mineral N and cumulative soil nitrate loss were simulated by the model reasonably well, with n-RMSE = 62 and 29%, respectively. In the CC-NF treatment, however, the model consistently overestimated soil nitrate loss. These outcomes can be used to improve our understanding of the long-term effects of fertilizer management practices on maize yield and soil properties in improved and degraded soils.     

The environmentally-sound management of agricultural phosphorus

Freshwater eutrophication is often accelerated by increased phosphorus (P) inputs, a greater share of which now come from agricultural nonpoint sources than two decades ago. Maintenance of soil P at levels sufficient for crop needs is an essential part of sustainable agriculture. However, in areas of intensive crop and livestock production in Europe and the U.S.A., P has accumulated in soils to levels that are a long-term eutrophication rather than agronomic concern. Also, changes in land management in Europe and the U.S.A. have increased the potential for P loss in surface runoff and drainage. There is, thus, a need for information on how these factors influence the loss of P in agricultural runoff. The processes controlling the build-up of P in soil, its transport in surface and subsurface drainage in dissolved and particulate forms, and their biological availability in freshwater systems, are discussed in terms of environmentally sound P management. Such management will involve identifying P sources within watersheds; targeting cost-effective remedial measures to minimize P losses; and accounting for different water quality objectives within watersheds. The means by which this can be achieved are identified and include developing soil tests to determine the relative potential for P enrichment of agricultural runoff to occur; establishing threshold soil P levels which are of environmental concern; finding alternative uses for animal manures to decrease land area limitations for application; and adopting management systems integrating measures to reduce P sources as well as runoff and erosion potential.     

Revegetation of eroded land and possibilities of carbon sequestration in Iceland

Carbon (C) sequestration within the context of the Kyoto protocol of the United Nations Framework Convention on Climate Change has great potential as an incentive for combating land degradation and desertification. Desertification continues to be a major threat to Iceland's natural resources. Revegetation in Iceland can both reduce C in the atmosphere by fixing C in vegetation and soil, and also reduce C emission by preventing further ecosystem damage, vegetation degradation and subsequent soil erosion. The sequestration potential in Iceland lies in the available land area and in the soil properties. Iceland has vast areas where vegetation can be enhanced or restored (10 000–45 000 km²), and the Andic nature of Icelandic soils tends to immobilize C. In the year 2001 the Soil Conservation Service worked on revegetation of roughly 13 000 ha or 130 km², resulting in C sequestration of about 8000–14000 Mg C in 2001. Reclamation of degraded land through changed land use, and/or seeding and fertilizing can promote sustainable development and healthier ecosystems, increase biological diversity and soil fertility, in addition to mitigating climate change through C sequestration.     

The role and function of organic matter in tropical soils

Soil organic matter (SOM) has many functions, the relative importance of which differ with soil type, climate, and land use. Commonly the most importantfunction of OM in soil is as a reserve of the nitrogen and other nutrients required by plants, and ultimately by the human population. Other important functions include: the formation of stable aggregates and soil surface protection; maintenance of the vast array of biological functions, including the immobilization and release of nutrients; provision of ion exchange capacity; and storage of terrestrial carbon (C). This paper considers the quantity and quality of SOM of soils in the tropics, which are estimated to contain one quarter of the C in the global pool in terrestrial soils, and supports strongly the use of analytical methods to characterizing labile SOM to develop valuable insights into C dynamics. As in other regions, the transformation of tropical lands for agriculture exploits SOM, and in particular nutrient reserves. The process of exploitation is accelerated in the tropics by the necessity to increase agricultural production, largely through agricultural intensification, to overcome inadequate nutrition, to satisfy population growth, and to cope with the limited reserves of arable land. Poverty has an overriding influence on the exploitation and degradation processes. Areas at greatest risk of land degradation are the infertile acid soils of the tropics, which, invariably, are cultivated by the poor. Soil organic matter has a central role in sustainable land management, but perspectives on the roles of SOM differ widely between farmers, consumers, scientists and policy-makers. Some consider SOM as a source of nutrients to be exploited, whereas others can afford to utilize it as a key component in the management of the chemical, biological, and physical fertility of soils. Still others see SOM as a dumping ground for excess nutrients and toxins, or as a convenient store for fossil fuel emissions, particularly CO₂. Farmers need sustainable land management systems that maintain OM and nutrient reserves. Nevertheless, many available practices, whether based on indigenous or scientific knowledge, do not meet social and economic criteria that govern farmer behaviour. Much scientific knowledge about the various roles of SOM does not reach farmers and other decision-makers in a form that can be used easily. The biggest challenge to researchers is to engage with clients to pinpoint gaps in knowledge and utilize new and existing information to devise decision support Systems tailored to their needs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Long-term impact of chronosequential land use change on soil carbon stocks on a Swedish farm

