Soil moisture and plant residue addition interact in their effect on extracellular enzyme activity

Water availability strongly affects soil microbial activity and community composition. In a laboratory incubation we investigated the combined effect of soil moisture potential (−10 kPa, −135 kPa, and <−1500 kPa) and plant residue addition on soil enzyme activities (protease, β-glucosidase, β-glucosaminidase and exocellulase) and phospholipid fatty acid (PLFA) profiles. Soil respiration was positively correlated with soil moisture potential and significantly increased with the addition of residue. In the unamended soil, enzyme activities were little affected by soil moisture potential, nor did they change much over time. The addition of residue, however, significantly increased enzyme activity at each moisture level. Furthermore, all four enzyme activities were considerably higher in the amended dry soil than in amended samples with a higher moisture potential. In contrast, in the amended dry soil, respiration and microbial biomass were reduced compared to the amended samples with a higher moisture potential. The low microbial biomass in the amended dry soil was mainly due to a decrease in Gram-negative bacteria, while the fungal biomass reached similar levels at all water potentials. Therefore, shifts in microbial community composition alone cannot explain the increased enzyme activities in the dry soil. Other factors, such as increased fungal activity, stronger interactions between enzymes and soil particles due to thinner water films, may have contributed to the observed effects. Our results suggest that under dry conditions, potential enzyme activities may be decoupled from microbial biomass and respiration in the presence of substrates.     

Grasses, litter, and their interaction affect microbial biomass and soil enzyme activity

Plant effects on ecosystem processes are mediated through plant-microbial interactions belowground and soil enzyme assays are commonly used to directly relate microbial activity to ecosystem processes. Live plants influence microbial biomass and activity via differences in rhizosphere processes and detrital inputs. I utilized six grass species of varying litter chemistry in a factorial greenhouse experiment to evaluate the relative effect of live plants and detrital inputs on substrate-induced respiration (SIR, a measure of active microbial biomass), basal respiration, dissolved organic carbon (DOC), and the activities of β-glucosidase, β-glucosaminidase, and acid phosphatase. To minimize confounding variables, I used organic-free potting media, held soil moisture constant, and fertilized weekly. SIR and enzyme activities were 2-15 times greater in litter-addition than plant-addition treatments. Combining live plants with litter did not stimulate microbial biomass or activity above that in litter-only treatments, and β-glucosidase activity was significantly lower. Species-specific differences in litter N (%) and plant biomass were related to differences in β-glucosaminidase and acid phosphatase activity, respectively, but had no apparent effect on β-glucosidase, SIR, or basal respiration. DOC was negatively related to litter C:N, and positively related to plant biomass. Species identity and living plants were not as important as litter additions in stimulating microbial activity, suggesting that plant effects on soil enzymatic activity were driven primarily by detrital inputs, although the strength of litter effects may be moderated by the effect of growing plants.     

Logging residue harvest may decrease enzymatic activity of boreal forest soils

Nowadays conventional stem-only harvest where logging residues are left on the site is often displaced by whole-tree harvest, in which logging residues are harvested for use as bioenergy. Logging residues consist of tree branches and tops of stems with needles. The aim of this study was to evaluate the effect of logging residue harvest on soil enzyme activities involved in C, N and P cycling, namely β-glucosidase, β-glucosaminidase, protease and acid phosphatase in relation to other soil characteristics (i.e. soil respiration, net N mineralization, microbial biomass C and N). Soil samples were taken from the humus layer of five study sites, differing in fertility, dominating tree species and time elapsed after treatment. The study sites were Norway spruce (Picea abies, (L.) Karst) and Scots pine (Pinus sylvestris L.) stands in different parts of Finland. Four of the study sites were single-tree experiments, where thinning was performed 4–5 years before this study and 3–4 different doses of logging residues (from 0 up to 37.5 Mg ha⁻¹) were distributed on a circle around a single tree in 3 replicates. The last field experiment had been thinned twice, 23 and 13 years ago; the treatments in 3 replicates were whole-tree harvest and stem-only harvest. In the whole-tree harvest vs. stem-only harvest experiment, activities of β-glucosidase, β-glucosaminidase, acid phosphatase were similar in both treatments. In general, in the single-tree experiment with pine, enzymes raised the activity in response to increasing amount of logging residue. The pattern was less clear for the spruce single-tree experiment, but acid phosphatase and protease activities increased with the increase in amount of logging residue. In general, other soil characteristics were less affected than enzyme activities by logging residue removal; however, in some sites logging residues seemed to increase net C and N mineralization with increasing logging residue amount. Our results suggest that retaining logging residues on the site can increase soil enzyme activities and C and N mineralization.     

