Unit Of Microbial Biomass Research Articles

Microbial metabolic capacity regulates the accrual of mineral-associated organic carbon in subtropical paddy soils

In general, the accrual of mineral-associated organic C (MAOC) level is limited by the finite area of reactive minerals. However, microbial metabolism probably affects MAOC accumulation and saturation because microbial necromass C constitutes the primary component of MAOC. To address this gap in current understanding, we used 18O–H2O incubation and biomarker analyses to quantify microbial C metabolism and necromass accumulation respectively, and quantify their relationship with MAOC and saturation levels across gradients of soil organic C (SOC) that ranged from 4.0 to 40.1 g kg−1 in subtropical paddy soils. Results showed that particulate organic C (POC) increased linearly as SOC increased whilst MAOC tended to saturation. Notably, MAOC accumulation showed a significant positive relationship to increases in microbial necromass C indicating that MAOC accumulation prior to saturation was closely linked to the activity of microbial C metabolism. Compared with low C soils, lower microbial growth efficiencies (qgrowth) and slower biomass turnover rates were observed in high C soils. Moreover, the necromass accumulation coefficient (i.e. necromass produced per unit of microbial biomass) was also lower in high C soils than in low C soils. The lower microbial growth efficiency in the high C soils was attributed to greater C/P and higher clay proportion, which decreased microbial biosynthesis and substrate accessibility. Microbial communities showed adaptive shifts in composition to modulate bacterial metabolic activity, with increases in the relative abundance of r-strategists and decreases in the K-strategists. Management strategies for actively increasing the size of the MAOC pool, by bolstering microbial anabolism and necromass accumulation, are essential for increasing the MAOC content in paddy soils where the MAOC is unsaturated. For high C soils, where the MAOC is already saturated, the use of structural and recalcitrant organic materials to increase the size of the POC pool is needed to enhance carbon sequestration efforts.

Phytin recovered from grain distillation can serve as a phosphorus fertilizer for maize and soybean

AbstractPhosphorus (P) recovery from waste streams can increase food system P use efficiency while simultaneousl mitigating point source P losses. Phytin is a P‐rich waste product generated from maize grain biorefineries, largely located in the US Midwest. However, since the majority of P in phytin is organic, phytin‐P is likely to have limited crop availability in soil following application, as it must first be mineralized to orthophosphate‐P by soil phosphatases. To evaluate the fertilizer potential of phytin recovered from a maize wet milling plant and test hypothesized mechanisms of P mineralization, a five‐step gradient of phytin substitution for monoammonium phosphate (MAP) (0%, 25%, 50%, 75%, and 100% substitution) was evaluated for maize (Zea mays L.) and soybean (Glycine max L.) growth in a P‐deficient Aquic Argiudoll. Irrespective of crop species, aboveground biomass at end of vegetative growth (VT stage) was similar for up to 75% phytin substitution as MAP, but was 21% lower for maize and 49% for soybean when phytin was fully substituted for MAP. Soil microbial biomass carbon (C), nitrogen (N), and P, as well as activities of phosphomonoesterase and phosphodiesterase were invariant across the phytin substitution gradient, suggesting negligible mineralization of phytin P. Full substitution of MAP with phytin lowered soil microbial biomass C:N by 121% for maize and by 153% for soybean, and soil phosphatase activities per unit microbial biomass C were 24% higher under soybean. Our results indicate that phytin can be partially substituted for highly water‐soluble P fertilizers for the two major crop species of the US Midwest in which phytin waste generation is co‐located.

Altered rainfall greatly affects enzyme activity but has limited effect on microbial biomass in Australian dryland soils

