Carbon dynamics in a 60?year fallowed loamy-sand soil compared to that in a 60?year permanent arable or permanent grassland UK soil

Aims

To determine if the soil microbial biomass in a 60?year fallow soil of the Highfield Ley-Arable Experiment at Rothamsted Research, UK, had maintained its ability to mineralise soil organic matter and added substrates compared to biomasses in a grassland and arable soil of the same experiment.

Materials and methods

Three soils of the same type: a 60 y permanent fallow, arable and grassland, were incubated (25°C, 40% WHC) with and without 1. a labile substrate (yeast extract, C/N ratio 3.6) or 2. more resistant ryegrass, (< 2?mm, C/N ratio 14.6). Measurements included biomass C, ATP, PLFAs and substrate C mineralization.

Results

Mean biomass C and ATP concentrations were:grassland.arable.fallow, as expected. However, substrate C mineralization was less in the grassland than fallow soil, opposite to that expected. Microbial biosynthesis efficiency (measured as biomass C and ATP) was similar in all soils. However, microbial community structure differed significantly between soils and treatments.

Conclusions

The extent of mineralization of both substrates were unrelated to initial microbial community structure, size or soil management. Thus, the biomass in the fallow soil maintained full metabolic capacity (assessed by CO₂-C evolution) compared to permanent arable or grassland soils.     

Greenhouse gas emissions from a Cu-contaminated soil remediated by in situ stabilization and phytomanaged by a mixed stand of poplar,willows, and false indigo-bush

Phytomanagement of trace element-contaminated soils can reduce soil toxicity and restore soil ecological functions, including the soil gas exchange with the atmosphere. We studied the emission rate of the greenhouse gases (GHGs) CO₂, CH₄, and N₂O; the potential CH₄ oxidation; denitrification enzyme activity (DEA), and glucose mineralization of a Cu-contaminated soil amended with dolomitic limestone and compost, alone or in combination, after a 2-year phytomanagement with a mixed stand of Populus nigra, Salix viminalis, S. caprea, and Amorpha fruticosa. Soil microbial biomass and microbial community composition after analysis of the phospholipid fatty acids (PLFA) profile were determined. Phytomanagement significantly reduced Cu availability and soil toxicity, increased soil microbial biomass and glucose mineralization capacity, changed the composition of soil microbial communities, and increased the CO₂ and N₂O emission rates and DEA. Despite such increases, microbial communities were evolving toward less GHG emission per unit of microbial biomass than in untreated soils. Overall, the aided phytostabilization option would allow methanotrophic populations to establish in the remediated soils due to decreased soil toxicity and increased nutrient availability.     

Ecosystem properties and microbial community changes in primary succession on a glacier forefront

We studied microbial community composition in a primary successional chronosequence on the forefront of Lyman Glacier, Washington, United States. We sampled microbial communities in soil from nonvegetated areas and under the canopies of mycorrhizal and nonmycorrhizal plants from 20- to 80-year-old zones along the successional gradient. Three independent measures of microbial biomass were used: substrate-induced respiration (SIR), phospholipid fatty acid (PLFA) analysis, and direct microscopic counts. All methods indicated that biomass increased over successional time in the nonvegetated soil. PLFA analysis indicated that the microbial biomass was greater under the plant canopies than in the nonvegetated soils; the microbial community composition was clearly different between these two types of soils. Over the successional gradient, the microbial community shifted from bacterial-dominated to fungal-dominated. Microbial respiration increased while specific activity (respiration per unit biomass) decreased in nonvegetated soils over the successional gradient. We proposed and evaluated new parameters for estimating the C use efficiency of the soil microbial community: “Max” indicates the maximal respiration rate and “Acc” the total C released from the sample after a standard amount of substrate is added. These, as well as the corresponding specific activities (calculated as Max and Acc per unit biomass), decreased sharply over the successional gradient. Our study suggests that during the early stages of succession the microbial community cannot incorporate all the added substrate into its biomass, but rapidly increases its respiration. The later-stage microbial community cannot reach as high a rate of respiration per unit biomass but remains in an “energy-saving state,” accumulating C to its biomass. Received: 4 June 1998 / Accepted: 11 January 1999     

Impacts of chronic nitrogen additions vary seasonally and by microbial functional group in tundra soils

