Size Of Microbial Biomass Research Articles

Mixing of pine and arbuscular mycorrhizal tree species changed soil organic carbon storage by affecting soil microbial characteristics

Pure and mixed pine forests are found all over the world. The mycorrhizal type affects soil microbial activity and carbon sequestration capacity in pure forests. However, the effects of mycorrhizal type on microbial characteristics and carbon sequestration capacity in pine mixed forests remain untested. Further, making it difficult to predict carbon storage of the conversion from pure pine forests to mixed forests at larger scales. Herein, a meta-analysis showed that the contents of soil microbial biomass, mineral-associated organic carbon, and soil organic carbon in pine mixed forests with introduced arbuscular mycorrhizal tree species (PMAM) increased by 26.41 %, 58.55 %, and 27.41 %, respectively, compared to pure pine forests, whereas those of pine mixed forests without arbuscular mycorrhizal tree species (PMEcM) remained unchanged. Furthermore, the effect size of microbial biomass, mineral-associated organic carbon and organic carbon contents in subsoil of PMAM are 56.48 %, 78.49 % and 43.05 %, respectively, which are higher than those in topsoil. The improvement of carbon sinks throughout the PMAM soil profile is positively correlated with increases in microbial biomass and mineral-associated organic carbon in subsoil, according to regression analysis and structural equation modelling. In summary, these results highlight that the positive effects of introducing arbuscular mycorrhizal tree species rather than ectomycorrhizal tree species into pure pine forests on soil microbial biomass and carbon sequestration. The positive link between microbial biomass, mineral-associated organic carbon, and soil organic carbon suggests an underlying mechanism for how soil microorganisms store carbon in pine mixed forests. Nevertheless, our findings also imply that the soil carbon pool of PMAM may be vulnerable under climate change. Based on the above findings, we propose that incorporating mycorrhizal type of tree species and soil thickness into mixed forests management and biodiversity conservation.

The nutritional composition and cell size of microbial biomass for food applications are defined by the growth conditions

BackgroundIt is increasingly recognized that conventional food production systems are not able to meet the globally increasing protein needs, resulting in overexploitation and depletion of resources, and environmental degradation. In this context, microbial biomass has emerged as a promising sustainable protein alternative. Nevertheless, often no consideration is given on the fact that the cultivation conditions affect the composition of microbial cells, and hence their quality and nutritional value. Apart from the properties and nutritional quality of the produced microbial food (ingredient), this can also impact its sustainability. To qualitatively assess these aspects, here, we investigated the link between substrate availability, growth rate, cell composition and size of Cupriavidus necator and Komagataella phaffii.ResultsBiomass with decreased nucleic acid and increased protein content was produced at low growth rates. Conversely, high rates resulted in larger cells, which could enable more efficient biomass harvesting. The proteome allocation varied across the different growth rates, with more ribosomal proteins at higher rates, which could potentially affect the techno-functional properties of the biomass. Considering the distinct amino acid profiles established for the different cellular components, variations in their abundance impacts the product quality leading to higher cysteine and phenylalanine content at low growth rates. Therefore, we hint that costly external amino acid supplementations that are often required to meet the nutritional needs could be avoided by carefully applying conditions that enable targeted growth rates.ConclusionIn summary, we demonstrate tradeoffs between nutritional quality and production rate, and we discuss the microbial biomass properties that vary according to the growth conditions.

Climatic Controls on Soil Carbon Accumulation and Loss in a Dryland Ecosystems

AbstractArid and semiarid ecosystems drive year‐to‐year variability in the strength of the terrestrial carbon (C) sink, yet there is uncertainty about how soil C gains and losses contribute to this variation. To address this knowledge gap, we embedded C‐depleted soil mesocosms, containing litter or biocrust C inputs, within an in situ dryland ecosystem warming experiment. Over the course of one year, changes in microbial biomass and total soil organic C pools were monitored alongside hourly measurements of soil CO2 flux. We also developed a biogeochemical model to explore the mechanisms that gave rise to observed soil C dynamics. Field data and model simulations demonstrated that water exerted much stronger control on soil biogeochemistry than temperature, with precipitation events triggering large CO2 pulses and transport of litter‐ and biocrust‐derived C into the soil profile. We expected leaching of organic matter would result in steady accumulation of C within the mineral soil over time. Instead, the size of the total organic C pool fluctuated throughout the year, largely in response to microbial growth: increases in the size of microbial biomass were negatively correlated with the quantity of C residing in the top 2 cm, where most biogeochemical changes were observed. Our data and models suggest that microbial responses to precipitation events trigger rapid metabolism of dissolved organic C inputs, which strongly limit accumulation of autotroph‐derived C belowground. Accordingly, changes in the magnitude and/or frequency of precipitation events in this dryland ecosystem could have profound impacts on the strength of the soil C sink.

