C and N storage in living trees within Finland since 1950s

Living biomass contains 45 to 60% carbon and 0.05 to 3% nitrogen, in dry weight. Like throughout Europe, the amount of living biomass in Finnish forests has increased on average over the last decades, largely because of changes in forest management. The storage of organic C and N in biomass has also increased.Changes in biomass vary between regions. Data were analysed on changes in the last 30–40 years in C and N storage in living trees in Finland, subdivided into 20 regions. Tree biomass increased in 17 regions, and decreased in 3 regions. The storage rate varied between -170 and +480 kg C ha^-1 a^-1, and between –0.5 and +1.2 kg N ha^-1 a^-1.Nitrogen accumulation in trees was less than 15% of atmospheric N deposition in all regions. Although the eventual increase of the nitrogen concentration in tree tissues was omitted, it is not possible that living biomass has been the major sink for atmospheric N deposition to forests. A hypothesis is presented that the main sink is litter layer and organic soil. Carbon can also be accumulating in soils essentially faster than hitherto estimated in analyses of carbon budgets of European forests.Died on September 2, 1994.     

Pine Forest Floor Carbon Accumulation in Response to N and PK Additions: Bomb <Superscript>14</Superscript>C Modelling and Respiration Studies

The addition of nitrogen via deposition alters the carbon balance of temperate forest ecosystems by affecting both production and decomposition rates. The effects of 20 years of nitrogen (N) and phosphorus and potassium (PK) additions were studied in a 40-year-old pine stand in northern Sweden. Carbon fluxes of the forest floor were reconstructed using a combination of data on soil ¹⁴C, tree growth, and litter decomposition. N-only additions caused an increase in needle litterfall, whereas both N and PK additions reduced long-term decomposition rates. Soil respiration measurements showed a 40% reduction in soil respiration for treated compared to control plots. The average age of forest floor carbon was 17 years. Predictions of future soil carbon storage indicate an increase of around 100% in the next 100 years for the N plots and 200% for the NPK plots. As much as 70% of the increase in soil carbon was attributed to the decreased decomposition rate, whereas only 20% was attributable to increased litter production. A reduction in decomposition was observed at a rate of N addition of 30 kg C ha^–1 y^–1, which is not an uncommon rate of N deposition in central Europe. A model based on the continuous-quality decomposition theory was applied to interpret decomposer and substrate parameters. The most likely explanations for the decreased decomposition rate were a fertilizer-induced increase in decomposer efficiency (production-to-assimilation ratio), a more rapid rate of decrease in litter quality, and a decrease in decomposer basic growth rate.     

模拟氮沉降对华西雨屏区天然常绿阔叶林凋落叶分解过程中基质质量的影响

为理解氮沉降对华西雨屏区天然常绿阔叶林凋落物分解过程的影响,采用立地控制实验和凋落物分解袋法,研究了低氮沉降(L,50 kg N hm~(-2)a~(-1))、中氮沉降(M,150 kg N hm~(-2)a~(-1))和高氮沉降(H,300 kg N hm~(-2)a~(-1))对华西雨屏区天然常绿阔叶林凋落叶分解过程中基质质量的影响。结果表明:N沉降抑制了凋落叶的分解,并随着N沉降量的增加,抑制作用增强。N沉降遏制了凋落叶的C、N释放和纤维素降解,促进了P释放。N沉降提高了凋落叶的C/P比,中氮和高氮处理提高了凋落叶C/N比。N沉降显著增加了凋落叶N、木质素和纤维素的含量,分解1年后,各N沉降处理的木质素/N和纤维素/N均显著高于对照。N沉降提高了质量残留率与C/N、木质素/N和纤维素/N的相关性,降低了与C/P的相关性。可见,模拟N沉降显著影响了华西雨屏区天然常绿阔叶林凋落叶分解过程中的基质质量,进而影响了凋落叶的分解过程。     

Multiyear fate of a 15N tracer in a mixed deciduous forest: retention,redistribution, and differences by mycorrhizal association

