Cycling of soil carbon in a Japanese red pine forest II. Changes occurring in the first year after a clear-felling

Cycling of soil carbon in the first year after a clear-felling was compared with that before the felling in a Japanese red pine forest in Hiroshima Prefecture, west Japan. The daily mean temperature at the soil surface in summer was increased after the felling in comparison to that before felling, and the water content of both the A₀ layer and the surface mineral soil was decreased due to the loss of the forest canopy. The rate of weight loss of the A₀ layer was reduced after felling. However, accumulation of the A₀ layer rapidly decreased because of the lack of litter supply to the forest floor. Low soil respiration after felling was mainly caused by the cessation of root respiration. Analysis of annual soil carbon cycling was then conducted using a compartment model. The relative decomposition rate of the A₀ layer decreased whereas that of humus and dead roots in mineral soil increased to some extent after felling. The accumulation of carbon in mineral soil, however, increased slightly due to the supply of humus from roots killed by the felling.     

Cycling of soil carbon in a Japanese red pine forest I. Before a clear-felling

Cycling of soil carbon was measured synthetically and quantitatively throughout a year in two Japanese red pine forest stands on mid- and foot-slopes at Mt. Takao, Hiroshima Prefecture, west Japan. There was no distinct difference of soil temperature along the slopes, but the soil water content was higher on the foot-slope than on the midslope. The carbon flow (litterfall, soil respiration, etc.) rates were larger on the foot-slope than on the mid-slope, but there was no significant difference of the accumulation of soil carbon (A₀ layer or human in mineral soil) between the areas. The results of the analysis of soil carbon cycling based on a compartment model show that the relative decomposition rate of A₀ layer and humaus in mineral soil increased 1.4–1.5 fold from the mid- to the foot-slopes, corresponding to the soil moisture condition. The relative decomposition rate of A₀ layer was, however, about one-third of that in a evergreen oak forest. This fact suggests that the great resistance of needle litter to decomposition is one of the main limiting factors of the cycling of soil carbon and prevents the fertilization of mineral soil in the pine forest, which was also proven by the simulation of dynamics of soil carbon cycling.     

Long-term tillage effects on soil organic carbon and dissolved organic carbon in a purple paddy soil of Southwest China

The impact of conservation tillage practices on soil carbon has been of great interest in recent years. Conservation tillage might have the potential to enhance soil carbon accumulation and alter the depth distribution of soil carbon compared to conventional tillage based systems. Changes in the soil organic carbon (SOC) as influenced by tillage, are more noticeable under long-term rather than short-term tillage practices. The objective of this study was to determine the impacts of long-term tillage on SOC and dissolved organic carbon (DOC) status after 19 years of four tillage treatments in a Hydragric Anthrosol. In this experiment four tillage systems included conventional tillage with rotation of rice and winter fallow system (CTF), conventional tillage with rotation of rice and rape system (CTR), no-till and ridge culture with rotation of rice and rape system (NT) and tillage and ridge culture with rotation of rice and rape system (TR). Soils were sampled in the spring of 2009 and sectioned into 0–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm depth, respectively.Tillage effect on SOC was observed, and SOC concentrations were much larger under NT than the other three tillage methods in all soil depths from 0 to 60 cm. The mean SOC concentration at 0–60 cm soil depth followed the sequence: NT (22.74 g kg^?1) > CTF (14.57 g kg^?1) > TR (13.10 g kg^?1) > CTR (11.92 g kg^?1). SOC concentrations under NT were significantly higher than TR and CTR (P < 0.01), and higher than CTF treatment (P < 0.05). The SOC storage was calculated on equivalent soil mass basis. Results showed that the highest SOC storage at 0–60 cm depth presented in NT, which was 158.52 Mg C ha^?1, followed by CTF (106.74 Mg C ha^?1), TR (93.11 Mg C ha^?1) and CTR (88.60 Mg C ha^?1). Compared with conventional tillage (CTF), the total SOC storage in NT increased by 48.51%, but decreased by 16.99% and 12.77% under CTR and TR treatments, respectively. The effect of tillage on DOC was significant at 0–10 cm soil layer, and DOC concentration was much higher under CTF than the other three treatments (P < 0.01). Throughout 0–60 cm soil depth, DOC concentrations were 32.92, 32.63, 26.79 and 22.10 mg kg^?1 under NT, CTF, CTR and TR, and the differences among the four treatments were not significant (P > 0.05). In conclusion, NT increased SOC concentration and storage compared to conventional tillage operation but not for DOC.     

