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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. [ABSTRACT FROM AUTHOR]

Grasslands are one of the major sinks of terrestrial soil organic carbon (SOC). Understanding how environmental and management factors drive SOC is challenging because they are scale-dependent, with large-scale drivers affecting SOC both directly and through drivers working at small scales. Here we addressed how regional, landscape and grazing management, soil properties and nutrients, and herbage quality factors affect 20 cm depth SOC stocks in mountain grasslands in the Pyrenees. Taking advantage of the high variety of environmental heterogeneity in the Pyrenees, we built a dataset (n=128) that comprises a wide range of environmental and management conditions. This was used to understand the relationship between SOC stocks and their drivers considering multiple environments. We found that temperature seasonality (difference between mean summer temperature and mean annual temperature; TSIS) was the most important geophysical driver of SOC in our study, depending on topography and management. TSIS effects on SOC increased in exposed hillsides, slopy areas, and relatively intensively grazed grasslands. Increased TSIS probably favours plant biomass production, particularly at high altitudes, but landscape and grazing management factors regulate the accumulation of this biomass into SOC. Concerning biochemical SOC drivers, we found unexpected interactive effects between grazer type, soil nutrients and herbage quality. Soil N was a crucial SOC driver as expected but modulated by livestock species and neutral detergent fibre contenting plant biomass; herbage recalcitrance effects varied depending on grazer species. These results highlight the gaps in knowledge about SOC drivers in grasslands under different environmental and management conditions. They may also serve to generate testable hypotheses in later/future studies directed to climate change mitigation policies. [ABSTRACT FROM AUTHOR]

Given their organic matter (OM) depletion, agricultural soils can act as carbon (C) sinks if adequate management practices are implemented. OM stabilisation in highly OM-depleted agricultural soils may depend upon the allocation of OM inputs among particle size fractions that differ in their capacity to stabilise OM. In a set of vegetable garden fields, we determined the magnitude of the differences in soil C and N content between organically and conventionally managed fields and the incorporation of the increased C and N pools to the fine fractions as an indication of the stability of the soil OM accrual. It was carried out in a stockless scenario in which exogenous OM was only used in organically managed fields for the last 20 years (as opposed to conventional management only using mineral fertilisers). Organic fertilisation caused a notable increase in soil organic C and N stocks compared with mineral-fertilised soils. Such increase remained significant below the plough depth. C and N content increased at all fractions, but the relative contribution of the fine-silt-plus-clay fraction to total C and N decreased at all depths. We concluded that organic management increases soil OM storage, but overall, the stability of the increased OM stocks decreases slightly. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We studied carbon balances and carbon stocks of mountain rangelands and meadows in a network of 8 eddy covariance sites and 14 sites with biomass data in Europe. Net ecosystem exchange of pastures and extensively managed semi-natural rangelands were usually close to zero, while meadows fixed carbon, with the exception of one meadow that was established on a drained peatland. When we accounted for off-site losses and inputs also the carbon budget of meadows approached zero. Soil carbon stocks in these ecosystems were high, comparable to those of forest ecosystems, while carbon stocks in plant biomass were smaller. Since soil carbon stocks of abandoned mountain grasslands are as high as in managed ecosystems, it is likely that the widespread abandonment of mountain rangelands used currently as pastures will not lead to an immediate carbon sink in those ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: Given the high turnover of fine roots in mountain grasslands, knowledge of their decomposition rates and the capacity of mountain grassland soils to stabilize root-derived C are central to understand the role of these ecosystems as potential C sinks. Here we studied the decomposition of fine roots in mountain grasslands and estimated the rates at which root-C and -N incorporated into protected pools at two soil depths. For this purpose, we incubated standard 13C- and 15N-labelled wheat roots mixed with unlabelled soil at 5 and 20 cm depth in two mountain grassland sites. Particle size fractionation allowed the quantification of the labelled wheat root-C and -N allocated to each size fraction (coarse sand, fine sand and silt plus clay sized) as well as their incorporation rates into the finest fraction. Between 62% and 78% root-C remained in the soil after one year of field incubation, faster decomposition being registered at the warmest site. In the following two years, roots decomposed much more slowly. In contrast to reports in the literature, our results indicate that decay rates during the first year were highest in the deep layer. The incorporation of wheat root-derived organic matter into the silt plus clay size fraction was also much greater during the first year of decomposition than in the following two years and also slightly higher in the deep soil than in topsoil. The incorporation rates of root-13C and root-15N into this fraction also suggest that the wheat-derived organic matter associated with this fraction was N-enriched and less recalcitrant (i.e., less resistant to acid hydrolysis) than that recovered from the coarser fractions. Furthermore, recalcitrant organic matter incorporated much more slowly than labile organic matter did. We conclude that the conditions of the subalpine grassland subsoil are more favourable for root decomposition than the topsoil and that the organic matter that incorporates into the protected pool is characterised by a high N content and low biochemical recalcitrance. [Copyright &y& Elsevier]