The impact of atmospheric nitrogen deposition on forest ecosystems depends in large part on its fate. Past tracer studies show that litter and soils dominate the short‐term fate of added ¹⁵N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a ¹⁵N‐ tracer over 5–6 years in a mixed deciduous stand that was evenly composed of trees with ectomycorrhizal and arbuscular mycorrhizal associations. The tracer was expected to slowly mineralize from its main initial fate in litter and surface soil, with some ¹⁵N moving to trees, some to deeper soil, and some net losses. Recovery of added ¹⁵N in trees and litterfall totaled 11.3% both 1 and 5–6 years after the tracer addition, as ¹⁵N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree ¹⁵N recovery amounted to 12–14 kg C per kg of N deposition. Tree ¹⁵N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of ¹⁵N from detrital to plant pools. In both years, ectomycorrhizal trees gained 50% more of the tracer than did trees with arbuscular mycorrhizae. Much of the ¹⁵N recovered in wood occurred in tree rings formed prior to the ¹⁵N addition, demonstrating the mobility of N in wood. Tracer recovery rapidly decreased over time in surface litter material and accumulated in both shallow and deep soil, perhaps through mixing by earthworms. Overall, results showed redistribution of tracer ¹⁵N through trees and surface soils without any losses, as whole‐ecosystem recovery remained constant between 1 and 5–6 years at 70% of the ¹⁵N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.     

Nitrogen deposition and plant species interact to influence soil carbon stabilization

Anthropogenic nitrogen (N) deposition effects on soil organic carbon (C) decomposition remain controversial, while the role of plant species composition in mediating effects of N deposition on soil organic C decomposition and long‐term soil C sequestration is virtually unknown. Here we provide evidence from a 5‐year grassland field experiment in Minnesota that under elevated atmospheric CO₂ concentration (560 ppm), plant species determine whether N deposition inhibits the decomposition of soil organic matter via inter‐specific variation in root lignin concentration. Plant species producing lignin‐rich litter increased stabilization of soil C older than 5 years, but only in combination with elevated N inputs (4 g m^?2 year^?1). Our results suggest that N deposition will increase soil C sequestration in those ecosystems where vegetation composition and/or elevated atmospheric CO₂ cause high litter lignin inputs to soils.     

川西亚高山不同森林生态系统碳氮储量及其分配格局

森林采伐和恢复是影响森林碳氮储量的重要因素。以川西亚高山岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林为研究对象,采用样地调查和生物量实测的方法,研究了不同森林生态系统各组分碳、氮储量及其分配特征。结果表明岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林生态系统碳储量分别为611.18、252.31、363.07 tC/hm~2和239.06 tC/hm~2;氮储量分别为16.44、12.11、15.48 tN/hm~2和8.92 tN/hm~2。恢复林分与原始林碳储量在土壤—植被的分配格局发生了变化,而氮储量未发生变化。岷江冷杉原始林以植被碳储量为主,恢复林分以土壤为主,氮储量均以土壤为主。乔木层碳储量分别占生态系统总储量的56.65%、17.63%、13.57%和22.05%,土壤层(0—80 cm)分别占32.03%、69.87%、76.20%和72.12%;土壤层氮储量占生态系统总储量的76.80%—92.58%。植物残体碳氮储量分别占生态系统总储量的4.40%—9.83%和2.94%—7.08%,林下植被所占比例最小。空间格局上,岷江冷杉原始林植被部分具有较高的碳储量,应进行保护。3种恢复林分具有较高的碳汇潜力,且地上/地下碳储量较低,表明其碳汇潜力尤其表现在地上部分。天然次生林利于土壤有机碳的积累,而人工林乔木层碳储量较高。     

Effects of nitrogen addition on litter decomposition, soil microbial biomass, and enzyme activities between leguminous and non-leguminous forests

