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.
The effects of 4 years of simulated nitrogen deposition, as nitrate (NO₃⁻) and ammonium (NH₄⁺), on microbial carbon turnover were studied in an ombrotrophic peatland. We investigated the ...mineralization of simple forms of carbon using MicroResptrade mark sign measurements (a multiple substrate induced respiration technique) and the activities of four soil enzymes involved in the decomposition of more complex forms of carbon or in nutrient acquisition: N-acetyl-glucosaminidase (NAG), cellobiohydrolase (CBH), acid phosphatase (AP), and phenol oxidase (PO). The potential mineralization of labile forms of carbon was significantly enhanced at the higher N additions, especially with NH₄⁺ amendments, while potential enzyme activities involved in breakdown of more complex forms of carbon or nutrient acquisition decreased slightly (NAG and CBH) or remained unchanged (AP and PO) with N amendments. This study also showed the importance of distinguishing between NO₃⁻ and NH₄⁺ amendments, as their impact often differed. It is possible that the limited response on potential extracellular enzyme activity is due to other factors, such as limited exposure to the added N in the deeper soil or continued suboptimal functioning of the enzymes due to the low pH, possibly via the inhibitory effect of low phenol oxidase activity.
•Soil microbial diversity was measured at 81 sites across Europe.•Arable sites with slightly higher pH on average and lower soil organic carbon content result in a lower overall substrate ...utilisation.•The MicroResp method is a viable biological indicator method for microbial activity that is capable of discriminating between land-use and key soil properties across Europe.
A European “transect” was established to assess soil microbial activity, using the MicroResp™ method, as part of a larger project looking at soil biodiversity and function across Europe. 81 sites were sampled across five biogeographical zones described and mapped in the EEA report (EEA, 2012) and included the following classes; Boreal, Atlantic, Continental, Mediterranean and Alpine, three land-use types (Arable, Grass and Forest) incorporating a wide range of soil pH, soil organic carbon (org C) and texture. Seven carbon substrates were used to determine multiple substrate induced respiration (MSIR), incorporating; acids, bases, sugars and amino acids. Substrates included: d-(+)-galactose, l-malic acid, gamma amino butyric acid, n-acetyl glucosamine, d-(+)-glucose, alpha ketogluterate, citric acid and water. MicroResp™ results showed discrimination of land-use type over a large spatial scale and response to soil pH and soil organic carbon. Substrates behaved differently depending upon combinations of land-use and soil properties specifically the greater utilisation of carboxylic acid based substrates in arable sites.
Soil microbial communities are responsive to abiotic and biotic conditions within the heterogeneous soil environment. In montane plant communities, vegetation can create distinctive microenvironments ...that have unique microbial responses. Here, we ask how soil microbial activity and functional diversity were influenced by the type and diversity of montane plant species, and the morphological and chemical traits of their associated root systems, that are expected to influence soil properties. Along an elevational gradient (1400-2400 m a.s.l.) in the French Alps, we investigated microbial global catabolic activity (i.e. microbial activity) and catabolic diversity (i.e. functional diversity) in bulk and rhizosphere soil beneath three plant species (Vaccinium myrtillus, Juniperus communis and Picea abies) using multiple substrate-induced respiration. We also measured soil physical and chemical properties, plant diversity, climatic factors and morphological and chemical traits of roots in bulk soil (‘community’ level traits, where several plant species were pooled together) and of individual plants (‘species’ level, where roots of single species were excavated). At lower elevations, global catabolic activity in the rhizosphere was higher than in bulk soil, but converged in the nutrient-poor, colder soils found at higher elevations, although changes in catabolic diversity were negligible. Variations in soil texture, cation exchange capacity, carbon and nitrogen content and pH were associated with the global catabolic activity, but these soil properties had minimal effects on catabolic diversity. Climatic variables were related to microbial activity beneath V. myrtillus only and warmer mean annual temperatures increased activity. Plant root traits at the community level in bulk soil had less effect on global catabolic activity than abiotic factors, with thicker roots, high root lignin content and low cellulose content influencing microbial activity, but not altering catabolic diversity. At the species level, more dense root tissue decreased global catabolic activity, reflecting changes in chemical composition. Overall, our results show that soil physical and chemical properties were the main drivers of microbial activity, but that vegetation created distinctive microenvironments that refined these relationships, mainly through modifications in root chemical traits.