Abstract: Altered rates of native soil organic matter (SOM) mineralisation in the presence of labile C substrate (‘priming’), is increasingly recognised as central to the coupling of plant and soil-biological productivity and potentially as a key process mediating the C-balance of soils. However, the mechanisms and controls of SOM-priming are not well understood. In this study we manipulated microbial biomass size and composition (chloroform fumigation) and mineral nutrient availability to investigate controls of SOM-priming. Effects of applied substrate (13C-glucose) on mineralisation of native SOM were quantified by isotopic partitioning of soil respiration. In addition, the respective contributions of SOM-C and substrate-derived C to microbial biomass carbon (MBC) were quantified to account for pool-substitution effects (‘apparent priming’). Phospholipid fatty acid (PLFA) profiles of the soils were determined to establish treatment effects on microbial community structure, while the 13C-enrichment of PLFA biomarkers was used to establish pathways of substrate-derived C-flux through the microbial communities. The results indicated that glucose additions increased SOM-mineralisation in all treatments (positive priming). The magnitude of priming was reduced in fumigated soils, concurrent with reduced substrate-derived C-flux through putative SOM-mineralising organisms (fungi and actinomycetes). Nutrient additions reduced the magnitude of positive priming in non-fumigated soils, but did not affect the distribution of substrate-derived C in microbial communities. The results support the view that microbial community composition is a determinant of SOM-mineralisation, with evidence that utilisation of labile substrate by fungal and actinomycete (but not Gram-negative) populations promotes positive SOM-priming. [Copyright &y& Elsevier]

We evaluated the chemical properties controlling phosphate (PO4-P) sorption characteristics of alpine soils at elevations of 1725 to 2900 m above sea level in four European mountain ranges (the Tatra Mountains, Slovakia and Poland; the Alps, Austria; the Pyrenees, Spain and France; and the Rila Mountains, Bulgaria). PO4-P sorption isotherms were determined, ranging in concentration from 0 to 6.5 mM KH2PO4 at original soil pH, and were evaluated using the Langmuir equation. PO4-P sorption maxima (Xmax) varied from 6 to 145 mmol kg-1 in the dry (105°C) fine soil fraction (<2 mm). Concentrations of Al oxohydroxides, determined as oxalate extractable aluminum (Alo), ranged between 36 and 770 mmol kg-1, and were the dominant parameter affecting the soil's ability to adsorb PO4-P. In the Tatra Mountains, Rila, and Alps, oxalate-extractable iron (Feo) was also important, while in the Pyrenees we found a significant correlation (p < 0.01) between Xmax and total organic C concentrations. The ability of Al and Fe oxohydroxides to adsorb PO4-P was higher in areas with more acidic soils (the Tatra Mountains) than in areas with higher soil pH (the Pyrenees), and in the more acidic uppermost organic soil horizons than in mineral soil horizons. A simple model based only on two basic soil characteristics (Alo and pH) explained 81 to 89% of the Xmax variability observed in individual mountain areas and 74% of the Xmax variability associated with all samples. [ABSTRACT FROM AUTHOR]

Dynamic modelling of hydrochemistry is a valuable tool to study and predict the recovery of surface waters from acidification, and to assess the effects of confounding factors (such as delayed soil response and changing climate) that cause hysteresis during reversal from acidification. The availability of soil data is often a limitation for the regional application of dynamic models. Here we present a method to upscale site-specific soil properties to a regional scale in order to circumvent that problem. The method proposed for upscaling relied on multiple regression models between soil properties and a suite of environmental variables used as predictors. Soil measurements were made during a field survey in 13 catchments in the Pyrenees (NW Spain). The environmental variables were derived from mapped or remotely sensed topographic, lithological, land-cover, and climatic information. Regression models were then used to model soil parameters, which were supplied as input for the biogeochemical model MAGIC (Model for Acidification of Groundwater In Catchments) in order to reconstruct the history of acidification in Pyrenean lakes and forecast the recovery under a scenario of reduced acid deposition. The resulting simulations were then compared with model runs using field measurements as input parameters. These comparisons showed that regional averages for the key water and soil chemistry variables were suitably reproduced when using the modelled parameters. Simulations of water chemistry at the catchment scale also showed good results, whereas simulated soil parameters reflected uncertainty in the initial modelled estimates. [Copyright &y& Elsevier]

