Effects of substrate availability on the temperature sensitivity of soil organic matter decomposition

Soil carbon is a major component in the global carbon cycle. Understanding the relationship between environmental changes and rates of soil respiration is critical for projecting changes in soil carbon fluxes in a changing climate. Although significant attention has been focused on the temperature sensitivity of soil organic matter decomposition, the factors that affect this temperature sensitivity are still debated. In this study, we examined the effects of substrate availability on the temperature sensitivity of soil respiration in several different kinds of soils. We found that increased substrate availability had a significant positive effect on temperature sensitivity, as measured by soil Q ₁₀ values, and that this effect was inversely proportional to original substrate availability. This observation can be explained if decomposition follows Michaelis–Menten kinetics. The simple Q ₁₀ model was most appropriate in soils with high substrate availability.     

Sensitivity of organic matter decomposition to warming varies with its quality

The relationship between organic matter (OM) lability and temperature sensitivity is disputed, with recent observations suggesting that responses of relatively more resistant OM to increased temperature could be greater than, equivalent to, or less than responses of relatively more labile OM. This lack of clear understanding limits the ability to forecast carbon (C) cycle responses to temperature changes. Here, we derive a novel approach (denoted Q_10?q) that accounts for changes in OM quality during decomposition and use it to analyze data from three independent sources. Results from new laboratory soil incubations (labile Q_10?q=2.1 ± 0.2; more resistant Q_10?q=3.8 ± 0.3) and reanalysis of data from other soil incubations reported in the literature (labile Q_10?q=2.3; more resistant Q_10?q=3.3) demonstrate that temperature sensitivity of soil OM decomposition increases with decreasing soil OM lability. Analysis of data from a cross‐site, field litter bag decomposition study (labile Q_10?q=3.3 ± 0.2; resistant Q_10?q=4.9 ± 0.2) shows that litter OM follows the same pattern, with greater temperature sensitivity for more resistant litter OM. Furthermore, the initial response of cultivated soils, presumably containing less labile soil OM (Q_10?q=2.4 ± 0.3) was greater than that for undisturbed grassland soils (Q_10?q=1.7 ± 0.1). Soil C losses estimated using this approach will differ from previous estimates as a function of the magnitude of the temperature increase and the proportion of whole soil OM comprised of compounds sensitive to temperature over that temperature range. It is likely that increased temperature has already prompted release of significant amounts of C to the atmosphere as CO₂. Our results indicate that future losses of litter and soil C may be even greater than previously supposed.     

The temperature dependence of organic matter decomposition: seasonal temperature variations turn a sharp short‐term temperature response into a more moderate annually averaged response

The temperature dependence of organic matter decomposition is a critically important determinant of any long‐term changes of soil‐carbon stocks in response to global warming. Because of practical experimental constraints, most knowledge of this temperature dependence is based on short‐term studies. These studies generally show a strong temperature dependence of organic matter decomposition. At the same time, many modelling studies, especially global studies, or studies that investigate the effects of climate change, use longer time steps, such as annual. It is investigated here to what extent the use of short‐term temperature dependencies are appropriate, or how they may need to be modified, for application over longer time steps. The work indicated that for global applications, it is critically important to explicitly consider seasonal temperature variations. Across the globe, observed annual mean temperature and the annual temperature range are negatively correlated. Inclusion of this correlation means that the strong short‐term temperature dependence becomes much weaker when data are expressed as annual averages for the temperatures experienced across the globe. For short‐term responses, the temperature dependence of organic matter decomposition is greater at low than high temperature and deviates strongly from an assumption of a constant Q₁₀ across temperature. For annually averaged values, this pattern also weakens, and temperature dependencies change only slightly with temperature. Using short time steps for simulations leads to the expectation of more positive changes (sequestration) in soil carbon especially for cold regions of the globe than would be predicted for simulations at annual time steps without explicit consideration of seasonal temperature variations. These considerations help to reconcile some of the apparent differences in temperature dependencies obtained by different workers using different approaches.     