•Soil physical and chemical properties were the main drivers of microbial activity.•Microbial activity converged in bulk and rhizosphere soils at higher elevations.•Plant species identity refined relationships with microbial activity and abiotic factors.
In order to gain more knowledge regarding the microbial community and its influence on carbon sequestration in subsoil, two depth profiles with different natural soil organic carbon (SOC) ...concentrations were sampled. This makes it possible to investigate the extent to which natural SOC availability or other subsoil specific conditions influence the composition and the functional diversity of the microbial community and in return how the microbial community composition affects SOC sequestration under these conditions. Soil samples were taken at four different depths on two neighbouring arable sites; one Colluvic Cambisol with high SOC concentrations (8–12 g kg−1) throughout the profile and one Haplic Luvisol with low SOC concentrations (3–4 g kg−1) below 30 cm depth. Multi-substrate-induced-respiration (MSIR) was used to identify shifts in functional diversity of the microbial community along the profiles. The amino sugars muramic acid and glucosamine were measured as indicators for bacterial and fungal residues and ergosterol was determined as a marker for saprotrophic fungi. Discriminant analysis of respiration values obtained from the 17 substrates used in the MSIR revealed that substrate use in subsoil differed significantly from that in topsoil and also differed highly between the two subsoils. Amino sugar analysis and the ratio of ergosterol to microbial biomass C showed that fungal dominance decreased with depth. MSIR clearly demonstrated that not only the fungi to bacteria ratio but also substrate use of the microbial community changed with depth according to substrate availability.
•Depth profiles of a Colluvic Cambisol and a Haplic Luvisol differed in multiple microbial parameters.•Saprotrophic fungal dominance over bacteria decreased with depth, more so in the Cambisol.•Increased SOC contents in the Cambisol subsoil did not increase microbial biomass.•C and N stocks controlled the response of soil microorganisms to added substrates.•N stocks differentiated substrate utilization between topsoil and subsoil.
Decomposition of soil organic matter (SOM) is regulated by microbial activity, which strongly depends on the availability of carbon (C) and nitrogen (N). Yet, the special role of N on soil organic ...carbon (SOC) mineralization is still under discussion. The recent concept of microbial N mining predicts increasing SOC mineralization under N-deficiency, which is in contrast to the generally accepted stoichiometric decomposition theory.
Following this concept we hypothesized that spatio-temporal patterns of microbial activity are controlled by SOC and N contents, but that microorganisms maintain their functionality to mineralize C under conditions of N deficiency because of microbial N mining.
To test this hypothesis, we added glucose to an arable soil that had experienced increasing losses of C3-derived SOM after one, three, and seven years of bare fallow and measured spatio-temporal patterns of substrate-induced respiration (SIR). The SIR measurements were performed with and without additions of mineral N. Selected samples were treated with C4 sugar in order to trace the source of CO2 emissions (sugar vs. SOC-derived) by natural 13C abundance measurements. Sugar additions were repeated after the first SIR experiment to derive information on changing N availability.
The results showed that spatial patterns of SIR were not consistently regulated by SOC and N. On a temporal scale, the maximum microbial growth peak declined by 47% from one year bare fallow to seven years bare fallow but soils often developed a second growth phase in the 7th year of fallow. Intriguingly, the maximum microbial growth peak increased again when N was added together with the glucose and no second growth peak occurred. A similar effect was observed after repeated sugar additions but without N additions. The 13C experiment revealed a slightly higher contribution of SOC-derived CO2 in N-deficient samples (16.7%) than in N-fertilized samples (14.6%).
