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.     

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.     

Temporal and spatial trends in soil organic carbon stocks following maize cultivation in semi-arid Tanzania,East Africa

Conversion of native ecosystems to agro-ecosystems influences the amount, quality and turnover of soil organic carbon (SOC). As most agro-ecosystems are not in a steady state in terms of the content of SOC, the time scale and feedback mechanisms of changes in SOC are highly relevant for predicting future soil fertility and potential rates of soil carbon losses or sequestration. This paper focuses on changes in land use linked to measured changes in the distribution of total stocks of SOC and the δ¹³C signature in the upper 0.5 m of cultivated soils in the semi-arid parts of Tanzania. Based on documented land use changes since 1950s using remote sensing data, 12 sampling sites along two transects were selected to represent semi-natural/natural savannah and maize fields cultivated for up to five decades. Comparisons between sites representing a chronosequence of well-drained soils showed that soils cultivated the last 50 years have in average less than 50% SOC compared to soils which have never been cultivated. Variations between sites were significant and a reduction in SOC could not be established at sites near present or former villages which have received substantial manure despite a long cultivation history or along a chronosequence representing wetter and more fine-grained soils. Spatial variations in land use changes were parameterized based on remote sensing data and successfully validated against sampling sites. Site-specific rates of soil element loss following cultivation were extrapolated to the study area and uncertainties related to scaling up were discussed.     

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.     

ICBM regional model for estimations of dynamics of agricultural soil carbon pools

Swedish arable land covers 3 Mha and its topsoil contains about 300 Mton C. The mineral soils seem to be close to steady-state, but the organic soils (about 10% of total arable land) have been estimated to lose ca. 1 Mton/year. We have devised a conceptual model (ICBMregion), using national agricultural crop yield/manuring statistics and allometric functions to calculate annual C input to the soil together with a five-parameter soil carbon model (ICBMr), calibrated using long-term field data. In Sweden, annual yield statistics are reported for different crops, for each of eight agricultural regions. Present topsoil carbon content and regional distribution of soil types have recently been measured. We use daily weather station data for each region together with crop type (bulked from individual crop data) and soil type to calculate an annual soil climate parameter for each crop/soil type permutation in each region. We use 14 soil types and 9 crop types, which gives 126 parameter sets for each year and region, each representing a fraction of the region's area. For each year, region, crop and soil type, ICBMregion calculates the change in young and old soil carbon per hectare, and sums up the changes to, e.g., national changes. With eight regions, we will have 1008 parameter sets per year, which easily can be handled, and what-if scenarios as well as comparisons between benchmark years are readily made. We will use the model to compare the soil C pools between the IPCC benchmark year 1990 and the present. In principle, we use inverse modelling from the sampled, recent soil C pools to estimate those in 1990. In the calculations, soil climate and yield for each year from 1990 onwards are taken into account. Then we can project soil C balances into the future under different scenarios, e.g., business as usual, land use change or changes in agricultural crops or cultivation practices. Projections of regional climate change are also available, so we can quite easily make projections of soil C dynamics under, e.g., different climate scenarios. We can follow the dynamic effects of carbon sequestration efforts – and estimate their efficiency. The approach is conceptually simple, fairly complete, and can easily be adapted to different needs and availability of data. However, perhaps the greatest advantage is that the results from this comprehensive approach used for, e.g., a 10-year period, can be condensed into a very simple spreadsheet model for calculating effects of management/land use changes on C stocks in agricultural soils.     

The impact of altered management on long-term agricultural soil carbon stocks – a Swedish case study

Land use in general and particularly agricultural practices can significantly influence soil carbon storage. In this paper, we investigate the long-term effects of management changes on soil carbon stock dynamics on a Swedish farm where C concentrations were measured in 1956 at 124 points in a regular grid. The soil was re-sampled at 65 points in 1984 and at all grid points in 2001. Before 1956 most of the fodder for dairy cattle was produced on the farm and crop rotations were dominated by perennial grass leys and spring cereals with manure addition. In 1956 all animals were sold; crop rotations were thereafter dominated by wheat, barley and rapeseed. Spatial variation in topsoil C concentration decreased significantly between 1956 and 2001. C stocks declined in fields with initially large C stocks but did not change significantly in fields with moderate C stocks. In the latter fields, soil C concentrations declined from 1956 to 1984, but increased slightly thereafter according to both measurements and simulations. Thus, the decline in C input due to the altered management in 1956 was partly compensated for by increasing crop yields and management changes, resulting in increased C input during the last 20 years. A soil carbon balance model (ICBM) was used to describe carbon dynamics during 45 years. Yield records were transformed to soil carbon input using allometric functions. Topsoil C concentrations ranging between 1.8 and 2.4% (depending on individual field properties) seemed to be in dynamic equilibrium with C input under recent farming and climatic conditions. Subsoil C concentrations seemed to be unaffected by the management changes.     

