Applying composts is useful for increasing soil carbon (C) stocks and improving agricultural productivity. In order to understand the effects of composts on soil organic matter (SOM) formation and mineralisation, 13C-labelled plant residues, previously composted or not, were incubated in an arable soil. The amount of 13C was quantified in the CO2 evolved, the dissolved organic carbon (DOC) and in the microbial biomass (MB). Composting decreased (from 29 to 9%) the proportion of the plant residue labile pool and increased the residence time of both labile and more stable pools from 21 to 34 days and 1.5–5.5 years, respectively. At the beginning of the incubation, the amounts of 13C in the DOC and MB were significantly higher when adding fresh residues than composted ones to the soil. A priming effect on SOM mineralisation (+21% over 3 years) was only observed for non-composted residues. These differences were attributed to changes in the chemical composition of plant materials during composting (less sugars and lipids, more lignins). In terms of C budget, the total loss of CO2 (including the composting process and the SOM priming by fresh residues) was comparable for both treatments after 600 days of incubation.

Returning and addition of organic material to soil is a key to protecting the soil, plants, and the environment. A study aimed to elucidate the effect of residual biomass application on some indicators of soil physical properties, abundance of earthworms and soil microbial activities was conducted in Kebun Percobaan (KP) Natar, BPTP Lampung from February to July 2017. The treatments were three types of crops biomass residues, i.e., maize stover, rice straw, and soybean stover (fresh or compost). The dosage rates were 0, 2.5, 5.0, 7.5, and 10 t/ha. The treatments were arranged in a randomized block design with three replicates. Upland rice (Inpago 9 variety) was planted after two weeks application of biomass residues treatments. The results showed that application of crops biomass residues improved bulkdensity and significantly increased soil water content at the high dose of biomass residues (7.5 or 10 t/ha). The amount and weight of earthworms with added of compost biomass was significantly correlated with soil water content (r values 0.491 and 0.376, respectively). The dose of biomass residues had a significant effect on soil respiration that the highest soil respiration was obtained in maize compost biomass treatment (at 12 weeks observation) was 31.7 and rice straw compost (at 8 weeks observation) which was 30.19 mg/hour/m2 C-CO2.

Soil net nitrogen mineralization rate (Nₘᵢₙ), which is critical for soil nitrogen availability and plant growth, is thought to be primarily controlled by climate and soil physical and/or chemical properties. However, the role of microbes on regulating soil Nₘᵢₙ has not been evaluated on the global scale. By compiling 1565 observational data points of potential net Nₘᵢₙ from 198 published studies across terrestrial ecosystems, we found that Nₘᵢₙ significantly increased with soil microbial biomass, total nitrogen, and mean annual precipitation, but decreased with soil pH. The variation of Nₘᵢₙ was ascribed predominantly to soil microbial biomass on global and biome scales. Mean annual precipitation, soil pH, and total soil nitrogen significantly influenced Nₘᵢₙ through soil microbes. The structural equation models (SEM) showed that soil substrates were the main factors controlling Nₘᵢₙ when microbial biomass was excluded. Microbe became the primary driver when it was included in SEM analysis. SEM with soil microbial biomass improved the Nₘᵢₙ prediction by 19% in comparison with that devoid of soil microbial biomass. The changes in Nₘᵢₙ contributed the most to global soil NH₄⁺‐N variations in contrast to climate and soil properties. This study reveals the complex interactions of climate, soil properties, and microbes on Nₘᵢₙ and highlights the importance of soil microbial biomass in determining Nₘᵢₙ and nitrogen availability across the globe. The findings necessitate accurate representation of microbes in Earth system models to better predict nitrogen cycle under global change.

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 CO2 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.

CORE IDEAS: Cover crops improve soil microbial biomass compared to no cover crop and influence soil microbial community structure.Soil microbial communities change both spatially and temporally.Results of the study imply that integration of CC in a regular corn–soybean rotation can improve soil quality and environmental benefits. Although cover crops (CC) are believed to play a major role in soil quality improvement, the effects of CC on microbial populations and community structure is not well understood. The objective of this study was to quantify CC effects on soil microbial biomass and community structure under a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. The study was conducted at the Chariton County Cover Crop Soil Health Research and Demonstration Farm (CCSH) in Missouri, USA, where CC were first established in 2012. Soils were sampled in 2016, 2017, and 2018 from the 0‐ to 10‐cm depth layer using a grid sampling design and phospholipid fatty acid (PLFA) profiles were determined. Microbial biomass and microbial community structure (total fungi, total bacteria, rhizobia, gram (−), and actinomycetes biomass), as estimated from the PLFA biomarkers, were significantly greater (P < 0.05) in the CC treatment compared to no cover crop (NCC) in 2016 and 2018 (2.4‐ and 1.7‐fold larger, respectively). Within the CC treatment, differences by soil type were also observed, finding that the silt loam soil supported greater total microbial biomass than the loam soil in 2018. Spatial distribution patterns of total microbial biomass, bacteria biomass and fungi biomass differed with time. Overall, this study demonstrated that the CC treatment affected the soil microbial community biomass and structure, which has potential environmental, production, and soil quality benefits.