Anthropogenic nitrogen (N) deposition is an expanding problem that affects the functioning and composition of forest ecosystems, particularly the decomposition of forest litters. Legumes play an important role in the nitrogen cycle of forest ecosystems. Two litter types were chosen from Zijin Mountain in China: Robinia pseudoacacia leaves from a leguminous forest (LF) and Liquidambar formosana leaves from a non-leguminous forest (NF). The litter samples were mixed into original forest soils and incubated in microcosms. Then, they were treated by five forms of N addition: NH₄ ⁺, NO₃ ^?, urea, glycine, and a mixture of all four. During a 6-month incubation period, litter mass losses, soil microbial biomass, soil pH, and enzyme activities were investigated. Results showed that mixed N and NO₃ ^?-N addition significantly accelerated the litter decomposition rates of LF leaves, while mixed N, glycine-N, and urea-N addition significantly accelerated the litter decomposition rates of NF leaves. Litter decomposition rates and soil enzyme activities under mixed N addition were higher than those under single form of N additions in the two forest types. Nitrogen addition had no significant effects on soil pH and soil microbial biomass. The results indicate that nitrogen addition may alter microbial allocation to extracellular enzyme production without affecting soil microbial biomass, and then affected litter decomposition process. The results further reveal that mixed N is a more important factor in controlling litter decomposition process than single form of N, and may seriously affect soil N cycle and the release of carbon stored belowground.     

Impact of Agricultural Land-use Change on Carbon Storage in Boreal Alaska

Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest‐field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m^?2 lower than C stocks in original forest soils. This suggests that soil C losses from land‐use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60‐year‐old grassland showing net ecosystem C gain of 2.1 kg m^?2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land‐use change were smaller than reductions in C stocks (34% or 0.31 kg m^?2), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high‐C forest‐floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m^?2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land‐use change. However, by selecting relatively C‐poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.     

Ecological and site historical aspects of N dynamics and current N status in temperate forests

Ecological developments during Holocene age and high atmospheric depositions since industrialization have changed the N dynamics of temperate forest ecosystems. A number of different parameters are used to indicate whether the forests are N‐saturated or not, most common among them is the occurrence of nitrates in the seepage water below the rooting zone. The use of different definitions to describe N saturation implies that the N status of ecosystems is not always appropriately assessed. Data on N dynamics from 53 different German forests were used to classify various development states of forest ecosystems according to the forest ecosystem theory proposed by Ulrich for which N balances of input – (output plus plant N increment) were used. Those systems where N output equals N input minus plant N increment are described as (quasi‐) Steady State Type. Those forests where N output does not equal N input minus plant N increment as in a ‘transient state.’ Forests of the transient state may lose nitrogen from the soil (Degradation Type) or gain nitrogen [e.g., from atmospheric depositions (Accumulation Type)]. Forest ecosystems may occur in four different N states: (a) (quasi‐) Steady State Type with mull type humus, (b) Degradation Type with mull type humus, (c) Accumulation Type with moder type humus, and (d) (quasi‐) Steady State Type with moder type humus. Forests with the (quasi‐) steady state with mull type humus in the forest floor (n= 8) have high‐soil pH values, high N retention by plant increment, high N contents in the mineral soils, and have not undergone large changes in the N status. Forests of the Degradation Type lose nitrogen from the mineral soil (currently degradation is occurring on one site). Most forests that have moder or mor type humus and low‐soil pH values, and low N contents in the mineral soil have gone through the transient state of organic matter loss in the mineral soils. They accumulate organic matter in the forest floor (accumulation phase, currently 21 sites are accumulating 6–21 kg N ha^?1 yr^?1) or have reached a new (quasi‐) steady state with moder/mor type humus (n= 15). N retention in the accumulation phase has significantly increased in soil with N deposition (r²= 0.38), soil acidity (considering thickness of the forest floor as indices of soil acidity, r²= 0.43) and acid deposition (sulfate deposition, r²= 0.39). Retention of N (4–20 kg N ha^?1 yr^?1) by trees decreased and of soils increased with a decrease in the availability of base cations indicating the important role of trees for N retention in less acid soils and those of soils in more acid soils. Ecosystem theory could be successfully applied on the current data to understand the dynamics of N in temperate forest ecosystems.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Anthropogenic N Deposition Increases Soil C Storage by Decreasing the Extent of Litter Decay: Analysis of Field Observations with an Ecosystem Model