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.     

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.     

Effect of long-term application of inorganic fertilizer and organic amendments on soil organic matter and microbial biomass in three subtropical paddy soils

Soil organic matter (SOM), microbial carbon (C_mic), and microbial nitrogen (N_mic) status affected by the application of inorganic fertilizer and organic amendments in subtropical paddy soils were investigated. Soil samples were collected from the plow layer of three long-term (17 years) field experiments at Xinhua, Ningxiang, and Taojiang counties in Hunan Province, China. Results showed that, compared to the control, application of inorganic fertilizer alone showed no significant effect on soil organic C (SOC), total N (N_tot), C_mic and N_mic. The application of inorganic fertilizer along with manure or straw significantly increased SOC and N_tot and soil C_mic and N_mic contents for all three sites, while following an application of inorganic fertilizer along with straw only for two sites. C_mic and N_mic were closely correlated with SOC and N_tot, respectively. In conclusion, application of inorganic fertilizer along with manure or straw is an effective way of enhancing SOM and microbial biomass in subtropical paddy soils.     

Characterization of potentially bioreactive soil organic carbon and nitrogen by acid hydrolysis

Studies determined the potential of acid hydrolysis for estimating the bioreactive fraction of organic carbon in soils (SOC). Three soils (clay loam, silt loam, and sandy loam) were hydrolyzed with 1 M or 6 M HCl under reflux for up to 24 h. Results showed that 1.7 to 3.2 % of SOC could be liberated as CO₂ from the acid hydrolysis of soil. This readily hydrolyzed fraction should be a part of the bioreactive SOC. Higher amounts of soluble SOC and N as well as CO₂ were released from all soils by 6 M HCl than by 1 M HCl. Soluble SOC and N contents in both 1 M HCl and 6 M HCl hydrolysates of all soils increased rapidly during the initial 2 hours of hydrolysis, and then increased very gradually. The amounts of CO₂-C evolved correlated with the amounts of NH₄-N released during the acid hydrolysis (r = > 0.88). The ratio of SOC to soluble N was lower in 6 M HCl hydrolysate than in 1 M HCl. Hydrolysis of soil by 1 M HCl for 4 h appeared to be a promising approach for estimating the more bioreactive pools of SOC and N. This revised version was published online in August 2006 with corrections to the Cover Date.     

Total and organic soil carbon in cropping systems of Nepal

The significance of soil organic matter (SOM) in sustaining agriculture has long been recognized. The rate of change depends on climate, cropping system, cropping practice, and soil moisture. A 3-yr on-farm study was conducted in two major agro-ecologies (hills with warm-temperate climate and plains with subtropical climate) of Nepal. The soils in warm-temperate climate are Lithic subgroups of Ustorthents with well-drained loamy texture, and in subtropical climate are Haplaquepts with imperfectly drained loamy texture. Farmers’ predominant cropping systems were selected from different cultivation length in addition to a reference sample collected from adjacent virgin forest. The objectives were to examine the effect of cultivation length and cropping system on total carbon, KMnO₄-oxidizable soil C, C storage, and C/N ratio in two climatic scenarios: warm-temperate and subtropical. A large difference in KMnO₄-oxidizable soil organic C was observed due to the effect of cultivation length and cropping system. However, TC remained similar during the 3-year study. The decrease in KMnO₄-oxidizable C due to cultivation was more in the surface layer (43–56%) than in the subsurface layer (20–30%). Total C in uncultivated, < 10-year cultivated, and > 50-year cultivated soil was 22, 13, and 10 g kg⁻¹ in warm-temperate climate and 10, 6, and 5 g kg⁻¹ in subtropical climate, respectively. During the 3-year study period in both climates, large changes in soil C were observed for KMnO₄-oxidizable C but not for TC, confirming our earlier work on the usefulness of the KMnO₄ oxidized fraction for detecting a relatively short-term increase or decrease in soil C pool. The TC storage in uncultivated, < 10-year cultivated, and > 50-year cultivated soil was 38, 25, and 19 Mg ha⁻¹ in warm-temperate climate and 22, 15, and 12 Mg ha⁻¹ in subtropical climate, respectively. The rice–wheat and maize–potato cropping systems were good in storing soil C of 30 and 20 Mg ha⁻¹ for 0–15-cm soil depth in warm-temperate climate. The rice–wheat cropping system was also good in storing soil C in subtropical climate (19 Mg ha⁻¹) compared with other cropping systems studied.     

