Dissolved organic carbon and nitrogen dynamics in temperate coniferous forest plantations

Dissolved organic matter (DOM) plays a central role in driving many chemical and biological processes in soil; however, our understanding of the fluxes and composition of the DOM pool still remains unclear. In this study we investigated the composition and dynamics of dissolved organic carbon (DOC) and nitrogen (DON) in five temperate coniferous forests. We subsequently related our findings to the inputs (litterfall, throughfall, atmospheric deposition) and outputs (leaching, respiration) of C and N from the forest and to plant available sources of N. With the exception of NO₃^?, most of the measured soil solution components (e.g. DOC, DON, NH₄⁺, free amino acids, total phenolics and proteins) progressively declined in concentration with soil depth, particularly in the organic horizons. This decline correlated well with total microbial activity within the soil profile. We calculated that the amount of C lost by soil respiration each day was equivalent to 70% of the DOC pool and 0.06% of the total soil C. The rapid rate of amino acid mineralization and the domination of the low molecular weight soluble N pool by inorganic N suggest that the microbial community is C‐ rather than N‐limited and that C‐limitation increases with soil depth. Further, our results suggest that the forest stands were not N‐limited and were probably more reliant on inorganic N as a primary N source rather than DON. In conclusion, our results show that the size of the DON and DOC pools are small relative to both the amount of C and N passing through the soil each year and the total C and N present in the soil. In addition, high rates of atmospheric N deposition in these forests may have removed competition for N resources between the plant and microbial communities.     

The contribution of fresh litter to dissolved organic carbon leached from a coniferous forest floor

The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, ¹³C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre⁻¹ during the first sampling period in May to 5 mg litre⁻¹ in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre⁻¹ from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m⁻², whereas only 2.2 and 0.9 g m⁻² were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter‐derived carbon, but of DOC produced in these two horizons themselves. Water‐extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one‐third native carbon from external sources at the experimental site and two‐thirds of the labelled litter. In contrast, the ¹³C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.     

The effects of organic carbon on acid rain in a temperate forest in Japan

From recent studies, we noticed that stemflow had an acidity that differed from that of precipitation or throughfall. Organic substances, supplied from the tree surface, would be one of the factors that modifies the acidity of rain. The objectives of this study were to determine the DOC concentration and to clarify the influence of dissolved organic carbon (DOC) on acidity in precipitation, throughfall and stemflow. Throughfall and stemflow were measured in sugi [Cryptomeria japonica D. Don], hinoki [Chamaecyparis obutusa Endl.] and kojii [Castanopsis cuspidata (Thumb.) Schottky.] stands. All samples were analyzed for their pH, electric conductivity (EC), major inorganic anions and cations and DOC concentration.The annual average of DOC was highest in stemflow, and that of throughfall and precipitation were one-third and one-tenth of stemflow, respectively. The averages of DOC in stemflow in two coniferous, sugi and hinoki stands, were higher than that of broadleaved kojii stand. DOC concentration was low in summer and high in winter in all stands. In Stemflow, pH and DOC were negatively correlated, while EC and DOC in stemflow were positively correlated in all stands. However in throughfall, there was no evident relationship between pH, DOC and EC. This relationship was not explained by the cause of organic acid.     

Impacts of acid precipitation on coniferous forest ecosystems

This paper summarizes the results from current studies in Norway. One main approach is the application of artificial acid ‘rain’ and of lime to field plots and lysimeters. Application during two growth seasons of 50 mm mo^?1 of ‘rain water’ of pH 3 to a podzol soil increased the acidity of the humus and decreased the base saturation. The reduction in base saturation was mainly due to leaching of Ca and Mg. Laboratory experiments revealed that decomposition of pine needles was not affected by any acid ‘rain’ treatment of the field plots. Liming slightly retarded the decomposition. No nitrification occurred in unlimed soils (pH 4.4-4.1). Liming increased nitrification. The soil enchytraeid (Ohgochaeta) fauna was not much affected by the acidification. Germination of spruce seeds in acidified mineral soil was negatively affected when soil pH was 4.0 or lower. Seedling establishment was even more sensitive to increasing soil acidity. Analysis of throughfall and stemflow water in southernmost Norway reveals that the total deposition of H₂SO₄ beneath spruce and pine is approximately two times the deposition in open terrain. A large part of this increase is probably due to dry deposition. Increased acidity of the rain seems to increase the leaching of cations from the tree crowns. Tree-ring analysis of spruce (Picea abies (L.) Karst.) and pine (Pinus sylvestris L.) has been based on comparisons between regions differently stressed by acid precipitation and also between sites presumed to differ in sensitivity to acidification. No effect that can be related to acid precipitation has yet been detected on diameter growth.     

