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Soils can be sources or sinks of carbon depending on the balance between carbon inputs from plants and losses from the decomposition of soil organic matter (SOM). A good understanding of the temperature sensitivity of SOM decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon, and therefore accelerate or mitigate the rate of increasing atmospheric carbon dioxide (CO2) concentration. We provide new evidence to show that the response of SOM decomposition to temperature may be constrained by substrate availability to microbial decomposers. We used laboratory incubations of a grassland soil to compare the temperature sensitivity of SOM decomposition with unmodified substrate availability with that of the same soil in which substrate availability was reduced by adding allophane, a clay-size mineral with a high capacity for binding SOM. In the soil with no added allophane, the decomposition rate increased about 7-fold over the temperature range from 1 to 40 °C. With added allophane, decomposition rate increased only about 3-fold over the same temperature range. We then used a non-disruptive, natural abundance isotopic technique at our field site to partition total soil respiration into CO2 efflux from newly released, 13C-depleted SOM (root respiration and rhizosphere decomposition) from CO2 efflux from older 13C-enriched SOM from the decomposition of more stable SOM. We found no increase in the decomposition rate of the 13C-enriched pool of SOM between 11 and 28 °C. That finding contrasts with most previous studies that have generally reported strong increases in SOM decomposition with temperature. We hypothesised that the large temperature sensitivity observed in laboratory incubations was due to substrate becoming readily available as a result of the disturbance involved in collecting soil samples. In undisturbed field conditions, the limiting step for the decomposition of the more stable SOM pool may be the rate at which decomposable substrate becomes available for decomposition. Our findings will have important implications for the feedbacks between soil carbon storage and the rate of increase in atmospheric CO2 concentration mediated by global warming.

Soils are the largest pool of carbon (C) in terrestrial ecosystems with labile C being particularly vulnerable to loss. In this study we incubated a range of soils in both the short- (minutes) and long-term (months) to assess the loss of labile soil C by measuring the isotopic signature of soil respired CO 2 (∂ 13 CO 2 ). Strong temporal trends in ∂ 13 CO 2 values were observed following soil disturbance: ∂ 13 CO 2 rapidly changed from a range of −22.5 to −23.9‰ to −25.8 to −27.5‰ during short-term incubations and reverted back to the initial values in long-term incubations. The shifts in ∂ 13 CO 2 over the course of soil incubations were consistent with changes in labile C substrate utilization following the disturbance of sampling the soil. An independent experimental approach which immobilised labile soil C onto allophane and included chemical extractions as a measure of extractable C in soils also confirmed this interpretation. Collectively, these results indicate that the isotopic analysis of respired CO 2 can be a powerful technique which enables us to probe mechanisms and examine the consequences of disturbance on the labile component of soil C.

Quantifying the loss of soil carbon through respiration has proved difficult, due to the challenge of measuring the losses associated with the turnover of soil organic matter (SOM) as distinct from autotrophic components. In forest ecosystems the δ13C value of respiration from turnover of SOM (δ13CRSOM) is typically 2–4‰ enriched compared with that from roots and associated microbes (δ13CRROOTS), with that from the litter (δ13CRLITTER) lying between the two. We measured soil respiration at 50 locations in a forest soil and then used differences in isotopic signatures to quantify the proportion of soil respiration arising from the turnover of SOM (fRSOM) at a subset of 30 locations, chosen randomly. The soil surface CO2 efflux was collected using an open chamber system supplied with CO2-free air and the δ13C signature (δ13CRS) measured, giving a mean (±SD) value across the site of −26.1 ± 0.58‰. The values of δ13CRROOTS, δ13CRLITTER and δ13CRSOM were measured at each location by incubation of roots, litter and root-free soil and collection of the CO2 for isotopic analysis. δ13CRSOM became progressively depleted with length of incubation (1.5‰ after 8 h), so CO2 was collected after 20 min. The mean value of δ13CRLITTER was −27.2 ± 0.68 ‰, which was indistinguishable from δ13CRROOTS of −27.6 ± 0.51‰, while δ13CRSOM was −25.1 ± 0.88‰. δ13CRROOTS and δ13CRSOM measured at each location were used as the end points of a two component mixing model to calculate fRSOM, giving a mean value for fRSOM of 0.61 ± 0.28. It was not possible to estimate fRSOM using the total C contents of the roots and soil which were significantly depleted in 13C in comparison to their respired CO2. However, at seven locations the δ13CRS was slightly enriched compared with δ13CRSOM (mean 0.3‰), which was not considered significantly different so fRSOM was constrained to 1.0. If these seven rings were excluded mean fRSOM was 0.49 ± 0.20. We have shown the possibility of using natural abundance 13C discrimination to quantify fRSOM in a forest soil with an input of carbon only from C3 photosynthesis.

Plants link atmospheric and soil carbon pools through CO2 fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous 13C-labelling to soil-grown Lolium perenne, imposing atmospheric CO2 concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific 13C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO2 concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO2 and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study.

We used natural gradients in soil and vegetation δ 13 C signatures in a savannah ecosystem in Texas to partition soil respiration into the autotrophic (Ra) and heterotrophic (Rh) components. We measured soil respiration along short transects from under clusters of C 3 trees into the C 4 dominated grassland. The site chosen for the study was experiencing a prolonged drought, so an irrigation treatment was applied at two positions of each transect. Soil surface CO 2 efflux was measured along transects and CO 2 collected for analysis of the δ 13 C signature in order to: (i) determine how soil respiration rates varied along transects and were affected by localised change in soil moisture and (ii) partition the soil surface CO 2 efflux into Ra and Rh, which required measurement of the δ 13 C signature of root- and soil-derived CO 2 for use in a mass balance model. The soil at the site was unusually dry, with mean volumetric soil water content of 8.2%. Soil respiration rates were fastest in the centre of the tree cluster (1.5 ± 0.18 μmol m −2 s −1 ; mean ± SE) and slowest at the cluster–grassland transition (0.6 ± 0.12 μmol m −2 s −1 ). Irrigation produced a 7–11 fold increase in the soil respiration rate. There were no significant differences ( p > 0.5) between the δ 13 C signature of root biomass and respired CO 2 , but differences ( p 2 and soil when sampled at the edge of the clusters and in the grassland. Therefore, end member values were measured by root and soil incubations, with times kept constant at 30 min for roots and 2 h for soils. The δ 13 C signature of the soil surface CO 2 efflux and the two end member values were used to calculate that, in the irrigated soils, Rh comprised 51 ± 13.5% of the soil surface CO 2 efflux at the mid canopy position and 57 ± 7.4% at the drip line. In non-irrigated soil it was not possible to partition soil respiration, because the δ 13 C signature of the soil surface CO 2 efflux was enriched compared to both the end member values. This was probably due to a combination of the very dry porous soils at our study site (which may have been particularly susceptible to ingress of atmospheric CO 2 ) and the very slow respiration rates of the non-irrigated soils.