Soil microbial communities perform critical functions in ecosystem processes. These functions can be used to assess the impact of agricultural practices on sustainable crop production. In this five-year study, the effect of various agricultural practices on soil microbial diversity and activity was investigated in a summer rainfall area under South African dryland conditions. Microbial diversity and activity were measured in the 0-15 cm layer of a field trial consisting of two fertilizer levels, three cropping systems, and two tillage systems. Using the Shannon-Weaver and Evenness diversity indices, soil microbial species richness and abundance were measured. Microbial enzymatic activities: β-glucosidase, phosphatase and urease, were used to evaluate ecosystem functioning. Cluster analysis revealed a shift in soil microbial community diversity and activity over time. Microbial diversity and activity were higher under no-till than conventional tillage. Fertilizer levels seemed to play a minor role in determining microbial diversity and activity, whereas the cropping systems played a more important role in determining the activity of soil microbial communities. Conservation agriculture yielded the highest soil microbial diversity and activity in diversified cropping systems under no-till.

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Soil quality of pastures changes through time because of management practices. Excessive soil disturbance usually leads to the decline in soil quality, and this has resulted in concerns about kikuyu (Pennisetum clandestinum)–ryegrass (Lolium spp.) pasture systems in the southern Cape region of South Africa. This study aimed to understand the effects of tillage on soil quality. The soil management assessment framework (SMAF) and the locally developed soil quality index for pastures (SQIP) were used to assess five tillage systems and were evaluated at a scale inclusive of variation in topography, pedogenic characteristics and local anthropogenic influences. Along with assessment of overall soil quality, the quality of the physical, chemical and biological components of soil were considered individually. Soil physical quality was largely a function of inherent pedogenic characteristics but tillage affected physical quality adversely. Elevated levels of certain nutrients may be warning signs to soil chemical degradation; however, tillage practice did not affect soil chemical quality. Soil disturbance and the use of herbicides to establish annual pastures has lowered soil biological quality. The SQIP was a more suitable tool than SMAF for assessing soil quality of high-input, dairy-pasture systems. SQIP could facilitate adaptive management by land managers, environmentalists, extension officers and policy makers to assess soil quality and enhance understanding of processes affecting soil quality.

Conversion of natural rangeland to minimum-tillage kikuyu (Pennisetum clandestinum) based pastures for dairy production in the southern Cape of South Africa, may be beneficial to soil biological quality. The objective was to evaluate whether 19 years of minimum-till kikuyu-ryegrass pasture had altered the distribution and quality of biological properties formerly developed under natural rangeland. An irrigated minimum-till kikuyu-ryegrass pasture soil was compared to virgin soil with natural rangeland. Soil organic matter, soil organic C, active C, microbial biomass C, total N and enzymatic activities (β-glucosidase, urease and alkaline phosphatase) behaved similarly by having higher values in the surface layers of the cultivated pasture soil than in virgin soil, decreased with depth until they become similar at the 200–300 mm depth. Acid phosphatase activity was similar (P > 0.05) between soils. Vertical distribution of potentially mineralizable N was similar (P > 0.05) at 0–100 mm soil depth, but higher (P ≤ 0.01) in the cultivated pasture soil than in the virgin soil. The microbial indicated along with stratification ratios for different biological indicators that the cultivated pasture soil’s ecosystem functionality improved. Soil microbial functional diversity and carbon source utilisation patterns of the cultivated pasture soil and virgin soil was influenced by plant species present and root exudate composition. The soil microbial diversity, as shown by the Shannon-Weaver and Enrichment Indices, was significantly altered between cultivated pasture and the virgin soil, especially at different soil depths. A general appraisal of biological soil properties indicated that conversion of natural fynbos vegetation to irrigated minimum-till kikuyu-ryegrass pasture after 19 years of cultivation on a podzolic soil beneficial.

Soil quality of tillage systems receives much attention worldwide, although few studies attempt to link soil quality to yield. Partial least‐squares regression analysis is a suitable method to construct predictive models around plural, highly collinear factors, such as soil quality and its effects on yield. This study aimed at identifying the soil quality properties which best model pasture herbage yield by relating soil quality indicators with variations in yield as a result of soil disturbance caused by tillage. The study was conducted on kikuyu (Pennisetum clandestinum) over‐sown with annual ryegrass (Lolium multiflorum) using different tillage methods. Tillage methods involved over‐sowing kikuyu with ryegrass using a minimum‐tillage seed drill, eradication of kikuyu with herbicide and ryegrass sown with a minimum‐tillage planter, shallow and deep disturbance, and a control. Most changes in soil quality indicators as a result of soil tillage were observed shortly after tillage and occurred mostly at the 0–100 mm soil layer. Few of these effects were still visible 420 d after tillage. Microbiological indicators changed most in response to the treatments, but unlike the chemical and physical indicators, microbiological indicators have no causal link to pasture production. The contribution of individual soil quality indicators to variance in pasture productivity could not be isolated and should thus be seen as complex processes which affect yield. Although mechanisms of how some soil quality indicators affect yield is clear, more research is required to determine mechanisms of how a combination of multiple soil quality indicators affects yield.