Previous studies have shown that fertilization with nitrogen depresses overall microbial biomass and activity in soil. In the present study we broaden our understanding of this phenomenon by studying the seasonality of responses of specific microbial functional groups to chronic nitrogen additions in alpine tundra soils. We measured soil enzyme activities, mineralization kinetics for 8 substrates, biomass of 8 microbial functional groups, and changes in N and carbon pools in the soil. Our approach allowed us to compare the ability of the soil microbial biomass to utilize various substrates in addition to allowing us to estimate changes in biomass of microbial functional groups that are involved in carbon and nitrogen cycling. Overall microbial activity and biomass was reduced in fertilized plots, whereas pools of N in the soil and microbial biomass N were higher in fertilized plots. The negative effects of N were most prominent in the summer. Biomass of the dominant microbial functional groups recovered in fertilized soils during the winter and nitrogen storage in microbial biomass was higher in fertilized soils in the autumn and winter than in the summer. Microbial immobilization of N may therefore be a significant sink for added N during autumn and winter months when plants are not active. One large microbial group that did not recover in the winter in fertilized soils was phenol mineralizers, possibly indicating selection against microbes with enzyme systems for the breakdown of phenolic compounds and complex soil organic matter. Overall, this work is a step towards understanding how chronic N additions affect the structure and biogeochemical functioning of soil microbial communities.     

Biochemical properties of soils of undisturbed and disturbed mangrove forests of South Andaman (India)

Studies on soil quality of mangrove forests would be of immense use in minimizing soil degradation and in adopting strategies for soil management at degraded sites. Among the various parameters of soil quality, biological and biochemical soil properties are very sensitive to environmental stress and provide rapid and accurate estimates on changes in quality of soils subjected to degradation. In this study, we determined the general and specific biochemical characteristics of soils (0-30 cm) of inter-tidal areas of 10 undisturbed mangrove forest sites of S. Andaman, India. In order to determine the effects of disturbance, soils from the inter-tidal areas of 10 disturbed mangrove forest sites were also included in the study. The general biochemical properties included all the variables directly related to microbial activity and the specific biochemical parameters included the activities of extracellular hydrolytic enzymes that are involved in the carbon, nitrogen, sulfur and phosphorus cycles in soil. The pH, clay, cation exchange capacity, Al₂O₃ and Fe₂O₃ levels exhibited minimum variation between the disturbed and undisturbed sites. In contrast, organic C, total N, Bray P and K levels exhibited marked variation between the sites and were considerably lower at the disturbed sites. The study also revealed marked reductions in microbial biomass and activity at the disturbed sites. In comparison to the undisturbed sites, the levels of all the general biochemical parameters viz., microbial biomass C, microbial biomass N, N flush, basal respiration, metabolic quotient (qCO₂), ATP, N mineralization rates and the activities of dehydrogenase and catalase were considerably lower at the disturbed sites. Similarly, drastic reductions in the activities of phosphomonoesterase, phosphodiesterase, ß-g1ucosidase, urease, BAA-protease, casein-protease, arylsulfatase, invertase and carboxymethylcellulase occurred at the disturbed sites due mainly to significant reductions in organic matter/substrate levels. The data on CO₂ evolution, qCO₂ and ATP indicated the dominance of active individuals in the microbial communities of undisturbed soils and the ratios of biomass C:N, ATP:biomass C and ergosterol:biomass C ratios indicated low N availability and the possibility of fungi dominating over bacteria at both the mangrove sites. Significant and positive correlations between soil variables and biochemical properties suggested that the number and activity of soil microorganisms depend mainly on the quantity of mineralizable substrate and the availability of nutrients in these mangrove soils.     

Temporal variations in some plant and soil P pools in two pasture soils of widely different P fertility status

