BG Activity Research Articles

Long‐Term Manure Application Effects on Nutrients and Selected Enzymes Involved in Their Cycling

Core Ideas We assessed long‐term manure rates and application times on soil nutrients and enzymes at two sites. Increasing manure rate enhanced soil fertility but hydrolase enzymes showed a quadratic trend. Manure application prior to period of heavy rainfall (spring and fall) was associated with phosphorus losses. Peroxidase and phenol oxidase activities declined with increasing manure application rates. Oxidative enzyme activities were good indices of long‐term soil organic carbon storage. We investigated how soil nutrients (soil organic carbon [SOC], nitrogen [N], and Mehlich‐3‐extractable phosphorus [P‐M3]) and enzymes involved in their cycling were impacted by 10 yr of beef cattle manure application at two sites in Nova Scotia, Canada: a dikeland site (silty clay loam) and an upland site (sandy loam site), under timothy (Phleum pratense L.) cover. Manure was applied at four rates (from 0 to 300 kg total N ha−1) at different times (spring, summer, early fall, and late fall). Activities of hydrolytic enzymes: β‐glucosidase (BG), cellobiohydrolase (CB), leucine aminopeptidase (LAP), and phosphomonoesterase (PME) and oxidative enzymes: phenol oxidase (PO), peroxidase (PP), were quantified. We hypothesize that manure application will increase soil nutrients and BG, CB, LAP, and PME activities and will decrease PO and PP activities. After 10 yr of manure application, compared to a control, soil pH, SOC, N, and P‐M3 increased linearly by 41, 45, and 125%, respectively, with increasing manure rate, while BG, CB, PME, and LAP activities showed a quadratic response. Conversely, enzymes degrading less labile components (PP and PO) declined with increasing manure rate. Higher SOC and P‐M3 contents and higher LAP activity were found at the upland site, whereas higher PME, PP, and PO activities were found at the dikeland site. Inverse relationships between PME activity and P‐M3 content and between PP activity and SOC or N content were found, whereas a positive correlation between N content and LAP activity was observed. Regular manure application is a good way to sustain soil fertility, with the extent of soil C storage controlled by soil type. The optimal rate and timing of manure application is essential, since application prior to periods of heavy rainfall is associated with phosphorus losses. Our results demonstrated that oxidative enzymes (PP and PO) are good indices of long‐term SOC storage. The lower their activity, the higher is the SOC sequestration.

Serum β-glucuronidase activity as a biomarker for acute cholinesterase inhibitor pesticide poisoning

β-glucuronidase (BG) activity is a promising biomarker for diagnosis and prognosis after exposure to organophosphorous (OP) pesticides. The aim of this study was to evaluate the changes in serum BG activity in patients with acute OP poisoning and to determine whether these changes correlate with the severity of poisoning. Thirty patients with anticholinesterase pesticide poisoning were included, besides 10 healthy volunteers as a control group. Serum activities of butyrylcholinesterase (BuChE) and BG were measured for each subject on admission, then after 12 and 24 h. Serum levels of BuChE and BG in poisoned patients were significantly different from the control subjects; these differences persisted in repeated measurements. Moreover, the serum levels showed significant differences within each group of the three time points. A significant negative correlation was found between the serum activities of BuChE and BG in all groups at the three time points. In conclusion, serum BG activity seems a reliable marker for OP poisoning even when measured at 24 h after poisoning.

