Nitrogen (N) deposition to soils is globally rising, but its effect on soil organic carbon (SOC) turnover is still uncertain. Moreover, common theories of stoichiometric decomposition and microbial N ...mining predict opposing effects of N supply on SOC turnover. We hypothesized that the effect of N deposition on SOC turnover depends on initial soil nutrient conditions. Thus, we sampled tropical forests and rubber gardens with pronounced gradients of nutrient availability from the topsoil to the deep subsoil (up to 400 cm) and measured substrate-induced respiration (SIR) for 30 days in four treatments (C, CN, CP, CNP additions). A natural 13C abundance approach was conducted to quantify priming effects (PE) of the added C on SOC mineralization. For this purpose we assessed the 13CO2 isotope composition after adding a C4 sugar to the C3 soil; to correct for isotopic fractionation a treatment with C3 sugar additions served as control. We found that nutrient additions to topsoil did neither alter cumulative CO2 release within 30 days (SIRacc) nor PE (PE = 1.6, i.e., sugar additions raised the release of SOC-derived CO2 by a factor of 1.6). In the upper subsoil (30–100 cm), however, both CN and CP additions increased SIRacc (by 239% and 92%, respectively) and the PE (PE = 5.2 and 3.3, respectively) relative to the treatments that received C only (PE = 1.7), while CNP additions revealed the largest increase of SIRacc (267%) and PE (PE = 6.0). In the deep subsoil (>130 cm depth), only the CNP addition consistently increased SIRacc (by 871%) and PE (PE = 5.2) relative to only C additions (PE = 2.0). We conclude that microbial activity was not limited by nutrients in the topsoil but was co-limited by both N and P in the subsoil. The results imply that microbes mine nutrients from previously unavailable pools under the conditions that 1) deficiency actually exists, 2) co-limitation is alleviated, and 3) nutrient reserves are present. Yet, as opposed to microbial nutrient mining theories, we showed that the subsoil PE is highest when nutrient supply matches microbial demand. As a result also N deposition might exert variable effects on SOC turnover in tropical soils: it might have no effect in nutrient-rich topsoils and in co-limited subsoils without P reserves but might increase SOC turnover in co-limited subsoils with potentially acquirable P reserves.
•Microbes are able to compensate for single nutrient deficiency by N or P mining.•Nutrient mining is constrained by co-limitation or absence of nutrient reserves.•Priming effects are largest in subsoils when nutrient supply matches microbial demand.•Soil nutrient conditions regulate the effect of N deposition on SOC turnover.
Carbon use efficiency (CUE) describes the relative partitioning of carbon (C) between anabolic and catabolic processes within the soil microbial community. Further, it represents a major factor ...regulating the amount of C cascading through the trophic levels of the soil food web. How CUE relates to C supply, however, remains poorly understood. The primary aim of this study was to determine how CUE varies across a range of spatial scales as a function of C substrate supply. Our secondary aim was to understand how variations in substrate CUE influences the interpretation of community level physiological profiles (CLPP). Using 16 different 14C-labelled substrates (including amino acids, sugars, organic acids and amino sugars) and soils collected at the field, regional and continental scale, we measured the rate of substrate uptake and mineralization from which we calculated CUE. Across all soils (n = 114) and substrates (n = 16), the average CUE for the microbial community was 0.568 ± 0.004 (range 0.492–0.794). While the partitioning of substrate-C within the biomass (immobilization/mineralization) over 72 h was highly conserved for some substrates (e.g. glucose), others showed a wide variability in CUE across the samples (e.g. valine). In the context of the CLPP methodology, we showed that individual sites could be statistically separated from each other, irrespective of whether the statistical analysis was based on microbial substrate uptake rate or mineralization rate. However, our results do suggest that caution is needed when ascribing observed CLPP differences to the importance of individual C pathways operating in soil due to the wide variation of CUE between substrates. In conclusion, we present new mechanistic evidence to support the paradigm that variation in ecosystem CUE may in part reflect differences in the types of C supplied to the microbial biomass.
