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Sequestering carbon (C) into stable soil pools has potential to mitigate increasing atmospheric carbon dioxide concentrations. Carbon accrues in grassland soil restored from cultivation, but the amount of physically protected C (here measured as microaggregate-within-macroaggregate C) and predominant mechanisms of accrual are not well understood. We modeled the rate of physically protected carbon accrued in three mesic temperate perennial restored grasslands from cross-continental regions using datasets with a wide range of restoration ages from northeast Kansas, USA; southeast Nebraska, USA; and northeast Free State, South Africa. Further, we investigated major controls on the amount of physically protected C in each site using structural equation modeling. Variables in the structural equation model were root biomass, root C:N ratio, soil structure (indicated by bulk density, percent of macroaggregates on a per whole soil mass basis, and percent of microaggregate-within-macroaggregates on a per macroaggregate mass basis), microbial composition (indicated by microbial biomass C, total phospholipid fatty acid [PLFA] biomass, and PLFA biomass of arbuscular mycorrhizae fungi [AMF] biomass), and microaggregate-within-macroaggregate C on a per whole soil mass basis. Across all sites, physically protected C accrued at a rate of 16 ± 5 g m−2 year−1. Data from South Africa fit an a priori metamodel developed for northeast KS that hypothesized physically protected C could be explained as a function of microbial composition, soil structure, root C:N ratio, and root biomass (listed in order of strength of direct effect on physically protected C). In contrast to the model-based hypothesis, root C:N ratio was the strongest influence (negative) on physically protected C in South Africa. The lesser effect of AMF on physically protected C in South Africa was consistent with lower AMF biomass in arid environments. The hypothesized model did not fit southeast Nebraska data possibly due to high (~ 30%) clay content. Overall, these results suggest that physically protected C in soil with moderate amounts of clay (more than 10% and less than 30%) can be predicted with knowledge of roots (biomass and C:N ratio), microbial biomass, and soil aggregation.

This study aimed to identify the factors that influence smallholder farmers’ decisions to adopt four different conservation agriculture (CA) practices (i.e. minimum tillage, intercropping, cover cropping and crop rotation) in Mozambique. A non-probability sampling approach, incorporating both purposive and accidental sampling types, was followed. Three agro-ecological regions, followed by four provinces, were purposely selected. In addition, Accidental sampling was used to select 616 smallholder farmers from 38 communities in the three agro-ecological regions where CA projects were historically implemented by several NGO institutions. A questionnaire was administered to the 616 selected smallholder farmers. A descriptive logit model was applied in STATA to determine the probability of respondents adopting CA practices. The findings show that 44.6% of smallholder farmers adopted one or more of the CA practices, and 55.4% did not. It was also clear that most farmers did not adopt all components CA. Results obtained revealed that household size, animal ownership, communication assets (such as television, radio, and cell phone) and group membership had a positive influence on CA adoption. Interestingly, female-headed households were more likely to adopt CA. Awareness of soil health decline is an important factor determining adoption. The study concluded that the reasons for adoption are site-specific and a ‘blanket approach’ to promote adoption of CA is unlikely to be successful.

Conservation agriculture (CA) practices are advocated to reduce soil degradation, resulting in more sustainable food production as compared to conventional tillage (CT). In this study, the short-term effects of two tillage systems in combination with cropping patterns on selected soil physical parameters on four experimental sites in Mozambique were studied. The study sites differ according to their climatic conditions, soil types, and crop adaptation. Tillage systems evaluated were CA and CT, while the cropping pattern had four levels of sole cropping and three levels of intercropping. In general, soil physical properties showed significant changes due to the tillage systems, but the cropping pattern and their interaction with tillage systems did not yield significant impacts on the soil physical properties. CA increased bulk density, penetration resistance, and saturated hydraulic conductivity as compared to CT. A significant difference due to the tillage system was observed across the four sites, and in general, evaporation was higher in CT compared to CA. The presence of crop residues in CA contributed to lower evaporation. Thus, in the short term, CA practices could be a sustainable option to conserve soil water through higher infiltration and less evaporation.

