Integrated Soil Fertility Management: Effects On Soil C and N Stabilization.

Soil organic matter is important to improve and sustain soil fertility in tropical agroecosystems.  The combined use of organic residue and fertilizer inputs is advocated for its positive effects on short-term nutrient supply, but the effect of the integrated use on long-term stabilization of soil organic C and N is unknown.  We conducted a 545-d soil incubation with maize (Zea mays) residue and urea fertilizer to examine the stabilization of C and N in four soils differing in texture (sand, sandy loam, clay loam, and clay) from sub-Saharan Africa.  The inputs were enriched in ¹³C and ¹⁵N in a mirror-labelling design to trace the fate of residue-C and N, and fertilizer-N in combination.  We hypothesized that combining inputs would enhance the stabilization of C and N relative to either input alone across a range of soil textures.  The treatments were destructively sampled to assess input-derived C and N stabilization in soil aggregate fractions.  The combination treatment had a significant but small (2% of residue-applied C) increase in residue-C stabilized in the total soil after 90 d, but this increase did not persist after 181 or 545 d.  While combining residue and fertilizer decreased the amount of residue-N stabilized within soil aggregates (7% less at 545 d), it increased the stabilization of fertilizer-N (20% more at 545 d).  The increased amount of fertilizer-N stabilized was significantly greater than the amount of residue-N lost in the combined input treatments in the three finer textured soils, indicating an interactive increase in the stabilization of new N.  Our results indicate that combining residue with fertilizer inputs can increase the short-term stabilization of N, which has the potential to improve soil fertility.  However, benefits to N stabilization from combining organic residues and fertilizer seem to be less in coarser textured soils.