Recent meta-analyses of experimental studies simulating increased anthropogenic nitrogen (N) deposition in forests reveal greater soil carbon (C) storage under elevated levels of atmospheric N deposition. However, these effects have not yet been included in ecosystem-scale models of soil C and N cycling and it is unclear whether increased soil C storage results from slower decomposition rates or a reduced extent of decomposition (for example, an increase in the amount of litter entering slowly decaying humus pools). To test these alternatives, we conducted a meta-analysis of litter decomposition data. We then used the results from our meta-analysis to model C and N cycling in four sugar maple forests in Michigan using an ecosystem process model (TRACE). We compared model results testing our alternative hypotheses to field data on soil C storage from a 17-year N deposition experiment. Using data from published litter decomposition studies in forests, we determined that, on average, exogenous N inputs decreased lignin decomposition rates by 30% and increased cellulose decomposition by 9%. In the same set of litter decomposition studies increased exogenous N availability increased the amount of litter entering slowly decaying humus pools in a manner significantly related to the lignocellulose index of decaying litter. Incorporating changes to decomposition rates in TRACE did not accurately reproduce greater soil C storage observed in our field study with experimentally elevated N deposition. However, when changes in the extent of decomposition were incorporated in TRACE, the model produced increased soil C storage by increasing the amount of litter entering the humus pool and accurately represented C storage in plant and soil pools under experimental N deposition. Our modeling results and meta-analysis indicate that the extent of litter decay as humus is formed, rather than slower rates of litter decay, is likely responsible for the accumulation of organic matter, and hence soil C storage, under experimental N deposition. This effect should be incorporated in regional to global-scale models simulating the C balance of forest ecosystems in regions receiving elevated N deposition.     

模拟N沉降下不同林龄马尾松林凋落叶分解-土壤C、N化学计量特征

模拟N沉降对森林生态系统的影响是当今全球变化生态学研究的一个热点问题,土壤碳库对N沉降比较敏感,N沉降增加了凋落叶分解过程中外源N含量,间接影响凋落叶分解的化学过程并改变凋落叶分解速率,因此,研究模拟N沉降下凋落叶分解-土壤C-N关系对预测森林C吸存有重要意义。利用原位分解袋法研究了模拟N沉降下三峡库区不同林龄马尾松林(Pinus massoniana)凋落叶分解过程中凋落叶-土壤C、N化学计量响应及其关系;N沉降水平分对照(CK,0 g m~(-2)a~(-1))、低氮(LN,5 g m~(-2)a~(-1))、中氮(MN,10 g m~(-2)a~(-1))和高氮(HN,15 g m~(-2)a~(-1))。结果表明:分解540 d后,N沉降促进20年生和30年生马尾松林凋落叶分解,46年生马尾松林中仅低氮处理促进凋落叶分解,4种处理均是30年生分解最快,说明同一树种起始N含量低的凋落叶对N沉降呈正响应,N沉降处理促进起始N含量低的凋落叶分解,起始N含量高的凋落叶分解过程中易达到"N饱和"。N沉降抑制20年生和46年生凋落叶C释放(低于对照0.62%—6.69%),促进30年生C释放(高于对照0.28%—5.55%);30年生和46年生林分N固持量均高于对照(高于对照0.15%—21.34%),20年生则低于对照(5.70%—13.87%),说明模拟N沉降处理促进起始C含量低的凋落叶C释放和起始N含量低的凋落叶N固持。N沉降处理下仅30年生马尾松林土壤有机碳较对照增加,且土壤有机质与凋落叶C、N和分解速率呈正相关,与凋落叶C/N比呈显著负相关;土壤总氮与凋落叶分解速率、凋落叶N含量呈正相关,土壤有机碳/总氮比与凋落叶C、N含量呈正相关;对照处理中凋落叶分解指标对土壤养分影响顺序是分解速率凋落物C含量凋落物C/N比凋落物N含量,低、中、高氮处理中则是凋落物C含量分解速率凋落物N含量凋落物C/N比。研究表明低土壤养分含量马尾松林对N沉降呈正响应,N沉降促进低土壤养分马尾松林凋落叶分解并提高土壤肥力;凋落叶质量和土壤养分含量低的生态系统土壤C对N沉降响应更显著。     