Agricultural practices and land use significantly influence soil carbon storage. The processes that are affected by land use and management are generally understood, but uncertainties in projections are high. In this paper, we investigate the long-term effects of chronosequential land use change from grassland to cropland and vice versa on soil carbon stock dynamics in four fields on a Swedish farm. Between 1850 and 1920, three of the fields were converted from grassland into cropland, and one was converted back to grassland in 1971. The fourth (control) field is a grassland that has never been ploughed. In 1937, the four fields were sampled at 111 points in a regular grid (25 or 50 m) and the dried soil samples were stored at our Department. In 1971 and 2002, the original grid points were revisited and re-sampled. Land use changes affected the soil C stock significantly. In 1937, carbon stocks were significantly smaller in the arable fields than in the grassland soil. In the field that was converted from arable back to grassland, soil C increased significantly at an average rate of about 0.4 Mg ha⁻¹ year⁻¹. A soil C balance model (ICBM) driven by standard meteorological data and soil carbon input estimated from yield records described soil carbon dynamics reasonably well, although the range of simulated relative changes in C stocks between 1937 and 2002 in the four fields (from −7.4 to +8.8%) was narrower than those measured (from −19.5 to +16.5%). There are only few long-term studies in Northern Europe available for quantifying the effect of land use change on soil carbon stocks and the results presented here are therefore useful for improving predictions of changes in soil carbon driven by land use change.     

Impact of agriculture on soil consumption of atmospheric CH4 and a comparison of CH4 and N2O flux in subarctic, temperate and tropical grasslands

Increasing concentrations of methane (CH₄) in the atmosphere are projected to account for about 25% of the net radiative forcing. Biospheric emissions of CH₄ to the atmosphere total approximately 400 Tg C y^-1. An estimated 300 Tg of CH₄-C y^-1 is oxidized in the atmosphere by hydroxyl radicals while about 40 Tg y^-1 remains in the atmosphere. Approximately 40 Tg y^-1 of the atmospheric burden is oxidized in aerobic soils. Research efforts during the past several years have focused on quantifying CH₄ sources while relatively less effort has been directed toward quantifying and understanding the soil sink for atmospheric CH₄. Recent research has demonstrated that land use change, including agricultural use of native forest and grassland systems has decreased the soil sink for atmospheric methane. Some agricultural systems consume atmospheric CH₄ at rates less than 10% of those found in comparable undisturbed soils. While it has been necessary to change land use practices over the past centuries to meet the required production of food and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Changes that have ensued in these cycles have and will impact the atmospheric concentrations of CH₄ and N₂O. Since CH₄ and N₂O production and consumption are accomplished by a variety of soil microorganisms, the influence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may already exist to limit the effect of land use change and agriculture on trace gas fluxes. It is therefore important to understand how management and land use affect trace gas fluxes and to observe the effect of new technology on them. This paper describes the role of aerobic soils in the global CH₄ budget and the impact of agriculture on this soil CH₄ sink. Examples from field studies made across subarctic, temperate and tropical climate gradients in grasslands are used to demonstrate the influence of nutrient cycle perturbations on the soil consumption of atmospheric CH₄ and in increased N₂O emissions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics

