Nutrient addition to soil can strongly affect interactions at the root-soil interface, which play a central role in terrestrial ecosystem functions. Controversies, however, remain on whether or not soils sequester more carbon (C) with nutrient addition (for example in the context of increasing atmospheric CO2 concentrations), and what is the role of plant traits and growth strategies in these impacts. In this study, we conducted a laboratory manipulation experiment focused on the effects of a grass (Lolium perenne L.) species, a legume (Lotus corniculatus L.) species, and their mixture with or without nutrient addition on plant biomass, root-derived respiration, soil organic matter (SOM) decomposition, and microbial community structure. L. perenne biomass and root-derived respiration were more responsive than L. corniculatus biomass and root-derived respiration to nutrient addition. The decomposition of SOM decreased, and the priming effect (PE) was negative, both with and without nutrient addition, in both plant species. Nutrient addition, however, impacted the magnitude of PE to a lesser extent in L. corniculatus than in L. perenne and in monocultures than in the mixture. With nutrient addition, fungi were more abundant and utilized a greater proportion of root-derived C than bacteria. In conclusion, the current study suggests that nutrient addition may promote a positive soil C-balance through reduced SOM decomposition, and that plant species mixtures with increased root-derived C-flow (representing root and rhizomicrobial respiration) suppressed SOM decomposition to a greater extent.

Both plant species and CO2 concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N-2-fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO(2)) and elevated (eCO(2)) CO2 levels. Plants were grown from seed, under continuous C-13-labelling atmospheres (400 and 800 mu mol mol(-1) CO2), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root- and soil-derived components. Root-derived DOC (0.53 +/- 0.34 and 0.26 +/- 0.29 mu g mL soil solution(-1)) and soil-derived CO2 (6.14 +/- 0.55 and 5.04 +/- 0.44 mu g CO2-C h(-1)) were on average two times and 22% higher at eCO(2) than at aCO(2), respectively. Plant species differed in exudate production at aCO(2) (0.11 +/- 0.11, 0.10 +/- 0.18, and 0.58 +/- 0.58 mu g mL soil solution(-1) for Plantago, Festuca, and Lotus, respectively) but not at eCO(2) (0.20 +/- 0.28, 0.66 +/- 0.32, and 0.75 +/- 0.15 mu g mL soil solution(-1) for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO2 levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO2 levels. A higher abundance of actinobacteria and G-positive bacteria occurred in unplanted (8.07 +/- 0.48 and 24.36 +/- 1.18 mol%) and Festuca-affected (7.63 +/- 0.31 and 23.62 +/- 0.69 mol%) soil than in Plantago- (7.04 +/- 0.36 and 23.41 +/- 1.13 mol%) and Lotus-affected (7.24 +/- 0.17 and 23.13 +/- 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.

We hypothesized that the rhizosphere priming effect (RPE) of soil organic matter by mutant barley lacking root hairs is dependant on a large network of symbiotic arbuscular mycorrhizal fungi (AMF). We thus predicted that fertilizing with phosphate-P would reduce AMF abundance and, in turn, reduce RPE of mutant barley. We packed microcosms with a P-responsive soil in which we grew mutant barley lacking root hairs as well as wild type barley and narrowleaf plantain, each possessing root hairs. One set of microcosms was fertilized with phosphate-P while another set was not fertilized. The plants were grown in a labelling chamber with 13C-depleted CO2. Soil respiration and δ13C of headspace CO2 were measured after 3, 4 and 5 weeks and RPE was calculated using an isotope mass balance approach. Root hair length was measured and soils were analyzed for the 16:1ω5 neutral lipid fatty acid (i.e. AMF biomarker). AMF abundance was greater, whereas RPE was lower, in mutant barley soil under low-P than under high-P conditions. In the other two plant-types, P had no effect on AMF or on RPE. As our results contradict our prediction, we propose an alternative explanation based on plant N demand under high-P.

Temperate forest soils are important carbon (C) sinks, where the C-stock is largely determined by the balance of leaf inputs and losses through respiration. However, studies dealing with leaf inputs to coniferous forest soils are limited although coniferous forests are widespread through the Northern temperate zone. In this study, we focused on the effects of soluble, insoluble and whole-tissue coniferous needle fractions on soil organic matter (SOM) decomposition and C storage in soil fractions. In addition, the effect of future increased C input was tested by applying a doubled amount of the soluble fraction (whole-tissue + soluble fraction). C-13-labelled needles were produced from spruce seedlings in growth chambers and needle fractions were added to the coniferous forest soil in laboratory microcosms. CO2 respired during incubation from the microcosms was partitioned into needle- and SOM-derived components. After seven months, soils were destructively harvested and analysed for C content in soil fractions and microbial community composition. The soluble, insoluble and whole-tissue fractions resulted in cumulative priming (increased SOM-derived CO2 relative to unamended controls) of 25 +/- 8%, 40 +/- 1%, and 39 +/- 7%, respectively. The doubled soluble-C addition caused a slightly lower priming (38 +/- 2%) than the whole-tissue fraction alone. The addition of needle fractions did not significantly affect the C content of soil fractions. However, the soluble fraction retained in soil was mainly found adsorbed onto mineral particles, whereas the insoluble and whole-tissue fractions occurred mainly as free particulate organic matter or adsorbed onto mineral particles. The insoluble and whole-tissue fraction led to increased fungal abundance and decreased abundance of G(+) bacteria and actinobacteria. All the fractions were primarily incorporated into fungal biomass after seven months suggesting that fungi were the main consumers of all needle fractions after the labile C had been depleted. When considering all the C gains and losses, the addition of all needle fractions resulted in net soil C increase. This suggests that, although the input of the coniferous needles leads to some C losses through the priming of SOM decomposition, these C losses are compensated by new C storage either in SOM fractions or microbial biomass.

