Biological quality of a podzolic soil after 19 years of irrigated minimum-till kikuyu–ryegrass pasture

Abstract

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.