二氧化碳升高与干旱对植物生理、土壤碳及土壤酶活的影响

Global climate models have indicated high probability of drought occurrences in the coming future decades due to the impacts of climate change caused by a mass release of CO2.Thus,climate change regarding elevated ambient CO2 and drought may consequently affect the growth of crops.In this study,plant physiology,soil carbon,and soil enzyme activities were measured to investigate the impacts of elevated CO2 and drought stress on a Stagnic Anthrosol planted with soybean (Glycine max).Treatments of two CO2 levels,three soil moisture levels,and two soil cover types were established.The results indicated that elevated CO2 and drought stress significantly affected plant physiology.The inhibition of plant physiology by drought stress was mediated via prompted photosynthesis and water use efficiency under elevated CO2 conditions.Elevated CO2 resulted in a longer retention time of dissolved organic carbon (DOC) in soil,probably by improving the soil water effectiveness for organic decomposition and mineralization.Drought stress significantly decreased C:N ratio and microbial biomass carbon (MBC),but the interactive effects of drought stress and CO2 on them were not significant.Elevated CO2 induced an increase in invertase and catalase activities through stimulated plant root exudation.These results suggested that drought stress had significant negative impacts on plant physiology,soil carbon,and soil enzyme activities,whereas elevated CO2 and plant physiological feedbacks indirectly ameliorated these impacts.     

The carbon we do not see—the impact of low molecular weight compounds on carbon dynamics and respiration in forest soils: a review

Dissolved organic matter (DOM), typically quantified as dissolved organic carbon (DOC), has been hypothesized to play many roles in pedogenesis and soil biogeochemical cycles, however, most research to date concerning forest soils has focussed on the high molecular weight (HMW) components of this DOM. This review aims to assess the role of low molecular weight (LMW) DOM compounds in the C dynamics of temperate and boreal forest soils focussing in particular on organic acids, amino acids and sugars. The current knowledge of concentrations, mineralization kinetics and production rates and sources in soil are summarised. We conclude that although these LMW compounds are typically maintained at very low concentrations in the soil solution (<50 μM), the flux through this pool is extremely rapid (mean residence time 1-10 h) due to continued microbial removal. Due to this rapid flux through the soil solution pool and mineralization to CO₂, we calculate that the turnover of these LMW compounds may contribute substantially to the total CO₂ efflux from the soil. Moreover, the production rates of these soluble transitory compounds could exceed HMW DOM production. The possible impact of climate change on the behaviour of LMW compounds in soil is also discussed.     

Fast turnover of low molecular weight components of the dissolved organic carbon pool of temperate grassland field soils

Large amounts of low molecular weight (LMW;<250 Da) carbon (C) are lost from roots into the rhizosphere as a consequence of root turnover and exudation. Their rates of turnover after release into the soil remain poorly understood. We extracted soil solution from a temperate grassland Eutric Cambisol, isotopically labeled the glucose and amino acid components, and then re-injected the solution back into the soil. We followed the subsequent evolution of ¹⁴CO₂ and incorporation of the LMW C into the soil microbial biomass or grasses for 48 h. The experiments were performed both on grazed and un-grazed swards in the field, and in the laboratory. In the field, we showed that glucose and amino acids had short half-lives (t_1/2) in soil solution (t_1/2=20-40 min), but that they persisted in soil microbes for much longer. A first-order double exponential model fitted the experimental data well and gave rate constant (k) values of 1.21-2.14 h⁻¹ for k₁ and 0.0025-0.0048 h⁻¹ for k₂. Only small amounts of the added ¹⁴C were recovered in plant biomass (<5% of total added to soil) indicating that plant roots are poor competitors for LMW dissolved organic C (DOC) in comparison to soil microorganisms. The first phase of glucose and amino acid mineralization in the laboratory was slower (t_1/2=40-60 min) than measured in the field reinforcing the importance of making flux measurements in situ. Whilst grazing stimulated below-ground respiration, it exerted only a small influence on the turnover of LMW DOC suggesting that the increase in respiration was due to increased root respiration and not turnover of soil organic matter (SOM). Our results suggest that some components of the LMW DOC pool are turned over extremely rapidly (ca. 4000 times annually).     

