大兴安岭北部不同海拔天然林土壤胞外酶化学计量特征及其季节动态

研究了大兴安岭北部奥克里堆山不同海拔(750～1420 m)天然林土壤胞外酶活性(EEA)和酶计量比的变化特征及影响机制。结果表明: 海拔、季节及其交互作用对β-葡萄糖苷酶(BG)、N-乙酰-β-氨基葡萄糖苷酶(NAG)、亮氨酸氨基肽酶(LAP)和酸性磷酸酶(AP)活性均存在显著影响。5月,BG和NAG活性随海拔升高呈逐渐增大趋势,AP活性随海拔升高呈先升高后降低的趋势。7月,NAG活性随海拔升高而增大,AP活性则先升高后降低。9月,不同海拔上NAG活性变化幅度较大,在1420 m处活性最高,为124.22 nmol·h^-1·g^-1。随着海拔的升高,酶化学计量比ln(BG)∶ln(NAG+LAP)呈降低的趋势。除海拔830 m外,7月化学计量比值最高。土壤C、N、P转化酶活性对数转换后的比值为1∶1.25∶0.82。海拔和土壤温度是影响土壤胞外酶的主要因素,土壤温度与BG、NAG和AP呈显著正相关。ln(BG)∶ln(NAG+LAP)和ln(NAG+LAP)∶ln(AP)与pH分别呈显著正相关和负相关,与DOC分别呈显著负相关和正相关,而ln(BG)∶ln(AP)受土壤容重的影响较大。     

Effects of desertification on the C:N:P stoichiometry of soil,microbes, and extracellular enzymes in a desert grassland

为探讨荒漠草地沙漠化对“土壤-微生物-胞外酶”系统生态化学计量的影响机理, 该研究采用空间序列代替时间演替的方法, 研究了宁夏盐池荒漠草地沙漠化过程中土壤、土壤微生物及土壤胞外酶碳(C)、氮(N)、磷(P)生态化学计量的变异特征。结果表明: (1)随着荒漠草地沙漠化的不断加剧, 土壤C、N、P含量和土壤C:P、N:P均呈降低趋势, 而土壤C:N逐渐增加。(2)荒漠草地沙漠化过程中, 土壤微生物生物量C (MBC):微生物生物量P (MBP)、微生物生物量N (MBN):MBP和土壤β-葡萄糖苷酶(BG):N-乙酰氨基葡萄糖苷酶(NAG)逐渐降低, 而土壤BG:磷酸酶(AP)和NAG:AP基本表现为增加趋势。(3)随着荒漠草地沙漠化程度的加剧, 土壤微生物C利用效率CUE_C:N和CUE_C:P与土壤微生物N利用效率NUE_N:C和土壤微生物P利用效率PUE_P:C的变化趋势相反。(4)荒漠草地土壤、土壤微生物生物量和土壤胞外酶C:N化学计量(C:N, MBC:MBN, BG:NAG)与土壤、土壤微生物生物量和土壤胞外酶N:P化学计量(N:P, MBN:MBP, NAG:AP)显著负相关, 而土壤和胞外酶C:N化学计量(C:N, BG:NAG)与土壤和胞外酶C:P化学计量(C:P, BG:AP)显著正相关。土壤N:P与土壤MBN:MBP显著正相关, 而与土壤NAG:AP显著负相关。分析表明, 荒漠草地沙漠化过程中土壤微生物生物量及胞外酶活性随着土壤养分的变化而发生变化; 微生物-胞外酶C:N:P生态化学计量与土壤养分存在协变关系, 为理解荒漠草地土壤-微生物系统C、N、P循环机制提供理论依据。     

Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m² in the 7-year-old stand to a high of 254 g C/m² in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization = 1.853−0.276 × age × e ^{−0.814 × age}; R ² = 0.381; P = 0.002). Total N mineralization decreased from 2.8 g N/m² in the 1-year-old stand to a minimum value of 0.5 g N/m² in the 14-year-old stand, and then increased to about 1.5 g N/m² in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.     

