Review and meta-analysis of organic matter in cultivated soils in southern Africa

Abstract

Soil organic matter (SOM) is tightly linked to soil quality, but cultivation generally causes a decline in SOM, reducing soil quality and releasing carbon dioxide into the atmosphere. Internationally, countries are expected to reduce their greenhouse gas (GHG) emissions, and compile and update GHG inventories. Many developing countries, such as those in southern Africa, do not have reliable information regarding SOM levels in cultivated soils, changes over time and best management practices to maintain or even restore SOM. A review was conducted to collate published research on SOM [or its indicator elements, carbon (C) and nitrogen (N)] in cultivated fields in southern Africa. A total of 84 publications were assessed, and information such as date, location, SOM/soil organic carbon (SOC) and clay content and key findings were recorded. Most of the publications (90 %) were published post-1990. The spatial distribution of these studies is very non-uniform. A decline of 25–53 % in SOC due to cultivation was evident for cultivated fields in different precipitation zones in southern Africa, with SOC ranging from 0.7 to 2.5 %. These values correlate relatively well with existing SOC maps. A strong correlation between SOC and clay content in natural soil is weakened following cultivation. Carbon sequestration and GHG emission dynamics under various management systems and for different climatic regions, as well as how field crop production can play a role in mitigating climate change, are poorly understood. Intensified monitoring by multi-institutional collaboration is recommended to address this issue.