Soil Extracellular Enzyme Activities Research Articles

Determination of soil phenanthrene degradation through a fungal-bacterial consortium.

Fungal-bacterial consortia enhance organic pollutant removal, but the underlying mechanisms are unclear. We used stable isotope probing (SIP) to explore the mechanism of bioaugmentation involved in polycyclic aromatic hydrocarbon (PAH) biodegradation in petroleum-contaminated soil by introducing the indigenous fungal strain Aspergillus sp. LJD-29 and the bacterial strain Pseudomonas XH-1. While each strain alone increased phenanthrene (PHE) degradation, the simultaneous addition of both strains showed no significant enhancement compared to treatment with XH-1 alone. Nonetheless, the assimilation effect of microorganisms on PHE was significantly enhanced. SIP revealed a role of XH-1 in PHE degradation, while the absence of LJD-29 in 13C-DNA indicated a supporting role. The correlations between fungal abundance, degradation efficiency, and soil extracellular enzyme activity indicated that LJD-29, while not directly involved in PHE assimilation, played a crucial role in the breakdown of PHE through extracellular enzymes, facilitating the assimilation of metabolites by bacteria. This observation was substantiated by the results of metabolite analysis. Furthermore, the combination of fungus and bacterium significantly influenced the diversity of PHE degraders. Taken together, this study highlighted the synergistic effects of fungi and bacteria in PAH degradation, revealed a new fungal-bacterial bioaugmentation mechanism and diversity of PAH-degrading microorganisms, and provided insights for in situ bioremediation of PAH-contaminated soil.IMPORTANCEThis study was performed to explore the mechanism of bioaugmentation by a fungal-bacterial consortium for phenanthrene (PHE) degradation in petroleum-contaminated soil. Using the indigenous fungal strain Aspergillus sp. LJD-29 and bacterial strain Pseudomonas XH-1, we performed stable isotope probing (SIP) to trace active PHE-degrading microorganisms. While inoculation of either organism alone significantly enhanced PHE degradation, the simultaneous addition of both strains revealed complex interactions. The efficiency plateaued, highlighting the nuanced microbial interactions. SIP identified XH-1 as the primary contributor to in situ PHE degradation, in contrast to the limited role of LJD-29. Correlations between fungal abundance, degradation efficiency, and extracellular enzyme activity underscored the pivotal role of LJD-29 in enzymatically facilitating PHE breakdown and enriching bacterial assimilation. Metabolite analysis validated this synergy, unveiling distinct biodegradation mechanisms. Furthermore, this fungal-bacterial alliance significantly impacted PHE-degrading microorganism diversity. These findings advance our understanding of fungal-bacterial bioaugmentation and microorganism diversity in polycyclic aromatic hydrocarbon (PAH) degradation as well as providing insights for theoretical guidance in the in situ bioremediation of PAH-contaminated soil.

Short rotation willow to restore degraded marginal land and enhance climate resiliency within the Prairie Pothole Region: A potential nature-based solution

Short rotation willow (SRW) is a land management strategy involving the cultivation of rapidly growing, biomass-rich herbaceous-woody plants. This practice holds promise for renewable energy production, water quality preservation, carbon sequestration, greenhouse gas (GHG) mitigation, enhancement of soil extracellular enzyme activities (EEAs), and promotion of overall soil health. The rapid growth of SRW demands substantial water and nutrient resources, posing concerns when cultivated in marginal riparian lands within the Prairie Pothole Region (PPR), potentially leading to alterations in groundwater table (GWT) depth fluctuations, elevated soil salinity levels, and disruptions to biogeochemical cycles. Hence, this study comprehensively evaluated the effects of establishing SRW as a degraded marginal riparian land use practice in the PPR and attempted to answer several vital questions in the field and microcosm scale on soil hydrology, salinity, nutrients, soil organic carbon (SOC), GHG emissions, and EEAs involved in biogeochemical cycling. In a field experiment, the effects of SRW were evaluated by measuring the depth to GWT, groundwater and soil electrical conductivity (EC), macronutrients (N, P, K, and S), and SOC content in different fractions and chemical compositions during the first rotation (3-year cycle) compared with adjacent annual crop and pasture in two semi-arid PPR sites. In a microcosm experiment, GHG (CO2, CH4, and N2O) emissions and EEAs [β-glucosidase (BG), N-acetyl glucosaminidase (NAG), and alkaline phosphatase (AP)] were measured in intact soil cores treated with declining water tables and different groundwater salinity levels. No consistent land use impacts on GWT or soil EC were observed between sites. Land use in site B significantly impacted GWT depth, implying site-specific factors, such as topography and soil characteristics, may be dominant over land use effects. Under SRW, the levels of macronutrients in the soil varied but did not significantly reduce the overall nutrient content of the soil. Total SOC was highest in pasture; light fraction organic carbon and particulate organic carbon followed a similar land use pattern, i.e., pasture > SRW = annual crop. Land uses affected GHG emissions significantly in the order of pasture > annual crop = SRW. GHG emission varied with salinity and GWT but there was no interaction with land use practices. Soil EEAs were significantly impacted by different land uses, i.e., pasture > annual crop = SRW, suggesting that the effects resulted from associated SOC. Our microcosm experiment suggests that the SRW land use practice holds promise as a sustainable Nature-Based Solution for enhancing climate resiliency in PPR. It exhibits a lower global warming potential compared to annual crop and pasture. Therefore, widespread implementation of the SRW land use practice in degraded marginal land could help mitigate the effects of climate change in the region.

