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In order to understand the coupling of carbon (C) and nitrogen (N) cycles, it is necessary to understand C and N-use efficiencies of microbial soil organic matter (SOM) decomposition. While important controls of those efficiencies by microbial community adaptations have been shown at the scale of a soil pore, an abstract simplified representation of community adaptations is needed at ecosystem scale. Therefore we developed the soil enzyme allocation model (SEAM), which takes a holistic, partly optimality based approach to describe C and N dynamics at the spatial scale of an ecosystem and time-scales of years and longer. We explicitly modelled community adaptation strategies of resource allocation to extracellular enzymes and enzyme limitations on SOM decomposition. Using SEAM, we explored whether alternative strategy-hypotheses can have strong effects on SOM and inorganic N cycling. Results from prototypical simulations and a calibration to observations of an intensive pasture site showed that the so-called revenue enzyme allocation strategy was most viable. This strategy accounts for microbial adaptations to both, stoichiometry and amount of different SOM resources, and supported the largest microbial biomass under a wide range of conditions. Predictions of the holistic SEAM model were qualitatively similar to precitions of the SYMPHONY model, which explicitly represents competing microbial guilds. With adaptive enzyme allocation under conditions of high C/N ratio of litter inputs, N that was formerly locked in slowly degrading SOM pools was made accessible, whereas with high N inputs, N was sequestered in SOM and protected from leaching. The findings imply that it is important for ecosystem scale models to account for adaptation of C and N use efficiencies in order to represent C-N couplings. The combination of stoichiometry and optimality principles is a promising route to yield simple formulations of such adaptations at community level suitable for incorporation into land surface models. [ABSTRACT FROM AUTHOR]

Abstract: Forest ecosystems in Europe have been affected by human activities for many centuries. Here we investigate, if current forest soil organic carbon stocks are influenced not only by present ecological conditions and land use, but also by land management in the past. Based on the forest management history of the Hainich-Dün region a total of 130 inventory plots were selected in age-class forest and selectively cut forests under present management practice. The age-class forest originated from (1) former coppice-with-standards, (2) former selectively cut forests and (3) afforestation. The selectively cut forest contains “early regulated” forest where selective cutting has been practised for centuries, and forest, which was managed as coppice-with-standards through the 18th and the 19th centuries. We hypothesise that past management influences present soil organic carbon stocks. Density fractionation of soils in three physical fractions (HF: heavy fraction, o-LF: occluded light fraction, f-LF: free light fraction) was carried out to increase the probability of detecting long-lasting effects of management history. No detectable differences in soil organic carbon (SOC) stocks, as measured in kgm−2 ground area, of the mineral soil and the heavy fractions, were found between present and historical forest management types (average total organic carbon (OC) stocks of mineral soil: 9.7±2.3kgm−2; average OC stocks of the organic layer: 0.5±0.3kgm−2; average total inorganic carbon (IC) stocks of mineral soil: 5.0±3.7kgm−2). The variation of samples was overlapping. There was no consistent trend with management history. The upper mineral soil (0–30cm) contained about 74% of total SOC, with f-LF contributing 24% in 0–10cm and 20% in 10–30cm, and o-LF 9% in 0–10cm and 6% in 10–30cm. The HF contained 85% (0–10cm) and 86% (10–30cm) of SOC stocks in the bulk soil. There was a significant decrease of total SOC stocks in the 0–10 and 10–30cm depth increment with increasing abundance of beech. Mean 14C concentrations in the HF were 102.0 pMC in 0–10cm, and 93.4 pMC in 10–30cm, corresponding to a mean 14C age of around 100years and 550years, respectively. Modelling C-dynamics based on the present measurements reveals that disturbances depleting 50% of soil C-stocks would equilibrate after 80years. Thus, there is no memory effect of 19th century forest management. We conclude that past and present management has no detectable effect on present SOC. [Copyright &y& Elsevier]

