Changes In Soil Enzyme Activities Research Articles

Rhizosheath drought responsiveness is variety-specific and a key component of belowground plant adaptation.

Biophysicochemical rhizosheath properties play a vital role in plant drought adaptation. However, their integration into the framework of plant drought response is hampered by incomplete mechanistic understanding of their drought responsiveness and unknown linkage to intraspecific plant-soil drought reactions. Thirty-eight Zea mays varieties were grown under well-watered and drought conditions to assess the drought responsiveness of rhizosheath properties, such as soil aggregation, rhizosheath mass, net-rhizodeposition, and soil organic carbon distribution. Additionally, explanatory traits, including functional plant trait adaptations and changes in soil enzyme activities, were measured. Drought restricted soil structure formation in the rhizosheath and shifted plant-carbonfrom litter-derived organic matter in macroaggregates to microbially processed compounds in microaggregates. Variety-specific functional trait modifications determined variations in rhizosheath drought responsiveness. Drought responses of the plant-soil system ranged among varieties from maintaining plant-microbial interactions in the rhizosheath through accumulation of rhizodeposits, to preserving rhizosheath soil structure while increasing soil exploration through enhanced root elongation. Drought-induced alterations at the root-soil interface may hold crucial implications for ecosystem resilience in a changing climate. Our findings highlight that rhizosheath soil properties are an intrinsic component of plant drought response, emphasizing the need for a holistic concept of plant-soil systems in future research on plant drought adaptation.

Effects of stand ages on soil enzyme activities in Chinese fir plantations and natural secondary forests.

Forest type and stand age are important biological factors affecting soil enzyme activities. However, the changes in soil enzyme activities across stand ages and underlying mechanisms under the two forest restoration strate-gies of plantations and natural secondary forests remain elusive. In this study, we investigated the variations of four soil enzyme activities including cello-biohydrolase (CBH), β-1,4-glucosidase (βG), acid phosphatase (AP) and β-1,4-N-acetylglucosaminidase (NAG), which were closely associated with soil carbon, nitrogen, and phosphorus cycling, across Cunninghamia lanceolata plantations and natural secondary forests (5, 8, 21, 27 and 40 years old). The results showed that soil enzyme activities showed different patterns across different forest types. The acti-vities of AP, βG and CBH in the C. lanceolata plantations were significantly higher than those in the natural secon-dary forests, and there was no significant difference in the NAG activity. In the plantations, AP activity showed a decreasing tendency with the increasing stand ages, with the AP activity in the 5-year-old plantations significantly higher than other stand ages by more than 62.3%. The activities of NAG and CBH decreased first and then increased, and βG enzyme activity fluctuated with the increasing stand age. In the natural secondary forests, NAG enzyme activity fluctuated with the increasing stand age, with that in the 8-year-old and 27-year-old stand ages being significantly higher than the other stand ages by more than 14.9%. βG and CBH enzyme activities increased first and then decreased, and no significant difference was observed in the AP activity. Results of the stepwise regression analyses showed that soil predictors explained more than 34% of the variation in the best-fitting models predicting soil enzyme activities in the C. lanceolata plantations and natural secondary forests. In conclusion, there would be a risk of soil fertility degradation C. lanceolata plantations with the increasing stand age, while natural secondary forests were more conducive to maintaining soil fertility.

Vermiremediation of heavy metals (HMs)-contaminated agricultural land: synergistic changes in soil enzyme activities and earthworm's growth parameters.