Effect of temperature on below-ground N-dynamics in a weedy model ecosystem at ambient and elevated atmospheric CO2 levels

Model multispecies terrestrial communities composed of four trophic levels (plants, herbivores, parasitoids, decomposers) were established in the Ecotron controlled environment facility. Two experimental runs enabled us to investigate the effects of enhanced temperature on below-ground microbial processes (N-mineralisation, urease, arginine deaminase, protease activity and potential denitrification) in both ambient and elevated (ambient +200 ppm) CO₂ atmospheres.The enzyme activities involved in nitrogen cycling showed weak responses to elevated temperature in both experimental runs. In the Ambient CO₂ Run, protease and arginine deaminase values tended to be lower in elevated temperature; on the other hand, N-mineralisation, urease and denitrification enzyme activity (DEA) were higher. In the Elevated CO₂ Run, all microbial variables showed higher activities at elevated temperature, although only the results for DEA and arginine deaminase were statistically significant. The interaction between higher temperature and elevated CO₂ weakly affected root growth and tissue C:N ratio, limiting feedbacks into the microbial community.Besides temperature and CO₂, substrate availability, water stress and successional development regulated the response of the soil microbes. The supply of organic carbon and nitrogen in the soil allowed plant growth and maintenance of the microbial population. Nitrogen competition between vegetation and microbes restricted net microbial growth. The increase of dissolved organic carbon (DOC) at higher CO₂ and temperature levels significantly favoured DEA. The high water regime in the soil also favoured DEA and inhibited oxidation of organic compounds, as indicated by low levels of enzyme activity. Additionally, water stress decreased rooting density in the soil; this resulted in negative feedback into microbial processes. We conclude that water stress and soil nitrogen deficiency caused an early levelling-off of both microbial population growth and activity rates during the early part of the model ecosystem's development.     

Temporal and spatial dynamics of soil solution C and N concentrations during Lolium perenne L. sward establishment and the effects of elevated CO2 and N additions

There is now clear evidence for a prolonged increase in atmospheric CO₂ concentrations and enrichment of the biosphere with N. Understanding the fate of C in the plant-soil system under different CO₂ and N regimes is therefore of considerable importance in predicting the environmental effects of climate change and in predicting the sustainability of ecosystems. Swards of Lolium perenne were grown from seed in a Eutric Cambisol at either ambient (ca. 350 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric pCO₂ and subjected to two inorganic N fertilizer regimes (no added N and 70 kg N ha⁻¹ month⁻¹). After germination, soil solution concentrations of dissolved organic C (DOC), dissolved inorganic N (DIN), dissolved organic N (DON), phenolics and H⁺ were measured at five depths down the soil profile over 3 months. The exploration of soil layers down the soil profile by roots caused transient increases in soil solution DOC, DON and phenolic concentrations, which then subsequently returned to lower quasi-stable concentrations. In general, the addition of N tended to increase DOC and DON concentrations while exposure to elevated pCO₂ had the opposite effect. These treatment effects, however, gradually diminished over the duration of the experiment from the top of the soil profile downwards. The ambient pCO₂ plus added N regime was the only treatment to maintain a notable difference in soil solution solute concentration, relative to other treatments. This effect on soil solution chemistry appeared to be largely indirect resulting from increased plant growth and a decrease in soil moisture content. Our results show that although plant growth responses to elevated pCO₂ are critically dependent upon N availability, the organic chemistry of the soil solution is relatively insensitive to changes in plant growth once the plants have become established.     

