Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15—20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10°C in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d^—1 with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH₄, NO₃, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L⁻¹) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.     

马尾松-麻栎混交林土壤溶解性有机碳、氮空间分布特征

以河南省驻马店西部低山丘陵区马尾松—麻栎混交林为研究对象,分析土壤有机碳(SOC)、溶解性有机碳(DOC)和溶解性有机氮(DON)储量及其对坡向、土层深度(0—10cm和10—20cm)及胸高断面积的响应规律。结果表明:坡向和胸高断面积显著影响SOC和DOC储量(p0.05),且不同坡向土壤中SOC、DOC和DON的影响因素不同。阳坡SOC储量主要限制性因素是速效钾,阴坡是全氮;阳坡与阴坡土壤DOC和DON储量主要受全氮的影响。另外,土层深度亦对SOC、DOC和DON储量有显著影响(p0.05),且随土层加深而降低,呈表层富集的现象。相关性分析表明,SOC、DOC和DON储量与土壤物理性质即土壤湿度、紧实度及土壤中颗粒组成存在显著的相关性。SOC、DOC和DON两两之间呈显著正相关关系,并均与土壤全氮、有效氮(铵态氮、硝态氮和碱解氮)、速效磷和速效钾等含量也存在显著的相关性(p0.01),表明土壤养分含量是影响马尾松—麻栎混交林土壤有机碳储量及溶解性有机碳、氮空间分布的重要因素。     

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH₄NO₃-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day⁻¹ g C⁻¹ and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.     

Contribution of dissolved organic nitrogen to N leaching from four German agricultural soils

Dissolved organic nitrogen (DON) substantially contributes to N leaching from forest ecosystems. However, little is known about the role of DON for N leaching from agricultural soils. Therefore, the aim of our study was to quantify the contribution of DON to total N leaching from four agricultural soils. Concentrations and fluxes of DON and mineral N were monitored at two cropped sites (Plaggic Anthrosols) and two fallow plots (Plaggic Anthrosol and Gleyic Podzol) from November 1999 till May 2001 by means of glass suction plates. The experimental sites were located near the city of Münster, NW Germany. Median DON concentrations in 90 cm depth were 2.3 mg l^—1 and 2.0 mg l^—1 at the cropped sites and 1.6 mg l^—1 and 1.3 mg l^—1 at the fallow sites. There was only a slight (Anthrosols) or no (Gleyic Podzol) decrease in median DON concentrations with increasing depth. Total N seepage was between 19 kg N ha^—1 yr^—1 and 46 kg N ha^—1 yr^—1 at the fallow sites and 16—159 kg N ha^—1 yr^—1 at the cropped sites. For the fallow plots, DON seepage contributed 10—21 % to the total N flux (4—5 kg DON ha^—1 yr^—1), at the cropped sites DON seepage was 6—21 % of the total N flux (6—10 kg DON ha^—1 yr^—1). Thus, even in highly fertilized agricultural soils, DON is a considerable N carrier in seepage that should be considered in detailed soil N budgets.     

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.     

