Adsorption and desorption of organotin compounds in organic and mineral soils

Organotin compounds (OTC) are deposited from the atmosphere into terrestrial ecosystems and can accumulate in soils. We studied the adsorption and desorption of methyltin and butyltin compounds in organic and mineral soils in batch experiments. The adsorption and desorption isotherms for all species and soils were linear over the concentration range of 10–100 ng Sn ml^?1. The strength of OTC adsorption correlated well with the carbon content and cation exchange capacity of the soil and was in the order mono‐ > di‐ > tri‐substituted OTCs and butyltin > methyltin compounds. The OTC adsorption coefficients were much larger in organic soils (K_d > 10⁴) than in mineral soils. The adsorption and desorption showed a pronounced hysteresis. Trimethyltin adsorption was partly reversible in all soils (desorption 2–12% of the adsorbed amounts). Dimethyltin, tributyltin and dibutyltin exhibited reversible adsorption only in mineral soils (desorption 4–33% of the adsorbed amounts). Mono‐substituted OTCs adsorbed almost irreversibly in all soils (desorption < 1% of adsorbed amounts). Trimethyltin was more mobile and more bioavailable in soils than other OTCs. It might therefore be leached from soils and accumulate in aquatic ecosystems. The other OTCs are scarcely mobile and are strongly retained in soils.     

Mercury and Methylmercury in Upland and Wetland Acid Forest Soils of a Watershed in NE-Bavaria,Germany

Mercury (Hg) and methylmercury (CH₃Hg⁺) are global pollutants, but little information is available on their distribution and mobility in soils and catchments of Central Europe. The objective of this study was to investigate the pools and mobility of Hg and CH₃Hg⁺ in different forest soils. Upland and wetland forest soils, soil solutions and runoff were sampled. In upland soils the highest contents of total-Hg were found in the Oh layer of the forest floor (>400 ng g^-1) and the storage of non geogenic total-Hg (calculated for 60 cm depth) was about 120 mg/m². The storage of total-Hg was one order of magnitude lower in wetland soils as compared to the upland soils. By far the largest proportion of total-Hg in soils was bound in immobile fractions. The depth gradients of CH₃Hg⁺ did not correspond to those of total-Hg and the highest contents of CH₃Hg⁺ in upland soils were observed in the litter layer of the forest floor and in the Bsv horizon. The CH₃Hg⁺ content of the wetland soils was generally much higher in comparison with upland soils. CH₃Hg⁺ in solution was found in the forest floor percolates of upland soils and in wetland soils, but not in soil solutions from mineral soil horizons. Gaseous losses of Hg as well as methylation of Hg are likely in wetland soils. The latter might be highly relevant for CH₃Hg⁺ levels in runoff.     

Short‐Term Recovery of Ammonium‐15Nitrogen Applied to a Temperate Forest Inceptisol and Ultisol in East Tennessee,USA

Abstract

The short‐term fate and retention of ammonium (NH₄)‐¹⁵nitrogen (N) applied to two types of forest soils in east Tennessee was investigated. Four ridgetop forests, predominantly oak (Quercus spp.), were studied. Five applications of NH₄‐¹⁵N tracer were made to the forest floor at 2‐ to 4‐week intervals over a 14‐week period in 2004. Nitrogen‐15 recovery in the forest floor, fine roots (<2 mm), and the mineral soil (0–20 cm) was calculated at 6, 21, and 42 weeks after the last application. Most of the ¹⁵N was retained in the forest floor and the mineral soil, with only small amounts (≤2%) found in roots from both soil layers. Recovery of NH₄‐¹⁵N was greater in Inceptisols, which had a wider carbon (C)‐to‐N ratio than Ultisols. For both soil types, higher NH₄‐¹⁵N recoveries and long retention times (half‐lives>100 weeks) indicated the forest floor is an effective filter for atmospheric N inputs.     