高锰酸根褪色光度法测定土壤中的有机碳

建立了以KMnO。测定土壤中有机碳的分光光度法。在硫酸介质中,有机碳能快速、定量的还原高锰酸根,在加入有机碳前后,高锰酸钾溶液在波长525nm处的吸光度发生明显变化,且吸光度之差△A与加入的有机碳的浓度成正比。结果表明,有机碳浓度在0．40—28．00mg／L范围内与溶液吸光度差值呈良好线性关系,线性回归方程为△A=0．0466p＋0．058（mg／L）,相关系数r=0．9987,检出限为0．170mg／a,相对标准偏差（RSD）为0．390％,表观摩尔吸光系数ε=8．39X10^3L／（mol·em）。用于测定国家标准土壤样品中有机碳的含量,相对标准偏差小于2．90％。     

Soil organic carbon,total nitrogen and grain yields under long-term fertilizations in the upland red soil of southern China

A long-term experiment with various fertilizations was carried out during 1990–2006 in a double cropping system rotated with wheat (Triticum Aestivium L.) and corn (Zea mays L.) in the red soil of southern China. The experiment consisted of eight treatments: non-fertilization (CK), nitrogen–phosphorus fertilization (NP), phosphorus–potassium fertilization (PK), nitrogen–phosphorus–potassium fertilization (NPK), pig manure (M), pig manure and NPK fertilization (NPKM), high rates of NPKM (hNPKM), and straw returned with inorganic fertilizers (NPKS). Applications of manure (i.e., M, NPKM and hNPKM) significantly increased soil organic carbon (SOC) and total nitrogen contents. Applications of inorganic fertilizers without manure showed small influences on SOC, but resulted in declines of soil total nitrogen over the long-term experiment. Grain yields were more than doubled under fertilizations for both wheat and corn, with the highest under the NPKM and hNPKM treatments and the lowest under non-fertilization. Long-term cropping practices without fertilization or with unbalanced fertilizations (e.g., NP and PK) caused low grain yields. The balanced fertilization of NPK increased grain yields. However, such practice was not able to maintain high grain yields during the last few years of experiment. Our analyses indicate that both wheat and corn grain yields are significantly correlated with SOC, total and available nitrogen and phosphorus. However, the relationships are stronger with total nitrogen (r = 0.5–0.6) than with available nitrogen (r = 0.26–0.3), indicating the importance of maintaining soil total nitrogen in agricultural practice.     

Fractions of organic carbon in soils under different crop rotations, cover crops and fertilization practices

We investigated the long-term effects (13–48 years) of crop rotations, cover crops and fertilization practices on soil organic carbon fractions. Two long-term experiments conducted on a clay loam soil in southeastern Norway were used. From the crop rotation experiment, two rotations, one with two years grain + four years grass and the second with grain alone (both for 6 years), were selected. Each rotation was divided into moderate fertilizer rate (30–40 kg N ha^–1), normal fertilizer rate (80–120 kg N ha^–1) and farmyard manure (FYM 60 Mg ha^–1 + inorganic N at normal rate). Farmyard manure was applied only once in a 6-year rotation, while NPK was applied to every crop. The cover crop experiment with principal cereal crops consisted of three treatments: no cover, rye grass and clover as cover crops. Each cover crop was fertilized with 0 and 120 kg ha^–1 N rates. Soil samples from both experiments were taken from 0–10 cm and 10–25 cm depths in the autumn of 2001. The classical extraction procedure with alkali and acid solution was used to separate humic acid (HA), fulvic acid (FA) and humin fractions, while H₂O₂ was used to separate black carbon (BC) from the humin fraction. The rotation of grain + grass showed a significantly higher soil organic carbon (SOC) compared with grain alone at both depths. Farmyard manure application resulted in significantly higher SOC than that of mineral fertilizer only. However, cover crops and N rates did not affect SOC significantly. Organic carbon content of FA, HA and humin fractions accounted for about 29%, 25% and 44% of SOC, respectively. The rotation of grain+grass gave a higher C content in HA and humin fractions, and a lower C in the FA fraction as compared with the rotation with grain alone. Farmyard manure increased HA and humin fractions more than did chemical fertilizers. Clover cover crop increased the C proportion of humin more than rye grass and no cover crop. No significant differences in C contents of FA, HA and humin fractions were observed between N rates. Effects of cover crop and N rates as well as fertilization with NPK on black carbon (BC) content were significant only at 10–25 cm depths. Farmyard manure increased the BC fraction compared with chemical fertilizers. Clover crop also enhanced the accumulation of the BC fraction. Application of 120 kg N ha^–1 resulted in a significant increase of the BC fraction.     