Drought‐resistant fungi control soil organic matter decomposition and its response to temperature

Microbial‐mediated decomposition of soil organic matter (SOM) ultimately makes a considerable contribution to soil respiration, which is typically the main source of CO₂ arising from terrestrial ecosystems. Despite this central role in the decomposition of SOM, few studies have been conducted on how climate change may affect the soil microbial community and, furthermore, on how possible climate‐change induced alterations in the ecology of microbial communities may affect soil CO₂ emissions. Here we present the results of a seasonal study on soil microbial community structure, SOM decomposition and its temperature sensitivity in two representative Mediterranean ecosystems where precipitation/throughfall exclusion has taken place during the last 10 years. Bacterial and fungal diversity was estimated using the terminal restriction fragment length polymorphism technique. Our results show that fungal diversity was less sensitive to seasonal changes in moisture, temperature and plant activity than bacterial diversity. On the other hand, fungal communities showed the ability to dynamically adapt throughout the seasons. Fungi also coped better with the 10 years of precipitation/throughfall exclusion compared with bacteria. The high resistance of fungal diversity to changes with respect to bacteria may open the controversy as to whether future ‘drier conditions’ for Mediterranean regions might favor fungal dominated microbial communities. Finally, our results indicate that the fungal community exerted a strong influence over the temporal and spatial variability of SOM decomposition and its sensitivity to temperature. The results, therefore, highlight the important role of fungi in the decomposition of terrestrial SOM, especially under the harsh environmental conditions of Mediterranean ecosystems, for which models predict even drier conditions in the future.     

Transplantation of organic surface horizons of boreal soils into warmer regions alters microbiology but not the temperature sensitivity of decomposition

Changes in soil carbon, the largest terrestrial carbon pool, are critical for the global carbon cycle, atmospheric CO₂ levels and climate. Climate warming is predicted to be most pronounced in the northern regions and therefore the large soil carbon pool residing in boreal forests will be subject to larger global warming impact than soil carbon pools in the temperate or the tropical forest. A major uncertainty in current estimates of the terrestrial carbon balance is related to decomposition of soil organic matter (SOM). We hypothesized that when soils are exposed to warmer climate the structure of the ground vegetation will change much more rapidly than the dominant tree species. This change will alter the quality and amount of litter input to the soil and induce changes in microbial communities, thus possibly altering the temperature sensitivity of SOM decomposition. We transferred organic surface soil sections from the northern borders of the boreal forest zone to corresponding forest sites in the southern borders of the boreal forest zone and studied the effects of warmer climate after an adaptation period of 2 years. The results showed that initially ground vegetation and soil microbial community structure and community functions were different in northern and southern forest sites and that 2 years of exposure to warmer climate was long enough to cause changes in these ecological indicators. The rate of SOM decomposition was approximately equally sensitive to temperature irrespective of changes in vegetation or microbial communities in the studied forest sites. However, as temperature sensitivity of the decomposition increases with decreasing temperature regime, the proportional increase in the decomposition rate in northern latitudes could lead to significant carbon losses from the soils.     

Rhizosphere priming effect increases the temperature sensitivity of soil organic matter decomposition