Losses of carbon (C) stocks in terrestrial ecosystems and increasing concentrations of greenhouse gases in the atmosphere are challenges that scientists and policy makers have been facing in the recent past. Intensified agricultural practices lead to a reduction in ecosystem carbon stocks, mainly due to removal of aboveground biomass as harvest and loss of carbon as CO₂ through burning and/or decomposition. Evidence is emerging that agroforestry systems are promising management practices to increase aboveground and soil C stocks and reduce soil degradation, as well as to mitigate greenhouse gas emissions. In the humid tropics, the potential of agroforestry (tree-based) systems to sequester C in vegetation can be over 70 Mg C ha^–1, and up to 25 Mg ha^–1 in the top 20 cm of soil. In degraded soils of the sub-humid tropics, improved fallow agroforestry practices have been found to increase top soil C stocks up to 1.6 Mg C ha^–1 y^–1 above continuous maize cropping. Soil C accretion is linked to the structural development of the soil, in particular to increasing C in water stable aggregates (WSA). A review of agroforestry practices in the humid tropics showed that these systems were able to mitigate N₂O and CO₂ emissions from soils and increase the CH₄ sink strength compared to cropping systems. The increase in N₂O and CO₂ emissions after addition of legume residues in improved fallow systems in the sub-humid tropics indicates the importance of using lower quality organic inputs and increasing nutrient use efficiency to derive more direct and indirect benefits from the system. In summary, these examples provide evidence of several pathways by which agroforestry systems can increase C sequestration and reduce greenhouse gas emissions.     

Assessing causes of recent organic carbon losses from cropland soils by means of regional-scaled input balances for the case of Flanders (Belgium)

Several recent reports on cropland soil organic carbon (SOC) stock changes throughout Europe indicate a general continuing loss of SOC from these soils. As most arable soils in Europe are not in an equilibrium situation because of past changes in land-use and management practices, shifts in both have been suggested to drive this decline of SOC stocks. A lack of data has prevented the unambiguous verification of the contribution of these factors to SOC loss. First, this study focused on recent evolutions in management options for SOC sequestration in Flanders and showed that despite such practices have increased since 1990, their current contribution is still limited. Strikingly, their expansion is at odds with the reported general losses of SOC (−0.48 t OC ha⁻¹ year⁻¹ on average). We used very detailed datasets of livestock numbers, N-application rates and cropping surfaces to calculate regional shifts in input of effective OC from animal manure application, cereal straw incorporation and crop residue incorporation which amounted to −0.094, −0.045 and −0.017 t OC ha⁻¹ year⁻¹, respectively. Shifts in management were identified to have potentially brought about but a third of the recent loss of SOC in the study area, although for central West-Flanders and the Eastern border of Flanders larger impacts of management were observed. This study suggests other influences such as land-use change and climate change to be involved as well. We estimated that another 10%–45% of the loss of SOC could potentially be attributed to land-use changes from grassland to cropland during the 1970–1990 period and about 10% to the observed temperature increase. While being a regional-scaled case study, these findings may be relevant to other European regions in particular (Denmark, The Netherlands, North-West Germany, Brittany and the North-West of France, the Po-valley in Italy and parts of England), with similar climate and intensity of agriculture, and where comparable trends in farming management may well have taken place.     

A conceptual model for conducting nutrient audits at national, regional, and global scales

Mining of nutrients from the soil, particularly in developing countries, is a major problem, causing soil degradation and threatening long-term food production. This paper develops a methodology for carrying out nutrient audits, which includes the calculation of nutrient balances and an evaluation of trends in nutrient depletion/enrichment. Nutrient balances for arable farming are constructed for 197 countries for 1996 and for the world and two specific countries – a developed/enriching country (Japan) and a developing/depleting country (Kenya) for the period 1961 – 1996.The results indicate that nutrient efficiency is approximately 50% for N, 40% for P, and 75% for K. In some countries in Western Europe and in Japan and the Republic of Korea, with large, mixed farming systems, there is a surplus of N, P, and K. However, in almost all other countries, food production is currently dependent on depleting large quantities of nutrients from soil reserves and this is likely to continue. The world average soil depletion of nutrients in 1996 was estimated to be 12.1 kg N ha^–1, 4.5 kg P ha^–1, and 20.2 kg K ha^–1. The depletion of K is particularly severe and could ultimately lead to a serious loss of crop productivity in several countries. There is an urgent need to investigate this issue further. Analytical tools, such as the nutrient audit model described, can play an important role in assessing the problem, and in developing sustainable nutrient management policies, strategies, and practices.