Soil Organic Matter Priming Research Articles

Combined effects of rhizodeposit C and crop residues on SOM priming, residue mineralization and N supply in soil

Fluxes of rhizodeposit carbon (C) to soil stimulate microbial activity affecting soil organic matter (SOM) decomposition and, in turn, nutrient fluxes in soil. In agricultural soils, residues from previous crops also have major impacts on SOM and nutrient cycling, and their turnover by microbes is likely to be indirectly impacted by rhizodeposition. However, the combined effects of rhizodeposit C and inputs of C from dead plant materials in soil on native SOM decomposition are unclear. In this study, we assessed (i) the individual and combined effects of barley rhizodeposition and ryegrass root residue inputs (as a model for residue input from previous crop) on SOM mineralization, (ii) the intraspecies variation within barley in impacting residue mineralization, and (iii) whether genotypes that stimulate high mineralization rates of plant residues in soil also directly benefit through increased nutrient uptake from these residues. We continuously applied 13C depleted CO2 to selected barley recombinant chromosome substitution lines (RCSLs) to trace the flow of barley root-derived C in surface soil CO2 efflux, soil microbial biomass and soil particle-size fractions. In addition, 13C and 15N enriched ryegrass root residues were mixed into soil to trace the mineralization of residue-derived C and the residue-derived nitrogen (N) uptake by plants. Our results show (i) genotype-specific variation in impacting total soil CO2 efflux and its component sources: SOM-derived C, barley root-derived C and/or ryegrass residue-derived C, (ii) residue effects on total C and SOM-derived C respired as CO2, (iii) genotype-residue combined effects on SOM primed C, that were very similar to the sum of primed C caused by planting or residue addition alone (except for the last sampling date), and (iv) that plant uptake of residue released N between genotypes was linked to genotype impacts on residue mineralization. These results suggest that impacts of plant rhizodeposition and residue inputs had additive effects on SOM priming. Furthermore, these results demonstrate, for the first time, genotype differences in impacting the mineralization of recent plant-derived organic materials in soil, and reveal that this process directly contributes to plant nutrition.

Bacterial carbon use plasticity, phylogenetic diversity and the priming of soil organic matter.

Microorganisms perform most decomposition on Earth, mediating carbon (C) loss from ecosystems, and thereby influencing climate. Yet, how variation in the identity and composition of microbial communities influences ecosystem C balance is far from clear. Using quantitative stable isotope probing of DNA, we show how individual bacterial taxa influence soil C cycling following the addition of labile C (glucose). Specifically, we show that increased decomposition of soil C in response to added glucose (positive priming) occurs as a phylogenetically diverse group of taxa, accounting for a large proportion of the bacterial community, shift toward additional soil C use for growth. Our findings suggest that many microbial taxa exhibit C use plasticity, as most taxa altered their use of glucose and soil organic matter depending upon environmental conditions. In contrast, bacteria that exhibit other responses to glucose (reduced growth or reliance on glucose for additional growth) clustered strongly by phylogeny. These results suggest that positive priming is likely the prototypical response of bacteria to sustained labile C addition, consistent with the widespread occurrence of the positive priming effect in nature.

Critical comparison of the impact of biochar and wood ash on soil organic matter cycling and grassland productivity

