Extractable Organic C Research Articles

How the Birch effect differs in mechanisms and magnitudes due to soil texture

Global climate change is predicted to intensify temperature and precipitation extremes, exposing soils to frequent and/or intense alternate wetting and drying regimes. Rewetting a dry soil causes a burst of CO2 known as the “Birch effect”. This spike in respiration can be attributed to: (i) release of cellular solutes (metabolites) accumulated during drought due to rapid increase in water potential upon re-wetting; (ii) sudden death of certain microbes serving as carbon (C) sources for surviving microbes, leading to microbial community shifts; and (iii) release of physically protected C due to aggregate breakdown upon repeated drying-wetting. The relative importance of these mechanisms may change in different soil textures, but very few studies examine all three processes. In this study, we evaluated the effects of repeated drying and wetting cycles (transient state moisture conditions) on the Birch effect and elucidated the mechanisms that contributed to this effect in different textured soils. Soils of three distinct textures (sandy, loamy, and clayey) were incubated for 140 days under five alternate cycles of drying (10% WHC) and wetting (100% WHC) conditions. Control soils were held at 55% WHC for the same duration. Microbial biomass C (MBC), extractable organic C (EOC), metabolites, microbial community structure, and changes in aggregate associated C were determined at each time point. Sandy soil had the lowest respiration rate followed by loamy and clayey soils, and cumulative CO2 loss was higher under transient moisture state compared to steady state. In sandy and clayey soils, changes in bacterial abundance controlled the Birch effect, while in loamy soil, the release of aggregate protected C majorly controlled the Birch effect.

Differential Organic Carbon Mineralization Responses to Soil Moisture in Three Different Soil Orders Under Mixed Forested System

Soil microbial respiration is one of the largest sources of carbon (C) emissions to the atmosphere in terrestrial ecosystems, which is strongly dependent on multiple environmental variables including soil moisture. Soil moisture content is strongly dependent on soil texture, and the combined effects of texture and moisture on microbial respiration are complex and less explored. Therefore, this study examines the effects of soil moisture on the mineralization of soil organic C Soil organic carbon in three different soils, Ultisol, Alfisol and Vertisol, collected from mixed forests of Georgia, Missouri, and Texas, United States , respectively. A laboratory microcosm experiment was conducted for 90 days under different moisture regimes. Soil respiration was measured weekly, and destructive harvests were conducted at 1, 15, 60, and 90 days after incubation to determine extractable organic C (EOC), phospholipid fatty acid based microbial community, and C-acquiring hydrolytic extracellular enzyme activities (EEA). The highest cumulative respiration in Ultisol was observed at 50% water holding capacity (WHC), in Alfisol at 100% water holding capacity, and in Vertisol at 175% WHC. The trends in Extractable Organic Carbon were opposite to that of cumulative microbial respiration as the moisture levels showing the highest respiration showed the lowest EOC concentration in all soil types. Also, extracellular enzyme activities increased with increase in soil moisture in all soils, however, respiration and EEA showed a decoupled relationship in Ultisol and Alfisol soils. Soil moisture differences did not influence microbial community composition.

Soil organic carbon cycling in response to simulated soil moisture variation under field conditions

The combination of extended dry periods and high intensity rainfall, common in the southeastern US, leads to greater variability in soil moisture and consequently increases uncertainty to microbial processes pertinent to soil carbon (C) mineralization. However, field-based findings on soil moisture sensitivity to soil C cycling are very limited. Therefore, a field experiment was conducted in 2018 and 2019 on a soybean (Glycine max L.) cropland in the southeastern US with three soil moisture treatments: drought (simulated using rainout-shelter from June to October in each year), rainfed (natural precipitation), and irrigated (irrigation and precipitation). Soil respiration was measured weekly from May to November in both years. Soil samples were collected multiple times each year from 0–5, 5–15, and 15–30 cm depths to determine microbial biomass C (MBC), extractable organic C (EOC), hydrolytic enzyme activities, and fungal abundance. The cumulative respiration under drought compared to other treatments was lower by 32% to 33% in 2018 and 38% to 45% in 2019. Increased MBC, EOC, and fungal abundance were observed under drought than other treatments. Specific enzyme activity indicated fewer metabolically active microbes under drought treatment compared to rainfed and irrigated treatments. Also, maintenance of enzyme pool was observed under drought condition. These results provide critical insights on microbial metabolism in response to soil moisture variation and how that influences different pools of soil C under field conditions.