Drylands support a substantial proportion of the worlds biodiversity and are important to food production but are sensitive to changes in rainfall regimes. Altered rainfall has been shown to impact plant growth and soil microbial activity in drylands but the longer-term effect on belowground communities and biogeochemical cycles remains uncertain. We explored how four years of reduced and increased rainfall influenced soil total and available carbon (C), nitrogen (N) and phosphorus (P) content, microbial biomass and potential extracellular enzyme activity under field conditions at six dryland sites in eastern Australia. The study coincided with a severe 3-year drought that resulted in low standing plant biomass and soil C content at all sites. Microbial biomass attributes varied considerably across sites, with rainfall treatment effects limited to decreased fungal biomass and lower fungal:bacterial ratios in semi-arid Nyngan and reduced fungal:bacterial ratios and microbial biomass C in semi-arid Quilpie in reduced treatments compared with increased rainfall plots. Similarly, available soil C, N and P varied considerably among sites, with more available N and P at four and all sites, respectively, in reduced rainfall treatments particularly when compared with increased rainfall treatments. Rainfall treatments consistently influenced enzyme activity across all sites, with higher rates in increased rainfall plots indicative of greater microbial activity and enhanced nutrient cycling. Enzymatic activity associated with N cycling showed a negative relationship with available N while enzymes associated with P cycling related positively to available C and negatively to available P. This indicates that microbes invested more in production of enzymes associated with less available nutrients. Enzyme activity was not related to microbial biomass suggesting a disconnect between biomass and enzyme production and that rainfall treatments altered the ecosystem's specific enzyme activity (activity per unit of microbial biomass). Our results suggest that altered rainfall consistently impacted dryland ecosystem function, but that microbial biomass is a poor proxy for rainfall-induced changes in soil processes.

Extracellular polymeric substances are closely related to land cover, microbial communities, and enzyme activity in tropical soils

Extracellular polymeric substances (EPS) form the main matrix of microbial biofilms and play a crucial role in maintaining microbial life. However, factors influencing EPS concentration and production in soil are poorly understood. Here we show that EPS are closely related to microbial communities and nutrient acquisition in tropical forest and cropland soils with varying iron-aluminum-manganese concentrations and total reserve in base cations. We found under homogenized moisture and temperature conditions that EPS concentration and production efficiency (i.e., EPS per unit of microbial biomass) depend more on land cover than on geochemical soil properties. EPS concentration and production efficiency were higher in cropland than in forest soil and were related to the higher relative abundance of microbial sequences identified as Paenibacillaceae, Ramlibacter, Chaetosphaeria, Burkholderiaceae, and Xanthobacteraceae, pointing to potential EPS producers. In contrast, lower EPS concentration in forest soil was related to the higher relative abundance of microbial sequences associated with e.g., Gemmatimonas and Massilia, suggesting potential EPS degradation. We also found that EPS production efficiency was positively related to microbial investment in nutrient acquisition, implying that EPS production likely follows the same principles as extracellular enzyme activity. That is, EPS production may increase when resources are scarce to facilitate nutrient acquisition, and decrease when resources are abundant. Overall, microbial community composition and resource demand seem to control EPS degradation and accumulation in tropical soils, which could influence microbially-driven carbon and nutrient cycling.

Soil Enzyme Activity in Natural and Ploughed Catenas as Parameters of Physiological State of Microbial Communities

The study of soil microbial biomass and enzymatic activity of natural and anthropogenically transformed ecosystems was carried out. The catenas of virgin luvisols and chernozems of the Belogorye Natural Reserve and the catenas of arable soils were studied under similar geomorphological and lithological conditions. The activities of enzymes involved in the cycles of carbon (β-glucosidase and xylosidase), nitrogen (chitinase), and phosphorus (acid and alkaline phosphatase) was studied. It has been established that a decrease in soil microbial biomass as a result of ploughing is not accompanied by an equivalent decrease in the enzymatic activity of the soil. Differences in the enzymatic activity of different soils types were revealed, which indicates differences in the structure of the microbial community and the type of phytocenoses. Patterns of changes in the enzymatic activity of soils in watershed areas, in the transit and accumulative parts of catenas have been established. The values of specific enzymes activities (enzymatic activities per unit of microbial biomass) were calculated. The obtained patterns of changes in the specific enzymatic activity of arable soils indicate that, despite the loss of organic matter and a decrease in microbial biomass as a result of plowing, the physiological efficiency of the microbial community of agrochernozem is higher than in virgin soil. High specific enzymatic activity in arable soils is associated with higher rate of enzyme production by soil microorganisms due to land use changes.

