Environmental stress response limits microbial necromass contributions to soil organic carbon

The majority of dead organic material enters the soil carbon pool following initial incorporation into microbial biomass. The decomposition of microbial necromass carbon (C) is, therefore, an important process governing the balance between terrestrial and atmospheric C pools. We tested how abiotic stress (drought), biotic interactions (invertebrate grazing) and physical disturbance influence the biochemistry (C:N ratio and calcium oxalate production) of living fungal cells, and the subsequent stabilization of fungal-derived C after senescence. We traced the fate of ¹³C-labeled necromass from ‘stressed’ and ‘unstressed’ fungi into living soil microbes, dissolved organic carbon (DOC), total soil carbon and respired CO₂. All stressors stimulated the production of calcium oxalate crystals and enhanced the C:N ratios of living fungal mycelia, leading to the formation of ‘recalcitrant’ necromass. Although we were unable to detect consistent effects of stress on the mineralization rates of fungal necromass, a greater proportion of the non-stressed (labile) fungal necromass C was stabilised in soil. Our finding is consistent with the emerging understanding that recalcitrant material is entirely decomposed within soil, but incorporated less efficiently into living microbial biomass and, ultimately, into stable SOC.     

Fate of gram-negative bacterial biomass in soil—mineralization and contribution to SOM

Soil microorganisms contribute to the formation of non-living soil organic matter (SOM) by metabolic transformation of plant-derived material. After cell death, their biomass components with a specific molecular character become incorporated into SOM imprinting its chemical properties, although this process has not yet been quantified. In order to elucidate the contribution to SOM formation, we investigated the fate of gram-negative bacterial model biomass (Escherichia coli usually introduced into soil with manure or feces) during incubation of soil with isotopically (¹³C) and genetically (lux gene) labeled cells. The decline of living cells was monitored by the loss of bioluminescence. The carbon turnover and mineralization was balanced by bulk soil stable isotope analysis, and the persistence of nucleic acids was investigated by PCR amplification of the lux gene. During incubation, the number of viable E. coli cells decreased rapidly (99.9% within the first 42 d) serving as substrate for other microorganisms or for the formation of SOM, and bioluminescent cells could only be detected during the first 56 d. However, the lux gene was still detected after 224 d, which indicates stabilization of DNA in SOM. Although the survival of E. coli in soil is limited, only about 65% of the added labeled biomass carbon was mineralized to ¹³CO₂ and 51% remained in soil after 224 d with an average ¹³C recovery of 117%. The amount of ¹³C found in the PLFA representative of living cells had decreased to 25% of the initial value, suggesting a proportional decrease of the ¹³C in the soil microbial biomass. The extent of this decrease is higher than the mineralization of the bulk E. coli C and thus the difference of around 25% has to be stabilized as metabolites, or in non-living SOM. The data provide evidence that the genetic information and a considerable part of the carbon from dying bacterial biomass were retained in both the soil microbial food web and in non-living SOM.     

Species richness of ectomycorrhizal hyphal necromass increases soil CO2 efflux under laboratory conditions

The ectomycorrhizal mycelium is a large component of boreal and temperate forest soil microbial biomass and the resulting necromass is likely to be an important source of nutrients for saprotrophic microorganisms. Here we test the effects of species richness of ectomycorrhizal mycelial biomass on short-term CO₂ efflux by amending forest soil with necromass from 8 fungal species added separately and in mixtures of 2, 4 and 8 species. All additions of necromass rapidly increased soil CO₂ efflux compared to unamended controls but CO₂ efflux increased significantly with species richness. Efflux of CO₂ did not correlate with the carbon (C) or nitrogen (N) contents or the C:N ratio of the added necromass. The study demonstrates that species diversity of dead ectomycorrhizal fungal hyphae can have important consequences for soil CO₂ efflux, and suggests decomposition of hyphae is regulated by specific constituents of the nutrient pools in the necromass rather than the total quantities added.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Rhizodeposition: Its contribution to microbial growth and carbon and nitrogen turnover within the rhizosphere