•Microbial CUE varied greatly between C substrates.•CUE was highly conserved across soils for some C substrates, but not for others.•CUE does not need to be accounted for to separate soils based on CLPP.•CUE does need to be accounted for when interpreting C use in CLPP.
To study vegetation feedbacks of nutrient addition on carbon sequestration capacity, we investigated vegetation and ecosystem CO2 exchange at Mer Bleue Bog, Canada in plots that had been fertilized ...with nitrogen (N) or with N plus phosphorus (P) and potassium (K) for 7–12 years. Gross photosynthesis, ecosystem respiration, and net CO2 exchange were measured weekly during May–September 2011 using climate‐controlled chambers. A substrate‐induced respiration technique was used to determine the functional ability of the microbial community. The highest N and NPK additions were associated with 40% less net CO2 uptake than the control. In the NPK additions, a diminished C sink potential was due to a 20–30% increase in ecosystem respiration, while gross photosynthesis rates did not change as greater vascular plant biomass compensated for the decrease in Sphagnum mosses. In the highest N‐only treatment, small reductions in gross photosynthesis and no change in ecosystem respiration led to the reduced C sink. Substrate‐induced microbial respiration was significantly higher in all levels of NPK additions compared with control. The temperature sensitivity of respiration in the plots was lower with increasing cumulative N load, suggesting more labile sources of respired CO2. The weaker C sink potential could be explained by changes in nutrient availability, higher woody : foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N‐only fertilization for both shrub biomass production and decomposition suggest that the bog ecosystem is N‐P/K colimited rather than N‐limited. Negative effects of further N‐only deposition were indicated by delayed spring CO2 uptake. In contrast to forests, increased wood formation and surface litter accumulation in bogs seem to reduce the C sink potential owing to the loss of peat‐forming Sphagnum.
The extensive use of silver nanoparticles (AgNPs) in consumer and medical products leads inevitably to release of such particles into environment and soil resources. This study was conducted to ...provide evidences for biological effects of AgNPs in two calcareous soils with different textures and salinity levels. Basal respiration (BR) and substrate-induced respiration (SIR), as indicators of soil microbial activity and biomass, respectively, were determined in the calcareous soils spiked with a dilution series of AgNPs and AgNO3. Urease and alkaline phosphatase activities were also measured in the spiked and control soils. Finally, dose–response approach was used to model the sensitivity of the soil biological properties to AgNPs and AgNO3 contamination. The results revealed that the effects of AgNPs and AgNO3 on the soil respiration and enzyme activities depended on Ag dose and soil type. For instance, soil respiration was not affected or even stimulated by low doses of AgNPs and AgNO3, but negatively affected by high doses (>20mg Ag kg−1). Soil urease and phosphatase activities were generally inhibited in the presence of AgNPs and AgNO3, though in low Ag concentrations there was no inhibition or even stimulation. Generally, the ecological dose (ED) values of AgNPs were smaller than those of AgNO3, suggesting that AgNPs have more negative effects than AgNO3 on the soil microbial and enzyme activities, at the same level of Ag dose. The results also revealed that suppression of microbial and enzyme activities by AgNPs and Ag ions are greater in the soil with lower clay content and ionic strength.
•AgNPs and AgNO3 significantly affected soil urease and phosphatase activities•AgNPs had more negative effects than Ag+ on soil enzyme activities•AgNPs and Ag+ had no effect or even stimulated soil respiration at low concentrations•AgNPs and Ag+ negatively affected SIR at high concentrations (≥10mg Ag kg−1)•soil urease and phosphatase activities were more sensitive than respiration to Ag
Forest fires can alter the biological properties of soils. There is increasing evidence that fires cause a shift in soil microbial communities, which play a central role in forest carbon and nutrient ...cycling. In this study, we evaluate the effect of soil heating on soil microbial functions. We hypothesised that fire reduces the catabolic functional diversity of soil, and that post‐fire plant growth enhances its recovery. To test this, we experimentally heated a forest soil at 200°C (T200) or 450°C (T450). Heated and unheated soils were then incubated in tubs with or without live grass (Lolium perenne L.). We determined the functional profiles by measuring the substrate‐induced respiration (SIR) using the Microresp™ technique and analysed nutrient availability at the end of the incubation. At both temperatures, soil heating altered the respiration responses to substrate additions and the catabolic functional diversity of soils. Functional diversity was initially reduced in T200 soils but recovered at the end of the incubation. In contrast, T450 soils initially maintained the catabolic functional diversity, but decreased at the end of the incubation. Heating‐induced nutrient availability stimulated the growth of grass, which in turn increased the response to several substrates and increased the functional diversity to values similar to the unheated controls. Our results suggest that fire‐driven alteration of soil microbial communities has consequences at a functional level, and that the recovery of plant communities enhances the recovery of soil microbial functions.