Sorption to Biochar Impacts β-Glucosidase and Phosphatase Enzyme Activities

Extracellular enzymes catalyze biogeochemical reactions in soil, cycling carbon and nutrients in agricultural systems. Enzymes respond quickly to soil management, including organic amendment inputs, such as biochar, a charcoal-like solid byproduct of bioenergy production. In a previous agricultural field trial, a pine biochar amendment caused an approximately 40% decrease in the enzyme activities of β-glucosidase (BG) and phosphatase (PHOS). The large surface area of the pine biochar has the potential to sorb nutrients and other organic molecules. To test if sorption caused decreased enzyme activity, we used a laboratory assay to quantify the activity of two sorbed enzymes: BG and acid PHOS, involved in the cycling of carbon and phosphorous. The enzymes were incubated with three solid phases: (1) the high surface area pine biochar, (2) the agricultural soil, and (3) a low surface area grass biochar, for an additional comparison. We quantified the sorbed enzymes at pH 6, 7, and 8, using a Bradford protein assay, and measured the immobilized enzyme activities via high-throughput fluorometric analysis. After sorption onto pine biochar, detectable BG and PHOS activity levels dropped by over 95% relative to the soil, supporting direct sorption as one mechanism that reduces enzyme activity in biochar amended soil. This laboratory assay demonstrated that sorption could account for the lack of priming of native soil organic matter and changes in soil phosphorous cycling after pine biochar addition.

Effect of Green Manure Application on Soil Enzyme Activity and Nutrient Dynamics in a Sugarcane Field of Kitadaito, Okinawa, Japan

To achieve sustainable sugarcane cultivation in subtropical cropland of Okinawa, Japan, it is necessary to improve and maintain the soil fertility by organic matter application. In most islands of Okinawa, green manure (GM) application is commonly used to improve the soil organic matter content because of its cost effectiveness. In this study, we evaluated the effect of application of different GMs, i.e., Crotalaria juncea (CRO) and Sesbania cannabina (SES), on soil nutrient dynamics in relation to soil enzymes, which are a sensitive indicator of land management in a sugarcane field of Kitadaito, Okinawa. We evaluated the fluctuation of soil enzymes (dehydrogenase [DH], β-glucosidase [BG], and phosphomonoesterase [PHO]), soil nutrients (extractable carbon [Ext-C] and nitrogen [Ext-N], and available phosphorus [Avail-P]) with environmental conditions after GM application for seven months. We also measured the sugarcane growth parameters seven months after planting. Application of CRO and SES clearly increased the BG and PHO activities throughout the experimental period and increased the sugarcane number (ca. 11%-20%), but they did not increase the DH activity and soil nutrients (e.g., Ext-C and Avail-P). Application of CRO also increased Ext-N, but SES application had no clear effect, possibly because of the different GM quality and decomposition patterns. Thus, our results showed that CRO is the preferred GM species in terms of improvement of soil enzyme and nutrient conditions for GM-sugarcane cropping systems in Kitadaito.

Linkages between straw decomposition rate and the change in microbial fractions and extracellular enzyme activities in soils under different long-term fertilization treatments.

In order to study the linkages between straw decomposition rate and the change in soil biological properties after straw addition to different fertilized soils, we collected soils from three long-term fertilization treatments (no-fertilizer, CK; nitrogen, phosphorus, and potassium fertilizers, NPK; NPK plus straw (S), NPKS), and incubated maize straw with these soils at 25°C for 75 days. The average straw carbon dioxide (CO2) emission rate in the CK+straw (S), NPK+S, and NPKS+S treatments was 0.58±0.51, 0.66±0.53, and 0.74±0.58 μg C g−1soil h−1, respectively. The average increase in the contents of fungi, bacteria, and Actinomycetes under straw addition treatments than the control soils (CK, NPK, and NPKS, respectively) changed in the order of CK+S≤NPK+S <NPKS+S, while bacteria and Actinomycetes peaked later in the CK+SthanNPK+S and NPKS+S treatments. Bacterial abundance unchanged, Actinomycetes abundance decreased, but fungal abundance significantly increased in soils after straw addition. The average increase in the activities of β-glucosidase (BG), β-D-cellobiosidase (CB), and β-xylosidase (XYL) differed as: CK+S<NPK+S ≤ NPKS+S, and the highest activities and increments of them occurred later in the CK+S than NPK+S and NPKS+S treatments. Straw CO2 emission rate was poorly correlated with changes in the contents of microbial fractions across all straw addition treatments, but it was significant positively correlated with the increased activities of BG, CB, and XYL under the NPK+S and NPKS+S treatments. Our results indicated that chemical fertilization and straw return soils differently increased straw decomposition because of the different increases in microbial fractions and soil enzyme activities when compared to the no-fertilizer soil, and the decomposition process was more closely correlated with C-obtaining enzymes than microbial fractions.