Effect of land use on forest floor and soil of a Quercus suber L. forest in Gallura (Sardinia,Italy)

In order to determine the effect of land use on forest floor and soil, two adjacent sites with different land use were investigated in Gallura (northern Sardinia, Italy). One site is a Quercus suber L. forest mainly used for cork production and the other is an open Quercus suber L. forest where livestock is put out to graze. In each site one soil profile was studied to characterize the mineral soil, and five humus profiles were opened along a vegetation transect, were studied to characterize the forest floor. Samples of L, F and H horizons of the forest floor and of the A mineral horizons were collected and analysed for each profile. In the site mainly used for cork production well‐developed ectorganic (L, F and H) horizons are always present, with a total thickness ranging from 5·2 to 9·5 cm. Humus profile is of the Moder type, while mineral soils have an A–C profile, generally 50 cm deep. Organic matter content in the forest floor ranges from 1·76–3·72 kg m⁻² and nutrients content in the mineral soil is high. In the site used chiefly for grazing the ectorganic horizons are very poorly developed, with a total thickness ranging from 1–3 cm, except for some islands under the Quercus suber L. canopy where the total thickness may reach 5·3 cm. Humus profile is of the Mull type, but the used classification system seems not appropriate when the tree density is below a critical limit. Mineral soils have an A–C profile 20–25 cm deep. The organic matter content in the forest floor ranges from 0·45 to 1·84 kg m⁻², while nutrient content in the mineral soil maintains at high level, even higher than in the former case for C, N and Ca, probably in relation with higher supply of cattle excreta. Sheet erosion is evident in the site. It is concluded that cork production will maintain a sustainable forest floor development in cork–oak forest ecosystem, whereas cattle grazing, fires and ploughing in cork–oak forests may be considered to trigger off severe soil degradation processes. Copyright © 2000 John Wiley & Sons, Ltd.     

Short‐term response on the quantity and quality of rhizo‐deposited carbon from Norway spruce exposed to low and high N inputs

The amount and composition of rhizo‐deposition as an important component of dissolved organic carbon (DOC) in forest soils is largely unknown. The objective of our study was to examine one component of DOC production in forest soils, namely rhizo‐deposition, under conditions of low, moderate, and high inorganic‐nitrogen (N) input. Two‐year‐old Picea abies saplings were incubated in hydroponic solution for a period of 2 weeks, comprising an adaptation and an experimental period of each 1 week. Treatments included 0 (zero N), 1, 10 (moderate N), and 100 (high N) mg N L^–1 nitrogen addition split equally between nitrate and ammonium. Quantity and quality of dissolved‐organic‐carbon (DOC) production were investigated. Cumulative DOC production ranged between 3.7 and 8.8 mg C (g root dry weight)^–1 (7 d)^–1 and was significantly reduced in zero‐ and high‐N treatments. In turn, the specific UV absorbance as a measure of the aromaticity increased significantly in zero‐ and high‐N treatments, indicating a reduced contribution of microbial products to rhizo‐deposited C. Mineralization of rhizo‐deposited DOC was not significantly different between N treatments at day 1 of the experimental period, but by day 7, 69% of rhizo‐deposition in zero‐N hydroponic solution was mineralizable compared to 37%–49% for the moderate‐ and high‐N treatments. Increasing N input tended to result in decreased degradability. We conclude from this short‐term experiment that rhizo‐deposited DOC from Norway spruce displays a fast response to changes in inorganic N.     