Drivers of the microbial metabolic quotient across global grasslands

AbstractAimThe microbial metabolic quotient (MMQ; mg CO2‐C/mg MBC/h), defined as the amount of microbial CO2 respired (MR; mg CO2‐C/kg soil/h) per unit of microbial biomass C (MBC; mg C/kg soil), is a key parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Here, we experimentally tested hypotheses about the individual and interactive effects of multiple nutrient addition (nitrogen + phosphorus + potassium + micronutrients) and herbivore exclusion on MR, MBC and MMQ across 23 sites (five continents). Our sites encompassed a wide range of edaphoclimatic conditions; thus, we assessed which edaphoclimatic variables affected MMQ the most and how they interacted with our treatments.LocationAustralia, Asia, Europe, North/South America.Time period2015–2016.Major taxaSoil microbes.MethodsSoils were collected from plots with established experimental treatments. MR was assessed in a 5‐week laboratory incubation without glucose addition, MBC via substrate‐induced respiration. MMQ was calculated as MR/MBC and corrected for soil temperatures (MMQsoil). Using linear mixed effects models (LMMs) and structural equation models (SEMs), we analysed how edaphoclimatic characteristics and treatments interactively affected MMQsoil.ResultsMMQsoil was higher in locations with higher mean annual temperature, lower water holding capacity and lower soil organic C concentration, but did not respond to our treatments across sites as neither MR nor MBC changed. We attributed this relative homeostasis to our treatments to the modulating influence of edaphoclimatic variables. For example, herbivore exclusion, regardless of fertilization, led to greater MMQsoil only at sites with lower soil organic C (< 1.7%).Main conclusionsOur results pinpoint the main variables related to MMQsoil across grasslands and emphasize the importance of the local edaphoclimatic conditions in controlling the response of the C cycle to anthropogenic stressors. By testing hypotheses about MMQsoil across global edaphoclimatic gradients, this work also helps to align the conflicting results of prior studies.

Trade-off between microbial carbon use efficiency and specific nutrient-acquiring extracellular enzyme activities under reduced oxygen

• Reduced oxygen increased microbial metabolic quotient (qCO2).• Reduced oxygen enhanced microbial specific C-, N- and P-acquiring enzyme activity.• Reduced oxygen increased microbial C relative to N and P limitation. • Reduced oxygen increased microbial N relative to P limitation. • Specific enzyme activity was positively related to qCO2 under reduced oxygen.Mangroves are one of the most ecologically sensitive ecosystems to global climate change, which have cascading impacts on soil carbon (C), nitrogen (N) and phosphorus (P) cycling. Moreover, mangroves are experiencing increasing N and P loadings and reduced oxygen availability due to intensified climate change and human activities. However, both direct and interactive effects of these perturbations on microbially mediated soil C, N and P cycling are poorly understood. Here, we simultaneously investigated the effects of N and P loadings and reduced oxygen on microbial biomass, microbial respiration, and extracellular enzyme activities (EEAs) in mangrove soils. We calculated the microbial metabolic quotient (qCO2), which is regarded as a useful inverse metric of microbial C use efficiency (CUE). Our results show that reduced oxygen significantly increases both qCO2 and microbial specific EEAs (enzyme activity per unit of microbial biomass) for C-, N- and P-acquisition regardless of N or P loadings. Furthermore, we found that qCO2 positively correlated with microbial specific EEAs under reduced oxygen, whereas no clear relationship was detected under ambient oxygen. These results suggest that reduced oxygen increases microbial specific EEAs at the expense of increasing microbial respiration per unit biomass, indicating higher energy cost per unit enzyme production.

Microbial metabolic limitation and carbon use feedback in lead contaminated agricultural soils

Potentially toxic elements (PTEs) pollution causes a great threat to microbial metabolism, which plays a vital role in studying soil nutrient cycling and predicting carbon (C) storage in agroecosystems. However, the responses of microbial metabolism characteristic to heavy metal contamination and the mechanisms through which microbial metabolism mediate nutrient cycling and C dynamics in contaminated soil remain elusive. Here, we performed an incubation experiment over 80 days to investigate the variations in microbial metabolic limitation under various Pb levels (0, 100, 500, 800, 1500, 2000, and 3000 mg Pb kg−1 dry soil) in cropland soil using extracellular enzymatic stoichiometry, and to reveal the impact of Pb stress on soil C storage by associating with microbial metabolic quotients (qCO2) and C use efficiency (CUE). The results showed microbial relative C limitation and phosphorus (P) limitation were observed in Pb-contaminated soils. Pb addition enhanced the microbial relative C limitation by approximately 7.3%, while decreasing the P limitation by approximately 12.3%. Furthermore, Pb addition led to higher qCO2 (from 8.75 to 108 μg C kg−1 MBC−1 d−1) duo to the increase of microbial relative C limitation, suggesting that the more CO2 was released of per unit of microbial biomass C. The increase of microbial relative C limitation reduced CUE (from 0.35 to 0.10) because of the change in microbial metabolism from growth to respiration maintenance under Pb stress. Consequently, the CUE and qCO2 together determined the loss of soil C. Our study reveals that microbial relative C limitation is the dominant driver of soil C loss and provides important knowledge of microbial metabolic limitation regulating soil C turnover in PTEs-contaminated agricultural soils.