In this investigation the heavy metals (Cu, Zn, Mn, Cr and Ni) remediation potential of Eisenia fetida was studied in the crude oil polluted soil. The potential of E. fetida was evaluated based on the decrease in concentrations of Cu, Zn, Mn, Cr and Ni, and improvement in the soil enzyme activities at the end of 90 days of experimental trials. Moreover, soil health quality, inter-relationship between the enzyme activities and the growth parameters of E. fetida and synergistic relation among the enzyme activities were also evaluated through G-Mean and T-QSI indices, chord plot analysis and principal component analysis (PCA) to confirm the performance of E. fetida during vermiremediation. The results revealed that the soil treated with E. fetida showed a reduction in the concentration of Cu, Zn, Mn, Cr and Ni by 17.4% 19.45%, 9.44%, 23.8% and 9.6% respectively by end of the experimental trials. The cellulase, amylase, polyphenol oxidase, peroxidase, urease, dehydrogenase and catalase activities in the E. fetida-treated soil were enhanced by 89.83%, 99.17%, 142%, 109.9%, 92.9%, 694.3% and 274.5% respectively. The results of SEM-EDS revealed enhancement in the O, K, Na, Mg and P content by 62.36%, 96.2%, 97.9%, 93.7% and 98.2% respectively by the end of the experimental trial. The G-Mean and T-QSI indices also confirmed the improvement in soil enzyme activities thereby indicating the positive influence of E. fetida on soil decontamination process. The chord plot indicated the interrelationship between the earthworm's growth parameters and enzyme activities of the soil as indicated by the high linkage between the nodes. Finally, the PCA confirmed the negative effect of the heavy metals on the soil enzyme activities and synergistic interrelationship between the enzyme activities during the vermiremediation process. Thus, this study demonstrated the changes in the soil enzyme activities and their interconnected influences during vermiremediation of crude oil sourced heavy metals from polluted soil.

Importance of Considering Enzyme Degradation for Interpreting the Response of Soil Enzyme Activity to Nutrient Addition: Insights from a Field and Laboratory Study

Soil enzyme activity can be affected by both production and degradation processes, as enzymes can be degraded by proteases. However, the impact of nutrient addition on enzyme activity is often solely attributed to changes in enzyme production without fully considering degradation. In this study, we demonstrate that the activities of β-1,4-glucosidase (BG), β-D-cellobiohydrolase (CBH), β-1,4-xylosidase (BX), and β-1,4-N-acetyl-glucosaminidase (NAG) in two tropical plantations exhibited comparable levels between nitrogen (N)- and phosphorus (P)-fertilized soils and the unfertilized control under field conditions. However, it was observed that the reduction in enzymatic activity was significantly higher in the fertilized soils during short-term laboratory incubation in the acacia plantation. Additionally, the eucalyptus plantation exhibited a similar tendency, although statistical significance was not achieved due to the high variance of the data. The results show that the interruption of the natural, continuous supply of organic matter or non-soil microbial-derived enzymes, which typically occurs under field conditions, leads to a more significant reduction in soil enzyme activities in fertilized soils compared to unfertilized control. This may be attributed to the higher abundance of protease in fertilized soils, resulting in faster enzyme degradation. Interestingly, P fertilization alone did not have a similar effect, indicating that N fertilization is likely the main cause of the larger decreases in enzyme activity during incubation in fertilized soils compared to unfertilized control soils, despite our study site being poor in P and rich in N. These findings highlight the importance of considering enzyme degradation when investigating material dynamics in forest ecosystems, including the impact of nutrient addition on enzyme activity, as enzyme production alone may not fully explain changes in soil enzyme activity.

Changes in soil biological and chemical environment during wheat growth stages under nitrogen and straw management practices in rice-wheat system

Crop residues are a neglected source of sustainable agriculture since they are produced in large quantities, yet farmers frequently burn them. The purpose of this study was to assess the effects of nitrogen (N) application rate and residue management strategies for wheat production on changes in soil enzyme activity, microbial count, and chemical properties at various growth stages. Different combinations of N fertilizer rates, tillage, and residue management practices for wheat were investigated in split-plot design. The main plots were assigned for two N applications rates (N75 and N100, 75 and 100% recommended dose of N, whereas the subplots were assigned for seven treatments as: (1) conventional tillage (CT)-rice residue removed (R), (2) sowing with rotoseeder (RS)-R, (3) sowing with Happy Seeder (HS)+ loose rice residue removed (R50), (4) HS + 100% rice residue retained (R100), (5) RS + R50, (6) RS + R100, (7) CT + R100. The results revealed that N100 and HS + R100 increased wheat yield, chemical and biochemical properties, with the exception of polyphenol oxidase and peroxidase. All biochemical characteristics were greater during vegetative and reproductive growth stages than before seeding and harvesting. Dehydrogenase, polyphenol oxidase, and flourescein diacetate were found as the most sensitive indicators using principle component analysis.