Animal effects on dissolved organic carbon bioavailability in an algal controlled ecosystem

1. Animals exert both direct and indirect controls over elemental cycles, linking primary producer-based (green) and decomposer-based (brown) food webs through top-down trophic interactions and bottom-up element regeneration. Where animals are aggregated at high biomass, they create hotspots of elemental cycling. The relative importance of animal control on elemental cycling depends on animal biomass, species functional traits (i.e. feeding mode and stoichiometry), and their overlap.
2. We evaluated how animal community complexity affects the mechanisms regulating energy flow to the brown food web. We conducted a mesocosm experiment where we varied the biomass and overlap of animals with different life history and stoichiometric traits (stream fish and mussels) and measured how this influenced the quantity and fraction of labile carbon available to microbes. We used linear models and structural equation modelling to evaluate direct (excretion) and indirect (herbivory, nutrient availability, and nutrient stoichiometry) effects of animals on bioavailable dissolved organic carbon (BDOC) concentration.
3. In experimental stream mesocosms, we found support for both direct (DOC excretion) and indirect (grazing) animal influences on BDOC concentration. Although we found that snail, fish, and mussel biomass increased nutrient concentrations, neither nutrient concentration nor stoichiometry had a significant effect on BDOC concentration. This has been due to the high background nutrient concentration context of our stream mesocosm water. Snails, probably due to their high biomass and small body size, exerted a significant positive direct control on BDOC concentration. Fish and mussels exerted a significant negative indirect control on BDOC via their effects (grazing and bioturbation) on algal biomass.
4. Our results imply that primary consumers with different feeding strategies provide a key mechanism regulating the flow of DOC into the brown food web through direct (excretion) and indirect (grazing) controls on primary producers. This highlights that animals can provide important controls on the production of bioavailable organic energy supporting microbes in aquatic ecosystems, but the importance of these controls depends on the nutrient context and the distribution of primary producer and animal biomasses.

     

Effects of increased atmospheric CO2, temperature,and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.)

Root exudation has been hypothesized as one possible mechanism that may lead to increased inputs of organic C into the soil under elevated atmospheric CO₂, which could lead to greater long-term soil C storage. In this study, we analyzed exudation of dissolved organic C from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO₂ (35.0 and 70.0 Pa) × 2 temperature (26° and 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. We also analyzed the decomposition rates of root exudate to estimate gross rates of exudation. Elevated CO₂ did not affect root exudation of organic C. A 4 °C increase in temperature and N fertilization did, however, significantly increase organic C exudation rates. Approximately 60% of the exudate decomposed relatively rapidly, with a turnover rate of less than one day, while the remaining 40% decomposed more slowly. These results suggest that warmer climates, as predicted for the next century, may accelerate root exudation of organic C, which will probably stimulate rapid C cycling and may make a minor contribution to intermediate to more long-term soil C storage. However, as these losses to root exudation did not exceed 1.2% of the net C fixed by Robinia pseudoacacia, root exudation of organic C appears to have little potential to contribute to long-term soil C sequestration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of Norway spruce seedlings on the nutrient availability in mineral soil and forest floor material

A greenhouse experiment was performed to evaluate the effect of Norway spruce (Picea abies (L.) Karst.) seedlings on net nutrient availability in five different growing media containing F- or H-layer and mineral soil originating from a haplic podzol in northern Sweden. The initial total amounts of eight nutrient elements (N, K, P, Ca, Mg, Mn, Fe, Zn) and exchangeable amounts of the same elements were analyzed in pots with or without spruce seedlings. In the planted pots seedling nutrient uptake was also estimated. After 26 weeks, higher net nutrient availability with seedlings was found in 25 out of the 40 (62%) growing media and nutrient element combinations. A positive seedling effect on net nutrient availability might be explained by rhizodeposition stimulating the soil microorganism activity and accelerating the weathering of minerals or by seedling roots promoting the nutrient providing processes through changes in soil chemical and physical properties. Nitrogen availability was primarily affected by what part of the forest floor the growing medium contained although the positive response to seedling presence was apparent. The positive net availability response of P, Ca, Mg, Mn, Fe and Zn to seedling presence was on the other hand relatively strong. In the case of P, K, and Zn the growing medium composition (if the F- and H-layer was pure or mixed with mineral soil) was also an important factor for the estimated net availability. Pure F-and H-layer provided greater P- and K-availability while the availability of Zn increased when mineral soil was added. The influence of growing plants ought to be considered when soil samples are used for assessing the nutrient availability.     