Wood represents the single most important source of renewable energy worldwide and depending on the mechanism of energy production can lead to the production of by-products with vastly different properties (i.e. wood ash (WA) from incineration and biochar (BC) from pyrolysis). These are typically applied to land, however, a critical comparison of their impact on soil quality and carbon (C) cycling is lacking. To address this, we generated biochar (450 °C) and wood ash (870 °C) from the same mixed hardwood feedstock and added it to an agricultural grassland at comparable rates under both laboratory and field conditions (10 t ha−1 and 571 kg ha−1 for BC and WA, respectively). We hypothesized that alkaline, nutrient-rich wood ash would stimulate microbial activity, resulting in the loss of soil organic matter (SOM), while biochar which is recalcitrant to microbial attack would promote the stabilization of native SOM. The effects on the soil microbial community and soil C and N cycling were determined over 1 year. Overall, biochar promoted soil quality by enhancing nutrient availability (P and K), moisture retention and increasing soil C content. However, it was also associated with an increase in below-ground CO2 loss. As plant productivity was unaffected and laboratory incubations of biochar with 14C-labelled SOM showed no indication of priming, we deduce that this CO2 originates from the biochar itself. This is supported by the lack of effect of biochar on soil N cycling, microbial biomass and community structure. Wood ash had almost no effect on either soil quality or vegetation quality (yield and foliar nutrient content) under field conditions but did induce negative SOM priming under both laboratory and field conditions. We conclude that when applied at field-relevant rates, neither amendment had a detrimental effect on native SOM cycling. While wood ash promotes the retention of native SOM, biochar may be a better strategy for enhancing SOM levels because of its intrinsic recalcitrant character, however, this needs to be offset against the reduced amount of energy derived from pyrolysis in comparison to incineration.

Winter ecology of a subalpine grassland: Effects of snow removal on soil respiration, microbial structure and function

Seasonal snow cover provides essential insulation for mountain ecosystems, but expected changes in precipitation patterns and snow cover duration due to global warming can influence the activity of soil microbial communities. In turn, these changes have the potential to create new dynamics of soil organic matter cycling. To assess the effects of experimental snow removal and advanced spring conditions on soil carbon (C) and nitrogen (N) dynamics, and on the biomass and structure of soil microbial communities, we performed an in situ study in a subalpine grassland in the Austrian Alps, in conjunction with soil incubations under controlled conditions. We found substantial winter C-mineralisation and high accumulation of inorganic and organic N in the topsoil, peaking at snowmelt. Soil microbial biomass doubled under the snow, paralleled by a fivefold increase in its C:N ratio, but no apparent change in its bacteria-dominated community structure. Snow removal led to a series of mild freeze-thaw cycles, which had minor effects on in situ soil CO2 production and N mineralisation. Incubated soil under advanced spring conditions, however, revealed an impaired microbial metabolism shortly after snow removal, characterised by a limited capacity for C-mineralisation of both fresh plant-derived substrates and existing soil organic matter (SOM), leading to reduced priming effects. This effect was transient and the observed recovery in microbial respiration and SOM priming towards the end of the winter season indicated microbial resilience to short-lived freeze-thaw disturbance under field conditions. Bacteria showed a higher potential for uptake of plant-derived C substrates during this recovery phase. The observed temporary loss in microbial C-mineralisation capacity and the promotion of bacteria over fungi can likely impede winter SOM cycling in mountain grasslands under recurrent winter climate change events, with plausible implications for soil nutrient availability and plant-soil interactions.

Priming of soil organic matter: Chemical structure of added compounds is more important than the energy content

The addition of easily degradable compounds to soil (e.g. root exudates, plant residues) can result in priming effects (PE), a short-term change in the turnover of soil organic matter (SOM). Although PE are recognized to be large enough to be taken into account into the ecosystem carbon balance, the exact mechanisms are still unknown. Here, we examined the effect of two characteristics of added compounds on PE, namely metabolic usable energy for microbes and resemblance to recalcitrant fractions of soil organic matter (SOM). For this purpose, glucose, cellobiose (energy rich compounds, low resemblance with recalcitrant SOM) and vanillic acid (energy-poor compound, higher resemblance with recalcitrant SOM) were selected. In addition the effect of mineral nitrogen (N) on PE was tested. 13C labelled compounds were mixed with sandy soil from an ex-arable site. To separate the effect of energy content from that of resemblance to SOM, the amount of carbon and the amount of energy content of added compounds was kept constant in treatments, respectively. The community structure of microbes that were able to use added compounds was evaluated using stable isotope probing (DNA-SIP) combined with qPCR and Illumina sequencing.When corrected for energy content, vanilic acid induced the highest CO2 respiration and PE. DNA-SIP revealed that bacterial classes like β- and γ-Proteobacteria, that are known to harbour many opportunistic bacteria, responded quickly (5 h) with incorporation of 13C from added substrates, whereas classes like Acidobacteria and Actinobacteria responded over a longer incubation time. In treatments where the energy-level of added compounds was kept constant, vanillic acid caused an increase in DNA copy numbers of bacteria and fungi using native SOM after prolonged incubation. The contribution of fungi to PE was minor, reflecting the low F:B ratio of the soil used for the experiment. Different substrates resulted in different PE but appeared to stimulate the growth of similar bacterial groups. This suggests that the added compounds stimulate different enzyme systems within similar bacterial taxa. Although combined addition of mineral nitrogen (ammonium nitrate) and organic compounds caused a slightly extra increase in PE in most treatments, this might be an artefact as addition of mineral N only decreased respiration. Overall our results indicate that the effect of chemical structure of added compounds on PE is much larger than the effect of energy-content.