We conclude that the first SIR peak was related to the supply of immediately available N while the second growth phase indicated a delayed release of N, due to N mining from SOM. Hence, microbes were able to compensate for initial N limitation and there was no significant change in the overall substrate-induced CO2 release with proceeding time under fallow.
•Spatial patterns of SIR parameters change with increasing time of bare-fallow.•Indications for microbial nitrogen mining in N-deficient soils are presented.•Microbes maintain their ability to mineralize glucose after 7 years of bare fallow.•SIR is a useful tool to examine microbial nitrogen acquisition strategies.
•Similar soil quality levels under trees and grassland irrespective to tree distance.•Improved soil quality at 0–5 compared with 5–20 cm soil depth within grassland ACS.•No effects of increased plant ...species diversity on soil microbial indices.
Effects of silvo-arable alley cropping systems (ACS) on soil functions have frequently been investigated, however, less is known about the effects of silvo-grassland ACS. Conversion of arable land to grassland ACS may have a high capability to rebuild soil fertility, while increased grassland plant diversity may further foster improvements. The objective of the study was to assess the impact of willows (Salix spp.) and grassland plant species diversity on soil ecology of an ACS established on former arable land. Thus, soil quality indices, like soil organic carbon (SOC), microbial biomass C and N, fungal abundance and microbial functional diversity, were quantified in formerly ploughed Eutric (Stagnic) Cambisols in two soil depths (0–5, 5–20 cm) of a temperate grassland ACS (Lower Saxony, Germany). To evaluate potential tree effects on alleyways, distance transects were analysed by repeated measures mixed effects models, considering abiotic factor (pH, clay content) variability. Possible changes of soil quality indices within the former ploughed soil layer were deduced by a comparison of both soil depths at each distance. Linear contrasts were calculated for comparisons of grassland diversity levels. Results showed significantly higher SOC contents (16.2–16.9 mg g−1) and soil microbial properties (e.g. MBC: 453-462 μg g−1, ergosterol: 4.3-4.4 μg g−1) in upper topsoils under trees and grassland compared with lower topsoil layers (11.6–12.0 mg g−1, 262-277 μg g−1 and 1.7-1.8 μg g−1 for SOC, MBC and ergosterol, respectively). No effects of plant species diversity on microbial properties have been detected to date. Similar levels of soil quality indices in upper topsoils under trees and grassland are ascribed to the cessation of tillage and a permanent vegetation cover ongoing for 5 years, as aboveground tree litter inputs into the alleyways were minor. Hence, irrespective of the diversity level of grassland vegetation, grassland ACS may have a high potential to increase soil quality for matching the requirement of sustainability of agroecosystems. However, as solely topsoils have been investigated, henceforth, the consideration of subsoils is vital to assess the effect of deep tree roots in grassland ACS.
Occasional one-time tillage (strategic tillage, ST) is an effective tool for managing weeds and crop diseases in no-till and conversative farming systems. However, there is limited understanding of ...the impacts of ST on soil microbiome and their associated soil processes, particularly in dryland agriculture. This study aims to quantify the effect of one-off ST - after three years - on soil microbiomes and functions in a long-term no-till farming system under crop stubble and fertilizer management practices. The results showed that ST had marginal effects on microbial richness and diversity, enzyme activities, and catabolic function, but significantly affected the abundance of some microbial taxa (Actinobacteria, Firmicutes, Verrucomicrobia, Basidiomycota and Ascomycota) that are relevant to carbon (C) degradation. Stubble retention, regardless of tillage and fertilizer management, mainly increased the abundance of copiotrophs such as Proteobacteria (e.g., Rhizobiales) and Actinobacteria (e.g., Streptomyces and Micromonosporaceae), and affected Ascomycota and Basidiomycota. Among the management practices, stubble retention was the main factor that contributed to increased richness and diversity of the soil bacterial and fungal community. Supplementary fertilizer application, regardless of tillage and stubble management had minimal impact on bacterial and fungal richness and diversity, enzyme activity and catabolic function. The variation in bacterial community structure was influenced mainly by soil pH (c.a. 10%), while only a small but significant effect (< 7%; P = 0.001) was attributed to tillage and stubble management. Wheat grain yields ranged between 5 and 5.3 t ha-1 and were not affected by tillage, stubble, nor fertilizer management practices. Similarly, these management practices did not influence total soil C or nitrogen concentrations. Our findings show that strategic tillage, when used to address specific constraints in no-till systems in dryland agriculture, does not have a significant effect on total soil C, microbial ecology nor catabolic function.