At the lower parts of the forest belt at Mt. Kilimanjaro, selective logging has led to a mosaic of mature forest, old secondary forests (∼60 years), and old clearings (∼10 years) covered by shrub vegetation. These variations in the vegetation are reflected by differences in nutrient leaching from the canopy and in both amount and quality of litter reaching the ground, thereby also influencing mineralization rates and the composition of seepage water in litter percolate and soil solution. The aim of this study was to investigate how above-and belowground nutrient dynamics vary between regeneration stages, and if forest regeneration at the clearings is hampered by a deterioration of abiotic site conditions. K, Mg, Ca, Na and N compounds were analysed in rainfall, throughfall, organic layer percolate and the soil solution to a depth of 1. 00 m at three clearings, three secondary forest and four mature forest sites. Element fluxes via throughfall showed only small variations among regeneration stages except for K and NO3-N. With 57–83 kg ha−1 a−1and 2. 6–4. 1 kg ha−1 a−1 respectively, K and NO3-N fluxes via throughfall were significantly higher at the clearings than at the mature forest sites (32–37 and 0. 7–1. 0 kg ha−1 a−1 for K and NO3-N). In organic layer percolate and in soil solution at 0. 15-m soil depth, concentrations of K, Mg, Ca and N were highest at the clearings. In the organic layer percolate, median K concentrations were e.g. 7. 4 mg l−1 for the clearings but only 1. 4 mg l−1 for the mature forests, and for NO3-N, median concentrations were 3. 1 mg l−1 for the clearings but only 0. 92 mg l−1 for the mature forest sites. Still, differences in annual means between clearings and mature forests were not always significant due to a high variability within the clearings. With the exception of NO3-N, belowground nutrient concentrations in secondary forests ranged between concentrations in mature forests and clearings. Vegetation type-specific differences decreased with increasing soil depths in the soil solution. Overall, the opening of the forest led to a higher spatial and seasonal variation of nutrient concentrations in the seepage water. These results suggest differences in both mineralization rates and in nutrient budgeting at different regeneration stages. Since nutrient availability was highest at the clearings and no compaction of the soil was observed, deterioration of soil properties did not seem to be the main reason for the impeded regeneration on the clearings. [Copyright &y& Elsevier]

Soils are the largest terrestrial organic carbon pool and the largest terrestrial source of atmospheric CO 2. Non-phototrophic CO 2 fixation by microbes re-fixes and recycles CO 2 respired in soils. Our previous study showed that in temperate deciduous forest soil profiles, rates of dark CO 2 fixation were proportional to microbial biomass, irrespective of soil depth. However, the amount and quality of organic matter entering different soil depths vary for different temperate forest types and these influence microbial communities with unknown consequences for CO 2 fixation rates. To test whether differences in the amount and quality of SOC caused by tree species affect dark CO 2 fixation rates with depth, we conducted a study using acidic soils from two forest plots from the Schorfheide-Chorin Exploratory, Germany. These soils, dominated by either beech (deciduous) or pine (coniferous) tree stands differ in their SOC content and quality. We traced the incorporation of 2% (v:v) 13C–CO 2 label into microbial biomass and estimated the CO 2 fixation rates relative to microbial biomass carbon content (in μg C g MBC−1 d−1) across the soil profiles. The rates of dark CO 2 fixation per g MBC were similar across the beech soil profiles, but significantly lower in the pine soils at the B2 and BC horizons, suggesting that while dark CO 2 fixation rates are linked to MBC in deciduous forest soils, other factors influence dark CO 2 fixation rates in coniferous forest soils. The pine subsoils had low SOC content and quality, with a microbial community enriched with heterotrophic fermenters like Chloroflexi that are predicted to have a lower potential for heterotrophic CO 2 fixation compared to the other dominant bacteria phyla. In contrast, the beech soil profiles, characterized by higher SOC inputs, featured higher fractions of copiotrophs like Proteobacteria, Acidobacteria, and Actinobacteria predicted to have high heterotrophic CO 2 fixation potential. We thus speculate that in contrast to the beech soils, lower SOC inputs in the pine subsoils affected the community composition, leading to lower CO 2 fixation rates. We further made comparisons to soils dominated by mixed deciduous trees and featuring a higher MBC and SOC content. Over this range of temperate forest soils, CO 2 fixation rates were highest in the mixed deciduous forest soils, with MBC and Shannon index (used as a proxy parameter for community composition) showing the strongest correlations with the varying CO 2 fixation rates. Our study suggests that predictions of dark CO 2 fixation rates need to consider tree-species specific or site-specific conditions, as these may alter root carbon inputs and affect microbial community composition and their metabolic CO 2 fixation potentials in soil. • Dark CO 2 fixation rates correlate with MBC in beech soil profiles. • In pine subsoils, CO 2 fixation rates per gram MBC are lower than in beech soils. • Lower rates in pine subsoils reflect distinct subsoil communities and SOC inputs. • Comparing 3 temperate forests, microbial communities control CO 2 fixation rates. • SOC & vegetation effects on the community explain their control on fixation rates. [ABSTRACT FROM AUTHOR]

