Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus taeda) forest exposed to elevated atmospheric [CO2] and N fertilization

Forests return large quantities of C to the atmosphere through soil respiration (R-soil), which is often conceptually separated into autotrophic C respired by living roots (R-root) and heterotrophic decomposition (R-het) of soil organic matter (SOM). Live roots provide C sources for microbial metabolism via exudation, allocation to fungal associates, sloughed-off cells, and secretions such as mucilage production, suggesting a coupling between the activity of roots and heterotrophs. We addressed the strength of root effects on the activity of microbes and exo-enzymes by removing live-root-C inputs to areas of soil with a trenching experiment. We examined the extent to which trenching affected metrics of soil heterotrophic activity (proteolytic enzyme activity, microbial respiration, potential net N mineralization and nitrification, and exo-enzyme activities) in a forest exposed to elevated atmospheric [CO2] and N fertilization, and used automated measurements of R-soil in trenched and un-trenched plots to estimate R-root and R-het components. Trenching decreased many metrics of heterotrophic activity and increased net N mineralization and nitrification, suggesting that the removal of root-C inputs reduced R-het by exacerbating microbial C limitation and stimulating waste-N excretion. This trenching effect was muted by N fertilization alone but not when N fertilization was combined with elevated CO2, consistent with known patterns of below-ground C allocation at this site. Live-root-C inputs to soils and heterotrophic activity are tightly coupled, so root severing techniques like trenching are not likely to achieve robust quantitative estimates of Rroot or R-het. (c) 2012 Elsevier B.V. All rights reserved.