Soil Aggregation and Organic Carbon Dynamics in Long-Term Tillage and Manure Amendment.

Changes in soil properties influenced by different management practices occur slowly. This study evaluated 70 yr of moldboard plowing (MP), manure (M) addition and different commercial fertilizer nitrogen (F) rates on soil organic C (SOC), water stable aggregate (WSA), and aggregate-associated-C. The Knorr- Holden plots (Scottsbluff, NE) have been managed in continuous-corn (Zea mays L.) and furrow irrigated since 1912. Soil type was Tripp sandy loam (coarse-silty, mixed, superactive, mesic Aridic Haplustolls). Soil samples were collected from the 0-5, 5- 10, 10-15, and 15-30 cm and fractionated into different aggregate-size classes (>1000, 500-1000, 250-500, and 53-250 μm) by wet sieving. Manure amendment increased the SOC, approximately 1.7 fold, than F treatment in the 0-30 cm depth. The combination of F+M further enhanced surface (0-15 cm) SOC for 90+M by approximately 36% and for 180+M by 16% compared with the subsurface 15-30 cm depth. Macroaggregates increased with M and the combination of F+M compared to F with the corresponding increase in the amounts of microaggregate with the F and no-N treatment. The mass of aggregate-associated-C was greater in microaggregates than in macroaggregates indicating greater amounts of soil microaggregates presence. A positive and significant correlation between SOC and aggregate-associated-C was observed. Overall, the long-term manure addition, with or without F, increased SOC and enhanced aggregate stability.