INFLUENCE OF SOIL TREATMENT ON THE NUMBER OF EARTHWORMS AND THE SIZE OF MICROBIAL BIOMASS UNDER THE CONDITIONS OF AGRICULTURAL INTENSIFICATION

Objective. To establish the influence of soil treatment methods on the number of earthworms and microbial biomass in dark-chestnut heavy loam soil with observance of the following crop rotation: soybean – winter wheat – corn; verify the hypothesis of the negative influence of traditional soil treatment technologies on the number of earthworms. Methods. Analytical, field, laboratory, system analysis, statistical. Results. During the studies conducted in 2018–2021 at Askanian State Agricultural Experimental Station of the Institute of Irrigated Agriculture of the NAAS, the positive influence of zero soil treatment technologies on the studied parameters versus with traditional technologies was established. A 2.5-fold increase in the number of earthworms and a 1.1-fold increase in microbial biomass was reported with zero soil treatment technologies. The interaction between soil treatment factors and cover crops in the structure of crop rotation was 23.7 %. Soil treatment technologies had a negligible influence on the size of microbial biomass — at the level of 1.4 %. The significance of the action of cover crops in crop rotation was 10.0 %. The experiment also showed a higher number of earthworms in the soil with zero soil treatment technologies with cover crops and traditional technologies for control, due to better live environment and favourable conditions for reproduction and distribution of the population depending on soil treatment technologies. Changes in the density of the soil layer 0–30 cm were within the measurement error (for zero soil treatment technologies the studied parameter was 1.23 g/cm3, for traditional — 1.24 g/cm3). Compaction of 0–20 cm soil layer with zero and traditional soil treatment technologies was reported. The content of soil organic matter by the years of studies under zero soil treatment technologies increased to 3.0 %, which is 1.2 times higher than with traditional technologies. Conclusion. Under the conditions of stationary field experiment the positive influence of zero soil treatment technologies on early changes of biological properties of soil, which were characterized by increase in number of earthworms and size of microbial biomass, has been established. The positive influence of cover crops on soil quality was determined, which was characterized by signs of biota activation in the 0–30 cm layer and improvement of biological and physical properties of the soil.

Altitude and Vegetation Affect Soil Organic Carbon, Basal Respiration and Microbial Biomass in Apennine Forest Soils

Both altitude and vegetation are known to affect the amount and quality of soil organic matter (SOM) and the size and activity of soil microbial biomass. However, when altitude and vegetation changes are combined, it is still unclear which one has a greater effect on soil chemical and biochemical properties. With the aim of clarifying this, we tested the effect of altitude (and hence temperature) and vegetation (broadleaf vs pine forests) on soil organic carbon (SOC) and soil microbial biomass and its activity. Soil sampling was carried out in two adjacent toposequences ranging from 500 to 1000 m a.s.l. on a calcareous massif in central Italy: one covered only by Pinus nigra J.F. Arnold forests, while the other covered by Quercus pubescens Willd., Ostrya carpinifolia Scop. and Fagus sylvatica L. forests, at 500, 700 and 1000 m a.s.l., respectively. The content of SOC and water-extractable organic carbon (WEOC) increased with altitude for the pine forests, while for the broadleaf forests no trend along the slope occurred, and the highest SOC and WEOC contents were observed in the soil at 700 m under the Ostrya carpinifolia forest. With regard to the soil microbial community, although the size of the soil microbial biomass (Cmic) generally followed the SOC contents along the slope, both broadleaf and pine forest soils showed similar diminishing trends with altitude of soil respiration (ΣCO2-C), and ΣCO2-C:WEOC and ΣCO2-C:Cmic ratios. The results pointed out that, although under the pine forests’ altitude was effective in affecting WEOC and SOC contents, in the soils along the broadleaf forest toposequence this effect was absent, indicating a greater impact of vegetation than temperature on SOC amount and pool distribution. Conversely, the similar trend with altitude of the microbial activity indexes would indicate temperature to be crucial for the activity of the soil microbial community.