Seasonal variations of soil microbial biomass and activity in warm- and cool-season turfgrass systems

Plant growth can be an important factor regulating seasonal variations of soil microbial biomass and activity. We investigated soil microbial biomass, microbial respiration, net N mineralization, and soil enzyme activity in turfgrass systems of three cool-season species (tall fescue, Festuca arundinacea Schreb., Kentucky bluegrass, Poa pratensis L., and creeping bentgrass, Agrostis palustris L.) and three warm-season species (centipedegrass, Eremochloa ophiuroides (Munro.) Hack, zoysiagrass, Zoysia japonica Steud, and bermudagrass, Cynodon dactylon (L.) Pers.). Microbial biomass and respiration were higher in warm- than the cool-season turfgrass systems, but net N mineralization was generally lower in warm-season turfgrass systems. Soil microbial biomass C and N varied seasonally, being lower in September and higher in May and December, independent of turfgrass physiological types. Seasonal variations in microbial respiration, net N mineralization, and cellulase activity were also similar between warm- and cool-season turfgrass systems. The lower microbial biomass and activity in September were associated with lower soil available N, possibly caused by turfgrass competition for this resource. Microbial biomass and activity (i.e., microbial respiration and net N mineralization determined in a laboratory incubation experiment) increased in soil samples collected during late fall and winter when turfgrasses grew slowly and their competition for soil N was weak. These results suggest that N availability rather than climate is the primary determinant of seasonal dynamics of soil microbial biomass and activity in turfgrass systems, located in the humid and warm region.     

Modelling in situ activities of enzymes as a tool to explain seasonal variation of soil respiration from agro-ecosystems

Understanding in situ enzyme activities could help clarify the fate of soil organic carbon (SOC), one of the largest uncertainties in predicting future climate. Here, we explored the role of soil temperature and moisture on SOM decomposition by using, for the first time, modelled in situ enzyme activities as a proxy to explain seasonal variation in soil respiration. We measured temperature sensitivities (Q₁₀) of three enzymes (β-glucosidase, xylanase and phenoloxidase) and moisture sensitivity of β-glucosidase from agricultural soils in southwest Germany. Significant seasonal variation was found in potential activities of β-glucosidase, xylanase and phenoloxidase and in Q₁₀ for β-glucosidase and phenoloxidase activities but not for xylanase. We measured moisture sensitivity of β-glucosidase activity at four moisture levels (12%–32%), and fitted a saturation function reflecting increasing substrate limitation due to limited substrate diffusion at low water contents. The moisture response function of β-glucosidase activity remained stable throughout the year. Sensitivity of enzymes to temperature and moisture remains one of the greatest uncertainties in C models. We therefore used the response functions to model temperature-based and temperature and moisture-based in situ enzyme activities to characterize seasonal variation in SOC decomposition. We found temperature to be the main factor controlling in situ enzyme activities. To prove the relevance of our modelling approach, we compared the modelled in situ enzyme activities with soil respiration data measured weekly. Temperature-based in situ enzyme activities explained seasonal variability in soil respiration well, with model efficiencies between 0.35 and 0.78. Fitting an exponential response function to in situ soil temperature explained soil respiration to a lesser extent than our enzyme-based approach. Adding soil moisture as a co-factor improved model efficiencies only partly. Our results demonstrate the potential of this new approach to explain seasonal variation of enzyme related processes.     

Tebuconazole application decreases soil microbial biomass and activity

A short-term mesocosm experiment was conducted to ascertain the impact of tebuconazole on soil microbial communities. Tebuconazole was applied to soil samples with no previous pesticide history at three rates: 5, 50 and 500 mg kg⁻¹ DW soil. Soil sampling was carried out after 0, 7, 30, 60 and 90 days of incubation to determine tebuconazole concentration and microbial properties with potential as bioindicators of soil health [i.e., basal respiration, substrate-induced respiration, microbial biomass C, enzyme activities (urease, arylsulfatase, β-glucosidase, alkaline phosphatase, dehydrogenase), nitrification rate, and functional community profiling]. Tebuconazole degradation was accurately described by a bi-exponential model (degradation half-lives varied from 9 to 263 days depending on the concentration tested). Basal respiration, substrate-induced respiration, microbial biomass C and enzyme activities were inhibited by tebuconazole. Nitrification rate was also inhibited but only during the first 30 days. Different functional community profiles were observed depending on the tebuconazole concentration used. It was concluded that tebuconazole application decreases soil microbial biomass and activity.     

