Initial assessment of selected biological soil health indicators in organic versus conventional cropping systems in field trials in South Africa.

Agricultural sustainability is built on the premise of promoting soil health. Agricultural practices such as tillage, crop rotation, cover crops and fertilization have been credited with shifts in the soil food web which affects many aspects of soil health. The aim of the current study was to investigate the effects of conventional versus organic cultivation systems on selected biological soil health indicators within the existing long-term comparative farming systems field trial at the George Campus of the Nelson Mandela University in South Africa's Western Cape Province. Our results show that microbial diversity, evenness and enzyme activity was highest in the organic monocrop cabbage treatment. Richness of nematode predators was a function of the abundance of nematode prey that could be improved by organic inputs. The highest nematode diversity was correlated with organic treatments which also indicated the highest fertility (enrichment index). Organic treatments also had the better soil food web structure (structure index) while for the majority of treatments, the decomposition channel was fungal (high channel index). The control and conventional cabbage treatments had more nematodes associated with stressful conditions, possibly because of toxicity from chemicals used in these cropping systems. Nematode populations (functional guilds) were more sensitive to cropping systems than microbial populations (functional diversity) although both biological indicators resulted in similar conclusions. In terms of biological diversity, rotation treatments had an intermediate effect while monocrop treatments performed better. We conclude that the use of organic amendments and cover crops relative to conventional systems has the potential to improve soil quality in the long term, through improved biodiversity and higher organic matter content.