Aggregate-related changes in living microbial biomass and microbial necromass associated with different fertilization patterns of greenhouse vegetable soils

Knowledge on soil aggregation and microbial-driven soil C dynamics at the aggregate scale is beneficial for long-term sequestration of C in greenhouse vegetable production (GVP) systems. Here, we used an eight-year fertilization experiment to compare the effects of organic vs. chemical fertilization on soil aggregate stability, as well as living microbial biomass, microbial necromass, and soil C dynamics at the aggregate scale. Relative to chemical fertilization treatment, organic amendments (e.g., manure and/or straw) could improve soil physical quality (as indicated by the value of mean weight diameter), increase microbial biomass and residues, as well as enhance the contributions of microbes to soil organic C (SOC) accumulation within large macroaggregates, small macroaggregates, microaggregates, and silt/clay fractions. Microbial biomass and residues were unevenly distributed among aggregates under different fertilization patterns, i.e., organic amendments made microbial biomass and fungal residues enriched from in silt/clay fractions to in macroaggregates. The low proportions of microbial residue C in SOC in microaggregates demonstrated that the microhabitat of microaggregates limits microbial necromass contributions to SOC accumulation. The changes of microbial biomass were closely related to extractable organic C (EOC), while the variations of fungal and bacterial residues were intimately associated with its corresponding microbes (i.e., fungal and bacterial PLFAs) and enzymes. Moreover, microbial associated ratios (e.g., fungal/bacterial PLFAs) were largely influenced by aggregates and strongly associated with soil chemical associated ratios (e.g., EOC/EON). Our findings provide useful insights on soil microbial-driven C dynamics at the aggregate scale in GVP systems under different fertilization patterns in China.

Interspecific plant competition increases soil labile organic carbon and nitrogen contents

Plant competition can impose species-specific effects on the dynamics of soil carbon (C) and nitrogen (N) through rhizosphere processes and litter input. Therefore, it is crucial to quantify these effects in various terrestrial ecosystems for a better understanding of the mechanisms. Here, we collected subsoils containing low N from a subtropical forest and planted eight dominant tree species (two deciduous and six evergreens) in these soils in a greenhouse to explore the effects of interspecific plant competition on plant growth, soil C and N contents, and soil C and N mineralization rates after the plants had grown for 12 months. Soil labile organic C and N contents were represented by soil extractable organic C (EOC) and extractable organic N (EON) contents. We assessed the magnitude of the interspecific plant competition via the relative interaction intensity (RII) index, which was calculated from the biomass of seedlings in the mixed treatments compared with the single treatments. Our results showed that interspecific plant competition had species-specific effects on plant biomass and soil total C and N contents as well as soil C mineralization rates, whereas it tended to decrease soil N mineralization rates. However, interspecific plant competition significantly decreased plant C and N contents, and significantly increased soil EOC and EON contents with increasing RII. After classifying the communities into two functional types (i.e., deciduous–evergreen versus evergreen–evergreen), similar relationships were observed. These findings address the importance of quantifying interspecific plant competition on soil labile organic C and N contents, which is helpful for understanding soil C and N cycling in forest ecosystems.

Rare rather than abundant microbial communities drive the effects of long-term greenhouse cultivation on ecosystem functions in subtropical agricultural soils

Long-term greenhouse cultivation has an adverse effect on ecosystem functions such as soil carbon (C) and nitrogen (N) pools and greenhouse gas (GHG) emissions, but the underlying microbial mechanisms still remain unclear. Here, different sites under long-term greenhouse cultivation in a subtropical agricultural ecosystem were selected to measure soil C and N contents, extractable organic C (EOC) and N (EON) contents, and potential GHG emissions. Metagenomic analysis and 16S rRNA high-throughput sequencing were used to measure microbial communities. The results showed that long-term greenhouse cultivation increased soil salinity, and significantly increased soil total C and N contents, EOC and EON contents, and N2O emission potentials, although it significantly decreased CO2 emission and CH4 oxidation potential compared with the ambient control. Changes in soil CH4 oxidation and N2O emission potential exhibited similar patterns in the corresponding key functional genes based on according to our metagenomic analysis. In addition, long-term greenhouse cultivation did not change microbial diversity, although it clearly affected soil microbial community composition. Soil microbial communities were further classified into rare and abundant microbial taxa. Rare rather than abundant microbial taxa could adequately explain the changes in ecosystem functions, except for CH4 oxidation potential across the treatments. To our knowledge, this is the first study to quantify the importance of microbial subcommunities to ecosystem functions on the basis of microbial co-occurrence network analysis under greenhouse cultivation in agricultural ecosystems. Overall, our results indicated that rare rather than abundant microbial taxa could act as indicators of variations in ecosystem functions under long-term greenhouse cultivation in subtropical agricultural soils, which might be useful for better management practices and improving crop yields in agricultural ecosystems.