Microsite assessment of forest soil nitrogen,phosphorus, and potassium supply rates in‐field using ion exchange membranes

Abstract

A new method for microsite assessment of soil nutrient supply in forest soil was developed. The method involves the use of ion exchange membranes to assess differences in soil nitrogen (N), phosphorus (P), and potassium (K) supply rates in‐field over small depth increments in the forest floor (i.e., the L, F, and H horizons). Ion exchange membranes were buried and retrieved from the forest floor in an aspen forest stand in Saskatchewan, Canada. Small (6 mm diameter) sections of the membrane were cut out and ion concentration on the sections measured to provide a nutrient supply rate at that location. Soil nutrient supply rates at the site ranged from 4.6–6.0, 7.3–8.5, 11.6–21.5, and 122–196μg 10 cm²#lb2 h^‐1 for NH₄ ⁺‐N, NC₃ ^‐‐N, P, and K, respectively. On average, the highly humified H horizon had the highest N and P supply rates, followed by the F horizon, with the surface litter (L horizon) having the lowest N supply rates. The simplicity and sensitivity of the procedure make this method appropriate for in‐field assessment of differences in soil nutrient supply over small vertical and horizontal distance and was especially appropriate for the forest floor horizons in forest soils.     

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant‐derived carbohydrates seem easily degradable. Carbohydrates and N‐rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co‐)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from <30 y in solution to >90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co‐)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM‐derived soil C ranges from 20 to 55 Mg ha^–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co‐)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM‐derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co‐)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co‐)precipitation with Al and Fe to DOM retention.     

Runoff Development and Soil Erosion in a Wet Tropical Montane Setting under Coffee Cultivation

The western interior portions of Puerto Rico offer optimal climatic conditions for coffee cultivation. However, land and water degradation result when abrupt topographic relief and high annual rainfall combine with forest conversion for coffee farming. Small‐scale rainfall simulation experiments were conducted to quantify runoff and erosion from four land surface types (i.e., mulched, weed‐covered, and bare soils under active cultivation, and unpaved roads) representative of coffee farms in Puerto Rico. Results show that mulch‐covered soils had runoff coefficients similar to those from undisturbed forested conditions (~4%), and that they eroded at rates about a quarter of those for bare cultivated soils. Weed‐covered soils had surprisingly high runoff coefficients (~70%), yet their erosion rates were only three‐fourths of those for bare soils. Annualized erosion rates from unpaved roads were 65 Mg ha⁻¹ y⁻¹, or ten times greater than bare soils and about a hundred times higher than weed‐ or mulch‐covered surfaces. Farm‐scale sediment production estimates amount to ~11 Mg ha⁻¹ y⁻¹, about two‐orders of magnitude higher than under forested conditions. At the farm‐scale, only 2 – 8% of the total sediment is potentially attributable to cultivated hillslopes. In contrast, unpaved roads may account for over 90% of the sediment budget, even though they comprise only 15% of the farm surface area. Therefore, while providing mulch or a vegetative cover to bare cultivated soils should be part of effective soil management, mitigating the effects of coffee cultivation on downstream water resources must focus on the unpaved road network as the primary sediment source. Copyright © 2016 John Wiley & Sons, Ltd.     

USE OF FORMERLY PLOWED LAND IN NEW ENGLAND TO MONITOR THE VERTICAL DISTRIBUTION OF LEAD,ZINC AND COPPER IN MINERAL SOIL

In this study, we used once-plowed lands that have returned to forest for over 50 years to study the vertical distribution of meteorologically-deposited lead. These mineral soils were an essentially homogeneous 20 cm-thick layer when last plowed. As such, they were effectively a “clean slate” upon which pollutants deposited since the last plowing can be measured without the confounding aspects of well-developed natural soil horizons and the spatial heterogeneity of native forest soils. The concentration and amount of lead as well as copper and zinc, biologically active metals, were measured at five sites in New England. In the mineral soil, copper content ranged from 25 mg cm depth^-1 m^-2 at 0–2 cm depth to 37 mg cm depth^-1 m^-2 at 6–8 cm depth, but showed no consistent pattern with depth at all sites. Zinc concentrations and amounts increased with depth in the mineral soil to 14 μg g^-1 and 167 mg cm depth^-1 m^-2, respectively. In contrast, lead showed a decrease with depth from 350 mg cm depth^-1 m^-2 at 0–2 cm depth to 102–108 mg cm depth^-1 m^-2 between 10 and 20 cm depth. At all five sites, decreases in lead concentration with depth were correlated with decreases in the amount of organic mater. Amounts of total lead deposited since the abandonment from plowing have been estimated at 1.4 g m^-2 in rural sites. Thirty-five percent of this presumably anthropogenically-derived lead was in the forest floor; the remaining 65% was in the upper mineral soil.     

