Greening summer fallow : agronomic and edaphic implications of legumes in dryland wheat agroecosystems

Abstract

Adopting nitrogen (N)-fixing legumes into crop rotations is an accessible, ecological practice capable of increasing agricultural sustainability. Nonetheless, in northern Great Plains (NGP) wheat systems, proper water use management and the realization of N benefits are barriers to legumes replacing summer fallow. Legumes should also be able to mitigate legacies of soil organic matter losses from summer fallow. We conducted a participatory field-scale study in north-central Montana, assessing the viability of no-till, early-terminated legume green manures (LGMs) as summer fallow replacements. Soil water and nitrogen were measured to evaluate LGM effects on subsequent wheat crops. Farmers were interviewed to elucidate perspectives and challenges of adopting LGMs. Compared to fallow, LGMs depressed subsequent wheat yields by 6% (0.24 Mg ha -¹), and lowered grain protein at sites where wheat was fertilized with N (9 g kg -¹); grain protein was increased at unfertilized sites (5 g kg -¹). Absent rotational benefits from LGMs were attributed to dry conditions in the LGM year leading to low LGM biomass N and reduced N mineralization potential in soils, rather than soil water limitation to subsequent wheat. Farmers were curious about possible longterm benefits from LGMs, but expressed that the economic viability of LGMs appeared tenuous in the short-term. We also examined attributes and processes in soils from an eight-year-old rotation study containing fallow-wheat, continuous wheat, and legume-inclusive no-till rotations. We examined potentially mineralizable C and N (PMC and PMN), microbial biomass-C and wet aggregate stability (WAS). Nitrogen fertilizer was also added to a duplicate set of soils, and effects on C and N mineralization were evaluated. Legume-inclusive systems generally had higher levels of soil parameters, and had 26-50% greater PMN than wheat-only systems. Systems returning the most crop residue C to the soils had higher WAS regardless of legumes. Nitrogen additions depressed C and N mineralization. Results of these studies suggest that in NGP agroecosystems, LGMs can avoid limiting soil water available to subsequent wheat when terminated early and managed as no-till crops, but that legumes should be viewed as an investment in soil quality which may precipitate rotational N benefits more reliably after three or more appearances in rotation.