Long-term Laboratory Incubations Research Articles

Study in a long-term laboratory experiment of the potential susceptibility to mineralization of organic matter in post-agrogenic light gray soils

Relevance. Since 2021, work has begun in Russia to assess the fertility of unused arable land and its involvement in agricultural circulation. Changes in the farming system of post-agrogenic soils can lead to uncontrolled mineralization of newly formed soil organic matter (SOM) under fallow vegetation and significant CO2 emissions into the atmosphere. Studies of the nature of SOM accumulation under fallow vegetation and assessment of its potential susceptibility to mineralization are relevant, since they can become the basis for the development of agrotechnical methods for returning unused lands to arable circulation with the maximum preservation of their fertility.Methods. In a long-term laboratory incubation experiment, we studied the dynamics of changes in the intensity of basal (BR) and substrate-induced respiration (SIR) in postagrogenic soils to assess the potential susceptibility to SOM mineralization with a change in land use. Layered samples (0–10 and 10–20 cm) were used from the old arable horizons of two fallow plots with different humus conditions. The results of the incubation experiment were compared with the results of assessing the quantitative content and qualitative composition of SOM.Results. In the 0–10 cm layer, the respiration rates are higher than in the 10–20 cm layer. The results of the assessment of the respiration intensity are consistent with the assessment of the qualitative composition of SOM. The accumulation of SOM occurs mainly in the upper part of postagrogenic soils due to mobile easily oxidized organic compounds of a fulvic nature. When developing agrotechnical methods for returning fallow lands to arable circulation, it is necessary to focus primarily on basic processing technologies that ensure maximum preservation of potentially easily mineralized material accumulated in the upper layer.

Quantification of Organic Carbon Sequestered by Biogenic Iron Sulfide Minerals in Long-Term Anoxic Laboratory Incubations.

Organic carbon sequestration in sedimentary environments controls oxygen and carbon dioxide concentrations in the atmosphere. While minerals play an important role in the preservation of organic carbon, there is a lack of understanding about the formation and stability of organo-mineral interactions in anoxic environments, especially those involving authigenic iron sulfide minerals. In this study, we quantified organic carbon and nitrogen sequestered in biogenic iron sulfide minerals co-precipitated with sulfate-reducing bacteria (SRB) in freshwater and marine conditions in long-term laboratory experiments. The amounts of C and N associated with biogenic iron sulfide minerals increased with increasing cell biomass concentrations available in the media. C and N levels stabilized over the first 2 months of incubation and remained stable for up to 1 year. Crystalline mackinawite (FeS) formed in all experimental conditions and transformed to greigite only in some experimental conditions. We did not find evidence that this mineral transformation affected C and N levels, neither could we identify the factors that controlled greigite formation. Pyrite did not form in our experimental conditions. While C concentrations in minerals correlated with concentrations of reduced sulfate in both the freshwater and marine media, removal of OC by iron sulfide minerals was more efficient in freshwater than marine conditions. Removal of OC by iron sulfide minerals was also more efficient when cells were present (SRB biomass) in comparison with abiotic incubations with organic mixtures (e.g., tryptone, yeast extract, and casamino acids). Our study highlights the potential for biogenic iron sulfide minerals to quantitatively contribute to organic carbon preservation in anoxic environments.

Plant Species and Defoliation Effects on Soil Nitrogen Mineralization in a Semiarid Rangeland of Argentina

The objective of this study was to evaluate the effects of moderate defoliation and grass species on soil nitrogen (N) mineralization in a semiarid grassland of northeastern Patagonia, Argentina. Studied species were Poa ligularis and Nassella tenuis (desirable/preferred by cattle) and Amelichloa ambigua (undesirable/non preferred). Two defoliations were made to a 5 cm stubble height during the growing season. Hypotheses were that (1) net N mineralization and N availability in the soil are increased by a moderate defoliation and greater species forage quality, and (2) potential N mineralization is higher in the soil beneath the desirable than undesirable species. In 2013 and 2014, in situ net N mineralization was estimated using the tube incubation technique under field conditions. Potential N mineralization was estimated by long-term laboratory incubations. Defoliation treatments did not affect the soil inorganic N dynamics. The soil under A. ambigua showed a greater in situ net N mineralization than other species, but only in 2013. Poa ligularis presented the highest initial inorganic N and potentially mineralizable N values. However, the opposite was recorded in this specie for the mineralization constant rate. These results demonstrated that moderate defoliations did not affect soil N availability, and presence of the desirable perennial grasses increased the potential N mineralization pool. Sustainable management practices that promote the persistence of these species in plant community are important to maintain soil fertility on semiarid grasslands.