Highlights
Soil experimental heating altered microbial functions and reduced soil functional diversity.
Soil heating also increased nutrient availability, enhancing plant growth.
Growth of plants promoted the recovery of soil functional diversity.
Post‐fire recovery of functional diversity may be related to the recovery of photosynthetic tissues.
Microbial and enzyme functioning depends on the quality of substrates, which strongly differ in bare soil and in the hotspots of microbial activity such as the rhizosphere and detritusphere. We ...established a field experiment to determine the effects of contrasting substrate quality, namely, soil organic matter, maize shoot litter (detritusphere) and maize rhizodeposits (rhizosphere) on microorganisms and their extracellular enzymes in an arable soil. Kinetic parameters (Vmax and Km) of four hydrolytic extracellular enzymes: β-cellobiohydrolase, β-glucosidase, acid phosphate and β-xylosidase were analyzed in 0–10 and 10–20 cm to elucidate the effects of substrate content on substrate affinity and catalytic efficiency (Vmax/Km). Living roots increased microbial biomass by 179% and microbial respiration by 100% compared to fallow soil. Lower enzyme affinities to substrates (e.g. 93% for β-glucosidase) in rooted soil pointed to the domination of r-strategists, which are favored in the decomposition of labile organics common in the rhizosphere. No differences in catalytic properties of cellulolytic enzymes were detected between bulk and litter-treated soil, indicating the recalcitrance of organics in both treatments. The rhizosphere and detritusphere effects on enzyme kinetics were negligible in 10–20 cm, except β-glucosidase. The reduction of Km of all enzymes in 10–20 cm versus the upper 10 cm indicated increasing substrate affinity with depth. Nonetheless, the catalytic efficiency increased from 0 to 10 to 10–20 cm (e.g. up to 420% for acid phosphatase), reflecting changes in properties and functioning of enzymatic systems. This pointed to a shift towards a more K-selected microbial community with higher affinity and more efficient substrate utilization. It also indicated the microbial adaptation to decreasing substrate contents with depth by altered enzyme functioning. Overall, the catalytic properties of cellulolytic enzymes were much more strongly affected by plants (substrate quality in the rhizosphere and detritusphere compared to bare fallow) than by depth (substrate content).
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•Living roots increased microbial biomass by 179% and microbial respiration by 100% in rooted topsoil compared to fallow soil.•The catalytic efficiency (Vmax/Km) of acid phosphatase was higher in rhizosphere and detritusphere compared to fallow.•The Km reduction of all enzymes in 10–20 cm versus the upper 10 cm indicated increasing substrate affinity with depth.•The catalytic efficiency increased from 0–10 to 10–20 cm for β-glucosidase, acid phosphatase and β-xylosidase.
Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO2 can cause a shift in the structure and activity of the microbial community responsible for the ...turnover of organic matter in soil. We investigated the long‐term effect of elevated CO2 in the atmosphere on microbial biomass and specific growth rates in root‐free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (μ) were determined using the substrate‐induced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher μ were observed under elevated vs. ambient CO2, depending on site, plant species and N fertilization. The μ‐values increased linearly with atmospheric CO2 concentration at all three sites. The effect of elevated CO2 on rhizosphere microorganisms was plant dependent and increased for: Brassica napus=Triticum aestivum<Beta vulgaris<Populus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The μ‐value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO2 was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO2. We conclude that the more abundant available organics released by roots at elevated CO2 altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast‐growing r‐selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO2 concentration.