Altitudinal Gradients Modify the Ecological Stoichiometry and Enzyme Activities of Soil in a Natural Secondary Quercus spp. Forest of the Dabie Mountains

Understanding the vertical distribution and driving mechanisms behind soil carbon (C), nitrogen (N), and phosphorus (P) contents and enzyme activities along elevation gradients is of great significance for the healthy and sustainable management of forest ecosystems. For this study, the 0–20 cm soil-layer samples of different natural Quercus spp. secondary forests from eight altitude gradients (ranging from 250 to 950 m) were investigated to quantify their physicochemical properties, ecological stoichiometry characteristics, and enzyme activities. The results indicated that the soil nutrient content of natural secondary Quercus spp. forests in the Dabie Mountains was low, with average soil organic carbon (SOC) and total phosphorus (TP) contents of 19.86 ± 3.56 g·kg−1 and 0.68 ± 0.10 g·kg−1, respectively, which were 19.14% and 12.82% lower, respectively, than the Chinese average. In terms of vertical spatial distribution, the SOC, total nitrogen (TN), and TP contents of the soil at high altitudes (≥750 m) were greater than those at middle- and low-altitude areas and reached the maximum value at or near the top of the mountain (850–950 m). The stoichiometric attributes of the soil ecosystem fluctuated with the higher altitudes in vertical space; however, the fluctuation range was not significant. The C:N, N:P, and C:P ratios reached their maximum values at altitudes of 250, 750, and 850 m, respectively. However, the overall average value remained generally lower than the national average; thus, in forest management, attention should be paid to the supplementation of the soil with C and P. The activities of soil sucrase, urease, acid phosphatase, and catalase were interconnected across the overall space, and increased with altitude. The SOC, TP, and pH were the main factors that influenced the changes in soil enzyme activities.

Changes in Soil Enzyme Activity and Microbial Population with Incorporation of Crop Residue along with Microbial Consortium in Black Cotton Soils

Changes in Soil Enzyme Activity and Microbial Population with Incorporation of Crop Residue along with Microbial Consortium in Black Cotton Soils

Implementation of different forest management methods in a natural forest: Changes in soil microbial biomass and enzyme activities

Forest management measures have changed soil characteristics, and these changes vary with respect to different management methods. The purpose of our study was to determine the effects of forest management on the soil ecological properties, especially the microbial biomass and enzyme activity. A comprehensive soil quality assessment was used to implement the Soil Quality Index (SQI) Model. Soil samples were collected from a natural oak forest (Quercus aliena var. acuteserrata) after 6 years of different management methods. Three forest management methods were implemented: selective cutting to remove the trees affecting target trees (CNFM), selective cutting to optimizing the stand’s spatial structure (SBFM), selective cutting to harvest large-diameter timber and reduce the stand density (SFCS), using unmanaged plots as a reference (NT, control plots). Here, the results showed that microbial biomass C (MBC) and microbial entropy (Cmic/Corg) were both higher in the CNFM and SBFM compared to the NT. CNFM had larger the ratios for MBC and MBN than the NT, while a smaller content of microbial biomass P in the SBFM. Phosphatase and protease activities were impacted by forest management, which was manifested as decreases inprotease and increases in phosphatase. Changes in soil enzyme activity may have been caused by soil organic matter and total nitrogen (TN), and the soil microbial biomass was sensitive to changes in TN, available phosphorus, urease and catalase. SQI value decreased in the order: NT > SBFM > CNFM > SFCS, which demonstrate that SFCS had strongly negative effects on comprehensive soil quality compared to CNFM and SBFM. Our finding suggest that forest management strategies that optimizing the spatial structure of forests by selective logging increased efficiency of microbial C and phosphatase activity and had less intensely negative effects on soil quality.

Changes in soil microecology of gangue reclamation areas after 10 years of in situ restoration with herbaceous plants (Artemisia sacrorum and Imperata cylindrica) and trees (Populus spp.)

Changes in the soil ecological microenvironment of coal gangue contaminated areas were evaluated after 10 years of in situ restoration with Artemisia sacrorum, Imperata cylindrica, and Populus spp. The changes in soil enzyme activity, ion spectrum, bacterial community structure and function, and metabolic spectrum were analyzed in an unrestored coal gangue pollution area (Control), an Artemisia sacrorum + Imperata cylindrica remediation area (T1), and a Populus remediation area (T2). Phytoremediation significantly increased the soil acid phosphatase activity by 4.62–7.36 times, and decreased the electrical conductivity by 6.2–85.5%. The overall reduction trend for heavy metals in the soil was shallow layer > middle layer > deep layer, with significant reductions seen for Cs, As, Co, Pb, Ni, U, and Cr. The number and diversity of shallow soil microbial communities increased significantly, with Actinobacteria, which participate in the carbon and nitrogen cycles, and Proteobacteria, which can tolerate/metabolize multiple heavy metals, becoming the dominant species in the restoration area. Microbial genes for glycosyltransferase, transcriptional regulators, and toluene degradation changed significantly. Phytoremediation by both remediation strategies significantly changed the metabolic level of lipids and lipid-like molecules in the soil, while significantly enriching the pathways of ABC transporters and galactose metabolism, which are essential for soil detoxification, ion transport, and carbon cycling. Bacteria such as Thiobacillus and Pseudarthrobacter were important drivers of soil metabolism. In general, the in situ phytoremediation of soil contaminated by coal gangue improved the overall soil ecological microenvironment.

