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SOIL BIOLOGY & BIOCHEMISTRY

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

^a Dep. of Plant Sciences, Univ. of California, One Shields Avenue, Davis, CA 95616
^b Soil Research Institute, Academy Post Office, Kwadaso, Kumasi, Ghana
^c Tropical Soil Biology and Fertility Inst. of CIAT, P.O. Box 30677, Nairobi, Kenya

^* Corresponding author (sjfonte{at}ucdavis.edu).

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 µm). Small macroaggregates (2000–250 µm) were further separated into coarse particulate organic matter (>250 µm), microaggregates within macroaggregates (53–250 µm), and macroaggregate-occluded silt and clay (<53 µm). 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.

Abbreviations: SOM, soil organic matter