不同施肥对雷竹林渗漏水中可溶性有机碳、氮流失的影响

【目的】雷竹(Phyllostachys praecox f. preveynalis)是一种在我国亚热带地区被广泛引种栽培的优良笋用竹。为了提高竹笋产量,农民不合理地大量施用化肥,已造成土壤盐化、酸化,地力破坏,导致土壤磷钾大量残留,特别是氮磷的大量流失已造成了周边水体严重污染。虽然土壤DOC和DON在土壤全碳、全氮含量中所占的比例很小,但却是土壤有机质中最重要和最活跃的部分。因此本研究的目的旨在通过全年动态监测雷竹林渗漏水中溶解性有机碳(DOC)和溶解性有机氮(DON)浓度的变化,探明减量施用化肥和有机肥对减少雷竹林氮渗漏淋失负荷的作用,以便为解决雷竹生产上的面源污染问题提供理论依据。【方法】试验设置了5个处理为对照(CK)、常规施肥(CF)、减量有机肥(DO)、减量无机肥(DI)和减量有机无机肥(DOI),3次重复,随机区组设计,小区面积为100 m2。试验于5月18日、 9月7日、11月9日分别施用肥料总量的40%、30%和30%,施肥后均进行浅翻,深度5 cm左右。【结果】 不同施肥雷竹林中DOC及DON平均浓度为33.7~45.5 mg/L和6.6~12.6 mg/L,DOC和DON的渗漏流失负荷为84.5~138.2 kg/hm2和17.2~46.3 kg/hm2。DOC渗漏流失负荷大小顺序为常规施肥(138.2 kg/hm2)减量有机肥(133.7 kg/hm2)减量无机肥(120.9 kg/hm2)不施肥(99.8 kg/hm2)减量有机无机肥(84.5 kg/hm2),而DON渗漏流失负荷大小顺序为减量有机肥(46.3 kg/hm2)常规施肥(35.3 kg/hm2）减量有机无机肥(34.8 kg/hm2)减量无机肥(31.1 kg/hm2)不施肥(17.2 kg/hm2)。渗漏水中DOC(mg/L)与DON(mg/L)之间不存在显著相关性。【结论】大幅减少化肥和有机肥用量,并推广有机肥和无机肥配施,不仅维持了雷竹竹笋的较高产量,还能减少土壤养分损失,具有经济和环境双重效益,是雷竹合理施肥的发展方向。     

Evidence of dissolved trivalent manganese in acidic forest soils

Background

Evidence of trivalent manganese (Mn³⁺) in the aqueous phase of soils is unknown so far although this strong oxidant has large environmental implications.

Aims

We aimed to modify a spectrophotometric protocol (porphyrin method) and to discriminate between Mn²⁺ and Mn³⁺ in the aqueous phase of forest soils based on kinetic modeling.

Methods

We investigated manganese speciation in 12 forest floor solutions and 41 soil solutions from an acidic forest site by adjusting pH and correcting for absorbance.

Results

The solutions showed broad ranges in pH (3.4−6.3), dissolved organic carbon (DOC, 1.78−77.1 mg C L⁻¹), and total Mn (Mn_T, 23.9−908 µg L⁻¹). For acidic solutions, a pH-buffer was added to increase the pH of the solutions to 7.5−8.0, and background absorption was corrected for colored solutions, that is, solutions high in DOC. This was done to accelerate the reaction kinetics and avoid overestimation of Mn_T concentrations. After the pH and color adjustments, the comparison of Mn_T concentrations between the porphyrin method and optical emission spectrometry showed good agreement. Trivalent Mn, which is stabilized by organic ligands, constitutes significant proportions in both forest floor solutions (10−87%) and soil solutions (0.5−74%).

Conclusions

The dissolved Mn³⁺ is present in acidic forest soils. Thus, we revise the paradigm that this species is not stable and encourage to apply the revised method to other 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.     

Repeated freeze–thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of –3°C, –8°C, and –13°C. The control columns were kept at constant +5°C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d^–1. Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m^–2 during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with –8°C and –13°C being most effective. In these treatments, only 3.1 and 4.0 g N m^–2 were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze–thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.     

Biodegradation kinetics of monosaccharides and their contribution to basal respiration in tropical forest soils

Low molecular weight (LMW) organic compounds in soil solution are easily biodegradable and could fuel respiration by soil microorganisms. Our main aim was to study the mineralization kinetics of monosaccharides using ¹⁴C-radiolabelled glucose. Based on these data and the soil solution concentrations of monosaccharides, we evaluated the contribution of monosaccharides to basal respiration for a variety of tropical forest soils. Further, the factors controlling the mineralization kinetics of monosaccharides were examined by comparing tropical and temperate forest soils. Monosaccharides comprised on average 5.2 to 47.7% of dissolved organic carbon in soil solution. Their kinetic parameters (V _max and K_M ), which were described by a single Michaelis-Menten equation, varied widely from 11 to 152?nmol?g^?1?h^?1 and 198 to 1294?µmol?L^?1 for tropical soils, and from 182 to 400?nmol?g^?1?h^?1 and 1277 to 3150?µmol?L^?1 for temperate soils, respectively. The values of V _max increased with increasing microbial biomass-C in tropical and temperate soils, while the K_M values had no correlations with soil biological or physicochemical properties. The positive correlation between V _max values and microbial biomass-C indicates that microbial biomass-C is an essential factor to regulate the V _max values in tropical and temperate forest soils. The biodegradation kinetics of monosaccharides indicate that the microbial capacity of monosaccharide mineralization far exceeds its rate at soil solution concentration. Monosaccharides in soil solution are rapidly mineralized, and their mean residence times in this study were very short (0.4–1.9?h) in tropical forests. The rates of monosaccharide mineralization at actual soil solution concentrations made up 22–118% of basal respiration. Probably because of the rapid and continuous production and consumption of monosaccharides, monosaccharide mineralization is shown to be a dominant fraction of basal respiration in tropical forest soils, as well as in temperate and boreal forest soils.     