模拟氮沉降对华西雨屏区撑绿 杂交竹凋落物分解的影响

从2008年1月至2010年1月,对华西雨屏区撑绿杂交竹(Bambusa pervariabilis × Dendrocala mopsi)人工林进行了模拟氮沉降试验,氮沉降水平分别为对照(CK, 0 g · m^-2 · a^-1)、低氮(5 g · m^-2 · a^-1)、中氮(15 g · hm^-2 · a^-1)和高氮(30 g · m^-2 · a^-1)。利用凋落袋法对杂交竹凋落叶和凋落箨进行原位分解试验,并在每月下旬定量地对各处理施氮(NH₄NO₃)。结果表明,自然状态下杂交竹凋落叶和凋落箨分解95%所需时间分别为2.9,1.5 a;氮沉降显著抑制了凋落叶的分解,在分解后期3个氮沉降处理凋落叶无灰分质量残留率均显著大于对照,氮沉降对凋落箨分解无明显影响;氮沉降显著抑制了凋落叶中木质素和纤维素的分解。杂交竹凋落叶在分解后期质量损失缓慢,处于较稳定状态,氮沉降的增加使得凋落物的残留率稳定在一个更高的水平,表明氮沉降的增加可能会使更多的凋落物残体和稳定有机质留存于杂交竹林土壤中,从而增加杂交竹林土壤碳贮存。     

黄土区不同类型土壤可溶性有机氮的含量及特性

测定了黄土高原地区不同生态系统土壤中可溶性有机氮（SON）和游离氨基酸的含量,并分析了其与土壤其他性质之间的关系。结果表明,黑垆土、红油土和淋溶褐土中SON的平均含量分别为24．75、39．10mg／kg和41．80mg／kg,占可溶性总氮（TSN）的51．25％、68．28％和68．57％,分别为土壤全氮的2．54％、3．75％和4．00％;土壤游离氨基酸的含量分别为7．18、7．42mg／kg和7．41mg／kg,分别占SON的30．53％、19．23％和17．50％,占全氮的0．74％、0．71％和0．71％．方差分析结果表明,土壤类型对土壤SON含量及其在TSN和全氮中所占的比例、游离氨基酸在SON中所占的比例有显著影响,而对游离氨基酸的含量及其占土壤全氮比例的影响未达显著水平。枯枝落叶层中SON含量（248．26mg／kg）为农田土壤（24．75mg／kg）的10倍左右,且林地土壤0～20cm土层SON含量（31．03mg／kg）显著的高于当地农田,两种生态系统0～20cm土层土壤中游离氨基酸含量（7．18～7．32mg／kg）相当,但均极显著低于枯枝落叶层中游离氨基酸平均含量（18．57mg／kg）。相关分析结果表明,TSN、SON以及游离氨基酸与全氮、硝态氮、铵态氮、有机质等各养分之间均有极显著的相关性。     