Abstract: Although a large amount of soil carbon (C) is stored in subsurface soils, most studies on soil C dynamics focus on the upper layers. The aim of this study is to assess the factors that regulate C mineralization in mountain grassland soils under standard laboratory conditions to compare regulation mechanisms at surface and subsurface horizons. For this purpose soil samples of surface and subsurface horizons from 35 locations were incubated under laboratory conditions, CO2 efflux rates were measured and microbial biomass C (MBC) and net N mineralization were determined. We also analysed the samples for pH, extractable C after fumigation (Cfe), potentially mineralizable N (PMN), reactive and non-reactive P, sum of exchangeable bases and clay content in order to assess the influence of soil characteristics on C mineralization. The influence of climate of each site on soil C mineralization under the same laboratory conditions was also explored for surface and subsurface horizons. C mineralization in surface horizons related positively with Cfe content, suggesting that microbial activity in this horizon was mainly regulated by the availability of C. By contrast, in subsurface horizons, C mineralization related with PMN and was independent of measured C fractions, suggesting that microbial activity in subsurface horizons was limited by the availability of N and that the available forms of C were more stable in these horizons. The effects of local climate on laboratory C mineralization were significant in both soil horizons, with lower rates of C mineralization being recorded in soils from wetter and warmer sites. This fact, suggested that the C stabilisation mechanisms in mountain grassland soils may be affected by the climate in which soils develop. [Copyright &y& Elsevier]

Accumulation of soil carbon is mainly controlled by the balance between litter production and litter decomposition. Usually In Mediterranean forests there are contrasting conditions in the distribution of faunal activity and the moss layer that may have different effects on litter decomposition. Decomposition and faunal activity were studied by exposing litter of contrasting quality (Pinus halepensis Mill. and Quercus ilex L.) for 3.5 yr in three Mediterranean pine forests of the eastern Iberian Peninsula. The effects of mosses on decomposition and on faunal activity were studied by exposing P. halepensis litter either on moss patches or directly on the forest floor. Faecal pellet production was used as an indication of faunal activity. Water availability or soil characteristics seem to limit faunal activities in the drier sites. Faecal pellets were not found during the first stages of decomposition and in all sites they appeared when about a 30% of the initial litter had decomposed. Under wet conditions faecal pellet production was very high and a mass balance suggested that soil faunal activity may result in a net flow of organic matter from the lower organic horizons to the surface Oi horizon. Mosses slightly increased mass loss of pine litter probably as a consequence of high potentially mineralizable nitrogen in the Oa horizon of moss patches and also, perhaps, as a consequence of the higher moisture content measured in the Oi horizon needles sampled among the mosses. In contrast, moss patches reduced faunal activity. The effect of litter quality on mass loss was not always significant, suggesting an interaction between litter quality and site conditions. During the first stages of decomposition there was N immobilisation in P. halepensis litter (poorer in N) and N release from Q. ilex litter (richer in N). In conclusion, in these forests soil microclimate and/or N availability appear to be more important controlling litter decomposition than the distribution of faunal activity. [Copyright &y& Elsevier]

Sugarcane filtercake is a nutrient-rich residue produced prior to sugarcane distillation and is commonly disposed of by applying directly to agricultural fields, often causing high decomposition and leaching rates. Transforming this material into biochar could improve its stability in the soil. In this 92-day incubation study, filtercake biochar produced at 400 °C (BC400) and 600 °C (BC600) was used to trace biochar stability when mixed with two soils with different organic matter levels: an agricultural field (1.2% carbon (C)) and a forest (2.8% C) soil. Based on δ13C isotope analysis, biochar decreases in the field soil mostly occurred in the coarse silt fraction. In contrast, biochar decreases in forest soil appeared to be more equally distributed in all particle size fractions. A negative priming effect in biochar-amended soils was noticeable, mainly in the forest soil. Cumulative CO2 emissions were greater in soils with BC400 than in those with BC600 for both field and forest soils, while adding biochar increased CO2 emissions only in field soils. This increase did not appear to affect native soil organic matter pools. High-temperature filtercake biochar could thus be a more stable alternative to the current practice of raw filtercake applications. [ABSTRACT FROM AUTHOR]

Microorganisms are vital in mediating the earth's biogeochemical cycles; yet, despite our rapidly increasing ability to explore complex environmental microbial communities, the relationship between microbial community structure and ecosystem processes remains poorly understood. Here, we address a fundamental and unanswered question in microbial ecology: 'When do we need to understand microbial community structure to accurately predict function?' We present a statistical analysis investigating the value of environmental data and microbial community structure independently and in combination for explaining rates of carbon and nitrogen cycling processes within 82 global datasets. Environmental variables were the strongest predictors of process rates but left 44% of variation unexplained on average, suggesting the potential for microbial data to increase model accuracy. Although only 29% of our datasets were significantly improved by adding information on microbial community structure, we observed improvement in models of processes mediated by narrow phylogenetic guilds via functional gene data, and conversely, improvement in models of facultative microbial processes via community diversity metrics. Our results also suggest that microbial diversity can strengthen predictions of respiration rates beyond microbial biomass parameters, as 53% of models were improved by incorporating both sets of predictors compared to 35% by microbial biomass alone. Our analysis represents the first comprehensive analysis of research examining links between microbial community structure and ecosystem function. Taken together, our results indicate that a greater understanding of microbial communities informed by ecological principles may enhance our ability to predict ecosystem process rates relative to assessments based on environmental variables and microbial physiology.