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.     

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.     

Long-term effects of fallow systems and lengths on crop production and soil fertility maintenance in West Africa

In the development of short fallow systems as alternatives to shifting cultivation in West Africa, a long-term trial was established at the International Institute of Tropical Agriculture (IITA) on an Alfisol in the forest-savanna transitional zone of southwestern Nigeria, comparing three fallow systems; natural regrowth fallow, cover crop fallow and alley cropping on soil productivity and crop yield sustainability. The natural fallow system consisted of natural regrowth of mainly Chromolaena odorata shrub as fallow vegetation. The cover crop fallow system consisted of Pueraria phaseoloides, a herbaceous legume as fallow vegetation. The alley cropping system consisted of woody hedgerows of Leucaena leucocephala as fallow vegetation. The fallow lengths were 0 (continuous cropping), 1, 2 and 3 years after 1 year of maize/cassava intercropping. Biomass produced from natural fallow and cover crop fallow was burnt during the land preparation. Fertilizer was not applied throughout the study. Without fertilizer application, maize yield declined from above 3.0 t ha^–1 to below 0.5 t ha^–1 during 12 years of cultivation (1989–2000) on a land cleared from a 23-year old secondary forest. Temporal change in cassava tuber yield was erratic. Mean maize grain yields from 1993–2000 except for 1999 were higher in cover crop fallow system (1.89 t ha^–1) than in natural fallow system (1.73 t ha^–1), while natural fallow system outperformed alley cropping system (1.46 t ha^–1). During the above 7 years, mean cassava tuber yield in cover crop system (7.7 t ha^–1) did not differ from natural fallow system (8.2 t ha^–1), and both systems showed higher cassava tuber than the alley cropping system (5.7 t ha^–1). The positive effect of fallowing on crop yields was observed for both crops in the three systems, however, insignificant effects were seen when fallow length exceeded 1 year for cover crop and alley cropping, and 2 years for natural fallow. Soil pH, organic carbon, available P and exchangeable Ca, Mg and K decreased considerably after 12 years of cultivation, even in a 3-year fallow subplot. After 12 years, soil organic carbon (SOC) within 0–5 cm depth in alley cropping (13.9 g kg^–1) and natural fallow (13.7 g kg^–1) was higher than in cover crop fallow (11.6 g kg^–1). Whereas significant increase in SOC with either natural fallow or alley cropping was observed only after 2 or 3 years of fallow, the SOC in the 1-year fallow alley cropping subplot was higher than that in continuous cropping natural fallow subplot. It can be concluded from our study that in transforming shifting cultivation to a permanent cropping, fallow with natural vegetation (natural fallow), herbaceous legumes (cover crop fallow) and woody legumes (alley cropping) can contribute to the maintenance of crop production and soil fertility, however, length of fallow period does not need to exceed 2 years. When the fallow length is reduced to 1 year, a better alternative to natural regrowth fallow would be the cover crop for higher maize yield and alley cropping for higher soil organic matter. For fallow length of 2 years, West African farmers would be better off with the natural fallow system.     

Is soil quality improvement by legume cover crops a function of the initial soil chemical characteristics?

