Soil Organic Carbon Fractions In A Long-term Experiment And The Potential For Their Use As A Diagnostic Assay In Highland Farming Systems Of Central Kenya Highlands.

Abstract

Soil organic matter fractions provide insight into soil quality and the maintenance of crop productivity in smallhold cropping systems. Soil organic carbon (SOC) pools were measured and compared to crop performance during the eighteenth year of a long-term field experiment established on a Humic Nitisol at the Kenya Agricultural Research Institute. Maize stover retention (+S), cattle manure (+M, 10 t ha-1 yr-1) and fertilizer inputs (+F, 120 kg N and 23 kg P ha-1 yr-1) were compared in a 2 x 3 factorial arrangement in a manner that is broadly reflective of farmer resource management options. Maize (Zea mays) and beans (Phaseolus vulgaris) were cultivated in rotation during the long (March-June) and short (October-December) rains of each year, respectively. Maize yields ranged between 1.21 (+S) and 5.21 t ha-1(+SMF). Beans produced between 0.14 (-S) and 0.77 t ha-1(+SMF). Total crop yields were significantly affected by manure and fertilizer addition (P<0.001) but not stover retention (P = 0.13). Soil microbial biomass carbon (MBC, r = 0.63), particulate organic carbon (POC, r = 0.82) and its density fractions varied significantly with combined maize and bean yields in plots not receiving fertilizers. Only MBC was significantly correlated with crop yields in the inorganically fertilised treatments (r = 0.50). Livestock manure contributed to POC to a greater extent than did retention of maize stover or the addition of fertilizers. Of the different Ludox density fractions of POC, the lightest (<1.13 g cm-3) was best correlated with crop yields (r = 0.73) and the heaviest fraction (>1.37 g cm-3) least correlated with yield (r = 0.37). When 190 Central Kenyan smallhold 'households' were surveyed, 79% were classified as maize-bean farmers and 79% of these relied on combinations of stover, manure and fertilizers to maintain soil fertility. Thus, the long-term experiment at Kabete is broadly applicable to the surrounding farming community. Soil organic matter fractions were more favourably influenced by addition of livestock manure than the retention of maize stover, suggesting that the widespread farmer practice of harvesting maize stover as animal feed and returning animal wastes may effectively contribute to soil fertility management.