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.

Rhizodeposition is an important source of substrate for microbial communities, supporting activities including soil organic matter (SOM) and nutrient cycling. Therefore, it is a potential trait of interest for crop plants, particularly in the context of variety selection for sustainable production systems. However, we do not have a good understanding of (i) whether there is significant variation in root-C deposition between varieties of important agricultural crops and (ii) whether variation in C deposition between varieties leads to major differences in C cycling in soil. In two experiments, we assessed variations in C deposition amongst barley genotypes and their respective impacts on microbial activity and SOM dynamics. In experiment 1, we applied 13C–CO2 labelling to selected barley recombinant chromosome substitution lines (RCSLs) and traced root-derived C in surface soil CO2 efflux, soil microbial biomass-C (MBC), soil solution, and soil particle-size fractions. In experiment 2, we conducted MicroResp analysis using 15 ecologically relevant C substrates to assess the impacts of barley genotypes on microbial activity. Soil respiration measurements (partitioned into plant- and SOM-derived components) revealed genotype-specific effects on plant-derived C, SOM-derived C and total C respired as CO2. For particle-size fractionation, we found that incorporation of plant-derived C to the silt-and-clay fraction varied between genotypes, indicating differences in relative stabilization of root-derived C as a result of barley genotype. Our data did not indicate genotype effects on total MBC size or dissolved organic-C (DOC) in soil solutions, but significant differences in plant-derived MBC and DOC were observed. MicroResp analysis showed differential utilization of 7 substrates (glucose, trehalose, lignin, arabinose, alanine, aminobutyric acid and lysine) revealing variation in community level physiological profiles (CLPPs) of soil microbes as impacted by barley genotypes. Furthermore, we found significant clustering of microbial CLPPs as a function of RCSLs and parent lines (Caesarea 26-24 and Harrington) suggesting a strong plant genetic control of the barley microbiome, and that this genetic control is heritable. Our results demonstrate barley genotype-specific effects on soil processes, revealing the potential for germplasm selection and variety improvement in barley to support sustainable production systems.

Soil organic matter ( SOM ) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO 2 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 (3 atm% 13 C) addition (in the range 0–1 mg glucose g −1 soil day −1 ) were applied for 8 days. Soil CO 2 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 (>0.5 mg 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.

Plant-bacterial interactions in the rhizosphere are important in mediating soil nutrient transformations. Plants supply carbon-rich substrates to rhizobacteria as root exudates and bacteria mobilise soil-bound phosphate for plant nutrition. This study aimed to probe the specificity of the plant effect on bacterial gene expression in P-starved rhizosphere conditions.

Microbial communities in soil are highly species-rich, recognition of which has led to the view that functional redundancy within communities may buffer many impacts of altered community structure on soil functions. In this study we investigated the impact of long-term (>50 years) exclusion of plant-inputs (bare-fallow treatment) on soil microbial community structure and on the ability of the microbial biomass to mineralise tracer additions of C-13-labelled plant-derived C-substrates. Exclusion of plant-inputs resulted in depletion of soil organic matter (SOM) and a reduction in microbial biomass size. The microbial community structure was also strongly affected, as indicated by the distinct phospholipid fatty acid (PLFA) profiles in bare-fallow and grassland soils. Mineralisation of labile plant-derived substrates was not perturbed by the bare-fallow treatment. The incorporation of labile plant-derived C into PLFA biomarkers was found to differ between soils, reflecting the distinct community structures of the soils and indicating that these substrates were utilised by a broad range of microbial groups. In contrast, the mineralisation of recalcitrant plant-derived substrates was reduced in bare-fallow soil and the fate of substrate-derived C within PLFA biomarkers was, initially, similar between the soils. These results indicate that utilisation of these recalcitrant substrates was a function restricted to specific groups, and that exclusion of plant-derived inputs to soil had reduced the capacity of bare-fallow microbial communities to utilise this substrate type. Therefore, the study suggests that long-term selective pressure on microbial communities, resulting in altered community structure, may also result in altered functional attributes. This structure function relationship was apparent for utilisation of recalcitrant plant-derived substrates, but not for the more widely distributed attribute of labile C-substrate utilisation. (C) 2011 Elsevier Ltd. All rights reserved.

