Increased Soil Microbial Biomass Research Articles

Soil enzyme activities of typical plant communities after vegetation restoration on the Loess Plateau, China

Soil enzyme activities can indicate soil quality due to the catalyst roles enzymes play in various soil biochemical processes. However, little is known about the variations in soil enzyme activities among different plant communities with diverse environmental conditions. Here, the activities of four soil enzymes, catalase (CA), urease (UA), polyphenol oxidase (POA) and saccharase (SA), were investigated in shrub lands, forest lands and abandoned cropland on the Loess Plateau, China. Soil microbial biomass (MB) that contained bacteria, fungi and actinomycetes was also analyzed to determine the variations in enzyme activities. The results showed that soil enzyme activities varied significantly among different plant communities. Land uses, climate factors, topography and soil properties are all related to variations in enzyme activities. Specifically, soil properties including soil nutrients and MB were stable driving factors. With increasing soil MB, UA increased linearly, but CA, POA and SA increased exponentially, which might have resulted from soil nutrient deficiencies. Different plant communities with diverse microbial metabolisms performed differently in terms of soil enzyme activities, for example, in comparison to the other studied species, Hippophae rhamnoides had the greatest advantages and could improve the soil environment more effectively. Therefore, we suggest that the afforestation of Hippophae rhamnoides is an appropriate scenario for vegetation restoration in the typical hilly region of the Loess Plateau. AbbreviationsUnlabelled TableCAcatalase activityUAurease activityPOApolyphenol oxidase activitySAsaccharase activityMBmicrobial biomassSLshrub landFLforest landACabandoned croplandMATmean annual temperatureMAPmean annual precipitationSOCsoil organic carbonTNtotal nitrogenANavailable nitrogenTPtotal phosphorousAPavailable phosphorousTKtotal potassiumAKavailable potassium

Effects of mowing dominant grasses on root exudation and soil nitrogen cycling in a natural sod culture apple orchard

We evaluated the effects of mowing dominant grasses on root exudation and soil nitrogen (N) cycling by assessing metabolomics analysis of root exudates, microbial metabolism, the abundance of N-cycling-related prokaryotes, and different forms of N concentrations in soil. The treatments included Polygonum aviculare L. mowing (T1), Digitaria sanguinalis (L.) Scop. mowing (T2), and no mowing as the controls (CK1 and CK2). The results showed that compared with the no mowing control (CK1 and CK2), T1 and T2 root exudates contained 223 (178 up-regulated, 45 down-regulated) and 183 (40 up-regulated, 143 down-regulated) differential metabolites, respectively. The average well colour development (AWCD) could reflect the microbial metabolic activity. The AWCD values of T1 were increased while that of T2 decreased on the 2<sup>nd</sup> day after mowing. The variation in root exudates was the main reason for the change in soil AWCD values and carbon utilisation of T1 and T2 on the 2<sup>nd</sup> day after mowing. Mowing increased soil microbial biomass N content significantly in the T1 and T2 topsoil. The NO<sub>3</sub><sup>–</sup>-N and NH<sub>4</sub><sup>+</sup>-N contents in the 0–10 cm soil increased on the 2<sup>nd</sup> day after T1 mowing with an increase in the nitrogenase iron protein gene (nifH), glutamate dehydrogenase gene (gdh), ammonia monooxygenase gene (amoA) of ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB) abundance. However, NO<sub>3</sub><sup>–</sup>-N content decreased on the 2<sup>nd</sup> day after T2 mowing following a decrease in AOA-amoA and AOB-amoA gene abundance. The results of this study will facilitate the optimisation of sod culture orchard N management, reduction of N fertiliser input, and improvement of N utilisation efficiency.