The importance of dissolved organic carbon fluxes for the carbon balance of a temperate Scots pine forest

Efforts to increase our understanding of the terrestrial carbon balance have resulted in a dense global network of eddy covariance towers, which are able to measure the net ecosystem exchange of CO₂, H₂O and energy between ecosystems and the atmosphere. However, the typical set-up on an eddy covariance tower does not monitor lateral CO₂- and carbon fluxes such as dissolved organic carbon (DOC). By ignoring DOC fluxes eddy covariance-based CO₂ balances overestimate the carbon sink of ecosystems as part of the DOC drains into the inland waters and get respired outside the footprint of the eddy covariance tower. In this study we quantify 7 years (2000-2006) of DOC fluxes from a temperate Scots pine forest in Belgium and analyse its inter-annual variability. On average, 10 gC m⁻² year⁻¹ is leached from the pine forest as DOC. If the DOC fluxes are considered relative to the gross ecosystem carbon fluxes we see that DOC fluxes are small: 0.8 ± 0.2% relative to gross primary productivity, 1.0 ± 0.3% relative to ecosystem respiration, and (2.4 ± 0.4%) relative to soil respiration. However, when compared to net fluxes such as net ecosystem productivity and net biome productivity the DOC flux is no longer negligible (11 ± 7% and 17%, respectively), especially because the DOC losses constitute a systematic bias and not a random error. The inter-annual variability of the DOC fluxes followed that of annual water drainage. Hence, drainage drives DOC leaching at both short and long time scales. Finally, it is noted that part of the carbon that is leached from the ecosystem as DOC is respired or sequestered elsewhere, so the physical boundaries of accounting should always be reported together with the carbon budget.     

Release of Carbon in Different Molecule Size Fractions from Decomposing Boreal Mor and Peat as Affected by Enchytraeid Worms

Terrestrial export of dissolved organic carbon (DOC) to watercourses has increased in boreal zone. Effect of decomposing material and soil food webs on the release rate and quality of DOC are poorly known. We quantified carbon (C) release in CO₂, and DOC in different molecular weights from the most common organic soils in boreal zone; and explored the effect of soil type and enchytraeid worms on the release rates. Two types of mor and four types of peat were incubated in laboratory with and without enchytraeid worms for 154 days at +?15 °C. Carbon was mostly released as CO₂; DOC contributed to 2–9% of C release. The share of DOC was higher in peat than in mor. The release rate of CO₂ was three times higher in mor than in highly decomposed peat. Enchytraeids enhanced the release of CO₂ by 31–43% and of DOC by 46–77% in mor. High molecular weight fraction dominated the DOC release. Upscaling the laboratory results into catchment level allowed us to conclude that peatlands are the main source of DOC, low molecular weight DOC originates close to watercourse, and that enchytraeids substantially influence DOC leaching to watercourse and ultimately to aquatic CO₂ emissions.     

Carbon dynamics determined by natural C abundance in microcosm experiments with soils from long-term maize and rye monocultures

Understanding carbon dynamics in soil is the key to managing soil organic matter. Our objective was to quantify the carbon dynamics in microcosm experiments with soils from long-term rye and maize monocultures using natural ¹³C abundance. Microcosms with undisturbed soil columns from the surface soil (0-25 cm) and subsoil (25-50 cm) of plots cultivated with rye (C₃-plant) since 1878 and maize (C₄-plant) since 1961 with and without NPK fertilization from the long-term experiment ‘Ewiger Roggen’ in Halle, Germany, were incubated for 230 days at 8 °C and irrigated with 2 mm 10⁻² M CaCl₂ per day. Younger, C₄-derived and older, C₃-derived percentages of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass (C_mic) and CO₂ from heterothropic respiration were determined by natural ¹³C abundance. The percentage of maize-derived carbon was highest in CO₂ (42-79%), followed by C_mic (23-46%), DOC (5-30%) and SOC (5-14%) in the surface soils and subsoils of the maize plots. The percentage of maize-derived C was higher for the NPK plot than for the unfertilized plot and higher for the surface soils than for the subsoils. Specific production rates of DOC, CO₂-C and C_mic from the maize-derived SOC were 0.06-0.08% for DOC, 1.6-2.6% for CO₂-C and 1.9-2.7% for C_mic, respectively, and specific production rates from rye-derived SOC of the continuous maize plot were 0.03-0.05% for DOC, 0.1-0.2% for CO₂-C and 0.3-0.5% for C_mic. NPK fertilization did not affect the specific production rates. Strong correlations were found between C₄-derived C_mic and C₄-derived SOC, DOC and CO₂-C (r≥0.90), whereas the relationship between C₃-derived C_mic and C₃-derived SOC, DOC and CO₂-C was not as pronounced (r≤0.67). The results stress the different importance of former (older than 40 years) and recent (younger than 40 years) litter C inputs for the formation of different C pools in the soil.     

