Smallholder farmer management impacts on particulate and labile carbon fractions of granitic sandy soils in Zimbabwe

Crop production in maize-based smallholder farming systems of Southern Africa is hampered by lack of options for efficiently managing limited and different quality organic nutrient resources. This study examined impacts of farmers' short- and long-term organic resource allocation patterns on sizes and quality of soil organic matter (SOM) fractions. Farmers' most- (rich) and least- (poor) productive fields were studied for two seasons under low (450-650 mm yr(-1)) to high (> 750 mm yr(-1)) rainfall areas in Zimbabwe, on Lixisols with similar to 6% clay and 88% sand. Rich fields received 0.5-14 Mg C ha(-1)compared with < 4 Mg C ha(-1) for poor fields, and the differences were reflected in soil particulate organic matter (POM) fractions. Organic inputs were consistent with resource endowments, with well-endowed farmers applying at least five times the amounts used by resource-constrained farmers. Rich fields had 100% more macro-POM (250-2,000 mu m diameter) and three times more meso-POM (53-250 mu m) than poor fields. Application of high quality (> 25 mg N kg(-1)) materials increased labile C (KMnO4 oxidizable) in top 60 cm of soil profile, with 1.6 Mg C ha(-1) of Crotalaria juncea yielding labile C amounts similar to 6 Mg C ha(-1) of manure. Labile C was significantly related to mineralizable N in POM fractions, and apparently to maize yields (P < 0.01). Farmers' preferential allocation of nutrient resources to already productive fields helps to maintain critical levels of labile SOM necessary to sustain high maize yields.