Soil extractable organic C and N contents, methanotrophic activity under warming and degradation in a Tibetan alpine meadow

The Tibetan alpine meadow ecosystem is an important part of the Eurasian grasslands and is experiencing intense warming at approximately three times the global warming rate and rapid degradation. However, little is known about the effect of warming and degradation and their interactions on ecosystem functions like soil carbon (C) and nitrogen (N) pools and methane (CH4) uptake in this region. Here, we selected a long-term simulated warming site in a Tibetan alpine meadow with different degradation levels. After 4 years of warming, we analyzed soil total C (TC) and total N (TN) contents, extractable organic C (EOC) and extractable organic N (EON) contents as well as methanotrophic activity, abundance and community structure. Soil EOC and EON contents were measured through hot water extraction, whereas methanotrophic activity was measured along a gradient of CH4 concentrations in laboratory incubations. Michaelis–Menten kinetics analysis [maximal rate of velocity (Vmax) and half-saturation constant (Km)] was used to quantify changes in methanotrophic activity among the treatments. Active methanotrophic communities in the natural soils were measured via DNA-based stable isotope probing (SIP). The results showed that warming significantly increased soil EON contents, whereas degradation significantly decreased soil TC and TN contents, and EOC and EON contents. Methanotrophic activity was significantly lower at different levels of degradation but no significant effects were observed under warming. Changes in soil methanotrophic abundance among the treatments followed the same trend, but warming and degradation had no interactive effects on methanotrophic activity and abundance. Active methanotrophic communities in the natural meadow soils were dominated by Methylosinus (a Type II methanotroph). In conclusion, our results indicate that soil C and N pools and CH4 oxidation capability were influenced more strongly by degradation than warming. However, warming may have an additional effect on the stability of these important ecosystem processes, regardless of degradation in this region.

Extreme drought slightly decreased soil labile organic C and N contents and altered microbial community structure in a subtropical evergreen forest

It is predicted that climate extremes such as drought will become more frequent and prolonged by the end of this century, so it is vital that we improve our understanding of the effects of extreme drought on the dynamics of soil labile C and N contents and microbial communities in terrestrial ecosystems. Using a 70% rainfall reduction manipulation experiment to simulate extreme drought in a subtropical evergreen forest of eastern China, we collected soil samples over all four seasons and analyzed the dynamic changes in soil labile organic C and N contents and microbial communities via high-throughput sequencing during the period from June 2016 to February 2017. Soil labile organic C and N contents were represented by soil extractable organic C (EOC) and extractable organic N (EON) contents, microbial biomass C (MBC) and microbial biomass N (MBN) contents. We also measured soil potential CO2 and N2O emissions by using laboratory incubation under drought conditions. The results showed that drought significantly decreased soil pH, while it slightly decreased soil EOC and EON contents, and MBC and MBN contents by 13.4%, 15.7%, 11.1% and 15.2% respectively, though it had no significant effects on these compared with the control plots. The dominant bacterial phyla across all soils were Proteobacteria (32.52–41.30%), Acidobacteria (34.47–42.64%) and Actinobacteria (6.52–8.16%). Drought greatly altered the soil microbial community structure underlying soil C and N cycling: (1) drought significantly increased the relative abundance of Acidobacteria, which was associated with lower soil pH in the drought plots; (2) drought had a significantly lower relative abundance of Proteobacteria, which was supported by the lower soil labile organic C and N contents under drought. Redundancy analysis showed that the indirect drought-induced effects of soil EOC and EON contents from two sampling times played a weightier role in influencing the patterns of soil microbial communities than the direct drought-induced effects of soil moisture. We found that drought significantly decreased soil potential CO2 and N2O emissions, confirming that drought decreased soil microbial activity. Overall, our results suggest that extreme drought slightly decreased soil labile C and N contents, and altered the microbial community structure, mainly through indirect drought-induced pathways.

