Modelling N mineralisation from sludge-amended soils across agro-ecological zones: A case study from South Africa

The expected rate of nitrogen (N) mineralisation is key to sludge recommendation rates for agricultural lands. The presence of a combination of factors (biotic and abiotic) that contribute to the variation in N release at different sites is the main obstacle to sludge application rate recommendations. Nonetheless, the use of a single annual N mineralisation rate across sites and agro-ecological zones is a common practice around the world. The aim of this study was to investigate whether a single annual N release rate could be used for sludge recommendation across sites within an agro-ecological zone. To achieve the stated aim, the following hypotheses were tested using the SWB-Sci computer model: Under dryland cropping, cumulative annual N mineralisation from sludge-amended soils (1) will remain unchanged across agro-ecological zones and between sites within an agro-ecological zone, (2) will not vary between and within seasons at a specific site and (3) will not vary across soil textures within a site. Model simulations showed that annual N mineralisation rates varied significantly (P > 0.05) across agro-ecological zones, ranging from 61 kg ha(-1) in arid to 103.6 kg ha(-1) in super-humid zones. Similarly, annual N mineralisation rates varied significantly between sites within an agro-ecological zone (98.6 kg ha(-1) in Durban vs. 81.4 kg ha(-1) in Cape Town in the humid zone). Annual N mineralisation rates did not differ significantly (P < 0.05) between seasons within a site, but varied significantly within a season in the humid and super-humid agro-ecological zones. Nitrogen mineralisation varied significantly between soil textures, ranging from 97.8 kg ha(-1) (clay soil) to 106.3 kg ha(-1) (clay loam) in the super-humid agro-ecological zone. Therefore, sludge recommendation rates for use in agricultural lands should be site specific and soil specific for sustainable ecosystem services and functions. Further model validation on N mineralisation using field data is recommended. (C) 2015 Elsevier B.V. All rights reserved.