Managing N availability and losses by combining fertilizer-N with different quality residues in Kenya

The integrated soil fertility management paradigm, currently advocated in Sub-Saharan Africa for rehabilitating its soils, recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. Residue quality may be a controlling factor for any beneficial interactions. The objectives of this study were to determine the effect of different quality organic residues and mineral fertilizer on N cycling under field conditions in Embu, Kenya. We hypothesized that combining low quality residue with mineral N would reduce potential system losses of N by synchronizing N release with plant uptake. Residue treatments consisted of a control (no residue input), high quality tithonia (Tithonia diversifolia) residue (C to N ratio of 13:1) and low quality maize (Zea mays) stover residue (C to N ratio of 42:1) applied at a rate of 1.2 Mg C ha−1. Subplots of each residue treatment received either 0 or 120 kg N ha−1 in a split-application, and maize was cultivated each season. During the 11th growing season of the trial (March–September 2007), we monitored soil mineral N, potential gross mineralization and nitrification rates, and plant N content. Extractable mineral N in the soil profile varied with residue and fertilizer inputs throughout the growing season. The tithonia treatments showed early season N release of 22 kg N ha−1 in the upper 30 cm of the soil profile. The maize + fertilizer treatment displayed an immobilization of 34 kg N ha−1 after the application of N fertilizer. However, the lower mineral N of the maize + fertilizer treatment did not reduce crop N uptake, as mineral N in the other fertilizer treatments was leached from the upper soil (0–60 cm) at 57 d after planting. The interactive effect on crop yield and N uptake of combining residue with fertilizer-N changed from negative to positive as residue quality decreased. The benefit of combining low quality residue with N fertilizer in reducing N losses indicates that this soil fertility management strategy should be adopted in environments subject to high N leaching losses.