Cellular and extracellular C contributions to respiration after wetting dry soil

Wetting of dry soil triggers a pulse of microbial respiration that has been attributed to two broad mechanisms: (1) recycling of microbial cellular carbon (C), and (2) consumption of extracellular organic C made available to microbes by wetting. We evaluated these two mechanisms by measuring cumulative CO2 release, changes in the size and chemical composition of microbial biomass, and water-extractable organic carbon (WEOC) concentrations following artificial wetting of soil sampled from two depths at each of seven sites across California spanning a range of geologic parent materials. In samples collected from surface soil (0–10 cm depth), we found that cumulative CO2 release after wetting in the laboratory was most strongly correlated with microbial biomass. In these samples, the relative abundance of trehalose—a putative microbial osmolyte—decreased from 25% (SD = 12) to 16% (SD = 7) of the chloroform-labile fraction of the microbial biomass after wetting. This suggested a role for osmolyte consumption in generating the respiration pulse. In subsoil (40–50 cm depth, or sampled at contact with rock), however, the cumulative CO2 release after wetting was unrelated to microbial biomass and more strongly related to WEOC. The concentrations of selected microbial biomass constituents (e.g. trehalose and amino acids) in WEOC were negligible (< 1%), suggesting that cell lysis was not important in generating WEOC in this study. The amount of WEOC relative to total organic C was greatest in subsoil, and negatively related to ammonium oxalate-extractable Fe (Pearson’s R = 0.42, p < 0.01), suggesting a role for soil mineralogical properties in controlling WEOC release. Together, these findings suggest that microbial cellular C and extracellular C jointly contribute to the respiration pulse, and that their relative contribution depends on depth.

Soil microbes become a major pool of biological phosphorus during the early stage of soil development with little evidence of competition for phosphorus with plants

We aimed to quantify the pool size of soil microbial biomass P (Pmic) during the early stage of soil development up to 125 years after glacial retreat in the Gongga Mountains, China and relate the pool size of Pmic to the plant P (Pplant) pools in the ecosystem. We determined the pool sizes of P in soil microbes, plants and soils and the P fluxes with plant uptake and litterfall in successional ecosystems at five study sites along the 125-year Hailuogou glacial retreat chronosequence. Moreover, we estimated the flux of P cycled through microbial biomass (Pmic cycling) based on literature data. We also approached the likelihood of P competition between plants and soil microbes based on the P status of the plants, soils and soil microbes. The size of the Pmic pools (0.2–8.3 g m−2) in the organic layer and top 10 cm of the mineral soils was comparable to that of the Pplant pools (0.3–9.1 g m−2) at all study sites along the Hailuogou chronosequence. Based on the literature, the Pmic cycling at our study site (0.3–13.5 g m−2 year−1 if estimated based on temporal fluctuations of Pmic, 5.2–268 g m−2 year−1 if estimated based on the isotope dilution method) was at least one order of magnitude larger than the Pplant uptake (not detected-0.36 g m−2 year−1) and the Pplant return by litterfall (not detected-0.16 g m−2 year−1). Although Pmic became a major pool of biological P, we did not find indications of P competition between plants and soil microbes as indicated by the positive relationships between the concentrations of Pmic and plant-available P in soils and the P-rich status of plants and soil microbes. Soil microbial biomass already becomes a major P pool in the early stage of soil development. Our estimations based on the literature suggest that Pmic cycling is probably the largest P flux in the studied up to 125-year ecosystems. Plants likely did not suffer P competition with microbes, in part due to the preferential decomposition of the P-rich compounds from dead microbial biomass which led to net P mineralization.

Soil Microbial Biomass Size and Nitrogen Availability Regulate the Incorporation of Residue Carbon into Dissolved Organic Pool and Microbial Biomass

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM‐derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass‐derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue‐derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass‐derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils.Core IdeasRyegrass residue addition primed the production of SOM‐derived DOC and MBC.Residue quality affected ryegrass‐derived DOC in crop soils.N addition reduced ryegrass‐derived DOC in grass soils.N addition increased ryegrass‐derived MBC in crop soils.Initial MB and N availability regulated the incorporation of residue C into DOC and MBC.

Effect of the amount of organic trigger compounds, nitrogen and soil microbial biomass on the magnitude of priming of soil organic matter.

