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Background: Microplastics (MP, plastic particles <5 mm) are ubiquitous in arable soils due to significant inputs via organic fertilizers, sewage sludges, and plastic mulches. However, knowledge of typical MP loadings, their fate, and ecological impacts on arable soils is limited. Aims: We studied (1) MP background concentrations, (2) the fate of added conventional and biodegradable MP, and (3) effects of MP in combination with organic fertilizers on microbial abundance and activity associated with carbon (C) cycling, and crop yields in an arable soil. Methods: On a conventionally managed soil (Luvisol, silt loam), we arranged plots in a randomized complete block design with the following MP treatments (none, low‐density polyethylene [LDPE], a blend of poly(lactic acid) and poly(butylene adipate‐co‐terephthalate) [PLA/PBAT]) and organic fertilizers (none, compost, digestate). We added 20 kg MP ha–1 and 10 t organic fertilizers ha–1. We measured concentrations of MP in the soil, microbiological indicators of C cycling (microbial biomass and enzyme activities), and crop yields over 1.5 years. Results: Background concentration of MP in the top 10 cm was 296 ± 110 (mean ± standard error) particles <0.5 mm per kg soil, with polypropylene, polystyrene, and polyethylene as the main polymers. Added LDPE and PLA/PBAT particles showed no changes in number and particle size over time. MP did not affect the soil microbiological indicators of C cycling or crop yields. Conclusions: Numerous MP occur in arable soils, suggesting diffuse MP entry into soils. In addition to conventional MP, biodegradable MP may persist under field conditions. However, MP at current concentrations are not expected to affect C turnover and crop yield. [ABSTRACT FROM AUTHOR]

Microplastics (MP, plastic particles between 0.1 and 5000 μm) contaminate agricultural soils through the application of organic fertilizers, sewage sludge, and plastic mulch. MP surfaces and the MP-soil interface provide specific habitats for soil microorganisms—the plastisphere. Microorganisms in the plastisphere may benefit from utilizing MP as a carbon (C) source. Hydrolyzable MP with ester bonds are susceptible to enzymatic depolymerization by hydrolysis. In a microcosm experiment, we investigated MP biodegradation of small and large (< 0.5 mm and 0.5–2 mm respectively), hydrolyzable (a poly(lactic acid)/poly(butylene co-adipate terephthalate) blend, PLA/PBAT) and non-hydrolyzable (low-density polyethylene, LDPE) polymers, and the effects of these MP on microorganisms in dry and wet MP-amended soil. MP affected neither abundance and composition of the main soil microbial groups (fungi, Gram-negative, and Gram-positive bacteria), specific activities of ß-glucosidase, ß-xylosidase, lipase, and phenoloxidase, nor respiration in MP-amended soil. Only large PLA/PBAT particles in dry soil were significantly mineralized (15.4% of initial PLA/PBAT-C after 230 days). PLA/PBAT mineralization coincided with enhanced lipase and ß-glucosidase activities on the surfaces of individual PLA/PBAT particles extracted from the soil after incubation (compared to LDPE and non-incubated PLA/PBAT particles). We detected cracks on the surfaces of PLA/PBAT particles using scanning electron microscopy, indicating initiation of MP biodegradation, presumably due to depolymerization by lipases. Results suggest that the PLA/PBAT plastisphere is a polymer-specific habitat for lipase-producing soil microorganisms. Our study demonstrates that analyzing biogeochemical interactions within polymer-specific plastispheres is essential to assess MP fate and their impacts on microbially driven soil processes. [ABSTRACT FROM AUTHOR]