Microbial decomposition of soil organic matter is mediated by quality and quantity of crop residues: mechanisms and thresholds

Crop residue quality and quantity have contrasting effects on soil organic matter (SOM) decomposition, but the mechanisms explaining such priming effect (PE) are still elusive. To reveal the role of residue quality and quantity in SOM priming, we applied two rates (5.4–10.8 g kg−1) of 13C-labeled wheat residues (separately: leaves, stems, roots) to soil and incubated for 120 days. To distinguish PE mechanisms, labeled C was traced in CO2 efflux and in microbial biomass and enzyme activities (involved in C, N, and P cycles) were measured during the incubation period. Regardless of residue type, PE intensity declined with increasing C additions. Roots were least mineralized but caused up to 60% higher PE compared to leaves or stems. During intensive residue mineralization (first 2–3 weeks), the low or negative PE resulted from pool substitution. Thereafter (15–60 days), a large decline in microbial biomass along with increased enzyme activity suggested that microbial necromass served as SOM primer. Finally, incorporation of SOM-derived C into remaining microbial biomass corresponded to increased enzyme activity, which is indicative of SOM cometabolism. Both PE and enzyme activities were primarily correlated with residue-metabolizing soil microorganisms. A unifying model demonstrated that PE was a function of residue mineralization, with thresholds for strong PE increase of up to 20% root, 44% stem, and 51% leaf mineralization. Thus, root mineralization has the lowest threshold for a strong PE increase. Our study emphasizes the role of residue-feeding microorganisms as active players in the PE, which are mediated by quality and quantity of crop residue additions.

An invasive wetland grass primes deep soil carbon pools.

Understanding the processes that control deep soil carbon (C) dynamics and accumulation is of key importance, given the relevance of soil organic matter (SOM) as a vast C pool and climate change buffer. Methodological constraints of measuring SOM decomposition in the field prevent the addressing of real-time rhizosphere effects that regulate nutrient cycling and SOM decomposition. An invasive lineage of Phragmites australis roots deeper than native vegetation (Schoenoplectus americanus and Spartina patens) in coastal marshes of North America and has potential to dramatically alter C cycling and accumulation in these ecosystems. To evaluate the effect of deep rooting on SOM decomposition we designed a mesocosm experiment that differentiates between plant-derived, surface SOM-derived (0-40cm, active root zone of native marsh vegetation), and deep SOM-derived mineralization (40-80cm, below active root zone of native vegetation). We found invasive P.australis allocated the highest proportion of roots in deeper soils, differing significantly from the native vegetation in root:shoot ratio and belowground biomass allocation. About half of the CO2 produced came from plant tissue mineralization in invasive and native communities; the rest of the CO2 was produced from SOM mineralization (priming). Under P.australis, 35% of the CO2 was produced from deep SOM priming and 9% from surface SOM. In the native community, 9% was produced from deep SOM priming and 44% from surface SOM. SOM priming in the native community was proportional to belowground biomass, while P.australis showed much higher priming with less belowground biomass. If P.australis deep rooting favors the decomposition of deep-buried SOM accumulated under native vegetation, P.australis invasion into a wetland could fundamentally change SOM dynamics and lead to the loss of the C pool that was previously sequestered at depth under the native vegetation, thereby altering the function of a wetland as a long-term Csink.

