Soil microclimates influence annual carbon loss via heterotrophic soil respiration in maize and switchgrass bioenergy cropping systems

Abstract

Heterotrophic soil respiration (RH) is the primary pathway of carbon (C) loss from litter and soil organic matter, and thus RH partially determines ecosystem C storage. Because RH is sensitive to soil temperature and moisture, aboveground factors that influence soil microclimate, such as plant structure and residue management, may in turn affect belowground C loss via RH, but this relationship has not been quantified. We examined multiyear soil microclimate differences to 1-m depth, measured seasonal trends of RH, and parameterized crop-specific microclimate-RH models to quantify the effect of soil microclimate differences on annual RH in temperate no-till maize and switchgrass bioenergy cropping systems. Summertime soil temperatures were typically warmer in maize compared to switchgrass, likely resulting from lower leaf area index (LAI) in maize. In contrast, winter soil temperatures were usually warmer in switchgrass than maize, due in part to more consistent snow retention within the switchgrass litter stubble. Daily soil temperature ranges were less extreme in the perennial switchgrass system compared to the annual no-till maize system. Soil moisture near the soil surface was usually lower in maize than switchgrass, but the opposite was true below about 50 cm. RH showed strong seasonal trends, with warmer and drier soil conditions generally leading to higher RH in both crops. Modeled scenarios indicated that the differences in crop-specific soil microclimates accounted for 4 to 17% of the annual RH flux, with the dominant soil microclimate effects on RH occurring during the summer. Thus, the soil microclimate serves as a strong coupling between aboveground properties and belowground C loss via RH in temperate agroecosystems. Agricultural management practices such as planting date, plant density, and residue management could be targeted to promote soil microclimates that reduce RH, thereby reducing gaseous belowground C losses and potentially enhancing ecosystem C storage.