Biochar and Sulphur Enriched Digestate: Utilization of Agriculture Associated Waste Products for Improved Soil Carbon and Nitrogen Content, Microbial Activity, and Plant Growth

A number of agriculture residues may be used either directly or after suitable treatment as amendments to improve soil quality. Such materials include biochar made of agriculture residues, digestate or elemental sulphur obtained from biogas desulphurisation. The joint use of these materials via pre-incubation may be more advantageous than only mixing prior the application to soil. In this study, digestates were mixed with amendments and incubated for 6 weeks before application to soil in a short-term pot experiment with lettuce (Lactuca sativa). The following treatments were tested: control digestate, digestate + biochar, digestate + elemental sulphur, digestate + biochar + elemental sulphur. The biochar-enriched digestate significantly increased soil microbial biomass, soil C:N, fresh above ground biomass, fresh and dry root biomass. Elemental sulphur-enriched digestate caused highest arylsulfatase and phosphatase, increased urease, microbial biomass in soil and fresh root biomass. Amendment of digestate + biochar + sulphur led to the significantly highest total soil carbon, microbial biomass, β-glucosidase, urease, and increased C:N ratio, arylsulfatase in soil and root biomass. It mitigated the adverse effect of either biochar or elemental sulphur on soil respiration. Properties of digestates were apparently affected by pre-incubation. This approach in digestate fertilizer production may contribute to sustainable farming.

Effects of in situ freeze-thaw cycles on winter soil respiration in mid-temperate plantation forests

As an important factor regulating soil carbon cycle, freeze-thaw cycle significantly affects winter soil respiration in temperate regions. However, few in situ studies have been carried out to evaluate the effect of freeze-thaw cycle on soil respiration. Here, a field experiment was conducted to explore the response of winter soil respiration to freeze-thaw cycle and the underlying mechanisms in larch and Chinese pine plantation forests in a mid-temperate region. These results indicated that CO2 emissions during the freeze-thaw period accounted for 18.89–18.94% and 0.79–1.00% of the cumulative winter CO2 emissions and the annual soil CO2 emissions, respectively. Soil respiration rates during the thawing phase were 1.54–3.95 times higher than those during the freezing phase, which was mainly due to the increase of soil microbial biomass upon thawing. This effect declined during the second freeze-thaw cycle compared to the first freeze-thaw cycle due to the exhaustion of resources for microbes. The different responses of soil CO2 flux to freeze-thaw cycle between the two types of forests were mainly because of the difference in the thickness of litter layer, which plays an important role in regulating soil temperature and enzyme activity. These results suggest the intensity and frequency of freeze-thaw cycle strongly affect soil carbon emissions during the freeze-thaw cycle period. Therefore, these factors should be considered in laboratory studies and model simulations under climate change scenarios.

Changes in soil microbial community and activity caused by application of dimethachlor and linuron

Soil microorganisms and their activities are essential for maintaining soil health and fertility. Microorganisms can be negatively affected by application of herbicides. Although effects of herbicides on microorganisms are widely studied, there is a lack of information for chloroacetamide herbicide dimethachlor. Thus, dimethachlor and well known linuron were applied to silty-loam luvisol and their effects on microorganisms were evaluated during112 days long laboratory assay. Dimethachlor and linuron were applied in doses 1.0 kg ha−1 and 0.8 kg ha−1 corresponding to 3.33 mg kg−1 and 2.66 mg kg−1 respectively. Also 100-fold doses were used for magnification of impacts. Linuron in 100-fold dose caused minor increase of respiration, temporal increase of soil microbial biomass, decrease of soil dehydrogenase activity, and altered microbial community. Dimethachlor in 100-fold dose significantly increased respiration; microbial biomass and decreased soil enzymatic activities. Microbial composition changed significantly, Proteobacteria abundance, particularly Pseudomonas and Achromobacter genera increased from 7 to 28th day. In-silico prediction of microbial gene expression by PICRUSt2 software revealed increased expression of genes related to xenobiotic degradation pathways. Evaluated characteristics of microbial community and activity were not affected by herbicides in recommended doses and the responsible use of both herbicides will not harm soil microbial community.

Does fresh farmyard manure introduce surviving microbes into soil or activate soil-borne microbiota?

Manure inputs into soil strongly affect soil microbial communities leading to shifts in microbial diversity and activity. It is still not clear whether these effects are caused mainly by the survival of microbes introduced with manure or by activation of the soil-borne microbiome. Here, we investigated how the soil microbiome was changed after the introduction of fresh farmyard cattle manure, and which microorganisms originating from manure survived in soil. Manure addition led to a strong increase in soil microbial biomass, gene copies abundances, respiration activity, and diversity. High-throughput sequencing analysis showed that higher microbial diversity in manured soil was caused mainly by activation of 113 soil-borne microbial genera which were mostly minor taxa in not-fertilized soil. Two weeks after manure input, 78% of the manure-associated genera were not detected anymore. Only 15 of 237 prokaryotic genera that originated from manure survived for 144 days in soil, and only 8 of them (primarily representatives of Clostridia class) were found in manured soil after winter. Thus, an increase in microbial biomass and diversity after manure input is caused mainly by activation of soil-borne microbial communities, while most exogenous microbes from manure do not survive in soil conditions after few months.

