Short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China

The Sanjiang Plain, one of the largest freshwater marshes in China, has experienced intensive cultivation over the past 50 years. However, there were few reports of short-term dynamics of soil carbon and nitrogen and CO₂ emission after tillage. In this paper, we studied the short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China. The results showed that response of carbon and nitrogen dynamic to tillage was different for intact wetland and soil cultivated for 10 years. Tillage was followed by immediate and significant increases in CO₂ efflux, which peaked at 0.25 h after tillage, four times higher than control in the wetland soils; while, only 2.5 times higher than control in the cultivated soils. Although, dissolved organic C (DOC) increased, the relative stability of microbial biomass C (MBC) pools together with the decreased respiration in the wetland soil suggested that the tillage did not lead to a burst in microbial activity and growth. Other factors such as moisture content before and after tillage may play an important role in determining microbial activity in the intact wetland. On the contrary, although dissolved organic C did not change, MBC pools, and soil respiration increase after tillage, suggesting tillage led to an increase in microbial activity and growth in the cultivated soil. Tillage initiated changes in soil aeration that was an important factor affecting soil microbiology in the long history of cultivation. Net N mineralization and nitrification occurred in both wetland and cultivated soils, but at different rates after tillage that in the intact wetland soil was higher than cultivated soil. Macroaggregates in the wetland soil would be expected to contain larger amounts of organic matter, and thus release a larger source of newly available substrate for microbes after tillage. In the intact wetland soil, ammonium, nitrate, and dissolved organic N (DON) concentrations were significantly negatively correlated to soil moisture (p < 0.01), suggesting high soil moisture in the natural wetland was not in favor of N mineralization.     

Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by ¹⁵N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable<grassland<forest soil while DON and gross N fluxes increased in the order of forest<arable<grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (P<0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment.     

Free amino sugar reactions in soil in relation to soil carbon and nitrogen cycling

Amino sugars represent a major constituent of microbial cell walls and hydrolyzed soil organic matter. Despite their potential importance in soil nitrogen cycling, comparatively little is known about their dynamics in soil. The aim of this study was therefore to quantify the behaviour of glucosamine in two contrasting grassland soil profiles. Our results show that both free amino sugars and amino acids represented only a small proportion of dissolved organic N and C pool in soil. Based upon our findings we hypothesize that the low concentrations of free amino sugars found in soils is due to rapid removal from the soil solution rather than slow rates of production. Further, we showed that glucosamine removal from solution was a predominantly biotic process and that its half-life in soil solution ranged from 1 to 3 h. The rates of turnover were similar to those of glucose at low substrate concentrations, however, at higher glucosamine concentrations its microbial use was much less than for glucose. We hypothesized that this was due to the lack of expression of a low affinity transport systems in the microbial community. Glucosamine was only weakly sorbed to the soil's solid phase (K_d=6.4±1.0) and our results suggest that this did not limit its bioavailability in soil. Here we showed that glucosamine addition to soil resulted in rapid N mineralization and subsequent NO₃⁻ production. In contrast to some previous reports, our results suggest that free amino sugars turn over rapidly in soil and provide a suitable substrate for both microbial respiration and new biomass formation.     

The contribution of soil organic matter fractions to carbon and nitrogen mineralization and microbial community size and structure