Short-term responses of soil enzyme activities and stoichiometry to litter input in Castanopsis carlesii and Cunninghamia lanceolata plantations.

Litter input triggers the secretion of soil extracellular enzymes and facilitates the release of carbon (C), nitrogen (N), and phosphorus (P) from decomposing litter. However, how soil extracellular enzyme activities were controlled by litter input with various substrates is not fully understood. We examined the activities and stoichiometry of five enzymes including β-1,4-glucosidase, β-D-cellobiosidase, β-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase and acidic phosphatase (AP) with and without litter input in 10-year-old Castanopsis carlesii and Cunninghamia lanceolata plantations monthly during April to August, in October, and in December 2021 by using an in situ microcosm experiment. The results showed that: 1) There was no significant effect of short-term litter input on soil enzyme activity, stoichiometry, and vector properties in C. carlesii plantation. In contrast, short-term litter input significantly increased the AP activity by 1.7% in May and decreased the enzymatic C/N ratio by 3.8% in August, and decreased enzymatic C/P and N/P ratios by 11.7% and 10.3%, respectively, in October in C. lanceolata plantation. Meanwhile, litter input increased the soil enzymatic vector angle to 53.8° in October in C. lanceolata plantations, suggesting a significant P limitation for soil microorganisms. 2) Results from partial least squares regression analyses showed that soil dissolved organic matter and microbial biomass C and N were the primary factors in explaining the responses of soil enzymatic activity to short-term litter input in both plantations. Overall, input of low-quality (high C/N) litter stimulates the secretion of soil extracellular enzymes and accelerates litter decomposition. There is a P limitation for soil microorganisms in the study area.

Thinning enhances forest soil C storage by shifting the soil toward an oligotrophic condition

Forests are significant carbon reservoirs, with approximately one-third of this carbon stored in the soil. Forest thinning, a prevalent management technique, is designed to enhance timber production, preserve biodiversity, and maintain ecosystem functions. Through its influence on biotic and abiotic factors, thinning can profoundly alter soil carbon storage. Yet, the full implications of thinning on forest soil carbon reservoirs and the mechanisms underpinning these changes remain elusive. In this study, we undertook a two-year monitoring initiative, tracking changes in soil extracellular enzyme activities (EEAs), microbial communities, and other abiotic parameters across four thinning intensities within a temperate pine forest. Our results show a marked increase in soil carbon stock following thinning. However, thinning also led to decreased dissolved organic carbon (DOC) content and a reduced DOC to soil organic carbon (SOC) ratio, pointing toward a decline in soil carbon lability. Additionally, fourier transform infrared spectroscopy (FTIR) analysis revealed an augmented relative abundance of aromatic compounds after thinning. There was also a pronounced increase in absolute EEAs (per gram of dry soil) post-thinning, implying nutrient limitations for soil microbes. Concurrently, the composition of bacterial and fungal communities shifted toward oligotrophic dominance post thinning. Specific EEAs (per gram of soil organic matter) exhibit a significant reduction following thinning, indicating a deceleration in organic matter decomposition rates. In essence, our findings reveal that thinning transitions soil toward an oligotrophic state, dampening organic matter decomposition, and thus bolstering the soil carbon storage potential of forest. This study provides enhanced insights into the nuanced relationship between thinning practices and forest soil carbon dynamics, serving as a robust foundation for enlightened forest management strategies.