The temperature sensitivity of soil organic matter (SOM) decomposition has been a crucial topic in global change research, yet remains highly uncertain. One of the contributing factors to this uncertainty is the lack of understanding about the role of rhizosphere priming effect (RPE) in shaping the temperature sensitivity. Using a novel continuous ¹³C‐labeling method, we investigated the temperature sensitivity of RPE and its impact on the temperature sensitivity of SOM decomposition. We observed an overall positive RPE. The SOM decomposition rates in the planted treatments increased 17–163% above the unplanted treatments in three growth chamber experiments including two plant species, two growth stages, and two warming methods. More importantly, warming by 5 °C increased RPE up to threefold, hence, the overall temperature sensitivity of SOM decomposition was consistently enhanced (Q₁₀ values increased 0.3–0.9) by the presence of active rhizosphere. In addition, the proportional contribution of SOM decomposition to total soil respiration was increased by soil warming, implying a higher temperature sensitivity of SOM decomposition than that of autotrophic respiration. Our results, for the first time, clearly demonstrated that root–soil interactions play a crucial role in shaping the temperature sensitivity of SOM decomposition. Caution is required for interpretation of any previously determined temperature sensitivity of SOM decomposition that omitted rhizosphere effects using either soil incubation or field root‐exclusion. More attention should be paid to RPE in future experimental and modeling studies of SOM decomposition.     

Sources of plant-derived carbon and stability of organic matter in soil: implications for global change

Alterations in forest productivity and changes in the relative proportion of above‐ and belowground biomass may have nonlinear effects on soil organic matter (SOM) storage. To study the influence of plant litter inputs on SOM accumulation, the Detritus Input Removal and Transfer (DIRT) Experiment continuously alters above‐ and belowground plant inputs to soil by a combination of trenching, screening, and litter addition. Here, we used biogeochemical indicators [i.e., cupric oxide extractable lignin‐derived phenols and suberin/cutin‐derived substituted fatty acids (SFA)] to identify the dominant sources of plant biopolymers in SOM and various measures [i.e., soil density fractionation, laboratory incubation, and radiocarbon‐based mean residence time (MRT)] to assess the stability of SOM in two contrasting forests within the DIRT Experiment: an aggrading deciduous forest and an old‐growth coniferous forest. In the deciduous forest, removal of both above‐ and belowground inputs increased the total amount of SFA over threefold compared with the control, and shifted the SFA signature towards a root‐dominated source. Concurrently, light fraction MRT increased by 101 years and C mineralization during incubation decreased compared with the control. Together, these data suggest that root‐derived aliphatic compounds are a source of SOM with greater relative stability than leaf inputs at this site. In the coniferous forest, roots were an important source of soil lignin‐derived phenols but needle‐derived, rather than root‐derived, aliphatic compounds were preferentially preserved in soil. Fresh wood additions elevated the amount of soil C recovered as light fraction material but also elevated mineralization during incubation compared with other DIRT treatments, suggesting that not all of the added soil C is directly stabilized. Aboveground needle litter additions, which are more N‐rich than wood debris, resulted in accelerated mineralization of previously stored soil carbon. In summary, our work demonstrates that the dominant plant sources of SOM differed substantially between forest types. Furthermore, inputs to and losses from soil C pools likely will not be altered uniformly by changes in litter input rates.     

Soil structural aspects of decomposition of organic matter by micro-organisms

Soil architecture is the dominant control over microbially mediated decomposition processes in terrestrial ecosystems. Organic matter is physically protected in soil so that large amounts of well-decomposable compounds can be found in the vicinity of largely starving microbial populations. Among the mechanisms proposed to explain the phenomena of physical protection in soil are adsorption of organics on inorganic clay surfaces and entrapment of materials in aggregates or in places inaccessible to microbes. Indirect evidence for the existence of physical protection in soil is provided by the occurrence of a burst of microbial activity and related increased decomposition rates following disruption of soil structures, either by natural processes such as the remoistening of a dried soil or by human activities such as ploughing. In contrast, soil compaction has only little effect on the transformation of ¹⁴C-glucose. Another mechanism of control by soil structure and texture on decomposition in terrestrial ecosystems is through their impact on microbial turnover processes. The microbial population is not only the main biological agent of decomposition in soil, it is also an important, albeit small, pool through which most of the organic matter in soil passes. Estimates on the relative importance of different mechanisms controlling decomposition in soil could be derived from results of combined tracer and modelling studies. However, suitable methodology to quantify the relation between soil structure and biological processes as a function of different types and conditions of soils is still lacking.     