Variations in leaf litter decomposition across contrasting forest stands and controlling factors at local scale

Aims Litter decomposition is a critical pathway linking the above- and belowground processes. However, factors underlying the local spatial variations in forest litter decomposition are still not fully addressed. We investigated leaf litter decomposition across contrasting forest stands in central China, with objective to determine the spatial variations and controlling factors in forest floor leaf litter decomposition in relation to changes in forest stands in a temperate forest ecosystem.Methods Leaf litter decomposition was studied by using litterbag method across several typical forest stand types in Baotianman Nature Reserve, central China, including pure stands of Quercus aliena var. acuteserrata, Q. glandulifera var. brevipetiolata and Q. variabilis, respectively, and mixed pine/oak stands dominated by Pinus armandii and Q. aliena var. acuteserrata, as well as stands of pure Q. aliena var. acuteserrata trees ranging in stand age from ~40 to>160 years. Measurements were made on litter mass remaining and changes in litter chemistry during decomposition over a 2-year period, along with data collections on selective biotic and environmental factors. A reciprocal transplant experiment involving Q. aliena var. acuteserrata and Q. variabilis was concurrently carried out to test the occurrence of 'home-field advantage (HFA)' in local forests when only considering contrasting oak tree species. Correlation analyses and path analyses were performed to identify the dominant drivers and their relative contributions to variations in leaf litter decomposition.Important findings Significant variations were found in the rate of leaf litter decomposition among stands of different tree species but not among stand age classes. The values of decay constant, k, varied from 0.62 in Q. aliena var. acuteserrata stands to 0.56 in Q. variabilis stands. The reciprocal litter transplant experiment showed that the rate of leaf litter decomposition was on average 5% slower in home-fields than on reciprocal sites. Path analysis identified litter acid-unhydrolyzable residue (AUR) to N ratio, soil microbial biomass carbon (MBC), soil pH and soil organic carbon (SOC) as most prominent factors controlling the rate of leaf litter decomposition, collectively accounting for 57.8% of the variations; AUR/N had the greatest negative effect on k value, followed by weaker positive effects of SOC and MBC. Our findings suggest that tree species plays a primary role in affecting forest floor leaf litter decomposition by determining the litter quality, with site environment being a secondary factor contributing to the local variations in leaf litter decomposition in this temperate forest ecosystem.     

Carbon and nitrogen storage in an age-sequence of <Emphasis Type="Italic">Pinus densiflora</Emphasis> stands in Korea

The carbon (C) and nitrogen (N) storage capabilities of Pinus densiflora in six different stand ages (10, 27, 30, 32, 44, and 71 years old) were investigated in Korea. Thirty sample trees were destructively harvested and 12 were excavated. Samples from the above and belowground tree components, coarse woody debris (CWD), forest floor, and mineral soil (0–30 cm) were collected. Tree biomass was highest in the 71-year-old stand (202.8 t ha⁻¹) and lowest in the 10-year-old stand (18.4 t ha⁻¹). C and N storage in the mineral soil was higher in the 71-year-old stand than in the other stands, mainly due to higher soil C and N concentrations. Consequently, the total ecosystem C and N storage (tree+forest floor+CWD+soil) was positively correlated with stand age: increasing from a minimum in the 10 year old stand (18.8 t C ha⁻¹ and 1.3 t N ha⁻¹) to a maximum in the 71-year-old stand (201.4 t C ha⁻¹ and 8.5 t N ha⁻¹). The total ecosystem C storage showed a similar sigmoidal pattern to that of tree C storage as a function of the age-sequence, while N storage in the CWD, forest floor and mineral soil showed no significant temporal trends. Our results provide important insights that will increase our understanding of C and N storage in P. densiflora stands and our ability to predict changes according to stand age in the region.     

Microbial biomass C and N,and mineralizable-N,in litter and mineral soil under Pinus radiata on a coastal sand: Influence of stand age and harvest management

Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0–10 cm and 10–20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation, and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management stands were sampled in the fifth year after the second-rotation establishment.In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of P. radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor had been removed; they were higher in associated plots in which the original forest floor had been removed but fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments, viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C, and especially total N, in 0–10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly suggest that the microbial populations in mineral soil had a high metabolic activity.On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in the litter plus mineral soil (0–20 cm depth) after five years of second-rotation growth comprised 7.3% of the total ecosystem N; values in the other treatments ranged between 5.6 and 6.0%.Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous under-storey species, in conserving pools of potentially available N during the early stages of tree development.     

