Long-Term Tillage and Manure Influence on Soil Nitrogen and Organic Carbon Dynamics.

Long-term studies are valuable to improve our quantitative knowledge on soil managements and sustainability.  The objectives of this study were to assess the effects of 70 yr of moldboard plowing with the addition of manure (M) and commercial fertilizer (F) on soil organic C (SOC), soil total N (STN), particulate organic matter (POM) and its associated C (POM-C). The study was initiated in 1912 on a Tripp sandy loam (coarse-silty, mixed, superactive, mesic Aridic Haplustolls) with furrow irrigated continuous-corn (Zea mays L.).  In 2011, soil samples were collected from the 0-5, 5-10, 10-15, 15-30, 30-60, and 60-90 cm depths. On a fixed depth basis (FD), SOC and STN increased by 2.3 fold in the 0-30 cm compared with 30-60 cm and by 3 fold compared with 60-90 cm layer. Similar differences among depths were observed when SOC and STN were calculated on an equivalent mass (ESM) basis. Greater POM amount was observed, 52%, with 180+M and 36% with 90+M compared with F only at 180 and 90 kg N ha^-1, respectively.  The proportion POM-C to SOC, in the upper 0-15 cm, was 43% greater than 15-30 cm layer. Positive and significant correlations were found among SOC and POM-C.  In general, manure amendments improved SOC and soil POM in this long-term moldboard plowing and irrigated management system.