Soil Microbial Nutrient Research Articles

Coupling changes of soil functional gene abundances and extracellular enzyme activities across the diagnostic horizons of agricultural Isohumosols

Soil functional microbial taxa and extracellular enzymes are involved in a variety of biogeochemical cycling processes. Although many studies have revealed the vertical change patterns of microbial communities along soil profile, the general understanding of the coupling changes in the functional gene abundances (FGAs) and extracellular enzyme activities (EEAs) in soil profiles is still limited, which hinders us from revealing soil ecosystem processes. Herein, we comparatively investigated the FGAs and EEAs in the diagnostic A, B, and C horizons of soil profiles obtained from two suborders of Isohumosols (Mollisols), Ustic and Udic Isohumosols, in Northeast China based on quantitative real-time polymerase chain reaction and standard fluorometric techniques, respectively. The distribution patterns of both FGAs and EEAs significantly distinguished by the two soil suborders and were also separated from A to C horizon. Additionally, the variations of EEAs and FGAs were greater in Udic Isohumosols compared to Ustic Isohumosols along soil profiles, and greater changes were observed in C horizon than in A horizon. Both FGAs and EEAs correspondently decreased along the soil profiles. However, when normalized by soil organic carbon, the specific EEAs significantly increased in deep soil horizons, suggesting that microorganisms will input more resources to the production of enzymes to ensure microbial nutrient requirements under resource scarcity. More importantly, we revealed that soil microbial nutrient demands were limited by carbon (C) and phosphorus (P), and the C and P limitations significantly increased along soil profiles with a greater C limitation observed in Ustic Isohumosols than in Udic Isohumosols. Overall, our findings provided solid evidence showing the links between FGAs, EEAs, and microbial nutrient limitations, which would be helpful for a better understanding of the ecosystem processes in soil profiles.

Vegetation restoration of abandoned cropland improves soil ecosystem multifunctionality through alleviating nitrogen-limitation in the China Danxia.

The microbial requirement for nutrient resources can be estimated by soil extracellular enzyme stoichiometry (EES) and their stoichiometries. Implementing the Grain for Green Program has significantly impacted land use and soil nutrient management in the China Danxia. However, drivers of soil microbial nutrient limitation changes in abandoned cropland (AC) remained unclear after vegetation restoration. Here, according to vector analysis, we evaluated microbial nutrient limitation by studying soil EES across vegetation restoration types (naturally restored secondary forests (NF) and artificially planted forests (AF)) with AC as a control. Results showed both NF and AF soils averaged higher C- and P- acquiring enzyme, indicating rapid C and P turnover rates after vegetation restoration. However, vegetation restoration resulted in higher C requirement for microorganisms with higher enzyme C:N and vector length. In addition, microorganisms shifted from N- (< 45°) to P-limited (> 45°) conditions with enzyme N:P less than 1 after vegetation restoration, and NF exacerbated microbial P limitation compared to AF. Decreased N limitation following vegetation restoration could be contributed to improving soil ecosystem multifunctionality. The greater variation of EES was explained by the interaction of pH, soil nutrient, and microbial biomass than by any one of these factors alone, suggesting that both abiotic and biotic factors regulate microbial nutrient limitation and microbial process. Overall, our results revealed vegetation restoration could alleviate N limitation in the China Danxia, and thus enhance soil ecosystem by regulating lower microbial N limitation, which provide insight into nutrient management strategies under ecological restoration of degraded areas.

Divergent effects of short-term warming on microbial resource limitation between topsoil and subsoil in a young subtropical Chinese fir forest