Quantitative analysis of exudates from soil-living basidiomycetes in pure culture as a response to lead, cadmium and arsenic stress

Six different ectomycorrhizal fungi (Hebeloma velutipes, Piloderma byssinum, Paxillus involutus, Rhizopogon roseolus, Suillus bovinus and Suillus variegatus) and two saprotrophic fungi (Hypholoma fasciculare and Hypholoma capnoides) were exposed to metal stress induced by Pb, Cd and As. After pre-growth in a nutrient solution in Petri dishes, metal exposure was performed either in a nutrient rich solution or in a nutrient poor solution for seven days. The fungi were exposed to two different metal concentrations, low and high (Pb: 10 + 100 μM; Cd: 1 + 10 μM; As: 1 + 10 μM). Exudation of low molecular weight organic compounds (low molecular weight organic acids (LMWOA), amino acids and dissolved monosaccharides), as well as dissolved organic carbon was quantified as a potential response to the metal stress. The main LMWOA identified was oxalate. Oxalate exudation increased significantly in response to both low and high Pb and Cd concentrations, as well as low As exposure, relative to nutrient controls. Exposure to As and mixtures of metals (Pb + Cd, Pb + As) did not result in any significant increase in oxalate production compared to controls. The presence of a carbon source (glucose) in this study is likely to have been important for exudation of organic compounds. For the nutrient rich (+1 mM glucose) metal treatments exposure to Pb and Cd mainly increased exudation of oxalate and total amino acids. Production of dissolved monosaccharides, as well as DOC, did not increase significantly in response to metal exposure, irrespective of nutrient conditions. This may be explained by re-absorption of the organic compounds by the mycelium or by the fact that metals had no effect on exudation of these compounds.     

Elevation of atmospheric CO2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L.

Increased root exudation and a related stimulation of rhizosphere-microbial growth have been hypothesised as possible explanations for a lower nitrogen- (N-) nutritional status of plants grown under elevated atmospheric CO₂ concentrations, due to enhanced plant-microbial N competition in the rhizosphere. Leguminous plants may be able to counterbalance the enhanced N requirement by increased symbiotic N₂ fixation. Only limited information is available about the factors determining the stimulation of symbiotic N₂ fixation in response to elevated CO₂.In this study, short-term effects of elevated CO₂ on quality and quantity of root exudation, and on carbon supply to the nodules were assessed in Phaseolus vulgaris, grown in soil culture with limited (30 mg N kg⁻¹ soil) and sufficient N supply (200 mg N kg⁻¹ soil), at ambient (400 μmol mol⁻¹) and elevated (800 μmol mol⁻¹) atmospheric CO₂ concentrations.Elevated CO₂ reduced N tissue concentrations in both N treatments, accelerated the expression of N deficiency symptoms in the N-limited variant, but did not affect plant biomass production. ¹⁴CO₂ pulse-chase labelling revealed no indication for a general increase in root exudation with subsequent stimulation of rhizosphere microbial growth, resulting in increased N-competition in the rhizosphere at elevated CO₂. However, a CO₂-induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia was detected in 0.5-1.5 cm apical root zones as potential infection sites. Particularly in nodules, elevated CO₂ increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development. Nodule number, biomass and the proportion of leghaemoglobin-producing nodules were also enhanced. The release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO₂, independent of the N supply, and was already detectable at early stages of seedling development at 6 days after sowing.     

Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem

Experimentation with dynamics of soil carbon pools as affected by elevated CO₂ can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ¹³C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO₂ enrichment (FACE) experiment, in which elevated CO₂ (FACE: 560 μmol mol⁻¹) and ambient CO₂ (Control: 360 μmol mol⁻¹) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO₂ transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO₂ was higher than that under ambient CO₂, indicating that elevated CO₂ reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO₂ enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO₂ and mitigating harmful CO₂ under future atmospheric conditions.     

Low‐molecular‐weight organic acids enhance desorption of polycyclic aromatic hydrocarbons from soil

The impact of low‐molecular‐weight organic acids (LMWOAs) on desorption of phenanthrene and pyrene as representative polycyclic aromatic hydrocarbons (PAHs) from a contaminated soil was investigated by using a laboratory batch experiment. Three LMWOAs were used in this study and were citric, oxalic and malic acids. The LMWOAs in aqueous solution promoted desorption of PAHs from soil significantly and demonstrated an increasing trend as the concentration of LMWOAs increased. When compared with desorption of phenanthrene and pyrene from soil to water, the addition of LMWOAs enhanced desorption of test PAHs by up to 285 and 299%, respectively. Among the three LMWOAs studied, citric acid demonstrated the greatest efficiency in promoting PAH desorption from soil. Solutions of LMWOA continuously promoted PAH removal from soil during the multiple cycles of desorption. Overall, the experimental results suggest that LMWOAs in aqueous solution could disrupt soil organic matter (SOM)–metal cation–mineral linkages in soils, resulting in the release of SOM from soil and simultaneous increase of dissolved organic carbon (DOC) in solution. The loss of SOM from soil and increase of DOC in solution are responsible for the enhanced PAH desorption from soil. The positive correlation between DOC in solution and desorbed PAHs from soil suggests that the loss of SOM from soil plays an important role in LMWOA‐enhanced desorption of PAHs from soil.     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Carbon flux from decomposing 13C-labeled transgenic and nontransgenic parental rice straw in paddy soil

Purpose

Genetic modification of Bt rice may affect straw decomposition and soil carbon pool under flood conditions. This study aims to assess the effects of cry gene transformation in rice on the residue decomposition and fate of C from residues under flooded conditions.

Materials and methods

A decomposition experiment was set up using ¹³C-enriched rice straws from transgenic and nontransgenic Bt rice to evaluate the soil C dynamics and CH₄ or CO₂ emission rates in the root and non-root zones. The concentrations and stable carbon isotope compositions of the soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), CH₄, and CO₂ of the root and non-root zones were determined from 7 to 110 days after rice straw incorporation.

Results and discussion

Rice straw incorporation into soil significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates. The percentage of ¹³C-SOC remaining in the root zone was significantly lower than that in the non-root zone with rice straw decomposition. The DOC and MBC concentrations significantly increased in both the root and non-root zones between 0 and 80 days after rice straw incorporation. However, no significant differences were found after Bts (Bt rice straw added into soil) and Cks (nontransgenic Bt rice straw added into soil) incorporation in the root and non-root zones. This result may be attributed to the priming effects of sufficient oxygen and nutrients on straw degradation in the root zone.

Conclusions

Bt gene insertion did not affect the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates in both the root and non-root zones. However, rice straw incorporation and root exudation significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates.     

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2

Elevated atmospheric CO₂ tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO₂ concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO₂ plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO₂ stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO₂ increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO₂. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO₂ studies. Increased mineralization of stable soil organic carbon by a CO₂-induced priming effect may diminish the terrestrial carbon sink globally.     