Priming effects (PEs) are defined as short-term changes in the turnover of soil organic matter (SOM) caused by the addition of easily degradable organic compounds to the soil. PEs are ubiquitous but the direction (acceleration or retardation of SOM decomposition) and magnitude are not easy to predict. It has been suggested that the ratio between the amount of added PE-triggering substrate to the size of initial soil microbial biomass is an important factor influencing PEs. However, this is mainly based on comparison of different studies and not on direct experimentation. The aim of the current study is to examine the impact of glucose-to-microbial biomass ratios on PEs for three different ecosystems. We did this by adding three different amounts of 13C-glucose with or without addition of mineral N (NH4NO3) to soils collected from arable lands, grasslands and forests. The addition of 13C-glucose was equivalent to 15%, 50% and 200% of microbial biomass C. After one month of incubation, glucose had induced positive PEs for almost all the treatments, with differences in magnitude related to the soil origin and the amount of glucose added. For arable and forest soils, the primed C increased with increasing amount of glucose added, whereas for grassland soils this relationship was negative. We found positive correlations between glucose-derived C and primed C and the strength of these correlations was different among the three ecosystems considered. Generally, additions of mineral N next to glucose (C:N = 15:1) had little effect on the flux of substrate-derived C and primed C. Overall, our study does not support the hypothesis that the trigger-substrate to microbial biomass ratio can be an important predictor of PEs. Rather our results indicate that the amount of energy obtained from decomposing trigger substrates is an important factor for the magnitude of PEs.

Organic amendment effectively recovers soil functionality in degraded vineyards

Abstract The aim of this study was i) to elucidate the impact of land levelling in vineyards on soil fertility and crop development; and ii) to evaluate the effectiveness of amendments in the recovery of soil functionality and vine status in disturbed soils. A field trial was set up on 2 vineyards located in North East Italy and subjected to land levelling. Four treatments were compared and sampled for 3 years: non-amended undisturbed soil (US); non-amended disturbed soil (DS); disturbed soil amended with vermicompost (VC, 1.5 t C ha−1 y−1); disturbed soil amended with vine shoot compost (VS, 4 t C ha-1 y−1). Land levelling negatively impacted soil fertility. In particular, soil organic matter (SOM) and extractable N were decreased by about 20 and 55%, respectively, which were reflected in significant reduction of crop yield and grape quality. Soil displacement had also a negative impact on root spatial distribution and density. Soil amendment was effective in recovering (VC) or enhancing (VS) soil fertility in disturbed soils, in particular by improving water content, SOM, available N and microbial biomass size and activity. Amendments, particularly VS, caused an increase in grape yield, titratable acidity and total N and a decrease of total soluble solids, although differences were not always statistically significant. Our results showed that soil amendment is an effective management for a prompt recovery of degraded soil in vineyards, but a medium- to long-term application of amendments is needed to significantly enhance crop status and grape quality.

Soil microbial biomass size and soil carbon influence the priming effect from carbon inputs depending on nitrogen availability

Microbial biomass plays a critical role in soil organic carbon (SOC) mineralization. However, the effects of microbial biomass size on SOC mineralization are poorly understood. We investigated how the priming effect (PE) of plant residue inputs on native SOC mineralization responds to changes in microbial biomass size and nitrogen (N) availability in the same soil with a 23 y history of crops and grass cover with contrasting SOC contents. The size of the soil microbial biomass was changed by pre-incubating soils with glucose. The pre-incubated soils were then treated with 13C-labeled ryegrass residue combined with or without N to determine the PE. In all soils, the addition of ryegrass residue significantly increased cumulative carbon dioxide (CO2) production, whereas the addition of N decreased it compared to the control (no ryegrass or N addition). After a 42-day incubation, only 9–16% of the ryegrass carbon (C) was mineralized to CO2, which contributed approximately 55 and 34% to the total CO2 production in crop and grass soils, respectively. The addition of N decreased CO2 production during the ryegrass decomposition by 9–45%, while the change in soil microbial biomass size had no impact. In addition, a positive PE was generally found in the soils amended with ryegrass alone, while the application of ryegrass residue combined with N decreased the PE. Moreover, the priming effect was independent of the size of the microbial biomass in crop soil. However, we observed a significant interaction of microbial biomass size and N availability on the priming effect in a grass soil but not in a crop soil. Our results indicate that soil microbial biomass size and soil C influence the magnitude and direction of the priming effect from C inputs depending on N availability.