Biochar stimulates the decomposition of simple organic matter and suppresses the decomposition of complex organic matter in a sandy loam soil

AbstractIncorporating crop residues and biochar has received increasing attention as tools to mitigate atmospheric carbon dioxide (CO2) emissions and promote soil carbon (C) sequestration. However, direct comparisons between biochar, torrefied biomass, and straw on both labile and recalcitrant soil organic matter (SOM) remain poorly understood. In this study, we explored the impact of biochars produced at different temperatures and torrefied biomass on the simple C substrates (glucose, amino acids), plant residues (Lolium perenne L.), and native SOM breakdown in soil using a 14C labeling approach. Torrefied biomass and biochars produced from wheat straw at four contrasting pyrolysis temperatures (250, 350, 450, and 550 °C) were incorporated into a sandy loam soil and their impact on C turnover compared to an unamended soil or one amended with unprocessed straw. Biochar, torrefied biomass, and straw application induced a shift in the soil microbial community size, activity, and structure with the greatest effects in the straw‐amended soil. In addition, they also resulted in changes in microbial carbon use efficiency (CUE) leading to more substrate C being partitioned into catabolic processes. While overall the biochar, torrefied biomass, and straw addition increased soil respiration, it reduced the turnover rate of the simple C substrates, plant residues, and native SOM and had no appreciable effect on the turnover rate of the microbial biomass. The negative SOM priming was positively correlated with biochar production temperature. We therefore ascribe the increase in soil CO2 efflux to biochar‐derived C rather than that originating from SOM. In conclusion, the SOM priming magnitude is strongly influenced by both the soil organic C quality and the biochar properties. In comparison with straw, biochar has the greatest potential to promote soil C storage. However, straw and torrefied biomass may have other cobenefits which may make them more suitable as a CO2 abatement strategy.

Quantifying Charcoal Degradation and Negative Priming of Soil Organic Matter with a 14C-Dead Tracer

AbstractConverting biomass to charcoal produces physical and chemical changes greatly increasing environmental recalcitrance, leading to great interest in the potential of this carbon form as a long-term sequestration strategy for climate change mitigation. Uncertainty remains, however, over the timescale of charcoal’s environmental stability, with estimates varying from decadal to millennial scales. Uncertainty also remains over charcoal’s effect on other aspects of carbon biogeochemical cycling and allied nutrient cycles such as nitrogen. Radiocarbon is a powerful tool to investigate charcoal mineralization due to its sensitivity; here we report the results of a study using 14C-dead charcoal (pMC=0.137±0.002) in organic-rich soil (pMC=99.76±0.46), assessing charcoal degradation over 55 days of incubation. Using this method, we discriminated between decomposition of indigenous soil organic matter (SOM) and charcoal by microorganisms. SOM was the major source of carbon respired from the soil, but there was also a contribution from charcoal carbon mineralization. This contribution was 2.1 and 1.1% on days 27 and 55, respectively. We also observed a negative priming effect due to charcoal additions to soil, where SOM mineralization was repressed by up to 14.1%, presumably arising from physico-chemical interactions between soil and charcoal.

The solubility of carbon inputs affects the priming of soil organic matter

Aims We investigated the extent water soluble and insoluble (hexane soluble) plant residue fractions in influence the priming of soil organic carbon (SOC).