The effects of N and P additions on soil microbial properties in paired stands of temperate secondary forests and adjacent larch plantations in Northeast China

The conversion of secondary forests to larch plantations in Northeast China has resulted in a significant decline in soil available nitrogen (N) and phosphorus (P), and thus affects plant productivity and ecosystem functioning. Microbes play a key role in the recycling of soil nutrients; in turn, the availability of soil N and P can constrain microbial activity. However, there is little information on the relationships between available soil N and P and the microbial biomass and activity in larch plantation soil. We studied the responses of soil microbial respiration, microbial biomass and activity to N and P additions in a 120-day laboratory incubation experiment and assessed soil microbial properties in larch plantation soil by comparing them with the soil of an adjacent secondary forest. We found that the N-containing treatments (N and N + P) increased the concentrations of soil microbial biomass N and soluble organic N, whereas the same treatments did not affect microbial respiration and the activities of β-glucosidase, N-acetyl-β-glucosaminidase and acid phosphatase in the larch plantation. In addition, the concentration of microbial biomass P decreased with N addition in larch plantation soil. In contrast, N and N + P additions decreased microbial respiration, and N addition also decreased the activity of N-acetyl-β-glucosaminidase in the secondary forest soil. The P treatment did not affect microbial respiration in either larch plantation or secondary forest soils, while this treatment increased the activities of β-glucosidase and acid phosphatase in the secondary forest soil. These results suggested that microbial respiration was not limited by available P in either secondary forest or larch plantation soils, but microbial activity may have a greater P demand in secondary forest soil than in larch plantation soil. Overall, there was no evidence, at least in the present experiment, supporting the possibility that microbes suffered from N or P deficiency in larch plantation soil.     

Comparison of microbiological methods for evaluating quality and fertility of soil

Routine soil testing procedures that are rapid and accurate are needed to evaluate C and N mineralization in agricultural soils in order to determine soil quality and fertility. Laboratory methods were compared for their usefulness in determining soil microbial biomass and potential activity in a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) subjected to long-term tillage, crop sequence, and N-fertilizer management practices. The methods included basal soil respiration, net N mineralization during a 10-day incubation, soil microbial biomass C with the chloroform fumigation-incubation technique with and without subtracting a control value, soil microbial biomass N with the chloroform fumigation-incubation technique, substrate-induced respiration, and arginine ammonification. All methods were highly correlated with each other and, therefore, appear to adequately reflect soil microbial biomass and potential activity under laboratory conditions. The longer incubation times used with the basal soil respiration, N mineralization, and microbial biomass C and N assays resulted in higher correlations and lower variation among replications compared to the shorter incubation times used with substrate-induced respiration and arginine ammonification. The relatively rapid procedural time (3 h) required for the latter two assays could make these methods more attractive for routine soil testing, although multiple assays on the same sample may be necessary because these methods are less precise than the incubation methods that require 10 days.     

Einfluß variierter Temperatur und Feuchte auf mikrobielle Aktivitäten im Boden unter Laborbedingungen

Influence of varied soil temperature and moisture on microbial activities under laboratory conditions Under laboratory conditions the influence of temperature (10°C, 20°C, fluctuation from 5° to 30°C within 12 h with additional freezing for 3 days) and soil moisture (30%, 60% w.h.c., remoistening to 60% for 1 week) on several microbial activities was investigated. The biomass-related, glucose-induced short-term respiration and the dehydrogenase activity (TTC reduction) were higher at 10°C in most cases as compared to 20°C. Independent of freezing fluctuating temperature caused the lowest activities. The nitrogen mineralization (including nitrification), however, was affected in the opposite way. No marked influences were observed with β-glucosidase, arylsulfatase, and alkaline phosphatase. In the sandy loam nearly no effects of the soil moisture occurred and in the loamy sand especially the dehydrogenase activity was higher at 30% w.h.c., whereas the nitrogen mineralization was lower. From the results it can be concluded, that ecological conditions favouring mineralization without substrate addition may even reduce microbial biomass by decomposition.     

