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

Abstract

Effects of long-term (18-yr) applications of cattle manure (20 Mg ha−1 yr−1) and NPK fertilizer on the labile organic matter (OM) and its protection in water-stable aggregates were investigated in a Le Bras silt loam (Humic Gleysol). Soil from the 0- to 10-cm depth was sampled from the untreated control, NPK, manure and NPK + manure treatments and fractionated into four size classes of slaking-resistant aggregates (>1000 µm, 250–1000 µm, 53–250 µm, <53 µm). Intact and crushed macroaggregates (250–1000 and >1000 µm) and intact microaggregates (<250 µm) were incubated for 21 d at 25 °C, and mineralized C and N were determined. The amount of mineralized C in intact aggregates increased with increasing aggregate size irrespective of the agronomic treatments, but there was no consistent trend for total N. Manure application led to an increase in mineralized C in most aggregate fractions. Crushing the macroaggregates enhanced mineralization of C by 14 to 35% and N by 17 to 103%. Additional C and N rendered mineralizable by crushing represents a fraction of the macroaggregate-protected OM. Manure application increased the protected pools of C (up to threefold) and N (up to fourfold) located in the small macroaggregates (250–1000 µm). In contrast, NPK fertilization increased the pool of macroaggregate-protected N by 2.5-fold but had no effect on the protected C. We conclude that manure application contributed to the accumulation of macroaggregate-protected C and N, whereas mineral fertilizers increased the protected-N pool only. Macroaggregates can provide a mechanism for the protection of labile soil OM in an annually tilled cropping system and this mechanism is enhanced with long-term manure application. Key words: Aggregate-protected organic matter, manure application, mineralization, mineral fertilizer, water-stable aggregates