Woody encroachment is increasingly threatening savanna ecosystems, but it remains unclear how this is driven by different land tenures and management systems. In South Africa, communal land is mainly managed under continuous grazing, while commercial land is under rotational grazing. We hypothesize that woody encroachment has increased since the end of the Apartheid era in 1994, when rotational grazing systems in communal land switched to continuous grazing. To test this hypothesis, we sampled six subsites in each of three replicates of the two tenure systems in the savannah biome, South Africa, and monitored the degree of woody encroachment and effects on soil using remotely-sensed normalized difference vegetation index (NDVI), dendro-ecology, and grid-based soil analyses. The results confirmed that there has been a positive greening trend over the past 25 years in communal areas due to an increase in woody cover, particularly for tree in the 50–150 cm height class. These trees corresponded to Senegalia tree ages between 10 and 25 years. Greater woody cover in communal areas was accompanied by height-dependent elevated nutrient and organic matter concentrations that increased in the topsoils by a factor of up to 1.5, relative to the freehold systems. Isotopic analyses identified debris of C3 bushes and trees as main carbon input into the soil of communal areas, where the δ13C value of −20.1‰ was significantly lower than it was at private farms (−19.2‰). Isotopic values also indicated that this carbon input took place beyond the edges of the bush canopy. We conclude that the tenure-based management system in the communal areas has promoted woody encroachment, with the associated changes in soil properties homogenizing the savanna system.

Reseeding grasses into degraded cropland and using it as permanent secondary pasture may combat desertification and promote sequestration of soil organic carbon (SOC). Our objective was to assess the contribution of lignin and grass-derived carbon to the rates of SOC accrual upon such land-use change. We sampled secondary pastures of different age (1 to 31 years old), established on former degraded cropland at three agro-ecosystems in the semi-arid Highveld of South Africa; old degraded croplands and primary grassland used for cattle grazing served as controls. Topsoil samples (0–5 and 5–10 cm) were characterized by their natural 13C and 15N abundances and by their contents of lignin-derived phenols (VSC) after alkaline CuO oxidation. The results showed that the δ13C signature in secondary pasture soil was variable and not a sensitive indicator of land restoration in these sites. Soil δ15N values declined from 8 to 6‰ with increased duration of secondary pasture use and approached the values of the primary grassland, suggesting that N-use efficiency improved. The VSC contents increased in the surface soils and reached a new steady-state equilibrium after 38 years. Lignin sequestration was accelerated relative to total SOC in the 0–5 cm soil depth but delayed in the 5–10 cm soil depth, reflecting different amounts of depth-dependent inputs of plant debris into the pastures. Even if absolute VSC contents did not reach the respective concentrations in the primary grassland, the contribution of VSC to total SOC (g VSC kg− 1 C) was similar to that of the primary grassland. Decreasing ratios of syringyl to vanillyl structural units reflected that after 8–19 years all remaining lignin derived primarily from the new grassland vegetation. Lignin enhances SOC accumulation only in the very surface soil (0–5 cm) but not below this depth, rendering the restoration of subsoil C stocks difficult in these ecosystems.

Global diminishing water resources, especially due to climate change have serious impacts on evaporation (E) from the soil surface, transpiration (T) from plants (crops) and grain yield, which relates to water use efficiency of different crops. A study was conducted at Kenilworth over two wheat cropping seasons (2007 and 2008) with the objectives of: (i) evaluating the effect of soils and seasons on T, E and yield, and (ii) relating these parameters to transpiration efficiency coefficient. The treatments included two soil types and two soil surface treatments (bare and mulched), which were all replicated four times. Weekly irrigation was done using a surface drip system while maintaining the water table at a constant depth. Soil water content was monitored using a neutron probe. Neither soils nor seasons were found to significantly influence the partitioning of evapotranspiration (ET), and T varied from 74 to 76% of ET while E varied between 24 and 26%. Surface treatments caused significant differences in grain yield in both seasons. Reducing evaporative loss improves the water productivity of wheat, which has an important implication in dryland farming., Evaporation; Transpiration; Water use efficiency; Transpiration efficiency coefficient

In South Africa, maize is often produced on highly weathered soils with low organic matter contents. These soils are unable to supply sufficient nitrogen (N) and phosphorus (P) for sustainable maize production, resulting in the use of fertiliser sources containing either N or P. In some instances, however, the efficiency of these sources may be questioned. In a glasshouse pot experiment, young maize aerial plant material was analysed to evaluate the effect of band-applied N sources, P sources and P application rates on the quantitative nutrient concentration, as well as uptake of several nutrients. With the exception of potassium uptake, all parameters were significantly affected by the main effects only. Nutrient concentration measurements were inconsistent for N sources. However, total uptake proved to be more efficient with the application of limestone ammonium nitrate compared with that of urea. Both nutrient concentration and uptake were greater with the application of the orthophosphate sources mono...