Measurements of microbial community activities in individual soil macroaggregates

The functional potential of single soil macroaggregates may provide insights into the localized distribution of microbial activities better than traditional assays conducted on bulk quantities of soil. Thus, we scaled down enzyme assays for β-glucosidase, N-acetyl-β-d-glucosaminidase, lipase, and leucine aminopeptidase to measure of the enzyme potential of individual macroaggregates (250–1000 μm diameter). Across all enzymes, the smallest macroaggregates had the greatest activity and the range of enzyme activities observed in all macroaggregates supports the hypothesis that functional potential in soil may be distributed in a patchy fashion. Paired analyses of ATP as a surrogate for active microbial biomass and β-glucosidase on the same macroaggregates suggest the presence of both extracellular β-glucosidase functioning in macroaggregates with no detectable ATP and also of relatively active microbial communities (high ATP) that have low β-glucosidase potentials. Studying function at a scale more consistent with microbial habitat presents greater opportunity to link microbial community structure to microbial community function.     

Quantifying soil water effects on nitrogen mineralization from soil organic matter and from fresh crop residues

A loamy sand was incubated with and without addition of carrot leaves at six different water contents ranging from 6% to 20% (g 100 g^-1 dry soil) and N mineralization was monitored during 98 days. We calculated zero- and first-order rates for mineralization in the unamended soil and first-order rates for N mineralization in the residue-amended soil. Although N mineralization was strongly affected by soil moisture, rates were still important at 6% water content (corresponding to permanent wilting point), particularly in the residue-amended soil. Soil water content was recalculated as soil water tension and as percent water-filled pore space (%WFPS) and a parabolic, a logistic and a Gaussian-type function were fitted to the relation between N mineralization rates and water content, %WFPS or pF. Water potential was a less suitable parameter than either %WFPS or water content to describe the soil water influence on N mineralization, because N mineralization rates were extremely sensitive to changes in the water potential in the range of pF values between 1.5 and 2.5. In the residue-amended soil the Gaussian model yielded an optimum %WFPS of 56% for N mineralization, which is slightly lower than optimum values cited in literature. N mineralization in the unamended soil was more influenced by soil water than N mineralization from fresh crop residues. This could be explained by less water limitation of the microbial population decomposing the residues, due to the water content of the residues. The effect of the water contained in the residues was most pronounced in the lowest water content treatments. The water retention curves of both undisturbed and repacked soil were determined and suggested that extrapolation of results obtained during laboratory incubations, using disturbed soil, to field conditions will be difficult unless soil bulk density effects are accounted for, as is the case with the use of %WFPS.     

Seasonality of soil biological properties in a poplar plantation growing under elevated atmospheric CO2

Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO_2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO₂ is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO_2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314 m² plots treated either with atmospheric (control) or enriched (550 μmol mol⁻¹ CO₂) CO₂ concentration with FACE technology (free-air CO₂ enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001.

The aim of the present work was: (1) to determine if CO₂ enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO₂ level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO₂.

CO₂ enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO₂. All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO₂ level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment.     

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.     

Seasonality of soil biological properties in a poplar plantation growing under elevated atmospheric CO2

Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO_2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO₂ is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO_2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314 m² plots treated either with atmospheric (control) or enriched (550 μmol mol⁻¹ CO₂) CO₂ concentration with FACE technology (free-air CO₂ enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001.The aim of the present work was: (1) to determine if CO₂ enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO₂ level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO₂.CO₂ enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO₂. All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO₂ level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment.     

Soil biochemical activities and the geometric mean of enzyme activities after application of sewage sludge and sewage sludge biochar to soil

There is a need to improve the way in which wastes, such as sewage sludges, are managed and a potential way to proceed would be to transform them into biochar. On the other hand, there is a growing interest in the use of soil biochemical properties as indicators of soil quality because they are sensitive to alterations in soil management. Thus, we have studied the effect of a biochar obtained from sewage sludge on soil biochemical properties in an organic soil using two doses of biochar and comparing these results with the control soil and with soils amended with the same two doses of unpyrolyzed sewage sludge. Microbial biomass C, soil respiration, net N mineralization and several enzyme activities (dehydrogenase, β-glucosidase, phosphomoesterase and arylsulphatase) were monitored. The geometric mean of enzyme activities (GMea) was used as a soil quality index. Individual biochemical properties showed a different response to the treatments, while GMea showed an increase in the quality of soils amended with the high biochar dose and a decrease in those amended with a high sewage sludge dose. The geometric mean of enzyme activities was a suitable index to condense the whole set of soil enzyme values in a single numerical value, which was sensitive to management practices.     