模拟火干扰对森林土壤微生物活性及氮矿化的影响

火干扰产生热能从而诱导土壤有机质的化学氧化,改变碳和氮转换,对土壤的结构与功能产生严重影响,影响程度取决于火强度、火干扰持续时间和热渗透。在湖南省株洲市高枧林场选取马尾松次生林火烧迹地,按两种土壤、3个温度和3种土壤水势进行试验设计与方差分析,探讨火干扰对土壤微生物及氮矿化的影响。结果表明:无机氮的浓度与火强度和初始土壤有机质含量呈正相关关系;火干扰后短期内土壤碳和氮浓度较高,微生物生物量碳和潜在可矿化氮较低,温度和土壤水势对基础呼吸速率没有显著影响;当土壤温度达160℃时,未受火干扰土壤中潜在可矿化氮浓度迅速不稳定增加,温度达350℃时破坏90%的非微生物组织;土壤加热后水势对氮矿化过程有显著影响,水势越高,潜在可矿化氮损失越大,火干扰土壤的含水量与硝态氮之间呈正相关关系;培养14d期间,土壤火灾历史、热处理和土壤水势对微生物活性、碳和氮矿化有显著影响,-1.5 MPa水势下加热到380℃后两种土壤的微生物生物量碳含量最高,土壤水势和可溶性糖呈负相关关系;水势和火干扰之间的交互作用显著影响微生物活性和氮转换,低水势土壤中的微生物生物量碳、可溶性糖和潜在可矿化氮浓度较高。     

模拟氮沉降对克氏针茅草原土壤有机碳的短期影响

为更好地了解天然草地土壤有机碳对氮沉降增加的响应,2011年在内蒙古太仆寺旗的克氏针茅(Stipa krylovii)草原上开展了模拟氮沉降的控制实验,设置对照(CK)和5个模拟氮沉降(NO-3)处理,分别为2(N1)、5(N2)、10(N3)、25(N4)和50 g N m-2a-1(N5)。生长季末,采集每个样地中0—2 cm和2—10 cm深度土壤进行有机碳含量及组成的分析,并进行实验室矿化培养。结果表明,土壤颗粒态有机碳(POC)对氮添加响应敏感,N1和N2处理下的POC含量高于CK,N3、N4和N5处理则低于CK。5个模拟氮沉降处理下的矿质结合态有机碳(MOC)含量均高于对照,但差别不显著。不同氮沉降水平下0—2 cm土层的碳矿化潜势为N2N1N4N3CKN5,且N1,N2,N3和N4处理均显著高于CK和N5;2—10 cm土层的碳矿化潜势为N2N1N3CKN4N5,N1、N2和N3显著高于CK、N4及N5。不同施氮处理对群落净第一性生产力有明显影响,N5的净第一性生产力和地上生物量显著低于对照和其它施氮处理,N1的0—10 cm地下生物量显著高于对照和其它处理,N5的凋落物量显著高于对照。模拟氮沉降短期内对土壤总有机碳(SOC)含量无显著影响。     