Understanding the nutrient requirements of maize is especially important during early vegetative growth and development, particularly when produced on highly weathered soils with low organic matter contents. Therefore, the growth response of maize to different nitrogen (N) sources viz. limestone ammonium nitrate and urea, phosphorus (P) sources viz. monoammonium phosphate, nitrophosphate and ammonium polyphosphate, as well as P application levels viz. 0, 10, 20, 30 and 40 kg ha−1 during early growth were evaluated. In pot trials with a sandy loam topsoil, aerial and subsoil plant parameters were measured to evaluate the effect of treatment and treatment combinations of P and N fertilisers on the vegetative growth and development of maize during the first five weeks after emergence. Plants treated with limestone ammonium nitrate had significantly greater plant parameter measurements compared with urea, primarily ascribed to immediate availability after application in addition to ease of uptake. Monoammoniu...

Pre-1994 land policies caused South Africa to have a well-developed commercial agricultural sector and an underdeveloped communal agricultural sector. Both sectors are still operational today for either crop or livestock production despite significant political and social changes that have taken place since 1994. Climate, irrespective of agricultural sector, dictates together with soil the location of crop production regions and, together with vegetation, the location of livestock production regions. In recent years the production of crops and livestock has intensified. This intensification can be devastating to soil and vegetation because both natural resources are very susceptible to degradation. In addition to land degradation, climate change and land claims may impact negatively on future food production and security. A combined approach between agricultural and governmental stakeholders is therefore essential to counteract these potential negative impacts.

Land use and management should be addressed before pursuing a concise discussion on the roles of soil in ecosystems, and the nature of soils. The soils of southern Africa will be dealt with extensively, focusing on land and soil degradation in each of the southern African countries. Soil carries out a number of key roles in ecosystems that are essential to our well-being. As a result of soil-forming factors and processes active in southern Africa, a large range of soils with diverse properties are found in the region. The diversity of active soil-forming factors and processes in southern Africa have led to the development of a large number of soil groups. Soil texture is a very stable, and arguably the most important, physical characteristic of soil. It also correlates well with other soil properties. Land degradation is responsible for enormous losses to the economies of some countries as well as the escalating poverty of the majority of southern Africa's population.

The continuous application of phosphorus (P) to agricultural systems to ensure profitable crop productivity can lead to an accumulation of P in agricultural soils. While this long-term residual pool of soil P is desirable from an agronomic perspective, there is some concern about its possible impacts on surface water quality as it may lead to eutrophication. Since a better understanding of P transport processes provides useful information for the development of site-specific P management strategies, rainfall simulation studies (100 mm h−1 for 30 min) on runoff plots (2.1 m × 0.75 m) were conducted at 20 field sites to study the mobilisation of soil P from sugarcane fields of Mauritius. The research findings indicated that the edge-of-plot P losses were insignificant from an agronomic perspective but only small amounts of P can actually trigger eutrophication in freshwaters. The results also showed that total P concentrations in runoff are more strongly associated with runoff sediments (r2 = 0.92) than runoff volume (r2 = 0.49) indicating that a greater proportion of the P transported in runoff occurred as particulate P rather than dissolved P. Actually, about 89 % of total P loss in runoff waters was mobilised in particulate form, pointing to the importance of erosion as a mechanism for mobilising soil P. The research findings suggest that in addition to the current management practices aiming at reducing runoff volume, such as conservation tillage and trash cover, measures that reduce sediments in runoff, such as grassed waterways and riparian buffers, may further attenuate P losses.

Cultivated pastures provide increased productivity and contribute inter alia to food security. Rangelands in the southern Cape region of South Africa have had a low animal production potential and were therefore improved through time as cultivated pastures. Initially, annual pastures were established by conventional tillage methods, but from the 1990s permanent pastures were established on a minimum-tillage regime. Lime and fertiliser guidelines, which were developed for annual pastures established by conventional tillage methods, were followed on minimumtillage systems, despite changes in the soil physical properties and stratification of biological parameters. The study aim was to survey the soil fertility of irrigated minimum-till kikuyu–ryegrass pastures in the southern Cape region. This study highlights changes in soil fertility and identified potential detrimental effects of elevated levels of extractable phosphorus and zinc, which were drastically increased in the topsoil layer. The necessity for remedial and preventative strategies to mitigate nutrient loading in cultivated pasture soil are stressed. Fertiliser guidelines should be applied strictly and should fit the farming system. In doing so, the dual goals of economic and ecological sustainability should be achieved.Keywords: cultivated pastures, fertilisation, nutrient cycling, phosphorus, soil fertility