N2 fixation per unit microbial biomass increases with aridity

The aim of this study was to test the hypothesis that N2 fixation by free-living microorganisms is a quantitatively important process in arid and semiarid ecosystems and that N2 fixation per unit microbial biomass increases with increasing aridity. For this purpose, we studied soils along a precipitation gradient in Chile (ranging from 10 to 1084 mm mean annual precipitation), comprising the arid, semiarid, Mediterranean, and humid climate zone.We sampled three soil depth increments (0–10, 10–20, and 20–30 cm) at four sites along the precipitation gradient, and measured the rates of N2 fixation by free-living microorganisms (using 15N–N2) and microbial respiration at two soil water contents (30% and 65% soil water holding capacity (WHC)). In addition, we determined carbon (C) and nitrogen (N) contents in roots, microbial biomass (MB) and soil as well as the soil Δ14C and δ15N signature.We found that the soil total organic C (TOC) content and the total soil organic C-to-total N (TOC:TN) ratio increased with declining aridity from the arid to the humid site by a factor of 44 and 1.9, respectively. The rate of N2 fixation per unit microbial biomass (qN2) decreased with declining aridity from 51.0 ng N mg−1 MBC d−1 at the arid site to 2.3 ng N mg−1 MBC d−1 at the humid site, at 30% WHC. The ratio of respiration-to-N2 fixation increased very strongly with declining aridity along the precipitation gradient by a factor of about 1760 from the arid to the humid site. N2 fixation per m2 was higher at the humid site than at the arid site by a factor of two, while microbial respiration was 186 times higher at the humid than at the arid site. At 65% soil WHC, N2 fixation per m2 was highest at the Mediterranean site, which is in accordance with the low natural abundance soil δ15N at this site.In conclusion, the study shows that N2 fixation by free-living microorganisms is a quantitatively important process in (semi-)arid ecosystems and that qN2 is high in (semi-)arid ecosystems. The high N2 fixation rate relative to the soil TOC and the low C:N ratio of organic matter inputs to the soils are very likely the most important reasons why the soil TOC:TN ratio is typically low in arid and semiarid ecosystems.

The ratio of ligninase to cellulase increased with the reduction of plant detritus input in a coniferous forest in subtropical China

Soil extracellular enzymes play a central role in mediating the decomposition of soil organic matter (SOM), and the activities of cellulase and ligninase therein quantify the preference of microbial carbon (C) utilization. However, the responses of cellulase and ligninase activities to plant detritus input change remain uncertain. Here, we investigated the activities of cellulase and ligninase after two years of detritus input manipulations (the detritus input and removal treatment (DIRT) include a control without litter manipulation, CK; double litter, DL; no litter, NL; no roots, NR; no litter and no roots, NRNL) in a coniferous forest in subtropical China. The litter removal treatments had negligible effect on cellulase activity, while the DL treatment significantly increased it by 55.7% compared to the CK treatment. The NL and NRNL treatments increased the activity of ligninase by 60.1% and 46.9%, respectively. However, the DL treatment did not significantly affect the activity of ligninase. Consequently, the ratio of ligninase to cellulase significantly increased under the litter removal treatments. Notably, the specific enzyme activity (the amount of enzyme produced per unit microbial biomass) increased under the litter removal treatment, but the DL treatment did not significantly affect it. The increased ratio of ligninase to cellulase under the litter removal treatments was primarily driven by the increased recalcitrance of substrates and higher proportion of fungal and gram-positive bacterial community. Moreover, the specific enzyme activity and ratio of ligninase to cellulase were positively correlated with microbial metabolic quotient (qCO2). Overall, our results provided an empirical evidence that microorganisms could shift substrate-using strategy by upregulating the production of ligninase with the reduction of plant detritus input. More importantly, the enhanced activity or proportion of ligninase resulted in higher qCO2, and thereby can accelerate soil C loss once plant detritus input is decreased caused by some global change drivers.