Mineral surfaces in soil, where fungal communities contribute to the formation and turnover of mineral-associated OM (MAOM), are important interfaces for organic matter (OM) and nutrient cycling. The significance and contribution of secondary minerals such as goethite (iron oxide) or illite (clay mineral) to fungal community structure and functioning under natural conditions in forest soils remains elusive. We placed mineral containers filled with mixtures of secondary minerals (either goethite or illite) and quartz-sand in 30 forest topsoils across three regions in Germany for five years. The mineral samples in the containers were separated from the surrounding soil with 50-μm mesh barriers. Mineral and surrounding soil samples were analyzed for abundance and community composition of saprotrophic and ectomycorrhizal fungi based on phospholipid fatty acid profiles and amplicon sequencing, and for enzyme activities (β-glucosidase, β-xylosidase, N-acetylglucosaminidase, and acid phosphatase). Compared to the surrounding soil, fungal communities in the carbon-poor mineral samples were less diverse and communities were distinct. The community composition of saprotrophic fungi was affected by mineral type, likely reflecting variability in the minerals' capacities to bind OM. Higher relative microbial enzymatic nutrient acquisition was associated with a shift from saprotrophic towards ectomycorrhizal fungi, indicating a tight link between trophic fungal groups and their distinct functional roles in the mineralosphere. At a larger scale, we observed strong site effects, suggesting that environmental filters such as physico-chemical soil properties, tree species, and region shape the structuring of fungal communities more strongly than the specific mineralosphere conditions. • Minerals were buried in forests for five years to study mineral-fungal associations. • Mineral type affected saprotrophic but not ectomycorrhizal fungi. • Fungal colonization was driven by organic matter sorption. • Fungal trophic guild shifts were linked to enzymatic nutrient acquisition. • Environmental filters shaped fungal communities more than mineral type. [ABSTRACT FROM AUTHOR]

Soils substantially contribute to the terrestrial fluxes of CO 2 to the atmosphere. Dark CO 2 fixation, the microbial process by which pore space CO 2 is reduced to organic matter, may recycle and trap some of the CO 2 respired in soils before it can escape to the atmosphere. To evaluate its potential significance for global temperate forest soil carbon stocks, we quantified dark CO 2 fixation rates in a temperate beech forest soil down to 1 m depth over a range of 2–20% (v:v) headspace CO 2 concentrations, by tracing incorporation of a13C–CO 2 label into microbial biomass carbon and soil organic matter. We found that fixation rates under a concentration of 2% CO 2 decreased with depth from 0.86 to 0.06 μg C normalized to g(dw) soil−1 d−1. However, when dark CO 2 fixation rates were normalized to soil microbial biomass carbon, no significant differences between depths were observed. Higher CO 2 concentrations increased fixation rates, with a linear 2-fold increase between 2% and 10% CO 2. Molecular analysis revealed the dominance of heterotrophs, along with the presence of autotrophs mainly employing the Calvin Benson Bassham (CBB) pathway followed by the reductive citric acid (rTCA) pathway. Although community composition varied with depth, the relative fraction of autotrophs determined by qPCR of RuBisCO (cbbL IA, cbbL IC) and ATP-citrate lyase (aclA) genes remained stable at approximately 0.5% of the total community. Dark CO 2 fixed carbon accounted for up to 1.1% of microbial biomass carbon and up to 0.035% of soil organic carbon after 28 days. We estimated a fixation flux of 25 ± 7.2 g C m−2 yr−1 to 1 m depth for the Hainich forest soil under field conditions. Without this process, Hainich forest soil CO 2 emissions would be 5.6% higher, recycling a fraction of carbon large enough to potentially affect carbon isotope signatures in SOC. If this is held for all temperate forest soils globally, the annual rate of dark CO 2 fixation would be 0.26 ± 0.07 Pg C yr−1 to a depth of 1 m, without considering contributions from other biomes. In conclusion, microbial biomass carbon and CO 2 concentration appear to be the main drivers of dark CO 2 fixation in temperate forest soils, and dark CO 2 fixation may maintain Hainich forest soil carbon stocks by moderating a significant fraction of soil CO 2 emissions annually. • Rates of dark CO 2 fixation correlate with microbial biomass carbon down to 1 m soil depth. • Dark CO 2 fixation rates increase linearly with CO 2 concentrations between 2 and 10%. • Without an estimated CO 2 fixation of 25 ± 7.2 g C m−2 yr−1, Hainich forest soil CO 2 emission could be 5.6% higher. [ABSTRACT FROM AUTHOR]

