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 (Lam.) 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 kg N 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 μrn). Small macroaggregates (2000-250 μrn) were further separated into coarse paniculate organic matter (>250 μm), microaggregates within macroaggregates (53-250 μrn), and macroaggregate-occluded silt and clay (<53 μrn). 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-term SOM dynamics. © Soil Science Society of America.