Soil Organic Carbon and Nitrogen in Long-Term Manure Management System

Long-term studies are extremely beneficial to our understanding and evaluation of changes in soil quality and sustainability of specific management practices. The objectives of this study were to evaluate the effects of 70 yr of intensive traditional tillage. Effect of moldboard plowing with manure (M) and inorganic fertilizer (F) additions on soil organic carbon (SOC), soil total nitrogen (STN), particulate organic matter (POM) and its associated carbon (POM-C), and mineral-associated organic matter carbon (MAOM-C) were quantified. The Knorr-Holden 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.) production. Soil samples were collected in 2011 from the 0-to 5-, 5-to 10-, 10-to 15-, 15-to 30-, 30-to 60-, and 60-to 90-cm depths. On fixed-depth basis, SOC and STN in the 0-to 30-cm layer increased by 2.3-fold compared with the 30-to 60-cm layer and by 3-fold compared with the 60-to 90 cm layer. Similar differences in SOC and STN were observed when calculated on an equivalent soil mass (ESM) basis. The application of inorganic fertilizer plus manure (F+ M) increased POM content in the 0-to 30-cm depth by 27 and 34% with 90+ M and 180+ M, respectively compared with F alone at 90 and 180 kg N ha-1. In the 0-to 15-cm soil depth, the relative POM-C to SOC content was 30% greater than in the 15-to 30-cm soil depth. In the 15-to 30-cm depth, the MAOM-C was 7% higher than the surface 0 to 15 cm. The MAOM-C represented 69% of SOC content associated with different treatments. Positive and significant correlations were found among SOC, POM-C, and MAOM-C. Our results indicate that measuring SOC distribution among different fractions, POM-C and MAOM-C, is important to improve our knowledge of SOC distribution as influenced by type of fertilization used at different depths. Overall, long-term manure amendment should be combined with F to enhance SOC and STN content in a system using intensive tillage and furrow irrigation.