Effects of silver nanoparticle size, concentration and coating on soil quality as indicated by arylsulfatase and sulfite oxidase activities

Recently, the nanotechnology industry has seen a growing interest in integrating silver nanoparticles (AgNPs) into agricultural products, which increases soil exposure to these particles. This demands an investigation into the effect of AgNPs on soil health. Changes in soil enzyme activities upon exposure to AgNPs can serve as early indicators of any adverse effects that these particles may have on soil quality. This study aimed to determine the effects of AgNP size, concentration, coating, and exposure time on the activities of two sulfur cycle enzymes, arylsulfatase and sulfite oxidase. To investigate the sensitivity of soil enzyme activity to AgNP contamination, silt loam soil samples were treated with 30, 80, and 200 nm-sized AgNPs coated with citrate, lipoic acid, and polyvinylpyrrolidone at 1, 10, and 100 mg Ag kg -1 soil, with the changes in enzyme activities monitored at 3 h, 3 d, and 30 d after treatment. For comparison, the effects of silver (Ag) ions on the enzyme activities were studied under similar treatment conditions. For most of the concentrations tested, the inhibitory effects of AgNPs on different enzymes differed, with a much stronger effect on sulfite oxidase activity than on arylsulfatase activity. The AgNP concentration and exposure time played much important roles than coating type and particle size in the effects of AgNPs on soil enzyme activities.

Host mediated microbiome selection to study the changes in the nutrient cycling, root exudation and microbial population in the rhizosphere of soybean genotypes

The study was conducted with an aim to understand the changes in soil enzyme activity in the plant rhizosphere during host mediated selection of rhizosphere microbiome. Soil samples were collected from three different locations on a farm in the Reading area, UK. Two popular genotypes of Soybean (Glycine max) were chosen for the study, Kenchen and Siverka. Initial microbiome inoculum was prepared by mixing field soil with sterilized water and this was added to the autoclaved coir: sand mix. After four weeks of plant growth, plants were uprooted from the soil, rhizosphere soil collected for soil enzyme estimation and NGS analysis. Root exudates were collected and stored in freezer for analysis. Growth of plants in legume soils was significantly higher. The plants in legume soil produced nodules which confirmed the presence of rhizobia in these soils. Independent analysis within each soil type showed that, the interaction between soil type and genotype is very significant with respect to enzyme activity. The presence of microbiome along with plants had a significant effect on enzymatic activity. GCMS analysis of root exudates from plants in each soil showed variations in individual chromatograms. The difference in enzyme activity with plant growth shows that there are changes taking place in the soil. Further in this study, NGS analysis of rhizosphere soil will shed more light into the changes happening in the soil microbiome and if the changes are related to the enzymatic changes and plant root exudation.

Dynamic responses of soil enzymes at key growth stages in rice after the in situ remediation of paddy soil contaminated with cadmium and arsenic

The practical application of in situ remediation techniques requires an understanding of the dynamic changes in soil enzyme activity as indicators of soil fertility and health. Experiments were carried out in paddy soils co-contaminated with cadmium (Cd) and arsenic (As) at low (L) and high (H) levels. A calcium and iron (CaFe)-based amendment (limestone + iron powder + silicon fertilizer + calcium‑magnesium-phosphate fertilizer) was applied to the soil at concentrations of 0, 450, and 900 g·m−2 (labeled CK, T1, and T2, respectively), and sampling was conducted at the tillering (TS), booting (BS), filling (FS), and mature (MS) stages. In soil L, urease activity increased significantly by 15.8% under T1 treatment at the MS, catalase activity increased significantly under T2 treatment by 52.4% at the FS and 25.9% at the MS, and acid phosphatase activity increased significantly by 50.1%–65.9% at the TS. For soil H, urease activity increased by maximum values of 101.6% and 28.6% at the FS and MS, respectively. Catalase activity increased by 29.0% at the MS under T2 treatment, and acid phosphatase activity increased by maximum values of 40.5%, 16.0%, and 53.9% at the BS, FS, and MS, respectively. The results indicate that the changes in soil enzyme activity were mainly related to the rice growth stage, soil pH, and available Cd and As after the application of Ca-Fe-based amendment. Overall, at the FS and MS, the amendment increased the soil pH, soil enzyme activity, and cation exchange capacity and reduced the available Cd and As, which reduced the Cd and As contents in brown rice.

