Alley cropping affects perennial bioenergy crop root distribution, carbon, and nutrient stocks

Authors: Gamble, Joshua; JOHNSON, GREGG - University Of Minnesota; CURRENT, DEAN - University Of Minnesota; WYSE, DONALD - University Of Minnesota; ZAMORA, DIOMEDES - University Of Minnesota; SHEAFFER, CRAIG - University Of Minnesota

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2020
Publication Date: 9/1/2020
Citation: Gamble, J.D., Johnson, G., Current, D., Wyse, D., Zamora, D., Sheaffer, C. 2020. Alley cropping affects perennial bioenergy crop root distribution, carbon, and nutrient stocks. Agronomy Journal. 112(5):3718-3732. https://doi.org/10.1002/agj2.20350.
DOI: https://doi.org/10.1002/agj2.20350

Interpretive Summary: Agroforestry, the integration of woody perennials with crops or pastures, has been promoted as a key strategy to mitigate GHG emissions and adapt agricultural systems to the impacts of shifting climate. However, considerable uncertainty remains about the capacity for C sequestration in tree and crop root biomass in agroforestry systems adapted to the Upper Midwest U.S. Our goal was to quantify the belowground C, N, P, and K stocks of perennial biomass crops planted in an alley cropping configuration. Our objectives were to: 1) quantify root biomass C, N, P, and K of alley cropped woody and herbaceous perennial crops; 2) determine the spatial distribution and relative allocation of belowground biomass within alley systems; and 3) quantify changes in soil organic carbon (SOC) and system C stocks (root biomass C + SOC) associated with alley cropped woody and herbaceous perennial crops four years after establishment. Our results show that after four years of production, alley cropping with NM6 poplar and prairie cordgrass had among the highest C and nutrient accumulation two Minnesota sites. Belowground competition reduced root biomass of herbaceous crops in both willow and poplar alleys. However, poplars appear to be more competitive with herbaceous crops than willows, suggesting that long term productivity, C sequestration, and nutrient accumulation may be greater in willow alley cropping systems. Converting from annual grain crops to perennial biomass alley cropping resulted in small SOC losses, but net C gains should be realized over the long-term due to rapid C accrual and turnover in roots. Our findings are important for the design of bioenergy crop-based agroforestry systems because they demonstrate that 1) poplar based systems are more productive (and accrue more C) than willow systems in the short term, but 2) poplar are more competitive with herbaceous crops than willow, which suggests greater long-term system productivity in willow systems, and 3) prairie cordgrass generally had greater C and nutrient accrual in root biomass than intermediate wheatgrass, switchgrass, and a native polyculture. This information can inform the design of additional research or field scale agroforestry systems designed for biomass feedstock production. The findings of this research will be of interest to researchers, and producers/producer groups with interest in agroforestry and biomass/bioenergy production systems.

Technical Abstract: This study quantified root biomass distribution; root accrual of C, N, P, and K; and changes in soil organic carbon (SOC) associated with alley-cropped switchgrass (Panicum virgatum L.), prairie cordgrass (Spartina pectinata Bosc ex Link), intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth and Dewey ‘Rush’], and a native polyculture planted between rows of ‘NM6’ poplar (Populus maximowiczii × P. nigra) and ‘Fish Creek’ willow (Salix purpurea) at two Minnesota sites (Empire and Granada). After 4 yr since establishment, SOC declined at each site but was not influenced by species selection. NM6 poplar–prairie cordgrass systems had among the highest root biomass, C, and nutrient accrual, with up to 16.3 Mg root biomass, 7.0 Mg C, 175 kg N, 31 kg P, and 97 kg K ha-1. Fine roots were the largest fraction of belowground biomass, although course roots were also a large fraction for poplar and prairie cordgrass. Tree roots extended to 6 m into the crop alley, although 85–89% were within 1 m of tree rows, depending on tree species. Crop fine root biomass was reduced up to 67% at 1 m from tree rows and 20% at 3.5 m and was up to 142% greater in willow than in poplar alleys. Total root C was predominated by poplars at Empire regardless of herbaceous crop type, whereas the proportions of tree and crop root C varied by crop at Granada. These results suggest that prairie cordgrass is well suited to alley cropping and that, due to the competitive ability of poplars, productivity, C sequestration, and nutrient accrual may be greater in willow systems in the long term.