Temporal variations in plant production, plant P and some soil P (and N) pools were followed over 21 months in two New Zealand pasture soils of widely different P fertility status. Plant growth rates, and herbage composition at the high-fertility site, were closely linked to soil water use, with growth rates falling when soil water deficits exceeded 60 mm. Herbage P concentrations reflected P fertility, and varied with season, being generally higher in winter and lower in summer. A similar temporal pattern was also observed for labile organic P (NaHCO₃-extractable P₀) in both soils. In the low-fertility soil in spring, net mineralization was especially strong, but from early winter net immobilization occurred. Surprisingly, Olsen P also changed temporally in the high-fertility soil. The microbial biomass remained fairly constant throughout the year, whereas the P content of the biomass varied seasonally. Although microbial biomass was not a useful index of soil fertility, highest microbial P₀ contents coincided with periods of maximum labile P₀ mineralization, when herbage production was also at a peak. Net N-mineralization in the low-fertility soil, in contrast to the high-fertility soil, was low but varied seasonally, under standardised incubation conditions. Soil P and N dynamics were apparently synchronised in the low-fertility soil through soil microbial processes, with mineral N being negatively correlated with microbial P₀ in samples collected two months later. The results of this investigation suggest that the demands of rapid and sustained pasture growth in spring and early summer can best be met by maximising the build-up of organic matter during the preceding autumn and winter. This practice could help to alleviate the common problem of feed shortage in North Island hill country pastures in late winter-early spring.     

Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO₂ concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C‐inputs to soil (e.g. plant‐derived) can significantly affect mineralization of SOM (‘priming effects’) complicates prediction of environmental and land‐use change effects on SOM dynamics and soil C‐balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3 atm% ¹³C) addition (in the range 0–1 mg glucose g^?1 soil day^?1) were applied for 8 days. Soil CO₂ efflux was partitioned into SOM‐ and glucose‐derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass (‘apparent priming’) were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5 mg glucose g^?1 soil), priming was a soil‐specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C‐models. However, these response functions are soil‐specific and may be affected by several interacting factors at lower addition rates.     

Organic matter turnover in a sagebrush steppe landscape

Laboratory incubations of¹⁵N-amended soils from a sagebrush steppe in south-central Wyoming indicate that nutrient turnover and availability have complex patterns across the landscape and between microsites. Total and available N and P and microbial C and N were highest in topographic depressions characterized by tall shrub communities. Net and gross N mineralization rates and respiration were also highest in these areas, but microbial efficiencies expressing growth relative to respiration cost were highest in soils of exposed ridgetop sites (prostrate shrub communities). Similar patterns occurred between shrub and intershrub soils, with greater nutrient availability under shrubs, but lower microbial efficiencies under shrubs than between. Surface soils had higher soil nutrient pools and N mineralization rates than subsurface soils, but N and C turnover and microbial efficiencies were lower in those surface soils. All soils decreased in respiration, mineralization, and immobilization rates during the 30-day incubation period, apparently approaching a steady-state substrate use. Soil microbial activity of the high organic matter accumulation areas was apparently more limited by labile substrate.     

Microbial community utilization of recalcitrant and simple carbon compounds: impact of oak-woodland plant communities

Little is known about how the structure of microbial communities impacts carbon cycling or how soil microbial community composition mediates plant effects on C-decomposition processes. We examined the degradation of four ¹³C-labeled compounds (starch, xylose, vanillin, and pine litter), quantified rates of associated enzyme activities, and identified microbial groups utilizing the ¹³C-labeled substrates in soils under oaks and in adjacent open grasslands. By quantifying increases in non-¹³C-labeled carbon in microbial biomarkers, we were also able to identify functional groups responsible for the metabolism of indigenous soil organic matter. Although microbial community composition differed between oak and grassland soils, the microbial groups responsible for starch, xylose, and vanillin degradation, as defined by ¹³C-PLFA, did not differ significantly between oak and grassland soils. Microbial groups responsible for pine litter and SOM-C degradation did differ between the two soils. Enhanced degradation of SOM resulting from substrate addition (priming) was greater in grassland soils, particularly in response to pine litter addition; under these conditions, fungal and Gram + biomarkers showed more incorporation of SOM-C than did Gram – biomarkers. In contrast, the oak soil microbial community primarily incorporated C from the added substrates. More ¹³C (from both simple and recalcitrant sources) was incorporated into the Gram – biomarkers than Gram + biomarkers despite the fact that the Gram + group generally comprised a greater portion of the bacterial biomass than did markers for the Gram – group. These experiments begin to identify components of the soil microbial community responsible for decomposition of different types of C-substrates. The results demonstrate that the presence of distinctly different plant communities did not alter the microbial community profile responsible for decomposition of relatively labile C-substrates but did alter the profiles of microbial communities responsible for decomposition of the more recalcitrant substrates, pine litter and indigenous soil organic matter.     