Plastics have become an emerging pollutant threatening the sustainability of agroecosystems and global food security. Biochar, a pro-ecosystem/negative carbon emission technology can be exploited as a circular approach for the conservation of plastics contaminated agricultural soils. However, relatively few studies have focused on the effects of biochar on plant growth and soil biochemical properties in a microplastic contaminated soil. This study investigated the effects of a cotton stalk (Gossypium hirsutum L.) biochar on plant growth, soil microbes, and enzyme activity in PVC microplastic (PVC-MPs) contaminated soil. Biochar amendment increased shoot dry matter production in PVC-MPs contaminated soil. However, PVC-MPs alone significantly reduced the soil urease and dehydrogenase activity, soil organic and microbial biomass carbon, bacterial/fungal community percentage, and their abundance (16S rRNA and 18S rRNA genes, respectively). Interestingly, biochar amendment with PVC-MPs significantly alleviated the hazardous effects. Principal component and redundancy analysis of the soil properties, bacterial 16S rRNA genes, and fungal ITS in the biochar-amended PVC-MPs treatments revealed that the observed traits formed an obvious cluster compared to non-biochar treatments. To sum up, this study indicated that PVC-MPs contamination was not benign, while biochar shielded the hazardous effects and sustained soil microbial functionality. [Display omitted] • PVC-MPs reduced soil enzymatic activities and microbial abundance in soil-plant system. • Biochar increased shoot dry matter production in PVC-MPs contaminated soil. • Addition of biochar alleviated hazardous effects in PVC-MPs contaminated soil. [ABSTRACT FROM AUTHOR]

• Earthworms affect the distribution of Si among fractions in soil. • More Si extracted from earthworm casts than from undigested materials. • Silicon bound to SOM is the major contributor to additionally extractable Si in soil. • Aggregates occlude plant-available Si in the casts. It is known that earthworms can increase the content of water-extractable silicon (Si) in soil, thus contributing to the availability of Si for plants. However, effects of earthworms on other Si fractions in soil, such as adsorbed Si, Si bound to SOM, Si occluded in pedogenic oxides and amorphous silica have not yet been studied. Therefore, we investigated the effects of the endogeic earthworm Octolasion cyaneum Savigny on the fractionation of Si in soils and that of the epigeic earthworm Eisenia andrei Bouché on the release of Si from model substances (quartz, wheat straw, bioopal). We quantified the amounts of Si in different soil fractions and those released from model substances before and after passage through the earthworm gut by sequential Si extraction. The amounts of Si extracted from the earthworms' casts were generally larger than in the undigested samples. This was especially pronounced for Si bound to soil organic matter (SOM; up to 41%), and for Si in wheat straw (up to 71%) and quartz (up to 1730%). With the soils, the increase in extracted Si was pronounced for the more mobile fractions (Si bound to SOM and occluded in pedogenic oxides) at the expense of amorphous silica. The amounts of mobile and adsorbed Si (plant-available Si) in soil tended to decrease after the passage through the earthworm gut, possibly by occlusion of adsorbents in aggregates formed in the gut. Our results indicate that both mechanical weathering of ingested Si-containing particles and microbial processes, promoting aggregate formation, SOM transformation and mineral solubilization, contribute to the increased release of Si, which induced the redistribution of Si among fractions. [ABSTRACT FROM AUTHOR]

Soil protists are increasingly appreciated as essential components of soil foodwebs; however, there is a dearth of information on the factors structuring their communities. Here we investigate the importance of different biotic and abiotic factors as key drivers of spatial and seasonal distribution of protistan communities. We conducted an intensive survey of a 10 m2 grassland plot in Germany, focusing on a major group of protists, the Cercozoa. From 177 soil samples, collected from April to November, we obtained 694 Operational Taxonomy Units representing >6 million Illumina reads. All major cercozoan taxonomic and functional groups were present, dominated by the small flagellates of the Glissomonadida. We found evidence of environmental selection structuring the cercozoan communities both spatially and seasonally. Spatial analyses indicated that communities were correlated within a range of 3.5 m. Seasonal variations in the abundance of bacterivores and bacteria, followed by that of omnivores suggested a dynamic prey-predator succession. The most influential edaphic properties were moisture and clay content, which differentially affected each functional group. Our study is based on an intense sampling of protists at a small scale, thus providing a detailed description of the biodiversity of different taxa/functional groups and the ecological processes involved in shaping their distribution. [ABSTRACT FROM AUTHOR]