In situ restoration of soil ecological function in a coal gangue reclamation area after 10 years of elm/poplar phytoremediation

The soil ecological health risks and toxic effects of coal gangue accumulation were examined after 10 years of elm/poplar phytoremediation. The changes in soil enzyme activities, ionome metabolism, and microbial community structure were analyzed at shallow (5–15 cm), intermediate (25–35 cm), and deep (45–55 cm) soil depths. Soil acid phosphatase activity in the restoration area increased significantly by 4.36–7.18 fold (p < 0.05). Soil concentrations of the metal ions Cu, Pb, Ni, Co, Bi, U, and Th were significantly reduced, as were concentrations of the non-metallic element S. The repair effect was shallow > middle > deep. The soil community structure, determined by 16S diversity results, was changed significantly in the restoration area, and the abundance of microorganisms increased at shallow soil depths. Altererythrobacter and Sphingomonas species were at the center of the microbial weight network in the restoration area. Redundancy analysis (RDA) showed that S and Na are important driving forces for the microbial community distributions at shallow soil depths. The KEGG function prediction indicated enhancement of the microbial function of the middle depth soil layers in the restoration area. Overall, phytoremediation enhanced the biotransformation of soil phosphorus in the coal gangue restoration area, reduced the soil content of several harmful metal elements, significantly changed the structure and function of the microbial community, and improved the overall soil ecological environment.

Soil enzyme stoichiometry revealed the changes of soil microbial carbon and phosphorus limitation along an elevational gradient in a Pinus taiwanensis forest of Wuyi Mountains, Southeast China

Understanding changes in soil enzyme activities and ecoenzymatic stoichiometry is important for assessing soil nutrient availability and microbial nutrient limitation in mountain ecosystems. However, the variations of soil microbial nutrient limitation across elevational gradients and its driving factors in subtropical mountain forests are still unclear. In this study, we measured soil properties, microbial biomass, and enzyme activities related to carbon (C), nitrogen (N), and phosphorus (P) cycling in Pinus taiwanensis forests at different altitudes of Wuyi Mountains. By analyzing the enzyme stoichiometric ratio, vector length (VL), and vector angle (VA), the relative energy and nutrient limitation of soil microorganisms and its key regulatory factors were explored. The results showed that β-glucosaminidase (BG) activities increased along the elevational gradient, while the activities of β-N-acetyl glucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (AcP) and (NAG+LAP)/microbial biomass carbon (MBC) and AcP/MBC showed the opposite trend. Enzyme C/N, enzyme C/P, enzyme N/P, and VL were enhanced with increasing elevation, while VA decreased, indicating a higher degree of microbial P limitation at low elevation and higher C limitation at high elevation. In addition, our results suggested that dissolved organic carbon and microbial biomass phosphorus are critical factors affecting the relative energy and nutrient limitation of soil microorganisms at different elevations. The results would provide a theoretical basis for the responses of soil carbon, nitrogen, and phosphorus availability as well as the relative limitation of microbial energy and nutrition to elevational gradients, and improve our understanding of soil biogeochemical cycle process in subtropical montane forest ecosystems.