Soil specific enzyme stoichiometry reflects nitrogen limitation of microorganisms under different types of vegetation restoration in the karst areas

Knowledge about resource limitations faced by soil microorganisms is crucial for understanding ecosystem functions and processes. In recent decades, vegetation restoration has been carried out in the degraded karst areas, leading to the alteration in the status of microbial resource limitation (MRL). However, mechanisms underlying MRL in different karst ecosystems remain poorly understood. Here we investigated MRL based on the theory of soil extracellular enzyme stoichiometry. Soil carbon (C), nitrogen (N), and phosphorus (P) acquiring enzyme activity (glucosidase, cellobiohydrolase, leucine aminopeptidase, urease and alkaline phosphatase) per unit microbial biomass carbon (MBCE) and per unit soil organic carbon (SOCE) of four main vegetation types (natural community, NC; ecological forest, EF; abandoned cropland, AC and economic plantation, EP) at 0–20 cm depth were measured in the karst areas in southwestern China. Significantly higher MBCE levels were found in EP than the other three vegetation types (p < 0.05), with EF having significantly higher SOCE than NC (p < 0.05), suggesting the highest metabolic activity and soil organic carbon (SOC) utilisation efficiency in EP and EF, respectively. C- and N-specific enzyme activity (MBCE and SOCE) were strongly negatively correlated with microbial biomass and soil C, N contents (p < 0.05), suggesting microbial resource requirement promotes the production of specific enzyme in resource-deficient ecosystems. Homeostasis analysis of microbial biomass C∶N and resources C∶N for all communities showed no homeostasis, indicating the microorganisms may be autotrophic to meet their N demands. In addition, the specific enzyme C∶N ratios were less than 1, the N∶P ratios were greater than 1, and vector angles were all less than 45° in all four types of vegetation restoration, indicating clear N limitation. A homeostasis analysis, as well as extracellular enzymatic stoichiometry and vector analysis, all suggested that soil microorganisms in the four vegetation types were nitrogen-limited, with NC most severely affected. Overall, we suggest that nitrogenous fertilisers should be added to restore the balance of elements while recovering the karst ecosystems.

Organic amendments, deficit irrigation, and microbial communities impact extracellular polysaccharide content in agricultural soils

Soil extracellular polysaccharides (EPSac) can improve irrigation water productivity in agriculture by increasing soil aggregate stability and retaining water on and around plant roots. However, limited studies assay soil EPSac directly from field trials and to date no studies have examined the long-term confounding impacts of organic amendments and deficit irrigation on soil EPSac and the underlying microbial communities. We quantified soil aggregate stability, EPSac content, chemical properties, and microbial community compositions and abundances in separate field studies in turfgrass and tomato. Soil samples were collected 4–5 years after soil amendment with compost, biochar, their combination, or biosolids from plots that had received full and deficit irrigation regimens for 4 years. Amending the soil with compost, with or without biochar co-application, enhanced soil total carbon, aggregate stability, EPSac, and total soil microbial biomass and shifted microbial community structures, trends which were more pronounced under turfgrass than for tomato. In both sites deficit irrigation treatments had higher quantities of soil EPSac per unit of microbial biomass. Indicators of microbial physiological stress and ubiquitous classes of soil bacteria were associated with soil EPSac production. These results provide mechanistic understanding of organic amendment benefits in drought-impacted and deficit-irrigated agriculture.

Continental-scale patterns of extracellular enzyme activity in the subsoil: an overlooked reservoir of microbial activity

Chemical stabilization of microbial-derived products such as extracellular enzymes (EE) onto mineral surfaces has gained attention as a possibly important mechanism leading to the persistence of soil organic carbon (SOC). While the controls on EE activities and their stabilization in the surface soil are reasonably well-understood, how these activities change with soil depth and possibly diverge from those at the soil surface due to distinct physical, chemical, and biotic conditions remains unclear. We assessed EE activity to a depth of 1 m (10 cm increments) in 19 soil profiles across the Critical Zone Observatory Network, which represents a wide range of climates, soil orders, and vegetation types. For all EEs, activities per mass of soil correlated positively with microbial biomass (MB) and SOC, and all three of these variables decreased logarithmically with depth (p < 0.05). Across all sites, over half of the potential EE activities per mass soil consistently occurred below 20 cm for all measured EEs. Activities per unit MB or SOC were substantially higher at depth (soils below 20 cm accounted for 80% of whole-profile EE activity), suggesting an accumulation of stabilized (i.e. mineral sorbed) EEs in subsoil horizons. The pronounced enzyme stabilization in subsurface horizons was corroborated by mixed-effects models that showed a significant, positive relationship between clay concentration and MB-normalized EE activities in the subsoil. Furthermore, the negative relationships between soil C, N, and P and C-, N-, and P-acquiring EEs found in the surface soil decoupled below 20 cm, which could have also been caused by EE stabilization. This finding suggests that EEs may not reflect soil nutrient availabilities deeper in the soil profile. Taken together, our results suggest that deeper soil horizons hold a significant reservoir of EEs, and that the controls of subsoil EEs differ from their surface soil counterparts.