Abstract Functionally, ectomycorrhizal (ECM) and saprotrophic (SAP) fungi belong to different guilds, and they play contrasting roles in forest ecosystem C-cycling. SAP fungi acquire C by degrading the soil organic material, which precipitates massive CO 2 release, whereas, as plant symbionts, ECM fungi receive C from plants representing a channel of recently assimilated C to the soil. In this study, we aim to measure the amounts and identify the drivers of ECM and SAP fungal biomass in temperate forest topsoil. To this end, we measured ECM and SAP fungal biomass in mineral topsoils (0–12 cm depth) of different forest types (pure European beech, pure conifers, and mixed European beech with other broadleaf trees or conifers) in a range of about 800 km across Germany; moreover, we conducted multi-model inference analyses using variables for forest and vegetation, nutritive resources from soil and roots, and soil conditions as potential drivers of fungal biomass. Total fungal biomass ranged from 2.4 ± 0.3 mg g−1 (soil dry weight) in pure European beech to 5.2 ± 0.8 mg g−1 in pure conifer forests. Forest type, particularly the conifer presence, had a strong effect on SAP biomass, which ranged from a mean value of 1.5 ± 0.1 mg g−1 in broadleaf to 3.3 ± 0.6 mg g−1 in conifer forests. The European beech forests had the lowest ECM fungal biomass (1.1 ± 0.3 mg g−1), but in mixtures with other broadleaf species, ECM biomass had the highest value (2.3 ± 0.2 mg g−1) among other forest types. Resources from soil and roots such as N and C concentrations or C:N ratios were the most influential variables for both SAP and ECM biomass. Furthermore, SAP biomass were driven by factors related to forest structure and vegetation, whereas ECM biomass was mainly influenced by factors related to soil conditions, such as soil temperature, moisture, and pH. Our results show that we need to consider a complex of factors differentially affecting biomass of soil fungal functional groups and highlight the potential of forest management to control forest C-storage and the consequences of changes in soil fungal biomass. Highlights • Saprotrophic and ectomycorrhizal fungal biomass vary with forest tree composition. • Nutritive resources from soil and roots are the main drivers of fungal biomass. • Forest-related factors primarily influence the saprotrophic fungal biomass. • Soil conditions mainly influence ectomycorrhizal fungal biomass. [ABSTRACT FROM AUTHOR]