氮磷添加及林分密度对大叶相思林土壤化学性质的影响

大气氮(N)沉降随着人类的活动而日趋严重, 加上中国热带亚热带红壤普遍缺磷(P), 许多森林生态系统由于广泛使用磷肥而产生P富集, 直接影响了森林土壤化学特性。林分密度改变林地的光照、温度、湿度和凋落物持水量, 从而影响土壤特性。为了解外源性N和P添加与林分密度对大叶相思(Acacia auriculiformis)林地土壤化学性质的影响, 为大叶相思人工林的种植密度和土壤养分管理提供科学依据, 该研究于2013到2015年, 以4种不同密度(1 667、2 500、4 444和10 000 trees·hm ^-2)的10年生大叶相思人工林为研究对象, 分别进行添加N、P和N+P处理, 在试验结束时采集0-10 cm土壤, 对其pH、有机质含量、N含量、P含量和钾(K)含量进行了测定分析。结果表明: 施N和N+P均显著降低了土壤的pH和速效K含量, 显著提高了林地土壤的碱解N含量。施N还显著提高了林分土壤的全N含量, 施P显著提高了土壤pH, 降低了林分土壤的全N含量。施P和N+P显著提高了土壤有机质、全P和有效P含量。随着林分密度的增加, 各处理的土壤有机质、全N、碱解N、全P、有效P和速效K含量显著提高。N、P添加处理和密度处理对大叶相思林的土壤pH、有机质和N、P、K含量有显著的交互作用。总体来看, N添加、P添加、林分密度及其交互作用对大叶相思的土壤化学性质有显著影响。     

Indirect Effects of Nitrogen Amendments on Organic Substrate Quality Increase Enzymatic Activity Driving Decomposition in a Mesic Grassland

The fate of soil organic carbon (SOC) is determined, in part, by complex interactions between the quality of plant litter inputs, nutrient availability, and the microbial communities that control decomposition rates. This study explores these interactions in a mesic grassland where C and nitrogen (N) availability and plant litter quality have been manipulated using both fertilization and haying for 7 years. We measured a suite of soil parameters including inorganic N, extractable organic C and N (EOC and EON), soil moisture, extracellular enzyme activity (EEA), and the isotopic composition of C and N in the microbial biomass and substrate sources. We use these data to determine how the activity of microbial decomposers was influenced by varying levels of substrate C and N quality and quantity and to explore potential mechanisms explaining the fate of enhanced plant biomass inputs with fertilization. Oxidative EEA targeting relatively recalcitrant C pools was not affected by fertilization. EEA linked to the breakdown of relatively labile C rich substrates exhibited no relationship with inorganic N availability but was significantly greater with fertilization and associated increases in substrate quality. These increases in EEA were not related to an increase in microbial biomass C. The ratio of hydrolytic C:N acquisition enzymes and δ¹³C and δ¹⁵N values of microbial biomass relative to bulk soil C and N, or EOC and EON suggest that microbial communities in fertilized plots were relatively C limited, a feature likely driving enhanced microbial efforts to acquire C from labile sources. These data suggest that in mesic grasslands, enhancements in biomass inputs and quality with fertilization can prompt an increase in EEA within the mineral soil profile with no significant increases in microbial biomass. Our work helps elucidate the microbially mediated fate of enhanced biomass inputs that are greater in magnitude than the associated increases in mineral soil organic matter.     

Weather or weathering? Growth of Nothofagus dombeyi on volcanic soils differing in nitrogen and phosphorus concentrations

Aims We tested whether—in addition to weather conditions—the concentrations of nitrogen and phosphorus in the substrate have an effect on the radial stem increment of Nothofagus dombeyi trees in old-growth forest stands on volcanic soil at the western slopes of the Andes in South-Central Chile.Methods We took soil samples and tree increment cores from five proximate sites (1000–1300 m a.s.l.) that are located in the volcanic region of the Conguillío National Park and differ in the age of the substrate (Miocene—3500 years B.P.) and in its concentrations of nitrogen (N) and phosphorus (P). The soil samples were also analysed on their concentrations of other plant mineral nutrients, carbon (C) and nitrogen isotope ratios (δ 15 N). Tree-ring widths and the stem basal area increment (BAI) were related to climate parameters. In selected tree rings, the stable isotope ratios of carbon (δ 13 C) and oxygen (δ 18 O) were determined and related to growth and climate parameters.Important findings Consistent with theory, the soils on the oldest substrate showed the highest (least negative) δ 15 N values, but mineral N was the only nutrient whose concentration exhibited a straightforward (increasing) relationship with increasing substrate age. The BAI was largest on the soil with the highest concentration of plant-available P. In contrast to BAI, tree-ring chronologies did not differ among the study sites. However, tree-ring chronologies and BAI exhibited significantly positive correlations with summer precipitation, and negative correlations, with summer (December) temperature. A negative correlation was found between δ 13 C and precipitation anomalies in the growing season (November–March). We interpret the negative correlations between growth and temperature, and between δ 13 C and δ 18 O in the tree rings, as an impairment of net carbon assimilation by anomalously warm weather conditions during the growing season. We conclude that the growth of N. dombeyi is mainly affected by high temperature and low precipitation in spring and summer irrespective of the substrate's age, and enhanced by higher concentrations of plant-available P in the soil. Our results may be representative of N. dombeyi stands on volcanic substrate within their principal distribution range along the Andes of South America.     