Effects of Long-term Fertilization on Enzyme Activities in Profile of Paddy Soil Profiles

The enzyme activity, which is closely related to soil material cycling (mineralization, transformation, etc.), can reflect soil quality and nutrient status. In order to explore the effect of long-term fertilization on the enzyme activity in paddy soil profile (0-40 cm), soils with organic fertilizer and inorganic fertilizer, and non-fertilized soils were selected, and the carbon and nitrogen contents, and the activities of β-1,4-glucosidase (BG), and β-1,4-N-acetylglucosaminidase (NAG) in 10cm depths of soil were analyzed. The results showed that the activities of BG and NAG in the soils treated with inorganic fertilizer and organic fertilizer increased by 0.73-47.87 nmol·(g·h)-1 and 1.33-128.81 nmol·(g·h)-1, and 0.19-9.72 nmol·(g·h)-1 and 0.92-57.66 nmol·(g·h)-1, respectively, compared to those for non-fertilized soil. Soil enzyme activity decreased with increasing soil depth. Soil enzyme activity in soil from 0-20 cm was significantly higher than that of soil from 20-40 cm. Soil enzyme activities were significantly affected by long term fertilization at different soil depths. RDA analysis showed that soil carbon and nitrogen contents had significant positive relationships with the activities of BG and NAG in the 0-20 cm soil profiles, however, negative relationships were observed in the 20-40 cm soil profiles. The long-term application of organic fertilizer significantly increased soil biomass and enzyme activity, both of which decreased with the increase in soil depth. Long-term fertilization could increase soil nutrient contents, microbial biomass, and extracellular enzyme activities, which has important theoretical significance for optimizing farmland fertilizer management and improving soil productivity.

Effects of nitrogen and phosphorus fertilization on the ratio of activities of carbon-acquiring to nitrogen-acquiring enzymes in a primary lowland tropical rainforest in Borneo, Malaysia

ABSTRACTPrevious meta-analyses revealed that the ratio of activities of carbon (C)-acquiring enzyme to nitrogen (N)-acquiring enzymes in tropical forest ecosystems was nearly identical to those in other ecosystems, despite of the N-rich condition in tropical forests. This could be explained by microbes in tropical forest soils, which require a large amount of N to produce N-rich acid phosphatase (AP) for catalyzation of the organic form of phosphorus (P) and compensation for poor P availability in soils. Based on this idea, we hypothesized that experimental P fertilization would reduce the allocation to N-acquiring enzymes compared with that of C-acquiring enzymes, i.e. that it would increase the ratios of activities of β-1,4-glucosidase (BG) to β-1,4-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP). We tested this hypothesis using an experimental fertilization site with factorial N (100 kg ha−1 yr−1) and P (50 kg ha−1 yr−1) addition in a primary tropical lowland forest in Bornean Malaysia, where our earlier work demonstrated that P fertilization reduced AP activity. Contrary to our hypothesis, the BG:NAG and BG:(NAG + LAP) ratios were not altered by either N or P fertilizations. This result indicated that AP production was not a reason for the maintenance of a relatively high investment in N-acquiring enzyme at our study site. Rather, NAG and LAP production was likely driven by C acquisition, rather than N acquisition, as the target substrates contained C as well as N. This idea was supported by the fact that neither the BG:NAG ratio nor the BG:(NAG + LAP) ratio was elevated by N addition. We propose that the ratios of activities of BG to NAG and LAP do not necessarily indicate the ratio of C:N acquisition, at least in our N-rich tropical forest ecosystem.