Carbon and nitrogen limitations of soil microbial biomass in desert ecosystems

Microbial biomass nitrogen was measured in unamended (dry) and wetted soils in ten shrubland and grassland communities of the Chihuahuan desert, southern New Mexico, by the fumigation-extraction method. Microbial biomass-N in dry soils was undetectable. Average microbial biomass-N in wetted soils among all plant communities was 15.3 μg g^-1 soil. Highest values were found in the communities with the lowest topographic positions, and the minimum values were detected in the spaces between shrubs. Microbial biomass was positively and significantly correlated to soil organic carbon and extractable nitrogen (NH₄ ⁺ + NO₃ ^-). In a stepwise multiple regression, organic carbon and extractable nitrogen accounted for 40.9 and 5.6%, respectively, of the variance in microbial biomass-N among all the samples. Among communities, the soil microbial biomass was affected by the ratio of carbon to extractable nitrogen. Our results suggest a succession in the control of microbial biomass from nitrogen to carbon when the ratio of carbon to nitrogen decreases during desertification.     

Characterization of the rhizosphere microbial community from different Arabidopsis thaliana genotypes using phospholipid fatty acids (PLFA) analysis

Total and culturable rhizosphere microbial communities structure from three different genotypes of Arabidopsis thaliana growing on three different substrates was studied with phospholipid fatty acid analysis (PLFA) and multivariate statistical analyses: correspondence analysis (CA) and distance based redundancy analyses (db-RDA). In addition, microbial biomass from different groups (total bacteria, Gram+, Gram? and fungi) was calculated from biomarkers PLFA peak area, both from total and culturable microbial community. db-RDA analysis showed significant differences between soils but not between plant genotypes for culturable microbial community structure. Total microbial community was significantly different between soils, and also between plant lines in each soil. Biomass of different bacterial groups showed significant higher values in soil two rhizosphere irrespective of the plant line. In addition, significant differences between plant lines were also found for microbial biomass of different bacterial groups both in total and culturable microbial community. Throughout the work we have demonstrated that PLFA analysis has been able to show a different behaviour of total microbial community with regard to the culturable fraction analyzed in this work under the influence of plant roots. Microbial biomass of different microbial groups calculated with PLFA biomarkers was a suitable tool to detect differences between soils irrespective of the plant line, and differences in the same soil between plant lines. According to this data, a previous study should be carried out before GMPs are used in field conditions to evaluate the potential alterations that may take place on rhizosphere microbial communities structure which may further affect soil productivity. In conclusion, based on data presented in this work, GMPs alter rhizosphere microbial communities structure and this effect is different depending on the soil. Furthermore, total microbial community is affected to a greater extent than the culturable fraction analyzed.     

Positive effects of plant diversity on soil microbial biomass and activity are associated with more root biomass production

This study aims to explore relationships between plant diversity and soil microbial function and the factors that mediate the relationships. Artificial plant communities (1, 2, 4 and 8 species) were established filled with natural and mine tailing soils, respectively. After 12 months, the plant species richness positively affected the soil microbial functional diversity in both soil environments but negatively affected microbial biomass and soil basal respiration in the natural soil. The root biomass positively correlated with the microbial biomass, cultural bacterial activity and soil basal respiration in both soil environments. Moreover, the D_i (deviations between observed performances and expected performances from the monoculture performance of each species of mixture) of microbial biomass, cultural bacterial activity and soil basal respiration positively correlated with the D_i of root biomass in both soil environments. Consistent with stress-gradient hypothesis, the D_mix (over-function index) of aboveground biomass positively correlated plant species richness in the mine tailing soil. Results suggest that the root biomass production is an important mechanism that affects the effects of plant diversity on soil microbial functions. Different responses of soil microbial function to increasing plant diversity may be due to root biomass production mediated by other factors.     