气候因子对森林土壤有机碳影响的幅度效应研究

揭示不同幅度上气候因子对土壤有机碳(Soil organic carbon,SOC)影响的主控性变化,是预测未来气候变化对SOC演变趋势影响的基础。本文利用中国西南地区363个森林土壤剖面数据,基于大区、省和地级市3个幅度,研究了气候因子对森林SOC密度的影响随幅度变化的规律及不同幅度下的主控气候因子。结果表明,年均降水量与SOC密度的相关性均随着幅度的减小而减弱,而年均气温与SOC密度的相关性随幅度变化的规律不明显,有较强的区域差异。大区幅度上,SOC密度主要受年均降水量和年均气温的综合作用。省级幅度上,西藏自治区东部主控因子为年均降水量,而四川和云南两省为年均气温。地级市幅度上,各市的主控因子基本与其所属的省一致。气候因子对SOC密度变异的解释能力在大区幅度上约20%,且随着幅度的减小解释能力也逐渐减小。     

The effect of cation–anion interactions on soil pH and solubility of organic carbon

Alkaline soil is widely distributed and cultivated throughout the agricultural regions of the world. Organic carbon (OC) concentrations in alkaline soil are often small, partly because of the limitations of a high pH on the productivity of crops and pastures together with the effects of high pH on the chemistry of soil OC. Soil pH is often hypothesized to be a major factor in regulating OC turnover in agricultural soil, but there are few detailed studies on the effects of high pH on carbon cycling in alkaline soil. Sodium, K⁺, Mg²⁺ and Ca²⁺ are the major cations in alkaline soil, whereas Cl^?, , and are the major anions. The effect of different combinations of these cations and anions on soil pH and OC is not well described in the literature. The objectives of this study were to evaluate the effect of cations and anions on soil pH and to quantify the dissolution of OC in relation to these changes in pH. The results showed that (Na⁺, K⁺ and Mg²⁺) salts and CaCO₃ dominate in the pH range 7.0–8.5, and salts of Na⁺ and K⁺ dominate above pH 8.5. The amount of dissolved OC (DOC) increased significantly as pH increased. Therefore, the presence of large concentrations of and not only increased pH but also promoted the dissolution of soil OC. The concentration of Ca²⁺ modified this effect; large concentrations of Ca²⁺ increased the adsorption and reduced the concentration of DOC.     

Adsorption of dissolved organic carbon and sulfate by acid forest soils in the Fichtelgebirge,FRG

The adsorption of dissolved organic carbon (DOC) and sulfate was examined in mineral horizons of acid soils from damaged (Oberwarmensteinach) and healthy (Wülfersreuth) Norway spruce forested sites in the Fichtelgebirge (NE-Bavaria). The A horizons of both sites desorbed DOC at all levels added, whereas the B horizons (Bs and Bv) retained added DOC at levels > 5 mmol C kg^?1. An initial mass isotherm used on the B horizon data indicated that these soils have a greater affinity for DOC than B horizons from Spodosols in the northeastern U.S. Sulfate was only retained at high solution levels, and retention was pH dependent. Nitrate and sulfate additions (1000 μeq L^?1 anion) had minor effects on DOC adsorption. Overall, there was little difference in DOC or sulfate retention at the two sites, indicating atmospheric deposition inputs have not affected these processes.     

A carbon balance model for organic layers of acid forest soils

Organic layers of acid forest soils are highly dynamic carbon reservoirs. During forest succession the stored amount of organic carbon (OC) changes drastically. Because of feedback between OC storage in organic layers and in mineral soils and other compartments of the environment (plant, atmosphere, and groundwater), there is a strong need for applicable carbon balance models, particularly for organic layers. In this paper a simplified model for the carbon balance of organic layers (CABOLA model) of acid forest soils is presented. The model considers two horizons, the L and O horizon. Decomposition and transport processes are described by first order differential equations. C input into the organic layer is due to litter fall onto the L horizon. The governing equations are solved by integration. To demonstrate the model's capability of simulating the OC dynamics of organic layers, data on OC storage in organic layers of acid sandy forest soils with deep groundwater tables (Podzols) under pine stands were used. Together with literature data and some assumptions, these data were used for a first, rough estimation of the model parameters. Model calculations confirm that the CABOLA model is in principle able to simulate the dynamics of OC storage in organic layers during forest succession. Nevertheless, intensive research efforts will be necessary to independently parameterize the model for broad applications.     

