Growing spring peas to increase soil organic carbon in eastern Oregon’s dryland wheat systems

Wheat farmers in Eastern Oregon, part of the Inland Pacific Northwest (iPNW), face the challenge of maintaining healthy soil while sustaining crop production, especially in the dryland region of the iPNW. Due to limited moisture, a winter wheat–summer fallow (WW-SF) cropping system has been adopted widely in the region where land is left fallow for 13–14 months between two wheat crops. Although the WW-SF system provides reliable yields, it is associated with a decline in total soil carbon (C), increased soil erosion, increased herbicide-resistant weeds, and a decline in overall soil health. 

Long-term studies conducted at Oregon State University’s Columbia Basin Agricultural Research Center in Pendleton, OR (CBARC) show that dryland farming practices, particularly long fallow periods and intensive tillage, have resulted in significant losses of inherent soil organic C and essential soil nutrients in winter wheat systems. Over the past century, up to 60% of the inherent C and N have been lost (Ghimire et al., 2018). For decades, farmers have often left fields fallow to conserve soil moisture (Schillinger & Papendick, 2008). While that can help in the short term, it also leads to nutrient loss over time, weakening soil health and reducing crop productivity, creating both challenges and a valuable opportunity for restoring C in eastern Oregon. 

These challenges are encouraging farmers to diversify traditional WW-SF systems. One promising approach is to incorporate spring peas into a crop rotation strategy that can enhance soil fertility, increase soil C, provide better weed control, break disease and pest cycles, and increase long-term resilience. Spring peas are particularly well-suited for the cold, dry climate of eastern Oregon. They were historically a significant part of local cropping systems, commonly grown in rotation with wheat from the early to mid-1900s (Rasmussen & Smiley, 2019). Although to some extent studies have been done on how including spring peas into WW-SF systems affect C, N, crop yields, and weed control, research-based information on the role of spring peas in sequestrating long-term C in the dryland region is limited.

Therefore, this study explores (i) how legumes like spring peas fit into wheat rotations and (ii) how different tillage methods impact C compared with the WW-SF system. A key focus of this study is the different types of soil organic C fractions: (i) particulate organic matter (POM) that breaks down quickly and (ii) mineral-associated organic matter (MAOM) that binds to minerals and locks in carbon for the long term. This study focused on total C and MAOM-C under different treatments.

Methodology

Soils at the study site are classified as Walla Walla silt loam and receive 421 mm of annual precipitation. This study uses data from several long-term experiments (LTEs) conducted at CBARC, some of which have been ongoing since 1931. One of the LTEs, the winter wheat–spring pea rotation (WW-SP), established in 1963, was designed to evaluate the impact of rotating wheat with peas under varying tillage intensities on soil health and wheat productivity. The study focuses on two tillage practices within a WW-SP rotation: conventional tillage (CT), involving fall moldboard plowing since 1963, and no-till (NT), which began as minimum tillage and transitioned to full no-till by 1995.

In the WW-SP rotation, wheat was planted in October with 80 lb N/ac applied, and peas were sown between March and April. The CT system involved plowing in the fall to turn over the soil, incorporating the crop residue, and cultivating in the spring. To provide a comprehensive understanding, we also compared these rotations to a long-term CT WW-SF system established in 1931 at CBARC. In this system, wheat received 80 lb N/ac with fall plowing, spring cultivation, and residue incorporation practiced during fallow years.

To understand how soil C has changed over time, we analyzed archived soil samples collected as far back as 1963 and measured C levels in the top 12 inches (30 cm) of the soil. Total C and MAOM-C were measured using the dry combustion method. 

Main findings

Nearly 60 years of research indicate that rotating wheat with spring peas consistently increases soil C in the top 12 inches. When farmers shift from the traditional WW-SF system to a wheat–legume (WW-SP) rotation, soil C increases annually by at least 0.07 tons/ac under CT and 0.11 tons/ac under NT. This is a substantial improvement compared with the WW-SF system, which continued to lose C at a rate of 0.45 tons/ac annually.

One important finding is that including spring peas in the rotation resulted in an effective way to build durable, long-lasting soil C, specifically MAOM-C (Figure 1). Even in tilled systems, MAOM levels increased when spring peas were included in the rotation and continued to decline under continuous wheat–fallow systems. The shift from WW-SF to WW-SP increased bulk soil C by ~38% and MAOM-C by ~60%. This is significant given the major role MAOM plays in building more resilient soils. Although legumes in rotation contribute modest C gains, they play a crucial role in reversing C losses and enhancing the formation of the persistent organic matter fraction such MAOM-C. 

Spring pea rotations also helped soils retain more N near the topsoil, resulting in a reduced need for synthetic fertilizers and increased availability of N for the next crop. Contrary to common belief in these areas, legumes provided these benefits without depleting soil moisture or reducing the performance of subsequent wheat crops. Overall, these benefits make spring peas an effective, climate-smart tool for improving soil health and sequestering C for the long term in dryland wheat-based systems. 

Spring peas also offer flexibility for growers as they can be grown for grain, forage, or as green manure—all while benefiting soil health in the long-term. Wheat yields for WW-SF rotations respond poorly to moisture with no significant gains in dry years and lower performance in wet years compared with legume-integrated systems. As C markets and incentive programs expand, the measured gains in C and organic matter from WW-SP systems make this rotation an attractive option for potential credit opportunities. 

Eastern Oregon has lost a large amount of its soil C since the beginning of soil cultivation, but this loss creates a significant opportunity for restoration. Replacing summer fallow with spring peas is recommended as a practical step toward regenerative agriculture, improving soil health, crop performance, and climate resilience.