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

• Published in: Agricultural and forest meteorology Vol. 165; pp. 43 - 52
• Main Authors: Drake, J.E., Oishi, A.C., Giasson, M.-A., Oren, R., Johnsen, K.H., Finzi, A.C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2012
• Subjects: Carbon dioxide; couplings; Elevated; Elevated carbon dioxide; enzyme activity; excretion; exudation; Fertilization; Forest Science; Forests; fungi; Heterotrophic respiration; heterotrophs; inputs; metabolism; Microorganisms; mineralization; mucilages; nitrification; Nitrogen fertilization; nitrogen fertilizers; Pinus taeda; Priming; proteolysis; Root respiration; Roots; secretion; Skogsvetenskap; Soil (material); soil organic matter; soil respiration; Trenching; Nitrogen fertilization; Priming; Heterotrophic respiration; Trenching; Root respiration; Elevated carbon dioxide
• ISSN: 0168-1923; 1873-2240; 1873-2240
• DOI: 10.1016/j.agrformet.2012.05.017

► The coupling between live-root-C inputs to soils and microbial activity was explored. ► Removing roots exacerbates microbial C limitation and increases N excretion. ► Techniques that sever roots are unlikely to eliminate Rroot without affecting Rhet.
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.