Vertical distribution of soil extractable organic C and N contents and total C and N stocks in 78-year-old tree plantations in subtropical Australia.

Few studies have focused on the effects of long-term forest plantations on the soil profile of carbon (C) and nitrogen (N) stocks. In this study, we selected 78-year-old tree plantations that included three coniferous tree species (i.e., slash pine, hoop pine and kauri pine) and a Eucalyptus species in subtropical Australia. We measured soil extractable organic C (EOC) and N (EON) contents and total C and N stocks under different tree species on the forest floor and along a soil profile to 100cm depth. The results showed that Eucalyptus had significantly higher soil EOC contents (3.3Mgha-1) than the other tree species (EOC of 1.9-2.3Mgha-1) and had significantly higher EON (156kgha-1) contents than slash pine (107kgha-1). Eucalyptus had significantly higher soil C (58.9Mgha-1) and N (2.03Mgha-1) stocks than the other tree species (22.3-27.6MgCha-1 and 0.71-1.23MgNha-1) at 0-100cm depth. There were no differences in soil C stocks at the 0-100cm depth among the coniferous tree species. Forest floor C stocks had stronger effects on mineral soil total N stocks than fine root biomass, whereas fine root biomass exerted stronger effects on soil total C stocks at the 0-100cm depth than forest floor C and N stocks. Our results addressed large differences in soil C and N stocks under different tree species, which can provide useful information for local forest management practices in this region.

Root exudation patterns in a beech forest: Dependence on soil depth, root morphology, and environment

Forest subsoils may represent an important C sink in a warming world, but rhizodeposition as the key biogeochemical process determining the C sink strength of mature forests has not yet been quantified in subsoils. According to studies conducted in topsoil or laboratory experiments, soil C inputs by root exudation are increasing with increasing temperature and decreasing nutrient availability. We examined whether these relationships apply to forest subsoil by analyzing the response of root exudation to increasing soil depth up to 130 cm in a mature European beech (Fagus sylvatica L.) forest. In two subsequent growing seasons differing in temperature and precipitation, we investigated in situ root exudation with a cuvette-based method and analyzed root morphology, microbial biomass, and soil nutrient availability. We proved that root exudation greatly decreases with soil depth as a consequence of a significant decrease in root-mass specific exudation activity to nearly a fifth of topsoil activity. The decrease in specific metabolic activity from 312 mg C g−1 yr−1 in the topsoil to 80 mg C g−1 yr−1 at 130 cm depth was amplified by an exponential decrease in root biomass per soil volume, leading to a relative decrease in root exudation per volume in the deep subsoil to 2% of topsoil root exudation (1 g C 10 cm−1 m−2 yr−1 at 130 cm depth). Specific root area decreased and mean fine root diameter and root tissue density increased with soil depth, indicating a shift in primary root functionality from fibrous roots in the topsoil to pioneer roots in the subsoil. The decrease in root exudation was accompanied by decreases in soil microbial biomass, extractable organic C (EOC), and N and P availability and increases in the aromatic C portion in SOM, but it did not relate to seasonal differences in climatic conditions. More specifically, it responded positively to an increase in EOC and ETN in the topsoil, but remained at its minimum rate in the SOC-poor subsoil, probably due to a lower organic N and higher mineral N content. The vertical pattern of beech root exudation is in accordance with a strategy to maximize whole-tree carbon-use efficiency, as it reduces C loss by exudation in soil spots where positive priming effects are unlikely, but enhances C exudation where microbes can mine less bioavailable SOM. The exudation patterns further suggest that increased C allocation to root systems as a likely tree response to elevated atmospheric [CO2] may not lead to enhanced soil C input by root exudation to subsoils poor in SOM.

Response of labile organic C and N pools to plastic film removal from semiarid farmland soil