Effects of three biochar types on activity of β-glucosidase enzyme in two agricultural soils of different textures

ABSTRACTBiochar has attracted significant attention due to the long-lasting nature, and prominent influence on soil characteristics. This study was conducted to evaluate changes in the activity of β-glucosidase enzyme (BG) in loamy and sandy loam texture soils following two winter wheat growing seasons. The experimental design was a randomized complete block with three replicates and four treatments. The treatments were two soils, three biochars (rice husk, corn cob and bean harvest residue), five biochar rates (BR) and five levels of mineral fertilizers (FR) or dairy effluent (DE). The fertilizers were applied at the beginning of each season, while biochars were applied only at the beginning of experiment. Soil samples were collected following the second season and analyzed for the BG activity. Addition of biochars reduced the BG activity and the decline was higher in sandy loam compared to loamy soils. Negative effect of biochar to BG activity was greater at the highest BR (3.0%) than the lower BR. Fertilizer additions along with DE biochar had significant effect on BG activity that increased with higher FR. Given the importance of BG activity in soil organic matter decomposition, biochar application can be considered a sustainable way of increase in carbon sequestration.

Microbial resource allocation for phosphatase synthesis reflects the availability of inorganic phosphorus across various soils

According to the resource allocation model for extracellular enzyme synthesis, microorganisms should preferentially allocate their resources to phosphorus (P)-acquiring enzyme synthesis when P availability is low in soils. However, the validity of this model across different soil types and soils differing in their microbial community composition has not been well demonstrated. Here we investigated whether the resource allocation model for phosphatase synthesis is applicable across different soil types (Andosols, Acrisols, Cambisols, and Fluvisols) and land uses (arable and forest), and we examined which soil test P and/or P fraction microorganisms responded to when investing their resources in phosphatase synthesis in the soils. The ratio of alkaline phosphatase (ALP) to β-d-glucosidase (BG) activities in the arable soils and the ratio of acid phosphatase (ACP) to BG activities in the forest soils were significantly negatively related with the available inorganic P concentration. We also observed significant effects of available inorganic P, pH, soil types, and land uses on the (ACP + ALP)/BG ratio when the data for the arable and forest soils were combined and used in a stepwise multiple regression analysis. These results suggest that microbial resource allocation for phosphatase synthesis is primarily controlled by available inorganic P concentration and soil pH, but the effects of soil types and land uses are also significant.

A possible correlation between the basal ganglia motor function and the inverse kinematics calculation

The main hypothesis of this study, based on experimental data showing the relations between the BG activities and kinematic variables, is that BG are involved in computing inverse kinematics (IK) as a part of planning and decision-making. Indeed, it is assumed that based on the desired kinematic variables (such as velocity) of a limb in the workspace, angular kinematic variables in the joint configuration space are calculated. Therefore, in this paper, a system-level computational model of BG is proposed based on geometrical rules, which is able to compute IK. Next, the functionality of each part in the presented model is interpreted as a function of a nucleus or a pathway of BG. Moreover, to overcome existing redundancy in possible trajectories, an optimization problem minimizing energy consumption is defined and solved to select an optimal movement trajectory among an infinite number of possible ones. The validity of the model is checked by simulating it to control a three-segment manipulator with rotational joints in a plane. The performance of the model is studied for different types of movement including different reaching movements, a continuous circular movement and a sequence of tracking movements. Furthermore, to demonstrate the physiological similarity of the presented model to the BG structure, the neuronal activity of each part of the model considered as a BG nucleus is verified. Some changes in model parameters, inspired by the dopamine deficiency, also allow simulating some symptoms of Parkinson's disease such as bradykinesia and akinesia.

Spatio-Temporal Variability of Soil Beta-glucosidase Activity at the Arable Field Scale

Knowledge about the spatio-temporal variability of soil enzymatic properties is crucial in evaluating their structure-function relationship and their impact on ecosystem functions. The aim of this study was to assess the spatio-temporal variability of soil β-glucosidase (BG) activity against selected physico-chemical properties at the arable fild scale. A grid soil sampling (10×10 m) was used to measure the spatial variation of soil properties across a 0.4-ha fild. The BG activity was analyzed in soil samples that were collected from the upper 20 cm of Luvisols at 50 locations in April and August 2007 and 2008. Additionally, total organic carbon (CORG), total nitrogen (NTOT) concentrations, soil pHKCl as well as texture and water content were determined. The dataset was analyzed using classical statistical and geostatistical methods. Based on the analysis of variance, it was found that the BG activity, CORG, and water content showed signifiant differences between the four sampling dates. The BG activity showed a high contribution of nugget effect in sill (over 50%) and revealed a moderate spatial structure. The range of spatial autocorrelation calculated for the BG activity was between 15.0 and 61.0 m. Less intensive sampling grid should be recommended for soil BG activity in further studies concerning spatial variability in arable fild scale. In turn, more frequent sampling must be included in the sampling strategy in order to better understand whether the BG activity always shows permanent spatial patterns in soil or whether it is more randomized.