滇中常绿阔叶林凋落物养分释放及生态化学计量特征对模拟N沉降的响应

为研究N沉降下凋落物养分释放及生态化学计量特征,以滇中磨盘山常绿阔叶林为研究对象,利用尼龙网袋法布设凋落物(凋落叶、凋落枝)原位分解试验,设置不同施N处理:对照(CK,0 g N·m^-2·a^-1)、低氮(LN,5 g N·m^-2·a^-1)、中氮(MN,15 g N·m^-2·a^-1)和高氮(HN,30 g N·m^-2·a^-1)。结果表明: 模拟N沉降1年后,凋落叶、凋落枝和土壤的C、N含量均表现为随着N沉降量的增加而逐渐升高,增幅分别为0.3%～8.2%、4.9%～69.0%;C/N则表现为随着N沉降量的增加逐渐降低,降幅为0.8%～37.8%;凋落枝P含量、C/P、N/P在各处理下差异均不显著。处理时间与施N水平均显著影响凋落叶、凋落枝及土壤的N、P含量及C/N、C/P、N/P;1年分解过程中,凋落物C、N、P残留率依次呈释放、淋溶-富集-释放、淋溶-富集的模式,外源N显著抑制了凋落物C、N、P释放过程;土壤C、P含量与凋落物N、P含量呈显著正相关,土壤N含量与凋落物C、N含量呈显著正相关。N沉降下常绿阔叶林凋落物与土壤生态化学计量具有显著相关性,研究滇中常绿阔叶林凋落物分解和生态化学计量特征有助于了解森林生态系统凋落物分解过程对N沉降的响应机理。     

Nitrogen deposition promotes the production of new fungal residues but retards the decomposition of old residues in forest soil fractions

Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal‐ and bacterial‐derived microbial residues in soil. We made use of a 4‐year combined CO₂ enrichment and N deposition experiment in model forest ecosystems, providing a distinct ¹³C signal for ‘new’ and ‘old’ C in soil organic matter and microbial residues measured in density and particle‐size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The ¹³C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4 years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.     

Slow response of soil organic matter to the reduction in atmospheric nitrogen deposition in a Norway spruce forest

Global nitrogen (N) deposition rates in terrestrial environments have quadrupled since preindustrial times, causing structural and functional changes of ecosystems. Different emission reduction policies were therefore devised. The aim of our study was to investigate if, and over what timescale, processes of soil organic matter (OM) transformation respond to a decline in atmospheric N deposition. A N‐saturated spruce forest (current N deposition: 34 kg ha^?1 yr^?1; critical N load: 14 kg ha^?1 yr^?1), where N deposition has been reduced to 11.5 kg ha^?1 yr^?1 since 1991, was studied. Besides organic C and organic and inorganic N, noncellulosic carbohydrates, amino sugars and amino acids were determined. A decline in organic N in litter indicated initial effects at plant level. However, there were no changes in biomarkers upon the reduction in N deposition. In addition, inorganic N was not affected by reduced N deposition. The results showed that OM cycling and transformation processes have not responded so far. It was concluded that no direct N deposition effects have occurred due to the large amount of stored organic N, which seems to compensate for the reduction in deposited N. Obviously, the time span of atmospheric N reduction (about 14.5 years) is too short compared with the mean turnover time of litter to cause indirect effects on the composition of organic C and N compounds. It is assumed that ecological processes, such as microbial decomposition or recycling of organic N and C, react slowly, but may start within the next decade with the incorporation of the new litter.     