Effects of Enhanced-Efficiency Nitrogen Fertilizers on Soil Microbial Biomass and Respiration in Tropical Soil Under Upland Rice Cultivation

While over-use of N fertilizers can suppress microbial biomass, application of urease inhibitors is known to be a potential way to rebuilt microbial diversity and improve soil functions. However, the hypothesis of this study is that the application of N fertilizers regardless of the source would increase soil microbial biomass and reduce soil respiration. A two-year field experiment was conducted to assess the effects of enhanced-efficiency N sources on soil microbial biomass, and soil respiration. The experiment was set up in a randomized block design in a 3 × 4 + 1 factorial scheme, with four replicates. Treatments comprised three sources (conventional uncoated urea, NBPT (N-(n-butyl) thiophosphoric triamide)-treated urea, and polymer-coated urea) and four rates (30, 60, 90 and 120 kg ha-1) of N, in addition to a control treatment (no fertilizer application). Microbial biomass C (MBC) and microbial biomass N (MBN), and soil respiration (C-CO2 and qCO2) were determined in upland rice rhizosphere in each crop season. Responses of soil microbial properties to N fertilization were dependent on the N rates, but no significant effect of the N sources was observed. All measured parameters, except MBC in the first season and C-CO2 in the second season, were increased with increasing N rates. However, the application of N higher than 60 kg ha-1 suppressed soil microbial biomass, as well as soil respiration.  Therefore, the lack of response by added urease inhibitors to the N sources indicate that optimizing N rates for upland rice production is a far more effective option for improving soil microbial community than using enhanced-efficiency N sources.

Positive long-term impacts of restoration on soils in an experimental urban forest.

As urbanization increases worldwide, investments in nature-based solutions that aim to mitigate urban stressors and counter the impacts of global climate change are also on the rise. Tree planting on degraded urban lands-or afforestation-is one form of nature-based solution that has been increasingly implemented in cities around the world. The benefits of afforestation are, however, contingent on the capacity of soils to support the growth of planted trees, which poses a challenge in some urban settings where unfavorable soil conditions limit tree performance. Soil-focused site treatments could help urban areas overcome impediments to afforestation, yet few studies have examined the long-term (>5yr) effects of site treatments on soils and other management objectives. We analyzed the impacts of compost amendments, interplanting with shrubs, and tree species composition (six species vs. two species) on soil conditions and associated tree growth in 54 experimental afforestation plots in New York City, USA. We compared baseline soil conditions to conditions after 6 yr and examined changes in the treatment effects from 1 to 6 yr. Site treatments and tree planting increased soil microbial biomass, water holding capacity, and total carbon and nitrogen, and reduced soil pH and bulk density relative to baseline conditions. These changes were most pronounced in compost-amended plots, and the effects of the shrub and species composition treatments were minimal. In fact, compost was key to sustaining long-term changes in soil carbon stocks, which increased by 17% in compost-amended plots but declined in unamended plots. Plots amended with compost also had 59% more nitrogen than unamended plots, which was associated with a 20% increase in the basal area of planted trees. Improvements in soil conditions after 6 yr departed from the initial trends observed after 1 yr, highlighting the importance of longer-term studies to quantify restoration success. Altogether, our results show that site treatments and tree planting can have long-lasting impacts on soil conditions and that these changes can support multiple urban land management objectives.