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. ¹⁴C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural ¹³C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the ¹³C value of the roots represents the ¹³C value of the rhizosphere respired CO₂. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ¹³C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.     

Application of a rapid thin section method for observations on decomposing litter in mor humus form in a subalpine coniferous forest

Morphological changes in the decomposing litter ofAbies spp. andBetula spp. in a mor humus form were studied by a rapid thin section method. According to the morphological characteristics, the epidermis, mesophyll and vascular bundleof Abies needle litter were classified into four types: (i) newly fallen; (ii) slightly decomposed; (iii) moderately decomposed; and (iv) greatly decomposed. The distribution of these tissue types along the profile of the forest floor was then investigated. The morphological changes in other litter types, such as branches, scales andBetula leaves during decomposition were observed directly with microscope and electron microscope. Five vertical thin sections and 80 horizontal thin sections were used for these observations and investigations. the decomposition ofAbies litter was slower than that ofBetula litter. The relative decomposition rate of the tissues was in the order of: mesophyll>vascular bundle >epidermis inAbies needles; mesophyll≥epidermis>vascular bundles inBetula leaves; and inner bark >xylem>outer bark in bothAbies andBetula branches. The last remains of the litter were usually stomata, segments of seminiferous scale and outer bark ofAbies. The decomposition of plant litter occurred mainly within the L and F layers of the soil (0–5 cm in depth).Abies needles andBetula leaves completely disappeared at depths of 0–6 cm and 0–4 cm, respectively. Branches disappeared within the top of 5 cm and 6–8 cm forBetula and forAbies, respectively. The scales ofAbies were most slowly decomposed in the soil layers.     

Redox reactions of organic matter decomposition in a soft water lake

During a three year study (1985–1987) we used a mass balance approach to study the oxidation and reduction reactions related to decomposition of organic carbon in Mirror Lake, New Hampshire. The stoichiometry of the reactions allows us to calculate an electron transfer budget for the summer stratification period in the lake, as well as in benthic chambers and sealed jars.The average decomposition rate measured as dissolved inorganic carbon (DIC) production was 5.33 mmol m^–2 d^–1. The proportions of decomposition accounted for by the various electron acceptors varied both during the summer, as well as from year to year. On average, oxygen accounted for 43% of DIC production, while the processes involving sulfate, nitrate, iron and methane formation together accounted for 20%. Despite conservative assumptions we could not account for 37% of the DIC production. The general pattern, including excess DIC production, was also observed in chamber studies conducted over shallow-water sediments and in sealed-jar experiments.Data on burial rates of reduced iron minerals indicate that such minerals are not sufficient to account for the discrepancy in the electron budget. Our analysis suggests that another electron acceptor such as organic carbon reduction, either via fermentation or selective oxidation, is the most likely explanation of excess DIC production.     

Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus

In the present study, `natural <sup>13</sup>C tracer method' was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C<sub>3</sub> plant species, Helianthus annuus (sunflower), was grown in `C₄' soil in microcosms so that the CO₂ evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO₂ evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and <74%.     

Decomposition of 13C-labelled standard plant material in a latitudinal transect of European coniferous forests: Differential impact of climate on the decomposition of soil organic matter compartments