杉木人工林凋落物生态化学计量与土壤有效养分对长期模拟氮沉降的响应

凋落物分解的快慢和养分释放的速度决定了生态系统中土壤有效养分的供应。探讨全球变化条件下森林生态系统凋落物与土壤养分的变化规律,有利于深入认识凋落物-土壤相互作用的养分调控因素,从而揭示生态系统C、N、P循环。通过模拟氮沉降增加试验,分4个水平处理,分别为0、60、120、240 kg N hm~(-2)a~(-1)。模拟氮沉降13年后,分析了杉木人工林凋落物中不同组分(落叶、落枝、落果)生态化学计量与土壤有效养分(有效氮、碱解氮、速效磷、速效钾)的关系。结果表明:氮沉降(N1、N2和N3)显著提高了落叶和落枝的N含量,平均增幅分别为35.27%和32.21%;高水平氮沉降(N3)处理显著降低了落叶和落枝的C/N,平均降幅分别为25.95%和22.32%,但N3增加了落枝和落果N/P,平均增幅分别为38.4%和31.7%;氮沉降对凋落物各组分的C、P和C/P均影响不显著。氮沉降处理显著增加了土壤NO_3~--N和NH_4~+-N含量,均表现为N3N2N1N0,其中NO_3~--N含量更容易受氮沉降处理的影响,表现为更大的增幅。N2显著增加0—20 cm土层的碱解氮含量,N1显著降低0—20 cm土层的速效钾,但氮沉降对速效磷含量没有影响。凋落物生态化学计量与土壤有效养分之间的Pearson相关和冗余分析(RDA)表明,凋落物生态化学计量与土壤有效养分之间关系紧密,凋落物P含量(蒙特卡罗检验,P=0.018)和C/P比值(P=0.037)对土壤有效养分影响显著。凋落物中C/N比值、C/P比值与土壤有效养分呈显著负相关,其比值越高越不利于土壤有效养分的累积。     

模拟氮沉降对两种竹林不同凋落物组分分解过程养分释放的影响

利用原位分解袋法研究了华西雨屏区苦竹(Pleioblastus amarus)和撑绿杂交竹(Bambusa pervariabilis × Dendrocala mopsi)人工林几种凋落物组分在模拟氮沉降下分解过程中养分释放状态,试验周期为2 a。氮沉降水平分别为对照(CK, 0 g · m^-2 · a^-1)、低氮(5 g · m^-2 · a^-1)、中氮(15 g · m^-2 · a^-1)和高氮(30 g · m^-2 · a^-1),每月下旬定量地对各处理施氮(NH₄NO₃)。结果表明,苦竹林和杂交竹林凋落物主要由凋落叶、凋落箨和凋落枝组成,其中凋落叶约占80%;两个竹种凋落物在分解过程中养分元素释放的种间差异主要与初始养分元素含量有关;凋落物养分元素初始含量对元素释放模式和最终净释放率的大小具有重要的决定作用;目前,这两种竹林生态系统土壤氮输入主要以大气氮沉降(8.24 g · m^-2 · a^-1)为主,同时凋落物氮输入(苦竹和杂交竹林分别为1.93,5.07 g · m^-2 · a^-1)也是一个重要途径;模拟氮沉降对苦竹凋落物碳、磷、钾、钙元素和杂交竹凋落物碳、氮、磷、钾、钙、镁元素释放的抑制作用较弱,处理与对照之间元素总释放率差异一般小于10%;氮沉降显著抑制了苦竹林凋落物氮元素释放,减小幅度为19.0%-27.2%,但由于氮沉降增加对土壤肥力的直接改良作用,氮沉降的增加并不会因为凋落物分解速率的降低造成植物生长所需养分供应的减少;从短期来看,在氮沉降继续增加的情况下,该地区这类竹林生态系统的碳吸存能力仍可能会因为N沉降对植物生长的促进作用而增加。     

森林土壤氮素转换及其对氮沉降的响应

近几十年人类活动向大气中排放的含氮化合物激增 ,并引起大气氮沉降也成比例增加。目前 ,氮沉降的增加使一些森林生态系统结构和功能发生改变 ,甚至衰退。近 2 0 a欧洲和北美有关氮沉降及其对森林生态系统的影响方面的研究较多 ,而我国少有涉及。森林土壤氮素转换是森林生态系统氮素循环的一个重要的组成部分 ,而矿化、硝化和反硝化作用是其核心过程 ,氮沉降作为驱动因子势必改变森林土壤氮素转换速度、方向和通量。根据国外近 2 0 a有关研究 ,首先介绍了森林土壤氮素转换过程和强度 ,论述森林土壤氮素在生态系统氮素循环中的作用 ,然后在此基础上 ,介绍了氮沉降对森林土壤氮素循环的研究途径 ,探讨了氮沉降对森林土壤氮素矿化、硝化和反硝化作用的影响及其机理