Microbes depolymerize soil organic matter to supply themselves with carbon (C), and nutrients such as nitrogen (N) and phosphorus (P). In this way, microbial resource limitations play an important role in characterizing the biogeochemical cycle dynamics of the ecosystem. However, the effect of warming on microbial resource limitations, especially in resource-poor subsoils, remains unclear. Therefore, this study aimed to examine the effects of warming (+ 5 °C above ambient) on microbial resource limitation and explored their relationships with soil physicochemical properties and microbial community structure in topsoil (0–10 cm) and subsoil (40–60 cm) in a Chinese fir (Cunninghamia lanceolata) plantation in southern China. Microbial resource limitation with warming treatment was assessed via vector analysis of soil extracellular enzymatic stoichiometry after two years. We found warming aggravated microbial C limitation in the topsoil but alleviated microbial C limitation in the subsoil, while it shifted the relative nutrient limitation from P limitation to N limitation in the subsoil. Soil microbial C limitation was explained by soil properties (specifically, ammonium nitrogen) in the topsoil while by microbial community composition in the subsoil based on variance partition analysis. The soil microbial nutrient limitation was explained by soil properties in the topsoil and subsoil. The decrease in actinomycetes abundance in the warming treatment may have led to a decreased microbial C limitation in the subsoil. Our study highlighted the differences in warming effects between the topsoil and subsoil. We argue that the microbial resource demand of the subsoil should be further implemented in the study of soil biogeochemistry to improve the prediction of the impact of climate warming on soil C dynamics.

Determining changes in microbial nutrient limitations in bamboo soils under different management practices via enzyme stoichiometry

Moso bamboo (Phyllostachys edulis) is a plant species recognized for its extremely high carbon sequestration potential, and, along with soil microbes, play a key role in the carbon (C) cycle. Nevertheless, the metabolic constraints of soil microorganisms in the ecosystem have rarely been studied. We investigated changes in soil extracellular enzyme activities and microbial nutrients limitation in three different managed Moso bamboo plantations. The results showed that compared to nonmanagement (M0), extensive (M1) and intensive (M2) management significantly (p ≤ 0.05) decreased the activities of C- (β-1,4-glucosidase, BG) and N-acquiring (β-1,4-N-acetylglucosaminidase (NAG) and l-leucine aminopeptidase (LAP)) enzymes and significantly (p ≤ 0.05) increased the activity of soil P-acquiring enzymes (acid phosphatase, ACP). M1 and M2 exhibited significantly (p ≤ 0.05) decreased ratios of ln(BG):ln(ACP) and ln(NAG + LAP):ln(ACP) compared to M0. The ln(BG):ln(NAG + LAP) ratio in M2 was significantly (p ≤ 0.05) lower than in M0 bamboo plantations. M1 and M2 had a lower vector length and higher vector angle than M0, suggesting that management practices can reduce the microbial C and N limitations in bamboo plantations. In contrast, boosted activity of the C-acquiring enzyme, high levels of total organic C concentration, and increased microbial C constraint in M0 and M1 indicated that reducing disturbance benefits soil C stability and renders C hard to use by microorganisms. Random forest analyses indicated that total N and pH were the most important predictors of microbial C and N limitations in bamboo soils, respectively. This study revealed that the intensity of management practices markedly affected the balance between soil microbial nutrient requirements and C sequestration in Moso bamboo plantations.

Soil degradation regulates the effects of litter decomposition on soil microbial nutrient limitation: Evidence from soil enzymatic activity and stoichiometry.

Soil microorganisms could obtain energy and nutrients during litter decomposition with the help of soil extracellular enzymes. The litter types were among the most critical factors that affect soil extracellular enzyme activities. However, how litter types modulate the soil extracellular enzyme activity with grassland gradation is unclear. Here, we conducted a 240-day experiment of two different types of litter decomposition on soil extracellular enzyme activity and stoichiometry in different degraded grasslands. We found that C-acquiring enzyme activity and the enzyme stoichiometry of C/N were higher in Chloris virgata litter than in Leymus chinensis litter at lightly degraded level and C-acquiring enzyme activity in C. virgata was 16.96% higher than in L. chinensis. P-acquiring enzyme activity had the same trend with litter types in moderately and highly degraded levels and it was 20.71% and 30.89% higher in C. virgata than that in L. chinensis, respectively. The change of the enzyme stoichiometry with litter types was only showed in the enzyme stoichiometry of C/N at lightly degraded level, suggesting that litter types only affected the microbial C limitation in lightly degraded grassland. Almost all soil extracellular enzyme activities and extracellular enzyme stoichiometry, except the enzyme stoichiometry of N/P, decreased with grassland degraded level increasing. All vector angles were less than 45° suggesting that soil microorganisms were limited by N rather than by P during the decomposition process. Enzyme vector analysis revealed that soil microbial communities were co-limited by C and N during litter decomposition. Moreover, based on Random Forest (explaining more than 80%), we found that soil total nitrogen, total carbon, total phosphorus, dissolved organic C, pH and EC were important factors affecting soil enzyme activities by degradation levels. Our results emphasized that degradation levels could modulate the influences of litter types on soil extracellular enzyme activity. Our study enhanced our understanding in resource requirements for microbial communities to litter resources in degraded grassland and helped us to provide new ideas for improving degraded grassland ecosystems.

