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 (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.

Duke Scholars

Author Ram Oren Earth and Climate Sciences