水稻品种和砷污染对土壤溶解性有机碳氮的影响

选取有机质含量和pH不同的2种水稻土(黄泥田和红泥田),通过盆栽实验研究砷(As)污染条件下,种植9个水稻品种对土壤溶解性有机碳(DOC)和溶解性有机氮(DON)含量的影响,分析水稻品种、As污染和土壤类型的相对影响与交互作用.结果表明,水稻品种显著影响了土壤DOC和DON的变化,在水稻收获后,DOC平均含量的大小顺序为杂交稻(41.09±0.92 mg kg-1)＞籼稻(38.10±1.53 mg kg-1)＞粳稻(37.74± 1.37 mg kg-1);DON平均含量的大小顺序为粳稻(2.94± 0.40 mg kg-1)＞杂交稻(2.61±0.42 mg kg-1)＞籼稻(1.45± 0.17 mg kg-1).As污染降低了土壤DOC和DON的含量,但不同品种水稻的响应不同.与对照相比,As污染条件下,黄泥田和红泥田中DOC平均含量分别下降了14.4％和11.1％,DON平均含量分别下降了65.0％和44.7％;DOC在种植杂交稻后降幅最小,而DON在种植籼稻后降幅最小.在两种水稻土中,黄泥田的DOC和DON平均含量高于红泥田,在没有As污染条件下,分别高22.4％和45.8％,这与黄泥田有机质含量和pH高有关.水稻品种、As污染和土壤类型对DOC和DON变化的影响不同,3个因子对DOC变化的相对贡献率分别为7.7％、15.5％和27.6％,对DON变化的相对贡献率分别为14.7％、24.2％和2.0％.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

Release of nutrients dissolved organic carbon during decomposition of Chamaecyparis obtusa var. formosana leaves in a mountain forest in Taiwan

Forest ecosystems in Taiwan are periodically influenced by typhoons that cause large amounts of litter input to the soil. The potential rapid decomposition of such litter under the warm and moist climatic conditions in Taiwan may lead to nutrient losses via seepage. The goal of this study was to investigate the dynamics of C, N, K, Ca, Mg, and dissolved organic carbon (DOC) during decomposition of Chamaecyparis obtusa var. formosana leaves in a field study at the Yuanyang Lake site in N Taiwan. We simulated the effect of a typhoon by adding about three times the annual aboveground litterfall (totally 13,900 kg ha^–1) as fresh leaves. Litterbags were taken at 7 dates over 16 months, followed by detection of mass loss and element composition in the remaining litter. Aqueous extracts of the remaining litter were analyzed for DOC and major elements. The properties of DOC were characterized by fluorescence spectra and by its stability against microbial decomposition. The litter mass loss was 35% after 16 months. The losses of Ca after 16 months from the litter bags were about equivalent to mass loss (39%), while those of K and Mg reached 86% and 60% of the initial amount, respectively. From the 13,900 kg ha^–1 of litter applied in total, 59 kg K ha^–1 and 12 kg Mg ha^–1 were released in the 16 months decomposition period, most of it in the first 4 months. The total release of Ca amounted to 69 kg ha^–1 but was more evenly distributed throughout the 16 months of observation. The absolute amount of N in the decomposing litter increased by 37% while the C : N decreased from 69 to 34. Extrapolated to the manipulation treatment, this resulted in a N gain of 36 kg N ha^–1 within 16 months. The leaching of K and DOC in laboratory extractions followed an asymptotic function with highest leaching from the initial litter and subsequent decrease with time of decomposition. On the contrary, the leaching of Ca and Mg reached a maximum after 2–4 months of incubation. About 2% of the C was extractable with water from the initially incubated leaves. The bioavailability of the extracted DOC decreased with litter age. Our results indicate that the decomposition of large amounts of litter induces a high risk of K and Mg losses with seepage, but the risk for N losses is low. The sources of N accumulation in decomposing litter at this site require further studies. In the initial phase of litter decomposition, the release of DOC seems to be an important contribution to mass loss.     