Extramatrical mycelia of ectomycorrhizal fungi as moderators of carbon dynamics in forest soil

Extramatrical mycelia (EMM) of ectomycorrhizal (ECM) fungi are potentially extensive in soil and receive significant allocations of plant-derived carbon. Although losses from living EMM occur via respiration and exudation, EMM represents a considerable biomass component and potential carbon sink in many forest soils. ECM root tips and rhizomorphs may persist in soil for many months, but interactions between grazing arthropods and decomposers probably facilitate more rapid turnover of diffuse EMM. Elevated atmospheric CO₂ concentration [CO₂] is likely to increase carbon allocation to ECM fungi by their tree hosts. This will probably increase root colonization by ECM fungi and drive changes in their communities in soil. The likely effects of elevated [CO₂] and other climate change factors on the production and turnover of EMM production are difficult to predict from current evidence, and this hampers our understanding of their potential value as future carbon sinks. Responses of grazing soil arthropods to future climate change will have a strong influence on EMM turnover, along with the abilities of ECM fungi to store carbon in below-ground, and this should be seen as a priority area for future research.     

Heavy metal binding by hydrophobic and hydrophilic dissolved organic carbon fractions in a Spodosol A and B horizon

For a one year period intact Spodosol soil columns were percolated weekly with H₂O_deion, 1.58 mmol H₂SO₄ L^?1, and 0.79 mmol H₂SO₄ L^?1+0.64 mmol HNO₃ L^?1, respectively. Decomposition rates, soil organic carbon (OC) solubilization, dissolved organic carbon (DOC) fractions, and Cr-, Cu-, and Cd-binding by dissolved hydrophobic and hydrophilic acids were studied. Acid treatment reduced significantly OC respiration as well as OC solubilization in the humic layers. The reduced OC solubility at acid addition was more pronounced for the less polar hydrophobic compounds, resulting in a decrease of the hydrophobic acids (from ca. 65 to 40–45% of DOC), and in an increase of the hydrophilic acids (from ca. 25 to 40–45% of DOC). For B horizon leachates, DOC increased at acid treatment. Generally, hydrophobic acids were retained preferentially in the B horizon. Also in the B horizon output there was an increase of the hydrophilic acids as acidity increased (from ca. 40 to 50% of DOC). Differences between the two acid treatments were negligible. The degree of metal-organic complexes decreased in the order Cr >Cu >Cd, from A to B horizon leachates, and with increasing acidity. Hydrophilic acids were found to be the dominating ligands in complexing Cr and Cu. Actual Cr- and Cu-binding by hydrophilic acids exceeded that by hydrophobic acids 2–8 times. As the hydrophilic acids represented the most mobile DOC components in the soil columns, in particular with increasing acidity, significant amounts of Cr and Cu in the B horizon leachates were organically complexed, although a great proportion of the hydrophobic acids was retained in the B horizon.     

Fate of Chinese-fir litter during decomposition as a result of inorganic N additions

We conducted a controlled experiment to evaluate Chinese-fir litter decomposition and its response to the addition of inorganic N. Litter-derived CO₂, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were monitored during an 87-d incubation of a mixed soil–litter substrate using the ¹³C tracer technique. Litter C was mostly converted to CO₂ (47.4% of original mass), followed by MBC (3.6%), and DOC (1.0%), with 48% remaining unaltered in the soil. The litter decomposition rate significantly increased with the addition of inorganic N, although the effect depended on whether N was added as NH₄⁺ or NO₃⁻. Soil-derived CO₂, MBC, and DOC also increased following the combined addition of litter and N. The results showed that only a small percentage of litter C was retained as MBC or DOC and that the conversion rate depended, in part, on the form of inorganic N added to the Chinese-fir plantation soil.     

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT

Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at ?8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH₄ uptake and from 70 to 90% WFPS for N₂O and CO₂ emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N₂O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g^?1 C. Cumulative soil CO₂ emissions and CH₄ uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N₂O, CO₂, and CH₄ fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.