A greenhouse rhizobox experiment was carried out to investigate the fate and turnover of ¹³C‐ and ¹⁵N‐labeled rhizodeposits within a rhizosphere gradient from 0–8 mm distance to the roots of wheat. Rhizosphere soil layers from 0–1, 1–2, 2–3, 3–4, 4–6, and 6–8 mm distance to separated roots were investigated in an incubation experiment (42 d, 15°C) for changes in total C and N and that derived from rhizodeposition in total soil, in soil microbial biomass, and in the 0.05 M K₂SO₄–extractable soil fraction. CO₂‐C respiration in total and that derived from rhizodeposition were measured from the incubated rhizosphere soil samples. Rhizodeposition C was detected in rhizosphere soil up to 4–6 mm distance from the separated roots. Rhizodeposition N was only detected in the rhizosphere soils up to 3–4 mm distance from the roots. Microbial biomass C and N was increased with increasing proximity to the separated roots. Beside ¹³C and ¹⁵N derived from rhizodeposits, unlabeled soil C and N (native SOM) were incorporated into the growing microbial biomass towards the roots, indicating a distinct acceleration of soil organic matter (SOM) decomposition and N immobilization into the growing microbial biomass, even under the competition of plant growth. During the soil incubation, microbial biomass C and N decreased in all samples. Any decrease in microbial biomass C and N in the incubated rhizosphere soil layers is attributed mainly to a decrease of unlabeled (native) C and N, whereas the main portion of previously incorporated rhizodeposition C and N during the plant growth period remained immobilized in the microbial biomass during the incubation. Mineralization of native SOM C and N was enhanced within the entire investigated rhizosphere gradient. The results indicate complex interactions between substrate input derived from rhizodeposition, microbial growth, and accelerated C and N turnover, including the decomposition of native SOM (i.e., rhizosphere priming effects) at a high spatial resolution from the roots.     

Microbial contribution to SOM quantity and quality in density fractions of temperate arable soils

The formation of soil organic matter (SOM) very much depends on microbial activity. Even more, latest studies identified microbial necromass itself being a significant source of SOM and found microbial products to initiate and enhance the formation of long-term stabilized SOM. The objectives of this study were to investigate the microbial contribution to SOM in pools of different stability and its impact on SOM quality. Hence, four arable soils of widely differing properties were density-fractionated into free and occluded particulate organic matter (fPOM, oPOM < 1.6 g cm⁻³ and oPOM < 2.0 g cm⁻³) and mineral associated organic matter (MOM > 2.0 g cm⁻³) by using sodium polytungstate. These fractions were characterized by in-source pyrolysis-field ionization mass spectrometry (Py-FIMS). Main SOM compound classes of the fractions were determined and further SOM properties were derived (polydispersity, thermostability). The contribution of microbial derived input to arable soil OM was estimated from the hexose to pentose ratio of the carbohydrates and the ratio of C₄–C₂₆ to C₂₆–C₃₆ fatty acids. Additionally, selected samples were investigated by scanning electron microscopy (SEM) for visualizing structures as indicators for the origin of OM. Results showed that, although the samples differed significantly regarding soil properties, SOM composition was comparable and almost 50% of identifiable SOM compounds of all soils types and all density fractions were assigned to phenols, lignin monomers and alkylaromatics. Most distinguishing were the high contents of carbohydrates for the MOM and of lipids for the POM fractions. Qualitative features such as polydispersity or thermostability were not in general assignable to specific compounds, density fractions or different mean residence times. Only the microbial derived part of the soil carbohydrates could be shown to be correlated with high SOM thermostability (r² = 0.63**, n = 39). Microbial derived carbohydrates and fatty acids were both enriched in the MOM, showing that the relative contribution of microbial versus plant-derived input to arable SOM increased with density and therefore especially increased MOM thermostability. Nevertheless, the general microbial contribution to arable SOM is suggested to be high for all density fractions; a mean proportion of about 1:1 was estimated for carbohydrates. Despite biomolecules released from living microorganisms, SEM revealed that microbial mass (biomass and necromass) is a considerable source for stable SOM which is also increasing with density.     

Large‐scale identification of hot spots for soil carbon demand under climate change and bioenergy production

Global change scenarios predict an increased risk for declining amounts of soil organic matter (SOM) for Central Germany. Within this region the production of bioenergy is one important strategy to counteract the rising anthropogenic CO₂‐emissions. Both issues have a close connection: SOM is an important basis for soil productivity and requires a steady reproduction flux. Bioenergy production requires productive soils and partly consumes plant biomass C. Therefore, the available amount for SOM reproduction is reduced. This study provides a methodology for the large‐scale identification of areas with possible conflicts between bioenergy production and SOM reproduction based on (1) the prediction of climate change impact on SOM reproduction and (2) an analysis of the regional distribution of biogas plants. With the C demand index (CDI) and the capacity index (CAP), two indicators were developed which enable the identification of hot spots of high carbon demand for SOM reproduction due to climate change and the usage of bioenergy. As a result of low data requirements, the indicators are widely applicable and transferable to other large‐scale studies. The proposed methodology was applied to Central Germany as a pilot region. Results indicate a growing demand (10–40%) of fresh organic C from biomass for SOM production in comparison to the current level. The analysis reveals that the bioenergy C demand is not evenly distributed within the study region. It also shows some regional clustering. Furthermore, the analysis identifies certain hot spots of a high C demand, where a high capacity of biogas production may conflict with rising demands for biomass to mitigate climate change effects on SOM storage. The hot spot areas—identified and selected on a large scale—can subsequently be analyzed in more detail on a local to farm scale by using high‐resolution data and models which enable the quantification of soil C dynamics.     