The long-term role of organic amendments in building soil nutrient fertility: a meta-analysis and review

An exhaustive meta-analysis of 132 long-term (≥ 10 years) studies worldwide was carried out to determine the effects of the use of organic amendments (OA) and OA + inorganic fertiliser (IF) on soil nutrient fertility. The responses of (1) crop yield [over the whole duration of the period (yieldm) and at the end of the experiment (yieldf)], (2) soil organic carbon (OC), (3) size of microbial biomass, and (4) Olsen phosphorus (P) to OA and OA + IF compared with IF only (standard control) and no fertilisation (nil control) were investigated. The overall effect of OA alone on yield was significant when compared with the nil control, but not when compared with the standard control. Only when OA and IF were added to soils that met specific conditions (low initial fertility, sandy texture, near-neutral pH values, under tropical climate) they rendered a significantly greater yieldf than the corresponding standard controls. The continuous application of manure caused greater relative and absolute gains in soil OC than straw + IF but did not produce significant greater yields while causing a considerable increase in Olsen P over time. The use of OA and OA + IF increased the resilience of agronomic systems over that of IF alone, as inferred from the smaller coefficient of variation of crop yield over time. We conclude that while the use of OA along with IF provides some additional benefits on yields as compared with IF application alone (especially under the above-mentioned conditions), the selection of the OA type and application rate should be carefully considered in order to maximise the nutrient use efficiency and minimise any undesirable effects to the environment.

Is the fate of glucose-derived carbon more strongly driven by nutrient availability, soil texture, or microbial biomass size?

Increasing organic matter (OM) in soil promotes the delivery of vital ecosystem services, such as improving water retention, decreasing erosion, increasing plant productivity, and mitigating climate change through terrestrial carbon (C) sequestration. The formation of organo-mineral associations through microbial turnover of labile (i.e. easily decomposed) C is a potential pathway of soil C stabilization. However, association of added C with mineral surfaces may be impacted by soil clay content and/or by nutrient availability (due to higher microbial C use efficiency). We added 14C labeled glucose as a model labile substrate together with either ion exchange resin beads (to induce nutrient limitation), water (no additional nutrients), or four increasing concentrations of nitrogen, phosphorus, and sulfur in constant stoichiometric ratios to nine agricultural soils under the same climate and management but along a texture gradient from 3 to 40% clay. The soils with 14C-glucose and a nutrient treatment were incubated for 4 weeks during which the 14C was traced into CO2, microbial biomass, dissolved organic C (DOC), and soil organic C (SOC). Induced nutrient limitation (available C:N ratio around 300:1) reduced mineralization of glucose-derived C, particularly in soils with <15% clay. However, in soils with ≥15% clay, higher microbial biomass allowed for glucose-derived C mineralization despite nutrient limitation. Alleviating the nutrient limitation (available C:N < 50:1) allowed for greater transformation of added C into microbial biomass-C and SOC, particularly in soils with ≥21% clay, although further additions (down to C:N of 11:1) did not result in greater SOC or microbial biomass formation. Except under conditions of nutrient limitation (where C:N > 50:1), soil texture and starting microbial biomass size, not nutrient availability, were the drivers of SOC and microbial biomass formation during the incubation.

Tillage effects on soil microbial biomass, SOM mineralization and enzyme activity in a semi-arid Calcixerepts