Apparent kinetic properties of soil phosphomonoesterase and β‐glucosidase are disparately influenced by pH

AbstractExtracellular enzymes catalyze organic matter transformations in soils, yet the study of soil enzymes does not typically evaluate how enzyme kinetic properties that govern activity in situ may shift with environmental parameters such as pH. To test this, we quantified changes in apparent kinetic properties (maximum catalysis rate [AppVmax] and substrate concentration that produces 50% of Vmax [AppKm]) of soil phosphomonoesterase (PME) and β‐glucosidase (BG) as a function of pH by using dual gradients of buffer pH (3–12) and substrate concentrations in a selected set of diverse soils. A third kinetic property, apparent specificity constant (AppKa = AppVmax/AppKm) was also evaluated as a function of pH. Results indicated widely differing pH optima of soil PME and BG AppVmax and AppKm, decoupling of which entailed unpredictable pH optima of AppKa. Additionally, changes in soil enzyme activity with pH may not be driven by shifts in the specificity constant because pH–AppVmax relationships are largely unrelated to co‐occurring changes in AppKa. These results invalidate the assumption that Km of a soil enzyme is constant across pH and emphasize the importance of assaying soil enzyme activities at sufficiently high (5 × Km) substrate concentrations at a given pH to avoid the mischaracterizing pH optima.

Effects of Microplastics Addition on Soil Organic Carbon Mineralization in Citrus Orchard

To investigate the effects of microplastics on soil organic carbon mineralization and the changes in soil enzyme activities, an incubation experiment was conducted whereby single applications of either microplastics or straw, and combined application of both, were added to Dangyang citrus orchard soil. The results showed that the combined application of straw and microplastics significantly affected organic carbon mineralization in the soil, but the single addition of microplastics had no significant effect. Compared with straw alone, the application of a small combined amount of microplastics and straw significantly increased soil organic carbon mineralization by 8.20%, while medium and high amounts of the combined application significantly inhibited soil organic carbon mineralization. The lowest amount of organic carbon mineralization occurred with the highest amount of combined microplastics and straw, 10.13% lower than with straw alone. The addition of microplastics significantly reduced the activity of β-glucosidase. In particular, a high amount of microplastics significantly decreased the activity of β-glucosidase, compared with the control, by 20.52%, 43.93%, and 17.79% on the day 1, 6, and 35, respectively. However, straw application alleviated the inhibition effect of microplastic application on soil β-glucosidase activity. The soil organic carbon mineralization rate was significantly positively correlated with DOC, MBC and β-glucosidase activity.

Biofumigation With Pellets of Defatted Seed Meal of Brassica carinata: Factors Affecting Performance Against Phytophthora nicotianae in Pepper Crops

Phytophthora nicotianaeis the pathogen that causes root and crown rot disease in open field paprika pepper crops of Extremadura (central-western Spain). A field experiment was established during spring, a period compatible with the pepper crop cycle in this region, to evaluate the effects of biofumigation with pellets ofBrassica carinatadefatted seed meal, at the rate recommended by the manufacturer (3 tons ha−1), on the survival and infectivity ofP. nicotianaechlamydospores inoculum. Furthermore, three biofumigant rates (3, 6, and 20 tons ha−1) were assayed in laboratory experiments with natural soil inoculated with chlamydospores. In the field trial, the incorporation of pellets at 3 tons ha−1did not produce remarkable changes in soil enzyme activity or soil properties. In both the field and laboratory tests, survival and infectivity in the biofumigated treatment at 3 tons ha−1did not differ from those in the untreated control. On the contrary, the same rate added to autoclaved soil completely suppressed the inoculum, suggesting that the soil microbiota degradesB. carinatapellets before being effective againstP. nicotianae. Increasing the recommended rate to 6 tons ha−1decreased inoculum survival in the laboratory test and 20 tons ha−1completely inhibited the inoculum, although the economic value of such a high rate application is a factor to be assessed. In phytotoxicity tests on radish and white mustard seeds with several concentrations (100, 50, 25, 10, 5, and 0%) ofB. carinatapellets solution, no phytotoxic effect was observed with the two lowest concentrations, and no symptoms of phytotoxicity were found in the bioassays of pepper plants.

Salinization depresses soil enzyme activity in metal-polluted soils through increases in metal mobilization and decreases in microbial biomass.

