Legume-based rotation enhance long-term soil carbon storage in eastern Oregon dryland wheat systems

The potential for increasing soil carbon (C) storage in dryland croplands is still under debate, and there is a lack of comprehensive data regarding storage capacity under conservation management practices. In this study, we analyze the potential for sequestration and stabilization by soil minerals in agricultural land in Eastern Oregon when legume-based rotations are incorporated into cereal cropping systems. This study encompasses long-term experiments that compare the traditional conventional tillage (CT) with winter wheat-summer fallow (WW-SF) cropping system to winter wheat (Triticum aestivum L.) (WW) - spring pea (Pisum sativum L.) (SP) rotation (WW-SP) under both CT and no-till (NT). Soil C sequestration rates were estimated by analyzing mid-infrared spectral data in archived soil samples from the WW-SP and WW-SF systems. Partial least squares (PLS) models were trained to predict different properties in archived soil samples, such as total C and nitrogen (N) in bulk soil and C in mineral-associated organic matter (MAOM). Our findings show that over nearly 60 years in the WW-SP cropping system, soil C in the top 30 cm depth profile increased annually by at least 0.16 t C per hectare under CT and 0.25 t C per hectare under NT following conversion from WW-SF systems. The WW-SF cropping system was highly susceptible to C losses in the MAOM fraction, while the NT cereal legume-based rotation showed significantly higher C levels in this fraction. Furthermore, the NT WW-SP resulted in a 60% increase in C sequestration in this fraction following the incorporation of cereal-legume systems. Although the studied sub-basin represents less than 1% of the state's croplands, we estimate that incorporating legumes into the WW-SF system could sequester enough C to offset approximately 0.20% of Oregon's annual agricultural CO₂ emissions. Our findings indicate that incorporating legumes into arable WW-SF systems can effectively increase C stock and stabilize eastern Oregon's dryland farms, promising to offset GHG emissions.