Large-scale forest girdling shows that current photosynthesis drives soil respiration

Abstract

The respiratory activities of plant roots, of their mycorrhizal fungi and of the free-living microbial heterotrophs (decomposers) in soils are significant components of the global carbon balance, but their relative contributions remain uncertain. To separate mycorrhizal root respiration from heterotrophic respiration in aboreal pine forest, we conducted a large-scale tree-girdling experiment, comprising 9 plots each containing about 120 trees. Tree-girdling involves stripping the stem bark to the depth of the current xylem at breast height terminating the supply of current photosynthates to roots and their mycorrhizal fungi without physically disturbing the delicate root-microbe-soil system. Here we report that girdling reduced soil respiration within 1-2 months by about 54% relative to respiration on ungirdled control plots, and that decreases of up to 37% were detected within 5 days. These values clearly show that the flux of current assimilates to roots is a key driver of soil respiration; they are conservative estimates of root respiration, however, because girdling increased the use of starch reserves in the roots. Our results indicate that models of soil respiration should incorporate measures of photosynthesis and of seasonal patterns of photosynthate allocation to roots.

Complex effects of mammalian grazing on extramatrical mycelial biomass in the Scandes forest‐tundra ecotone

“…obs.). Moose can decrease mycorrhizal colonization in taiga ecosystems through winter browsing of willow and poplar twigs (Rossow et al., 1997), and, furthermore, browse the sites throughout the summer, which would have a greater impact on the flow of photosynthates to roots and, subsequently, mycorrhiza, as ECM fungi are dependent on newly fixed carbon (Ekblad et al., 2013; Högberg et al., 2001). Contrary to our hypothesis, however, we found no correlation between B. pubescens biomass and EMM biomass which could mean that other factors connected to herbivory, such as trampling and fertilization, may have influenced mycelial production, although no correlation to any of the measured soil variables corroborated a fertilization effect either.…”

Properties of soils influenced by ectomycorrhizal fungi in hybrid spruce [Picea glauca × engelmannii (Moench.) Voss]

“…Berthelin (1983) attributed the low pH in the mycorrhizosphere zone to oxidized inorganics (e.g., sulphur) and the production of organic acids (e.g., oxalic, carbonic, citric, acetic) by the activities of ECM. Singh et al (2003) and Högberg et al (2001) reported that respiration from ectomycorrhizal roots is equivalent to up to 65% of total soil respiration. This finding has significant implications with respect to the generation of exudates (e.g., organic acids) that can subsequently lower the pH and further enhance the rates of mineral weathering in ECS compared to N-ECM soils.…”

A new approach to trenching experiments for measuring root–rhizosphere respiration in a lowland tropical forest

“…root exudates and sloughed-off root cells; Andrews et al, 1999;Kuzyakov, 2006), consequently these components are usually estimated together (sensu Wiant, 1967) and we henceforth use the term 'rooterhizosphere respiration' to include both autotrophic root respiration and heterotrophic respiration in the rhizosphere. Tree-girdling has been used successfully to estimate rooterhizosphere respiration in a northern Scots pine forest (Högberg et al, 2001) and has the advantage of not changing the abiotic soil environment (Subke et al, 2006), however it has been less successful in studies of tropical trees because of large carbohydrate reserves in the existing root system of some species, which provide C for the maintenance of fine roots when the supply of C from aboveground assimilation is disrupted (Binkley et al, 2006;Nottingham et al unpublished data). Thus, most attempts at estimating rooterhizosphere respiration in tropical and subtropical forests have used either an indirect mass balance approach (e.g.…”