Pools and fluxes of osmolytes in moist soil and dry soil that has been re-wet

When dry soils are re-wet there is a pulse of soil respiration that could be partially fuelled by the osmolytes that had accumulated within microbial biomass during the preceding drying phase. We hypothesize that if the pulse of respiration in re-wet soil is because microbes respond to the hypo-osmotic shock of re-wetting by expelling osmolytes and then re-capture osmolytes for use as respiratory substrates: a) rewetting will lead to a massive flux of osmolytes from microbial biomass into the extracellular fraction of soil, and b) following rewetting there will be a decrease in the total pool of osmolytes (i.e. in microbial biomass + the extracellular fraction of soil) that is proportional to the extra C that is respired by rewet soil compared to permanently moist soil.We used mass spectrometry and isotope pool dilution to estimate intracellular and extracellular pools, and extracellular fluxes, of seven putative osmolytes (trehalose, betaine, carnitine, hercynine, acetyl-carnitine, choline, proline) and the protein amino acid alanine in a soil that was permanently moist and a soil that had been slowly dehydrated and was then re-wet to the same water content as the permanently moist soil.In moist soil, the influx and efflux of trehalose through the extracellular pool was around 50% faster than of alanine and proline, and all three compounds had mean residence times in the extracellular solution of less than 10 min. Quaternary ammonium compounds (betaine, carnitine, hercynine, acetyl-carnitine, choline) had gross fluxes 5 to 33 times slower than for trehalose, but despite these slower fluxes mean residence times were 45 min or less.Microbial biomass of dry soil had a three times larger pool of osmolyte-C than moist soil, while the extracellular fraction of dry soil had a six times larger pool of osmolyte-C than the moist soil. Microbial biomass of dry soil was 2.5–4 times smaller than for permanently moist soil, and thus per unit microbial biomass the amount of osmolyte-C in dry soil was 7.5 to 12 times greater than the moist soil. The contribution of intracellular osmolytes to solute potential was likely too small to maintain turgor in the dry soil, and thus it is possible the large extracellular and intracellular accumulation of osmolytes was functioning to stabilise membranes and foster desiccation tolerance. The fluxes of osmolytes into and out of solution resumed rapidly after re-wetting. There was no evidence that rewetting led to an increased flux of osmolytes from microbial to extracellular pools, as would be expected if re-wetting caused microbes to expel osmolytes or be lysed. Extracellular pools of most osmolytes changed little following re-wetting, whereas the extracellular pool of trehalose decreased significantly. Isotope pool dilution revealed that the constant extracellular concentrations of most osmolytes following rewetting was because rates of removal from solution balanced the rates of entry, while the decreasing extracellular pool of trehalose reflected that it was removed from solution almost three times faster than it entered solution. The decreasing extracellular and intracellular pools of osmolytes, especially trehalose, following rewetting could theoretically account for the faster respiration of re-wet soil than permanently moist soil for several hours. However, respiration of re-wet soil was faster than permanently moist soil for five days following re-wetting and over this longer time scale the respiration pulse must be fuelled by sources of C in addition to osmolytes.