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.     

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg^–1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content <0.2% did not affect yield significantly. At values >0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha^–1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.     

Agricultural activities and the global carbon cycle

The observed and projected increase in emission of greenhouse gases, with attendant effects on global warming and sea level rise, have raised interests in identifying mitigation options. Terrestrial C sequestration involves capture of atmospheric C through photosynthesis and storage in biota, soil and wetlands. Land use, vegetation and soil management have a strong impact on the biotic processes of C sequestration. Losses of C from the terrestrial ecosystems are exacerbated by deforestation, biomass burning, plowing, resource-based and subsistence agriculture, and practices that mine soil fertility and deplete the soil organic C (SOC) pool. Biomass burning may also produce charcoal, which is an inert carbon with long residence time. Practices that enhance C sequestration include afforestation and reforestation, conservation tillage and mulch farming, integrated nutrient management and adopting systems with high biodiversity. Net C sequestration within an ecosystem can be assessed by taking into account the hidden C costs of fertilizers, pesticides, tillage, irrigation and other input. Restoration of degraded soils and ecosystems has a vast potential of C sequestration. The Kyoto Protocol provides for C sequestration in terrestrial sinks and C trading through Clean Development Mechanisms. Terrestrial C sequestration, besides being a win–win strategy, offers a window of opportunity for the first few decades of the 21^st century. It is a natural process of reducing the rates of gaseous emissions while alternatives to fossil fuel take effect.     

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.     

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.     

Spatial and temporal variability in excessive soil phosphorus levels in eastern North Carolina

Numerous studies have shown that accumulation of excessive soil phosphorus raises the potential for phosphorus export and eutrophication of adjacent surface waters. Soil phosphorus data from the North Carolina Agronomy Division's database were analyzed for two-year periods spanning the decades of the 1980s and 1990s for 39 eastern North Carolina counties. Eastern North Carolina supported extensive row crop agriculture, rapidly growing intensive livestock industries, and a growing human population during these decades. Excessive soil phosphorus levels, defined as having a soil phosphorus index (P-I, based on Mehlich III testing) > 100, occurred in over 40% of almost a million samples reported for the three two-year periods analyzed. Excessive soil P-I levels were most frequent in central eastern North Carolina, declined in the 1980s and rose again in the 1990s. The distribution of row crop area with excessive soil P-I levels was very similar in time and space. Increases in the area harvested for cotton (+635%) and pasture (+523%) with excessive soil P-I levels were particularly large during the 1990s, when crop areas harvested associated with excessive soil P-I levels for other major crops (corn, tobacco, peanuts) declined. Residential and recreational land uses were associated with similarly high frequencies of excessive soil P-I levels, but these land uses were relatively unimportant (<5% area) compared to agricultural land use (~34%) in the region. Recent increases in fertilizer shipments (approximately twofold in the late 1990s) likely reflected increased cotton production. Rapid growth in concentrated animal production (almost twofold increase in total animal units (AU) between 1992 and 2001), with accompanying land application of wastes, accounted for increases in soil P-I values in pasturelands in the 1990s, particularly in central eastern North Carolina, where these activities were concentrated. The potential threat to water quality from export of excessive soil phosphorus is therefore greatest in this region. North Carolina is currently developing a Phosphorus Loss Assessment Tool (PLAT) in an attempt to manage the challenge posed by excessive soil phosphorus levels.