Effects of soil fertility management practices on soil aggregation, carbon and nitrogen dynamics in a long-term experimentat Kabete
Poor resource farmers cultivate steep slopes without soil conservation measures and apply insufficient plant nutrients thus degrading the soils. Integrated soil fertility management using organic and inorganic sources of nutrients is one of the approaches being advocated to farmers as a way of improving soil health and increasing production. This study was conducted to determine the effects of long-term use of inorganic fertilizer (NPK); inorganic fertilizer (NPK) with manure (F + M); inorganic fertilizer (NPK) with residue (F + R); residue and manure application on soil aggregates, organic carbon, and nitrogen and macrofauna in a humic nitisol soil under annual maize-bean crop rotation. Macrofauna (soil invertebrates) and soil samples were collected from the 31 year-old long-term experiment and assessed for stable aggregate size distribution, total soil organic C and N contents. The diversity, abundance and biomass of soil macrofauna (termites and earthworms) were also determined. The results of the study showed a significant increase in large (p=0.01) and small macroaggregates (p=0.002) in the 0-15 cm and 15-30 cm depths under inorganic fertilizer (NPK) combined with manure (F + M) treatment. Also, significant increase (p=0.003) in mean weight diameter (MWD) of soil aggregates, soil organic carbon in small macroaggregates (p=0.005) and microaggregates (p=0.044) in soil that received inorganic fertilizer (NPK) with manure (F + M) compared to control. In terms of biodiversity, use of inorganic fertilizer (NPK) and inorganic fertilizer (NPK) combined with manure led to increase in earthworm biomass which was positively correlated with large macroaggregates (r=0.397, p=0.017), silt and clay (r=0.385, p=0.020), C in small macroaggregates (r=0.474, p=0.003) and microaggregates (r=0.493, p=0.002). Long-term use of combined inorganic fertilizer (NPK) with manure improved the stability of the macroaggregates and increased mean weight diameter (MWD) in both 0-15 cm and 15-30 cm compared to all other treatments. Thus integration of fertilizers and animal manures would result in build up of soil organic matter in the long-term, thus contributing to C sequestration.