AbstractTo examine the effects of plastic film removal on grain yield and soil organic matter (SOM), a spring maize (Zea may L.) field experiment was conducted for 5 yr at Changwu Agricultural and Ecological Experimental Station of Northwest China. Compared with traditional plastic film mulching during entire growing stages (FM), plastic film removal at the silking stage (RM) resulted in a 6.3% higher average maize yield. Under the RM treatment, soil organic carbon and total nitrogen significantly increased after the 5‐yr cultivation in the 0‐ to 20‐cm layer. Significant increases in extractable organic C (EOC), KMnO4‐oxidizable C (KMnO4‐C) and C management index (CMI) in the 0‐ to 20‐cm layer, and light fraction organic C and EOC in the 20‐ to 40‐cm layer were observed in response to plastic film removal after the 1‐yr treatment; the responses were more significant after 5 yr. Under the RM treatment, significant increases in microbial biomass C, light fraction organic N, extractable organic N, KMnO4‐C and CMI were also observed after five years in the 20‐ to 40‐cm layer. Moreover, KMnO4‐C and EOC were much more sensitive than other labile SOM fractions to the application of RM, even after only 1 yr of cultivation. Therefore, compared with mulching for the whole growing season, plastic film removal at the maize silking stage is an effective option for increasing yields and enhancing SOM concentration and soil sustainability in the regions with semiarid monsoon climates that have sufficient rainfall during maize reproductive stages.

Relationships between labile soil organic matter and nematode communities in a California oak woodland

Labile soil organic matter (SOM) is an important energy source for below-ground ecosystems but the association of labile SOM and nematode communities is poorly characterised. In this study, soil nematode communities and nematode-derived indices of ecosystem function were characterised and related to SOM lability in an undisturbed riparian woodland (California, USA). SOM lability was assessed by microbial biomass C (MBC), permanganate-oxidisable C (POXC), extractable organic C (EOC), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The channel index, which measures the ratio of bacterial-feeding to fungal-feeding nematodes in cp groups 1 and 2, respectively, decreased with labile C fractions and aliphatic C-H enrichment (infrared absorbance at 2920 cm−1) but increased with aromatic C=C enrichment (1620 cm−1) and index of decomposition (2930:1620 cm−1), as did the nematode structure index. These results indicate that nematode communities respond to variation in labile C fractions and SOM composition across a heterogeneous natural landscape, which may reflect observed differences in SOM lability among woody plant species.

Carbon transfer from maize roots and litter into bacteria and fungi depends on soil depth and time

Plant-derived carbon (C) transfer to soil is one of the important factors controlling the size and structure of the belowground microbial community. The present study quantifies this plant-derived C incorporation into abiotic and biotic C pools in top- and subsoil in an arable field over five years. Stable isotope analysis was used to determine the incorporation of maize root and shoot litter C into soil organic C (SOC), extractable organic C (EOC), total microbial biomass (Cmic), ergosterol and phospholipid fatty acids (PLFAs). The following treatments were investigated: corn maize (CM), providing root- and shoot-derived C (without corncobs), fodder maize (FM), providing only root-derived C, and wheat plus maize shoot litter amendment (WL), providing only shoot-derived maize C. Wheat plants (W) without maize litter amendment served as control. Soil samples were taken each September directly before harvest from 2009 to 2013. During the experiment, the maize-derived C signal increased in SOC, EOC, Cmic, ergosterol, bacterial and fungal PLFAs in the topsoil (0–10 cm). Although total maize shoot C input was threefold lower than maize root C input, similar relative amounts of maize C derived from shoots and roots were incorporated into the different C pools in the WL and the FM treatments, indicating the importance of shoot-derived C sources for microorganisms in the topsoil. An additive effect of both C sources was found in the CM treatment with almost twice as much maize-derived C in the respective pools. Furthermore, the proportion of maize-derived C varied between the different pools with lower incorporation into the total SOC (17%) and total EOC (24%) pools and higher incorporation ratios of maize C into PLFAs of different microbial groups (29% in Gram-positive (Gr+) bacterial PLFA-C, 44% in Gram-negative (Gr−) bacterial PLFA-C, 69% in fungal PLFA-C and 78% in ergosterol) in the CM treatment in topsoil after five years. After the third and fifth vegetation periods, we also detected maize-derived C in the rooted zone (40–50 cm depth) and the root-free zone (60–70 cm depth). The maize-derived C incorporation was lower in subsoil C pools in comparison to topsoil C pools. In the root-free zone, the maize-derived C was found to be 2% in total SOC, 28% in total EOC, 9% in Gr+ bacterial PLFA-C, 20% in Gr− bacterial PLFA-C and 53% in fungal PLFA-C. Saprotrophic fungi incorporated maize-derived C in all soil depths to a greater degree than Gr+ and Gr− bacteria, indicating the importance of saprotrophic fungi in this agro-ecosystem.