Responses of litter decomposition to temperature along a chronosequence of tropical montane rainforest in a microcosm experiment

We conducted a microcosm experiment for studying the decomposition of Altingia obovata leaf litter by the decomposer community at 20 and 30°C from three forest stands (namely a 35-year-old secondary forest, a 47-year-old secondary forest, and a primary forest) of a tropical montane rainforest. Our results showed that rank-order of the litter decomposition among the three forest stands was not parallel to the stand age. At 20°C, the mass loss of A. obovata leaf litter from the primary forest was higher than those from the two secondary forests, of which the younger stand showed higher mass loss than did the older one. However, there were no differences in mass loss among these three stands at 30°C. The mass loss for the two secondary forest stands, but not for the primary forest stand, increased significantly from 20 to 30°C. The level of lignin decomposition among the three stands at 20°C corresponded to their forest stand age, i.e., the primary forest > the 47-year-old secondary forest > the 35-year-old secondary forest. A rise of 10°C in temperature significantly increased lignin decomposition for the two secondary forests, while the reverse was true for the primary forest. Carbohydrate decomposition was positively related to the temperature but not to the stand age. The different responses of litter decomposition to the forest stand age and temperature might be due to the differences in the microbial activities among the three forest stands.     

凋落物的树种多样性与杉木人工林土壤生态功能

通过模拟试验和野外调查对凋落物的树种多样性与杉木人工林土壤生态功能之间关系进行了研究。林分取样调查的结果表明 ,有两个树种凋落物覆盖的几个杉阔混交林土壤脲酶和蔗糖酶活性均明显高于只有杉木凋落物覆盖的杉木纯林土壤 ,酸性磷酸酶活性也呈现相同的变化趋势 ,这 3种土壤酶活性均以具有凋落物种类最多的次生常绿阔叶林土壤最高。除了土壤酶活性升高之外 ,杉阔混交林的土壤有机质和全氮含量也明显高于仅有一种杉木凋落物覆盖的杉木纯林土壤。采用杉木叶凋落物与不同阔叶树种凋落物处理土壤的模拟试验结果表明 ,在凋落物投放量和 (1 5NH4 ) 2 SO4 施用量相同的控制条件下 ,随着投放的凋落物树种组成的增加 ,土壤中 1 5N的残留量也随之增加 ,而其损失量却随之减少 ;土壤中杉木幼树对于 1 5N的吸收量以及杉木幼树的单株鲜重也随着处理凋落物组成树种的增加而增加 ,具有不同树种数量的凋落物处理之间差异显著 (p<0 .0 5 )。可见随着凋落物树种多样性的增加 ,不仅土壤有机质和全氮含量这两个基本的质量指标得到明显改善 ,而且土壤酶活性、土壤养分保蓄功能以及保证幼树良好生长等的生态功能明显改善     

Variation in protein complexation capacity among and within six plant species across a boreal forest chronosequence

We investigated among and within species variation in several litter chemical properties, including protein complexation capacity (PCC), for six plant species across a boreal forest chronosequence in northern Sweden across which stand fertility declines sharply with stand age. We hypothesized (1) that evergreen species which dominate in late-successional stands would exhibit higher PCCs than deciduous species that dominate in young stands, (2) that individual species would increase their PCCs in response to nutrient limitation as succession proceeds, and (3) that differences in PCC among litter types would determine their interactive effects with proteins on soil N and C mineralization. The data demonstrated a high PCC, but a low PCC per unit of soluble phenol, for two deciduous species that dominate in early-successional high fertility stands, providing mixed support for our first hypothesis. No species demonstrated a significant correlation between their PCC and stand age, which did not support our second hypothesis. Finally, a soil incubation assay revealed that litter extracts for three of the six species had negative interactive effects with added proteins on N mineralization rates, and that all six species demonstrated positive interactive effects with protein on C mineralization. This pattern did not provide strong support for our third hypothesis, and suggests that N immobilization was likely a more important factor regulating N mineralization than stabilization of proteins into tannin complexes. These data suggest that multiple interactive mechanisms between litter extracts and proteins likely occur simultaneously to influence the availability of N in soils.     