Soil microbial properties are considered important indicators for assessing and comparing soil quality and degradation under tillage systems due to their early responses to environmental changes. The main objective of this study was to determine the medium-term (6 years) influence of tillage practices on soil microbial attributes in a semi-arid Calcixerepts from Central Iran with a similar plant residue input rate. The experiment was organized as a randomized complete block design with three replicates of each tillage regime. Tillage treatments consisted of moldboard and disk plows as conventional tillage (CT), and chisel and rotary plows as reduced tillage (RT) regimes over a period of six years (2005–2011). Soil microbial biomass carbon C (MBC) and nitrogen (MBN) contents, C (CMR) and N (NMR) mineralization rates; and urease, alkaline phosphatase (ALP) and catalase activities at the 0–20cm depth were determined only in 2008, 2009 and 2011. Less disturbed soils by both chisel and rotary plows under RT had higher MBC (25–43%), MBN (7–13%), MBC/MBN (17%) and enzyme activities (urease 2.0-fold, ALP 31–53% and catalase 18–59%) when compared with more disturbed soils by moldboard and disk plows under CT. In contrast, RT soils had lower microbial turnover rate (9–17%), metabolic quotient (qCO2, 37%), CMR (17–30%) and NMR (24–28%) than CT soils. However, the magnitude of the changes in soil MBC, MBN, CMR, NMR and catalase activity due to tillage effect were more greater during the later years of tillage implementation. The positive effect of RT practices on soil microbial biomass and enzyme activity was most likely due to improved environmental conditions, since soil organic matter (SOM) content was unaffected by tillage treatments. Soils under RT systems were characterized by a higher microbial biomass and enzyme activity, but development of less metabolically active microorganisms; most probably as a result of changes in the microbial community composition and lower microbial turnover rate. Furthermore, increased microbial enzyme production and biomass size with RT does not necessarily lead to higher microbial activity and SOM mineralization rate. It is suggested that the reduced SOM mineralization under RT is largely controlled by changes in the composition of microbial community and microbial turnover. In conclusion, microbial attributes were improved under RT after six years, and hence are regarded as early and sensitive indicators of changes in soil quality induced by tillage systems in this semi-arid environment.

Hydrolytic soil enzymes and their response to fertilization: a short review

Enzymes are proteins that catalyze chemical reactions in living systems, transforming specific substrates into the products needed in biological cycles and for many edaphic processes. Soil enzymatic activities have been proposed as soil quality indicators, due to their relation with soil biology. Although the long-term effects of organic and mineral fertilization on physical and chemical soil properties have been previously studied, little is known about their effects on microbial community structure, microbial biomass carbon, microbial activity and enzymatic activity. Some studies report that organic and mineral fertilizers can affect, be it positively or negatively, microbial biomass size as well as soil microbial activity. This work examines the effect of fertilization on the enzymatic activity of soil hydrolases.

Microbiological indicators of soil quality and degradation following conversion of native forests to continuous croplands

Deforestation resulting from forest conversion to agricultural land use is an important issue worldwide. This phenomenon is known to influence the activity and size of soil microbial community due to changes in environmental conditions with subsequent losses of soil organic matter (SOM) and soil quality degradation. The objective of this study was to investigate the relationship between soil organic carbon (SOC) losses and enzyme activities following land use conversion from native forests to continuous croplands. The amount of soil microbial biomass carbon (SMBC) and the activity of five soil enzymes (i.e., urease, invertase, alkaline phosphatase, acid phosphatase and arylsulfatase) were measured in croplands derived from forests and adjacent natural forests all on similar soil type at Gorgan site located in Northeast Iran. The content of SMBC decreased (47–83%) with deforestation at both soil sampling depths (0–20 and 20–40cm). With the exception of phosphatases, the absolute activities of soil enzymes (activity on a soil mass basis) tended to decrease significantly (15–35%) with continuous cultivation. However, the specific enzyme activities expressed either per unit of SOC or SMBC tended to increase (about 1.5–5.5 times) with conversion of forestlands to croplands. The significant positive correlation between enzyme activity per SMBC and C turnover rate may imply that a faster C cycle and loss due to deforestation is related to a greater enzymatic activity by a smaller size of microbial biomass in cropland soils. In brief, the specific activities of soil enzymes could be used to reveal SOM losses and soil degradation in natural forest ecosystems, and to identify changes in soil quality and fertility following deforestation. Changes or improvements in soil management such as cessation of cultivation or implementing agricultural practices that stop or minimize soil disturbance are most likely needed to stop further soil degradation, restore soil quality and rebuild SOC stocks to offset CO2 emissions in these ecosystems.