Soil Carbon, Nitrogen and Phosphorus Dynamics as Affected by Solarization Alone or Combined with Organic Amendment

Soil solarization, alone or combined with organic amendment, is an increasingly attractive approach for managing soil-borne plant pathogens in agricultural soils. Even though it consists in a relatively mild heating treatment, the increased soil temperature may strongly affect soil microbial processes and nutrients dynamics. This study aimed to investigate the impact of solarization, either with or without addition of farmyard manure, in soil dynamics of various C, N and P pools. Changes in total C, N and P contents and in some functionally-related labile pools (soil microbial biomass C and N, K₂SO₄-extractable C and N, basal respiration, KCl-exchangeable ammonium and nitrate, and water-soluble P) were followed across a 72-day field soil solarization experiment carried out during a summer period on a clay loam soil in Southern Italy. Soil physico-chemical properties (temperature, moisture content and pH) were also monitored. The average soil temperature at 8-cm depth in solarized soils approached 55 °C as compared to 35 °C found in nonsolarized soil. Two-way ANOVA (solarization×organic amendment) showed that both factors significantly affected most of the above variables, being the highest influence exerted by the organic amendment. With no manure addition, solarization did not significantly affect soil total C, N and P pools. Whereas soil pH, microbial biomass and, at a greater extent, K₂SO₄-extractable N and KCl-exchangeable ammonium were greatly affected. An increased release of water-soluble P was also found in solarized soils. Yet, solarization altered the quality of soluble organic residues released in soil as it lowered the C-to-N ratio of both soil microbial biomass and K₂SO₄-extractable organic substrates. Additionally, in solarized soils the metabolic quotient (qCO₂) significantly increased while the microbial biomass C-to-total organic C ratio (microbial quotient) decreased over the whole time course. We argued that soil solarization promoted the mineralization of readily decomposable pools of the native soil organic matter (e.g. the microbial biomass) thus rendering larger, at least over a short-term, the available fraction of some soil mineral nutrients, namely N and P forms. However, over a longer prospective solarization may lead to an over-exploitation of labile organic resources in agricultural soils. Manure addition greatly increased the levels of both total and labile C, N and P pools. Thus, addition of organic amendments could represent an important strategy to protect agricultural lands from excessive soil resources exploitation and to maintain soil fertility while enhancing pest control.     

Decomposition of 14C-labeled cellulose substrates in litter and soil from a beechwood on limestone

The decomposition of three different ¹⁴C-labeled cellulose substrates (plant holocellulose, plant cellulose prepared from ¹⁴C-labeled beech wood (Fagus sylvatica) and bacterial cellulose produced by Acetobacter xylinum) in samples from the litter and mineral soil layer of a beechwood on limestone was studied. In a long-term (154 day) experiment, mineralization of cellulose materials, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass was in the order Acetobacter cellulose > holocellulose > plant cellulose in both litter and soil. In general, mineralization of cellulose, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass were more pronounced, but microbial biomass ¹⁴C declined more rapidly in litter than in soil. In short-term (14 day) incubations, mineralization of cellulose substrates generally corresponded with cellulase and xylanase activities in litter and soil. Pre-incubation with trace amounts of unlabeled holocellulose significantly increased the decomposition of ¹⁴C-labeled cellulose substrates and increased cellulase activity later in the experiment but did not affect xylanase activity. The sum of ¹⁴CO₂ production, ¹⁴C in microbial biomass, and ¹⁴C in water-soluble compounds is considered to be a sensitive parameter by which to measure cellulolytic activity in soil and litter samples in short-term incubations. Shorter periods than 14 days are preferable in assays using Acetobacter cellulose, because the decomposition of this substrate is more variable than that of holocellulose and plant cellulose.Offprint requests to: S. Scheu.     

Effects of Substrate Addition on Soil Respiratory Carbon Release Under Long-Term Warming and Clipping in a Tallgrass Prairie

Regulatory mechanisms of soil respiratory carbon (C) release induced by substrates (i.e., plant derived substrates) are critical for predicting ecosystem responses to climate change, but the mechanisms are not well understood. In this study, we sampled soils from a long-term field manipulative experiment and conducted a laboratory incubation to explore the role of substrate supply in regulating the differences in soil C release among the experimental treatments, including control, warming, clipping, and warming plus clipping. Three types of substrates (glucose, C₃ and C₄ plant materials) were added with an amount equal to 1% of soil dry weight under the four treatments. We found that the addition of all three substrates significantly stimulated soil respiratory C release in all four warming and clipping treatments. In soils without substrate addition, warming significantly stimulated soil C release but clipping decreased it. However, additions of glucose and C₃ plant materials (C₃ addition) offset the warming effects, whereas C₄ addition still showed the warming-induced stimulation of soil C release. Our results suggest that long-term warming may inhibit microbial capacity for decomposition of C₃ litter but may enhance it for decomposition of C₄ litter. Such warming-induced adaptation of microbial communities may weaken the positive C-cycle feedback to warming due to increased proportion of C₄ species in plant community and decreased litter quality. In contrast, clipping may weaken microbial capacity for warming-induced decomposition of C₄ litter but may enhance it for C₃ litter. Warming- and clipping-induced shifts in microbial metabolic capacity may be strongly associated with changes in plant species composition and could substantially influence soil C dynamics in response to global change.     

