Trends and methodological impacts in soil CO₂ efflux partitioning: A metaanalytical review

Abstract

Partitioning soil carbon dioxide (CO2) efflux (RS) into autotrophic (RA; including plant roots and closely associated organisms) and heterotrophic (RH) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the RH/RS ratio for increasing annual RS fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P<0.05) RH/RS ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the RH/RS ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of RS under which they had been measured was considered, with the exception of studies estimating RH by means of root mass regressions against RS, which resulted in consistently lower RH/RS estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect RH/RS ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO2 efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself.

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

“…It is very likely that rainfall seasonality combined with the differences in soil water content between trenched and untrenched soil masked decreases in soil respiration due to trenching: we were unable to estimate rooterhizosphere respiration during the driest months of the year (March and April 2008) as the soil water content in the trenched soil was 32e84% higher than in the untrenched soil due to the lack of uptake by roots. These large differences in soil water content most likely lead to increased heterotrophic respiration rates in the trenched plots during the dry season (Subke et al, 2006), resulting in similar respiration rates in trenched and untrenched soil in the CT and Lþ plots (Fig. 2).…”

“…The ability to partition soil respiration into these components is becoming increasingly important in the context of predicting the effects of environmental change because the sensitivity to disturbance of autotrophic and heterotrophic respiration is likely to differ (Subke et al, 2006). There is evidence that heterotrophic respiration and root respiration may respond differently to increased temperature (e.g.…”

“…Reviews and meta-analyses of respiration partitioning studies highlight the difficulties involved in the different methods employed and generally agree that isotopic labelling is the most accurate method with the fewest methodological artefacts (e.g. Hanson et al, 2000;Subke et al, 2006;Kuzyakov, 2006). However, isotope methods are costly and not readily applicable to most field studies (Kuzyakov, 2006) and thus litter and root exclusion techniques remain the most feasible way to partition soil respiration directly in the field.…”

“…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.…”

“…In trenching experiments a trench is cut around a block of soil to kill all roots and mycorrhizal hyphae within the area and a solid barrier is installed to exclude root regrowth; respiration rates in the root-free soil are then compared to respiration rates in control plots with roots present and rooterhizosphere respiration is estimated from the difference. Although this approach is fairly simple and can be carried out in most ecosystems, it is quite labour-intensive and entails several inherent problems in estimating rooterhizosphere respiration: i) decomposition of the cut roots can lead to high heterotrophic respiration (Hanson et al, 2000;Subke et al, 2006) which can last for several months to over a year, depending on soil type (Silver et al, 2005); ii) the presence or absence of roots in the soil leads to differences in soil water content which can greatly affect heterotrophic respiration rates (Subke et al, 2006); iii) seasonal differences in respiration rates, soil water content and soil temperature; and iv) roots have been shown to grow underneath installed barriers and recolonize root-free soil (Edwards and Norby, 1998;Tanner and Barberis, 2007). Estimates of rooterhizosphere respiration in tropical forests range widely from 27 to 76% of total soil respiration (Subke et al, 2006) and it is uncertain how much of this variation results from differences in methodology.…”