Wetting-drying cycles do not increase organic carbon and nitrogen mineralization in soils with straw amendment

Increasing soil organic matter (SOM) is of primary importance for maintaining soil fertility and mitigating climate change. Leaving crop residues on top of soil is not always an efficient means of increasing SOM because (i) of the high mineralization of the crop residues, (ii) crop residues may increase the mineralization of existing SOM (priming effect) and (iii) wetting-drying cycles may increase mineralization of SOM and crop residues. Little research has been carried out into these mechanisms under Sudano-Sahelian conditions where the rainfall is mostly irregular with wetting-drying cycles during the transition between the wet and the dry season. To evaluate the effect of wetting-drying cycles on the mineralization of SOM and crop residues and the priming effect, an agricultural soil from the North Region of Cameroon with or without (controls) 13C-labeled rice straw amendment as crop residues was either subjected to five wetting-drying cycles or maintained at constant water potential after a single rewetting event. Soil samples were incubated for 70days at 28°C and the CO2 and 13CO2 emissions and mineral N were monitored.Adding straw (+833μgCg−1 soil) increased the cumulative CO2 emissions from the soil (+921μgC-CO2g−1soil). A positive priming effect was observed (+92μgC-CO2g−1 soil). Only the first wetting cycle created a mineralization flush of the SOM and the straw. This flush did not recur probably because of a lack of labile SOM. However an extra addition of straw after 28days increased CO2 emissions but did not result in further mineralization flushes after re-wetting while SOM mineralization was not limited by N availability. We conclude that SOM depletion under Sudano-Sahelian conditions was not explained by SOM mineralization enhancement due to multiple rewetting events or to priming effect following crop residues addition. Indeed, SOM depletion could rather be explained by a high level of mineralization of both, SOM and crop residues, when the soil reached its water retention capacity.

Do rhizosphere priming effects enhance plant nitrogen uptake under elevated CO2?

Numerous studies suggest that rhizosphere priming can mediate decomposition of soil organic matter (SOM), but direct evidence of priming-induced soil SOM decomposition on plant N uptake under elevated CO2 (eCO2) is very rare. By using a continuous dual labelling technique with 13C-depleted CO2 and 15N-enriched soil, we investigated priming of SOM decomposition and its relationship with plant N uptake of C4 and C3 grasses from a grassland ecosystem under eCO2. We observed that eCO2 induced increases in plant biomass, plant N uptake, rhizosphere priming, and total SOM decomposition in both grasses at an early plant life stage. Increased total SOM decomposition was positively related with plant N uptake by both C4 and C3 grasses under eCO2. However, the C3 grass was more dependent on N acquired from rhizosphere priming of SOM than the C4 grass. Our findings highlight that plant N uptake could be enhanced under eCO2 via accelerated SOM decomposition, and rhizosphere priming effects on SOM decomposition could play a more important role in N availability of the C3 grass in comparison with the C4 grass.

Quality of fresh organic matter affects priming of soil organic matter and substrate utilization patterns of microbes.

Changes in biogeochemical cycles and the climate system due to human activities are expected to change the quantity and quality of plant litter inputs to soils. How changing quality of fresh organic matter (FOM) might influence the priming effect (PE) on soil organic matter (SOM) mineralization is still under debate. Here we determined the PE induced by two 13C-labeled FOMs with contrasting nutritional quality (leaf vs. stalk of Zea mays L.). Soils from two different forest types yielded consistent results: soils amended with leaf tissue switched faster from negative PE to positive PE due to greater microbial growth compared to soils amended with stalks. However, after 16 d of incubation, soils amended with stalks had a higher PE than those amended with leaf. Phospholipid fatty acid (PLFA) results suggested that microbial demand for carbon and other nutrients was one of the major determinants of the PE observed. Therefore, consideration of both microbial demands for nutrients and FOM supply simultaneously is essential to understand the underlying mechanisms of PE. Our study provided evidence that changes in FOM quality could affect microbial utilization of substrate and PE on SOM mineralization, which may exacerbate global warming problems under future climate change.