Interaction between earthworms and arbuscular mycorrhizal fungi on the degradation of oxytetracycline in soils

The interactive impact of earthworms (Eisenia fetida) and arbuscular mycorrhizal fungi (Rhizophagus intraradices, AM fungi) on the degradation of oxytetracycline (OTC) in soils was studied under greenhouse conditions. Treatments included maize plants inoculated vs. not inoculated with AM fungi and treated with or without earthworms at low (1 mg kg⁻¹ soil DM) or high (100 mg kg⁻¹ soil DM) OTC rates. The root colonization rate, the hyphal density of mycorrhizae, the residual OTC concentration in soils, catalase, dehydrogenase, urease, soil microbial biomass C, Shannon–Wiener index (H) for microbial communities from T-RFLP profiles were measured at harvest. The results indicated that earthworms and AM fungi would individually or interactively enhance OTC decomposition and significantly decreased the residual OTC concentration at both high and low OTC rates. Both earthworms and AM fungi could promote the degradation of OTC by increasing soil microbial biomass C at both high and low OTC rates. The effect of soil enzyme activity and soil microbial diversity on OTC decomposition was different between high and low OTC rates. Hyphomicrobium and Bacillus cereus were dominant bacteria, and Thielavia and Chaetomium were dominant phyla of fungi at all occasions. Earthworm activity stimulated the growth of Hyphomicrobium and Thielavia, while AM fungi may stimulate B. cereus, Thielavia and Chaetomium, resulting in greater OTC decomposition. The interaction between earthworms and AM fungi in affecting the degradation of OTC may be attributed to different mechanisms, depending on soil microbial biomass, function (enzyme activity) and communities (the abundance of Hyphomicrobium, B. cereus, Thielavia and Chaetomium) in the soil.     

Carbon mineralization rates at different soil depths across a network of European forest sites (FORCAST)

Most of the carbon (C) in terrestrial ecosystems is stored in the mineral soil layers. Thus, the response of the mineral soil to potential increases in temperature is crucial for the prediction of the impact of climate change on terrestrial ecosystems. Samples from three mineral soil layers were collected from eight mature forest sites in the European network CARBOEUROFLUX and were incubated at four temperatures (4, 10, 20 and 30°C) for c. 270 days. Carbon mineralization rates were related to soil and site characteristics. Soil water holding capacity, C content, nitrogen (N) content and organic matter all decreased with soil depth at all sites, with significantly larger amounts of organic matter, C and N in the top 0–5 cm of mineral soil than in the deeper layers. The conifer forest soils had significantly lower pH, higher C/N ratios and carbon contents in the top 5 cm than the broadleaf forest soils. Carbon mineralization rates decreased with soil depth and time at all sites but increased with temperature, with the highest rates measured at 30°C for all sites. Between 50 and 70% of the total C respired after 270 days of incubation came from the top 5 cm. The percentage C loss was small in all cases, ranging from 1 to 10%. A two‐compartment model was fitted to all data to derive the labile/active and slow/recalcitrant fractions, as well as their decomposition constants. Although the labile fraction was small in all cases, we found significantly larger amounts of labile C in the broadleaf forest soils than in the conifer forest soils. No statistically significant differences were found in the temperature sensitivity parameter Q₁₀ among sites, soil layers or between conifer and broadleaf soils. The average Q₁₀ for all soils was 2.98 (± 0.10). We found that despite large differences among sites, C mineralization can be successfully predicted as a combined function of site leaf area index, mean annual temperature and content of labile carbon in the soil (R² = 0.93).     

