Forest and Grassland Cover Types Reduce Heterotrophic but Not Autotrophic Respiration from Agricultural Systems.

Planting or retaining woody species in the agricultural landscape in the form of agroforestry has potential for mitigating greenhouse gas emissions, when a reduction in microbial (heterotrophic respiration, R_H) rather than root respiration (autotrophic respiration, R_A) is achieved. Soil R_A and R_H and their sensitivity to temperature changes were determined in forest and herbland (areas without trees) land cover types of three agroforestry systems (hedgerow, shelterbelt, and silvopasture) over two growing seasons in 2013 and 2014 in central Alberta, Canada. Over the two seasons, mean R_A from forest was 32% greater than that from herbland, while the R_H in the forest was 22% lower than that in the herbland. Sensitivity of R_A to temperature was consistently greater in forest (3.6) than in herbland cover type (3.4), but the opposite was found for R_H. Effects of agroforestry systems on the R_A and R_H vary seasonally. The contribution of R_H to total respiration was greater in hedgerow (59%) and shelterbelt (55%) than in silvopasture (50%), reflecting the contribution of R_H from the cropland within the hedgerow and shelterbelt systems. The temperature sensitivity of R_H was higher in hedgerow and shelterbelt systems than in silvopasture, indicating the greater potential for soil organic carbon loss when the climate warms in the future. We conclude that planting or retaining woody species in the agricultural landscape can be an effective measure to mitigate climate change_.