Conversion of cultivated land to grassland is globally practiced to reverse soil degradation, but belowground ecosystem response to restoration has never been compared between old and new world temperate grasslands. We used a chronosequence approach to model change in root biomass and quality (indexed by C:N ratio), microbial biomass and composition [indexed by phospholipid fatty acids (PLFAs)], soil aggregate structure, and soil C and N stocks in the South African Highveld and compared recovery of these variables to a grassland restoration chronosequence in the US tallgrass prairie. We hypothesized soil C recovery, and mechanisms promoting soil C and N accrual would be convergent between these distant temperate grasslands with similar growing season precipitation, history of cultivation, and undergoing restoration with C4-grasses. Total PLFA richness and concentrations of most microbial groups rose to represent uncultivated grassland in the highveld (similar to tallgrass prairie), but in contrast to tallgrass prairie, the fungi:bacteria ratio did not increase with restoration age. In the highveld, root biomass accumulation was lower, but root quality became more representative of the never-cultivated grassland than in restorations in tallgrass prairie. Soil aggregate recovery was slightly faster in tallgrass prairie, and the pattern of macroaggregate C recovery was divergent due to less depletion in cultivated soil and higher stock of C in the uncultivated soil relative to the highveld. More rapid restoration of total soil C and N stocks in the highveld was attributed to greater soil C saturation deficit at the onset of restoration, development of higher quality root systems that promote the microbial biomass and soil aggregation, and climate conditions (distinct periodicity of rainfall and high aridity) that likely impose more limitation to decomposition relative to the tallgrass prairie ecosystem.

Kikuyu (Pennisetum clandestinum)–ryegrass (Lolium spp.) cultivated pastures form the base for dairy production in the southern Cape region of South Africa. The impact of various methods and implements used to over-sow ryegrass on the productivity and botanical composition of the pasture is unknown. The study aimed to assess the impact of cultivation along a soil disturbance gradient on the productivity and botanical composition of kikuyu-based pastures. Five tillage treatments were assessed, namely kikuyu over-sown with ryegrass using a minimum till planter, eradication of kikuyu with herbicide and ryegrass sown with a minimum-till planter, shallow ( 150 mm) disturbance with a rotavator or conventional tillage, respectively, and a control. The autumn, spring, summer and annual productions of treatments with the least soil disturbance were the highest (P ≤ 0.05). Shallow or deep tillage resulted in intermediate annual productions and the lowest (P ≤ 0.05) was recorded for the herbicide t...

Prolonged arable cropping of subtropical sandy grassland soils resulted in substantial losses of water-stable aggregates. The objective of this study was to evaluate how rapidly and to which degree soil aggregates may be restored when degraded cropland soils are converted to secondary pasture lands. Hence, chronosequences of degraded cropland soils (more than 20 years of arable cropping) were sampled that had been converted to secondary pastures between 1 and 52 years ago in three agro-ecosystems of the South African Highveld. Primary grasslands as well as sites under long-term cropping served as references. The surface soils (Plinthusthalfs; 0–10 cm) were fractionated to aggregates of different sizes by wet sieving (8000–2800 μm, 2800–2000 μm, 2000–500 μm, 500–250 μm, 250–53 μm and 2000 μm) approached 85–94% of the levels of the primary grasslands. The stocks of organic matter bound to large macroaggregates recovered more slowly and reached only 50% of that of the primary grassland, which, however, exhibited also a slightly elevated clay content. The carbon concentration in the aggregates did not change significantly. Thus, the increase in C stocks in the secondary pasture soils was mainly due to a rebuilding of large macroaggregates, which contained more C than the smaller-sized aggregate classes, but not more C than the respective aggregates in the degraded cropland. We conclude that only the amount of large macroaggregates was restored upon land conversion, while their protective capacity was obviously not restored and thus not strong enough to account for a full sequestration of soil organic matter.