Potential fluxes of N2O and CH4 from soils of three forest types in Eastern Canada

We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N₂O) and methane (CH₄) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH₄NO₃ amendment and an elevated headspace CH₄ concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N₂O and CH₄ fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N₂O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N₂O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH₄ when incubated under an elevated headspace CH₄ concentration, except the poorly drained soils in the deciduous forest, which emitted CH₄. CH₄ consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N₂O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH₄ unlike the well-drained soils, which are net sinks of atmospheric CH₄. Because wetland soils can produce a substantial amount of CH₄ and N₂O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.     

Release Kinetics of Nonexchangeable Potassium by Different Extractants from Soils of Varying Mineralogy and Depth

Abstract

Nonexchangeable potassium (K) release kinetics of six major benchmark soil series of India as affected by mineralogy of clay and silt fractions, soil depth and extraction media was investigated. The cumulative release of nonexchangeable K was greater in smectitic soils (353 mg K kg^?1 at 0‐ to 15‐cm depth and 296 mg K kg^?1 at 15‐ to 30‐cm depth, averaged for 2 soils and 3 extractants) than in illitic (151 mg K kg^?1 at 0‐ to 15‐cm depth and 112 mg K kg^?1 at 15‐ to 30‐cm depth) and kaolinitic (194 mg K kg^?1 at 0‐ to 15‐cm depth and 167 mg K kg^?1 at 15‐ to 30‐cm depth) soils. Surface soils exhibited larger cumulative K release in smectitic and illitic soils, whereas subsurface soils had larger K release in kaolinitic soils. Among the extractants, 0.01 M citric acid extracted a larger amount of nonexchangeable K followed by 0.01 M CaCl₂ and 0.01 M HCl. The efficiency of citric acid extractant was greater in illitic soils than in smectitic and kaolinitic soils. Release kinetics of nonexchangeable K conformed fairly well to parabolic and first‐order kinetic models. The curve pattern of parabolic diffusion model suggested diffusion controlled kinetics in all the soils, with a characteristic initial fast rate up to 7 h followed by a slower rate. Greater nonexchangeable K release rates in smectitic soils, calculated from the first‐order equation (b=91.13×10^?4 h^?1), suggested that the layer edge and wedge zones and swelling nature of clay facilitated the easier exchange. In contrast to smectitic soils, higher release rate constants obtained from parabolic diffusion equation (b=39.23×10^?3 h^?1) in illitic soils revealed that the low amount of exchangeable K on clay surface and larger amount of interlayer K allowed greater diffusion gradients, thus justifying the better fit of first‐order kinetic equation in smectitic soils and parabolic diffusion equation in illitic soils.     

Mineralization of dissolved organic carbon in mineral soil solution of two forest soils

Dissolved organic carbon (DOC) constitutes an important carbon input flux to forested mineral soils. Seepage from mineral subsoils contains only small amounts of DOC because of mineralization, sorption or the formation of particulate organic matter (POM). However, the relation between these processes is largely unknown. Therefore, the objective of this study was to quantify the mineralization of DOC from different depths of forest soils, and to determine degradation rate constants for rapidly and slowly degradable DOC pools. Mineralization of DOC and formation of POM in mineral soil solution from two forested sites in northern Bavaria (Germany) were quantified in a 97 days laboratory incubation experiment. Furthermore, spectroscopic properties such as specific UV absorption and a humification index derived from fluorescence emission spectrometry were measured before and after incubation. DOC in all samples turned out to belong mainly to the stable DOC pool (> 95 %) with half‐lives ranging from years to decades. Spectroscopic properties were not suitable to predict the mineralization of DOC from mineral soils. However, together with data on DOC from the forest floor and long‐term data on DOC concentrations in the field they helped to identify the processes involved in C sequestration in mineral subsoils. Mineralization, formation of POM, and probably sorption seem all to be responsible for maintaining low concentrations of DOC in the upper mineral soil. DOC below the upper mineral soil is highly resistant to mineralization, and thus the further decrease of DOC concentrations in the subsoil as observed under field conditions cannot be attributed to mineralization. Our results suggest that sorption and to some minor extent the formation of POM may be responsible for C sequestration in the subsoil.     