Long-term effects of earthworms (Lumbricus rubellus Hoffmeister, 1843) on activity and composition of soil microbial community under laboratory conditions

Observations of earthworm colonization on sites where they were absent previously, such as on post-mining heaps, indicate that they may substantially alter soil conditions as well as soil microbial communities. However, long-term effects of earthworms on microbial activity of soil ecosystems remains poorly understood. To improve this situation, we set up microcosm experiments with clay only, with clay and litter, and with clay, litter and earthworms. Microcosms were sampled after 3, 60, 180 and 395 days of cultivation and divided into three subsamples: clay, litter, and drilosphere (earthworm tunnels and associates casts), when present. Microbial respiration, microbial biomass carbon, species composition, metabolic activity using BIOLOG, PLFA concentration, isoprenoid quinones content and DGGE fingerprints were analysed in all microcosms and at all subsamples. Microbial biomass carbon and basal soil respiration were significantly affected by type of microcosms and time of sampling. Microbial biomass in drilosphere was significantly higher than in clay and litter. The CFU (colony forming units) were significantly affected by presence of earthworms having significantly higher CFU in drilosphere than in all other subsamples. Analysis of PLFA indicated highest relative biomass of fungi and bacteria in clay from microcosms with earthworms. The DGGE analysis showed various compositions of microbial communities among subsamples from various treatments taken at different time. Finally, our results revealed significant effect of earthworms on activity and biomass of soil microbial community under long-term laboratory incubation.

Performance of physical and chemical methods in the co-reduction of internal phosphorus and nitrogen loading from the sediment of a black odorous river

The continuous release of nutrients from sediment is a major barrier to the remediation of black odorous rivers. This study used a long-term laboratory incubation experiment to investigate the effectiveness of sediment dredging, intermittent aeration, and in situ inactivation with modified clays to reduce the internal loading of sediment from a seriously polluted river. The results indicated that intermittent aeration and in situ inactivation were effective in reducing the TN and NH4+ concentrations in the water column. However, sediment dredging did not consistently reduce the TN and NH4+ concentrations in the water column. In contrast, the three methods were all effective in controlling the TP and PO43− concentrations in the water column. Except for dredging, >30% of NH4+ and 40% of PO43− fluxes from sediment were reduced when compared with a control sample after 120 days of remediation. Dredging induced a significant release of NH4+ from sediment. Dredging and aeration made nearly no change to the amount of extractable nitrogen in the sediment. However, inactivation may increase sediment-extractable ammonium in deep sediment layers with time due to vertical transportation of clay by intensive bioturbation. Dredging is the most effective way to reduce surface mobile phosphorus over time while the transported clays can reduce a large percentage of the mobile phosphorus in deeper sediment. The relative abundance of Nitrospira in the surface sediment increased significantly with each remediation measure, creating favorable conditions for the reduction of the ammonium released from sediment. Altogether, the results of this study indicated that clay inactivation is the best method for controlling the internal loading of both phosphorus and nitrogen in seriously polluted river sediment.

Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel

Abstract The release kinetics of nitrogen and phosphorous of a granulated fertilizer, encapsulated in a κ-carrageenan-based hydrogel (CBH), was evaluated in order to determine its release mechanism given the potential this hydrogel has as coating material for controlled release fertilizers (CFRs). The effect of pH on the release properties was also investigated. The relationship between the NH4+, NO3-, and PO43- release of encapsulated fertilizers and time was determined by short- and long-term laboratory incubations. The mechanism of the release of nutrient ions was determined by comparing the release data with the zero-order, first-order, Higuchi, Hixon-Crowell and Korsmeyer-Peppas kinetic models. The findings showed that the Korsmeyer-Peppas model could be used to describe the release characteristics of the nutrients in the encapsulated fertilizers and that non-Fickian diffusion is the main release mechanism. The experimental hydrogel showed a high water retention capacity able to absorb 300 times its weight water.