In the past decades, social-ecological systems (SESs) worldwide have undergone dramatic transformations with often detrimental consequences for livelihoods. Although resilience thinking offers promising conceptual frameworks to understand SES transformations, empirical resilience assessments of real-world SESs are still rare because SES complexity requires integrating knowledge, theories, and approaches from different disciplines. Taking up this challenge, we empirically assess the resilience of a South African pastoral SES to drought using various methods from natural and social sciences. In the ecological subsystem, we analyze rangelands' ability to buffer drought effects on forage provision, using soil and vegetation indicators. In the social subsystem, we assess households' and communities' capacities to mitigate drought effects, applying agronomic and institutional indicators and benchmarking against practices and institutions in traditional pastoral SESs. Our results indicate that a decoupling of livelihoods from livestock-generated income was initiated by government interventions in the 1930s. In the post-apartheid phase, minimum-input strategies of herd management were adopted, leading to a recovery of rangeland vegetation due to unintentionally reduced stocking densities. Because current livelihood security is mainly based on external monetary resources (pensions, child grants, and disability grants), household resilience to drought is higher than in historical phases. Our study is one of the first to use a truly multidisciplinary resilience assessment. Conflicting results from partial assessments underline that measuring narrow indicator sets may impede a deeper understanding of SES transformations. The results also imply that the resilience of contemporary, open SESs cannot be explained by an inward-looking approach because essential connections and drivers at other scales have become relevant in the globalized world. Our study thus has helped to identify pitfalls in empirical resilience assessment and to improve the conceptualization of SES dynamics.

Loss of productive agricultural land due to soil degradation poses a serious threat to agricultural output and sustainability. Soil degradation of cultivated pastures manifests as a long-term decline in production potential. There are concerns that soils under pastures in certain regions of South Africa are degrading as a result of mismanagement, which include practising continuous tillage, improper grazing management, injudicious application of fertilisers and poor irrigation management. Soil quality indicators, which include physical, chemical and biological parameters in soil, are science-based methods to monitor sustainability and protect soil. Soil quality data informs the land manager in the decision-making process to address practices that degrade agro-ecological systems and mitigate any loss of function. The aim of this paper is to provide a synthesis of the challenges and opportunities in soil management of pastures. The value of assessing soil chemical, physical and biological indicators to monitor soil quality is stressed.Keywords: fertilisation, grazing management, irrigation management, soil degradation, tillage

Converting grassland soils to cropland changes the turnover of aggregates and their protective effects on soil organic matter (SOM). Our aim was to elucidate how rapid soil aggregation and associated SOM contents change with increased duration of long-term cropping. The study was conducted in the South African Highveld in three agro-ecosystems with different cropping durations (0–98 years) after conversion of the native grassland. Soil samples (0–20 cm; Plinthustalfs) were analyzed for aggregate size distribution (six water-stable aggregate fractions) and the C and N contents in each single fraction. Additionally, sieved and ground bulk soil was oxidized with 0.33 M KMnO 4 . With an increase of cultivation time the aggregate size distribution changed remarkably: the large macroaggregates (2000–2800 μm) and peds (> 2800 μm) broke down into finer size classes (exponential rate constant k = 0.20 year −1 ), accompanied especially by an increase of the contents of small macroaggregates (250–2000 μm) and not of the microaggregates as previously reported for other soil types. The C and N concentrations decreased in all aggregates ( k = 0.05 to 0.52 year −1 ) and the rate of decline was fastest in the peds. Most C was lost from the 250–2000 μm and 53–250 μm sized fractions (about 60%). At the primary grassland, the peds (> 2800 μm) stored 47% of total C, but less than 10% after 90 years of cropping. Parts of this C were recovered in smaller aggregate fractions. Especially the C stored in the small macroaggregates (250–2000 μm) increased from 27 ± 1% to 61 ± 1% of total C. The total loss of SOM and large aggregates went along with an accelerated loss of its C resistant to oxidation. While 40% of soil C was not oxidizable with KMnO 4 from the grassland soil, the contents of this C resistant to chemical oxidation were reduced by up to 50% as cultivation proceeded. Grinding the soil hardly changed this amount. We conclude that the loss of soil structure made also resistant SOM available for decay, which might then be difficult to replace when a restoration of the sites is anticipated.

