Smallholder farmer management impacts on particulate and labile carbon fractions of granitic sandy soils in Zimbabwe
Nutrient Cycling in Agroecosystems
Department of Soil Science and Agricultural Engineering, University of Zimbabwe, Box MP 167, Mount Pleasant, Harare, Zimbabwe; Soil Fertility Consortium for Southern Africa (SOFECSA), CIMMYT Southern Africa, Box MP 163, Mount Pleasant, Harare, 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 ∼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 μm diameter) and three times more meso-POM (53-250 μm) 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. © 2008 Springer Science+Business Media B.V.
agricultural management; crop production; crop yield; farming system; maize; resource allocation; sandy soil; smallholder; soil carbon; soil organic matter; Africa; Southern Africa; Sub-Saharan Africa; Zimbabwe; Crotalaria; Crotalaria juncea; Zea mays