Nitrogen mineralisation in sugarcane soils in Queensland, Australia: I. evaluation of soil tests for predicting nitrogen mineralisation

We explored soil properties as indices of mineralisable nitrogen (N) in sugarcane soils and whether we could increase the accuracy of predicting N mineralisation during laboratory incubations. Utilising historical data in combination with samples collected during 2016, we: (i) measured mineralised N over the course of short-term (14 days) and long-term (301 days) laboratory incubations; (ii) compared models representing mineralisation; then (iii) related model parameters to measured soil properties. We found measures representing the labile organic N pool (Hydrolysable NaOH organic N; amino sugar Illinois soil N test) best related to short-term mineralised N (R2 of 0.50–0.57, P < 0.001), while measures of CO2 production (3, 7, 10 and 14 days) best related to longer-term mineralised N (R2 of 0.75–0.84, P < 0.001). Indices were brought together to model the active and slow pools of a two-pool mineralisation model in the statistical framework of a mixed-effects model. Of the models that relied on measurement of one soil property, cumulative CO2 production (7 days) performed the best when considering all soil types; in a cross-validation test, this model gave an external R2 of 0.77 for prediction of the 301-day mineralised N. Since the mixed-effects model accounts for the various sources of uncertainty, we suggest this approach as a framework for prediction of in-field available N, with further measurement of long-term mineralised N in other soils to strengthen predictive certainty of these soil indices.

Gas production from dredged sediment

The anaerobic degradation of sediment organic matter leads to considerable gas production in constructions made from sediments and in landfills where contaminated sediments are disposed of, inducing problems with the mechanical stability of constructions or necessitating extraction and treatment of gas. However, little is known about the magnitude of gas generation from dredged sediment, hence validated input parameters for gas production modelling are missing. On the occasion of drillings performed for the installation of inclinometers on a mono-landfill for contaminated dredged sediment, eleven waste layers of known were sampled. Samples were analysed for gas generation in a long-term laboratory incubation experiment carried out for 757 days. It was found that the residual gas potential of the deposited dredged material ranged between 2 and 12 m3 MgDW−1, relating to 3–11% of the organic matter being degraded. Correlation analyses with material properties suggest a strong role of nitrogen, either directly or as indicative parameter, with the gas potential increasing with total nitrogen content and the share of degradable carbon decreasing with increasing TOC/TN ratio. The by far greatest share of organic matter was bound in the heavy density fraction >1.4 g cm−3, suggesting that the readily available light organic matter pool had already been depleted during pre-treatment of the dredged sediment in dewatering fields and the subsequent years of landfilling. Consequently, the correlation of the remaining gas potential with heavy fraction nitrogen was even stronger than for bulk nitrogen. The gas potential as revealed from the long-term test correlated well with short-term values, but outreached the commonly applied potential measured for 21 days by the factor of four. The data improve the state of knowledge on gas production from the large mineral waste stream of dredged material and serve to improve gas production modelling for these types of wastes. The strong correlation of gas potential to TN suggests that TN may serve as a proxy to estimate total gas potential.

Bioavailable dissolved organic matter and its spatio-temporal variation in a river dominated tropical brackish water Lagoon, India

Bioavailable dissolved organic carbon (BDOC), nitrogen (BDON) and their degradation rate constants were measured for the Chilika Lagoon, India. Long-term laboratory incubation experiments (90 days) were conducted at a constant temperature (25 °C) to quantify the bioavailable dissolved organic matter (DOM) and the possible degradation rate coefficients. The results showed that 41 ± 12% of dissolved organic carbon (DOC) and 47 ± 17% of dissolved organic nitrogen (DON) were BDOC and BDON respectively, with their stoichiometry found to be higher than the Redfield ratio. A first order exponential non-linear fitting routine was used to estimate pool sizes. The degradation rate constant (k) for the BDOC varied from 0.127–0.329 d−1 and BDON from 0.043–0.306 d−1 during the study period. Half-lives of the BDOC and BDON ranged from 2.1–5.4 and 2.2–15.9 days, respectively. Overall, the results showed that a fraction of the labile DON was transported from the lagoon to the adjacent coastal sea.