Global warming will likely enhance greenhouse gas ( GHG) emissions from soils. Due to its slow decomposability, biochar is widely recognized as effective in long-term soil carbon (C) sequestration and in mitigation of soil GHG emissions. In a long-term soil warming experiment (+2.5 °C, since July 2008) we studied the effect of applying high-temperature Miscanthus biochar (0, 30 t/ha, since August 2013) on GHG emissions and their global warming potential ( GWP) during 2 years in a temperate agroecosystem. Crop growth, physical and chemical soil properties, temperature sensitivity of soil respiration ( R s), and metabolic quotient ( qCO2) were investigated to yield further information about single effects of soil warming and biochar as well as on their interactions. Soil warming increased total CO2 emissions by 28% over 2 years. The effect of warming on soil respiration did not level off as has often been observed in less intensively managed ecosystems. However, the temperature sensitivity of soil respiration was not affected by warming. Overall, biochar had no effect on most of the measured parameters, suggesting its high degradation stability and its low influence on microbial C cycling even under elevated soil temperatures. In contrast, biochar × warming interactions led to higher total N2O emissions, possibly due to accelerated N-cycling at elevated soil temperature and to biochar-induced changes in soil properties and environmental conditions. Methane uptake was not affected by soil warming or biochar. The incorporation of biochar-C into soil was estimated to offset warming-induced elevated GHG emissions for 25 years. Our results highlight the suitability of biochar for C sequestration in cultivated temperate agricultural soil under a future elevated temperature. However, the increased N2O emissions under warming limit the GHG mitigation potential of biochar. [ABSTRACT FROM AUTHOR]

Very few principles have been unraveled that explain the relationship between soil properties and soil biota across large spatial scales and different land-use types. Here, we seek these general relationships using data from 52 differently managed grassland and forest soils in three study regions spanning a latitudinal gradient in Germany. We hypothesize that, after extraction of variation that is explained by location and land-use type, soil properties still explain significant proportions of variation in the abundance and diversity of soil biota. If the relationships between predictors and soil organisms were analyzed individually for each predictor group, soil properties explained the highest amount of variation in soil biota abundance and diversity, followed by land-use type and sampling location. After extraction of variation that originated from location or land-use, abiotic soil properties explained significant amounts of variation in fungal, meso- and macrofauna, but not in yeast or bacterial biomass or diversity. Nitrate or nitrogen concentration and fungal biomass were positively related, but nitrate concentration was negatively related to the abundances of Collembola and mites and to the myriapod species richness across a range of forest and grassland soils. The species richness of earthworms was positively correlated with clay content of soils independent of sample location and land-use type. Our study indicates that after accounting for heterogeneity resulting from large scale differences among sampling locations and land-use types, soil properties still explain significant proportions of variation in fungal and soil fauna abundance or diversity. However, soil biota was also related to processes that act at larger spatial scales and bacteria or soil yeasts only showed weak relationships to soil properties. We therefore argue that more general relationships between soil properties and soil biota can only be derived from future studies that consider larger spatial scales and different land-use types. [ABSTRACT FROM AUTHOR]

A geostatistical approach using replicated grassland sites (10 m × 10 m) was applied to investigate the influence of grassland management, i.e. unfertilized pastures and fertilized mown meadows representing low and high land-use intensity (LUI), on soil biogeochemical properties and spatial distributions of ammonia-oxidizing and denitrifying microorganisms in soil. Spatial autocorrelations of the different N-cycling communities ranged between 1.4 and 7.6 m for ammonia oxidizers and from 0.3 m for nosZ-type denitrifiers to scales >14 m for nirK-type denitrifiers. The spatial heterogeneity of ammonia oxidizers and nirS-type denitrifiers increased in high LUI, but decreased for biogeochemical properties, suggesting that biotic and/or abiotic factors other than those measured are driving the spatial distribution of these microorganisms at the plot scale. Furthermore, ammonia oxidizers ( amoA ammonia-oxidizing archaea and amoA ammonia-oxidizing bacteria) and nitrate reducers ( napA and narG) showed spatial coexistence, whereas niche partitioning was found between nirK- and nirS-type denitrifiers. Together, our results indicate that spatial analysis is a useful tool to characterize the distribution of different functional microbial guilds with respect to soil biogeochemical properties and land-use management. In addition, spatial analyses allowed us to identify distinct distribution ranges indicating the coexistence or niche partitioning of N-cycling communities in grassland soil. [ABSTRACT FROM AUTHOR]