Plant Life History and Residue Chemistry Influences Emissions of CO2 and N2O From Soil – Perspectives for Genetically Modified Cell Wall Mutants

Vascular plants have lignified tissues that transport water, minerals, and photosynthetic products throughout the plant. They are the dominant primary producers in terrestrial ecosystems and capture significant quantities of atmospheric carbon dioxide (CO₂) through photosynthesis. Some of the fixed CO₂ is respired by the plant directly, with additional CO₂ lost from rhizodeposits metabolized by root-associated soil microorganisms. Microbially-mediated mineralization of organic nitrogen (N) from plant byproducts (rhizodeposits, dead plant residues) followed by nitrification generates another greenhouse gas, nitrous oxide (N₂O). In anaerobic soils, reduction of nitrate by microbial denitrifiers also produces N₂O. The plant-microbial interactions that result in CO₂ and N₂O emissions from soil could be affected by genetic modification. Down-regulation of genes controlling lignin biosynthesis to achieve lower lignin concentration or a lower guaiacyl:syringyl (G:S) ratio in above-ground biomass is anticipated to produce forage crops with greater digestibility, improve short rotation woody crops for the wood-pulping industry and create second generation biofuel crops with low ligno-cellulosic content, but unharvested residues from such crops are expected to decompose quickly, potentially increasing CO₂ and N₂O emissions from soil. The objective of this review are the following: 1) to describe how plants influence CO₂ and N₂O emissions from soil during their life cycle; 2) to explain how plant residue chemistry affects its mineralization, contributing to CO₂ and N₂O emissions from soil; and 3) to show how modification of plant lignin biosynthesis could influence CO₂ and N₂O emissions from soil, based on experimental data from genetically modified cell wall mutants of Arabidopsis thaliana. Conceptual models of plants with modified lignin biosynthesis show how changes in phenology, morphology and biomass production alter the allocation of photosynthetic products and carbon (C) losses through rhizodeposition and respiration during their life cycle, and the chemical composition of plant residues. Feedbacks on the soil environment (mineral N concentration, soil moisture, microbial communities, aggregation) affecting CO₂ and N₂O emissions are described. Down-regulation of the Cinnamoyl CoA Reductase 1 (CCR1) gene is an excellent target for highly digestable forages and biofuel crops, but A. thaliana with this mutation has lower plant biomass and fertility, prolonged vegetative growth and plant residues that are more susceptible to biodegradation, leading to greater CO₂ and N₂O emissions from soil in the short term. The challenge in future crop breeding efforts will be to select tissue-specific genes for lignin biosynthesis that meet commercial demands without compromising soil CO₂ and N₂O emission goals.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Soil Amendment with <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> CHA0: Lasting Effects on Soil Biological Properties in Soils Low in Microbial Biomass and Activity

Pseudomonas fluorescens strains are used in agriculture as plant growth-promoting rhizobacteria (PGPR). Nontarget effects of released organisms should be analyzed prior to their large-scale use, and methods should be available to sensitively detect possible changes in the environments the organism is released to. According to ecological theory, microbial communities with a greater diversity should be less susceptible to disturbance by invading organisms. Based on this principle, we laid out a pot experiment with field-derived soils different in their microbial biomass and activity due to long-term management on similar parent geological material (loess). We investigated the survival of P. fluorescens CHA0 that carried a resistance toward rifampicine and the duration of potential changes of the soil microflora caused by the inoculation with the bacterium at the sowing date of spring wheat. Soil microbial biomass (C _mic, N _mic) basal soil respiration (BR), qCO₂, dehydrogenase activity (DHA), bacterial plate counts, mycorrhiza root colonization, and community level substrate utilization were analyzed after 18 and 60 days. At the initial stage, soils were clearly different with respect to most of the parameters measured, and a time-dependent effect between the first and the second set point were attributable to wheat growth and the influence of roots. The effect of the inoculum was small and merely transient, though significant long-term changes were found in soils with a relatively low level of microbial biomass. Community level substrate utilization as an indicator of changes in microbial community structure was mainly changed by the growth of wheat, while other experimental factors were negligible. The sensitivity of the applied methods to distinguish the experimental soils was in decreasing order N _mic, DHA, C _mic, and qCO₂. Besides the selective enumeration of P. fluorescens CHA0 rif⁺, which was only found in amended soils, methods to distinguish the inoculum effect were DHA, C _mic, and the ratio of C _mic to N _mic. The sampling time was most sensitively indicated by N _mic, DHA, C _mic, and qCO₂. Our data support the hypothesis—based on ecosystem theory—that a rich microflora is buffering changes due to invading species. In other words, a soil-derived bacterium was more effective in a relatively poor soil than in soils that are rich in microorganisms.     