In situ effects of biochar field-aged for six years on net N mineralization in paddy soil

An understanding of nitrogen (N) mineralization is essential for tracing the supply of inorganic N for plant uptake. However, how microorganisms regulate N mineralization for carbon (C) sequestration under field-aged biochar amendment remains unclear. To address this, we investigated the soil net N mineralization rate (net Nmin), contents and hydrolytic enzyme activities of C and N, microbial biomass N, and native 15N values in bulk soil and aggregate size classes for six years after biochar application (20 and 40 t ha-−1) in a typical rice-wheat rotation. The results showed that aged biochar decreased net Nmin (normalized by total N content) by 10.5 %–69.9 %, and C and N hydrolytic enzyme activities per unit of microbial biomass C by 4.8–71.1 % and 24.0–77.8 %, respectively, compared with N fertilization in all soil aggregates except for ClayF size class (<2 μm). Microbial biomass N (MBN) increased by 21.5–130.9 % in soil aggregates, while the δ15N values decreased following biochar addition compared with those under N fertilization. The labile C:N ratios were higher in the bulk soil and MacroA size class (250−2000 μm) following biochar addition than under N fertilization, which would increase microbial N demand as evidenced by the lower enzymatic C:N ratios and higher MBN. Microorganisms obviously restrained net Nmin but did not increase N hydrolytic enzyme activity to meet their stoichiometric N demands. Structural equation modeling revealed that enzymatic C:N stoichiometry is a dominant indicator of net Nmin in bulk soil and the >53 μm size class, while the MBN is more important to net Nmin in the <53 μm size class. We conclude that the addition of aged biochar could meet microbial stoichiometric requirements and regulate extracellular enzyme production, resulting in the decline of net Nmin in soil aggregates, especially in MacroA size class.

Native broadleaf tree species stimulate topsoil nutrient transformation by changing microbial community composition and physiological function, but not biomass in subtropical plantations with low P status

Microbial communities and their associated enzyme activities affect carbon (C), nitrogen (N), and phosphorus (P) metabolism in soils. We used phospholipid fatty acids (PLFAs) analysis and extracellular enzyme activity assays to evaluate the effects on topsoil microbial community and nutrient transformation of converting Chinese fir (CF) plantations to native broadleaf (Castanopsis hystrix [CH] and Mytilaria laosensis [ML]) plantations in this study. We found that CH and ML plantations had significantly higher soil organic C (SOC), total N (TN), NH4+-N, soil C/N ratios (C/Nsoil), soil C/P ratios (C/Psoil) and soil N/P ratios (N/Psoil) but significantly lower total P (TP) compared to CF plantations. A distinct shift in soil microbial community composition, but not microbial biomass was evident 23 years after stand conversion. Redundancy analysis (RDA) showed that soil microbial community composition was substantially influenced by litterfall mass (LF), TP, N/Psoil, SOC, fine root biomass (FR), C/Nsoil, TN, litter C/N ratios (C/Nlitter), and C/Psoil; of these, LF appeared to be the primary driver of differences in soil microbial community composition (p < 0.05). In addition, the total activity of hydrolytic enzymes involved in C metabolism (β-glucosidase and cellobiohydrolase), N metabolism (N-acetyl-glucosaminidase), and P metabolism (acid phosphatase) increased significantly in the two native broadleaf plantations, whereas the total activity of oxidase (phenol oxidase) decreased significantly. Furthermore, the change trends of specific enzyme activity (i.e., enzymes per unit microbe) were similar to total enzyme activity. The dynamics of soil microbial enzymatic activity after stand conversion provide a functional link between changes in microbial community composition and soil nutrient transformation. Our findings suggest a mechanism in native, non-N2-fixing broadleaf tree species that stimulates soil nutrient transformation in subtropical planted forests with low P via increases in the quantity and quality of litter and the physiological function per unit of microbial biomass.

Sustained increase in soil respiration after nine years of warming in an alpine meadow on the Tibetan Plateau

Soil microbes are key determinants of soil carbon (C) dynamics. The response of the soil microbial community to climate warming modulates the feedback between ecosystem C cycling and future climate change. We conducted a long-term manipulative warming (1.6 °C increase of the soil temperature at 5 cm) experiment to examine the soil respiration, microbial biomass, and community composition at an alpine meadow site on the Qinghai-Tibetan Plateau. After nine years of warming, soil respiration (3.5 μmol m2 s−1 in control in the growing season) increased in the warmed plots. In the early growing season, the increase in heterotrophic respiration (Rh) accounted for more than 90% of the increase in soil respiration. The warming effect gradually decreased during the mid and late growing season (46%). Microbial biomass C and nitrogen declined significantly in the 0–10 cm to the 30–50 cm layer. Warming did not significantly affect microbial biomass C and N in any soil depth layer. Metabolic activity of microbes in terms of Rh per unit microbial biomass C significantly increased by 66% in warmed plots. The bacterial and fungal community composition did not significantly change in the warmed plots. The relative abundance of Actinobacteria decreased at 20–30 cm and 30–50 cm soil depths, but that of Cercozoa increased in all four soil layers. The relative Actinobacteria abundance was negatively correlated with Rh and metabolic activity in the 10–20 and 20–30 cm layers. Our results indicate a decrease in Actinobacteria abundance, increases in metabolic activity, and no substrate limitation sustained the positive warming effect on soil respiration throughout the last 9 years. This implies that climate warming could trigger a substantial loss of soil C to the atmosphere in the alpine meadow on the Qinghai-Tibet Plateau.

