Carbon loss estimates from cultivated peat soils in Norway: a comparison of three methods

Drainage and cultivation of peat soils stimulates soil organic matter (SOM) mineralization, which substantially increases CO₂ emissions from soils. Large uncertainties are associated with this CO₂ flux, and little data are available, especially in Norway. The objective of the present research was to estimate C losses from cultivated peatlands in West Norway by three independent methods: (1) long-term monitoring of subsidence rates, (2) changes in ash contents, and (3) soil CO₂ flux measurements. Subsidence of cultivated peat soils averaged about 2.5 cm year⁻¹. We estimated that peat loss and compaction were respectively responsible for 38% and 62% of the total subsidence during a 25-year period after drainage. Based on this estimate the corresponding C loss equals 0.80 kg C m⁻² year⁻¹. The observed increase in mineral concentration of the topsoil of cultivated peat is proportional to their C loss, providing no mineral particles other than lime and fertilizers are added to the soil. Using this novel approach across 11 sites, we estimated a mean C loss of 0.86 kg C m⁻² year⁻¹. Soil CO₂ flux measurements, corrected for autotrophic respiration, yielded a C loss estimate from cultivated peat soils of 0.60 kg C m⁻² year⁻¹. The three methods yielded fairly similar estimates of C losses from Norwegian cultivated peatlands. Cultivated peatlands in Norway cover an estimated 63,000 ha. Total annual C losses from peat degradation were estimated to range between 1.8 and 2 million tons CO₂ year⁻¹, which equals about 3–4% of total anthropogenic greenhouse gas emissions from Norway.     

Land use change and soil organic carbon dynamics

Historically, soils have lost 40–90 Pg carbon (C) globally through cultivation and disturbance with current rates of C loss due to land use change of about 1.6 ± 0.8 Pg C y⁻¹, mainly in the tropics. Since soils contain more than twice the C found in the atmosphere, loss of C from soils can have a significant effect of atmospheric CO₂ concentration, and thereby on climate. Halting land-use conversion would be an effective mechanism to reduce soil C losses, but with a growing population and changing dietary preferences in the developing world, more land is likely to be required for agriculture. Maximizing the productivity of existing agricultural land and applying best management practices to that land would slow the loss of, or is some cases restore, soil C. There are, however, many barriers to implementing best management practices, the most significant of which in developing countries are driven by poverty. Management practices that also improve food security and profitability are most likely to be adopted. Soil C management needs to considered within a broader framework of sustainable development. Policies to encourage fair trade, reduced subsidies for agriculture in developed countries and less onerous interest on loans and foreign debt would encourage sustainable development, which in turn would encourage the adoption of successful soil C management in developing countries. If soil management is to be used to help address the problem of global warming, priority needs to be given to implementing such policies.     

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 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.     

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.     

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.     

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.     

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.     

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 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.     

Long-term application effects of chemical fertilizer and compost on soil carbon under intensive rice–rice cultivation

From a long-term fertilizer experiment on rice–rice cropping in Typic Endoaquept, established at the Central Rice Research Institute, Cuttack, India in 1969, effects of application of composted manure (5 Mg ha⁻¹ year⁻¹) and chemical fertilizers (N, NP, NK, and NPK twice in a year), in series without compost (C₀) or with compost (C₁) on changes in soil carbon and microbial pools were examined by comparing the soils archived in 1984 and those sampled in 2004. Mean concentrations of soil organic carbon (SOC) varied between 5.5 and 7.6 g kg⁻¹ in 1984, and 6.8 and 10.8 g kg⁻¹ in 2004, respectively. Temporal increases in the total amounts of carbon, which reflect the carbon sequestration potential of the soil followed the order: C₁ + NK > C₁ + NP = C₁ + NPK > C₁ + N = C₁-control > C₀ + NP = C₀ + NK > C₀ + NPK > C₀-control > C₀ + N. Fractions of H₂O–C and K₂SO₄–C were higher in 1984, especially in those soil treated without compost. A reverse trend was observed in case of KMnO₄–C and carbohydrate–C fractions. The continuous application of compost enhanced microbial biomass carbon as well as active microbial biomass carbon in 2004. Long-term application of chemical fertilizers in combination, rather than N alone, had beneficial effects on soil carbon and microbial pools. Compost application, even once a year, invariably led to higher increments in both soil carbon and microbial pools and the combinations of chemical fertilizers with compost generally showed comparable effects in the long-term.     