Monitoring rhizospheric pH,oxygen, and organic acid dynamics in two short‐time flooded plant species

The rhizosphere of two flooding‐resistant plant species (Arundinella anomala Steud., Alternanthera philoxeroides Mart.) from Three Gorges Reservoir area (China) has been examined for reactions to waterlogging and submergence. Rhizosphere parameters were monitored in natural sediment substrate by means of a dual‐access floodable rhizobox, which allows monitoring of oxygen and pH dynamics noninvasively with planar optodes in high temporal and spatial resolution, as well as simultaneous low‐invasive soil‐solution sampling. Analysis of samples for low‐molecular‐weight organic acids (LMWOA) was done by capillary electrophoresis. Roots could be observed easily in situ during growth and exposure to flooding. The floodable rhizobox is therefore considered a valuable tool for root‐reaction monitoring also under flooding conditions. During waterlogging, both species exuded oxygen into their rhizosphere and showed diurnal rhythms of rhizospheric acidification. The pH of the rhizosphere of growing root tips decreased up to 0.8 units corresponding to higher LMWOA concentrations. These rhythms weakened during flooding, but gained maximum amplitude again rapidly after resurfacing. We conclude that the root system was still fully functioning during and after flooding, and that flooding poses no threat to the physiology of the root system of the study species.     

Diversity of total,nitrogen‐fixing and denitrifying bacteria in an acid forest soil

To assess the diversity of total, denitrifying and N₂‐fixing bacteria in a nitrogen (N)‐limited, acid forest soil, isolated DNA was analysed for the genes 16S rRNA, nosZ and nifH. Sequence information for these genes was obtained from clone libraries and from our TReFID computer program, which employs terminal restriction patterns for bacteria using multiple restriction enzymes. Both approaches indicated that Proteobacteria (α‐ and γ‐groups) and Acidobacteria dominated. A comprehensive list of bacteria retrieved from this soil is provided and compared with literature data on the bacterial community compositions from other sites. The study indicated that the current PCR conditions with the primers employed allowed retrieval of only a portion of the bacteria occurring in soils. Massive treatment of a soil plot with NH₄NO₃ caused an increase in the N content, which was rapidly followed by an enhancement of carbon (C) content. Thus the C/N ratio stayed below 16.0 and the soil remained N‐limited. This may explain why the bacterial diversity did not undergo drastic shifts as was tentatively inferred from the available data sets.     

Soil layer-specific variability in net nitrification and denitrification in an acid coniferous forest

 High spatial variation in nitrification potentials has been observed in forest soils, but explanations for this variability have remained speculative. In the present study we determined whether sample treatment, sample size, denitrification or small-scale variations in abiotic properties could explain spatial variation in nitrogen transformations in the organic horizon of a pine forest soil. Net nitrate production in homogenates of the organic horizon was extremely variable. Sample size (60–600 cm²) had no significant effect on nitrate production. In homogenised samples no increased nitrogen production was observed compared to intact incubated cores. High small-scale variation in nitrate production was observed in the litter (L) horizon. When this stratified L layer was subdivided, high net nitrate production was observed in moss (LM) and fragmented needles, whereas no net nitrate production was found in intact needles. The addition of acetylene, inhibiting nitrous oxide reductase, led to significant nitrous oxide production in the L layer. Low nitrous oxide production was found in the LM layer and none in the fragmentation layer. These results show that denitrification can explain part of the spatial variation and plays a major role in nitrogen transformations in the L layer. The relatively higher pH and the presence of fungi are suggested as factors responsible for high denitrification rates in the L layer. As a consequence homogenisation of the organic horizon could lead to highly variable nitrate production due to denitrifying activity from the needles being introduced into other layers. Received: 10 December 1999     

Effects of decreasing water potential on gross ammonification and nitrification in an acid coniferous forest soil

Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the ¹⁵N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of ¹⁵N, we compared different extraction and incubation times.T₀ times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg⁻¹ soil d⁻¹ and from 15.4 to 5.6 mg N kg⁻¹ soil d⁻¹ when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg⁻¹ soil d⁻¹ when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation.     