Earthworms are important members of the soil macrofauna. They modify soil physical properties, soil organic matter decomposition, and thus regulate carbon and nitrogen cycling in soil. However, their interactions with soil microorganisms are still poorly understood, in particular the effect of gut passage on the community structure of ingested microorganisms. Moreover, it is still unsolved, if earthworms, like many other soil-feeding invertebrates, possess an indigenous gut microbial community. Therefore, we investigated the bacterial and archaeal community structure in soil (with and without additional beech litter), gut, and fresh casts of Lumbricus terrestris, an anecic litter-feeding earthworm, by means of terminal-restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene fragments. Ecological indices of community diversity and similarity, calculated from the T-RFLP profiles, revealed only small differences between the bacterial and archaeal communities in soil, gut, and fresh casts under both feeding conditions, especially in comparison to other soil-feeding invertebrates. However, multivariate statistical analysis combining multidimensional scaling and discriminant function analysis proved that these differences were highly significant, in particular when the earthworms were fed beech litter in addition. Because there were no dominant gut-specific OTUs detectable, the existence of an abundant indigenous earthworm microbial community appears unlikely, at least in the midgut region of L. terrestris. [Copyright &y& Elsevier]

Climate change factors alter soil microbial biomass and respiration, which represent two key parameters of carbon (C) cycling in soils. While these interrelations are increasingly well described for soils in natural ecosystems, long-term studies conducted in temperate agricultural ecosystems do rarely exist. Here we used the 'Hohenheim Climate Change Experiment (HoCC)' to describe the response of microbial biomass and respiration to interacting effects of climate change factors. We were interested in whether effects would be season specific, and if ultimately, climate change factors would affect the total soil organic carbon (SOC) content in the arable soil. The HoCC experiment is an agroecosystem where soils were permanently warmed for one decade, and summer precipitation amount and frequency were reduced. We found that soil respiration was not affected by reduced summer precipitation, but strongly increased by warming. This increase was not season specific and persisted over the years. Further, warming continuously enhanced labile C availability, while microbial biomass did not differ between warmed and ambient temperature soils. Therefore, soil respiration stimulated by warming was independent of microbial biomass, and accordingly, we found an increase of the metabolic quotient. Despite the warming induced increase in soil respiration, no indications for a loss in SOC content of the arable soil was detected. These results suggest that in an arable soil with overall low SOC content, the persisting stimulation of respiration due to warming is driven by an agroecosystem specific labile C pool dynamic. • Warming persistently stimulated soil respiration over ten years across seasons. • Reduced summer precipitation only played a minor role within respiration dynamics. • Microbial biomass was largely unaffected by climate change factors across seasons. • Warming increased labile C availability which explains increase in respiration. • No SOC loss from warmed arable soil despite persistent increase in respiration. [ABSTRACT FROM AUTHOR]

Abstract: Elevated nitrogen (N) deposition can affect litter decomposition directly, by raising soil N availability and the quantity and quality of litter inputs, and indirectly by altering plant community composition. We investigated the importance of these controls on litter decomposition using litter bags placed in annual herb based microcosm ecosystems that had been subject to two rates of N deposition (which raised soil inorganic N availability and stimulated litter inputs) and two planting regimes, namely the plant species compositions of low and high N deposition environments. In each microcosm, we harvested litter bags of 10 annual plant species, over an 8-week period, to determine mass loss from decomposition. Our data showed that species differed greatly in their decomposability, but that these differences were unlikely to affect decomposition at the ecosystem level because there was no correlation between a species’ decomposability and its response to N deposition (measured as population seed production under high N, relative to low N, deposition). Litter mass loss was ∼2% greater in high N deposition microcosms. Using a comprehensive set of measurements of the microcosm soil environments, we found that the most statistically likely explanation for this effect was increased soil enzyme activity (cellobiosidase, β-glucosidase and β-xylosidase), which appears to have occurred in response to a combination of raised soil inorganic N availability and stimulated litter inputs. Our data indicate that direct effects of N deposition on litter input and soil N availability significantly affected decomposition but indirect effects did not. We argue that indirect effects of changes to plant species composition could be stronger in natural ecosystems, which often contain a greater diversity of plant functional types than those considered here. [Copyright &y& Elsevier]