A facilitation between large herbivores and ants accelerates litter decomposition by modifying soil microenvironmental conditions

AbstractLarge herbivores and insects commonly coexist and play important functional roles in grassland ecosystems. The interactive effects of these two animal groups in shaping ecosystem processes and functioning are poorly understood. In a semi‐arid grassland of northeastern China, we previously found a reciprocal facilitation between large herbivores (cattle;Bos tarurs) and ants: cattle grazing led to a twofold increase in ant mound abundance compared with ungrazed sites, while the presence of ant mounds, in turn, increased the foraging of cattle during the peak of the growing season.Here, by using a large‐scale, 4‐year (2010–2013) manipulative experiment, we further investigated how such a facilitation between large herbivores and ants can affect a key ecosystem process, litter decomposition. Using a set of small‐scale reciprocal translocation litterbag experiments, we separated the effects of litter quality and soil microenvironmental factors altered by cattle and ants on litter decomposition rates.A significant interaction between the experimental factors, cattle grazing and ant presence, showed that litter decomposition rate was at the highest levels when both cattle and ants were present, with only a small impact when each was present on its own. Mechanistically, cattle and ants exerted limited effects on litter quality (litter C:N ratio). However, these animals greatly altered soil microenvironments by increasing soil N availability, which in turn increased soil microbial biomass and accelerated decomposition process.Synthesis. Our results demonstrate how positive interactions between two groups of animals, large herbivores and ants, can affect decomposition rates, with important consequences for ecosystem carbon and nutrient cycling. Large herbivores, either domestic or wild, often coexist and interact frequently with a diverse of other fauna in terrestrial ecosystems. Assessing their interactive effects will help us to better understand their role in shaping ecosystem processes and functioning with important management implications.A freePlain Language Summarycan be found within the Supporting Information of this article.

Tillage, Mulching and Nitrogen Fertilization Differentially Affects Soil Microbial Biomass, Microbial Populations and Bacterial Diversity in a Maize Cropping System

Determination of biologically active components of the soil organic matter, such as soil microbial biomass carbon (C) and nitrogen (N) can be used as indicators for variations in soil productivity due to changes in soil management. Soil agronomic management practices bring about changes in the physical and chemical properties of the soil, resulting in variations in soil microbial biomass and microbial diversity. The effects of tillage, mulch and inorganic fertilizers on soil microbial biomass C and N, microbial populations and bacterial diversity were determined from the treatment combinations which had been applied for 5 years in Central Kenyan Highlands. The test crop used was maize (Zea mays L.). The study involved conventional and minimum tillage systems, mulching and inorganic fertilizers (120 kg N/ha). Tillage (P < 0.001), mulch (P < 0.001), and fertilizer (P = 0.009) significantly affected soil microbial biomass C and N whereby minimum tillage and mulch increased soil microbial biomass C and N. Interestingly, minimum tillage and mulch recorded the highest bacteria and fungi CFUs compared to conventional tillage and inorganic fertilizers. Only fertilizer and mulch (P < 0.001) had significant effect on actinobacteria CFUs. Amplified ribosomal DNA analysis (ARDRA) showed that the highest genetic distance of 0.611 was recorded between treatments conventional tillage + no mulch + no NPK fertilizer and conventional tillage + no mulch + NPK fertilizer. The results demonstrate that minimum tillage and mulching are attractive soil agronomic management practices since they increase soil microbial biomass and bacterial diversity in agricultural soils.

Effects of different rotational legume crops and residue management regimes on soil microbial properties and functions in a sugarcane farming system

AbstractBackground: Legume crop rotation in sugarcane cropping systems can improve soil health and fertility. However, their impacts on soil microbial community composition and function have not been clearly understood to date.Aims: This study aimed to assess responses of soil biochemical properties and composition of microbial communities driving C and N transformations in soils to different legume crop rotations and crop residue management practices.Methods: We investigated the effects of two rotational legume crops (soybean and peanut) and six different crop residue management practices on soil physicochemical and microbial properties as well as biological activities including C and N mineralisation, nitrification capacity and denitrification potential, in a subtropical sugarcane cropping system.Results: Incorporating high‐N soybean crop residues into soil significantly increased soil mineral N content in the first month compared to no‐till; no significant tillage effects were recorded with low‐N peanut residues. Both crop residues increased soil microbial biomass C, C mineralisation capacity and nitrification capacity. However, bacterial and archaeal composition and abundances of 16S rRNA and nitrification genes were not affected. Spraying a nitrification inhibitor (DMPG) onto the crop residues before tillage significantly suppressed soil nitrification capacity.Conclusions: Compared to peanut rotation, soybean rotation could supply significantly more N for the following sugarcane crop due to its much higher crop residue N inputs and faster N mineralisation. Spraying legume residues with DMPG before tillage may provide an effective strategy to inhibit formation and thus reduce the risk of losses through leaching or denitrification.