How relevant is recalcitrance for the stabilization of organic matter in soils?

Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long‐term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water‐extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction (<20 µm), indicating physico‐chemical stabilization. Compound‐specific analysis of ¹³C : ¹²C ratios of SOM pyrolysis products in soils with C3‐C4 crop changes revealed no compounds with mean residence times of > 40–50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low ¹⁴C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short‐term (60 d) and a long‐term (2 y) incubation experiment. This degradation was enhanced, when glucose was supplied as an easily available microbial substrate. Similarly, SOM mineralization in many soils generally increased after addition of carbohydrates, amino acids, or simple organic acids, thus indicating that stability may also be caused by substrate limitations. It is concluded that the presented results do not provide much evidence that the selective preservation of recalcitrant primary biogenic compounds is a major SOM‐stabilization mechanism. Old SOM fractions with slow turnover rates were generally only found in association with soil minerals. The only not mineral‐associated SOM components that may be persistent in soils appear to be black and fossil C.     

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.     

Alberta油砂地区在两种水文流域森林土壤酸化敏感性研究

Input of large amounts of N and S compounds into forest ecosystems through atmospheric deposition is a significant risk for soil acidification in the oil sands region of Alberta. We evaluated the sensitivity of forest soils to acidification in two watersheds （Lake 287 and Lake 185） with contrasting hydrological regimes as a part of a larger project assessing the role of N and S cycling in soil acidification in forest ecosystems. Fifty six forest soil samples were collected from the two watersheds by horizon from 10 monitoring plots dominated by either jack pine （Pinus banksiana） or aspen （Populus tremuloides）. Soils in the two watersheds were extremely to moderately acidic with pH （CaCl2） ranging from 2.83 to 4.91. Soil acid-base chemistry variables such as pH, base saturation, Al saturation, and acid-buffering capacity measured using the acetic acid equilibrium procedure indicated that soils in Lake 287 were more acidified than those in Lake 185. Acid-buffering capacity decreased in the order of forest floor 〉 subsurface mineral soil 〉 surface mineral soil. The most dramatic differences in percent Ca and Al saturations between the two watersheds were found in the surface mineral soil horizon. Percent Ca and Al saturation in the surface mineral soil in Lake 287 were 15% and 70%, respectively; the percent Ca saturation value fell within a critical range proposed in the literature that indicates soil acidification. Our results suggest that the soils in the two watersheds have low acid buffering capacity and would be sensitive to increased acidic deposition in the region.     

Capability of Loss‐on‐Ignition as a Predictor of Total Organic Carbon in Non‐Calcareous Forest Soils

Abstract

Accurate analyses of large numbers of soil samples are needed in order to reduce the uncertainty of carbon inventories. Loss‐on‐ignition (LOI) is still considered the most convenient assessment method, but its accuracy and precision for predicting total organic carbon (TOC) is questioned. However, our estimation of measurement precision for different samples showed comparable relative standard deviations (RSDs) for LOI and TOC determinations. Highest precision was found in forest floor samples (RSD<1.2%) and lowest (RSD 5–10%) in sandy soil samples low in organic matter. Forest floor samples (n=66) and non‐calcareous mineral soil samples (n=654) were used to calibrate and validate predictive equations. Excellent linear relationships were found. For a wide range of soils the bivariate predictive equation TOC=?0.1046 Clay+0.5936 LOI (r²=0.98) was developed and validated. After correction for clay content, slopes averaged remarkably close to the traditional 0.58 conversion factor.     