Relationship between residue quality,decomposition patterns,and soil organic matter accumulation in a tropical sandy soil after 13 years

The objectives of this study were to investigate decomposition patterns and soil organic matter (SOM) accumulation of incorporated residues (10 Mg ha⁻¹ year⁻¹) of different quality, and identify microbiological parameters sensitive to changes in SOM dynamics, in a 13-year-old field experiment on a sandy soil in Northeast Thailand. Mass loss was fastest in groundnut stover (high N), followed by rice straw (high cellulose) and tamarind (intermediate quality), and slowest in dipterocarp (high lignin and polyphenol) following a double exponential pattern. The decomposition rate k ₁ (fast pool) was positively correlated with cellulose (r = 0.70*) while k ₂ (slow pool) was negatively related to lignin (r = −0.85***) and polyphenol (r = −0.81**) contents of residues. Residue decomposition was sensitive to indigenous soil organic nitrogen (SON), particularly during later stages (R ² = 0.782**). Thirteen years’ addition of tamarind residues led to largest soil organic carbon (SOC) (8.41 Mg ha⁻¹) accumulation in topsoil (0–20 cm), while rice straw yielded only 5.54 Mg ha⁻¹ followed by the control (2.72 Mg ha⁻¹). The highest SON (0.78 Mg N ha⁻¹) was observed in the groundnut treatment. Increases in SOC were negatively correlated with cellulose content of residues (r = −0.92***) and microbial respiration (CO₂-C) losses, while SON was governed by organic N added. During later decomposition stages, there was a high efficiency of C utilization (low qCO₂) of decomposer communities especially under tamarind with the lowest qCO₂ and CO₂-C evolution loss. This study suggests that N-rich residues with low cellulose and moderate lignin and polyphenol contents are best suited to improve SOM content in tropical sandy soils.     

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.     

Quantifying soil organic carbon in forage-based cow–calf congregation-grazing zone interface

Recent concerns about global warming due to accumulations of atmospheric CO₂ have encouraged the achievement of better understanding of the roles of animal agriculture in mitigating CO₂ emissions. Grazing can accelerate and alter the timing of nutrient transfers, and increase the amount of nutrients cycled from plant to soil. Our reason for conducting this study is to test whether cattle congregation sites (CCS) typical on most Florida ranches, such as mineral feeders (MF), water troughs (WT), and shaded areas (SA) have higher soil organic carbon (SOC) than in other locations of pasture under foraged-based system. Baseline soil samples around the congregations zones (MF, WT, and SA) and grazing zones in established (>10 year), grazed cow–calf pastures were collected in the spring and fall of 2003, 2004, and 2005, respectively. Soil samples were collected from two soil depths (0–20 and 20–40 cm) at different locations around the CCS following a radial (every 90 degrees: N, S, E, and W) sampling pattern at 0.9, 1.7, 3.3, 6.7, 13.3, 26.7, and 53.3 m away from the approximate center of MF, WT, and SA. The levels of SOC varied significantly with CCS (P ≤ 0.001), distance away from the center of the CCS (P ≤ 0.05), sampling depth (P ≤ 0.001), sampling year (P ≤ 0.001) and the interaction of CCS and soil depth (P ≤ 0.001). Sampling orientations did not significantly affect the levels of SOC. The SA sites had the highest level of SOC of 3.58 g kg⁻¹, followed by WT sites (3.47 g kg⁻¹) and MF sites (2.98 g kg⁻¹). Results of our study did not support our hypothesis that cattle congregation sites typical on most ranches, such as MF, WT and SA, may have higher concentrations of SOC. The levels of SOC (averaged across CCS) within the congregation zone (3.42 g kg⁻¹) were not significantly (P ≤ 0.05) different from the concentrations of SOC at the grazing zone (3.16 g kg⁻¹).     

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.     

Organic carbon fractions affected by long-term fertilization in a subtropical paddy soil

Increasing evidence is showing a greater potential for carbon (C) sequestration in paddy soils than in upland soils. However, the mechanisms underlying long-term accumulation and protection of soil organic carbon (SOC) in paddy fields have not been well documented. In the present study, five soil C fractions were separated by physical fractionation in a subtropical paddy field following 27-year differential fertilization regimes (started in 1981). Results showed that, compared to the initial level, long-term rice (Oryza sativa L.) cropping increased SOC concentrations by 28.8, 30.1, 30.8, and 61.6% in the non-fertilized (CK), nitrogen (N), nitrogen-phosphorus-potassium (NPK), and NPK combined with farmyard manure (NPK + FYM) treatments, respectively. Application of FYM enhanced the formation of macroaggregates (>2,000 and 250–2,000 μm), whereas no significant differences in aggregate-size distribution were found among the CK, N, and NPK treatments. Inorganic fertilization (N and NPK) did not affect the concentration of either total SOC or any C fraction as compared with the CK, whereas application of FYM significantly increased the concentrations both in total SOC (25.5%) and in all C fractions, except coarse particulate organic matter (cPOM). Carbon in the paddy soil was dominated by free silt and clay (s + c_f) and intra-aggregate particulate organic matter within microaggregates (iPOM_m) in all treatments that accounted for 46.4–49.6% and 25.1–27.2% of the total SOC, respectively. Furthermore, the differences in C in the iPOM_m and s + c_f fractions between the CK and NPK + FYM treatments accounted for 53.2 and 38.8% of the differences in total SOC stocks, respectively. These results indicate that SOC originating from manure is stored mainly in fractions with slow turnover (i.e., iPOM_m and s + c_f), which may benefit the long-term C sequestration in paddy soils.     

