Fertilizer and Residue Quality Effects on Organic Matter Stabilization in Soil Aggregates

This study examined the influence of organic residue quality and N fertilizer on aggregate-associated soil organic matter (SOM) in maize (Zea mays L.) cropping systems of southern Ghana. Six residue treatments of differing quality [Crotalaria juncea L., Leucaena leucocephala (Lain.) de Wit, maize stover, sawdust, cattle manure, and a control with no residues added] were applied at 4 Mg C ha(-1) yr(-1) both with and without fertilizer N additions (120 kgN ha(-1) season(-1)). Soils (0-15 cm) were sampled 3 yr after study implementation and wet sieved into four aggregate size classes (8000-2000, 2000-250, 250-53, and < 53 mu m). Small macroaggregates (2000-250 mu m) were further separated into coarse particulate organic matter (> 250 pro), microaggregates within macroaggregates (53-250 mu m), and macroaggregate-occluded silt and clay (< 53 mu m). Nitrogen fertilizer additions reduced aggregate stability, as was evident from a 40% increase in the weight of the silt and clay fraction (P = 0.014) as well as a decrease in microaggregates across all residue types (P = 0.019). Fertilizer similarly affected C and N storage within these aggregate fractions, while the effects of residue quality were largely insignificant. Our results suggest that fertilizer effects on soil aggregation may have important implications for long-tern, SOM dynamics.