Decline in soil organic carbon and nitrogen limits yield in wheat-fallow systems

This study evaluated long-term trends of soil organic carbon (SOC), soil total nitrogen (N), and wheat (Triticum aestivum L.) yield to glean information for improving the sustainability of dryland winter wheat-summer fallow (WW-SF) systems in the inland Pacific Northwest (IPNW). Effects of fall burn (FB), no burn (NB), manure application (MN), and pea vine incorporation (PV) treatments on SOC, N, and wheat grain yield from 1931 to 2010 were evaluated in an on-going Crop Residue Long-term experiment (CR-LTE). A medium tall wheat variety (Rex-M) was planted during 1931-1966 and several semi-dwarf varieties were planted since then. We observed a slight but gradual decrease in wheat yield in all treatments from 1931 to 1966 and in FB and NB from 1967 to 2010. Wheat yield in PV and MN increased until 1995 and decreased thereafter. The SOC and N content in 0-30 cm depth declined in FB, NB, and PV treatments with the highest rates of SOC and N loss under FB. The SOC content in 0-30 cm depth in MN was maintained. Growers should match organic matter and nitrogen (N) inputs to C and N loss to maintain SOC, N and crop yields, and improve the sustainability of dryland WW-SF systems.