Short- and long-term carbon and nitrogen dynamics in Integrated Soil Fertility Management systems

Soil fertility decline is the most important constraint to crop production in Sub-Saharan Africa. The Integrated Soil Fertility Management paradigm recognizes the potential interactive benefits of the combined use of organic residue and mineral fertilizer inputs. However, these interactions may be controlled by the relationships between residue quality and soil C and N stabilization mechanisms. The overall objectives of this dissertation were to examine the short- to long-term C and N dynamics across a gradient of (1) inputs, (2) residue qualities, and (3) soil textures.;The combined results of these studies indicate that N fertilizer and increasing residue quality stimulate short-term C and N mineralization. Combining low quality residue and fertilizer inputs resulted in a significant negative interactive effect on potentially available N in all soils. However, this interactive effect changed from negative to positive with increasing residue quality. Under field conditions, this reduction in available N by combining low quality residue and N fertilizer reduced environmental N losses and created a positive interactive effect for crop N uptake. While input management manipulates short-term N dynamics, these practices do not influence the long-term soil stabilization of input-derived C and N. Rather, the long-term stabilization of soil organic matter is more dependent on environmental factors, such as soil texture, than fertilization or residue quality. In conclusion, capitalizing on interactions between fertilizer and low quality organic residues allows for the development of sustainable nutrient management practices.;These objectives were examined in three experiments comprising different levels of environmental control and different timescales of measurement. First, a laboratory incubation using inputs enriched in 13C and 15N was conducted to look at short-term C and N dynamics with combinations of residue and N fertilizer in four soil textures. Second, field trials at two Kenyan sites designed to examine the interaction of input combinations with three residue qualities was sampled for long-term changes in soil C and N stabilization. Lastly, short-term N dynamics of combining fertilizer with different quality residues were monitored in one of the field trials during a maize growing season.