Particulate and mineral-associated organic matter in water-stable aggregates as affected by mineral fertilizer and manure applications

Application of cattle manure generally improves soil structure and organic matter (OM) content. However, changes in forms and location of OM within the aggregate structure are less well known. The effects of long-term (18-yr) applications of cattle manure (20 Mg ha(-1) yr(-1)) and NPK fertilizer on the distribution of particulate and mineral-associated organic matter in water-stable aggregates were investigated in a Le Bras loam (Humic Gleysol). Soil samples from the 0- to 10-cm depth were taken from the untreated control, NPK, manure and NPK + manure treatments. They were separated into four aggregate-size fractions (>1000 mu m, 250-1000 mu m, 53-250 mu m, and <53 mu m) by slaking air-dried soil, followed by wet sieving. Particulate (>53 mu m) and mineral-associated OM (<53 mu m) were separated from water-stable aggregates >53 mu m by sieving after mechanical dispersion. After 18 yr, manure increased the OM level of the whole sail and favored formation of slaking-resistant macroaggregates (250-1000 mu m). This effect was primarily a result of the OM added by the manure. In contrast, NPK fertilizer did not affect soil OM level or macroaggregation. The increase in OM induced by manure application was observed primarily in macroaggregates, and both as mineral-associated and particulate OM. However, manure did not change OM located in the fraction <53 mu m confirming that recently deposited OM preferentially accumulates within the aggregate structure and not in the finely or non-aggregated material. Since previous studies have shown that most of the C in cattle manure is composed of coarse particles, we hypothesize that manure-derived OM first enters the soil primarily as particulate material, then, during decomposition, is transformed within the aggregate structure into mineral-associated material thereby contributing to aggregate stabilization.