喀斯特峰丛洼地土壤有机碳和氮素空间变异特征

本项试验在喀斯特典型峰丛洼地选择3个生态功能区,运用经典统计学和地统计学方法研究了土壤有机碳、全氮及碳氮比的空间变异特征。结果表明,随着生态系统由人工林(Ⅰ)次生林(Ⅱ) 原生林(Ⅲ)顺向演替,土壤有机碳、全氮和碳氮比平均值均呈现增加趋势,变异系数均在10.40％～80.94％之间,存在中等强度的变异性。人工林和原生林的土壤有机碳和全氮及人工林的碳氮比的块金值/基台值均＜25%,具有强烈的空间相关性;原生林碳氮比和次生林的3个土壤性质指标的块金值/基台值在32.6％～41.7％之间,具有中等的空间相关性;人工林各指标的变程明显大于次生林和原始林。Kriging插值结果表明,3个区域内有机碳和全氮的空间分布均具有显著的相似性,碳氮比变化则较不规则;人工林和原生林土壤有机碳、全氮含量呈片状平缓变化分布,次生林土壤有机碳、全氮含量及碳氮比呈支离破碎的斑块状分布。高度异质性的小生境决定了土壤有机碳、全氮和碳氮比的空间结构和格局,而受重度人类活动干扰的人工林和未受干扰的原生林空间异质性较次生林降低。     

Effects of nitrogen deposition on soil organic carbon fractions in the subtropical forest ecosystems of S China

Experiments were conducted between 2003 and 2008 to examine how N additions influence soil organic C (SOC) and its fractions in forests at different succession stages in the subtropical China. The succession stages included pine forest, pine and broadleaf mixed forest, and old‐growth monsoon evergreen broadleaf forest. Three levels of N (NH₄NO₃)‐addition treatments comprising control, low‐N (50 kg N ha^–1 y^–1), and medium‐N (100 kg N ha^–1 y^–1) were established. An additional treatment of high‐N (150 kg N ha^–1 y^–1) was established in the broadleaf mixed forest. Soil samples were obtained in July 2008 for analysis. Total organic C (TOC), particulate organic C (POC, > 53 μm), readily oxidizable organic C (ROC), nonreadily oxidizable organic C (NROC), microbial biomass C (MBC), and soil properties were analyzed. Nitrogen addition affected the TOC and its fractions significantly. Labile organic‐C fractions (POC and ROC) in the topsoil (0–10 cm) increased in all the three forests in response to the N‐addition treatments. NROC within the topsoil was higher in the medium‐N and high‐N treatments than in the controls. In the topsoil profiles of the broadleaf forest, N addition decreased MBC and increased TOC, while no significant effect on MBC and TOC occurred in the pine and mixed forests. Overall, elevated N deposition increased the availability of labile organic C (POC and ROC) and the accumulation of NROC within the topsoil irrespective of the forest succession stage, and might enhance the C‐storage capacity of the forest soils.     