The aims of this study were to: (i) assess the impact of hay and fertilizer application on organic matter (OM) fractions (dissolved organic matter (DOM), light fraction organic matter (LFOM, <1.0 g cm⁻³), heavy fraction OM (HFOM, <1.7 g cm⁻³)), carbon (C) and nitrogen (N) cycling processes and microbial community size and structure, and (ii) quantify the role of OM fractions to C and N cycling. Soil was collected in 2001 from a field experiment to which grass hay (1996) and/or fertilizer (1995 and 1999) had previously been applied. DOM-C (P<0.05) and DOM-N (P=0.07) were significantly higher in control and fertilized soil than hay and hay+fertilized soil. LFOM and HFOM C and N contents and C/N ratios were significantly (P<0.05) higher in hay+fertilized and hay amended soil than in control and fertilized soil. Potentially mineralizable-N (PMN), microbial biomass-C (MB-C), microbial biomass-N (MB-N) and microbial respiration (CO₂) were not affected by fertilizer and/or hay application. Gross N mineralization (Gross Min) and gross nitrification (Gross Nit) rates were significantly (P<0.05) higher in fertilized, hay, hay+fertilized soil than control soil. However, there was no significant difference between treatments in gross N immobilization rates. Results reported here highlight the importance of a labile fraction of the DOM pool to N and C cycling as its removal significantly (P<0.05) reduced PMN, MB-N, Gross Min and Gross Nit compared with whole soil in most or all treatments. In soil where DOM+LFOM were removed PMN was significantly (P<0.05) lower, but MB-C, Gross Min and Gross Nit was significantly (P<0.05) higher than in DOM removed soil. This suggests that LFOM plays an important role as a sink for mineral-N. Total soil phospholipid fatty acid (PLFA) concentration was significantly (P<0.05) higher in hay amended than control, fertilized and hay+fertilized soil. Principal components analysis was able to clearly discriminate between control, fertilized, hay+fertilized and hay amended soil. Soil amended with hay or fertilizer had a microbial community structure which differed from that of the control or hay+fertilized soils. Redundancy analysis with Monte Carlo permutation tests revealed that PLFA profiles were strongly correlated to differences in Gross Min, Gross Nit, MB-N, MB-C, MB-C/N ratio, total soil C and total soil C/N ratio. The results of this research suggest that changes in microbial structure are related to aspects of soil C and N pools and cycling.     

Short and long-term effects of elevated CO2 on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO₂ increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO₂ on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO₂ on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO₂ and using soil from the New Zealand FACE site that had >4 years exposure to CO₂ enrichment. Using ¹⁴CO₂ pulse labelling, the effects of elevated CO₂ on C allocation within the plant-soil system were studied. Under elevated CO₂ more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO₂, N availability to plants stayed unchanged. Soil previously exposed to elevated CO₂ exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO₂ was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.     

Effects of simulated nitrogen deposition on soil respiration components and their temperature sensitivities in a semiarid grassland

Nitrogen (N) deposition to semiarid ecosystems is increasing globally, yet few studies have investigated the ecological consequences of N enrichment in these ecosystems. Furthermore, soil CO₂ flux – including plant root and microbial respiration – is a key feedback to ecosystem carbon (C) cycling that links ecosystem processes to climate, yet few studies have investigated the effects of N enrichment on belowground processes in water-limited ecosystems. In this study, we conducted two-level N addition experiments to investigate the effects of N enrichment on microbial and root respiration in a grassland ecosystem on the Loess Plateau in northwestern China. Two years of high N additions (9.2 g N m⁻² y⁻¹) significantly increased soil CO₂ flux, including both microbial and root respiration, particularly during the warm growing season. Low N additions (2.3 g N m⁻² y⁻¹) increased microbial respiration during the growing season only, but had no significant effects on root respiration. The annual temperature coefficients (Q₁₀) of soil respiration and microbial respiration ranged from 1.86 to 3.00 and 1.86 to 2.72 respectively, and there was a significant decrease in Q₁₀ between the control and the N treatments during the non-growing season but no difference was found during the growing season. Following nitrogen additions, elevated rates of root respiration were significantly and positively related to root N concentrations and biomass, while elevated rates of microbial respiration were related to soil microbial biomass C (SMBC). The microbial respiration tended to respond more sensitively to N addition, while the root respiration did not have similar response. The different mechanisms of N addition impacts on soil respiration and its components and their sensitivity to temperature identified in this study may facilitate the simulation and prediction of C cycling and storage in semiarid grasslands under future scenarios of global change.     

