Long-Term Trend of Soil Organic Carbon Influenced By Crop Residue and Nitrogen Management Practices.

Maintenance of soil organic matter is important for sustainable crop production in dryland agroecosystems. Long-term trend of soil organic carbon (SOC), nitrogen (N), and crop yield of winter wheat-summer fallow system was monitored at Columbia Basin Agricultural Research Center, Pendleton OR. The experiment was established in 1931 and had nine treatments. Fall burn treatment (FB0) included burning of crop residue in late-September and spring burn treatments (SB0, SB45, and SB90) included burning of crop residue in late April – early May of the fallow year. No burn treatments (NB0, NB45, and NB90) included crop residue incorporation by moldboard plow tillage (0-20 cm) in late spring of the fallow year. Manure and pea vines were applied at 22.4 and 2.24 Mg ha-1 biennually, respectively. Results of this study revealed that SOC was maintained only on manure applied treatments. All other treatment lost SOC compared with SOC in 1931. Rate of SOC loss was the highest in fall burn treatment with 263 kg C loss ha-1 yr-1 and the lowest in pea vine incorporated treatment with 97 kg C loss ha-1 yr-1. The rate of SOC loss at 0-30 cm depth were in the order of FB0>SB0=SB45>SB90=NB0> NB45>NB90>Pea vine application. Soil N declined in all treatments and followed a trend similar to SOC loss with the least amount of N loss from manure applied plots. Crop yield was not influenced by crop residue and nutrient management treatments in 1931-1967, increased in 1968-1979, and decreased in 1980-2010 period across all treatments.