Animal manures increased maize yield by promoting microbial activities and inorganic phosphorus transformation in reclaimed soil aggregates

Animal manures positively affect soil microbial activity and inorganic phosphorus (Pi) accumulation. However, how different types of manures affect microbial-driven Pi transformation at the aggregate scale is largely unclear. Here, we conducted a field experiment focused on maize cultivation in a reclamation site. Five fertilization treatments included no fertilizer (CK), chemical fertilizer (CF), chicken manure (CHM), pig manure (PM), and cow manure (CM). We examined microbial biomass, soil extracellular enzyme activities associated with carbon and phosphorus cycling, and Pi fractions (Labile-P, Moderately labile-P, and Stable-P) in three aggregate sizes (> 2 mm large macro-aggregates, 0.25–2 mm small macro-aggregates, and < 0.25 mm micro-aggregates), as well as grain yield. The results showed that manures addition significantly increased the proportion of large macro-aggregates and aggregate stability compared to chemical fertilizer. Stable-P constituted the largest proportion among the Pi fractions, accounting for 63 %–72 %. However, manures reduced Stable-P (5.4 %–10.7 %) and increased Labile-P (111 %–190 %) in Pi, especially in micro-aggregates. Both chemical fertilizer and manures increased microbial biomass and enzyme activities in aggregates, with manures causing a greater enhancement of soil microbial biomass carbon (SMBC) and C-cycling enzymes, especially under pig manure, and improving soil microbial biomass phosphorus (SMBP) and P-cycling enzymes, especially under chicken manure. Phytase, phosphatase and SMBP accounted for 81.4 %, 81.2 % and 87 % of the differences in large macro-aggregates, small macro-aggregates, and micro-aggregates, respectively. The structural equation model (SEM) demonstrated that manures facilitated the transformation of Stable-P to Labile-P by regulating aggregates and microbial activity, which consequently increased yield. In summary, manures addition, particularly chicken manure, can enhance inorganic phosphate (Pi) accumulation and transformation, thereby mitigating phosphorus (P) limitation in calcareous reclamation soils.

Effect of stand age on soil microbial metabolic limitation and enzyme activities in Robinia pseudoacacia L. plantations in the loess hilly‐gully region, China

AbstractSoil heterotrophic microorganisms play crucial roles in soil biogeochemical cycles by secreting extracellular enzymes, but their metabolism is often limited by resource availability. Identifying major resources that limit soil microbial metabolism could help to create reasonable management practices for the maintenance of ecosystem function. Robinia pseudoacacia plantations perform crucial functions in reconstituting the ecological integrity of disturbed ecosystems; however, soil microbial metabolic limitations in R. pseudoacacia plantations remain poorly understood. Herein, R. pseudoacacia plantations of four age classes (10, 22, 37, and 47 years old) were sampled in the loess hilly‐gully region of northern Shaanxi Province in China. This study analyzed the effects of stand age on soil extracellular enzyme activities and soil microbial metabolic limitation. It also determined the main factor driving soil microbial metabolic limitation and extracellular enzyme activities. The results of ecoenzymatic stoichiometry showed that nitrogen (N) limited soil microbial metabolism in R. pseudoacacia plantations. Soil microbial metabolism at 10 and 22 years old were more carbon (C)‐ and N‐limited than that at 37 and 47 years old. Except for N‐ and phosphorus (P)‐acquiring enzyme activities at 10–20 cm, soil C‐, N‐, and P‐acquiring enzyme activities (in 0–10 and 10–20 cm of soil) at 10 and 22 years old were significantly higher than those at 37 and 47 years old. The enzymatic ratio of C:N:phosphorus(P) acquisition in all R. pseudoacacia plantations, regardless of stand age and soil depth, deviated from 1:1:1. Soil nutrients explained most of the variation (90.80%) in soil extracellular enzyme activities based on variation‐partitioning analysis. The partial least squares path model showed that soil nutrients had the highest total negative effect on microbial C limitation and microbial N/P limitation. Our results confirmed that microbial metabolic limitation was induced by soil resource deficits. The findings provide another visual that increases our understanding of soil microbial metabolic limitation in R. pseudoacacia plantations.