The effect of rice straw on the priming of soil organic matter and methane production in peat soils

Rice residue management often leads to increased methane (CH4) emissions but the outcomes of edaphic and management factors are not always predictable. Rice residue can act as a substrate for CH4 production; however the role it plays in priming (mineralization) of soil organic matter (SOM) to release additional substrates for CH4 production are not well established. We anaerobically incubated a highly organic soil with 13C-enriched rice straw for 3 months to investigate its priming effect (PE) on SOM and source of C for CH4 production. Anaerobic decomposition of SOM was accompanied by iron (Fe) reduction with minimal CH4 production when straw was absent. Straw addition enhanced Fe reduction and increased CH4 production concurrently with a clear succession of microbial community structure and function assessed with phospholipid fatty acid (PLFA) profiling. The PE on CH4 production from SOM was strong and positive during the entire experiment. Overall, PE on SOM (CO2 plus CH4 production) was slightly positive at the end of the experiment, associated with only a 32% mineralization of the added straw-C (as CO2 plus CH4). Straw addition also released large amounts of dissolved organic carbon (DOC) from SOM. Our results suggest that straw addition effects on PE of SOM and CH4 production can last for a long period of time showing that straw will cause non-linear response in CH4 production and potentially result in significant losses of soil C as DOC by leaching or direct exports in histosols.

Effect of biochar addition on C mineralisation and soil organic matter priming in two subsoil horizons

The impact of biochar on subsoil organic carbon mineralisation has never been assessed despite its susceptibility to downward transport after soil amendment. In this study, we analysed the potential mineralisation of biochar and plant material as well as their effect on native soil organic matter (SOM) decomposition in subsoil horizons. We used 13C-labelled biochar and plant material to allow disentangling substrate mineralisation and priming effects. The substrates were added to two mountain subsoils under different land use and incubated for 1 year under optimum conditions. We analysed for physical parameters and C mineralisation in the two soils. Moreover, microbial communities were assessed by phospholipid fatty acid (PLFA) analyses. Our results indicated contrasting potential C mineralisation of subsoils under different land use probably related to sampling depth, contribution of stabilised organic matter compounds, carbon content as well as quality. In general, very low proportions of biochar were mineralised in the two soils as compared to plant material. The mineralisation of each of the added substrates (biochar and plant material) was slightly, but significantly different in the two soils. Native C mineralisation was much higher after plant material addition than after biochar addition. Subsoil type influenced the kind and magnitude of priming effects for both added substrates. Biochar mineralisation and its priming effects in subsoil are small as compared to uncharred plant litter. We suggest that substrate mineralization and priming effects induced on subsoil organic matter are dependent on the composition of the added substrate, as well as soil parameters rather than microbial community characteristics.

Input of easily available organic C and N stimulates microbial decomposition of soil organic matter in arctic permafrost soil

Rising temperatures in the Arctic can affect soil organic matter (SOM) decomposition directly and indirectly, by increasing plant primary production and thus the allocation of plant-derived organic compounds into the soil. Such compounds, for example root exudates or decaying fine roots, are easily available for microorganisms, and can alter the decomposition of older SOM (“priming effect”). We here report on a SOM priming experiment in the active layer of a permafrost soil from the central Siberian Arctic, comparing responses of organic topsoil, mineral subsoil, and cryoturbated subsoil material (i.e., poorly decomposed topsoil material subducted into the subsoil by freeze–thaw processes) to additions of 13C-labeled glucose, cellulose, a mixture of amino acids, and protein (added at levels corresponding to approximately 1% of soil organic carbon). SOM decomposition in the topsoil was barely affected by higher availability of organic compounds, whereas SOM decomposition in both subsoil horizons responded strongly. In the mineral subsoil, SOM decomposition increased by a factor of two to three after any substrate addition (glucose, cellulose, amino acids, protein), suggesting that the microbial decomposer community was limited in energy to break down more complex components of SOM. In the cryoturbated horizon, SOM decomposition increased by a factor of two after addition of amino acids or protein, but was not significantly affected by glucose or cellulose, indicating nitrogen rather than energy limitation. Since the stimulation of SOM decomposition in cryoturbated material was not connected to microbial growth or to a change in microbial community composition, the additional nitrogen was likely invested in the production of extracellular enzymes required for SOM decomposition. Our findings provide a first mechanistic understanding of priming in permafrost soils and suggest that an increase in the availability of organic carbon or nitrogen, e.g., by increased plant productivity, can change the decomposition of SOM stored in deeper layers of permafrost soils, with possible repercussions on the global climate.

