| Soil management affects soil organic matter levels and aggregation, which can affect crop production. This study was conducted to determine the long-term (10 yr) effect of no-tillage and manure application on soil organic C and N, aggregates, soil N supplying capacity, corn yield, and N uptake. The field experiment was established in 1990 on a moderately well-drained Kennebec silt loams (Fine-silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). Treatments included tillage (no-tillage and chisel/disk) and N source (manure, NH4NO3 , and no N). Corn yields were not significantly different between tillage treatments or among N sources when applied at similar N rates. No-tillage (NT) and manure (M) significantly increased soil organic C and N. Six yr after N application were discontinued, total N uptake from N mineralization represented about 81 and 66 kg N ha-1 for M and fertilizer (F), respectively. In situ N mineralized from planting to tasselling at 0- to 30-cm depth accounted for 38 and 22% of N uptake from M and F, respectively. Predicted Nmin accounted for 77% of plant uptake of mineralized N. The difference in N mineralization resulted in corn yields of 61% for M and 47% for F relative to a well-fertilized reference. Predicted and in situ N mineralization was highly correlated (r = 0.82, p = 0.0001). The mechanism for the increased soil C and N was evaluated by determining distribution and C and N dynamics of aggregates. The amount of macroaggregates (>250 mum) was greater with NT and manure. Aggregate-associated C and N were significantly greater for macroaggregates than microaggregates and greater for M than F indicating physical protection of C within macroaggregates. Laboratory incubation of the soil showed that NT improved macroaggregate stability; thus the binding agents were more resistant to decomposition. Besides tillage, wet-dry cycling is known to affect the dynamics of aggregates and soil organic matter. Repeated wet-dry (WD) cycles significantly reduced cumulative C and N mineralization compared with constant soil water. The flush of mineralized C and N was mostly microbial in origin since aggregate distribution was unaffected by WD cycles. |