Soil extracellular enzyme activity linkage with soil organic carbon under conservation tillage: A global meta-analysis

Conservation tillage has a profound impact on soil organic carbon (SOC) and extracellular enzyme activity (EEA), thereby influencing the global soil nutrient cycle. However, there is currently a lack of comprehensive understanding regarding the key factors influencing changes in SOC and EEA, as well as their correlations. To address this gap, a meta-analysis was conducted using data from 78 published papers, encompassing 2005 comparisons from around the world. The goal was to quantify the effects of conservation tillage on SOC and EEA while assessing the effect sizes of categorical variables. The meta-analysis results revealed that conservation tillage significantly increased SOC and carbon (C)-, nitrogen (N)-, phosphorus (P)-, and sulfur (S)-related enzyme activities, with effect sizes of 1.34, 1.57, 1.50, 1.06, and 1.81, respectively. Additionally, conservation tillage resulted in a decrease in soil pH compared to conventional tillage. Furthermore, significant correlations were found between changes in SOC and N-, P-, and S-related enzyme activities. However, only β-glucosidase (βG) and β-xylosidase (βX) in C-related enzyme activities were found to be relevant to SOC. Moreover, the results from the structural equation model indicated that several factors directly influenced the EEA of C-, N-, P-, and S-related enzymes. Climatic factors such as latitude and mean annual temperature (MAT), agronomic practices (including fertilization and tillage duration), and soil properties (such as soil depth), were identified as having direct effects on the EEA of the respective enzymes. From a global perspective, conservation tillage was observed to enhance the potential activity of C-related enzymes, while the balanced activity of N-, P-, and S-related enzymes requires greater attention. Additionally, latitude, MAT, fertilization, tillage duration, and soil depth emerged as important aspects requiring careful consideration in the development of management strategies for conservation tillage.

Planting grass enhances relations between soil microbes and enzyme activities and restores soil functions in a degraded grassland.

Forage culture is a common way to restore degraded grasslands and soil functions, in which the reconstruction of the soil microbial community and its relationship with extracellular enzyme activity (EEAs) can characterize the recovery effects of degraded grasslands. However, the impacts of forage culture on the interaction between soil microbes and EEAs and whether the recovery effect of soil functions depends on the varying degradation statuses remain unclear. We conducted a plantation of a dominant grass, Leymus chinensis, in the soil collected from severe, moderate, light, and non-degradation statuses in the Songnen grassland in northeastern China. We measured soil microbial diversity and soil EEAs, and predicted microbial functional groups using FUNGuild. The results showed that L. chinensis culture promoted soil bacterial alpha diversity and soil EEAs only in the moderate degradation status, indicating a dramatic dependence of the recovery effects of the grass culture on degradation status of the grassland. After planting L. chinensis for 10 weeks, a decreasing trend in the chemoheterotrophy and nitrate-reduction microbial functional groups was found. In contrast, the abundance of the nitrogen (N)-fixing microbial functional group tended to increase. The positive correlation between soil EEAs and the nitrate-reduction and N-fixing microbial functional groups was enhanced by planting L. chinensis, indicating that grass culture could promote soil N cycle functions. We illuminate that grass culture may promote the restoration of soil functions, especially soil N cycling in degraded grasslands, and the recovery effect may depend on the grassland degradation status. We emphasized that selection of the plant species for restoration of grasslands needs to consider the restoration effects of microbial functional groups and soil functions.