Changes in Organic Matter of an Oxisol with the Addition of Soybean and Corn Residues,Nitrogen and Phosphorus

Plants with different photosynthetic cycles (C3 and C4) and different plant parts (root or shoot) contribute in different ways to soil organic carbon (SOC). In addition, phosphate and nitrogen fertilization also act differently on the SOC stock. In this study, roots or shoots of corn (C3) and soybean (C4) plants were incorporated into samples of an Oxisol, with or without the addition of nitrogen (N) and phosphorus (P) and had the emission of carbon (C)- carbon dioxide (CO₂) measured during 45 days. Subsequently, soil organic matter fractionation, carbon and nitrogen microbial biomass and 13C isotopic discrimination were performed. The greatest increment in SOC was observed by adding corn plant material rather than soybean material. For both crops, the greatest contribution to SOC was achieved by adding roots as compared to shoots. Phosphorus addition produced greater microbial activity, followed by the addition of NP and then the addition of only N.     

Distribution and dynamics of soil organic matter in an Antarctic dry valley

Terrestrial ecosystems in the Antarctic dry valleys function under extremely cold and dry climatic conditions that severely constrain C and N cycling and, like other polar regions, are likely to be sensitive to environmental change. To characterize the distribution and dynamics of soil organic C (SOC) and N in the various landscape elements of an Antarctic dry valley, we measured soil profile organic C and organic N stocks, inorganic N (NH₄-N and NO₃-N), soil CO₂ effluxes, water contents and soil temperatures in the Garwood Valley, a relatively small valley in southern Victoria Land. We also conducted laboratory measurements of basal respiration on soils collected from the Valley. SOC and respiration rates were low and SOC was highly stratified in the soil profile, with the largest values observed near the surface. Significant variations of SOC stocks and soil CO₂ effluxes were observed between landscape elements and spatial variability was closely related to the distance from the lake, the major site of primary production. The fastest rate of SOC turnover (residence time c. 30 years) was found in the soils at the lake edge, slower rates were found in landscape elements close to the lake (c. 52-67 years), and the slowest rates in other landscape elements (c. 84-123 years) further away. A mass balance of organic C indicates that the quantity of C fixed in the lake, accumulated on the lake edge, exposed and subsequently displaced on a 14-year basis can explain the near-surface SOC turnover within the entire valley. We conclude that the displacement of organic matter derived from the lake is an important external source for the microbial processes in these soils at a landscape scale. However, further investigations are needed in order to evaluate the importance of displaced C compared to other nutrients (e.g. N) on the spatial control of observed soil respiration rates.     

Soil organic matter quality as influenced by tillage,lime, and phosphorus

In Eastern Canada, cereal yields are often restricted by soil acidity and low fertility. Continuous cereal production can also lead to soil structural degradation. The addition of lime and fertilizers and the adoption of conversation tillage practices are proposed solutions which may have a positive impact on soil quality. The objective of the present work was to assess the impact of 3 years of different tillage practices and P additions, and of a single lime addition on organic C and total N, microbial biomass C, and on N mineralization at the surface layer (0–7.5 cm) of a Courval sandy clay loam (Humic Gleysol). The easily mineralizable N, total amount of N mineralized in 22.1 weeks, the rate of N mineralization, and microbial biomass C were significantly greater in the minimum tillage than in the moldboard plow treatment. Chisel plow treatment showed intermediate values. The ratios of potentially mineralizable N and of easily mineralizable to total soil N were also significantly larger under minimum tillage and chisel plowing than under moldboard plowing. The lime and P treatments had no significant effect on the measured soil quality parameters. The total amount of N mineralized per unit of biomass C decreased as the tillage intensity increased, suggesting a decrease in the efficiency of the biomass in transforming organic N into potentially plant-available forms and thus a loss in soil organic matter quality. The results of this study indicate that conservation tillage practices such as rototilling and chisel plowing are efficient ways of maintaining soil organic matter quality when old pastures are brought back into cultivation.     