Growth and Yield of Okra Using Bio-Stimulant of Golden Apple Snails Extracts and Fertilizer on Ultisol

Cultivation of okra on ultisol soils having poor physical, chemical and biological soil properties can be improved by administering a bio-stimulant extract of golden snails and combined with NPK fertilizer. This research objective was to study a combination of bio-stimulant treatment of golden apple snail extract and NPK fertilizer dosage in increasing the growth and yield of okra on ultisol soil. This study used a factorial completely randomized design with two factors. The first factor was Bio-stimulant golden apple snails extract consisting of 4 treatment, and the second factor was levels of NPK fertilizer consisting of 3 treatment. Fertilizers had a significant effect on increasing overall plant growth. The bio-stimulant extracts of golden apple snails contain organic materials that can improve physical properties (increasing aggregation, moisture retention, soil hydraulic conductivity), chemical (increasing soil organic carbon content) and biology (increasing soil microbial biomass). The nutrient content in NPK fertilizers can increase nutrients; the availability of nutrients in the soil is balanced and can be absorbed by plants so that the photosynthesis process in plants can run well and increase on growth and yield of okra. The combination of bio-stimulant extract of golden apple snails of 20 mL/L water and NPK fertilizer of 150 kg/ha (1 g/polybag) had the highest resulted in the average value of growth and yield of okra on the ultisol soils.

Amendment with biodiesel co-product modifies genes for N cycling (nirK, nirS, nosZ) and greenhouse gas emissions (N2O, CH4, CO2) from an acid soil

When applied to soils, ‘biodiesel co-product’ (BCP) functions as a labile source of organic C for microorganisms and promotes immobilisation of inorganic N (otherwise susceptible to leaching). However, the effects on biological N transformations and greenhouse gas (GHG) emissions, have not yet been investigated. Therefore, we investigated these effects with BCP amendment of soils at water contents ranging from 40 to 100% water holding capacity (WHC). The BCP applications significantly increased soil microbial biomass C, N, and ATP contents at all soil moistures, but maximally at 60% WHC. Amendment with BCP increased the abundance of nosZ genes coding for N2O reductase, and temporarily increased emissions of N2O up to day 3, but from day 7 onwards, emissions of N2O at all water contents almost stopped. Consequently, BCP suppressed total N2O emissions from waterlogged soil (100% WHC) by about 40% at 56 days, while the greatest emissions of N2O were still observed at 80% WHC. BCP significantly increased the abundance of ammonia oxidising archaea (AOA) and ammonia oxidising bacteria (AOB) at lower moisture contents (40% and 60% WHC) at day 7. In summary, our results suggest that BCP disproportionately increased the genetic capacity for the final reactions of denitrification by nosZ compared with those genes more implicated in early reactions of denitrification (nirS and nirK).

Changes in the Microbial Properties of Olive Cultivated Soils under Short, Medium and Long-term Irrigation with Treated Wastewater

Aims: In Tunisia, Climatic changes and water shortage has led to the reuse of treated municipal wastewater (TMWW) in the agricultural sector since the sixties. This work was intended to study the short, medium, and long-term impacts of this practice on soil microbial properties.
 Study Design: Five different experimental fields were chosen which had been irrigated with TMWW for 10, 20, 25, and 28 years, respectively. A pluvial irrigated field was selected as a control.
 Place and Duration of Study: The study was conducted in Zaouit Sousse (Tunisian Sahel region) located in the south of Sousse City (longitude: 35°47′, latitude: 10°38′ and of altitudes: 20 m N.G.T.). The soil sampling campaign was carried out at the end of the dry season (September 2014). This study was undertaken in a semi-arid area that is facing a water crisis (water shortage and irreversible seawater intrusion).
 Methodology: Soil fecal pollution indicators were determined with the most probable number MPN method. Bacterial and fungal enumeration was done by the plate count agar method. Pathogenic bacteria was determined using the conventional bacteria identification methods.
 Results: Irrigation with TMWW (for more than ten years) induced a significant increase in soil microbial biomass (heterotrophic bacteria and filamentous fungi). Soil microbial contamination was assessed by measuring Total and Fecal Coliforms, E. coli, and Faecal Streptococci at three studied soil layers (0-20; 20-40 and 40-60 cm) show’s a significant increase in TMWW irrigated plots compared to the control. Salmonella spp. and Shigella spp. screening revealed the absence of those pathogens in all studied soils. This result is true for the three soil horizons (0-20; 20-40 and 40-60 cm). This result seems to be due to the short survival period of these pathogens in the soil.
 Conclusion: TMWW irrigation had positive effects on soil fertility. However, this practice has led to a deterioration of the soil sanitary quality. The quality of the wastewater treated in Sousse Sud plant must be improved to ensure the reduction of emerging bacterial pathogens to non-detectable levels or to levels that have not been associated with human health risk.

