Effect of cultivation on the nitrogen fertility of selected dryland soils in South Africa

Abstract

This study is part of an investigation into the effect of cultivation on the organic matter content of topsoils (0 - 200 mm) from commercial farms practising dryland agriculture in the summer rainfall area of South Africa (24°-30°S; 24°-30°E). The aim was to examine the effects of cultivation, period of cultivation and soil properties on potential mineralizable and residual inorganic N as parameters of the nitrogen fertility of the soils under discussion. Virgin and cultivated soils were sampled at 50 sites. Of these, 27 were located within five ecotopes. The sites in each ecotope varied as to cultivation period. The remaining 23 sites were chosen to obtain a wider range of climatic conditions and soil properties. Incubation tubes, packed in triplicate with a mixture of a soil and filter sand, were leached before incubation (residual inorganic N) and again after 2, 5, 9, 14 and 22 weeks of incubation (mineralizable N) at 30°C with 0.01 M CaCl₂. The data was used to calculate cumulative net mineralization after 22 weeks (N ₂₂), nitrogen mineralization potential (N ₀) and rate constant (k). Paired t-tests showed that cultivation had a significant negative effect on mineralization characteristics, viz. a decrease of 48.5% in N₂₂ 37.6% in N ₀ and 22.6% in k. Residual inorganic N during sampling did not differ significantly between virgin (18.3 mg kg⁻¹) and cultivated (15.5 mg kg⁻¹) soils. Plant-available N, the sum of N ₂₂ and residual inorganic N differed significantly between virgin (61.9 mg kg⁻¹) and cultivated (38.8 mg kg⁻¹) soils. Although unique for each ecotope, the decline of N ₂₂ was rapid during the first few years of cultivation. Thereafter the rate decreased until an equilibrium was reached after 15 - 45 years of cultivation. The decline of N ₂₂ in ecotopes from the warmer, drier areas was higher than in the ecotopes from the cooler, wetter areas, but the percentage loss was larger in the latter areas. This reduction in nitrogen fertility status of cultivated soils may result in unsustainable crop production.