Exploring potential pine litter biodegradability as a natural tool for low-carbon forestry

Litter decomposition has a remarkable role in the terrestrial C cycle not only for being an important C source of CO2 effluxes but also for leading soil organic matter (OM) formation with remarkable influences on sustainability of forest ecosystems, either broadleaf or coniferous. Thus, in order to evaluate the decomposability of litter from humid-temperate coniferous forests, two of the most widespread types of pine forests in the NW of Spain were selected (Pinus radiata D. Don, Pinus pinaster Ait.). In addition to litter characterization and solid state 13C NMR chemical structural identification, potential biodegradability and isotopic dynamics during the decomposition of litter from these two types of pine plantations were determined by using long-term controlled laboratory incubations (≈1year) under optimal environmental conditions for microbial growth (28°C and 80% moisture content). For both pine species, CO2 fluxes released during the experimental decomposition process fitted a common litter biodegradability model that considers two OM pools (labile and recalcitrant). The labile C pool represented 7.9% or 8.6% of the litter total C content in radiata or maritime pine forests, respectively, with the recalcitrant fraction showing averaged values close to 92% of the litter C. This recalcitrant pool had instantaneous mineralization rates between 80 and 170 times lower than the corresponding labile fraction. Radiata pine litter presented significantly higher instantaneous mineralization rates of the labile C pool compared with maritime litter (possibly related with their interspecific compositional differences, mean C/N ratio of 48 for radiata and 70 for maritime pine litter), which resulted in lower residence times for the former than the latter with an average half-life of 18days for radiata litter and of 34days for maritime litter. Multilinear regression modelling by combining the C mineralization indices and the biochemical characteristics of the litter as predictor variables of its potential biodegradability pointed to the labile C pool as the main driver of the whole litter decay process, since this parameter alone explained more than the 60% of the total variance. For both species, a brief 13C depletion was detected at the beginning of litter incubation and a negative significant correlation between litter 13C contents and early C fluxes was observed. Despite the already mentioned differences between radiata and maritime pine litter layers, the overall general findings on potential biodegradability suggest a similar short-to-medium term behaviour of the soil organic inputs for both types of forest ecosystems under the envisaged global warming challenge.

Carbon and nitrogen mineralization in hierarchically structured aggregates of different size

Soil organic matter pools are turned over at different rates, but uncertainty persists regarding how far the hierarchically organized soil structure controls the mineralization dynamics. To better understand carbon and nitrogen mineralization in undisturbed aggregate classes (coarse aggregates: 2–6.3mm, fine aggregates: <2mm), we conducted a long-term (224-days) laboratory incubation experiment. A grassland soil (Haplic Cambisol) was chosen since its aggregates were not disturbed by tillage. The field-moist aggregate size classes were separated by a gentle dry-sieving method. We monitored the CO2–C and NH3–N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.5MK2SO4) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. By this approach, we could distinguish between the carbon and nitrogen mineralization processes of two soil aggregate size classes, relative to undisturbed bulk soil. The classes showed different aggregate architectures with variously sized subunits, but confirmed the aggregate hierarchy. For both aggregate classes, the recombined sum of respired CO2–C per unit of soil organic carbon equaled the bulk soil, proving that our aggregate separation preserved the original aggregates as intact functional units. Both aggregate size classes and the bulk soil respired only 4% soil organic carbon throughout the incubation period. The coarse aggregates, which mostly comprised small macroaggregates, mineralized more carbon per unit soil organic carbon than the fine aggregates (composed of microaggregates), indicating a higher bioavailability of soil organic matter in the coarse aggregates. Accordingly microbial metabolic efficiency was higher in coarse than in fine aggregates. Nitrogen mineralization was higher in fine aggregates than in coarse aggregates, but was impaired by carbon limitation as the incubation experiment proceeded. We conclude that bioavailability of soil organic matter was affected by different aggregate architectures in the different aggregate size classes. The lower bioavailability of soil organic matter in fine aggregates is due to an enhanced stabilization through cation bridging of the dolomitic soil material at the microaggregate level, whereas the higher soil organic matter bioavailability in coarse aggregates is explained by labile organic components at the macroaggregate level.