Soil enzymes secreted by microorganisms play a critical role in nutrient cycling, soil structure maintenance, and crop production. However, understanding of the linkage between soil enzyme kinetics ...and microbial metabolism and active microbial communities is remarkably limited. In this study, we measured the kinetics of three hydrolase enzymes, active microbial abundance and substrate-induced respiration (SIR) from 21 farmlands differing in their fertilities collected from the Loess Plateau, China. Results showed the high fertility soils had higher total organic carbon (TOC) and nutrient contents, potential microbial activity, the colony-forming units (CFU) of actinomycetes, and values of enzyme Vmax and Km than those of low fertility soils. We also observed that the CFU of fungi and other bacterial groups did not change with soil fertility status. Soil chemical properties explained 74.0% of the variance in Vmax and 28.3% of the variance in Km, respectively. Whereas, the abundance of main microbial groups and fungi/bacteria ratio only explained 10.2% and 7% of the variance of Vmax and Km, respectively. The interactive effect of soil properties and microbial community could explain 20.2% of the variance in Km. Our results suggest that the substrate availability would mainly drive enzyme kinetics compared to the abundance of active/potentially active microbes in the farmland soils.
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•Soil enzyme kinetics (SEK) were mainly driven by organic C and nutrient contents.•SEK rarely correlated with the active/potentially active main microbial groups.•SEK could predict the substrate induced respiration.
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.
Schematic presentation of expected mid-winter snow removal effects on soil microclimate and associated microbial activity. Indicated are the four a priori chosen sampling dates for the analyses of carbon and nitrogen cycling in the soil and the two soil incubations under advanced spring conditions in the presence of fresh plant-derived substrates. Continuous measurements of soil respiration integrate the overall microbial activity dynamics under the snow. Display omitted
•Snow cover is essential for alpine ecosystems, but it diminishes with climate change.•How do reduced snow cover and advanced growing season affect soil microbial activity?•We studied C and N cycling under snow in relation to microbial communities and activity.•Snow removal and advanced spring decreased SOM mineralisation and microbial biomass.•Preferential bacterial growth may interfere with nitrogen availability for plants.
Increasing temperatures may alter the stoichiometric demands of soil microbes and impair their capacity to stabilize carbon (C) and retain nitrogen (N), with critical consequences for the soil C and ...N storage at high latitude soils. Geothermally active areas in Iceland provided wide, continuous and stable gradients of soil temperatures to test this hypothesis. In order to characterize the stoichiometric demands of microbes from these subarctic soils, we incubated soils from ambient temperatures after the factorial addition of C, N and P substrates separately and in combination. In a second experiment, soils that had been exposed to different in situ warming intensities (+0, +0.5, +1.8, +3.4, +8.7, +15.9 °C above ambient) for seven years were incubated after the combined addition of C, N and P to evaluate the capacity of soil microbes to store and immobilize C and N at the different warming scenarios. The seven years of chronic soil warming triggered large and proportional soil C and N losses (4.1 ± 0.5% °C−1 of the stocks in unwarmed soils) from the upper 10 cm of soil, with a predominant depletion of the physically accessible organic substrates that were weakly sorbed in soil minerals up to 8.7 °C warming. Soil microbes met the increasing respiratory demands under conditions of low C accessibility at the expenses of a reduction of the standing biomass in warmer soils. This together with the strict microbial C:N stoichiometric demands also constrained their capacity of N retention, and increased the vulnerability of soil to N losses. Our findings suggest a strong control of microbial physiology and C:N stoichiometric needs on the retention of soil N and on the resilience of soil C stocks from high-latitudes to warming, particularly during periods of vegetation dormancy and low C inputs.
•Warming triggered large and proportional C and N losses from these subarctic soils.•Weakly sorbed organic substrates in soil minerals were depleted predominantly.•Warmed soils were able to sustain a lower microbial biomass.•Strict microbial C:N stoichiometric demands also constrained N retention.•This impaired soil N storage and increased its vulnerability to C losses.
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