Mineral surfaces in soil are an important interface for organic matter (OM) and nutrient cycling, with associated microorganisms contributing to the formation and turnover of mineral-associated OM (MAOM). However, the relevance of intrinsic (mineral type) versus extrinsic (land-use intensity) factors on the co-development of MAOM and microorganisms under natural conditions remains poorly understood. Mineral containers filled with mixtures of quartz-sand and pristine secondary minerals (goethite or illite) were exposed to 50 grassland topsoils of the Schwäbische Alb (Germany) along a land-use intensity gradient for five years. Mineral samples and soils were analyzed for organic carbon (OC) and nutrients (N and P), abundance and composition of major microbial groups based on phospholipid fatty acid profiles, as well as enzyme activities (β-glucosidase, β-xylosidase, N-acetylglucosaminidase, and acid phosphatase). Microorganisms colonized both mineral samples to the same extent, with goethite samples exhibiting greater MAOM accumulation and higher enzyme activities than illite samples. Both mineral samples differed from the overlying soils with greater relative abundances of fungi and Gram-negative bacteria and greater microbial acquisition of nutrients (N and P) relative to C as indicated by the stoichiometry of enzyme activities. Increasing land-use intensity was associated with decreasing C:N ratios and microbial abundances for goethite samples and increasing β-glucosidase activity for illite samples while the proportion of fungi was reduced in both mineral samples. We conclude that in the studied temperate grasslands the association of OM and microorganisms with secondary minerals is driven more by mineral type and reactivity than by differences in land-use intensity. The different minerals apparently formed distinct microhabitats with unique characteristics that differed in MAOM accumulation and microbial access to OC and nutrients, thus affecting microbial colonization and functionality. • Pristine minerals were buried in grasslands of varying land-use intensity to study co-development of MAOM and microbes. • Mineral type (goethite, illite) affected OC-normalized microbial biomass, fungi to bacteria ratios and enzyme activities. • Minerals were preferentially colonized by fungi and Gram-negative bacteria. • Mineral-associated microbes invested relatively more into nutrient than C acquisition. • Land-use intensity and surrounding soil properties were less important for microbial colonization than mineral type. [ABSTRACT FROM AUTHOR]

Land-use change and intensification play a key role in the current biodiversity crisis. The resulting species loss can have severe effects on ecosystem functions and services, thereby increasing ecosystem vulnerability to climate change. We explored whether land-use intensification (i.e. fertilization intensity), plant diversity and other potentially confounding environmental factors may be significantly related to water use (i.e. drought stress) of grassland plants. Drought stress was assessed using δ 13 C abundances in aboveground plant biomass of 150 grassland plots across a gradient of land-use intensity. Under water shortage, plants are forced to increasingly take up the heavier 13 C due to closing stomata leading to an enrichment of 13 C in biomass. Plants were sampled at the community level and for single species, which belong to three different functional groups (one grass, one herb, two legumes). Results show that plant diversity was significantly related to the δ 13 C signal in community, grass and legume biomass indicating that drought stress was lower under higher diversity, although this relation was not significant for the herb species under study. Fertilization, in turn, mostly increased drought stress as indicated by more positive δ 13 C values. This effect was mostly indirect by decreasing plant diversity. In line with these results, we found similar patterns in the δ 13 C signal of the organic matter in the topsoil, indicating a long history of these processes. Our study provided strong indication for a positive biodiversity-ecosystem functioning relationship with reduced drought stress at higher plant diversity. However, it also underlined a negative reinforcing situation: as land-use intensification decreases plant diversity in grasslands, this might subsequently increases drought sensitivity. Vice-versa, enhancing plant diversity in species-poor agricultural grasslands may moderate negative effects of future climate change. [ABSTRACT FROM AUTHOR]

Profiles of soil organic carbon (SOC) are often characterized by a steep increase of 14C age with depth, often leading to subsoil 14C ages of more than 1000 years. These observations have generally been reproduced in SOC models by introducing a SOC pool that decomposes on the time-scale of millennia. The overemphasis of chemical recalcitrance as the major factor for the persistence of SOC was able to provide a mechanistic justification for these very low decomposition rates. The emerging view on SOC persistence, however, stresses that apart from molecular structure a multitude of mechanisms can lead to the long-term persistence of organic carbon in soils. These mechanisms, however, have not been incorporated into most models. Consequently, we developed the SOC profile model COMISSION which simulates vertically resolved SOC concentrations based on representations of microbial interactions, sorption to minerals, and vertical transport. We calibrated COMISSION using published concentrations of SOC, microbial biomass and mineral-associated OC (MOC), and in addition, 14C contents of SOC and MOC of a Haplic Podzol profile in North-Eastern Bavaria, Germany. In order to elucidate the contribution of the implemented processes to the 14C age in different parts of the profile, we performed model-experiments in which we switched off the limitation of SOC decomposition by microbes, sorptive stabilization on soil minerals, and dissolved OC (DOC) transport. By splitting all model pools into directly litter-derived carbon and microbe-derived organic carbon, we investigated the contribution of repeated microbial recycling to 14C ages throughout the profile. The model-experiments for this site lead to the following implications: Without rejuvenation by DOC transport, SOC in the subsoil would be on average 1700 14C years older. Across the profile, SOC from microbial recycling is on average 1400 14C years older than litter-derived SOC. Without microbial limitation of depolymerization, SOC in the subsoil would be on average 610 14C years younger. Sorptive stabilization is responsible for relatively high 14C ages in the topsoil. The model-experiments further indicate that the high SOC concentrations in the Bh horizon are caused by the interplay between sorptive stabilization and microbial dynamics. Overall, the model-experiments demonstrate that the high 14C ages are not solely caused by slow turnover of a single pool, but that the increase of 14C ages along a soil profile up to ages >1000 years is the result of different mechanisms contributing to the overall persistence of SOC. The dominant reasons for the persistence of SOC are stabilization processes, followed by repeated microbial processing of SOC. [ABSTRACT FROM AUTHOR]