Earthworms regulate plants’ effects on soil microbial nutrient limitations: Examinations with contrasting soils and moisture

Soil microbial nutrient limitations significantly affect microbial processes and thus ecosystem functionality, whereas the response of soil microbial nutrient limitations to earthworms has rarely been addressed but is urgently needed due to the important role of earthworms in terrestrial ecosystems. By examining how earthworms regulate plants’ effects on microbial nutrient limitations under contrasting soil types and moisture conditions, we showed that plant presence reduced microbial carbon (C) limitation and such reduction was enhanced by earthworm. Plant presence increased soil microbial phosphorus (P) limitation in soils with earthworms in most cases. Additionally, the effects of plants on microbial nutrient limitations and their responses to earthworms were dependent on soil type (or soil nutrients) and moisture. These results suggested that earthworms have the potential to reduce soil microbial C limitation but enhance P limitation and highlighted the importance of nutrients and moisture in influencing the effects of earthworms and plants on microbial nutrient limitations.

Microbial nutrient limitation in rhizosphere soils of different food crop families: Evidence from ecoenzymatic stoichiometry

AbstractMicrobial metabolism directly participates in soil nutrient recycling and ecosystem stability. Although the rhizosphere soil is one of the most active habitats, information on how microorganisms acquire nutrients based on ecoenzymatic stoichiometry is lacking, especially for major food crops under field conditions. Thus, this study aimed to explore the soil nutrient properties and microbial biomass concentrations as well as extracellular enzymatic activities and focused on carbon (C), nitrogen (N), and phosphorus (P) cycling of different crop types, including potato (Solanaceae), maize (cereal), soybean (Leguminosae), and buckwheat (Polygonaceae) in the arid area of northern Shaanxi, China. Microorganisms in the rhizosphere soil of four crops were subjected to relative C and N limitation tests based on ecoenzymatic activities. Among the crops, potato and soybean exhibited the maximum limitations. Linear regression analysis of soil nutrients, microorganisms (stoichiometric homeostasis), and threshold elemental ratio synthetically supported microbial metabolic limitations. This finding coincided with the highest and lowest microbial carbon use efficiencies in soybean (0.58) and potato (0.51), illustrating the distinguishing physiological responses of microbes. Partial least squares path modeling further confirmed that the soil microbial nutrient limitations were significantly regulated by soil nutrient properties and extracellular enzyme activities. Taken together, soil microbial metabolism in agricultural ecosystems was co‐limited by relative C and N regardless of the main food crop families in this region in China (northern Loess Plateau). These observations clarified the nutrient cycling driven by soil elemental stoichiometry at the root–soil interface in arid and oligotrophic ecosystems.

Enzymatic C:N:P stoichiometry: Questionable assumptions and inconsistencies to infer soil microbial nutrient limitation

Enzymatic C:N:P stoichiometry: Questionable assumptions and inconsistencies to infer soil microbial nutrient limitation

Vegetation Restoration with Mixed N2-Fixer Tree Species Alleviates Microbial C and N Limitation in Surface Soil Aggregates in South Subtropical Karst Area, China

