Soil-Profile Organic Carbon Stock Changes with Increased Cropping Intensity and Reduced Tillage.

Decades of wheat-fallow rotation with intensive tillage have resulted in reduced soil organic carbon (SOC) storage in the Pacific Northwest dryland region. Research is needed to assess the impact of reduced tillage and intensified alternative cropping systems on soil-profile C accretion. Our objective was to determine the effects of tillage and cropping intensity on SOC stocks for Typic Haploxeroll profiles (0-100 cm) under three cropping systems.  A randomized complete block design long-term alternative tillage and crop intensity study was started at Pendleton in 1998 with the following treatments: 1) continuous winter wheat (Triticum aestivum L.) under direct seeding (NT), CWW/NT; 2) winter wheat–winter wheat–sudan (Sorghum sudanese L.) grass rotation under direct seeding, W–W–S/NT; and 3) winter wheat–fallow under sweep tillage, W–F/ST. Using a grid scheme, six geo-referenced cores per plot per block were collected in 2004 and 2008. Soil samples were sectioned at 0-5, 5-10, 10-20, 20-30, 30-60, 60-100, and 100-150 cm.  Total soil C and N, organic C, inorganic C and N, and ¹³C were determined at each soil depth. After 10 years of NT and increased crop intensity, significant increases in SOC were found down to the 100-cm depth.