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ13C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ13C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of 13CO2 to maize (Zea mays L.). Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested. The δ13C abundance before and after the continuous labeling was −24.20 ± 0.05 ‰ and −23.80 ± 0.05 ‰ in the rooted bulk soil. The labeling caused increases in δ13C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ13C abundance was enriched in the >2 mm and 1–2 mm aggregates (−23.17 ± 0.12 ‰ and −23.26 ± 0.05 ‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ13C abundance in pore water (−24.0 ± 0.01 ‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ13C abundance was in general greater in the horizontal cell than in the vertical cell. The δ13C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (−23.66 ± 0.11 ‰ and −23.5 ± 0.10 ‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ13C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell. Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.

Impact of future climatic conditions on the potential for soil organic matter priming

Terrestrial carbon (C) storage and turnover are of major interest under changing climatic conditions. We present a laboratory microcosm study investigating the effects of anticipated climatic conditions on the soil microbial community and related changes in soil organic matter (SOM) decomposition. Soil samples were taken from a heath-land after six years of exposure to elevated carbon dioxide (eCO2) in combination with summer drought (D) and increased temperature (T). Soil C-dynamics were investigated in soils from: (i) ambient, (ii) eCO2, and (iii) plots exposed to the combination of factors simulating future climatic conditions (TDeCO2) that simulate conditions predicted for Denmark in 2075. 13C enriched glucose (3 atom% excess) was added to soil microcosms, soil CO2 efflux was measured over a period of two weeks and separated into glucose- and SOM-derived C. Microbial biomass was measured using chloroform fumigation extraction, and compound-specific phospholipid fatty acid analysis was used to determine microbial community composition and substrate use. We observed that glucose additions induced SOM priming in ambient and eCO2 treated soils, but not in soil exposed to future climatic conditions. Climate treatments and glucose additions did not affect relative abundances of microbial functional groups but the fate of glucose through the microbial community was changed by climate treatments as revealed by the incorporation of 13C in PLFAs. Soil treated with eCO2 showed a high flow of glucose through gram-positive bacteria whereas in ambient and future soils utilization of glucose by actinomycetes and fungi (putative SOM-decomposers) was greater. Our results suggest that individual climate change factors may influence pathways of C-flux through microbial communities and therefore affect soil processes; these factors may counterbalance each other and maintain ecosystem stability. This highlights the importance of studying climate change factors in combination to fully assess consequences of environmental change on plant–soil systems.

Microbial utilisation of biochar-derived carbon

Whilst largely considered an inert material, biochar has been documented to contain a small yet significant fraction of microbially available labile organic carbon (C). Biochar addition to soil has also been reported to alter soil microbial community structure, and to both stimulate and retard the decomposition of native soil organic matter (SOM). We conducted a short-term incubation experiment using two 13C-labelled biochars produced from wheat or eucalypt shoots, which were incorporated in an aridic arenosol to examine the fate of the labile fraction of biochar-C through the microbial community. This was achieved using compound specific isotopic analysis (CSIA) of phospholipid fatty acids (PLFAs). A proportion of the biologically-available fraction of both biochars was rapidly (within three days) utilised by gram positive bacteria. There was a sharp peak in CO2 evolution shortly after biochar addition, resulting from rapid turnover of labile C components in biochars and through positive priming of native SOM. Our results demonstrate that this CO2 evolution was at least partially microbially mediated, and that biochar application to soil can cause significant and rapid changes in the soil microbial community; likely due to addition of labile C and increases in soil pH.

Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO2 concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C-inputs to soil (e.g. plant-derived) can significantly affect mineralization of SOM ('priming effects') complicates prediction of environmental and land-use change effects on SOM dynamics and soil C-balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3atm% (13) C) addition (in the range 0-1mg glucose g(-1) soil day(-1) ) were applied for 8days. Soil CO2 efflux was partitioned into SOM- and glucose-derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass ('apparent priming') were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5mg glucose g(-1) soil), priming was a soil-specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C-models. However, these response functions are soil-specific and may be affected by several interacting factors at lower addition rates.