Variation patterns and their driving factors in soil extracellular enzyme activities and stoichiometry along a 49-years vegetation restoration chronosequence

Variation patterns and their driving factors in soil extracellular enzyme activities and stoichiometry along a 49-years vegetation restoration chronosequence

Effects of degradable and non-degradable microplastics and oxytetracycline co-exposure on soil N2O and CO2 emissions

Microplastics and antibiotics are emerging contaminants that can elicit considerable adverse effects on carbon and nitrogen cycles and greenhouse gas emissions. The effect of the combined occurrence of antibiotics and different types of microplastics on soil ecosystem is poorly understood. We investigated the effects of oxytetracycline (OTC), polylactic acid (PLA) microplastics, and polyethylene (PE) microplastics alone and in combination on plant root growth traits, soil extracellular enzymatic activity, and nitrous oxide (N2O) and carbon dioxide (CO2) emissions from the soil. Plant root growth traits including root area, root surface area, root volume and root branches were negatively affected by all treatments; soil extracellular enzymatic activity was also negatively affected by all treatments expect for the cellobiohydrolase. The results showed that CO2 emission increased in all treatments, with a maximum increase of 505 % in the PLA treatment; however, N2O emissions increased in the OTC treatment by 44 % but decreased in all other treatments as compared to the control. Furthermore, partial least squares path modeling analysis suggested that soil extracellular enzymes significantly influenced soil N2O and CO2 emissions (P < 0.05) and had a strong correlation with root growth traits (P < 0.05). We thus concluded that changes in soil extracellular enzymatic activities and plant root growth traits, which were affected by different microplastic and antibiotic treatments, influenced soil CO2 and N2O gas emissions.

Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation.

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and β-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production.

Analyzing soil enzymes to assess soil quality parameters in long-term copper accumulation through a machine learning approach

Soil contamination by agrochemicals is a big concern for soil health and ecosystem functioning. This is especially true for non-degradable substances like heavy metals (HM) that, because of their long-term use, are reaching significant values today due to soil accumulation. Among agrochemicals, copper (Cu) has been important in fighting fungal diseases on perennial crops for centuries.Laboratory experiments can be useful to understand the highest potential toxic effect of Cu but need to reflect what happens with long-term application in a dynamic and living agroecosystem. This study uses multivariate data analysis and machine learning to investigate long-term Cu accumulation on soil quality parameters, especially soil extracellular enzymatic activities. We collected soil samples from 21 apple orchards in South Tyrol, Italy. The orchards had different concentrations of Cu. We took 315 samples in total and analyzed them for various soil properties. We also measured the concentrations of elements in apple leaves and the activities of soil extracellular enzymes. We depicted the effect of Cu on several enzymatic activities, shedding light on the effect of Cu on the soil microbial communities functionality. Our results show that Cu concentrations in the study area affect only phosphatase activity, showing effects above 60 mg kg−1 of available Cu. Protease activity was positively correlated with Cu, while soil organic matter and management mainly influenced the carbon (C) cycle enzymes. Phosphatase decrease could be of concern for the potential disruption of the Phosphorus (P) cycle in the soil and plays a role in plant nutrition, as seen by P concentration in apple trees' leaves.We demonstrated how machine learning can help interpret complex and multivariate environmental data and overcome some downsides of traditional statistical models.

Soil extracellular enzyme stoichiometry reveals the nutrient limitations in soil microbial metabolism under different carbon input manipulations

Changes in the quality and quantity of litter and root inputs due to climate change and human activities can influence below-ground biogeochemical processes in forest ecosystems. However, it is unclear whether and how much aboveground litter and root inputs affect soil microbial metabolism and nutrient limitation mechanisms. In this study, according to a 4-years field manipulation experiment, litter and root manipulations (control (CK), double litter input (DL), no litter (NL), no root (NR), and no inputs (NI)) were set up to analyze the extracellular enzyme activities and stoichiometric ratios characteristics of 0–10 cm and 10–20 cm soils, explore the metabolic limitations of microorganisms, and clarify the main driving factors restricting nutrient limitation. The results showed that the enzyme activities associated with the C cycling (β-1,4-glucosidase (BG), cellulose disaccharide hydrolase (CBH)) and N cycling (β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP)) in DL treatment were significantly higher than those in NR treatment. Moreover, enzyme activities related to P cycling are significantly higher in comparison to other treatments. The acid phosphatase (AP), which is related to the P cycle, showed the highest activity under NR treatment. In addition, there was no significant difference in soil microbial metabolic limitation by the different carbon inputs, which did not change the original nutrient limitation pattern. The main drivers of microbial nutrient metabolic limitation included soil physicochemical properties, soil total nutrients, and available nutrients, among which soil SWC and pH presented the greatest influence on microbial C limitation and soil total nutrients showed the greatest influence on microbial N limitation. Changes in soil carbon input altered soil extracellular enzyme activities and their stoichiometric ratios by affecting soil physicochemical properties, total nutrients. This study provides data for the understanding of material cycling in forest ecosystems under environmental change.

