| Perennial biomass production in agroforestry systems has been promoted as a strategy to increase productivity and ecosystem services from marginal agricultural lands. However, little is known about appropriate species combinations and production potential for biomass crops in agroforestry systems. Our objectives were to evaluate the potential for biomass feedstock production, nutrient uptake and accumulation, and carbon sequestration in alley cropping agroforestry systems at two Minnesota sites, and to determine how tree -- crop interactions influenced productivity in these systems. Short-rotation woody crops (SRWC) were hybrid poplar (Populus maximowiczii x P. nigra 'NM6') and shrub willow (Salix purpurea 'Fish Creek'). Herbaceous alley crops were switchgrass (Panicum virgatum L.), prairie cordgrass (Spartina pectinata Bosc ex Link), 'Rush' intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey cv. Rush), and an eleven species native polyculture.;After four years of growth, we found that NM6 poplar alley cropping systems maximized biomass yields at Empire (13.5 Mg ha-1 yr -1) and Granada, MN (9.6 Mg ha-1 yr-1), irrespective of herbaceous crop type. NM6 poplar -- intermediate wheatgrass systems showed the greatest potential for aboveground N, P, and K uptake (477, 62, and 301 kg ha-1), while NM6 poplar -- prairie cordgrass systems had among the highest root biomass, and root C, N, P, and K due to extensive coarse roots. Soil carbon declined slightly over the study period, although alley system roots sequestered up to 7.0 and 6.3 Mg C ha-1 at Empire, and Granada, respectively. At Empire, above-- and belowground biomass of herbaceous alley crops declined substantially with proximity to SRWC rows, as did soil water potential, soil NO3 -- N, and transmittance of photosynthetically active radiation (PAR). A mixed effects model with predictors for PAR and soil water potential best explained patterns in prairie cordgrass and native polyculture yield, suggesting that competition for light and water limited crop growth at this site.;Our results show that after four years of production, NM6 poplar and prairie cordgrass were among the best SRWC and herbaceous crop choices for biomass production, C sequestration, and nutrient accumulation in alley cropping systems. However, competition may limit the stand longevity of herbaceous crops, which could reduce the utility of these systems for biomass production and ecosystem services over time. |