Microbial carbon source utilization in rice rhizosphere soil with different tillage practice in a double cropping rice field

Carbon (C) plays an important role in maintaining soil fertility and increasing soil microbial community, but there is still limited information about how source utilization characteristics respond to soil fertility changes under double-cropping rice (Oryza sativa L.) system in southern China paddy field. Therefore, the effects of different short-term (5-years) tillage management on characteristics of C utilization in rice rhizosphere and non-rhizosphere soils under double-cropping rice field in southern China were investigated by using 18O incorporation into DNA. Therefore, a field experiment were included four tillage treatments: conventional tillage with crop residue incorporation (CT), rotary tillage with crop residue incorporation (RT), no-tillage with crop residue retention (NT), rotary tillage with crop residue removed as control (RTO). The results showed that soil microbial biomass C content with CT, RT, NT treatments were increased by 29.71–47.27% and 3.77–21.30% in rhizosphere and non-rhizosphere soils, compared with RTO treatment, respectively. Compared with RTO treatment, soil microbial basal respiration and microbial growth rate with CT treatment were increased 30.56%, 30.94% and 11.91%, 12.34% in rhizosphere and non-rhizosphere soils, respectively. The soil microbial C utilization efficiency were promoted with NT treatment. Compared with RTO treatment, the metabolic capacity of soil microorganism to exogenous C source with CT, RT and NT treatments were increased. The largest type of exogenous C source was saccharides, followed by amino acid and polymers, and complex compounds was the smallest. The redundancy analysis results indicated that tillage treatments significantly changed the utilization characteristics of soil microorganism to exogenous C source. Compared with RTO treatment, the grain yield of early rice and late rice with CT treatment were increased by 409.5 kg ha−1 and 387.0 kg ha−1, respectively. Therefore, the CT and RT treatments could significantly increase soil microbial biomass C content, but the NT treatment promote microbial C utilization efficiency in the double-cropping paddy field of southern China.

Nutrient Availability to Maize Crop (Zea mays L.) in Biochar Amended Alkaline Subtropical Soil

Biochar application variably affects nutrients availability, growth, and yield of crop plants based on their feedstock type and pyrolysis conditions. A pot experiment was carried out to evaluate the effect of biochars prepared from three different feed stocks viz. sugarcane filter cake (SF-BC), farmyard manure (FM-BC), and rice husk (RH-BC), on the availability of the plant nutrients in soil and the growth and nutrient uptake by maize crop (Zea mays L.). The biochars were applied separately to the soil at the rate of 0, 0.25, 0.5, and 1% separately, and the experiment was laid out according to two-factor factorial completely randomized design (CRD) maintaining three replications. Maize variety Monsanto-DK8031 was sown as the test crop. The concentration of total nitrogen (N), nitrate (NO3-N), ammonium (NH4-N), microbial biomass nitrogen (MBN), Olsen phosphorus (P), and extractable K increased with the addition of FM-BC, while the SF-BC increased soil microbial biomass phosphorus (MBP) and micronutrients (Zn, Mn, Cu, Fe) availability in the soil. Moreover, the biochar application significantly improved crop growth parameters (i.e., plant height, fresh plant weight and dry biomass) and also increased plant nutrient concentration and uptake. No negative impact of biochar was observed on plant growth or nutrients concentration. The macronutrient (total N, P, and K) contents and their uptakes were higher in plants grown with RH-BC, while the micronutrient (Mn, Cu, and Fe) contents were higher in plants grown with SF-BC. Biochar as an organic amendment has a great potential to improve soil fertility status and nutrients availability to crop plants in tropical alkaline soils; however, the effectiveness varies with the feed stock type and application rate.