The aim of this study, which was conducted in a humid savannah zone of central Côte d’Ivoire, was to examine changes in the quality of soil cultivated with herbaceous legume cover crops as a function of initial soil characteristics. Mucuna pruriens var utilis and Pueraria phaseoloides were used in a two side-by-side location experiment: a shrubby savannah (the savannah site or “SAV”) and a natural fallow dominated by Chromolaena odorata (the fallow site or “FAL”). The latter was mainly characterized by higher organic matter [organic carbon (C) 10 vs. 7.5 mg kg^?1; total nitrogen (N) 0.8 vs. 0.5 mg kg^?1) and total phosphorus (P) (282.3 vs. 168.3 mg kg^?1) contents in the upper soil layer (0–10 cm). After 8 months of growth, biomass production by M. pruriens was found to be 6.5 and 4.9 t dry matter (DM) ha^?1 at FAL and SAV, respectively. For P. phaseoloides, the values were 7.2 and 6.4 t DM ha^?1, respectively, in approximately the same period. The quantities of nutrients released by decomposing legume litter were higher at FAL than at SAV. Between-site differences in soil quality improvement were most noticeable in terms of available P, microbial biomass carbon (MBC) and MBC:total carbon (TC) ratio. The FAL site experienced a faster improvement of soil parameters under both legume species: available P increased from 18 to 58 mg kg^?1 under M. pruriens, and from 19 to 52 mg kg^?1 under P. phaseoloides; MBC increased from 88 to 185 mg kg^?1 under M. pruriens, and from 127 to 192 mg kg^?1 under P. phaseoloides. In contrast, the parameters remained constant over time at SAV. Soil C and N contents as well as C mineralization showed similar trends at both sites. Based on these results, we conclude that soil quality improvement under cover crops appears to be faster when the initial soil organic C, total N and P contents are adequate. These findings will be useful in assisting governmental decision-making on approaches to be taken for restoring soil fertility in low-input agricultural systems in West Africa.     

Soil organic carbon (SOC) management for sustainable productivity of cropping and agro-forestry systems in Eastern and Southern Africa

In Eastern and Southern Africa, the shifting from the no-external input agriculture (shifting cultivation through slash and burn) to intensified agricultural systems has resulted in widespread agro-ecosystems with high soil organic carbon and nutrient depletion. This is quite evident in farming systems with reduced fallow period or those that practice continuous cropping without or with little inputs. Long-term experiments indicate that losses of up to 0.69 t carbon ha⁻¹ yr⁻¹ in the soil surface layers are common. Such losses are commonly reported by farmers engaged in participatory community-based research development projects. This calls for judicious strategies for recapitalization or replacement of these depletions. Such strategies include direct SOC replenishment through addition of organic materials, notably manures, wastes, residues and plant litter; biomass transfer; incorporation of improved fallows in the farming systems. Indirect contribution to SOM replenishment or recapitalization may be achieved through inorganic fertilizations and amendments; legume integration in the production system; and combined inorganic and organic inputs. Research is required to identify inputs that help meet both nutrients availability while contributing to SOC build up at the same time. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contemporary land use/land cover types determine soil organic carbon stocks in south-west Rwanda

Soil organic carbon (SOC) constitutes a large pool within the global carbon cycle. Land use change significantly drives SOC stock variation. In tropical central and eastern Africa, how changes in land use and land cover impact on soil C stocks remains unclear. Variability in the existing data is typically explained by soil and climate factors with little consideration given to land use and management history. To address this knowledge gap, we classified the current and historical land cover and measured SOC stocks under different land cover, soil group and slope type in the humid zone of south-west Rwanda. It was observed that SOC levels were best explained by contemporary land cover types, and not by soil group, conversion history or slope position, although the latter factors explained partly the variation within annual crop land cover type. Lack of the influence of land use history on SOC stocks suggests that after conversion to a new land use/land cover, SOC stocks reached a new equilibrium within the timestep that was observed (25 years). For conversion to annual crops, SOC stocks reach a new equilibrium at about 2.5 % SOC concentration which is below the proposed soil fertility threshold of 3 % SOC content in the Eastern and central African region. SOC stock declined under transitions from banana-coffee to annual crop by 5 % or under transitions from natural forest to degraded forest by 21 % and increased for transitions from annual crops to plantation forest by 193 %. Forest clearing for agricultural use resulted in a loss of 72 %. Assuming steady states, the data can also be used to make inferences about SOC changes as a result of land cover changes. We recommend that SOC stocks should be reported by land cover type rather than by soil groups which masks local land cover and landscape differences. This study addresses a critical issue on sustainable management of SOC in the tropics and global carbon cycle given that it is performed in a part of the world that has high land cover dynamics while at the same time lacks data on land cover changes and SOC dynamics.     

