Ecosystem‐level controls on root‐rhizosphere respiration

Abstract

SummaryRecent advances in the partitioning of autotrophic from heterotrophic respiration processes in soils in conjunction with new high temporal resolution soil respiration data sets offer insights into biotic and environmental controls of respiration. Besides temperature, many emerging controlling factors have not yet been incorporated into ecosystem-scale models. We synthesize recent research that has partitioned soil respiration into its process components to evaluate effects of nitrogen, temperature and photosynthesis on autotrophic flux from soils at the ecosystem level. Despite the widely used temperature dependence of root respiration, gross primary productivity (GPP) can explain most patterns of ecosystem root respiration (and to some extent heterotrophic respiration) at within-season time-scales. Specifically, heterotrophic respiration is influenced by a seasonally variable supply of recent photosynthetic products in the rhizosphere. The contribution of stored root carbon (C) to root respiratory fluxes also varied seasonally, partially decoupling the proportion of photosynthetic C driving root respiration. In order to reflect recent insights, new hierarchical models, which incorporate root respiration as a primary function of GPP and which respond to environmental variables by modifying C allocation belowground, are needed for better prediction of future ecosystem C sequestration.

Biological and physical influences on soil <sup>14</sup>CO<sub>2</sub> seasonal dynamics in a temperate hardwood forest

“…For researchers primarily interested in an average growing season 14 C respiration value, this study corroborated previous work suggesting that seasonal variation in respired 14 CO 2 is substantial (Hicks Pries et al, 2013;Hirsch et al, 2003;Hopkins et al, 2013;Schuur and Trumbore, 2006). At a minimum, sampling time points at the beginning, middle, and end of the growing season are ideal to capture the seasonal progression of new C additions.…”

“…Our results indicated that heterotrophic 14 C is dynamic and sensitive to immediate substrate availability, and that experimental manipulations to isolate heterotrophic and autotrophic activity can substantially impact estimates of heterotrophic 14 C. Inputs of new photosynthates over the growing season, which have been shown to decrease the 14 C content of root respiration (Hopkins et al, 2013), may also lead to decreases in the 14 C content of microbial respiration. Studies that make use of 14 CO 2 measurements for examining disturbance or climatic change impacts should be interpreted with an understanding that respired 14 CO 2 can fluctuate seasonally by 40 ‰, and that this variability may reflect not only changes in root contributions, but possibly root impacts on 14 C of heterotrophic respiration as well.…”

“…If microbes in intact plots switched to newly available substrates, then the trenched plot would have no longer provided a good measure of heterotrophic 14 C for mixing-model partitioning. Hopkins et al (2013) have also shown that 14 C abundance in root respiration declines over the course of the growing season. While we measured root respiration at only a single time point and did not explicitly assess root respiration seasonal variability, the analysis by Hopkins et al suggests that the root 14 C end-member is non-static through time.…”

“…Based on our findings and recent analyses by Hopkins et al (2013), which show that root respiration from several forest sites becomes more similar to the atmosphere in 14 C content over the course of the growing season, we believe one of the more pronounced signals that can be detected using 14 CO 2 measurements is the relative contributions of current photosynthates. A persistent challenge, however, is detecting the amount and type of C respired by soil microbes.…”

Environmental forcing does not induce diel or synoptic variation in the carbon isotope content of forest soil respiration

“…The potential biological mechanisms that would lead to diel variation in δ 13 C of respiration have been recently reviewed (Werner and Gessler, 2011) and include variation in respiratory substrate, isotopic fractionation, and/or changes in relative amounts of component fluxes such as autotrophic (rhizospheric) and heterotrophic respiration. There is compelling evidence for a short-term linkage between photosynthesis and belowground respiration (Kuzyakov and Gavrichkova, 2010), almost certainly mediated by the roots and their symbionts (Hopkins et al, 2013;Savage et al, 2013). Several studies have highlighted isotopic signals in respiration from the soil or whole forests that appear to be influenced by photosynthetic response to humidity or water availability (see reviews by Bowling et al, 2008;Brüggemann et al, 2011), which might be expected to lead to diel changes in δ 13 C of soil efflux.…”