Soil labile carbon and nitrogen pools and microbial metabolic diversity under winter crops in an arid environment

The conservation farming systems coupled with stubble retention are now widely adopted in southern Australia to improve soil fertility. However, little information is available about the effects of winter crops on soil labile organic carbon (C) and nitrogen (N) pools, especially in an arid agricultural ecosystem. In this study, eight winter cover crop treatments were used to investigate their effects on soil labile organic C and N pools and microbial metabolic profiles and diversity in temperate Australia. These treatments included two legume crops (capello woolly pod vetch and field pea), four non-legume crops (rye, wheat, Saia oat and Indian mustard), and a mixture of rye and capello woolly pod vetch as well as a nil-crop control. At the crop flowering stage, soil and crop samples were collected from the field and we examined aboveground crop biomass, soil NH4+-N, NO3−-N, extractable organic C (EOC) and N (EON) concentrations using methods of 2M KCl and hot water, microbial biomass, biologically active organic C (CBio), and substrate-induced respiration (SIR) using the MicroResp method. Results showed that the crop treatments had lower soil moisture content, NO3−-N and the ratios of EOC to EON, but higher pH, NH4+-N, EOC, EON, CBio, microbial metabolic diversity index (H) and evenness index compared with the control. There were no significant differences in microbial biomass C and N among the treatments. Although no pronounced differences in EOC and EON concentrations were found between the legumes and non-legumes, the legume treatments had lower SIR and higher H than the non-legume treatments. Principal component analysis showed that soil microbial metabolic profiles under the crops were different from those of the control, and the crop treatments had a clear separation along principal component 2. In addition, redundancy analysis showed that soil pH and moisture content were the most important influencing factors, along with EON and crop biomass, determining the patterns of microbial metabolic profiles under the crops.

Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems

Abstract The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C org ), extractable organic C (EOC), microbial biomass (C mic ) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C org , EOC, C mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.

Effects of thermally dried and composted sewage sludges on the fertility of residual soils from limestone quarries

Limestone quarrying reduces the land's capacity to support a complete functional ecosystem. Adding sewage sludge to mining residues facilitates the establishment of a vegetation cover and can stimulate C and N cycling. We aimed to evaluate the effects of three composted and three thermally dried sewage sludges, on some biological properties of two types of debris (extraction soil and trituration soil) from a limestone quarry. Lysimeters filled with debris-sludge mixtures and control soils were sampled immediately after preparation and after being left in the open for 13 months. Total carbohydrates (TCH), 0.5 M K 2SO 4 extractable (ECH) carbohydrates, 0.5 M K 2SO 4 extractable organic C (EOC), microbial biomass carbon (MBC), microbial respiration (MR), β-glucosidase activity and β-galactosidase activity were determined immediately after sampling. The treated soils were also analyzed for their more general physicochemical characteristics. Adding sewage sludge clearly improved the physicochemical and biological properties of the residual soil and the effect of the type of sludge was greater than that of the type of soil. The sludge effect was generally more durable over the trituration soil. The sludge effect decreased the most in MR and EOC followed by MBC and ECH. Total carbohydrates showed the least enhancement but the sludge effect on this endpoint had the smaller decrease with time. Root exudates and plant debris contributed to β-glucosidase and β-galactosidase activities in the treated soils. Activities present in mixtures partly corresponded to enzymes free in the soil aqueous face. β-Glucosidase was also partly associated with humified organic matter. Thirteen months after sludge addition a fraction of the organic matter present in soils was still moderately labile. Results observed in BMC and MR suggests the sludge did not cause major toxic effects on residual soils. The sludge effect differed with the pre and post treatments of the sludges; thermal drying made the sludge organic matter more easily decomposable.

Rapid changes in carbon and phosphorus after rewetting of dry soil

Drying–rewetting (DRW) cycles are important for soil organic matter turnover; however, few studies have considered the short-term effects on nutrient availability. The pulses in soil respiration, extractable C, P and N pools were quantified after a single DRW cycle (ten sampling times over 49 h). Soil was pre-incubated with or without glucose (2.5 g kg−1) for 10 days to induce differences in the size and activity of the microflora and then either subjected to a single DRW cycle (7-day drying period) or kept constantly moist. A resin extractable P (Presin) method was used and compared to extraction of dissolved organic (DOP) and inorganic P (DIP) with a salt solution. The pulse in soil respiration, extractable organic C (EOC), Presin, DOP and DIP was immediate and greatest in the first 2 h. The Presin pulse was two to three times that measured by solution extraction (DIP). Also, Presin quantified temporal changes in P not apparent in DIP, indicating the advantage of anion-exchange membranes in quantifying short-term changes in P availability. The Presin pulse was smaller in the soil incubated with glucose showing that P pulses will be quantitatively smaller in a soil with an active microbial biomass. In contrast to P, pre-incubation with glucose did not alter EOC concentration or the pulse in EOC after rewetting. The Presin pulse had disappeared by 49 h after DRW despite continued elevated rates of respiration. The sustained increase in DIP following DRW may have implications for plant availability or environmental losses.