Dynamics of 13C‐labeled mustard litter (Sinapis alba) in particle‐size and aggregate fractions in an agricultural cropland with high‐ and low‐yield areas

The application of ¹³C‐labeled litter enables to study decomposition processes as well as the allocation of litter‐derived carbon to different soil C pools. ¹³Carbon‐labeled mustard litter was used in order to compare decomposition processes in an agricultural cropland with high‐yield (HY) and low‐yield (LY) areas, the latter being characterized by a finer texture and a lower organic‐C (OC) content. After tracer application, ¹³C concentrations were monitored in topsoil samples in particulate organic matter (POM) and in fine mineral fractions (silt‐ and clay‐sized fractions). After 568 d, approximately 5% and 10% of the initial ¹³C amount were found in POM fractions of LY and HY areas, respectively. Higher amounts were found in POM occluded in aggregates than in free POM. Medium‐term (0.5–2 y) storage of the initial ¹³C in fine silt‐ and clay‐sized fractions amounts to 10% in HY and LY soils, with faster enrichment but also faster disappearance of the ¹³C signal from LY soils. Amounts of 80%–90% of the added ¹³C were mineralized or leached in the observed period. Decomposition of free POM was faster in HY than in LY areas during the first year, but the remaining ¹³C amounts in occluded‐POM fractions were higher in HY soils after 568 d. High‐yield and low‐yield areas showed different ¹³C dynamics in fine mineral fractions. In LY soils, ¹³C amounts and concentrations in mineral‐associated fractions increased within 160 d after application and decreased in the following time period. In HY areas, a significant increase in ¹³C amounts did not occur until after 568 d. The results indicate initially faster decomposition processes in HY than in LY areas due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote a faster aggregate formation and turnover, leading to a closer contact between POM and mineral surfaces in this area. This favors the OC storage in fine mineral fractions in the medium term. Lower aggregate formation and turnover in the coarser textured HY soil leads to a delayed C stabilization in silt‐ and clay‐sized fractions.     

Dynamics and origin of the non-hydrolysable organic fraction in a forest and a cultivated temperate soil, as determined by isotopic and microscopic studies

We isolated the non‐hydrolysable macromolecular organic fraction (insoluble fraction resistant to drastic laboratory hydrolyses) from a temperate, loamy, forest soil (Lacadée, France) and from the soil of an adjacent plot cleared 35 years ago and continuously cropped with maize. The quantitative, morphological (light, scanning and transmission electron microscopy) and isotopic (bulk δ¹³C, individual compound δ¹³C and radiocarbon dating) features of these two non‐hydrolysable fractions were determined and compared. It appeared that: (i) extensive degradation of the non‐hydrolysable material inherited from the forest soil occurred upon cropping, revealing that its resistance to drastic laboratory hydrolyses is not paralleled by a great resistance to natural biodegradation triggered by change in land use; (ii) only small inputs of maize‐derived compounds occurred in the non‐hydrolysable fraction of the cultivated soil so that, in spite of extensive degradation, the forest‐inherited carbon still predominates; (iii) the non‐hydrolysable fractions of both soils comprise the same components (wood debris, spores, pollen, and, predominantly, granular organic aggregates), which correlate with the previously identified chemical components (charcoal, aliphatic lipid components and melanoidin‐like components); (iv) the non‐hydrolysable fraction of the cropped soil shows a greater contribution of aliphatic moieties, reflecting differential degradation of the components of the non‐hydrolysable material inherited from the forest soil; (v) this degradation resulted in enrichment in the oldest components; and (vi) no relationship is observed, in the two Lacadée soils, between resistance to drastic laboratory hydrolyses, on the one hand, and stability towards biodegradation in situ, on the other. These observations, added to recent ones on other types of soils, suggest that such a conspicuous uncoupling between non‐hydrolysable and stable carbon is probably a general feature of organic matter in soil as opposed to sedimentary organic matter.