Responses of soil microbial metabolic limitations to erosion under contrasting soil types and management practices

AbstractSoil microbial metabolism plays an important role in supporting soil services and functionalities but is usually restricted by the availability of soil nutrients. Although soil microbial metabolism limitation (SMML) has been widely addressed with regard to various driving factors, its response to erosion has not been examined but is urgently needed given the important influence of erosion on global environments. To understand how erosion affects SMML and whether erosion's effect varies with tillage practices and soil types, we conducted a simulated erosion experiment to compare eroded and noneroded plots with contrasting soil types (Mollisol vs. loess soils) and managements (conventional tillage versus no‐tillage in plots with crops versus without crops). We measured soil extracellular enzymatic activities and quantified SMML with extracellular enzymatic stoichiometry models. In both soils, the microbes were limited by carbon (C) and phosphorus (P), but not by nitrogen, and microbial C and P limitations were greater in Mollisol than loess soils. For the Mollisol, erosion significantly increased the relative C limitation, whereas the presence of crops eliminated this increase. For the loess soils, erosion enhanced the relative C limitation regardless of crop presence and tillage management. Erosion consistently increased microbial P limitation in plots with crops, with greater effects under no‐tillage (+2.9%) compared to conventional tillage (+0.7%) conditions regardless of soil type (p &lt; 0.05). In plots without crops, microbial P limitation was increased by erosion under no‐tillage (+2.1%) but was decreased under conventional tillage (−5.6%) (p &lt; 0.05). Additionally, conventional tillage exacerbated microbial C limitation but alleviated microbial P limitation. These results indicated that erosion shifted microbial C and P limitations depending on soil type, tillage practice and the presence of plants, highlighting the importance of considering all these factors when assessing soil microbial metabolic limitation at various spatial scales.

Altered intra-annual precipitation patterns affect the N-limitation status of soil microorganisms in a semiarid alpine grassland

The ongoing intensification of the hydrological cycle due to global climate change alters intra-annual precipitation variability. Changes in precipitation patterns lead to disparities in soil moisture; however, the responses of soil extracellular enzyme activities (EEAs) and microbial metabolism limitations to them is unclear. This study conducted an in situ field experiment in the alpine grasslands of the Kunlun Mountains to simulate the same amount of precipitation but change the distribution time of precipitation within the plant growing season. We examined the effects of variability in intra-annual precipitation patterns on the stoichiometry of topsoil (0–5 and 5–20 cm) properties, microbial biomass, and EEAs. Results showed that altered precipitation patterns significantly increased soil moisture’s disparity index (D) by 57 %–89 %. The activity of nitrogen-acquiring enzymes (L-leucine aminopeptidase) was increased by 20.82–32.08 % (P < 0.05) with increased variability of precipitation, while phosphorus-acquiring enzymes (Acid phosphatase) and carbon-acquiring enzymes (β-glucosidase) both decrease by 9.25–23.35 % and by 17.86–33.04 %, respectively (P < 0.05). We determined that soil microorganisms in the study region were metabolism-limited by nitrogen (vector angles < 45◦), and increased precipitation variability exacerbated nitrogen limitation (decrescent vector angles) and alleviated carbon limitation (shortened vector length). In addition, soil EEAs can be both interactively influenced by biotic and abiotic factors, with abiotic factors (i.e., D, soil organic carbon content, and soil total nitrogen content) having a stronger effect. Our study provides an effective strategy for regulating soil EEAs, microbial biomass, and microbial metabolism limitation and contributes to developing nutrient management and restoration strategies in semiarid alpine grassland ecosystems.