淹水培养条件下土壤微生物生物量碳、氮和可溶性有机碳、氦的动态

以洞庭湖区2个典型水稻土（红黄泥和紫潮泥）为对象,研究了25℃、淹水培养条件下稻草-硫铵配施和单施硫铵处理土壤微生物生物量碳、氮（SMBC、SMBN）和可溶性有机碳、氦（SDOC、SDON）的动态变化．结果表明,SMBC、SMBN和SDOC、SDON在培养前期达到峰值,之后降低,并趋于稳定．添加底物后,2种土壤不同处理土壤微生物生物量碳与有机碳（SMBC／TC）和土壤微生物生物量氮与全氮（SMBN／TN）的平均值都在2％-3％之间变化;可溶性碳与全碳（SDOC／TC）的平均值为1％左右,可溶性氮与全氮（SDON／TN）平均值为5％-6％．2种土壤中SMBC峰值单施硫铵处理最大,但与稻草-硫铵配施处理差异均不显著;SMBN、SDOC和SDON峰值稻草-硫铵配施最大．稻草．硫铵配施与单施硫铵处理中,低肥力红黄泥的SMBN、SDOC和SDON峰值差异显著;而高肥力紫潮泥SMBN和SDOC峰值差异不显著．前7d,SMBC／SMBN〈10;14d后,同一时刻单施硫铵处理SMBC／SMBN〉稻草．硫铵配施．不同处理的SDOC!SDON3d时最大．28d时最小．     

Belowground carbon cycling in a humid tropical forest decreases with fertilization

Only a small fraction of the carbon (C) allocated belowground by trees is retained by soils in long-lived, decay-resistant forms, yet because of the large magnitude of terrestrial primary productivity, even small changes in C allocation or retention can alter terrestrial C storage. The humid tropics exert a disproportionately large influence over terrestrial C storage, but C allocation and belowground retention in these ecosystems remain poorly quantified. Using mass balance and ¹³C isotope methods, we examined the effects of afforestation and fertilization, two land-use changes of large-scale importance, on belowground C cycling at a humid tropical site in Hawaii. Here we report that in unfertilized plots, 80% of the C allocated belowground by trees to roots and mycorrhizae was returned to the atmosphere within 1 year; 9% of the belowground C flux was retained in coarse roots and 11% was retained as new soil C. The gains in new soil C were offset entirely by losses of old soil C. Further, while fertilization early in stand development increased C storage in the litter layer and in coarse roots, it reduced by 22% the flux of C moving through roots and mycorrhizae into mineral soils. Because soil C formation rates related strongly to rhizosphere C flux, fertilization may reduce an already limited capacity of these forests to sequester decay-resistant soil C.     

Soil water, nutrient availability and sapling survival under organic and polyethylene mulch in a seasonally dry tropical forest

We examine the effect of mulches on the soil volumetric water content (SVWC), pH, carbon (C), total and mineral (NH₄ and NO₃) nitrogen (N), total and bicarbonate phosphorus (P), and on the survival and relative growth rate of three species, Ipomea wolcottiana Rose, Lonchocarpus eriocarinalis Micheli and Caesalpinia eriostachys Benth, in a degraded seasonally dry tropical forest (SDTF) area. Our study year was unusually dry, with only half of the mean annual rainfall. Sixteen plots (5 × 6 m) for each of our four treatments, mulches with alfalfa (Medicago sativa L.) straw, forest litter (SDTF litter), polyethylene and bare soil (control), were used. In each plot, 20 tree saplings were planted of each species. The SVWC was higher in plots mulched with polyethylene than in bare soil plots. The soil pH did not change with mulching, and there were no differences between treatments in the concentrations of soil organic C, total N, NO₃ and total P. However, soil concentrations of NH₄ were highest in plots with alfalfa straw and of bicarbonate P in plots with polyethylene. Sapling survival was higher in polyethylene mulch plots than in other mulching treatments, in the order I.␣wolcottiana > C. eriostachys > L. eriocarinalis. Sapling survival under organic mulches, alfalfa straw and forest litter were similar, and lowest in bare soil. The relative growth rate followed the order L. eriocarinalis < C. eriostachys < I. wolcotiana, and the growth rate of all species was greatest under polyethylene mulch. We conclude that a combination of polyethylene mulch with species of high growth rate is best for restoring seasonally dry tropical areas.     