Crop residue,manure and fertilizer in dryland maize under reduced tillage in northern China: II nutrient balances and soil fertility

A long-term experiment was carried out in the dryland of northern China to assess the effects of applications of maize stover, cattle manure and NP (1:0.44) fertilizer on partial nitrogen (N), phosphorus (P) and potassium (K) balances, extractable soil N (SEN), P and K, and soil organic matter (SOM) in a spring maize cropping system, under reduced tillage conditions. The experiment was set-up according to an incomplete, optimal design, with three factors at five levels and 12 treatments, including a control with two replications. Statistical analyses using multiple regression models showed that the partial N, P and K balances were strongly influenced by annual variations in the amounts of soil water at seeding (SWS) and growing season rainfall (GSR). Most treatments had positive P but negative N and K balances. Cumulative P and K balances were reflected in extractable soil P (P-Olsen) and K (exchangeable K), but the weak relationships indicated that the sorption of P and buffering of K were strong. Cumulative balances of effective organic carbon (C) were weakly related to soil organic C (SOC) content after 12 years. Negative C balances were related to decreases in SOC, but positive C balances were not translated into increases in SOC. The analysis of nutrient balances and soil fertility indices revealed that nutrient inputs in most treatments were far from balanced. It is concluded that the concepts of ‘ideal soil fertility level’ and ‘response nutrient management’ provide practical guidelines for improving nutrient management under the variable rainfall conditions of dry land areas in northern China.     

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.     

Particulate and active soil nitrogen fractions are reduced by sheep grazing in dryland cropping systems

Sheep (Ovis aries L.) grazing, a cost-effective method of weed control compared to herbicide application and tillage, may influence N cycling by consuming crop residue and weeds and returning N through feces and urine to the soil. The objective of this experiment was to evaluate the effect of sheep grazing compared to tillage and herbicide application for weed control on soil particulate and active soil N fractions in dryland cropping systems. Our hypothesis was that sheep grazing used for weed control would increase particulate and active soil N fractions compared to tillage and herbicide application. Soil samples collected at the 0–30 cm depth from a Blackmore silt loam were analyzed for particulate organic N (PON), microbial biomass N (MBN), and potential N mineralization (PNM) under dryland cropping systems from 2009 to 2011 in southwestern Montana, USA. Treatments were three weed management practices [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] as the main plot and two cropping sequences [continuous spring wheat (Triticum aestivum L.; CSW) and spring wheat–pea (Pisum sativum L.)/barley (Hordeum vulgare L.) mixture hay–fallow; W–P/B–F] as the split-plot factor arranged in randomized complete block with three replications. The PON and MBN at 0–30 cm were greater in the chemical or mechanical than the grazing treatment with CSW. The PNM at 15–30 cm was greater in the chemical or mechanical than the grazing treatment in 2009 and 2011 and at 5–15 cm was greater with W–P/B–F than CSW in 2010. From 2009 to 2011, PON at 0–30 cm and PNM at 15–30 cm reduced from 2 to 580 kg N ha^?1 year^?1 in the grazing and chemical treatments, but the rate varied from ?400 to 2 kg N ha^?1 year^?1 in the mechanical treatment. Lower amount of labile than nonlabile organic matter returned to the soil through feces and urine probably reduced soil active and coarse organic matter N fractions with sheep grazing compared to herbicide application and tillage for weed control. Reduction in the rate of decline in N fractions from 2009 to 2011 compared to the herbicide application treatment, however, suggests that sheep grazing may stabilize N fractions in the long-term if the intensity of grazing is reduced. Animal grazing may reduce soil N fractions in annual cropping systems in contrast to known increased fractions in perennial cropping systems.     

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.     

Soil organic matter,greenhouse gases and net global warming potential of irrigated conventional,reduced-tillage and organic cropping systems

Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011–March 2013, and were analyzed for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions, water-filled pore space (WFPS), soil nitrate (NO₃ ^?–N) and ammonium (NH₄ ⁺–N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO₂ emissions compared to 10.2 kg C ha^?1 day^?1 and 43% lower N₂O emissions compared to 17.5 g N ha^?1 day^?1 in CT during cropping season 2011, and no difference in CO₂ and N₂O emissions during cropping season 2012. The OR emitted 31% less N₂O, but 74% more CO₂ than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha^?1) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems.     