Soil extracellular enzyme activity and microbial resource limitation exhibited close relationships with groundwater table decline in desert wetlands

In groundwater-fed desert wetlands, groundwater table (GWT) decline leads to the loss of carbon and nutrients, followed by the expansion of desertification. How the GWT decline affects soil nutrient cycling and microbial resource limitation is still unclear. Here, we sampled soils along a steep GWT decline gradient in desert wetlands and surroundings traversing marshland (ML), sandy grassland (SG) and bare sandy land (SL) in the southern margin of Mu Us Sandy Land, China, then determined the extracellular enzyme activity (EEA, including carbon (C)-, nitrogen (N)- and phosphorus (P)-acquiring enzymes) and related soil environmental variables, and quantified the relative value of microbial metabolic limitation by vector analysis. Results showed that GWT decline eventually inhibited the activities of soil C-, N- and P-acquiring enzymes, which might be unfavourable for nutrient cycling. The relationship between soil microbial C limitation and GWT showed a downwards unimodal trend, despite the largest SOC value observed in ML. However, microbial P limitation was relieved by a slight GWT decline, and the continuous decline in GWT eventually shifted microorganisms from P to N limitation. We further provide evidence that DOC and C:P were the key factors explaining the variations in microbial C and nutrient limitations along the decline in GWT, respectively. Altogether, these findings suggest that soil EEA and microbial resource limitation exhibit close relationships with GWT decline in desert wetlands. Moreover, they highlight that alleviating microbial N limitation may be the primary solution for vegetation restoration and carbon sequestration in sandy land around wetlands.

Linkages of Enzymatic Activity and Stoichiometry with Soil Physical-Chemical Properties under Long-Term Manure Application to Saline-Sodic Soil on the Songnen Plain

Excess Na+ and high pH result in poor structures in Saline-Sodic soils, which reduces extracellular enzyme activity (EEA) and causes nutrient limitations. The application of manure improved the Physical-Chemical properties of soil and balanced the soil nutrient supply, which was reflected in the soil EEAs and stoichiometry. Five experimental treatments were designed according to the manure application duration as follows: manure application for 11 years (11a), 16 years (16a), 22 years (22a), and 27 years (27a) and a control treatment with no manure application (CK). The results of the redundancy analysis (RDA) showed that physical properties (mean weight diameter (MWD)) and EEA (β–glucosidase (BG)) significantly increased and bulk density (ρb) significantly decreased when the nutrient content increased. Additionally, soil pH, electrical conductivity (EC), exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR) significantly decreased after manure application. Based on stepwise multiple linear regression models (SMLR), total nitrogen (TN) was the dominant variable that significantly increased EEA, and the Mantel test showed that soil C:N significantly influenced enzyme stoichiometry. Furthermore, RDA showed that pH, soil C:N and TN were the main factors influencing EEAs and enzyme stoichiometry. Soil EEAs significantly increased with TN and decreased with pH and soil C:N, which affected enzyme stoichiometry. The enzyme stoichiometry increased from 1:2.1:1.2 and 1:2.7:1.5 to 1:1.7:1.2, and the vector angle (vector A) increased, which showed that the N limitation was relieved after the application of manure. The vector length (vector L) showed no significant difference in the C limitation at depths of 0–20 cm and significantly increased at depths of 20–40 cm. In conclusion, soil EEAs and stoichiometry improved with changes in TN and soil C:N, and pH decreased with changes in the soil structure after the application of manure, which accelerated the soil nutrient cycle and balanced the soil nutrient supply.

Product-inhibition feedbacks, not microbial population level tradeoffs or soil pH, regulate decomposition potential under nutrient eutrophication

Global eutrophication of growth-limiting nutrients for plants and microbes, including nitrogen (N) and phosphorus (P), can cause enzymatic feedbacks at different levels of biological organization that in turn drive soil organic matter (SOM) decomposition. Because land management practices, such as fire in mesic grasslands, also impact nutrient availability, it is important to understand how soil feedbacks may manifest under multifactorial change. Here, using a thirty-year field experiment that crosses N- and P-fertilization (alone and in combination) with contrasting fire treatments (annual burning and fire suppression) in a tallgrass prairie ecosystem, we evaluated evidence for three possible feedbacks (product-inhibition, enrichment of fast-growing copiotrophic taxa, and soil pH shift) by measuring soil extracellular enzyme activities (EEAs) and microbial community composition (16S rRNA genes) monthly for a year. Consistent with product-inhibition feedback predictions, both N-fertilization and fire suppression (which increases soil N availability) decreased N-acquiring relative to C-acquiring EEAs, suggesting reduced N limitation. P-fertilization did not lower acid phosphatase activity, thus the product-inhibition feedback hypothesis was not supported under P eutrophication conditions. Although N- and P-fertilization, alone and in combination, did decrease putatively oligotrophic populations in favor of putatively copiotrophic populations, the hypothesized tradeoff for faster growth, a concomitant decrease in all soil EEAs, was only observed in unburned soils fertilized with N. Finally, observed changes in EEAs were not driven by acidification, in contrast to other fertilization experiments. Overall, results suggest that higher N availability affects enzymatic feedbacks primarily through product-inhibition, but a threshold-like growth tradeoff (copiotrophic) response is possible under the combination of fire suppression and N-fertilization. Inherent soil properties may modulate site-specific responses to fertilization, as evidenced by weak pH and EEA responses to N- and P-fertilization, respectively. Finally, soil microbial community change following chronic fertilization did not reliably predict the “copiotrophic” feedback of reduced SOM decomposition potential.