Gross nitrogen mineralization and nitrification rates and their relationships to enzyme activities and the soil microbial biomass in soils treated with dairy shed effluent and ammonium fertilizer at different water potentials

 Gross N mineralization and nitrification rates and their relationships to microbial biomass C and N and enzyme (protease, deaminase and urease) activities were determined in soils treated with dairy shed effluent (DSE) or NH₄ ⁺ fertilizer (NH₄Cl) at a rate equivalent to 200 kg N ha^–1 at three water potentials (0, –10 and –80 kPa) at 20  °C using a closed incubation technique. After 8, 16, 30, 45, 60 and 90 days of incubation, sub-samples of soil were removed to determine gross N mineralization and nitrification rates, enzyme activities, microbial biomass C and N, and NH₄ ⁺ and NO₃ ^– concentrations. The addition of DSE to the soil resulted in significantly higher gross N mineralization rates (7.0–1.7 μg N g^–1 soil day^–1) than in the control (3.8–1.2 μg N g^–1 soil day^–1), particularly during the first 16 days of incubation. This increase in gross mineralization rate occurred because of the presence of readily mineralizable organic substrates with low C : N ratios, and stimulated soil microbial and enzymatic activities by the organic C and nutrients in the DSE. The addition of NH₄Cl did not increase the gross N mineralization rate, probably because of the lack of readily available organic C and/or a possible adverse effect of the high NH₄ ⁺ concentration on microbial activity. However, nitrification rates were highest in the NH₄Cl-treated soil, followed by DSE-treated soil and then the control. Soil microbial biomass, protease, deaminase and urease activities were significantly increased immediately after the addition of DSE and then declined gradually with time. The increased soil microbial biomass was probably due to the increased available C substrate and nutrients stimulating soil microbial growth, and this in turn resulted in higher enzyme activities. NH₄Cl had a minimal impact on the soil microbial biomass and enzyme activities, possibly because of the lack of readily available C substrates. The optimum soil water potential for gross N mineralization and nitrification rates, microbial and enzyme activities was –10 kPa compared with –80 kPa and 0 kPa. Gross N mineralization rates were positively correlated with soil microbial biomass N and protease and urease activities in the DSE-treated soil, but no such correlations were found in the NH₄Cl-treated soil. The enzyme activities were also positively correlated with each other and with soil microbial biomass C and N. The forms of N and the different water potentials had a significant effect on the correlation coefficients. Stepwise regression analysis showed that protease was the variable that most frequently accounted for the variations of gross N mineralization rate when included in the equation, and has the potential to be used as one of the predictors for N mineralization. Received: 10 March 1998     

Microbial biomass, enzyme and mineralization activity in relation to soil organic C, N and P turnover influenced by acid metal stress

This study focused on the potential of using soil microbial biomass, enzyme and mineralization activities involved in organic C, N and P turnover, to evaluate the quality of a subtropical agricultural soil affected by long-term acid metal stress. Fractions of C, N and P involved in soil organic matter, microbial biomass and mineralization processes were estimated. Total enzyme activity (FDA) and eight hydrolase activities (xylanase, amylase, β-glucosidase, invertase, N-acteyl-glucosaminidase, urease, alkaline and acid phosphatases) in different decomposition stages of organic C, N and P were selected to characterize the soil functional diversity. These biological datasets were compared with soil metal variables (total contents and free and ligand-complexed ions of Cu, Pb, Zn, Cd, Al and Mn), using principal component analyses, co-inertia and discriminant analyses. The multiple statistics indicate that the metal variables were significantly related with not only general biological factors, but also respective datasets of biomass, enzyme activities and mineralization rates (all P < 0.001). In general, metal variables were inversely related to parameters and indices of microbial biomass C, N and P, FDA and C-related polysaccharidase and heterosidase activities, and P mineralization. As comparison, metal variables exhibited positive relationships with parameters and indices of N-related N-acteyl-glucosaminidase, urease, ammonification, total N mineralization and metabolic quotient, compared with inhibited nitrification. Specifically, free and complexed metal cations showed higher bioavailability than total contents in most cases. Cu, Pb, Al and Mn had different ecotoxicological impacts than Cd and Zn did. Stepwise regression models demonstrated that metal variables are key stress factors, but most of them excluded soil pH. Furthermore, spatial distribution in land uses and of sampling sites clearly separated the soil samples in these models (P < 0.001). We conclude that such a statistical analysis of microbiological and biochemical indices can provide a reliable and comprehensive indication of changes in soil quality and organic nutrient cycling, after exposure to long-term acid metal stress.     