Responses of root exudation and nutrient cycling to grazing intensities and recovery practices in an alpine meadow: An implication for pasture management

The rhizosphere priming effect is caused by root carbon (C) exudation into the rhizosphere; the role of this effect in nutrient cycling and ecosystem recovery of natural grasslands as affected by different grazing intensities is still unknown. The objective of the present study was to investigate the relationships among root C exudation, rhizospheric microbial activity, and their influences on plant nutrient uptake during grazing and recovery periods. Field experiments were conducted in the Hongyuan Alpine Meadow to measure root exudation rate and nutrient cycling processes of the dominant species Elymus nutans. Three grazing intensities (an ungrazed control, moderate grazing and heavy grazing) were introduced for two months, following which all treatments received a recovery practice (no grazing for 21 days). Heavy grazing significantly decreased root exudation rate, soil nitrogen (N) mineralization rate, β-1,4-glucosidase (BG) activity, and foliar C concentration, while moderate grazing had no influence on these parameters compared to the control. After the 21 days of recovery, all these parameters, except N mineralization rate and foliar C concentrations in the heavy grazing treatment, returned to similar levels as in the control, whereas root exudation rate and BG activity rose to even higher levels. Meanwhile, moderate grazing significantly promoted root exudation rate, soil inorganic N concentration, net soil N mineralization rate, and β-N-acetylglucosaminidase (NAG) activity during the recovery stage as compared to the control. Foliar quality was also improved by the recovery practice, indicating that the high availability of N and P is a consequence of the positive root–microbe feedback and will ultimately benefit grazers. The flush of labile C released to the rhizosphere by grazed plants stimulated extracellular enzyme activities, enhanced soil N mineralization, and increased plant nutrient uptake. These results imply that reasonable (i.e. moderate) grazing followed by a recovery practice can effectively restore and strengthen grassland vegetation, and contribute to the sustainable use of alpine meadows such as Hongyuan.

Stoichiometric responses of soil microflora to nutrient additions for two temperate forest soils

The ratios of soil carbon (C) to nitrogen (N) and C to phosphorus (P) are much higher in Chinese temperate forest soils than in other forest soils, implying that N and P might limit microbial growth and activities. The objective of this study was to assess stoichiometric responses of microbial biomass, enzyme activities, and respiration to N and P additions. We conducted a nutrient (N, P, and N + P) addition experiment in two temperate soils under Korean pine (Pinus koraiensis) plantation and natural broadleaf forest in Northeast China and measured the microbial biomass C, N, P; the activities of β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), and acid and alkaline phosphomonoesterase (AP); and the microbial respiration in the two soils. Nitrogen addition increased microbial biomass N and decreased microbial biomass C-to-N ratio and microbial respiration in the two soils. Nitrogen addition decreased NAG activity to microbial biomass N ratio, P addition decreased AP activity to microbial biomass P ratio, and N, P, and N + P additions all increased BG activity to microbial biomass C ratio. These results suggest that microbial stoichiometry is not strictly homeostatic in response to nutrient additions, especially for N addition. The responses of enzyme activities to nutrient additions support the resource allocation theory. The N addition induced a decline in microbial respiration, implying that atmospheric N deposition may reduce microbial respiration, and consequently increase soil C sequestration in the temperate region.

Effects of nitrogen additions on soil hydrolase and oxidase activities in Pinus elliottii plantations.