Salinity may increase metal mobilization with a potentially significant consequence for soil enzymatic activity and nutrient cycling. The goal of this study was to investigate changes in soil enzyme activity in response to salinization of a clay loam soil artificially polluted with cadmium (Cd) and lead (Pb) during a 120-day incubation experiment. Soil samples were polluted with Cd (10 mg Cd kg-1), Pb (150 mg Pb kg-1), and a combination of Cd and Pb, then preincubated for aging and eventually salinized with three levels of NaCl solution (control, low and high). NaCl salinity consistently increased the mobilization of Cd (12-22%) and Pb (5-16%) with greater increases at high (17-22% for Cd, 9-16% for Pb) than low (12% for Cd, 5-7% for Pb) salinity levels. While the increased Cd mobilization was greater in co-polluted (22%) than Cd-polluted (17%) soils, the increase of Pb mobilization was lower in co-polluted (9%) than Pb-polluted (16%) soils at high salinity level. The salinity-induced increases in metal mobilization significantly depressed soil microbial respiration (up to 43%), microbial biomass content (up to 63%), and enzymatic activities (up to 87%). The multivariate analysis further supported that the increased soil electrical conductivity, Cd mobilization, and pH after salinization were the most important factors governing microbial activity and biomass in metal-polluted soils. Results showed that changes in microbial biomass and mobile metal pool with increasing salinity had a major effect on enzyme activities, particularly under the combined metals. This study indicated that the secondary salinization of metal-polluted soils would impose an additional stress on enzymatic activities as biochemical indicators of soil quality, and therefore should be avoided for the maintenance of soil microbial and biochemical functions, especially in arid regions. In metal-polluted soils, the observed responses of extracellular and intracellular enzymes to salinity can be used to advance our knowledge of microbial processes when modeling the carbon and nutrient cycling.

Microbial community structure and metabolome profiling characteristics of soil contaminated by TNT, RDX, and HMX

This experiment was conducted to evaluate the ecotoxicity of typical explosives and their mechanisms in the soil microenvironment. Here, TNT (trinitrotoluene), RDX (cyclotrimethylene trinitramine), and HMX (cyclotetramethylene tetranitramine) were used to simulate the soil pollution of single explosives and their combination. The changes in soil enzyme activity and microbial community structure and function were analyzed in soil, and the effects of explosives exposure on the soil metabolic spectrum were revealed by non-targeted metabonomics. TNT, RDX, and HMX exposure significantly inhibited soil microbial respiration and urease and dehydrogenase activities. Explosives treatment reduced the diversity and richness of the soil microbial community structure, and the microorganisms able to degrade explosives began to occupy the soil niche, with the Sphingomonadaceae, Actinobacteria, and Gammaproteobacteria showing significantly increased relative abundances. Non-targeted metabonomics analysis showed that the main soil differential metabolites under explosives stress were lipids and lipid-like molecules, organic acids and derivatives, with the phosphotransferase system (PTS) pathway the most enriched pathway. The metabolic pathways for carbohydrates, lipids, and amino acids in soil were specifically inhibited. Therefore, residues of TNT, RDX, and HMX in the soil could inhibit soil metabolic processes and change the structure of the soil microbial community.

Differential seasonal changes in soil enzyme activity along an altitudinal gradient in an alpine-gorge region

Information about the seasonal changes in soil enzyme activity along altitudinal gradients is key for understanding the responses of soil biochemical processes to ongoing climate change. Therefore, a 3-year in situ soil core incubation experiment was conducted along a 2431-m altitudinal gradient in an alpine-gorge region on the eastern Qinghai-Tibet Plateau, which includes dry valley shrubland, valley-mountain ecotone forest, subalpine coniferous forest, alpine coniferous forest and alpine meadow. The invertase, urease, and acid phosphatase activities in the soil organic layer (OL) and mineral layer (ML) were measured at five critical periods from 2013 to 2017. Higher soil enzyme activity was observed in the subalpine coniferous forest and alpine meadow, whereas lower activity was observed in the dry valley shrubland and valley-mountain ecotone forest. Overall, the seasonal sensitivity of soil enzyme activity decreased with increases in altitude. In the subalpine and alpine coniferous forests, the seasonal sensitivity of soil enzyme activity was significantly higher in the ML than in the OL. The key drivers of the seasonal variation in soil enzyme activity exhibited marked variation with changes in altitude, and these variations depended on the enzyme type and soil layer. Significant effects of temperature and precipitation on seasonal fluctuations in enzyme activities were observed at lower altitudes, whereas soil freeze-thaw cycles significantly affected the seasonal changes in acid phosphatase activity at higher altitudes. In addition, the seasonal variations in specific enzyme activities differed from those found for enzyme activities, which indicated that microorganisms might have mediated the effects of environmental factors on soil enzyme activity by altering the enzyme production efficiency. Briefly, the seasonal fluctuations in soil enzyme activity and their dominant drivers varied greatly among different altitudes in the alpine-gorge region, implying that soil biochemical processes will exhibit differential responses to ongoing climate change at different altitudes.