有机物料输入稻田提高土壤微生物碳氮及可溶性有机碳氮

土壤微生物量碳、氮和可溶性有机碳、氮是土壤碳、氮库中最活跃的组分,是反应土壤被干扰程度的重要灵敏性指标,通过设置相同有机碳施用量下不同有机物料处理的田间试验,研究了有机物料添加下土壤微生物量碳（soil microbial biomass carbon,MBC）、氮（soil microbial biomass nitrogen,MBN）和可溶性有机碳（dissolved organic carbon,DOC）、氮（dissolved organic nitrogen,DON）的变化特征及相互关系。结果表明化肥和生物碳、玉米秸秆、鲜牛粪或松针配施下土壤微生物量碳、氮和可溶性有机碳、氮显著大于不施肥处理（no fertilization,CK）和单施化肥处理,分别比不施肥处理和单施化肥平均高23.52%和12.66%（MBC）、42.68%和24.02%（MBN）、14.70%和9.99%（DOC）、22.32%和21.79%（DON）。化肥和有机物料配施处理中,化肥+鲜牛粪处理的微生物量碳、氮和可溶性有机碳、氮最高,比CK高26.20%（MBC）、49.54%（MBN）、19.29%（DOC）和32.81%（DON）,其次是化肥+生物碳或化肥+玉米秸秆处理,而化肥+松针处理最低。土壤可溶性有机碳质量分数（308.87 mg/kg）小于微生物量碳（474.71 mg/kg）,而可溶性有机氮质量分数（53.07 mg/kg）要大于微生物量氮（34.79 mg/kg）。与不施肥处理相比,化肥和有机物料配施显著降低MBC/MBN和DOC/DON,降低率分别为24.57%和7.71%。MBC和DOC、MBN和DON随着土壤有机碳（soil organic carbon,SOC）、全氮（total nitrogen,TN）的增加呈显著线性增加。MBC、MBN、DOC、DON、DOC+MBC和DON+MBN之间呈极显著正相关（P<0.01）。从相关程度看,DOC+MBC和DON+MBN较MBC、DOC、MBN、DON更能反映土壤中活性有机碳和氮库的变化,成为评价土壤肥力及质量的更有效指标。结果可为提高洱海流域农田土壤肥力,增强土壤固氮效果,减少土壤中氮素流失,保护洱海水质安全提供科学依据。     

Organic‐carbon and nitrogen stocks and organic‐carbon fractions in soil under mixed pine and oak forest stands of different ages in NE Germany

The objective of this work was to evaluate the C and N stocks and organic‐C fractions in soil under mixed forest stands of Scots pine (Pinus sylvestris L.) and Sessile oak (Quercus petraea [Matt.] Liebl.) of different ages in NE Germany. Treatments consisted of pure pine (age 102 y), and pine (age 90–102 y) mixed with 10‐, 35‐, 106‐, and 124‐y‐old oak trees. After sampling O layers, soils in the mineral layer were taken at two different depths (0–10 and 10–20 cm). Oak admixture did not affect total organic‐C (TOC) and N stocks considering the different layers separately. However, when the sum of TOC stocks in the organic and mineral layers was considered, TOC stocks decreased with increasing in oak age (r² = 0.58, p < 0.10). The microbial C (C_MB) was not directly correlated with increase of oak age, however, it was positively related with presence of oak species. There was an increase in the percentage of the C_MB‐to‐TOC ratio with increase of oak‐tree ages. On average, light‐fraction C (C_LF) comprised 68% of the soil TOC in upper layer corresponding to the highest C pool in the upper layer. C_LF and heavy‐fraction C (C_HF) were not directly affected by the admixture of oak trees in both layers. The C_HF accounted on average for 30% and 59% of the TOC at 0–10 and 10–20 cm depths, respectively. Despite low clay contents in the studied soils, the differences in the DCB‐extractable Fe and Al affected the concentrations of the C_HF and TOC in the 10–20 cm layers (p < 0.05). Admixture of oak in pine stands contributed to reduce topsoil C stocks, probably due to higher soil organic matter turnover promoted by higher quality of oak litter.     

Chemistry of soil organic matter as related to C : N in Norway spruce forest (Picea abies(L.) Karst.) floors and mineral soils

Due to high nitrogen deposition in central Europe, the C : N ratio of litter and the forest floor has narrowed in the past. This may cause changes in the chemical composition of the soil organic matter. Here we investigate the composition of organic matter in Oh and A horizons of 15 Norway spruce soils with a wide range of C : N ratios. Samples are analyzed with solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin, polysaccharides, and amino acid‐N. The data are investigated for functional relationships between C, N contents and C : N ratios by structural analysis. With increasing N content, the concentration of lignin decreases in the Oh horizons, but increases in the A horizons. A negative effect of N on lignin degradation is observed in the mineral soil, but not in the humus layer. In the A horizons non‐phenolic aromatic C compounds accumulate, especially at low N values. At high N levels, N is preferentially incorporated into the amino acid fraction and only to a smaller extent into the non‐hydrolyzable N fraction. High total N concentrations are associated with a higher relative contribution of organic matter of microbial origin.