We evaluated responses of hydrolase and oxidase activities in a subtropical Pinus elliottii plantation through a nitrogen (N) addition field experiment (dosage level: 0, 40, 120 kg N·hm-2·a-1). The results showed that N additions significantly decreased the carbon, nitrogen and phosphorus related hydrolase and oxidase activities. The activities of β-1,4-glucosidase (BG), cellobiohydrolase (CBH), β-1,4-N-acetylglucosaminidase (NAG) and peroxidase (PER) activities were decreased by 16.5%-51.1% due to N additions, and the decrease was more remarkable in the higher N addition treatment. The activities of α-1,4-glucosidase (aG), β-1,4-xylosidase (BX), acid phosphatase (AP) and phenol oxidase (PPO) were decreased by 14.5%-38.6% by N additions, however, there was no significant difference among the different N addition treatments. Soil enzyme activities varied obviously in different seasons. The activities of BG, NAG, BX, CBH, AP and PPO were in the order of March > June > October, and aG and PER activities were in the order of October > March > June. Most of the soil hydrolase and oxidase activities were positively correlated with soil pH, but negatively with NO3--N content. It indicated that N additions inhibited soil hydrolase and oxidase activities by reducing soil pH and increasing soil nitrification. N additions inhibited the soil organic matter mineralization and turnover in the subtropical area, and the effects were obvious with the increasing dosage of N additions.

Soil enzyme activity and stoichiometry in forest ecosystems along the North-South Transect in eastern China (NSTEC)

Soil enzymes, as indicators of microbial metabolism, play an important role in nutrient biogeochemistry at the ecosystem level. In this study, we present information from a comprehensive analysis of the latitudinal variations in, and stoichiometric relationships between, soil β-glucosidase (BG), N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (AP) in nine forest ecosystems along the North-South Transect in eastern China. The results showed that soil BG and NAG activities were higher in temperate forests than in subtropical and tropical forests. Soil AP activities were the opposite, which indicates that microbial nutrient demand in tropical forests was limited by the nutrient phosphorus (P). Soil BG and NAG activities were significantly and negatively correlated with mean annual temperature (MAT), mean annual precipitation (MAP), the soil carbon (C):P and soil nitrogen (N):P ratios, but not with the soil C:N ratio. Soil NAG and AP activities were inversely correlated with soil pH, and soil AP activity increased as soil pH decreased. The latitudinal variations in the C:N, C:P, and N:P acquisition ratios represented by ln(BG):ln(LAP + NAG):ln(AP) activities across ecosystems were significantly and negatively related to MAP and MAT. The C:P and N:P acquisition ratios were positively related to soil pH but negatively related to the soil C:P and N:P ratios. The C:N and C:P acquisition ratios were also negatively correlated with leaf C:N, C:P, and N:P ratios. This study provides useful information about environmental controls on enzyme stoichiometry, and also highlights the stoichiometric and energy limitations on the metabolism of soil microbes.

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

In order to explore the responses of soil enzyme activities and microbial community compositions to long-term nitrogen (N) addition in both bulk soil and microaggregate of chestnut soil, we conducted a 7-year urea addition experiment with N treatments at 6 levels (0, 56, 112, 224, 392 and 560 kg N ha–1 yr–1) in a temperate steppe of Inner Mongolia in China. Soil properties and the activities of four enzymes involved in carbon (C), nitrogen (N) and phosphorus (P) cycling were measured in both bulk soil and microaggregate, and phospholipid fatty acids (PLFAs) were measured in bulk soil. The results indicated that: 1) in bulk soil, N addition significantly decreased β-1,4-glucosidase (BG) and leucine aminopeptidase (LAP) activities at the treatment amounts of 224, 392 and 560 kg N ha–1 yr–1, and obviously suppressed β-1,4-N-acetylglucosaminidase (NAG) activity at the treatment amount of 560 kg N ha–1 yr–1. N addition enhanced total PLFAs (totPLFAs) and bacterial PLFAs (bacPLFAs) at the treatment amounts of 392 and 560 kg N ha–1 yr–1, respectively, but fungal PLFAs showed no response to N addition. The activities of BG, NAG and LAP were positively correlated with soil pH, but negatively correlated with the concentration of NH 4 + -N; 2) in microaggregate (53–250 μm), the activities of BG, NAG and AP showed no response to increased addition of N, but the significantly decreased LAP activity was observed at the treatment amount of 392 kg N ha–1 yr–1. These results suggested that enzyme activities were more sensitive to N addition than PLFA biomarkers in soil, and LAP activity in microaggregate may be a good indicator for evaluating N cycle response to long-term N addition.