Microbial carbon recycling – an underestimated process controlling soil carbon dynamics – Part 1: A long-term laboratory incubation experiment

Abstract. Independent of its chemical structure carbon (C) persists in soil for several decades, controlled by stabilization and recycling. To disentangle the importance of the two factors on the turnover dynamics of soil sugars, an important compound of soil organic matter (SOM), a 3-year incubation experiment was conducted on a silty loam soil under different types of land use (arable land, grassland and forest) by adding 13C-labelled glucose. The compound-specific isotope analysis of soil sugars was used to examine the dynamics of different sugars during incubation. Sugar dynamics were dominated by a pool of high mean residence times (MRT) indicating that recycling plays an important role for sugars. However, this was not substantially affected by soil C content. Six months after label addition the contribution of the label was much higher for microbial biomass than for CO2 production for all examined land use types, corroborating that substrate recycling was very effective within the microbial biomass. Two different patterns of tracer dynamics could be identified for different sugars: while fucose and mannose showed highest label contribution at the beginning of the incubation with a subsequent slow decline, galactose and rhamnose were characterized by slow label incorporation with subsequently constant levels, which indicates that recycling is dominating the dynamics of these sugars. This may correspond to (a) different microbial growing strategies (r and K-strategist) or (b) location within or outside the cell membrane (lipopolysaccharides vs. exopolysaccharides) and thus be subject of different re-use within the microbial food web. Our results show how the microbial community recycles substrate very effectively and that high losses of substrate only occur during initial stages after substrate addition. This study indicates that recycling is one of the major processes explaining the high MRT observed for many SOM fractions and thus is crucial for understanding the global soil C cycle.

Long-term reindeer grazing limits warming-induced increases in CO2 released by tundra heath soil: potential role of soil C quality

The current climate warming in the Arctic may increase the microbial degradation of vast pools of soil carbon (C); however, the temperature sensitivity of decomposition is often highly dependent on the quality of accumulated soil C. Grazing by reindeer (Rangifer tarandus L.) substantially affects the dominant vegetation and often increases graminoids in relation to dwarf shrubs in ecosystems, but the effect of this vegetation shift on the soil C quality has not been previously investigated. We analyzed the soil C quality and rate of microbially mediated CO2 release at different temperatures in long-term laboratory incubations using soils from lightly grazed dwarf shrub-dominated and heavily grazed graminoid-dominated tundra ecosystem. The soil C quality was characterized by solid-state cross-polarization magic angle spinning (CPMAS 13C NMR) spectroscopy, which showed a higher relative proportion of carbohydrate C under light grazing and higher relative proportion of aliphatic not-O-substituted C under heavy grazing. Initial measurements showed lower temperature sensitivity of the CO2 release in soils under light grazing compared with soil under heavy grazing, but the overall CO2 release rate and its temperature sensitivity increased under light grazing as the soil incubation progressed. At the end of incubation, significantly more carbohydrate C had been lost in soils under light grazing compared with heavy grazing. These findings indicate that there may be a link between the grazer-induced effects on soil C quality and the potential of soils to release CO2 to atmosphere. We suggest that vegetation shifts induced by grazing could influence the proportion of accumulated soil C that is vulnerable to microbial degradation under warming climate.

Soil respiration is not limited by reductions in microbial biomass during long-term soil incubations

Declining rates of soil respiration are reliably observed during long-term laboratory incubations. However, the cause of this decline is uncertain. We explored different controls on soil respiration to elucidate the drivers of respiration rate declines during long-term soil incubations. Following a long-term (707day) incubation (30 °C) of soils from two sites (a cultivated and a forested plot at Kellogg Biological Station, Hickory Corners, MI, USA), soils were significantly depleted of both soil carbon and microbial biomass. To test the ability of these carbon- and biomass-depleted (“incubation-depleted”) soils to respire labile organic matter, we exposed soils to a second, 42day incubation (30 °C) with and without an addition of plant residues. We controlled for soil carbon and microbial biomass depletion by incubating field fresh (“fresh”) soils with and without an amendment of wheat and corn residues. Although respiration was consistently higher in the fresh versus incubation-depleted soil (2 and 1.2 times higher in the fresh cultivated and fresh forested soil, respectively), the ability to respire substrate did not differ between the fresh and incubation-depleted soils. Further, at the completion of the 42day incubation, levels of microbial biomass in the incubation-depleted soils remained unchanged, while levels of microbial biomass in the field-fresh soil declined to levels similar to that of the incubation-depleted soils. Extra-cellular enzyme pools in the incubation-depleted soils were sometimes slightly reduced and did not respond to addition of labile substrate and did not limit soil respiration. Our results support the idea that available soil organic matter, rather than a lack microbial biomass and extracellular enzymes, limits soil respiration over the course of long-term incubations. That decomposition of both wheat and corn straw residues did not change after major changes in the soil biomass during extended incubation supports the omission of biomass values from biogeochemical models.