Carbon pulses but not phosphorus pulses are related to decreases in microbial biomass during repeated drying and rewetting of soils

Drying and rewetting cycles are known to be important for the turnover of carbon (C) in soil, but less is known about the turnover of phosphorus (P) and its relation to C cycling. In this study the effects of repeated drying-rewetting (DRW) cycles on phosphorus (P) and carbon (C) pulses and microbial biomass were investigated. Soil (Chromic Luvisol) was amended with different C substrates (glucose, cellulose, starch; 2.5 g C kg −1) to manipulate the size and community composition of the microbial biomass, thereby altering P mineralisation and immobilisation and the forms and availability of P. Subsequently, soils were either subjected to three DRW cycles (1 week dry/1 week moist) or incubated at constant water content (70% water filled pore space). Rewetting dry soil always produced an immediate pulse in respiration, between 2 and 10 times the basal rates of the moist incubated controls, but respiration pulses decreased with consecutive DRW cycles. DRW increased total CO 2 production in glucose and starch amended and non-amended soils, but decreased it in cellulose amended soil. Large differences between the soils persisted when respiration was expressed per unit of microbial biomass. In all soils, a large reduction in microbial biomass (C and P) occurred after the first DRW event, and microbial C and P remained lower than in the moist control. Pulses in extractable organic C (EOC) after rewetting were related to changes in microbial C only during the first DRW cycle; EOC concentrations were similar in all soils despite large differences in microbial C and respiration rates. Up to 7 mg kg −1 of resin extractable P (P resin) was released after rewetting, representing a 35–40% increase in P availability. However, the pulse in P resin had disappeared after 7 d of moist incubation. Unlike respiration and reductions in microbial P due to DRW, pulses in P resin increased during subsequent DRW cycles, indicating that the source of the P pulse was probably not the microbial biomass. Microbial community composition as indicated by fatty acid methyl ester (FAME) analysis showed that in amended soils, DRW resulted in a reduction in fungi and an increase in Gram-positive bacteria. In contrast, the microbial community in the non-amended soil was not altered by DRW. The non-selective reduction in the microbial community in the non-amended soil suggests that indigenous microbial communities may be more resilient to DRW. In conclusion, DRW cycles result in C and P pulses and alter the microbial community composition. Carbon pulses but not phosphorus pulses are related to changes in microbial biomass. The transient pulses in available P could be important for P availability in soils under Mediterranean climates.

Trihalomethane Formation Potential of Filter Isolates of Electrolyte‐Extractable Soil Organic Carbon

Certain organic C moieties of soil origin in drinking source waters of Sacramento-San Joaquin Delta (Delta) can react with chlorine to form trihalomethanes (THMs) during the disinfection process. Isolation and characterization of them and quantitation of their THM formation potential (THMFP) is necessary for developing effective strategies to reduce their influxes in Delta waters and for removing them during drinking water treatment. In this study, organic C from two Delta soils was extracted using deionized H(2)O and four Na- or Ca-based electrolytes of varying electrical conductivity values. Extracts were filtered into particulate, colloidal, fine colloidal, and soluble organic C for quantitation and THMFP determination. Results suggested that <1.5% of soil organic C was electrolyte-extractable. The soluble organic C fraction from both soils dominated in quantity and THMFP. Electrolyte effects were cation dependent. Sodium-based electrolytes at either conductivity level did not significantly decrease extractable organic C (EOC) or THMFP compared with deionized H(2)O. In contrast, Ca-based electrolytes reduced EOC and THMFP by >50% even at 1 dS m(-1). Further increase in Ca concentration did not significantly decrease EOC or THMFP. Most reduction in EOC and THMFP by Ca-based electrolytes occurred with the fractions other than the soluble organic C. Results suggested that under natural soil leaching and runoff conditions, the majority of THMFP is associated with organic C of <0.025 mum in diameter. Further molecular characterization of the fractions with high THMFP may help understand the nature of chlorine-reactive organic C from Delta soils.