火烧对马尾松林土壤酶活性和有机碳组分的影响

土壤酶参与土壤有机质矿化过程,林火能通过改变土壤中微生物的群落结构等来改变土壤酶的活性,进而影响土壤有机碳的动态过程。土壤有机碳库是陆地碳库的重要组成部分,火烧会使土壤有机碳组分发生变化,研究火烧后土壤有机碳组分的变化对于土壤有机碳库的管理具有重要意义。以我国中亚热带典型马尾松人工林火烧迹地为研究对象,通过对比研究,探讨了火烧对土壤酶活性和土壤有机碳组分的影响以及两者之间的关系。结果表明:(1)与对照相比,火烧后一年0—10 cm土层土壤p H值明显升高(P0.05),土壤总碳含量显著降低(P0.05),土壤全氮含量平均降低17.5%(P0.05)。0—10 cm土层和10—20 cm土层土壤含水量均显著低于对照(P0.05)。(2)相比对照,土壤β-葡萄糖苷酶活性在0—10 cm土层显著降低(P0.05),土壤酚氧化物酶活性和过氧化物酶活性显著升高(P0.05)。(3)火烧后一年0—10 cm土层土壤微生物量碳、土壤颗粒性有机碳、土壤易氧化碳含量比对照分别降低26.4%、30.9%和2.69%,但无显著差异,土壤溶解性有机碳含量则显著降低(P0.05);两个土层土壤不稳定有机碳含量和粘粒有机碳含量变化不显著。     

侵蚀退化地植被恢复过程中芒萁对土壤可溶性有机碳的影响

林下植被是生态系统的重要组分。通过对比分析红壤侵蚀区植被恢复过程中,林下有无芒萁覆盖地的土壤可溶性有机碳(DOC,Dissolved Organic Carbon)含量及其与地下根系生物量、地上植被淋溶液DOC含量的关系。结果表明:林下植被芒萁覆盖增加了地上叶片和地下根系生物量,土壤DOC含量及储量也显著增加(P0.05),芒萁覆盖对表层土壤(0—20cm)DOC的影响大于深层土壤(20—100cm)(P0.05);相关分析结果表明,林下芒萁覆盖地土壤DOC储量与细根生物量的垂直变化呈显著的正相关关系(P0.05),且随植被恢复年限的增加相关性显著增加,地下根系的垂直分布直接影响各土层DOC储量。不同植被恢复时期,林下芒萁覆盖地土壤DOC与鲜叶(马尾松+芒萁)和枯落物(马尾松+芒萁)淋溶液DOC均呈显著的正相关关系(P0.01),而林下裸露地土壤DOC仅与鲜叶(马尾松)淋溶液DOC呈显著的相关性(P0.01),林下芒萁覆盖地相对于裸露地枯落物淋溶液对土壤DOC储量的影响大于鲜叶。植被恢复过程中芒萁覆盖地土壤微生物生物量碳和微生物熵显著高于林下裸露地。因此,在植被恢复进程中,芒萁能够提供更多底物参与土壤物质与养分循环,对土壤DOC的贡献较大,为侵蚀区马尾松林恢复提供了重要的养分再吸收来源;同时芒萁覆盖增加了微生物活性,促进了微生物对土壤DOC的同化作用,提高了微生物碳源的利用率,对土壤有机碳的积累起着重要的作用。     

Correlations among indices of forest soil nutrient availability in fertilized and unfertilized loblolly pine plantations

Summary Various laboratory indices of N and P availability in forest soils correlated poorly among themselves and with on-site ion exchange resin (IER) estimates in both unfertilized and N+P fertilized loblolly pine plantations. IER nutrient availability estimates had greatest within-site variability than laboratory indices. Net nitrification was minimal in laboratory incubation of the mineral soil despite high rates of ammonification. In contrast, IER NO₃–N values were usually of the same magnitude as IER NH₄–N values. In both fertilized and unfertilized stands, at least one N availability index was negatively correlated with at least one P index. Soil N and P availabilities were generally higher on fertilized plots than on unfertilized plots 3.5 years after fertilization, and IER estimates showed the greatest number of plots with increased levels. The greater ability of the IER method to distinguish between fertilized and unfertilized plots indicated the method was affected by on-site factors that the laboratory methods do not assess.     