Stability of Soil Carbon Stocks Varies with Forest Composition in the Canadian Boreal Biome

Changes in forest composition as a result of forest management, natural disturbances, and climate change may affect the accumulation of soil organic carbon (SOC). We examined the influence of common boreal tree species (trembling aspen, black spruce, and jack pine), either in pure stands or in conifer-broadleaf mixtures, on the amount, distribution, and quality of SOC in two regions of the Canadian boreal biome. Long-term laboratory incubations were used to assess SOC quality by quantifying proportions of fast carbon (C) (that is, proportion of total C released during the first 100 days of incubation) and active C (that is, modeled proportion of total C that can be potentially released). Total amounts of SOC did not differ between stand types, but the effects of stand type on SOC stocks and quality differed with soil depth. Among stand types, aspen stands had the greatest relative proportion of total SOC in deeper mineral layers and the lowest amount of active C in the organic layer. For these reasons, the SOC stock that developed under aspen was more stable than in the other stand types. Although black spruce stands allowed a greater accumulation of SOC in surface layers, these stocks, however, might become more vulnerable to extra losses if environmental conditions are to become more favorable to decomposition in the future. Our work highlights that boreal forest composition influences the stability of SOC stocks and how climate change could alter this large C pool.

Long-term net N mineralization potential as an indicator for soil fertility: chances and constraints

The soil capacity to supply nitrogen (N) to crops through decomposition of soil organic matter (SOM) is a main indicator for soil fertility. Assessment of a soil net N mineralization potential via long-term laboratory incubations accounts for many of the influencing factors as it integrates the activity of soil micro-organisms and to some extent, the physical accessibility of SOM. A two-pool first-order kinetic equation was used to fit cumulative net N mineralization data from incubations. It allowed the identification of a fast mineralizable N pool, Nfast (N in fresh residues), and a slow mineralizable N pool, Nslow, which mainly consists of humified SOM. Since the latter represents long-term soil N supply and is a mostly inherent soil property, it could be used as an indicator for soil fertility. We will present results from different studies: • A watershed study (NW-Germany), where reliable pedotransfer functions for site-specific estimates of pool size Nslow were derived. • A study in the Kalahari region, where the approach helped to understand the relevance of the soil net N mineralization potential for groundwater pollution with nitrate. • A study aiming to develop an internet-service for field-specific simulations of current net N mineralization (NW-Germany).

Effect of pressure on silica solubility of diatom frustules in the oceans: Results from long-term laboratory and field incubations

The oceanic cycle of silicon (Si) has been studied extensively due to its close coupling to the oceanic carbon cycle and the biological CO2 pump. The oceanic Si cycle is dominated by the uptake of dissolved silicate (dSi) by planktonic organisms, predominantly diatoms, which use it to synthesize siliceous frustules. As oceanic waters are undersaturated with respect to biogenic silica (bSiO2) the frustules dissolve after death of the organisms, thereby regenerating dSi. Because the dissolution rate of bSiO2 depends on the degree of undersaturation, the thermodynamic solubility of bSiO2 is a key parameter controlling the recycling efficiency of nutrient Si in the water column and sediments. While an extensive body of data exists describing the dependence of bSiO2 solubility on temperature, the effect of pressure on the solubility of natural diatom frustules has never been measured directly. In this study, we conducted long-term (up to 22months) laboratory and field equilibration experiments to determine the solubility of cleaned frustules of a cultured marine diatom (Thalassiosira punctigera) in seawater, for pressures between 1 and 700bar, and temperatures between 2 and 21°C. According to our results, the solubility of the frustules decreases by about 10% when pressure increases from 1 to ~200bar. From 200bar on, the pressure dependence reverses, and at 700bar the solubility is about 15% higher than at atmospheric pressure. Integrated over an average oceanic water depth of 4000m, a drop in temperature of 15–20°C has a far more significant effect on the solubility of bSiO2 than a corresponding 400bar increase in pressure.