The environmental conditions under which the availability of inorganic nutrients such as nitrogen (N) and phosphorus (P) influence soil microbial growth are poorly understood, especially with regards to how fertilization changes specific aspects of microbial growth such as carbon-use efficiency (CUE). Microbial CUE is the fraction of C converted into biomass out of all C taken in and plays a critical role in global C budgets. Using the 18O labeled water method we tested short vs. long-term effects of N and/or P fertilization on microbial growth, CUE, and C, N, and P-acquiring enzyme activities in two soils from an oak-savanna, which differ in their soil organic matter (SOM) content. We hypothesized that soils with more SOM (from under tree canopies) would have higher microbial growth rates than soils with less SOM (from open grassland), and that microbial growth and CUE would increase with fertilization. We further hypothesized that these increases would be associated with a decrease in enzyme activity and a shift towards older SOM substrates in the short-term, in contrast to substrates from recently fixed C resulting from increased plant productivity in the long-term. We found that nutrient additions did not affect microbial growth or CUE in the relatively high SOM habitat on either time scale. In contrast, the low SOM habitat had lower growth and CUE when single nutrients were added, with significantly reduced growth when P alone was added, but was unchanged when N and P were added together. Our results show that short-term, stoichiometric imbalances can reduce microbial growth and that microbial growth at this site is limited not by nutrients but by the amount of C available to soil microbes. • Microbial growth, carbon-use efficiency, and enzyme activity were unchanged by short or long-term addition of N and P. • Soil organic matter content was a primary control on microbial growth and the short-term effect of adding N or P alone. • Addition of N and P together led to changes in microbially respired Δ14C signatures. [ABSTRACT FROM AUTHOR]

Management of forest sites has the potential to modulate soil organic matter decomposition by changing the catalytic properties of soil microorganisms within a soil profile. In this study we examined the impact of forest management intensity and soil physico-chemical properties on the variation of enzyme activities (β-glucosidase, β-xylosidase, α-glucosidase, phenol oxidase, N-acetyl-glucosaminidase, l-leucine aminopeptidase, phosphatase) in the topsoil and two subsoil horizons in three German regions (Schorfheide-Chorin, Hainich-Dün, Schwäbische Alb). The sandy soils in the Schorfheide-Chorin (SCH) showed lower ratios of the activity of carbon (C) acquiring enzymes (β-glucosidase) relative to nitrogen (N) acquiring enzymes (N-acetyl-glucosaminidase+ l-leucine aminopeptidase), and activity of C acquiring enzymes relative to phosphorous (P) acquiring enzymes (phosphatase) than the finer textured soils in the Hainich-Dün (HAI) and Schwäbische Alb (ALB), indicating a shift in investment to N and P acquisition in the SCH. All enzyme activities, except phenol oxidase activity, decreased in deeper soil horizons as concentrations of organic C and total N did, while the decrease was much stronger from the topsoil to the first subsoil horizon than from the first subsoil to the second subsoil horizon. In contrast, phenol oxidase activity showed no significant decrease towards deeper soil horizons. Additionally, enzyme activities responsible for the degradation of more recalcitrant C relative to labile C compounds increased in the two subsoil horizons. Subsoil horizons in all regions also indicate a shift to higher N acquisition, while the strength of the shift depended on the soil type. Further, our results clearly showed that soil properties explained most of the total variance of enzyme activities in all soil horizons followed by study region, while forest management intensity had no significant impact on enzyme activities. Among all included soil properties, the clay content was the variable that explained the highest proportion of variance in enzyme activities with higher enzyme activities in clay rich soils. Our results highlight the need for large scale studies including different regions and their environmental conditions in order to derive general conclusions on which factors (anthropogenic or environmental) are most influential on enzyme activities in the whole soil profile in the long term at the regional scale. [ABSTRACT FROM AUTHOR]

