Long-Term Crop Residue and Nitrogen Management Influences on Soil Profile Carbon and Nitrogen.

Crop residue burning or removal for alternative uses significantly impact soil organic carbon (SOC) in wheat-fallow systems in the Pacific-Northwest of USA. To evaluate the effects of crop residue management on soil profile C, soil samples were collected during summer 2010 from 0-10, 10-20, 20-30, and 30-60 cm soil depths of long-term wheat-fallow rotations at Columbia Basin Agricultural Research Center, Pendleton OR. The experiment was established in 1931 in wheat-fallow cropping system 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 loss from surface soil (0-10 cm) as well as 0-60 cm depth profile was the highest in fall burn treatment with 68% and 50% less SOC at 0-10 and 0-60 cm depths, respectively, compared with nearby undisturbed grassland. Crop residue incorporation and manure addition lost the least amount of SOC (8% and 20% less SOC at 0-10 and 0-60 cm depths, respectively) compared with the undisturbed grassland. Soil organic C loss from different treatments at 0-60 cm depth profile were in the order of FB0>SB0=SB45>SB90=NB0=NB45>NB90>Pea vine> Manure application. Organic matter addition along with crop residue incorporation is required to maintain SOC in dryland wheat-fallow system in the Pacific-Northwest.