Organic matter characteristics and C and N transformations in the humus layer under two tree species, Betula pendula and Picea abies

The aim of this study was to compare the effects of silver birch (Betula pendula Roth) and Norway spruce (Picea abies (L.) Karst.) on soil C and N transformations and on the characteristics of organic matter. Soil samples were taken from the humus layer of a replicated 35-year-old birch-spruce field experiment growing on Vaccinium myrtillus site type in middle-eastern Finland. The soil was a podzol and humus type mor. Soil pH was higher under birch (4.7) than under spruce (4.1). The C-to-N ratio was lower under birch (17) than under spruce (23). Per unit organic matter, microbial biomass C and N, net N mineralization and net nitrification were all higher in birch soil than in spruce soil. The rate of C mineralization (CO₂ production) was, however, the same regardless of tree species. Water-extracts were analyzed for the concentrations of dissolved organic C (DOC) and N (DON) and characterized according to molecular size distribution by ultrafiltration and according to chemical composition using a resin fractionation technique. The concentration of DON, in particular, was higher in birch soil than in spruce soil. The distribution of DOC and DON into different fractions based on molecular size or chemical composition was rather similar in both soils. The concentration of total phenolics, expressed as tannic acid equivalents, was higher in the humus layer under birch than in the humus layer under spruce, because the birch humus layer contained significantly more low-molecular weight (about <0.5 kD) phenolics than the spruce humus layer did. The concentration of proanthocyanidins (condensed tannins) was higher in spruce soil than in birch soil. The concentrations of the five most abundant phenolic acids showed that ferulic and p-coumaric acids were more abundant in spruce soil. Birch soil tended to contain slightly more nonvolatile sesquiterpenes than the spruce soil. The concentration of diterpenes was similar in both soils; but birch soil contained significantly more triterpenes, mainly sterols, than spruce soil did.     

大气CO2浓度升高对温带森林土壤微生物活性的影响

Microorganisms play a key role in the response of soil ecosystems to the rising atmospheric carbon dioxide (CO2) as they mineralize organic matter and drive nutrient cycling. To assess the effects of elevated CO2 on soil microbial C and N immobilization and on soil enzyme activities, in years 8 (2006) and 9 (2007) of an open-top chamber experiment that begun in spring of 1999, soil was sampled in summer, and microbial biomass and enzyme activity related to the carbon (C), nitrogen (N) and phosphorus (P) cycling were measured. Although no effects on microbial biomass C were detected, changes in microbial biomass N and metabolic activity involving C, N and P were observed under elevated CO2. Invertase and dehydrogenase activities were significantly enhanced by different degrees of elevated CO2. Nitrifying enzyme activity was significantly (P < 0.01) increased in the August 2006 samples that received the elevated CO2 treatment, as compared to the samples that received the ambient treatment. Denitrifying enzyme activity was significantly (P < 0.04) decreased by elevated CO2 treatments in the August 2006 and June 2007 (P < 0.09) samples. β-N-acetylglucosaminidase activity was increased under elevated CO2 by 7% and 25% in June and August 2006, respectively, compared to those under ambient CO2. The results of June 2006 samples showed that acid phosphatase activity was significantly enhanced under elevated CO2. Overall, these results suggested that elevated CO2 might cause changes in the belowground C, N and P cycling in temperate forest soils.     