Highlights: [•] PLFA data and enzyme activities were studied in managed grasslands at regional scale. [•] OC, pH and soil moisture impacted microbial parameters independent of study region. [•] Water stagnation modifies relations between OC and microbial biomass and activity. [•] Soil properties control soil microbiological properties more than management. [ABSTRACT FROM AUTHOR]

Most current soil organic matter (SOM) models represent the soil as a bulk without specification of the vertical distribution of SOM in the soil profile. However, the vertical SOM profile may be of great importance for soil carbon cycling, both on short (hours to years) time scale, due to interactions with the soil temperature and moisture profile, as well as on long (years to centuries) time scale because of depth-specific stabilization mechanisms of organic matter. It is likely that a representation of the SOM profile and surface organic layers in SOM models can improve predictions of the response of land surface fluxes to climate and environmental variability. Although models capable of simulating the vertical SOM profile exist, these were generally not developed for large scale predictive simulations and do not adequately represent surface organic horizons. We present SOMPROF, a vertically explicit SOM model, designed for implementation into large scale ecosystem and land surface models. The model dynamically simulates the vertical SOM profile and organic layer stocks based on mechanistic representations of bioturbation, liquid phase transport of organic matter, and vertical distribution of root litter input. We tested the model based on data from an old growth deciduous forest (Hainich) in Germany, and performed a sensitivity analysis of the transport parameters, and the effects of the vertical SOM distribution on temporal variation of heterotrophic respiration. Model results compare well with measured organic carbon profiles and stocks. SOMPROF is able to simulate a wide range of SOM profiles, using parameter values that are realistic compared to those found in previous studies. Results of the sensitivity analysis show that the vertical SOM distribution strongly affects temporal variation of heterotrophic respiration due to interactions with the soil temperature and moisture profile. [Copyright &y& Elsevier]

During the last decade, a paradigmatic shift regarding which processes determine the persistence of soil organic matter (SOM) took place. The interaction between microbial decomposition and association of organic matter with the soil mineral matrix has been identified as a focal point for understanding the formation of stable SOM. Using an improved version of the vertically resolved SOM model COMISSION (Ahrens et al., 2015), this paper investigates the effect of a maximum sorption capacity (Q max) for mineral-associated organic matter (MAOM) formation and its interaction with microbial processes, such as microbial decomposition and microbial necromass production. We define and estimate the maximum sorption capacity Q max with quantile regressions between mineral-associated organic carbon (MAOC) and the clay plus silt (<20 μm) content. In the COMISSION v2.0 model, plant- and microbial-derived dissolved organic matter (DOM) and dead microbial cell walls can sorb to mineral surfaces up to Q max. MAOC can only be decomposed by microorganisms after desorption. We calibrated the COMISSION v2.0 model with data from ten different sites with widely varying textures and Q max values. COMISSION v2.0 was able to fit the MAOC and SOC depth profiles, as well as the respective 14C gradients with soil depth across these sites. Using the generic set of parameters retrieved in the multi-site calibration, we conducted model experiments to isolate the effects of varying Q max , point-of-entry of litter inputs, and soil temperature. Across the ten sites, the combination of depolymerization limitation of microorganisms due to substrate scarcity in the subsoil and the size of Q max explain 14C depth gradients in OC. • The vertically explicit SOC model COMISSION was used to study 14C depth gradients. • A texture-based maximum sorption capacity was introduced to the COMISSION model. • One universal parameter set was calibrated to 14C and SOC data from ten sites. • Energy limitation and sorption capacity explain millennial 14C ages in the subsoil. [ABSTRACT FROM AUTHOR]