Differential responses of soil extracellular enzyme activity and stoichiometry to precipitation changes in a poplar plantation

Changes in precipitation patterns can significantly affect belowground processes. Although soil extracellular enzymes play a vital role in several biogeochemical processes, our knowledge of how precipitation changes affect soil extracellular enzyme activity (EEA) and stoichiometry remains insufficient. In this study, we investigated the activities of C-acquiring enzyme (β-1,4-glucosidase), N-acquiring enzymes (β-N-acetylglucosaminidase and leucine aminopeptidase), and P-acquiring enzyme (acid phosphatase) under different precipitation scenarios [ambient precipitation (CK), 30% decrease in precipitation (moderate DPT), 50% decrease in precipitation (extreme DPT), 30% increase in precipitation (moderate IPT), and 50% increase in precipitation (extreme IPT)] in a poplar plantation. We found soil EEA exhibited more pronounced increases to moderate IPT compared to moderate DPT (positive asymmetry), the opposite trend (negative asymmetry) was observed under extreme precipitation; whereas soil EEA C:N:P stoichiometry exhibited negative asymmetry at moderate precipitation changes, and exhibited positive asymmetry at extreme precipitation changes. Under moderate precipitation changes, the asymmetry of soil EEA was mainly regulated by asymmetries of respective microbial biomass and litter mass; the asymmetry of soil EEA stoichiometry was mainly regulated by asymmetries of respective soil stoichiometric ratios and litter mass. Furthermore, under extreme precipitation changes, the asymmetries of soil EEA and stoichiometry were best explained by the asymmetry of soil moisture. Our results provide the first evidence of double asymmetric responses of soil EEA and stoichiometry to precipitation changes and highlight the need to consider this asymmetry when modeling the dynamics of biogeochemical cycling in forest ecosystems.

Biochar and manure additions increased above‐ and belowground wood decomposition, and soil enzyme activities in a sandy loam soil

AbstractWhile biochar and manure can provide considerable benefits to soil properties, how these amendments may alter soil microbial activity and decomposition processes remains unknown. In a split‐split‐split‐plot experiment, we amended a sandy loam soil with three rates of manure (whole plot; 0, 3, 9 Mg ha−1) and biochar (split‐plot; 0, 2.5, 10 Mg ha−1), and installed three species of wood stakes (split‐split‐split plot; triploid poplar, Populus tomentosa Carr.; trembling aspen, Populus tremuloides Michx.; and loblolly pine, Pinus taeda L.) on the soil surface and in the mineral soil (split‐split plot) to serve as a substrate for microbial degradation. Wood stakes were sampled 3 years after installation to assess decomposition rates (mass loss), and changes in wood carbon (C) and nitrogen (N). In addition, soil extracellular enzyme activities at the 0–20 cm depth were examined. Biochar alone, especially 10 Mg ha−1, increased wood stake decomposition and moisture content on the soil surface and in the mineral soil. Manure at the rate of 9 Mg ha−1 increased soil N‐acetyl‐β‐D‐glucosaminidase, α‐glucosidase, and aryl sulfatase activities by 91%, 17%, and 48% respectively. Because of the synergistic benefits of biochar and manure, we suggest that, in this climatic regime and soil texture, 10 Mg ha−1 biochar can be used for soil C sequestration and soil quality improvement, and 9 Mg ha−1 manure can be used in combination with biochar to build soil organic matter in plantations.