Biodiversity, Nitrogen Deposition, and CO2 Affect Grassland Soil Carbon Cycling but not Storage

Grasslands are globally widespread and capable of storing large amounts of carbon (C) in soils, and are generally experiencing increasing atmospheric CO2, nitrogen (N) deposition, and biodiversity losses. To better understand whether grasslands will act as C sources or sinks in the future we measured microbial respiration in long-term laboratory incubations of soils collected from a grassland field experiment after 9 years of factorial treatment of atmospheric CO2, N deposition, and plant species richness on a deep and uniformly sandy soil. We fit microbial soil respiration rates to three-pool models of soil C cycling to separate treatment effects on decomposition and pool sizes of fast, slow, and resistant C pools. Elevated CO2 decreased the mean residence time (MRT) of slow C pools without affecting their pool size. Decreasing diversity reduced the size and MRT of fast C pools (comparing monocultures to plots planted with 16 species), but increased the slow pool MRT. N additions increased the size of the resistant pool. These effects of CO2, N, and species-richness treatments were largely due to plant biomass differences between the treatments. We found no significant interactions among treatments. These results suggest that C sequestration in sandy grassland soils may not be strongly influenced by elevated CO2 or species losses. However, high N deposition may increase the amount of resistant C in these grasslands, which could contribute to increased C sequestration.

Effects of carbon and nitrogen amendment on soil carbon and nitrogen mineralization in volcanic immature soil in southern Kyushu, Japan

Soil N mineralization is affected by microbial biomass and respiration, which are limited by available C and N. To examine the relationship between C and N for soil microbial dynamics and N dynamics, we conducted long-term laboratory incubation (150 days) after C and N amendment and measured changes in C and N mineralization, microbial biomass C, and dissolved C and N throughout the incubation period. The study soil was volcanic immature soil from the southern part of Japan, which contains lower C and N compared with other Japanese forest soils. Despite this, the area is covered by well-developed natural and plantation forests. Carbon amendment resulted in an increase in both microbial biomass and respiration, and net N mineralization decreased, probably due to increasing microbial immobilization. In contrast, N amendment resulted in a decrease in microbial respiration and an increase in net N mineralization, possibly due to decreased immobilization by microbes. Amendment of both C and N simultaneously did not affect microbial biomass and respiration, although net N mineralization was slightly increased. The results suggested that inhibitory effect on microbial respiration by N amendment should be reduced if carbon availability is higher. Thus, soil available C may limit microbial biomass and respiration in this volcanic immature soil. Even in immature soil where C and N substrate is low, soil C, such as plant root exudates and materials from above- and belowground dead organisms, might help to maintain microbial activity and N mineralization in this study site.

Controls on soil carbon accumulation during woody plant encroachment: Evidence from physical fractionation, soil respiration, and δ 13C of respired CO 2

Woody plant encroachment into grasslands and savannas is a globally extensive land-cover change that alters biogeochemical processes and frequently results in soil organic carbon (SOC) accrual. We used soil physical fractionation, soil respiration kinetics, and the isotopic composition of soil respiration to investigate microbial degradation of accrued SOC in sandy loam soils along a chronosequence of C 3woody plant encroachment into a C 4-dominated grassland in southern Texas. Our previous work in this system demonstrated significant changes in the chemistry and abundance of lignin and aliphatic biopolymers within particulate soil fractions during the first 40 yrs of woody plant encroachment, indicating selective accrual of purportedly more recalcitrant plant chemicals. However, during the long-term soil laboratory incubation presented herein, a greater proportion of SOC was mineralized in soils from older woody stands (34–86 yrs) than in soils from younger woody stands (14–23 yrs) and grasslands, providing no evidence for greater biochemical recalcitrance as a controlling mechanism for SOC accrual. In addition, δ 13C values of respired CO 2 indicate that the mineralized SOC was predominately of C 3 origin from all woody stands along the chronosequence, and that respired CO 2 was primarily derived from the free light fraction (density <1.0 g/cm 3) and macroaggregate-sized soil fraction. Our data suggested that the location of SOC among soil fractions was more important than plant polymer chemistry in determining SOC turnover rates during incubation. Surprisingly, estimates of the size and turnover rate of the active SOC pool based on respiratory kinetics did not increase with woody encroachment, and the turnover rate of the slower SOC pool decreased, again supporting the notion that increases in biochemically recalcitrant biopolymers did not hinder decomposition in the lab. These data indicate environmental conditions that may allow for C accrual in the field were alleviated during the controlled incubation. Therefore, C accrual in these sandy loam soils following woody encroachment should not be assumed stable, and this factor should be taken into account when considering responses of SOC to climate change or when making management decisions regarding land cover impacts on SOC.