Microbial response time to sugar and amino acid additions to soil

Soil microbial respiration is derived predominantly from the turnover of carbohydrates and proteins in soil. In most agricultural ecosystems, these C compounds enter soil mainly from rhizodeposition (root exudation and turnover). Our aim was to determine how long it takes for the microbial population to reach their maximum mineralization potential after the addition of low-molecular-weight (MW) rhizodeposits to the soil. We added sugar in the form of glucose and amino acids in the form of glycine to an arable, grazed grassland, Eucalyptus forest and boreal forest soil and monitored CO₂ efflux over a 6-h period. Artificial rainwater amended (zero C addition) or unamended soils were used as controls. The Michaelis-Menten substrate utilization profiles showed vastly different patterns of microbial mineralization capacity and substrate affinity between the soils. However, in all soils we showed that activation of the soil microbial community to C addition occurred almost instantaneously (?60 s) with the average time taken to reach half maximal CO₂ production being 14±8 min for glucose and 10±8 min for glycine. After reaching their maximal mineralization potential, the rate of CO₂ evolution remained constant for the remainder of the experiment. Our results showed that while substrate uptake and mineralization within the soil microbial biomass was activated quickly, subsequent adaptation and upregulation of its C processing capacity did not occur at least in the short term. The fast rate of microbial activation and substrate use we partially attribute to the large degree of functional redundancy that exists within the soil microbial community for processing rhizodeposits.     

Wetting and drying events determine soil N pools in two Mediterranean ecosystems

To improve our knowledge of how nutrient cycling in Mediterranean environments responds to climate change, we evaluated the effects of the continuous changes in soil nitrogen (N) pools during natural wetting and drying events. We measured soil N pools (microbial biomass [MB-N], dissolved organic nitrogen [DON], NH₄⁺ and NO₃⁻) and N ion exchange resins at weekly intervals for one year in two contrasting Mediterranean ecosystems. All soil N fractions in both ecosystems showed high intraseasonal and interseasonal variability that was greater in inorganic soil fractions than in organic N soil fractions. MB-N, DON and resin-NH₄⁺ showed increased concentrations during wetting events. Only the soil NO₃⁻ and resin-NO₃⁻ showed the opposite trend, suggesting a different response to water pulses compared to the other soil variables. Our results show that N pools are continuously changing, and that this high variability is not associated with the total amount of organic matter and labile soil carbon (C) and N soil fractions found in each ecosystem. The highest variability was found for inorganic N forms, which suggests that organic N forms are more buffered in soils exposed to wetting-drying cycles. Our results suggest that the changes in wetting-drying cycles expected with global climate change may have a significant impact on the availability and turnover of organic and inorganic N.     

Moisture modulates rhizosphere effects on C decomposition in two different soil types

While it is well known that soil moisture directly affects microbial activity and soil organic matter (SOM) decomposition, it is unclear if the presence of plants alters these effects through rhizosphere processes. We studied soil moisture effects on SOM decomposition with and without sunflower and soybean. Plants were grown in two different soil types with soil moisture contents of 45% and 85% of field capacity in a greenhouse experiment. We continuously labeled plants with depleted ¹³C, which allowed us to separate plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration measurements. We observed an overall increase in soil-derived CO₂-C efflux in the presence of plants (priming effect) in both soils. On average a greater priming effect was found in the high soil moisture treatment (up to 76% increase in soil-derived CO₂-C compared to control) than in the low soil moisture treatment (up to 52% increase). Greater plant-derived CO₂-C and plant biomass in the high soil moisture treatment contributed to greater priming effects, but priming effects remained significantly higher in the high moisture treatment than in the low moisture treatment after correcting for the effects of plant-derived CO₂-C and plant biomass. The response to soil moisture particularly occurred in the sandy loam soil by the end of the experiment. Possibly, production of root exudates increased with increased soil moisture content. Root exudation of labile C may also have become more effective in stimulating microbial decomposition in the higher soil moisture treatment and sandy loam soil. Our results indicate that moisture conditions significantly modulate rhizosphere effects on SOM decomposition.     