Soil extracellular enzyme stoichiometry (EES) is the essential predictor in nutrient status and resource limitation of soil microorganisms, whose metabolism has a vital role in biogeochemical cycling and ecosystem function. However, little is known about how N2-fixer tree species with different planting patterns affect soil nutrient resources in terms of extracellular enzyme activity (EEA) or EES within aggregates in degraded karst ecosystems. In this study, we evaluated soil EEA and EES related to carbon (C), nitrogen (N), and phosphorus (P) cycles across two eight-year-old pure plantations of legume species [Dalbergia odorifera T. Chen (PD) and Acrocarpus fraxinifolius Wight ex Arn. (PA)] and a mixed plantation of the two tree species listed above (MP). Meanwhile, a nearby undisturbed shrubland was used as a control (CK). We concluded that the activities of C-, N-, and P-acquiring enzyme increased to different degrees in the N2-fixer tree species stands (particularly in MP) compared to CK in all aggregates. Compared to CK, MP significantly increased by 39.0%, 54.0%, 39.3%, and 24.8% in total C-acquiring EEA, 41.1%, 60.5%, 47.8%, and 12.5% in total N-acquiring EEA, and 100.4%, 79.7%, 69.2%, and 56.4% in total P-acquiring EEA within &gt;2 mm, 1–2 mm, 0.25–1 mm, and &lt;0.25 mm aggregates, respectively. Furthermore, the logarithmic transformed ratio of C-, N-, and P-acquiring enzyme activities was 1.20:1.08:1, which deviated from the global ratio (1:1:1). Vector analysis of EEA showed that the vector length (VL) within aggregates was significantly lower than that of CK in all stands of N2-fixer species except PD; while in all treatments, vector angle (VA) was &lt;45° for all aggregate sizes, except in MP, where VA reached 45° for &lt;0.25 mm aggregate. These indicated soil microbes were limited by C and N together. However, MP significantly alleviated microbial C and N limitation than CK (p &lt; 0.05). There were obvious positive relationships between enzyme C:N, C:P, and N:P ratios. VL was markedly negatively linked to VA. EES was markedly related to most soil nutrients and microbial biomass stoichiometry ratios. Changes in soil EEA and EES were primarily driven by available phosphorus (AP), microbial biomass carbon (MBC), soil C:N and MBN:MBP ratios. Together, our results demonstrate the influences after introducing N2-fixer tree species (particularly MP) for vegetation recovery on soil microbial nutrient limitation and ecological processes in aggregate level and will contribute to the development of ecological restoration practices and fertility management in degraded karst ecosystems of southwest China.

Plant-soil-enzyme C-N-P stoichiometry and microbial nutrient limitation responses to plant-soil feedbacks during community succession: A 3-year pot experiment in China.

Studying plant-soil feedback (PSF) can improve the understanding of the plant community composition and structure; however, changes in plant-soil-enzyme stoichiometry in response to PSF are unclear. The present study aimed to analyze the changes in plant-soil-enzyme stoichiometry and microbial nutrient limitation to PSF, and identify the roles of nutrient limitation in PSF. Setaria viridis, Stipa bungeana, and Bothriochloa ischaemum were selected as representative grass species in early-, mid-, and late-succession; furthermore, three soil types were collected from grass species communities in early-, mid-, and late-succession to treat the three successional species. A 3-year (represents three growth periods) PSF experiment was performed with the three grasses in the soil in the three succession stages. We analyzed plant biomass and plant-soil-enzyme C-N-P stoichiometry for each plant growth period. The plant growth period mainly affected the plant C:N in the early- and late- species but showed a less pronounced effect on the soil C:N. During the three growth periods, the plants changed from N-limited to P-limited; the three successional species soils were mainly limited by N, whereas the microbes were limited by both C and N. The plant-soil-enzyme stoichiometry and plant biomass were not significantly correlated. In conclusion, during PSF, the plant growth period significantly influences the plant–soil–microbial nutrient limitations. Plant-soil-enzyme stoichiometry and microbial nutrient limitation cannot effectively explain PSF during succession on the Loess Plateau.

Adaptive Pathways of Microorganisms to Cope With the Shift From P- to N-Limitation in Subtropical Plantations.

Ecological stoichiometry is increasingly acknowledged as one of the main control factors for microbial activity and diversity. Soil carbon/nitrogen (C/N) and carbon/phosphorus (C/P) ratios are usually much higher than microbial nutrient requirements and vary with planting density and stand age in forestlands. However, how microorganisms cope with such stoichiometric imbalances and how they regulate nutrient cycling remain unclear. Here, 5- and 35-year-old experimental Cunninghamia lanceolata [Lamb.] Hook plantations with five planting densities (1,667, 3,333, 5,000, 6,667, and 10,000 stems ha−1) were used to explore the underlying mechanism of the response of microorganisms to stoichiometric imbalances. We found that (i) enzyme activity and microbial biomass and diversity increased with planting density at age 5 but decreased at age 35; (ii) soil microorganisms were P-limited at age 5, but gradually shifted from P- to N-limitation during the development of plantations from 5 to 35 years; and (iii) significantly negative relationships between microbial biomass stoichiometry and enzymatic stoichiometry were observed. The adaptive pathways of soil microorganisms to cope with stoichiometric imbalances include (i) adjusting the stoichiometry of microorganisms and enzymes; (ii) changing the relative abundance of the dominant microbial phyla; and (iii) increasing the ratio of fungal to bacterial diversity. These results highlight how to use the ecological stoichiometry method to identify soil microbial nutrient limitations with planting density during the development of plantations. By underlining the important role of stoichiometry on microbial growth and activity, these findings furthermore emphasize the dependency of organic matter transformation and nutrient cycling on the microbial community.