Stream water nutrient and organic carbon exports from tropical headwater catchments at a soil degradation gradient

Carbon and nutrient losses were quantified from four small headwater catchments in western Kenya in the year 2008. They include a forested catchment and three catchments under maize continuously cultivated for 5, 10 and 50 years following forest conversion. The C isotopic composition of dissolved organic C (DOC) in stream discharge suggested that soil organic C (SOC) derived from the original forest rather than OC from maize may have contributed to a large extent to watershed OC losses, even 50 years after the forest was removed. Flow-weighted stream water concentrations of DOC and coarse particulate OC, all N species, total P, K and Na significantly (P < 0.05) increased in streams after forest conversion and long-term cultivation. Solute concentrations increased despite the fact that soil contents decreased and total water flow increased indicating mobilization of C and N, P and K from soil with progressing cultivation. In contrast, Ca and Mg concentrations in stream water did not systematically change after deforestation and cultivation, and may be controlled by geochemical weathering rather than by changing water flow paths or topsoil contents. All OC and nutrient exports increased with longer cultivation over decadal time scales (P < 0.05) to the same or greater extent than through deforestation and the first years of cultivation. Fluvial OC and total N losses were 2 and 21 % of total SOC and total N decline, respectively, in the top 0.1 m over 50 years. Fluvial OC losses therefore played a minor role, and SOC losses were mainly a result of microbial mineralization. Resulting total N losses by stream discharge, however, were large with 31 kg ha^?1 year^?1 after 50 years of continuous cropping in comparison to fertilization of 40 kg N ha^?1 year^?1. Most (91 %) of the N losses occurred as NO₃ ^?. In contrast, P losses by stream discharge were negligible in comparison to plant uptake. Water losses should be managed to reduce soil fertility declines especially through large N export from agricultural headwater catchments. However, stream concentrations of both P (0.01–0.15 mg L^?1) and N (0.4–4.8 mg L^?1) were moderate or low with respect to possible consequence for human health and not responsible for eutrophication observed in Lake Victoria.     

Organic matter recovery in sandy soils under bush fallow in southern Mozambique

Bush fallow under shifting cultivation is commonly practised by subsistence farmers in southern Mozambique. This farming system is likely to persist due to the existence of large sparsely inhabited areas, coupled with financial limitations preventing smallholders from buying fertilisers. The bush fallow is intended to naturally restore soil production capacity lost during the cropping period. This study quantified soil organic matter (OM) content and investigated factors determining its dynamics in sandy soils under varying duration of bush fallow in southern Mozambique. Agroecosystems representing rainfall regions of <400, 400–600, 600–800, 800–1,000, >1,000 mm and a transitional agroecosystem of 400–800 mm were selected. Within each agroecosystem, five land uses (virgin, cultivated, <5 years fallow, 5–15 years fallow and >15 years fallow) were identified and compared for three shallow soil layers (0–50, 50–100 and 100–200 mm). Organic C and total N were used as indicators of soil OM and the study examined whether OM content increased with higher rainfall or longer bush fallow. Higher rainfall did not consistently increase soil OM content, which was highest in the wettest agroecosystem but did not decline gradually towards the driest. For example, the lowest OM content was found in the second wettest agroecosystem (800–1,000 mm rainfall). Duration of bush fallow was also inconsistent, as OM content continued to decrease for some time after cultivated land was abandoned to bush fallow and an increase in OM content occurred sooner in wetter agroecosystems than drier at a 600 mm rainfall threshold. After 15 years of bush fallow, OM content was still lower than in the virgin land of the agroecosystems. Relative to virgin land, both the loss and gain of OM were smaller in the drier agroecosystems. Further studies are needed to establish optimum bush fallow periods for full recovery of soil OM in southern Mozambique.     

