Soil organic carbon fractions under smallholder farmer conservation agriculture in Eastern and Southern Africa

Mr Stuart Irvine-Brown1, Dr Joseph Eyre2, Dr Daniel Rodriguez2

1Queensland Dept. Agriculture and Fisheries, Nambour, Australia, 2Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, St Lucia, Australia

Soil organic carbon (SOC) is a measurable component of soil organic matter and is used as an important indicator for soil health and fertility. This is due to its function through microbial mineralisation processes to assist plant nutrition, improve soil structure, enhance water infiltration and retention, and build resilience against soil erosion. The use of conservation agriculture (CA) has been promoted via zero tillage, residue retention, and crop rotation as a means to raise and retain SOC levels above that of conventional agricultural practices (CP) in the Americas, Europe and Australia.  However, it is disputed that CA can provide similar benefits to smallholder farmers in Eastern and Southern Africa.  This is due to the limited availability of crop residues for retention in such low productivity systems, and the capacity to accurately measure changes in SOC and its constituent physical fractions.  We investigated SOC content via fractionation on soil from three depths in the uppermost 0.3 m under establishment phase CA (zero tillage and residue retention for 4 years) and compared this with CP in continuous maize subsistence agriculture at paired sites across an agro-ecological gradient in Eastern and Southern Africa.  We used Dumas method after a series of chemical and physical size fractionation processes to assess SOC content of the various fractions in conjunction with the use of Australian MIR spectral calibrations to fractionate SOC.  Strong agreement between measured and predicted values gives confidence in the ability of MIR spectral calibrations from Australia to be used to assess SOC fractions of soils from Eastern and Southern Africa.  We found significant increases in particulate (POC) and humic (HOC) organic carbon concentrations in shallow soil (0-0.05 m) from CA treatments with high residues when compared with CP although variable presence of resistant charcoal carbon (ROC) confounded predictions.


I am a soil scientist and horticulturist working across the state of Queensland on horticultural agronomy research and industry development for extension on issues of sustainability, resource base conservation and improvement to catchment scale water quality. My interests lie in sustainable international agriculture and natural resource management with a key focus on innovation and diversity in sub-tropical and tropical horticultural production systems linked to soil science.

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