Growing season carbon dynamics differ in intermediate wheatgrass monoculture versus biculture with red clover

Abstract

Perennial crops can improve the ecological and economic sustainability of agroecosystems because of their potential to provide diverse ecosystem services including carbon storage. Intermediate wheatgrass (IWG; Thinopyrum intermedium) is a stress-tolerant grain and forage species that can be grown in bicultures with legumes for symbiotic nitrogen fixation that provide additional ecosystem services but also compete with IWG for resources and may diminish field-scale carbon uptake. An eddy covariance (EC) tower was installed in December 2018 in an IWG field in Wisconsin – in which half of the field was frost seeded with red clover (Trifolium pratense L.) – to investigate how perennial grain bicultures and monocultures differ in carbon accumulation compared to monocultures. Using a combination of spatially-partitioned carbon and energy fluxes, collected biomass samples, and harvest and manure estimates, we found that IWG monocultures were larger carbon sinks (–538 to –580 g C m−2 yr−1) compared to bicultures (–458 to –520 g C m−2 yr−1), due to greater photosynthetic activity during the growing season and following harvests in August each year. In contrast, evapotranspiration rates were greater in bicultures compared to monocultures (by 0.3–1.4 kg H2O m−2 day−1), specifically during summer. Grain and forage harvest resulted in carbon loss which was not recovered until 30 days post-harvest. Carbon loss was greater for bicultures (by 20 g C m−2 month−1). Net ecosystem carbon balance (NECB) estimates suggested that the IWG monoculture accumulated more carbon (306 ± 88 C m−2), whereas the biculture was on average carbon neutral (7 g ± 131 C m−2), when biomass removal and manure additions were accounted for. Our study demonstrates the complexities of quantifying carbon budgets in dynamic agricultural systems over short time scales, and the importance of assessing crop multifunctionality within a site's ecological and economic context.