Application of organic materials to agricultural soils offers the potential to divert these from conventional waste streams and to reduce the use of mineral fertilisers for crop production. In addition to direct impacts of organic materials on soil physico-chemical conditions, such amendments may also affect the structure and functioning of soil food webs and the coupling of plant- and soil biological activity. In this study we investigated the impact of cattle slurry and municipal green compost applications on soil microbial and nematode communities. Specifically, we applied steady-state C-13-labelling to quantify the impacts of the amendments on transfer of plant-derived C through components of the soil food web and to determine the activity of the soil biota in mineralising amendment- and soil organic matter derived-C. Both the slurry and compost amendments significantly promoted barley N-acquisition and growth. The municipal compost amendment, but not slurry application, strongly affected soil microbial community structure and increased nematode abundance, relative to unamended soil. This was associated with increased flux of plant-derived C through bacterial (particularly Gram negative) populations in compost-amended soil and was associated with increased mineralisation of amendment- and soil organic matter-derived substrates. For slurry-amended soils, the amendment did not alter soil community structure and mineralisation of soil C was not coupled to the flux of plant-derived C through below-ground pools. The results are indicative of N-supply to barley in compost-amended soil being dependent on the functioning of the soil food web (including C-flux to it from living roots), whereas N-supply from slurry was independent of this coupling between plant and soil processes. We suggest that plant/food web interactions are important targets for development of effective management strategies for organic waste applications to agricultural soils in the context of optimising the balance between nutrient supply to crops and the susceptibility of nutrients to losses from soil. (C) 2011 Elsevier B.V. All rights reserved.

Plant inputs of organic material to soil are thought to be key determinants of microbial activity, community composition and processes. However, the identity of organisms utilising these chemically diverse inputs is not well understood. In this study, we applied tracer amounts of highly enriched, 13C-labelled plant tissue fractions (whole, insoluble and soluble) to soil cores that either allowed or prevented access to roots and mycorrhizal fungi. For all tissue fractions, C derived from the additions was detected rapidly (<2 h after additions) in soil respiration. The additions did not alter microbial community structure, but their fate was strongly dependent on the addition type. The mineralisation of the soluble fraction was the most rapid and was recovered predominantly in bacterial PLFA biomarkers. In contrast, the insoluble addition was mineralised more slowly and recovered in fungal biomarkers to a greater extent. The presence of roots and/or mycorrhizas did not significantly affect rates of mineralisation or the biological fate of additions. A second harvest indicated that the distribution of substrate-derived C within the microbial community was still distinct (i.e. dependent on the form of addition) after 49 d, but with accumulation of 13C within the enchytraeid biomass. The results indicate that under the conditions of this experiment, transfer of C between microbial groups is relatively slow, suggesting that the range of chemical forms of plant inputs are likely important in maintaining microbial community structure in soils.

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.

The aims of the study were to determine group specificity in microbial utilization of root-exudate compounds and whole rhizodeposition; quantify the proportions of carbon acquired by microbial groups from soil organic matter and rhizodeposition, respectively; and assess the importance of root-derived C as a driver of soil microbial community structure. Additions of 13C-labelled root-exudate compounds to organic soil and steady-state labelling of Lolium perenne, coupled to compound-specific isotope ratio mass spectrometry, were used to quantify group-specific microbial utilization of rhizodeposition. Microbial utilization of glucose and fumaric acid was widespread through the microbial community, but glycine was utilized by a narrower range of populations, as indicated by the enrichment of phospholipid fatty acid (PLFA) analysis fractions. In L. perenne rhizospheres, high rates of rhizodeposit utilization by microbial groups showed good correspondence with increased abundance of these groups in the rhizosphere. Although rhizodeposition was not the quantitatively dominant C source for microbes in L. perenne rhizospheres, relative utilization of this C source was an important driver of microbial group abundance in organic soil.

Root proliferation as a response to exploit zones of nutrient enrichment in soil has been demonstrated for a wide range of plant species. However, the effectiveness of this as a strategy to acquire nutrients is also dependent on interactions with the soil microbial community. Specifically, C-flow from roots modifies microbial activity and probably the balance between nutrient mineralization and immobilization processes in the rhizosphere. In this study, near-natural abundance 13C-labelling and gene-reporter methods were applied to determine the effects of uneven nitrate supply to roots of Hordeum vulgare on assimilate partitioning and root exudation. Plants were initially grown in uniform nitrate supply in split-root, sand microcosms after which one treatment continued to receive uniform supply, and the other received nitrate to one root compartment only. At the time of imposing the treatments, the CO2 supplied to the plants was switched to a cylinder source, providing a distinct delta13C-signature and allowing the fate of new assimilate within the plants to be determined. The labelling approach allowed quantification of the expected preferential allocation of new C-assimilate to roots in enriched nitrate, prior to any measurable effect on whole biomass or root architecture. Biosensor (lux-marked Pseudomonas fluorescens 10586 pUCD607) bioluminescence, quantified spatially by CCD imaging, demonstrated that root exudation was significantly increased for roots in enriched nitrate. This response of root exudation, being primarily associated with root apices and concurrent with enhanced assimilate supply, strongly suggests that C-flow from roots is an integral component of the proliferation response to nitrate.

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of 14 C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of 14 C to roots. In parallel, N fertilisation increased the root soluble 14 C and the 14 C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).