¹³C labelled plant material was incubated in situ over 2 to 3 years in 8 conifer forest soils located on acid and limestone parent material along a north-south climatic transect from boreal to dry Mediterranean regions in western Europe. The objectives of the experiment were to evaluate the effects of climate and the soil environment on decomposition and soil organic matter dynamics. Changes in climate were simulated using a north-to-south cascade procedure involving the relocation of labelled soil columns to the next warmer site along the transect.Double exponential, decay-rate functions (for labile and recalcitrant SOM compartments) vs time showed that the thermosensitivity of microbial processes depended on the latitude from which the soil was translocated. Cumulative response functions for air temperature, and for combined temperature and moisture were used as independent variables in first order kinetic models fitted to the decomposition data. In the situations where climatic response functions explained most of the variations in decomposition rates when the soils were translocated, the climate optimised decomposition rates for the local and the translocated soil should be similar. Differences between these two rates indicated that there was either no single climatic response function for one or both compartments, and/or other edaphic factors influenced the translocation effect. The most northern boreal soil showed a high thermosensitivity for recalcitrant organic matter compartment, whereas the labile fraction was less sensitive to climate changes for soils from more southern locations. Hence there was no single climatic function which describe the decay rates for all compartments. At the end of the incubation period it was found that the heat sum to achieve the same carbon losses was lower for soils in the north of the transect than in the south. In the long term, therefore, for a given heat input, decomposition rates would show larger increases in boreal northern sites than in warm temperate regions.The changes in climate produced by soil translocation were more clearly reflected by decomposition rates in the acid soils than for calcareous soils. This indicates that the physicochemical environment can have important differential effects on microbial decomposition of the labile and recalcitrant components of SOM.     

Bacterivorous grazers facilitate organic matter decomposition: a stoichiometric modeling approach

There is widespread empirical evidence that protist grazing on bacteria reduces bacterial abundances but increases bacteria-mediated decomposition of organic matter. This paradox has been noted repeatedly in the microbiology literature but lacks a generally accepted mechanistic explanation. To explain this paradox quantitatively, we develop a bacteria-grazer model of organic matter decomposition that incorporates protozoa-driven nutrient recycling and stoichiometry. Unlike previous efforts, the current model includes explicit limitation, via Liebig's law of minimum, by two possible factors, nutrient and carbon densities, as well as their relative ratios in bacteria and grazers. Our model shows two principal results: (1) when the environment is carbon limiting, organic matter can always be decomposed completely, regardless of the presence/absence of grazers; (2) when the environment is nutrient (such as nitrogen) limiting, it is possible for organic matter to be completely decomposed in the presence, but not absence, of grazers. Grazers facilitate decomposition by releasing nutrients back into the environment, which would otherwise be limiting, while preying upon bacteria. Model analysis reveals that facilitation of organic matter decomposition by grazers is positively related to the stoichiometric difference between bacteria and grazers. In addition, we predict the existence of an optimal density range of introduced grazers, which maximally facilitate the decomposition of organic matter in a fixed time period. This optimal range reflects a trade-off between grazer-induced nutrient recycling and grazer-induced mortality of bacteria.     

Reduction of the temperature sensitivity of soil organic matter decomposition with sustained temperature increase

The degree to which microbial communities adjust their decomposition of soil carbon over time in response to long-term increases in temperature is one of the key uncertainties in our modeling of the responses of terrestrial ecosystems to warming. To better understand changes in temperature sensitivity of soil microbial communities to long-term increases in soil temperature, we incubated 27 soils for one year with both short-term and long-term manipulations of temperature. In response to increasing temperature short-term from 20 to 30 °C, respiration rates increased more than threefold on average across soils. Yet, in response to long-term increases in temperature, respiration rates increased approximately half as much as they did to short-term increases in temperature. Short-term Q₁₀ of recalcitrant C correlated positively with long-term Q₁₀ measured between 10 and 20 °C, yet there was no relationship between short-term Q₁₀ and long-term Q₁₀ between 20 and 30 °C. In all, under laboratory conditions, it is clear that there is reduction in the temperature sensitivity of decomposition to long-term increases in temperature that disassociate short- and long-term responses of microbial decomposition to temperature. Determining the fate of soil organic matter to increased temperature will not only require further research on the controls and mechanisms of these patterns, but also require models to incorporate responses to both short-term and long-term increases in temperature.     