Prolonged intensive arable cropping of semiarid grassland soils in the South African Highveld resulted in a significant loss of C, N and associated living and dead microbial biomass. To regenerate their soils, farmers converted degraded arable sites back into secondary pastures. The objective of this study was to clarify the contribution of microorganisms to the sequestration of C and N in soil during this regeneration phase. Composite samples were taken from the topsoils of former arable land, namely Plinthustalfs, which had been converted to pastures 1–31 years ago. Amino sugars were determined as markers for microbial residues in the bulk soil and in selected particle-size fractions. The results showed that when C and N contents increased during the secondary pasture usage, the amino sugar concentration in the bulk soil (0–5 cm) recovered at similar magnitude and reached a new steady-state level after approximately 90 years, which corresponded only to 90% of the amino sugar level in the primary grassland. The amino sugar concentration in the clay-sized fraction recovered to a higher end level than in the bulk soil, and also at a faster annual rate. This confirms that especially the finer particles contained a high amount of amino sugars and were responsible, thus, for the restoration of microbially derived C and N. The incomplete recovery of amino sugars in bulk soil can only in parts be attributed to a slightly coarser texture of secondary grassland that had lost silt through wind erosion. The soils particularly had also lost the ability to restore microbial residues below 5 cm soil depth. Overall, the ratios of glucosamine to muramic acid also increased with increasing duration of pasture usage, suggesting that fungi dominated the microbial sequestration of C and N whereas the re-accumulation of bacterial cell wall residues was less pronounced. However, the glucosamine-to-muramic acid ratios finally even exceeded those of the primary grassland, indicating that there remained some irreversible changes of the soil microbial community by former intensive crop management.

SUMMARY Sustainability is socially constructed, and striving for a sustainable agricultural system requires addressing ecological, social and economic dimensions simultaneously. The objective of this study was to assess the sustainability of dryland farming systems of Eastern Kenya based on farmers’ perceptions of their farming environment and the implications for rural livelihoods. Farmers’ perceptions were elicited through semi-structured interviews in which questionnaires were administered, on single visits, to 30 selected farm households. The study showed that soil fertility and yields of staple food crops (maize) have declined in the past decade and that current farming systems are not able to produce adequate food and income to the dependent households. Thus, the farming system is showing symptoms of unsustainability. Improving farming system sustainability in this dryland area will ultimately require integration of technical and policy options that take cognisance of farmers’ abilities, opportunities and socio-economic circumstances.

A study was carried out to investigate soil quality of dryland farming systems of Mbeere District, Eastern Kenya and its implications on agricultural sustainability. Soil sampling was done in years 2002 and 2004 from 30 study farms and same sampling sites and from paired adjacent land-use systems (bush fallow and cultivated fields). Physical, chemical and biological laboratory analyses were carried out. Factor analysis showed that the main soil quality indicators for evaluating sustainability of the dryland farming systems of the district were texture; organic matter, exchangeable bases (Ca, Mg, K, and Na); extractable nutrients (P and Zn); and acidity (pH). Cultivated soils were characterised by low pH, nutrient deficiencies (N, P, Cu, and Zn) and low organic matter and a decline in organic C, total N, exchangeable K and extractable P across time. They were also lower in exchangeable bases and microbial biomass C than adjacent bush fallow soils. We conclude that the soils are degraded and farmin...

A study was conducted in the semi-arid Mbeere district, Eastern Kenya, to determine spatial and temporal variability of crop yields and climatic factors and their impacts on sustainability of semi-arid agriculture. Spatial aspects were assessed by conducting a survey of on-farm crop yields and using a computer model (QUEFTS) to predict maize yields from soil chemical indices. Temporal aspects were studied using time-series data (rainfall, temperature and crop yields). The study did not find a significant correlation between farmers' actual yields and QUEFTS predictions, and soil nutrients were thus not the only factors influencing maize yields in the study area. Other yield-reducing factors (climate and management) not accounted for in the QUEFTS model played a role. Grain yields of staple food crops were highly variable, fluctuating in time and space, and suboptimal. Particular aspects of rainfall (e.g. the short rains, for cowpeas and beans) were more important than mean annual rainfall in determining crop yields, and factors other than rainfall and soil fertility apparently played a role. The observed low grain yields, large yield gaps and high rainfall variability challenge the sustainability of these farming systems. Copyright © 2008 John Wiley & Sons, Ltd

At least one third of South Africa's total grain crop, mainly maize and wheat, is produced in the Free State. Crop production potential and sensitivity to degradation depend largely on the interaction of climate and soil. Climatic conditions vary from arid in the south and southwestern region to nearly sub-humid in the extreme east on the edge of the Drakensberg escarpment. A map of soil distribution in the Province shows groups of land types, each one characterised by a particular soil distribution pattern. The soil components of the map units is also presented. The crop production potential of the Province is presented in the form of a map. Delineations are based on interpretation of land type data and range from arable to non-arable. The total potentially arable area of the Free State is 3.82 million ha. In spite of a detailed understanding of the Free State's soils, degradation continues to occur, especially on cultivated land, where the most serious forms are erosion, acidification and organ...