Soil carbon sequestration, plant nutrients and biological activities affected by organic farming system in tea (Camellia sinensis (L.) O. Kuntze) fields

There is growing interest in investigations into soil carbon (C) sequestration, plant nutrients and biological activities in organic farming since it is regarded as a farming system that could contribute to climate mitigation and sustainable agriculture. However, most comparative studies have focused on annual crops or farming systems with crop rotations, and only a few on perennial crops without rotations, e.g. tea (Camellia sinensis (L.) O. Kuntze). In this study, we selected five pairs of tea fields under organic and conventional farming systems in eastern China to study the effect of organic farming on soil C sequestration, plant nutrients and biological activities in tea fields. Soil organic C, total nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg), available nutrients, microbial biomass, N mineralization and nitrification were compared. Soil pH, organic C and total N contents were higher in organic tea fields. Soil microbial biomass C, N and P, and their ratios in organic C, total N and P, respectively, net N mineralization and nitrification rates were significantly higher in organic fields in most of the comparative pairs of fields. Concentrations of soil organic C and microbial biomass C were higher in the soils with longer periods under organic management. However, inorganic N, available P and K concentrations were generally lower in the organic fields. No significant differences were found in available calcium (Ca), Mg, sodium (Na), iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn) concentrations between the two farming systems. These findings suggest that organic farming could promote soil C sequestration and microbial biomass size and activities in tea fields, but more N-rich organic fertilizers, and natural P and K fertilizers, will be required for sustainable organic tea production in the long term.

Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO2 concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C-inputs to soil (e.g. plant-derived) can significantly affect mineralization of SOM ('priming effects') complicates prediction of environmental and land-use change effects on SOM dynamics and soil C-balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3atm% (13) C) addition (in the range 0-1mg glucose g(-1) soil day(-1) ) were applied for 8days. Soil CO2 efflux was partitioned into SOM- and glucose-derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass ('apparent priming') were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5mg glucose g(-1) soil), priming was a soil-specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C-models. However, these response functions are soil-specific and may be affected by several interacting factors at lower addition rates.

Microbial responses to multi-factor climate change: effects on soil enzymes

The activities of extracellular enzymes, the proximate agents of decomposition in soils, are known to depend strongly on temperature, but less is known about how they respond to changes in precipitation patterns, and the interaction of these two components of climate change. Both enzyme production and turnover can be affected by changes in temperature and soil moisture, thus it is difficult to predict how enzyme pool size may respond to altered climate. Soils from the Boston-Area Climate Experiment (BACE), which is located in an old field (on abandoned farmland), were used to examine how climate variables affect enzyme activities and microbial biomass carbon (MBC) in different seasons and in soils exposed to a combination of three levels of precipitation treatments (ambient, 150% of ambient during growing season, and 50% of ambient year-round) and four levels of warming treatments (unwarmed to ~4°C above ambient) over the course of a year. Warming, precipitation and season had very little effect on potential enzyme activity. Most models assume that enzyme dynamics follow microbial biomass, because enzyme production should be directly controlled by the size and activity of microbial biomass. We observed differences among seasons and treatments in mass-specific potential enzyme activity, suggesting that this assumption is invalid. In June 2009, mass-specific potential enzyme activity, using chloroform fumigation-extraction MBC, increased with temperature, peaking under medium warming and then declining under the highest warming. This finding suggests that either enzyme production increased with temperature or turnover rates decreased. Increased maintenance costs associated with warming may have resulted in increased mass-specific enzyme activities due to increased nutrient demand. Our research suggests that allocation of resources to enzyme production could be affected by climate-induced changes in microbial efficiency and maintenance costs.

Soil microbial biomass influence on strontium availability in mine soil

The main aim of our work was to assess whether strontium (Sr) affects soil microbial biomass size and activity, and the involvement of said biomass in the availability process of the metal. In addition, information concerning the distribution and mobility of the stable element within ecosystems may allow the prediction of the behaviour of its radioisotope counterpart, 90Sr. Samples were collected in the surroundings of a strontium mine and characterised for total and diethylene triamine pentaacetic acid (DTPA)-extractable Sr, total organic C (TOC), microbial biomass C (MBC), MBC/TOC ratio and metabolic quotient (qCO2). Moreover, MBC and DTPA-extractable Sr were measured during a 45-day incubation experiment of samples soils amended with maize. Overall, increased levels of total Sr had a negative effect on both TOC and MBC. DTPA-extractable Sr was significantly correlated to MBC/TOC suggesting a possible role of soil microbial biomass in the mobilisation of the element. The synthesis of new microbial biomass after soil amendment was negatively affected by the initial content of DTPA-extractable Sr. Conversely, there was a linear positive relationship between newly formed MBC and DTPA-extractable Sr during the incubation, indicating that soil microbial biomass may promote the mobilisation of Sr. These findings indicate that soil amendment with easily degradable organic substrate significantly increases Sr mobility and availability.