不同土地利用方式下土壤微生物群落水平生理图谱(CLP)与土壤碳、氮有效性之间的相互关系

The goal of this work was to assess soil microbial respiration, determined by the assay of community-level physiological profiling in an oxygen-sensitive microplate (O₂-CLPP), in response to endogenous C and several individual C substrates in the soils with different organic C contents (as a function of soil type and management practice). We also used the O₂-CLPP to determine the respiratory response of these soils to endogenous C and amended C substrates with N addition. A respiratory quotient (RQ) was calculated based on the ratio of the response to endogenous soil C vs. each C-only substrate, and was related to total organic carbon (TOC). For assessing N availability for microbial activity, the effect of N supplementation on soil respiration, expressed as N_ratio, was calculated based on the response of several substrates to N addition relative to the response without N. Soils clustered in 4 groups after a principal component analysis (PCA), based on TOC and their respiratory responses to substrates and endogenous C. These groups reflected differences among soils in their geographic origin, land use and C content. Calculated RQ values were significantly lower in natural forest soils than in managed soils for most C-only substrates. TOC was negatively correlated with RQ (r = - 0.65), indicating that the soils with higher organic matter content increased respiratory efficiency. The N addition in the assay in the absence of C amendment (i.e., only endogenous soil C present) had no effect on microbial respiration in any soil, indicating that these soils were not intrinsically N-limited, but substrate-dependent variation in N_ratio within soil groups was observed.     

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO₂ alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO₂ induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO₂. However, the long turnover times of many SOC pools - decades to centuries - make the detection of changes in the soil's pool size difficult. Long-term CO₂ enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO₂ has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO₂; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO₂ influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO₂. Specifically, we need to understand how elevated CO₂ influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.     

THE EFFECTS OF GRINDING ON MICROBIAL AND NON-MICROBIAL ORGANIC MATTER IN SOIL

Grinding more than doubled the respiration rate of two silt loam soils, one arable and one grassland. The increases were smaller when the grinding treatment was given to portions of soils that had previously been fumigated with CHCI₃and incubated, a treatment that greatly decreased microbial biomass. The results indicate that the flush of decomposition caused by grinding was in part derived from killed organisms and in part from enhanced decomposition of non-biomass sections of the soil organic matter. Grinding killed about a quarter of the biomass in both soils. Carbon from killed organisms accounted for a quarter of the extra CO₂–C evolved after grinding in the arable soil and almost half in the grassland soil. The extra non-biomass organic matter decomposing after grinding amounted to about 0.5% of the soil organic carbon in both soils. This non-biomass material rendered decomposable by grinding had a higher C/N ratio than the organic matter decomposing in unground soil.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Changes in soil microbial biomass carbon and enzyme activities under elevated CO2 affect fine root decomposition processes in a Mongolian oak ecosystem

The relationships between soil microbial properties and fine root decomposition processes under elevated CO₂ are poorly understood. To address this question, we determined soil microbial biomass carbon (SMB-C) and nitrogen (SMB-N), enzymes related to soil carbon (C) and nitrogen (N) cycling, the abundance of cultivable N-fixing bacteria and cellulolytic fungi, fine root organic matter, lignin and holocellulose decomposition, and N mineralization from 2006 to 2007 in a Mongolian oak (Quercus mongolica Fischer ex Ledebour) ecosystem in northeastern China. The experiment consisted of three treatments: elevated CO₂ chambers, ambient CO₂ chambers, and chamberless plots. Fine roots had significantly greater organic matter decomposition rates under elevated CO₂. This corresponded with significantly greater SMB-C. Changes in the activities of protease and phenol oxidase under elevated CO₂ could not explain the changes in fine root N release and lignin decomposition rates, respectively, while holocellulose decomposition rate had the same response to experimental treatments as did cellulase activity. Changes in cultivable N-fixing bacterial and cellulolytic fungal abundances in response to experimental treatments were identical to those of N mineralization and lignin decomposition rates, respectively, suggesting that the two indices were closely related to fine root N mineralization and lignin decomposition. Our results showed that the increased fine root organic matter, lignin and holocellulose decomposition, and N mineralization rates under elevated CO₂ could be explained by shifts in SMB-C and the abundance of cellulolytic fungi and N-fixing bacteria. Enzyme activities are not reliable for the assessment of fine root decomposition and more attention should be given to the measurement of specific bacterial and fungal communities.