Biochar stimulates the decomposition of simple organic matter and suppresses the decomposition of complex organic matter in a sandy loam soil

Incorporating crop residues and biochar has received increasing attention as tools to mitigate atmospheric carbon dioxide (CO₂) emissions and promote soil carbon (C) sequestration. However, direct comparisons between biochar, torrefied biomass, and straw on both labile and recalcitrant soil organic matter (SOM) remain poorly understood. In this study, we explored the impact of biochars produced at different temperatures and torrefied biomass on the simple C substrates (glucose, amino acids), plant residues (Lolium perenne L.), and native SOM breakdown in soil using a ¹⁴C labeling approach. Torrefied biomass and biochars produced from wheat straw at four contrasting pyrolysis temperatures (250, 350, 450, and 550 °C) were incorporated into a sandy loam soil and their impact on C turnover compared to an unamended soil or one amended with unprocessed straw. Biochar, torrefied biomass, and straw application induced a shift in the soil microbial community size, activity, and structure with the greatest effects in the straw‐amended soil. In addition, they also resulted in changes in microbial carbon use efficiency (CUE) leading to more substrate C being partitioned into catabolic processes. While overall the biochar, torrefied biomass, and straw addition increased soil respiration, it reduced the turnover rate of the simple C substrates, plant residues, and native SOM and had no appreciable effect on the turnover rate of the microbial biomass. The negative SOM priming was positively correlated with biochar production temperature. We therefore ascribe the increase in soil CO₂ efflux to biochar‐derived C rather than that originating from SOM. In conclusion, the SOM priming magnitude is strongly influenced by both the soil organic C quality and the biochar properties. In comparison with straw, biochar has the greatest potential to promote soil C storage. However, straw and torrefied biomass may have other cobenefits which may make them more suitable as a CO₂ abatement strategy.     

Land use and climatic factors structure regional patterns in soil microbial communities

Aim  Although patterns are emerging for macroorganisms, we have limited understanding of the factors determining soil microbial community composition and productivity at large spatial extents. The overall objective of this study was to discern the drivers of microbial community composition at the extent of biogeographical provinces and regions. We hypothesized that factors associated with land use and climate would drive soil microbial community composition and biomass.
Location  Great Basin Province, Desert Province and California Floristic Province, California, USA.
Methods  Using phospholipid fatty acid analysis, we compared microbial communities across eight land-use types sampled throughout the State of California, USA ( n = 1117).
Results  The main factor driving composition and microbial biomass was land-use type, especially as related to water availability and disturbance. Dry soils were more enriched in Gram-negative bacteria and fungi, and wetter soils were more enriched in Gram-positive, anaerobic and sulphate-reducing bacteria. Microbial biomass was lowest in ecosystems with the wettest and driest soils. Disturbed soils had less fungal and more Gram-positive bacterial biomass than wildland soils. However, some factors known to influence microbial communities, such as soil pH and specific plant taxa, were not important here.
Main conclusions  Distinct microbial communities were associated with land-use types and disturbance at the regional extent. Overall, soil water availability was an important determinant of soil microbial community composition. However, because of the inclusion of managed and irrigated agricultural ecosystems, the effect of precipitation was not significant. Effects of environmental and management factors, such as flooding, tillage and irrigation, suggest that agricultural management can have larger effects on soil microbial communities than elevation and precipitation gradients.