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 organic carbon fractions after 16-years of applications of fertilizers and organic manure in a Typic Rhodalfs in semi-arid tropics

Agricultural soils can act as a potential sink of the increased carbon dioxide in the atmosphere if managed properly by application of organic manures and balanced fertilizers. However, the rate of carbon (C) sequestration in soils is low in warm climates and thus the short term changes in soil organic carbon (SOC) contents are almost negligible. Therefore, the knowledge about other C fractions that are more sensitive or responsive and indicative of the early changes in SOC can help to determine the effect of the management practices on soil C sequestration. The objective of this study was to determine the soil C sequestration after 16-years of applications of chemical fertilizers and farmyard manure (FYM) to rice (Oryza sativa)—cowpea (Vigna unguiculata) rotation system in a sandy loam soil (Typic Rhodalfs). The treatments were—(1) one control (no fertilizer or FYM); (2) three chemical fertilizer treatments [100 kg N ha⁻¹ (N), 100 kg N ha⁻¹ + 50 kg P₂O₅ ha⁻¹ (NP), 100 kg N ha⁻¹ + 50 kg P₂O₅ ha⁻¹ + 50 kg K₂O ha⁻¹ (NPK)]; (3) one integrated treatment [(50 kg N ha⁻¹ + 25 kg P₂O₅ ha⁻¹ + 25 K₂O ha⁻¹) + (50 kg N ha⁻¹ from FYM)]; and (4) one organic treatment at10 Mg ha⁻¹ FYM. Compared to the control treatment, the increase in SOC was 36, 33, and 19% greater in organic, integrated, and NPK treatments. The 16-years application of fertilizers and/or FYM resulted in much greater changes in water soluble C (WSC), microbial biomass C (MBC), light fraction of C (LFC), and particulate organic matter (POM) than SOC. Of the SOC, the proportion of POM was highest (24–35%), which was followed by LFC (12–14%), MBC (4.6–6.6%), and WSC (0.6–0.8%). The application of fertilizers and/or FYM increased the mean weight diameter of soil aggregates; thus provided physical protection to SOC from decomposition. Our results suggests that the application of fertilizers and/or FYM helps to sequester C in the soil and that the labile fractions of C can be used as indicators to determine the amount of C sequestered as a result of different management practices.     

Soil phosphorus stocks and distribution in chemical fractions for long-term sugarcane,pasture, turfgrass,and forest systems in Florida

Phosphorus distribution and stability in soils of the Everglades Agricultural Area (EAA) of south Florida is important because of changing land uses. We investigated the effects of land use on P distribution in the soil profile and between chemical fractions for a histosol of the Florida Everglades. Labile, Fe–Al bound, Ca-bound, humic–fulvic acid, and residual P pools in 0–15, 15–30, and 30–45 cm depths were determined for drained soils planted to sugarcane (Saccharum sp.) for 50 yr, pasture for 100 yr, turfgrass for 60 yr, and forest for 20 yr. The P concentrations of all chemical fractions decreased with depth in the profile, indicating accumulation in surface soil resulting from oxidation and fertilization. Trends in P distribution between chemical fractions were similar between land uses. Labile P comprised less than 1% of total P. Fe–Al bound P averaged 2.9% of the total P for turfgrass and forest, but 11.4 and 9.6% for sugarcane and pasture. Increasing soil disturbance and long-term fertilization increased P allocation to inorganic fractions, as Ca-bound P contained 49% of total P for sugarcane but 28% for other land uses. Total P stocks in the soil profile (0–45 cm) averaged 1,323, 2,005, 2,294, and 2,317 kg P ha⁻¹ for pasture, sugarcane, turfgrass, and forest, respectively. Under current land uses P in organic fractions represents an unstable pool since the soil is prone to oxidation under drained conditions. In contrast, P sequestered in inorganic fractions is more stable under current land uses, thus sugarcane cultivation and incorporation of bedrock CaCO₃ into surface soil by tillage will enhance long-term P sequestration.