We know little about the mechanisms that cause rapid losses in the soil organic N pool during cropping. As the analysis of amino acid enantiomers can provide insight into both the fate of microbial N and the ageing of cells in the environment, we used this technique as a tool to examine how the pool of protein-bound N in subtropical Plinthosols responds to increasing duration of arable cropping. The samples comprised bulk soils (0–20 cm) and clay fractions from each of three agro-ecosystems in semiarid South Africa; the sites have been cropped for periods varying from 0 to 98 years. The amino acid enantiomers contributed 34% to the total N content. With increasing number of years a piece of land has been cropped, the amino acid concentrations declined bi-exponentially to about 30% of their initial level in the native grasslands. Changes of the remaining soil protein-N pool were indicated by alterations in the d-content of individual amino acids. As the years of arable cropping increased, the proportions of d-alanine and d-glutamic acid increased relative to the respective l-enantiomers. This was attributed to an accumulation of N in residues of bacterial cell walls. In contrast, the d/l-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land.

Degradation of soil as a consequence of land use poses a threat to sustainable agriculture in South Africa, resulting in the need for a soil protection strategy and policy. Development of such a strategy and policy require cognisance of the extent and impact of soil degradation processes. One of the identified processes is the decline of soil organic matter, which also plays a central role in soil health or quality. The spatial variability of organic matter and the impact of grazing and burning under rangeland stock production are addressed in this first part of the review. Data from uncoordinated studies showed that South African soils have low organic matter levels. About 58% of soils contain less than 0.5% organic carbon and only 4% contain more than 2% organic carbon. Furthermore, there are large differences in organic matter content within and between soil forms, depending on climatic conditions, vegetative cover, topographical position and soil texture. A countrywide baseline study to quantify organic matter contents within and between soil forms is suggested for future reference. Degradation of rangeland because of overgrazing has resulted in significant losses of soil organic matter, mainly as a result of lower biomass production. The use of fire in rangeland management decreases soil organic matter because litter is destroyed by burning. Maintaining or increasing organic matter levels in degraded rangeland soils by preventing overgrazing and restricting burning could contribute to the restoration of degraded rangelands. This restoration is of the utmost importance because stock farming uses the majority of land in South Africa.

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.

Despite our growing knowledge on plants' functional responses to grazing, there is no consensus if an optimum level of functional aggregation exists for detecting grazing effects in drylands. With a comparative approach we searched for plant functional types (PFTs) with a consistent response to grazing across two areas differing in climatic aridity, situated in South Africa's grassland and savanna biomes. We aggregated herbaceous species into PFTs, using hierarchical combinations of traits (from single- to three-trait PFTs). Traits relate to life history, growth form and leaf width. We first confirmed that soil and grazing gradients were largely independent from each other, and then searched in each biome for PFTs with a sensitive response to grazing, avoiding confounding with soil conditions. We found no response consistency, but biome-specific optimum aggregation levels. Three-trait PFTs (e.g. broad-leaved perennial grasses) and two-trait PFTs (e.g. perennial grasses) performed best as indicators of grazing effects in the semi-arid grassland and in the arid savanna biome, respectively. Some PFTs increased with grazing pressure in the grassland, but decreased in the savanna. We applied biome-specific grazing indicators to evaluate if differences in grazing management related to land tenure (communal versus freehold) had effects on vegetation. Tenure effects were small, which we mainly attributed to large variability in grazing pressure across farms. We conclude that the striking lack of generalizable PFT responses to grazing is due to a convergence of aridity and grazing effects, and unlikely to be overcome by more refined classification approaches. Hence, PFTs with an opposite response to grazing in the two biomes rather have a unimodal response along a gradient of additive forces of aridity and grazing. The study advocates for hierarchical trait combinations to identify localized indicator sets for grazing effects. Its methodological approach may also be useful for identifying ecological indicators in other ecosystems.