Tillage and Manure Effects on Soil and Aggregate-Associated Carbon and Nitrogen

Abstract

In agricultural systems, maintenance of soil organic matter (SOM) has long been recognized as a strategy to reduce soil degradation. No-tillage and manure amendments are management practices that can increase SOM content and improve soil aggregation. We investigated the effects of 10-yr of different tillage systems and N sources on soil aggregate-size distribution and aggregate-associated C and N. The study was a split-plot design replicated four times. The main plot treatment was tillage (no-tillage, NT; conventional tillage, CT) and the subplot treatment was N source (manure, M; NH4NO3 fertilizer, F). The experiment was established in 1990 on a moderately well-drained Kennebec silt loam (Fine-silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). In 1999, soil samples were collected (0- to 5-cm depth) from the field treatments and separated into four aggregate-size classes (>2000, 250–2000, 53–250, and 20–53 μm) by wet sieving. Labile C and N content of all aggregate-size fractions were measured using 28-d laboratory incubations of intact and crushed aggregates. No-tillage and M treatments significantly increased total C and N and the formation of macroaggregates. Conventional tillage in comparison with NT significantly reduced macroaggregates with a significant redistribution of aggregates into microaggregates. Aggregate protected labile C and N were significantly greater for macroaggregates, (>2000 and 250–2000 μm) than microaggregates (53–250 and 20–53 μm) and greater for M than F indicating physical protection of labile C within macroaggregates. No-tillage and M a lone each significantly increased soil aggregation and aggregate-associated C and N; however, NT and M together further improved soil aggregation and aggregate-protected C and N.