Soil fertility effects of repeated application of sewage sludge in two 30-year-old field experiments

Recirculation of plant nutrients from waste materials back to agriculture is necessary in a sustainable food production system. In this study we investigated the long-term effect of direct land application of sewage sludge (SS) on soil fertility on two Swedish farms, where field experiments were set up in 1981 with three rates of SS (0, 4 and 12 Mg dry matter ha^?1 every 4 years) and three rates of mineral fertilisation, in a factorial design with four replicates. At one site, SS application tended to increase crop yield over time but, when plant nutrients were not limiting, the effect was only significant for spring barley. We deduced that improved soil structure was the main driver for this fertility enhancement after SS application, as indicated by lower soil bulk density and higher soil carbon concentration. After 30 years, soil organic carbon stocks to 0.40 m depth differed by up to 17 Mg ha^?1 between treatments. According to carbon balance calculations, retention of carbon derived from SS ranged between 18 and 20% for the soil layers analysed at the two sites. Soluble phosphorus (P) increased with sludge application rate, but represented only around 2% of the expected residual P according to P-balance calculations. The fertiliser value of nitrogen was also low, with only 3–8% nitrogen use efficiency. Heavy metals such as copper, zinc and mercury showed moderate accumulation in the soil, but elevated levels of metals were not detected in crops, even at the highest dose of SS. These results show that SS represents a valuable resource for improving soil fertility in terms of soil organic matter and soil structure, but its efficiency for nutrient cycling is very low within the time frame considered in the study.     

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.     

Crop yields and soil organic carbon fractions as influenced by straw incorporation in a rice–wheat cropping system in southeastern China

In agro-ecosystems, the relationship between soil fertility and crop yield is mediated by manure application. In this study, an 8-year field experiment was performed with four fertilizer treatments (NPK, NPKM₁, NPKM₂, and NPKM₃), where NPK refers to chemical fertilizer and M₁, M₂, and M₃ refer to manure application rates of 15, 30, and 45 Mg ha^?1 year^?1, respectively. The results showed that the NPKM (NPKM₁, NPKM₂, and NPKM₃) treatments produced greater and more stable yields (4.95–5.45 Mg ha^?1 and 0.59–0.75) than the NPK treatment (4.01 Mg ha^?1 and 0.50). Crop yields under the NPKM treatments showed two trends, with a rate of decrease of 0.48–0.83 Mg ha^?1 year^?1 during the first 4 years and a rate of increase of 0.10–0.25 Mg ha^?1 year^?1 during the last 4 years. The soil organic carbon (SOC) significantly increased under all treatments. The estimated annual SOC decomposition rate was 0.35 Mg ha^?1 year^?1 and the equilibrium SOC level was 6.22 Mg ha^?1. Soil total nitrogen (N), available N, total phosphorus (P) and available P under the NPKM treatments increased by 0.15–0.26, 15–33, 0.17–0.66 and 45–159 g kg^?1, respectively, compared with the NPK treatment. Manure application mainly influenced crop yield by affecting the soil TN, available N, and available P, which accounted for up to 64% of the crop yield variation. Taken together, applying manure can determine or at least improve the effects of soil fertility on crop yield in acidic soils in South China.     

Organic Matter Management as an Underlying Cause for Soil Fertility Gradients on Smallholder Farms in Zimbabwe