Soil extracellular enzyme activities, soil carbon and nitrogen storage under nitrogen fertilization: A meta-analysis

Nitrogen (N) fertilization affects the rate of soil organic carbon (SOC) decomposition by regulating extracellular enzyme activities (EEA). Extracellular enzymes have not been represented in global biogeochemical models. Understanding the relationships among EEA and SOC, soil N (TN), and soil microbial biomass carbon (MBC) under N fertilization would enable modeling of the influence of EEA on SOC decomposition. Based on 65 published studies, we synthesized the activities of α-1,4-glucosidase (AG), β-1,4-glucosidase (BG), β-d-cellobiosidase (CBH), β-1,4-xylosidase (BX), β-1,4-N-acetyl-glucosaminidase (NAG), leucine amino peptidase (LAP), urease (UREA), acid phosphatase (AP), phenol oxidase (PHO), and peroxidase (PEO) in response to N fertilization. The proxy variables for hydrolytic C acquisition enzymes (C-acq), N acquisition (N-acq), and oxidative decomposition (OX) were calculated as the sum of AG, BG, CBH and BX; AG and LAP; PHO and PEO, respectively. The relationships between response ratios (RRs) of EEA and SOC, TN, or MBC were explored when they were reported simultaneously. Results showed that N fertilization significantly increased CBH, C-acq, AP, BX, BG, AG, and UREA activities by 6.4, 9.1, 10.6, 11.0, 11.2, 12.0, and 18.6%, but decreased PEO, OX and PHO by 6.1, 7.9 and 11.1%, respectively. N fertilization enhanced SOC and TN by 7.6% and 15.3%, respectively, but inhibited MBC by 9.5%. Significant positive correlations were found only between the RRs of C-acq and MBC, suggesting that changes in combined hydrolase activities might act as a proxy for MBC under N fertilization. In contrast with other variables, the RRs of AP, MBC, and TN showed unidirectional trends under different edaphic, environmental, and physiological conditions. Our results provide the first comprehensive set of evidence of how hydrolase and oxidase activities respond to N fertilization in various ecosystems. Future large-scale model projections could incorporate the observed relationship between hydrolases and microbial biomass as a proxy for C acquisition under global N enrichment scenarios in different ecosystems.

Nitrogen and phosphorus regulation of soil enzyme activities in acid forest soils

The activities of soil extracellular enzymes are important in governing rates of organic matter decomposition and nutrient cycling in forest ecosystems. Measurements of soil enzyme activities can provide insights on microbial function, in terms of how much energy microbial communities are investing to acquire particular nutrients from OM substrates. In ecosystems enriched with nitrogen (N), phosphorus (P) supply may play an important role in regulating microbial activity, enzyme production, and organic matter decomposition. The response of extracellular enzyme activity to chronic N-enrichment was assessed at a long-term paired watershed N-enrichment experiment, the Bear Brook Watershed in Maine (BBWM) in hardwood and softwood forest types. Specifically, we measured the ambient (extant) activity of C hydrolyzing β-glucosidase (BG) and xylosidase (XYLO), N hydrolyzing N-acetylglucosaminidase (NAG), and P hydrolyzing acid phosphatase (AP) in the watershed subjected to chronic N-enrichment, and in the reference watershed. Secondly, in a series of soil incubations, we characterized the extent to which microbial C and P acquisition were regulated by N and P availability. In these incubations, we measured BG and AP activity response to acute (high-dose fertilizer) nutrient amendments. We hypothesized that soil enzyme activities would respond more to enhanced P availability than N, particularly in the N-enriched watershed. Our results from extant enzyme activity measurements suggest that chronic N-enrichment inhibited rather than stimulated extant soil hydrolytic enzyme activities, which could reflect suppression of microbial biomass and activity. In the acute nutrient amendment incubations, our data indicate that inorganic P was more important than N in regulating soil microbial C and P acquisition in soils from both the N-enriched and reference watersheds. Our results also indicate that the extent to which P availability regulated microbial acquisition of P in O horizon soils was greatest in softwood soils subjected to chronic N-enrichment. Findings from this study suggest that both forest type and soil inorganic P availability could be more important in influencing soil biological response to N pollution than previously recognized.