Effects of stand age and soil organic matter quality on soil bacterial and fungal community composition in Larix gmelinii plantations, Northeast China

AbstractIt is of great interest to elucidate the biogeographic patterns of soil microorganisms and their driving forces, which are fundamental to predicting alterations in microbial‐mediated functions arising from environmental changes. Although the vertical movement of dissolved organic matter (DOM) drives the cycle of nutrients such as soil carbon, in the restored ecosystem, the relationship between DOM and soil microbial nutrient utilization remains to be determined. Here, we investigated the changes of soil microbial community at 0–40 cm depth profile in three stages (10, 30, 50 years) of succession in Larix olgensis plantations and the fluorescence spectrum composition of DOM. With the increase of soil depth, the signal source of microorganisms increases. In a coniferous forest soil environment, the possible main source of DOM in deep soil is the production of microbial metabolism. Difficulty in the decomposition of organic matter determines the distribution and composition of microorganisms. Increasing forest age makes bacteria and fungi more specific and bacterial‐fungal associations greater. Overall, our work contributes to the understanding of factors underlying microbial community distribution in plantation forests and the importance of DOM quality in the building microbial community.

Soil Enzyme Stoichiometric Characteristics of Pinus massoniana Plantations at Different Stand Ages in Mid-subtropical Areas

Soil enzyme activity is an important index to characterize the nutrient requirements and nutrient limitations of soil microorganisms. In this study, Pinus massoniana plantations of different stand ages (9, 17, 26, 34, and 43 a) in mid-subtropical China were taken as the research object; the activities of β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), leucine amino-peptidase (LAP), acid phosphatase (AP), polyphenol oxidase (POX), and peroxidase (POD) were determined; and soil enzyme stoichiometric ratios were also calculated to investigate the soil microbial nutrient limitations of P. massoniana plantation development. The results showed that the activities of BG, NAG, AP, POX, and POD were enhanced with the increase in stand age, and the activity of LAP was the lowest at 17 a, which showed a significant difference and fluctuated among the neighboring stand ages. The soil enzyme C:N:P stoichiometric ratio was 1:1:0.56, which deviated from the global ecosystem enzyme C:N:P stoichiometric ratio (1:1:1). The enzyme C:N increased, whereas the enzyme N:P decreased, with increasing stand age, and both ratios tended to be stable after 17 a. There was no significant difference in enzyme N:P among different stand ages. The vector length of enzyme stoichiometry was not significantly different among the five stand ages. The vector angles increased with the increase in stand ages and tended to be stable after 17 a of stand age, but the angles were less than 45°. Redundancy analysis (RDA) revealed that soil carbon quality index and pH were the main factors influencing soil enzyme activity and the associated stoichiometric ratio. Our findings indicated that P. massoniana plantation soil microorganisms at different growth stages were all subjected to N limitation, and the N limitation was alleviated with the increase in stand age; however, the P requirement was gradually enhanced. Therefore, the management of P. massoniana plantations should take care to increase nitrogen fertilizer at the early growth stage of P. massoniana, and more phosphorus fertilizers need to be applied with nitrogen at the late growth stage in order to maintain the productivity and sustainable development of P. massoniana plantations.