Management of spatial and temporal variability of soil fertility within fields and farms is one major challenge for increasing farm-level crop productivity in smallholder agriculture. A study was conducted across 120 on-farm field sites in three agro-ecological regions of Zimbabwe to identify management factors influencing the formation of within-field/farm soil fertility gradients. Using farmer participatory research approaches, host farmers were put into three classes according to resource endowment, namely, Resource-endowed, Intermediate and Resource-constrained farmers. Each host farmer identified the most (rich) and least (poor) productive field or field section, which were then studied over two years. Farmer criteria for defining soil fertility ranged from colour through elements of soil structure to crop response following external nutrient inputs. The fertility ranking of fields consistently matched with laboratory indices, with rich fields containing significantly more soil organic carbon (SOC) and nutrients than the corresponding poor fields. Fertility gradients were evident within and across farms belonging to different farmer classes. The mean SOC content for rich fields were >6.0 g kg⁻¹ compared with <4.6 g kg⁻¹ for the designated poor fields. Rich fields belonging to Resource-endowed farmers had 16–28% more SOC than those belonging to their resource-constrained counterparts, suggesting differences in organic matter management. Differences in SOC and fertility status between rich and poor fields were wider in two of the study areas which had more than 70 years of cultivation in contrast to the third site which had been under smallholder farming for only 20 years, suggesting that the observed fertility gradients are a cumulative effect of years of differential management practices by different farmer classes. Analysis of potential benefits from in situ organic biomass inputs suggested that the processes of organic matter capture and utilization discriminated against Resource-constrained farmers. About 50% of in situ biomass, preferentially maize stover, was lost in three dry season months, and up to 72% of potentially recyclable N is lost from poor fields managed by Resource-constrained farmers. In contrast, Resource-endowed farmers incorporated more than 1.5 t C, 25 kg N and 5 kg P ha⁻¹ season⁻¹ because of their access to draught power during the early dry season. Such inputs could make a difference on these nutrient-depleted soils. Intermediate farmers represented a diverse transitional group whose size and variability could be indicative of the dynamism of technology usage. It was concluded that management of soil fertility gradients to increase crop productivity on smallholder farms hinges on increasing the capacity and efficiency with which organic matter is generated and utilized by different farmer weaclth groups across temporal scales.     

Long-term rice rotation,tillage, and fertility effects on near-surface chemical properties in a silt-loam soil

A majority of the rice (Oryza sativa L.) produced in the United States occurs on alluvial soils in the lower Mississippi River Valley, lower coastal plains of Louisiana and Texas, and in the Sacramento River Valley of California. Rice is a staple grain of global importance, so ensuring the sustainability of rice production systems is vital to feeding the world’s population and protecting economic livelihoods. Therefore, a study was conducted at the Rice Research and Extension Center near Stuttgart, Arkansas to evaluate the long-term effects of rice-based crop rotations [with corn (Zea mays L.), soybean (Glycine max L.), and winter wheat (Triticum aestivum L.)], tillage (conventional and no-tillage), and soil fertility treatments (optimal and sub-optimal) after 11 years (1999–2010) on soil organic matter (SOM) content, soil pH, and Mehlich-3 extractable nutrient contents in the top 10 cm of a Dewitt silt loam (fine, smectitic, thermic, Typic Albaqualf). Results revealed increases in SOM (9–14 %) and extractable Mn (68–220 %), Fe (82 %), and Na (37–76 %) contents in most tillage–fertility–rotation treatments over time. Soil P (fourfold to eightfold) and K (twofold to threefold) contents increased in rotations with wheat and soil pH (9 %) increased under sub-optimal fertility. In contrast, extractable Ca (22 %), S (30 %), and Cu (27 %) contents decreased over time in all treatment combinations and Zn (twofold) contents decreased under continuous rice. Understanding the decadal effects field management practices have on soil chemical properties will provide insight into the longer-term economic and environmental sustainability of rice-based cropping systems.     

Soil aggregates response to tillage and residue management in a double paddy rice soil of the Southern China

Soil aggregate stability is a key indicator of soil quality and environmental sustainability of agroecosystems. The protection of organic material within aggregates against microbial decomposition is regarded as an important process in soil organic carbon stabilization but detailed knowledge about this process is still lacking. The objective of our study was to examine the multiple year effects of plow tillage with residue removed (PT0), plow tillage with residue incorporation (PT), rotary tillage with residue retention (RT), and no-till with residue retention (NT) on soil water stable aggregates (WSA) under a double rice (Oryza sativa L.) cropping system in the Southern China. Results showed that the NT system increased the proportion of >2 mm aggregate fraction, and reduced the proportion of <0.053 mm aggregates at 0–5 cm depth in 2011. Compared with PT0 and PT, significantly higher large macroaggregate (>2 mm) associated-C contributions to TOC were observed in the surface layer (0–10 cm depth) under RT and NT. A significant positive correlation between TOC and macroaggregate (>2 and 2–0.25 mm) associated-C was observed at 0–20 cm soil depth in the paddy rice ecosystem. Therefore, conversion to NT, could enhance the formation of stable macroaggregate, macroaggregates associated-C, and total C contents in paddy soil of Southern China.     

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.