Net nitrification rate and presence of Nitrosospira cluster 2 in acid coniferous forest soils appear to be tree species specific

The impact of four coniferous tree species and their corresponding soil factors on N transformation rates and presence of ammonia-oxidising bacteria (AOB) was studied in an acid pine forest soil (Appelscha, The Netherlands). Pine soil had a relatively low net nitrification rate, while spruce, fir and larch soils showed high net nitrification rates. 16S rRNA and amoA sequences were only found in soils with high nitrification rates and belonged solely to Nitrosospira cluster 2. We conclude that tree species, possibly through their effects on soil C/N ratios, determines the presence of Nitrosospira cluster 2. Whenever AOB are present, however, the AOB community composition appears to be similar.     

Type of organic carbon amendment influences pH changes in acid sulfate soils in flooded and dry conditions

Purpose

Acid sulfate soils (ASS) are common in wetlands and can pose an environmental threat when they dry because oxidation of pyrite may cause strong acidification. Addition of organic matter can stimulate sulfate reduction during wet periods and minimize acidification during dry periods. However, the effect of the organic amendment may depend on its composition.

Materials and methods

Three wetland acid sulfate (sulfuric, hypersulfidic, and hyposulfidic) soils collected from different depth in one profile were used. The soils, unamended or amended with 10 g C kg^?1 as glucose, wheat straw, pea straw, or Phragmites litter, were incubated for 18 weeks under flooded conditions (“wet period”) followed by 10 weeks during which the soils were maintained at 100 % of maximum water-holding capacity (“dry period”).

Results and discussion

During the wet period, the pH decreased in the control and with glucose to pH 3–4, but increased or was maintained in residue-amended soils (pH at the end of the wet period about 7). In the dry period, the pH of the control and glucose-amended soils remained low, whereas the pH in residue-amended soils decreased. However, at end of the dry period, the pH was higher in residue-amended soils than in the control or glucose-amended soils, particularly with pea straw (C/N 50).

Conclusions

Amendment of acid sulfate soils with plant residues (particularly those with low to moderate C/N ratio) can stimulate pH increase during flooding and reduce acidification under oxidizing conditions.

     

Effect of particle-size distribution, soil organic carbon content and organo-mineral aluminium complexes on acid phosphatases of seasonally flooded forest soils

We studied the acid phosphatase activity (APA) and its relationships with some soil physico-chemical properties along a seasonally flooded forest gradient. The soil samples were collected during the non-flooded period in three zones subjected to different flooding periods: a zone inundated 8 months per year (MAX), a zone inundated 5 months per year (MED), and a zone inundated 2 months per year (MIN). In the MAX zone APA was low and negatively correlated with the fine earth fraction of soil. In this zone, clay minerals appeared to reduce the enzymatic activity. In the MIN zone acid phosphatase had a relatively higher activity, which was positively correlated with the soil organic C content and with Al associated with the organic matter. The highest value of APA was obtained in the MED zone, and no correlation was found between edaphic factors and the enzymatic activity in this zone. However our results are restricted to a single sampling date and, therefore, they do not take into account the seasonal dynamics of acid phosphatase in relation to other soil factors over time.     

Ecological characteristics of a nitrogen-saturated coniferous forest in The Netherlands

Wekerom forest shows a high nitrogen (N) load, and the first signs of N saturation. This characterization is based on the high N content of the needles, the high nitrate-N (NO₃-N) mobilization and low cation mobilization from the organic horizon. The N cycle in this forest has been transformed into an „open flow” system, in which the ammonium-N, deposited in large quantities from the atmosphere, is transformed into NO₃-N, which is leached into the groundwater. Decomposition of deeper organic layers, such as the fragmented litter and humus layers, is thought to provide additional NH₄-N, which explains the high NO₃-N output. Together with a reduction in the number and vitality of the pine trees, there is an increase in the number of nitrophilous plants, such as Deschampsia flexuosa and Rumex acetosella. The ectomycorrhizal and litter-decaying fungi are specific, N-resistant species. Soil fauna are classified as common inhabitants of dry, acid, nutrient poor forests. Received: 26 June 1997