Extracellular Enzyme Stoichiometry and Microbial Metabolism Limitation During Vegetation Restoration Process in the Middle of the Qinling Mountains, China

Clarifying the characteristics of soil microbial nutrient limitation and its driving mechanisms during vegetation restoration after farmland abandonment has important implications for revealing soil nutrient cycling and maintaining ecosystem stability. To determine the limitation of soil microbial nutrients and its relationship with soil properties along a chronosequence of abandoned farmland in the middle of the Qinling Mountains, the soil physicochemical properties and five enzyme activities (β-1,4-glucosidase (BG), cellobiohydrolase (CBH), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (AP)) were measured, and models of extracellular enzymatic activity were applied. The results showed that the activities of BG, CBH, NAG, LAP, and AP were significantly increased following farmland abandonment. With the increasing years of abandonment, the ratios of (BG+CBH)/(NAG+LAP) and (BG+CBH)/AP significantly decreased, whereas the ratio of (NAG+LAP)/AP increased. Correlation analysis showed that most soil physicochemical properties were significantly correlated with extracellular enzyme activities and extracellular enzymatic stoichiometry. The vector length of extracellular enzymatic stoichiometry decreased with the increase in abandonment years, indicating that the limitation of soil microorganisms on carbon (C) was reduced. Moreover, the vector angles (>45°) showed a decreasing trend, indicating that microbial metabolisms were limited by phosphorus (P) and gradually decreased. Regression analysis showed that the C and P limitations were significantly related to total nutrients, available nutrients, nutrient ratio, and soil physical properties. Partial least squares path modeling (PLS-PM) revealed that the C and P limitations were directly regulated by nutrient ratio. PLS-PM further showed that soil total nutrients indirectly affected soil microbial C and P limitations by affecting nutrient ratio, and nutrient ratio affected the soil metabolism limitation via available nutrients and pH. Our study suggests that the characteristics of microbial metabolism during the vegetation restoration process reflect the mechanism of microorganism-mediated soil nutrient cycling, which provides a theoretical basis for revealing the community dynamics and stability during the vegetation restoration process and maintaining the regional ecological environment security in the Qinling Mountains.

Soil ecoenzymatic stoichiometry reveals microbial phosphorus limitation after vegetation restoration on the Loess Plateau, China

Exploring the limitations of soil microbial nutrient metabolism would help to understand the adaptability and response mechanisms of soil microbes in semi-arid ecosystems. Soil ecoenzymatic stoichiometry is conducive to quantifying the nutrient limitations of microorganisms. To quantify microbial nutrient limitation during plant restoration, we measured soil physicochemical properties, microbial biomass, and the activities of four enzymes (ꞵ-1,4-glucosidase, leucine aminopeptidase, ꞵ-1,4-N-acetylglucosaminidase, and alkaline phosphatase) in the soils of the northern Loess Plateau. Vegetation restoration patterns significantly affected soil properties, microbial biomass, enzymatic activity, and associated stoichiometry. Soil enzymatic activity increased significantly after vegetation restoration, especially in Robinia pseudoacacia plantations (RP). Correlation analysis showed that soil nutrients (C and N), moisture and pH were significantly correlated with ecoenzymatic activities and their stoichiometries. Vector-threshold element ratio (VT) model analysis revealed that microbial nutrient metabolism was limited by P, and soil microbial C limitation was significantly weakened after vegetation restoration, particularly in RP. Correlation analysis indicated that microbial nutrient limitations represented by the VT model were significantly correlated with soil moisture, nutrients, and associated stoichiometry. Therefore, the soil microbial community was mainly limited by P rather than N in vegetation restoration on the Loess Plateau via the VT model, and this limitation was primarily associated with the variation in soil properties. In addition, the soil microbial C limitation was significantly negatively correlated with microbial nutrient (P or N) limitation, which illustrated that soil microbial nutrient metabolism has strong stoichiometric homeostasis.

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.

Effect of high soil C/N ratio and nitrogen limitation caused by the long-term combined organic-inorganic fertilization on the soil microbial community structure and its dominated SOC decomposition

