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

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; NH 4 NO 3 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.