Drivers of microbial respiration and net N mineralization at the continental scale

The dominant pools of C and N in the terrestrial biosphere are in soils, and understanding what factors control the rates at which these pools cycle is essential in understanding soil CO₂ production and N availability. Many previous studies have examined large scale patterns in decomposition of C and N in plant litter and organic soils, but few have done so in mineral soils, and fewer have looked beyond ecosystem specific, regional, or gradient-specific drivers. In this study, we examined the rates of microbial respiration and net N mineralization in 84 distinct mineral soils in static laboratory incubations. We examined patterns in C and N pool sizes, microbial biomass, and process rates by vegetation type (grassland, shrubland, coniferous forest, and deciduous/broadleaf forest). We also modeled microbial respiration and net N mineralization in relation to soil and site characteristics using structural equation modeling to identify potential process drivers across soils. While we did not explicitly investigate the influence of soil organic matter quality, microbial community composition, or clay mineralogy on microbial process rates in this study, our models allow us to put boundaries on the unique explanatory power these characteristics could potentially provide in predicting respiration and net N mineralization. Mean annual temperature and precipitation, soil C concentration, microbial biomass, and clay content predicted 78% of the variance in microbial respiration, with 61% explained by microbial biomass alone. For net N mineralization, only 33% of the variance was explained, with mean annual precipitation, soil C and N concentration, and clay content as the potential drivers. We suggest that the high R² for respiration suggests that soil organic matter quality, microbial community composition, and clay mineralogy explain at most 22% of the variance in respiration, while they could explain up to 67% of the variance in net N mineralization.     

Phyllostachys edulis (moso bamboo) rhizosphere increasing soil microbial activity rather than biomass

Purpose

Rhizosphere and fertilization might affect soil microbial activities, biomass, and community. This study aimed to evaluate the impacts of Phyllostachys edulis (moso bamboo) rhizospheres on soil nutrient contents and microbial properties in a moso bamboo forest with different fertilizer applications and to link soil microbial activities with abiotic and biotic factors.

Materials and methods

The experiment included three treatments: (1) application of 45% slag fertilizer (45%-SF); (2) application of special compound fertilizer for bamboos (SCF); and (3) the control without any fertilizer application (CK). Simultaneously, bulk soils and 0.5, 2.5, 4.5, and 6.5-year-old (y) bamboo rhizosphere soils were selected. Soil nutrient contents were analyzed. Microbial activities were evaluated based on the activities of soil enzymes including β-glucosidase, urease, protease, phosphatase, and catalase. The total microbial biomass and community were assessed with the phospholipid fatty acids (PLFAs) method.

Results and discussion

In the CK and SCF treatments, organic matter contents of rhizosphere soils were significantly higher than those of bulk soils. Soil β-glucosidase, urease, protease, phosphatase, and catalase activities in rhizosphere soils were higher than those of bulk soils, with the sole exception of β-glucosidase of 0.5 y rhizosphere soil in the 45%-SF treatment. Compared with the CK treatment, fertilizer applications tended to increase soil total PLFAs contents and changed soil microbial community. Moso bamboo rhizospheres did not significantly increase the total microbial biomass. In the SCF treatment, the Shannon index of bulk soil was significantly lower than those of rhizosphere soils.

Conclusions

Our results suggested that both rhizospheres and fertilizer applications could change the soil microbial community structures and that moso bamboo rhizosphere could increase microbial activity rather than biomass in the forest soils with different fertilizer applications.

     

Soil microbial and biochemical characteristics in relation to soil management and fertility

From 1969 to 1974, two experimental fields were studied in terms of their soil microbial and biochemical characteristics in relation to soil management practice and crop yields. Of the various microbial and biochemical properties measured soil phosphatase, saccharase, β-glucosidase and urease activities, N mineralization and soil respiration were found valuable to characterize the soils. These characteristics increased with increasing soil organic matter, clay and CaCO₃ content. They also revealed a strong effect of organic fertilization. Multiple regression analyses indicated that the alkalinity and the humus content of the soil largely determined the magnitude of these characteristics. The regression analyses also showed that the yields of winter wheat were related positively to phosphatase activity while the sugarbeet yields were related negatively to soil urease activity.