Linkages between extracellular enzyme activities and the carbon and nitrogen content of grassland soils

Important biochemical reactions in soils are catalyzed by extracellular enzymes, which are synthesized by microbes and plant roots. Although enzyme activities can significantly affect the decomposition of soil organic matter and thus influence the storage and cycling of carbon (C) and nitrogen (N), it is not clear how enzyme activities relate to changes in the C and N content of different grassland soils. Here we address whether the activity of C-acquiring (β-1,4-glucosidase, BG) and N-acquiring (l-leucine aminopeptidase (LAP) and β-1,4-N-acetyl-glucosaminidase (NAG)) enzymes is linked to changes in the C and N content of a variety of human-managed grassland soils. We selected soils which have a well-documented management history going back at least 19 years in relation to changes in land use (grazing, mowing, ploughing), nutrient fertilization and lime (CaCO3) applications. Overall we found a positive relationship between BG activity and soil C content as well as between LAP + NAG activity and soil N. These positive relationships occurred across grasslands with very different soil pH and management history but not in intensively managed grasslands where increases in soil bulk density (i.e. high soil compaction) negatively affected enzyme activity. We also found evidence that chronic nutrient fertilization contributed to increases in soil C content and this was associated with a significant increase in BG activity when compared to unfertilized soils. Our study suggests that while the activities of C- and N-acquiring soil enzymes are positively related to soil C and N content, these activities respond significantly to changes in management (i.e. soil compaction and nutrient fertilization). In particular, the link between BG activity and the C content of long-term fertilized soils deserves further investigation if we wish to improve our understanding of the C sequestration potential of human-managed grassland soils.

Chronic nitrogen fertilization and carbon sequestration in grassland soils: evidence of a microbial enzyme link

Chronic nitrogen (N) fertilization can greatly affect soil carbon (C) sequestration by altering biochemical interactions between plant detritus and soil microbes. In lignin-rich forest soils, chronic N additions tend to increase soil C content partly by decreasing the activity of lignin-degrading enzymes. In cellulose-rich grassland soils it is not clear whether cellulose-degrading enzymes are also inhibited by N additions and what consequences this might have on changes in soil C content. Here we address whether chronic N fertilization has affected (1) the C content of light versus heavier soil fractions, and (2) the activity of four extracellular enzymes including the C-acquiring enzyme β-1,4-glucosidase (BG; necessary for cellulose hydrolysis). We found that 19 years of chronic N-only addition to permanent grassland have significantly increased soil C sequestration in heavy but not in light soil density fractions, and this C accrual was associated with a significant increase (and not decrease) of BG activity. Chronic N fertilization may increase BG activity because greater N availability reduces root C:N ratios thus increasing microbial demand for C, which is met by C inputs from enhanced root C pools in N-only fertilized soils. However, BG activity and total root mass strongly decreased in high pH soils under the application of lime (i.e. CaCO3), which reduced the ability of these organo-mineral soils to gain more C per units of N added. Our study is the first to show a potential ‘enzyme link’ between (1) long-term additions of inorganic N to grassland soils, and (2) the greater C content of organo-mineral soil fractions. Our new hypothesis is that the ‘enzyme link’ occurs because (a) BG activity is stimulated by increased microbial C demand relative to N under chronic fertilization, and (b) increased BG activity causes more C from roots and from microbial metabolites to accumulate and stabilize into organo-mineral C fractions. We suggest that any combination of management practices that can influence the BG ‘enzyme link’ will have far reaching implications for long-term C sequestration in grassland soils.