Decomposition of organic matter from 36 soils in a long-term pot experiment

The organic matter contents of thirty-six soils were measured annually for twenty years in a pot experiment. The soils originated mainly from arable land and varied in initial organic matter content, texture and pH. The soils were stored at an average air temperature of around 13 °C and every year each soil was mixed thoroughly. Throughout the experiment, soil moisture was kept between 50-70% of its water holding capacity. No organic matter was added during the experiment, so that gross soil organic matter decomposition could be assessed. Relative decomposition rates of soil organic matter decreased as time proceeded. Despite the wide range of soils studied, it was found that during the initial decades, the pattern of soil organic matter degradation was strongly correlated with the organic matter content of the soils at the start of the experiment. This means that during this period the time course of the organic matter content of the soils in our experiment can be estimated from the initial soil organic matter content alone.     

Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the ¹³C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if ¹³C discrimination during respiration could contribute to this pattern, we compared δ¹³C signatures of respired CO₂ from sieved mineral soil, litter layer and litterfall with measurements of δ¹³C and δ¹⁵N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in δ¹³C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As ¹³C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ¹³C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ¹³C of atmospheric CO₂ since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO₂ explains only a portion of the additional 1.0‰ increase in δ¹³C below a depth of 20 cm. The δ¹³C of the respired CO₂ was similar to that of the organic matter in the upper soil layers but became increasingly ¹³C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this ¹³C enrichment of the CO₂ as well as the residual increase in δ¹³C of the organic matter below a soil depth of 20 cm results from the increased contribution of ¹³C-enriched microbially derived C with depth. Our results suggest that ¹³C discrimination during microbial respiration does not contribute to the ¹³C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ¹³C of respired CO₂ are used to separate different components of soil respiration or ecosystem respiration.     

Impact of spruce forest and grass vegetation cover on soil micromorphology and hydraulic properties of organic matter horizon

Two organic matter horizons developed under a spruce forest and grass vegetation were chosen to demonstrate the impact of a different vegetation cover on the micromorphology, porous system and hydraulic properties of surface soils. Micromorphological studies showed that the decomposed organic material in the organic matter horizon under the grass vegetation was more compact compared to the decomposed organic material in the organic matter horizon under the spruce forest. The detected soil porous system in the organic matter horizon under the spruce forest consisted of two clusters of pores with different diameters that were highly connected within and between both clusters. The soil porous system in the organic matter horizon under the grass vegetation consisted of one cluster of pores with the larger diameters and isolated pores with the smaller diameter. The retention ability of the organic matter horizon under the grass vegetation was higher than the retention ability of the organic matter horizon under the spruce forest. Presented at the International Conference on Bioclimatology and Natural Hazards, Poľana nad Detvou, Slovakia, 17–20 September 2007.     

Rhizosphere priming effects on the decomposition of soil organic matter in C4 and C3 grassland soils

Using a natural abundance ¹³C method, soil organic matter (SOM) decomposition was studied in a C₃ plant – `C<sub>4</sub> soil' (C<sub>3</sub> plant grown in a soil obtained from a grassland dominated by C<sub>4</sub> grasses) system and a C<sub>4</sub> plant – `C₃ soil' (C₄ plant grown in a soil taken from a pasture dominated by C₃ grasses) system. In C₃ plant – `C<sub>4</sub> soil' system, cumulative soil-derived CO<sub>2</sub>–C were higher in the soils planted with soybean (5499 mg pot<sup>–1</sup>) and sunflower (4484 mg pot<sup>–1</sup>) than that in `C₄ soil' control (3237 mg pot^–1) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than `C<sub>4</sub> soil' control. In C<sub>4</sub> plant – `C₃ soil' system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO₂–C were lower in the soils planted with sorghum (2308 mg pot^–1) and amaranthus (2413 mg pot^–1) than that in `C<sub>3</sub> soil' control (2541 mg pot<sup>–1</sup>). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than `C₃ soil' control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C₃ plant – `C<sub>4</sub> soil' system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C<sub>4</sub> plant – `C₃ soil' system. Implications of rhizosphere priming effects on SOM decomposition were discussed.