A bioassay of the availability of residual 15N fertilizer eight years after application to a forest soil in interior British Columbia

Although a high proportion of fertilizer N may be immobilized in organic forms in the soil, no studies have examined the long-term availability of residual fertilizer ¹⁵N in forestry situations. We investigated this by growing lodgepole pine (Pinus contorta) seedlings in surface (0–10 cm) soil sample eight years after application of ¹⁵N-urea, ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ to lodgepole pine in interior British Columbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the ¹⁵N and 4.6% of the native N per pot. Most of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. In contrast to the greenhouse study, there was no significantuptake of ¹⁵N by trees in the field study after the first growing season, although half of the soil organic ¹⁵N was lost between one and eight years after fertilization. This indicates the need to understand the mechanisms which limit the uptake of mineral N by trees in the field, and the possible mismatch of tree demand and mineral N availability.     

Effects of grassland conversion to cropland and forest on soil organic carbon and dissolved organic carbon in the farming-pastoral ecotone of Inner Mongolia

Effects of grassland conversion to cropland and forest on soil organic carbon (SOC), dissolved organic carbon (DOC) in the farming-pastoral ecotone of Inner Mongolia were investigated by direct field sampling. SOC content and DOC content in soil decreased after grassland were shifted to forest or cropland, in the sequence of grassland soil > forest soil > cropland soil. SOC stock declined by 18% after grassland shifted from to forest. Reclamation of cropland for 10 years, 15 years and 20 years lost SOC in 0–30 cm soil layer, by 34%, 14% and 18%, respectively, compared with that of grassland. DOC in 3 soil layers was within 21.1–26.5 mg/L in grassland, 12.1–14.6 mg/L in forest soil, and 8.0–14.0 mg/L in cropland soil. Correlation analysis indicated that SOC content and DOC content were positively dependent on total nitrogen content (p < 0.05), but negatively on bulk density or land use type (p < 0.05). DOC was positively correlated SOC (p < 0.01). Moreover, SOC content could be quantitatively described by a linear combination of land use types (p = 0.000, r² = 0.712), and DOC content by a linear combination of two soil-related variables, land use types and SOC (p = 0.000, r² = 0.861).     

Effects of grassland conversion to cropland and forest on soil organic carbon and dissolved organic carbon in the farming-pastoral ecotone of Inner Mongolia

Effects of grassland conversion to cropland and forest on soil organic carbon (SOC), dissolved organic carbon (DOC) in the farming-pastoral ecotone of Inner Mongolia were investigated by direct field sampling. SOC content and DOC content in soil decreased after grassland were shifted to forest or cropland, in the sequence of grassland soil > forest soil > cropland soil. SOC stock declined by 18% after grassland shifted from to forest. Reclamation of cropland for 10 years, 15 years and 20 years lost SOC in 0–30 cm soil layer, by 34%, 14% and 18%, respectively, compared with that of grassland. DOC in 3 soil layers was within 21.1–26.5 mg/L in grassland, 12.1–14.6 mg/L in forest soil, and 8.0–14.0 mg/L in cropland soil. Correlation analysis indicated that SOC content and DOC content were positively dependent on total nitrogen content (p < 0.05), but negatively on bulk density or land use type (p < 0.05). DOC was positively correlated SOC (p < 0.01). Moreover, SOC content could be quantitatively described by a linear combination of land use types (p = 0.000, r² = 0.712), and DOC content by a linear combination of two soil-related variables, land use types and SOC (p = 0.000, r² = 0.861).     

Nitrogen resorption from needles of Pinus thunbergii Parl. growing along a topographic gradient of soil nutrient availability

To examine the relative importance of nutrient resorption in increasing the nutrient-use efficiency of Pinus thunbergii Parl., we investigated the nitrogen contents of green and senescent needles of P. thunbergii trees growing at five positions along a slope (LS, lower slope; TR, transitional site; MS, middle slope; US, upper slope; RG, ridge) and found that soil nitrogen availability tended to decrease upslope. Nitrogen concentration in green and senescent needles decreased upslope. Nitrogen resorption efficiencies (percentage change in nitrogen content between green and senescent needles) increased upslope from 43 to 77% with decreasing soil nitrogen availability. Nitrogen resorption efficiency was related to green needle dry mass per unit length, but there was no clear correlation between nitrogen resorption efficiency and nitrogen content in green needles. We concluded that the increase in nitrogen resorption efficiency of P. thunbergii enhanced the nitrogen-use efficiency as a response to the low nitrogen availability.     

Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest

Despite growing attention concerning therole of dissolved organic matter (DOM) inelement cycling of forest ecosystems, thecontrols of concentrations and fluxes of bothdissolved organic carbon (DOC) and nitrogen(DON) under field conditions in forest soilsremain only poorly understood. The goal ofthis project is to measure the concentrations and fluxes of DON, NH₄ ⁺, NO₃ ^–and DOC in bulkprecipitation, throughfall, forest floorleachates and soil solutions of a deciduousstand in the Steigerwald region (northernBavaria, Germany). The DOC and DONconcentrations and fluxes were highest inleachates originating from the Oa layer of theforest floor (73 mg C L^–1, 2.3 mg NL^–1 and about 200–350 kg C, 8–10 kg Nha^–1 yr^–1). They were observed to behighly variable over time and decreased in themineral topsoil (17 mg C L^–1, 0.6 mg NL^–1 and about 50–90 kg C, 2.0 to 2.4 kg Nha^–1 yr^–1). The annual variability ofDOC and DON concentrations and subsequentialDOC/DON ratios was substantial in allsolutions. The DOC and DON concentrations inthroughfall were positively correlated withtemperature. The DOC and DON concentrationsdid not show seasonality in the forest floorand mineral soil. Concentrations were notrelated to litterfall dynamics but didcorrespond in part to the input of DOC and DONfrom throughfall. The throughfall contributionto the overall element fluxes was higher forDON than for DOC. Concentrations and fluxes ofDON were significantly correlated to DOC inthroughfall and the Oi layer. However, thecorrelation was weak in Oa leachates. Inaddition, seasonal and annual variation ofDOC/DON ratios indicated different mechanismsand release rates from the forest floor forboth components. The concentrations of DOC andDON in forest floor leachates were in mostcases dependent neither on the pH value orionic strength of the solution, nor on thewater flux or temperature changes. As aconsequence, the DOC and DON fluxes from theforest floor into the mineral soil werelargely dependent on the water flux if annualand biweekly time scales are considered.     

不同火烈度对四川泸山林场云南松林土壤有机碳组分的影响

林火通过改变土壤理化性质显著影响土壤固碳能力及土壤有机碳各组分含量。阐明不同火烈度对云南松林土壤有机碳的影响,对于云南松林火后生态系统恢复及土壤碳库管理具有重要意义。以火烧2年后四川泸山林场云南松林0-5、5-10、10-15、15-20cm层土壤为研究对象,根据不同火烈度（未过火、低烈度、中烈度、高烈度）设置3块20m×30m样地,共12块样地,野外采集土壤样品,室内测定土壤全氮、容重等理化性质和土壤有机碳（Soil organic carbon,SOC）、土壤活性有机碳（易氧化有机碳,Readily oxidizable carbon,ROC;颗粒有机碳,Particulate organic carbon,POC;微生物生物量碳,Microbial biomass carbon,MBC;水溶性有机碳,Water-soluble organic carbon,WSOC）,采用方差分析研究不同火烈度下土壤有机碳各组分含量的差异性和变化趋势,并采用相关性分析和冗余分析从土壤理化性质角度探究林火对土壤有机碳的影响机制。研究结果表明：（1）不同火烈度显著影响土壤理化性质,其中土壤全氮、有效磷、阳离子交换量、毛管孔隙度、总孔隙度随火烈度增大而增加,而土壤pH、速效钾、碱解氮、容重随火烈度增大而减少。（2） SOC随火烈度增大而减少,其中5-10cm土层降幅最大为44.79%;4种土壤活性有机碳随火烈度增大而减小,其中0-5cm土层下降幅度最明显,各组分降幅大小依次为ROC （36.31%-61.31%） > POC （30.05%-53.61%） > MBC （20.60%-48.19%） > WSOC （13.47%-29.29%）,不同组分对林火的不同响应显著影响了土壤ROC、POC、WSOC的占比。（3）土壤有机碳及其活性组分与阳离子交换量、毛管孔隙度、总孔隙度、有效磷、碱解氮呈显著正相关（P<0.05）,与容重呈极显著负相关（P<0.01）。火烧后土壤有机碳的变化主要与阳离子交换量、容重、全氮、含水率有关,且阳离子交换量影响最大（解释率为61.7%）。