The application of organic fertilizers, such as straw and manure, is an efficient approach to maintain soil productivity. However, the effect of these organic fertilizers on soil microbial nutrient balance has not yet been established. In this study, the effects of the long-term combined organic-inorganic fertilization on microbial community were investigated by conducting a 30-year-long field test. Overall, the following five fertilizer groups were employed: inorganic NP fertilizer (NP), inorganic NK fertilizer (NK), inorganic NPK fertilizer (NPK), NPK + manure (MNPK), and NPK + straw (SNPK). The results indicated that the mean natural logarithm of the soil C:N:P acquisition enzyme ratio was 1.04:1.11:1.00 under organic-inorganic treatments, which showed a deviation from its overall mean ratio of 1:1:1. This indicates that microbial resources do not have a balance. Vector analysis (vector angle <45°) and threshold elemental ratio analysis (RC:N-TERC:N > 0) further demonstrated that the microbial metabolism was limited by Nitrogen (N) under SNPK and MNPK treatments. N limitation further influenced soil microbial community structure and its dominated SOC decomposition. Specifically, Microbial communities transformed into a more oligotrophic-dominant condition (fungal, Acidobacteria, Chloroflexi) from copiotrophic-dominant (Proteobacteria, Actinobacteria) condition with increasing N limitation. Lysobacter genus and Blastocatellaceae family, in the bacterial communities along with the Mortierella elongata species in fungal communities, were markedly associated with the N limitation, which could be the critical biomarker that represented N limitation. Both correlation analysis and partial least squares path modeling showed significant positive effects of N limitation on the ratio of bacterial functional genes (Cellulase/Amylase), involved in recalcitrant SOC degradation.

Arbuscular mycorrhizal fungi promote litter decomposition and alleviate nutrient limitations of soil microbes under warming and nitrogen application

Arbuscular mycorrhizal fungi (AMF) are vital for nutrient cycling and aboveground ecosystem stability. To understand soil microbe resource limitations and AMF feedback regulation of the soil microbial biomass and litter stoichiometry under warming and nitrogen (N) application conditions, a 3-year experiment involving AMF suppression by benomyl under warming and N application conditions was conducted in the Songnen meadow, in which soil microbial nutrient limitation was measured by ecoenzymatic stoichiometry. We assessed the soil microbes and stoichiometric variables of Leymus chinensis and Phragmites australis litter under various treatment conditions. Carbon (C) and phosphorus (P) are commonly limited nutrients in soil microbial communities in the Songnen meadow ecosystem. AMF alleviated soil microbe C and P limitations by decreasing the microbial C:P ratio under warming and N application conditions and by accelerating litter decomposition, which occurred independently of warming and N application. AMF play a critical role in balancing nutrient stoichiometry and are key to maintaining microbial metabolism and nutrient cycle homeostasis. AMF can shorten the litter nutrient cycle, which is of great significance for the absorption, utilization and storage of plant nutrients in the Songnen grassland ecosystem.

Disentangling drivers of soil microbial nutrient limitation in intensive agricultural and natural ecosystems

Characterized by continuous chemical fertilization, intensive agriculture generally reduces soil ecoenzymatic activities and nutrient mineralization, as well as alters the biomass production and microbial community composition. Soil acidification poses serious threats to the sustainable development of intensive agriculture. However, the mechanism of nutrient cycling and metabolism of soil microorganisms in response to soil acidification in intensive agriculture remains unclear. Herein, we studied the variations in ecoenzymatic stoichiometry of soil β-glucosidase (BG), cellobiohydrolase (CBH), N-acetylglucosaminidase (NAG) and acid phosphatase (AP) under different land use types and pH gradients of tea garden soils. The results revealed that natural forest and cropland soils had significantly higher BG and CBH activities than tea garden soils. Soil BG and CBH activities displayed significant positive correlations with soil pH, total nitrogen (TN) and phosphorus (TP), while soil NAG activity was significantly associated with nitrate nitrogen, total carbon (TC), TN, carbon: phosphorus (C:P) and nitrogen: phosphorus (N:P) ratios. Soil AP activity showed significant negative associations with pH, TP and C:N ratio, but was significantly positively correlated with TC, TN, C:P and N:P ratios. Enzyme vector model revealed that soil microorganisms are limited by P (enzyme vector angle >45°) regardless of land use types. Compared to natural forest soils, the P limitation of microorganisms in tea garden soils became increasingly serious with a decreasing pH gradient, as indicated by the significant increase in enzyme vector angle. Thus, the overall ecoenzymatic stoichiometry was shifted by soil pH. In summary, higher pH increased BG activity and decreased AP activity, but had no significant effect on NAG activity, suggesting co-limitation of soil microorganisms by C and P in this area. This study provides novel insights into the effect of soil acidification on ecoenzymatic stoichiometry, and also highlights the stoichiometric and energy limitations on the metabolism of soil microorganisms in agricultural ecosystems.

The effects of Vicia sativa planting density on soil microbial nutrient metabolism

The effects of Vicia sativa planting density on soil microbial nutrient metabolism