Tree species richness and water availability interact to affect soil microbial processes

The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0–10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp™ method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems.

Effect of Water Management on Rice Growth and Rhizosphere Priming Effect in Paddy Soils

The rhizosphere priming effect (RPE) caused by carbon inputs from crop rhizodeposits plays a key role in regulating the carbon emission flux and carbon balance of farmland soils. Due to frequent alternations between dry and wet conditions, CO2 and CH4 emissions and the RPE in paddy field ecosystems are significantly different to those of other ecosystems. Therefore, it is of great significance to determine the direction and intensity of the rice RPE under alternations of dry and wet to limit greenhouse gas emissions. In this study, using a 13C-CO2 continuous labeling method combined with a pot-based experiment, the response of rice growth and the RPE under alternating dry and wet and continuous flooding conditions was examined. The results showed that, compared with the continuous flooding treatment, the alternating dry and wet treatments significantly increased aboveground and root biomass and the root-to-root ratio, and also increased soil microbial biomass. Under continuous flooding conditions, fluxes of 13CO2 and 13CH4 increased with rice growth from 10.2 μg·(kg·h)-1 and 2.8 μg·(kg·h)-1 (63 d) to 16.0 μg·(kg·h)-1 and 3.2 μg·(kg·h)-1 (75 d), respectively. During the 12-day drying process, the emissions of 13CO2 and 13CH4 derived from rhizosphere deposited C decreased by 57.5% and 88.1%. Under continuous flooding conditions, the RPE for CO2 and CH4 were positive and increased with the growth of rice. Under the alternating dry and wet treatment, after 12 days of drying, the RPE for CO2 and CH4 was reduced from 0.29 mg·(kg·h)-1 and 12.3 μg·(kg·h)-1 (63 d) to -0.39 mg·(kg·h)-1 and 0.07 μg·(kg·h)-1 (75 d). Thus, alternating wet and dry treatment can effectively promote rice growth and reduce the cumulative emissions of CH4. Therefore, adopting appropriate field water management is of great significance for increasing rice yields and mitigating greenhouse gas emissions.

