Forest and perennial herbland cover reduce microbial respiration but increase root respiration in agroforestry systems

Agricultural land management practices have the potential to reduce carbon emissions from soils, especially when a reduction in microbial (heterotrophic respiration, R-H) rather than root respiration (autotrophic, R-A) is achieved. Soil R-A and R-H and their sensitivity to temperature changes were determined in the forestland and neighboring herbland (area without trees) soils of three agroforestry systems (hedgerow, shelterbelt, and silvopasture) over two growing seasons (May through September in 2013 and 2014). Over the two growing seasons, mean R-A from the forestland was 32% greater than that from the herbland, while the R-H in the forestland was 22% lower than that in the herbland. The sensitivity of RA to temperature was consistently greater in the forestland (3.6) than in the herbland (3.4), though the opposite was found for R-H. Effects of agroforestry system on R-A and R-H also varied seasonally. The contribution of RH to total soil respiration was greater in each of the hedgerow (59%) and shelterbelt (55%) systems than in the silvopasture system (51%), reflecting the high R-H from annual cropland within the hedgerow and shelterbelt systems. We found stronger control of R-H by temperature in the hedgerow and shelterbelt, suggesting that an increase in soil temperature in response to future climatic warming could reduce the amount of carbon held in these systems as compared to the silvopasture system. Overall, the inclusion or maintenance of perennial vegetation (forest and grassland) in an annually cropped agricultural landscape could result in a net reduction in soil R-H, and thereby mitigate losses of carbon from agricultural soils.