Few long-term fertilization experiments have been performed in forests, even though the effects of nitrogen (N) addition on soil microbial biomass are a cause for concern. Our objective was to examine the effects of repeated fertilization for 36 years on soil microbial biomass in two forest stands. We measured soil chemical properties and microbial biomass carbon (C) and N in soils in fertilized and non-fertilized plots in a birch stand (Betula maximowicziana Regel) and a fir stand (Abies sachalinensis Fr. Schmidt). We also performed lime amendments and a 21-day laboratory incubation, and measured microbial biomass to clarify the effects of acidification due to fertilization. Soil pH was significantly lower in fertilized plots in both stands, and soil microbial biomass C and N were lower (significantly so in the fir stand) in the fertilized plots after 36 years of repeated fertilization. In the laboratory incubation, lime amendment did not significantly affect the microbial biomass C, N, or C:N ratio, despite an increase of about 1 unit in soil pH. Our results therefore indicate that factors other than soil pH also have important effects on soil microbial biomass in repeatedly fertilized forest stands.

Soil microorganisms are critical for the maintenance of terrestrial biodiversity and ecosystem functions. Both plant diversity and water availability are individually known to influence soil microorganisms; however, their interactive effects remain largely unknown. Here, we investigated whether the effects of tree species mixtures on microbial biomass and composition were altered by water availability. This was accomplished by sampling soils in the growing season from stands that were dominated by Populus tremuloides and Pinus banksiana, respectively, and their relatively even mixtures under reduced (−25% throughfall), ambient, and added (+25% throughfall) water. Microbial community biomass and composition were determined by phospholipid fatty acid analysis. We found that water addition increased soil total microbial biomass and by individual groups, whereas water reduction had no effect. Under ambient water conditions, soil total microbial biomass, arbuscular mycorrhizal fungal, bacterial, gram-positive (GP) bacterial, and gram-negative (GN) bacterial biomass were significantly lower in mixtures than from those of constituent monocultures, but saprotrophic fungal biomass and the ratios of fungal/bacterial and GP/GN bacteria were not significantly affected by tree species mixtures. Water reduction increased species mixture effects on total and individual group microbial biomass from negative to neutral, while water addition only increased mixture effects on arbuscular mycorrhizal fungal and GP bacterial biomass. Across all water treatments, soil total and individual group microbial biomass significantly increased with the abundance of broadleaved trees, but only weakly with species richness. Further, microbial community compositions differed significantly with both overstory type and water treatment. Microbial community compositions exhibited strong associations with tree species richness, soil moisture, soil pH, and litterfall production, whereas microbial biomass did not. Our results suggest that higher species diversity is not always of benefit for soil microorganisms; however, mixed tree species have the potential to regulate ecosystem responses to climate change.

The seasonal dynamics of the structure of microbial biomass in a soddy-podzolic soil under fallow was assessed using luminescent microscopy. Samples from three soil horizons (P, 5‒15 cm, BEL, 30‒40 cm, and BT2, 50‒60 cm) were sampled monthly from March, 2017 to February, 2018, in the territory of Eldigino experimental station (Moscow oblast). In addition to microbial biomass measurement, soil temperature and moisture were recorded. The microbial biomass at all sampling times was dominated by fungi (up to 93%). Minimal microbial population and biomass were observed in the period from November to March. The biomass of prokaryotes increased twofold in May, the maximal values were observed in August and September. The length of actinomycete mycelium was maximal in July and August, when the soil water content was the lowest. Maximal fungal biomass was observed in July and September. Seasonal changes of microbial biomass were most pronounced in the upper soil horizon P, while they were more even in the BEL and BT horizons. Using regression analysis, we revealed a significant effect of temperature and sampling depth on the fungal and prokaryotic biomass. The results indicate substantial seasonal variations in biomass of soil microbiota, which should be taken into account when comparing soils sampled at different seasons.

Censuses of tropical forest plots reveal large variation in biomass and plant composition. This paper evaluates whether such variation can emerge solely from realistic variation in a set of commonly measured soil chemical and physical properties. Controlled simulations were performed using a mechanistic model that includes forest dynamics, microbe‐mediated biogeochemistry, and competition for nitrogen and phosphorus. Observations from 18 forest inventory plots in Guanacaste, Costa Rica were used to determine realistic variation in soil properties. In simulations of secondary succession, the across‐plot range in plant biomass reached 30% of the mean and was attributable primarily to nutrient limitation and secondarily to soil texture differences that affected water availability. The contributions of different plant functional types to total biomass varied widely across plots and depended on soil nutrient status. In Central America, soil‐induced variation in plant biomass increased with mean annual precipitation because of changes in nutrient limitation. In Central America, large variation in plant biomass and ecosystem composition arises mechanistically from realistic variation in soil properties. The degree of biomass and compositional variation is climate sensitive. In general, model predictions can be improved through better representation of soil nutrient processes, including their spatial variation.

Soil contains a large amount of organic matter, which constitutes the largest terrestrial carbon pool. Heterotrophic or microbial respiration (Rh) that results from microbial decomposition of soil organic carbon (SOC) constitutes a substantial proportion of soil C efflux. Whether soil microbial biomass is of primary importance in controlling Rh remains under debate, and the question of whether the microbial biomass-decomposition relationship changes with warming and nitrogen (N) deposition has rarely been assessed. We conducted an incubation experiment to test the relationship between Rh and the size of soil microbial communities in two layers of soil collected from a natural subtropical forest and to examine whether the relationship was affected by changes in temperature and by added N in different forms. The results showed that regardless of the added N species, the N load did not significantly affect Rh or the size of the soil microbial communities. These results could be due to a long-term N-rich soil condition that acclimates soil microbial communities to resist N inputs into the studied forest; however, warming may significantly stimulate SOC decomposition, reducing soil microbial biomass under high temperatures. A significant linear soil microbial biomass-decomposition relationship was observed in our study, with the coefficients of determination ranging from 54 to 70%. Temperature rather than N additions significantly modified the linear relationship between soil microbial biomass and respiration. These results suggest that warming could impose a more substantial impact than N addition on the relationship between soil microbial biomass and SOC decomposition.