Disentangling drivers of soil microbial potential enzyme activity across rain regimes: An approach based on the functional trait framework

The functional trait framework provides a powerful corpus of integrated concepts and theories to assess how environmental factors influence ecosystem functioning through community assembly. While common in plant ecology, this approach is under-used in microbial ecology. After an introduction of this framework in the context of microbial ecology and enzymology, we propose an approach 1) to elucidate new links between soil microbial community composition and microbial traits; and 2) to disentangle mechanisms underlying “total” potential enzyme activity in soil (sum of 7 hydrolase potential activities). We address these objectives using a terrestrial grassland ecosystem model experiment with intact soil monoliths from three European countries (Switzerland, France and Portugal) and two management types (Conventional-intensive and Ecological-intensive), subjected to 4 rain regimes (Dry, Wet, Intermittent and Normal) under controlled conditions in a common climate chamber. We found tight associations between proxies of microbial ecoenzymatic community-weighted mean traits (enzymatic stoichiometry and biomass-specific activity) and community composition, bringing new information on resource acquisition strategy associated with fungi, Gram positive and Gram negative bacteria. We demonstrate that microbial biomass explained most of the total enzyme activity before altered rain regimes, whereas adjustments in biomass-specific activity (enzyme activity per unit of microbial biomass) explained most variation under altered rain regime scenarios. Furthermore, structural equation models revealed that the variation of community composition was the main driver of the variation in biomass-specific enzyme activity prior to rain perturbation, whereas physiological acclimation or evolutionary adaptation became an important driver only under altered rain regimes. This study presents a promising trait-based approach to investigate soil microbial community response to environmental changes and potential consequences for ecosystem functioning. We argue that the functional trait framework should be further implemented in microbial ecology to guide experimental and analytical design.

Microbial biomass and respiration in rangeland soils of southern Siberia and western Mongolia

Southern Siberia and western Mongolia are characterized by very cold, dry winters, and short, warm summers with modest or little precipitation. In the driest region that we investigated (119 mm annual precipitation), the upper part of two catenas had a larger fraction of microbial biomass in soil organic matter (6–12%) than the four other catenas (<4%) in less dry regions (260 and 442 mm). All six catenas had values ≤4% at the lowest position near the shore of saline lakes. Respiration per unit microbial biomass was similar within and among the catenas, except for the lowest position in three catenas, where enhanced values indicated microbial stress, probably due to high salinity.

Weed Interference in Soybean Crop Affects Soil Microbial Activity and Biomass

ABSTRACT: Weeds and microorganisms interacting with their rhizosphere may influence nutrient absorption, which is an important factor for plant competition. The purpose of this study was to evaluate the microbiological activity, inorganic phosphorus solubilization (Pi) and acid phosphatase in the cultivated soil, in a combination of soybean (Glycine max) plants and weeds. Soybeans were cultivated in monoculture and in competition with Bidens pilosa, Brachiaria decumbens (Syn. Urochloa decumbens) and Eleusine indica, under two conditions: a) plants competing without contact between the roots b) plants competing with contact between the roots. A nylon screen with a 50 µm mesh was added to prevent contact between the roots of the species in competition so that the substratum could be separated in the vase. The experiment was conducted in randomized blocks, with four replications. The soybeans in competition with weeds led to lower oxidation of organic matter per unit of microbial biomass, resulting in a lower metabolic quotient, compared with the soybean monoculture. The contact between soybean roots and B. pilosa, B. decumbens and E. indica maintained a strong influence, raising the solubilization of Pi, respectively valued at 51, 39 and 31% in relation to the cultivation of each species with a nylon screen. Microbiological activity, inorganic phosphorus solubilization and acid phosphatase were altered by plant species, combinations of weeds and soybean plants in competition; by root contact in some cases. Thus, the microbiological activity of the soil can influence competition strategies and plant development.