杨树人工林幼林阶段林下植被管理对土壤微生物生物量碳、氮酶活性的影响

为了探讨林下植物物种多样性对杨树（Populus spp.）人工幼林阶段土壤微生物区系的影响，在林下设计了林下植被物种数量递增的3种处理，即清除林下植被（无林下植物）、保留单一林下植物（一种林下植物）和保留物种多样的自然林下植被（多种林下植物），于处理1年后采样分析土壤微生物生物量碳（MBC）和氮（MBN）含量、基于Biolog-ECO微平板的土壤微生物群落代谢特征以及与土壤碳、氮转化相关的胞外酶活性的差异，以期从土壤养分转化和供应的角度为科学管理人工林林下植被提供依据。结果表明，林下植被处理对8月份0-5 cm土层的土壤MBC、MBN含量以及酶活性有较大影响。与清除林下植被处理相比，保留单一林下植物处理8月份0-5 cm土层的土壤MBC、MBN含量以及β-葡萄糖苷酶、多酚氧化酶和芳基酰胺酶活性显著增加，增幅分别为27.91%、54.48%、14.74%、32.53%和6.20%，而保留物种丰富度高的自然林下植被处理林地土壤的上述指标进一步分别增加了4.88%、14.93%、9.22%、13.63%和12.86%。此外，林下植被处理还改变了土壤微生物群落代谢特征，8月份0-5 cm土层的土壤微生物Shannon指数随着林下植被物种数量的增加而显著增大。清除林下植被处理的土壤微生物主要利用的碳源包括部分糖类、氨基酸类和酯类，与清除林下植被处理相比，保留单一林下植物处理的土壤微生物提高了对上述几种碳源的利用能力，同时对糖类、氨基酸类、酯类和有机酸类碳源的利用种类范围明显扩大，而保留物种多样的自然林下植被的土壤微生物对31种碳源基本上可以全面有效利用。因此，保留林下植被，特别是提高林下植被的物种丰富度，有利于增加土壤微生物生物量，提高土壤微生物的代谢功能和分解活性，可以在一定程度上加快土壤的物质转化和养分循环功能。;Understory vegetation plays crucial roles on nutrient turnover and cycling in plantation soil by regulating soil microbial biomass and extracellular enzyme activity. Under normal condition, the understory vegetation generally contains diverse plant species in plantations. Whether increasing the species abundance of understory vegetation can change the effect of understory vegetation is still inconclusive. In this study, a diversity gradient of understory vegetation species was setup to study its effects on soil microbial biomass carbon (MBC) and nitrogen (MBN), soil microbial communities' metabolic profiles, and enzyme activities that related to soil C and N turnover in a popular plantation. Three understory treatments were designed including the understory vegetation removal, the retention of single main understory vegetation species, and the retention of natural diverse understory vegetation. The soils were sampled in June, August and October after one year treatment for the analysis of corresponding soil microbial properties. The results showed that the understory vegetation treatments had strong effects on 0-5 cm soil layer in August. Compared with understory vegetation removal, the retention of single main understory species resulted in significant increases in soil MBC and MBN contents and the activities of soil β-glucosidase, polyphenol oxidase and arylamidase by 27.91%, 54.48%, 14.74%, 32.53%, and 6.20%, respectively, in the 0-5 cm soil layer in August; moreover, the retention of natural understory vegetation showed further increases by 4.88%, 14.93%, 9.22%, 13.63%, and 12.86%, respectively, when compared with the retention of single main understory species. Understory vegetation treatments also changed soil microbial communities' metabolic profiles, soil microbial Shannon index of the 0-5 cm soil layer in August increased significantly with the increase in understory vegetation species. The main carbon sources utilized by soil microorganism with understory removal treatment were a few kinds of carbohydrates, amino acids and esters. By contrast, soil microorganisms under the retention of single main understory species treatment improved the utilization capacity of the above-mentioned carbon sources, and expanded the available carbon sources to some organic acids. At the same time, the retention of natural diverse understory vegetation enabled soil microorganisms to effectively utilize all of the 31 carbon source types. Therefore, the retention of understory vegetation, especially with high diversity, was beneficial to increase soil microbial biomass, improve soil microbial metabolism and activities, and to a certain extent accelerate the decomposition of soil organic matter and nutrient cycling.

Rhizosphere priming regulates soil organic carbon and nitrogen mineralization: The significance of abiotic mechanisms

The effect of living roots on the co-mineralization of soil organic carbon (SOC) and nitrogen (SON) and driving mechanisms of the rhizosphere priming effect (RPE) remains unclear. Moreover, it is still poorly understood whether the abiotic mechanisms, whereby roots accelerate soil organic matter (SOM) loss by destabilizing organo-mineral associations, are involved. Biotic and abiotic processes involved in the RPE and gross N mineralization (GNM) were investigated using three paddy soils (C3) under maize (C4 plant) cultivation. The soils had high and low total N (1.79 and 3.3 g kg−1 soil) as well as iron- (Fe-) (hydr-) oxide (1.1 and 2.2 g kg−1 soil) contents, which gave the following combinations: high-Fe/low-N, low-Fe/low-N, and low-Fe/high-N. The RPEs and GNM were measured using a 13C-natural abundance approach and 15N-pool dilution technique, respectively, on day 86 of maize cultivation. Living roots enhanced native SOC mineralization by 70.4–204% and GNM by 118–382%. As expected, biotic mechanisms contributed to RPEs (‘microbial activation’ and ‘microbial N-mining’) that was supported by the increase of soil microbial biomass C and extracellular enzyme activities in presence of plant, and the lower C/N ratio of primed SOM than the original SOM in unplanted soils. Higher RPEs and relative primed N mineralization via stronger ‘microbial N-mining’ were found in low-Fe/low-N vs low-Fe/high-N soils. In contrast to expectations, the RPEs and relative primed N mineralization were greater in high-Fe/low-N versus low-Fe oxide soils. The strongest decrease in SOM-derived Fe-bound C and increase in root-derived Fe-bound C were observed in the high-Fe soil, because root exudates liberated more C from Fe-bound SOM and co-precipitated with Fe oxides. This shows that the abiotic process was involved in RPEs whereby root exudates promoted soil C loss by releasing it from Fe-organic complexes. Thus, this study demonstrated that coupled biotic-abiotic processes could regulate RPEs.