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

Abstract

Forests return large quantities of C to the atmosphere through soil respiration (Rsoil), which is often conceptually separated into autotrophic C respired by living roots (Rroot) and heterotrophic decomposition (Rhet) 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 Rsoil in trenched and un-trenched plots to estimate Rroot and Rhet components. Trenching decreased many metrics of heterotrophic activity and increased net N mineralization and nitrification, suggesting that the removal of root-C inputs reduced Rhet 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 belowground 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 Rhet.

Citation

Drake, J.E.; Oishi, A.C.;Giasson, M. A.;Oren, R.; Johnsen, K.H.; Finzi, A.C. 2012.Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus taeda) forest exposed to elevated atmospheric [CO₂] and N fertilization. Agricultural Forest Meteorology 165: 43-52. 10 p.  10.1016/j.agrformet.2012.05.017