•Strategic tillage (ST) after 3-years did not affect microbial richness and diversity.•ST affected Actinobacteria, Firmicutes, and Verrucomicrobia.•ST increased Auriculariaceae but decreased Pezizales.•ST did not affect soil enzymatic activities or catabolic function.•Stubble retention increased bacterial and fungal richness and composition.
The applicability of DNA-based analysis of soil microbial biomass was proven under conditions when the common approaches – chloroform fumigation-extraction (CFE) and substrate-induced respiration ...(SIR) – are restricted. These restrictions include certain soil properties typical for arid areas (alkaline or carbonaceous soils) or limitation by sample preparation (frozen samples). To prove the suitability and correspondence of the methods, microbial biomass was determined by CFE, SIR and by DNA quantification in slightly alkaline Chernozem and strongly alkaline Calcisol of semi-arid climate under contrasting land use. Quantification of double-stranded DNA (dsDNA) revealed an excellent agreement (R2 = 0.96) with SIR-based microbial biomass (SIR-Cmic) for soils with pH lower than 8. DNA- and CFE-based microbial biomass (CFE-Cmic) correlated well (R2 = 0.97) for all studied soils. The conversion factors from dsDNA to SIR-Cmic of 5.1 and to CFE-Cmic of 4.4 were obtained. In alkaline soils (pH > 8), microbial biomass measured by SIR was strongly underestimated because of CO2 retention in soil solution due to high pH and CO32− exchange with carbonates.
Thus, the dsDNA quantification provides a simple and durable approach for microbial biomass analysis as alternative for SIR and CFE and can be successfully used in alkaline or calcareous semi-arid soils.
•Microbial biomass is strongly underestimated by SIR in alkaline soils.•Limited applicability of pH-based CO2 retention conversion factors.•DNA-based microbial biomass is a valid alternative to SIR and CFE.•The DNA-to-Cmic conversion factor is refined for soils of semi-arid climate.•DNA-based eco-physiological indexes revealed positive effect of agrogenic impact.
Tropical soils often contain less soil organic C (SOC) and microbial biomass C (MBC) than temperate soils and, thus, exhibit lower soil fertility. The addition of plant residues and N fertilizers can ...improve soil fertility, which might be reflected by microbial C use efficiency (CUE) and functional diversity. A 42-day incubation study was carried out, adding leaf litter of the C4 plant finger millet (
Eleusine coracana
Gaertn.) and inorganic
15
N fertilizer. The aim was to investigate amendment effects on CUE and functional diversity in a tropical Nitisol and a temperate Luvisol. At day 42, 28% of the millet litter-derived C (C4) added was mineralised to CO
2
C4 in the temperate Luvisol and only 18% in the tropical Nitisol, averaging all N treatments. In contrast, none of the different fractions used for calculating CUE values, i.e. CO
2
C4, MBC4, microbial residue C4, and particulate organic matter C4, differed between the soils in the N0 (no N addition) treatment. CUE values considering microbial residues varied around 0.63, regardless of soil type and sampling day, which needs further evaluation. Millet litter increased autochthonous SOC-derived CO
2
C3 production, but N addition did not. This priming effect was apparently not caused by N mining. The respiratory response to most substrates added by multi-substrate-induced respiration (MSIR) and, thus, functional diversity was higher in the Luvisol than in the Nitisol. Millet litter had positive and N addition negative effects on the functional diversity of Nitisol, indicating that MSIR is a useful tool for evaluating soil fertility.
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