Temperature and substrate controls on microbial phospholipid fatty acid composition during incubation of grassland soils contrasting in organic matter quality

Soil incubations are often used to investigate soil organic matter (SOM) decomposition and its response to increased temperature, but changes in the activity and community composition of the decomposers have rarely been included. As part of an integrated investigation into the responses of SOM components in laboratory incubations at elevated temperatures, fungal and bacterial phospholipid fatty acids (PLFAs) were measured in two grassland soils contrasting in SOM quality (i.e. SOM composition), and changes in the microbial biomass and community composition were monitored. Whilst easily-degradable SOM and necromass released from soil preparation may have fuelled microbial activity at the start of the incubation, the overall activity and biomass of soil microorganisms were relatively constant during the subsequent one-year soil incubation, as indicated by the abundance of soil PLFAs, microbial respiration rate (r), and metabolic quotient (qCO₂). PLFAs relating to fungi and Gram-negative bacteria declined relative to Gram-positive bacteria in soils incubated at higher temperatures, presumably due to their vulnerability to disturbance and substrate constraints induced by faster exhaustion of available nutrient sources at higher temperatures. A linear correlation was found between incubation temperatures and the microbial stress ratios of cyclopropane PLFA-to-monoenoic precursor (cy17:0/16:1ω7c and cy19:0/18:1ω7c) and monoenoic-to-saturated PLFAs (mono/sat), as a combined effect of temperature and temperature-induced substrate constraints. The microbial PLFA decay patterns and ratios suggest that SOM quality intimately controls microbial responses to global warming.     

Decadal Nitrogen Fertilization Decreases Mineral‐Associated and Subsoil Carbon: A 32‐Year Study

Crop residues and manure are important sources of carbon (C) for soil organic matter (SOM) formation. Crop residue return increases by nitrogen (N) fertilization because of higher plant productivity, but this often results only in minor increases of SOM. In our study, we show how N fertilization and organic C additions affected SOM and its fractions within a 32‐year‐long field‐experiment at Puch, Germany. Five organic additions, no‐addition (control), manure, slurry, straw and straw + slurry, were combined with three mineral N fertilization rates (no, medium and high fertilization), which resulted in 1·17–4·86 Mg C‐input ha^‐1 y^‐1. Topsoil (0–25 cm) SOM content increased with N fertilization, mainly because of the C in free light fraction (f‐LF). In contrast, subsoil (25–60 cm) SOM decreased with N fertilization, probably because of roots' relocation in Ap horizon with N fertilization at the surface. Despite high inputs, straw contributed little to f‐LF but prevented C losses from the mineral‐associated SOM fraction (ρ > 1·6 g cm^‐3) with N fertilization, which was observed without straw addition. Above (straw) and belowground (roots) residues had opposite effects on SOM fractions. Root C retained longer in the light‐fractions and was responsible for SOM increase with N fertilization. Straw decomposed rapidly (from f‐LF) and fueled the mineral‐associated SOM fraction. We conclude that SOM content and composition depended not only on residue quantity, which can be managed by the additions and N fertilization, but also on the quality of organics. This should be considered for maintaining the SOM level, C sequestration, and soil fertility. Copyright © 2016 John Wiley & Sons, Ltd.     

Soil organic matter composition and soil lightness

Relationships between soil lightness, soil organic matter (SOM) composition, content of organic C, CaCO₃, and texture were studied using 42 top‐soil horizons from different soil types located in southern Germany. SOM composition was determined by CPMAS ¹³C NMR spectroscopy, soil color was measured by diffuse‐reflectance spectrophotometry and given in the CIE L*a*b* color coordination system (Commission Internationale de l'Eclairage, 1978). Multiple‐regression analysis showed, that soil lightness of top‐soil horizons is principally determined by OC concentration, but CaCO₃ and soil texture are also major variables. Soil lightness decreased with increasing OC content. Carbonate content had an important effect on soil lightness even at low concentrations due to its lightening property. Regressions between soil lightness and organic C content were strongly linear, when the soils were differentiated according to texture and CaCO₃ content. The aryl‐C content was the only SOM component which correlated significantly with soil lightness (r_S = –0.87). In the linear regressions carried out on the different soil groups, soil aryl‐C content was a more significant predictor for soil lightness than total OC content.