滇西并流河谷区土壤酶活性化学计量学特征与环境因子的关系

横断山河谷区具有极高的景观异质性,气候与植被类型多样化程度较高。为探讨土壤C、N、P、S四种生物元素在滇西怒江、澜沧江、金沙江及元江并流河谷区的区域循环特征,在各河谷的森林、草地、农田中分别取浅层(0～10 cm)土样,测定了土壤中C、N、P、S的循环酶,即β-葡萄糖苷酶(BG)、N-乙酰-β-D-氨基葡萄糖苷酶(NAG)、酸性磷酸酶(AP)、硫酸脂酶(SU)活性,分析了土壤酶活性及其化学计量学特征与环境因素之间的关系。结果表明: 不同流域和不同土地类型下AP、NAG活性均有显著差异;4种酶活性之间均呈显著正相关,BG、NAG、SU活性由东南向西北随采样点的海拔升高而逐渐升高;在各流域土壤中,酶活性的生态化学计量比均为AP∶SU > BG∶SU > NAG∶SU > BG∶NAG > BG∶AP > NAG∶AP;与各流域内的林地和草地相比,农田土壤BG∶NAG较高,而NAG∶AP较低(元江流域除外);农田土壤中AP∶SU、BG∶SU、NAG∶SU在元江流域小于草地和林地,在澜沧江流域和金沙江流域则大于林地而小于草地。土壤酶活性及其化学计量学特征受到土壤理化性质、气候及区位的综合影响,其中土壤理化性质的影响最大。农业活动对C∶N∶P相关酶化学计量学特征具有显著影响,降低了土壤中N分解酶与其他酶活性的计量比,表现为增加了BG∶NAG,降低了NAG∶AP,农业活动对其他酶化学计量学特征的影响较小。     

Variation of carbon and nitrogen stoichiometry along a chronosequence of natural temperate forest in northeastern China

Aims Carbon (C) and nitrogen (N) stoichiometry contributes to understanding elemental compositions and coupled biogeochemical cycles in ecosystems. However, we know little about the temporal patterns of C:N stoichiometry during forest development. The goal of this study is to explore the temporal patterns of intraspecific and ecosystem components' variations in C:N stoichiometry and the scaling relationships between C and N at different successional stages.Methods Along forest development in a natural temperate forest, northeastern China, four age gradients were categorized into ca. 10-, 30-, 70- and 200-year old, respectively, and three 20 m × 20 m plots were set up for each age class. Leaves, branches, fine roots and fresh litter of seven dominant species as well as mineral soil at depth of 0–10 cm were sampled. A Universal CHN Elemental Analyzer was used to determine the C and N concentrations in all samples.Important findings Intraspecific leaf C, N and C:N ratios remained stable along forest development regardless of tree species; while C, N concentrations and C:N ratios changed significantly either in branches or in fine roots, and they varied with tree species except Populus davidiana (P < 0.05). For ecosystem components, we discovered that leaf C:N ratios remained stable when stand age was below ca. 70 years and dominant tree species were light-demanding pioneers such as Betula platyphylla and Populus davidiana, while increased significantly at the age of ca. 200 years with Pinus koraiensis as the dominant species. C:N ratios in branches and fresh litter did not changed significantly along forest development stages. C concentrations scaled isometrically with respect to N concentrations in mineral soil but not in other ecosystem components. Our results indicate that, leaf has a higher intraspecific C:N stoichiometric stability compared to branch and fine root, whereas for ecosystem components, shifts in species composition mainly affect C:N ratios in leaves rather than other components. This study also demonstrated that C and N remain coupled in mineral soils but not in plant organs or fresh litter during forest development.     

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。     

Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Aims Understanding carbon (C) and nitrogen (N) dynamics and their dependence on the stand density of an even-aged, mature forest provides knowledge that is important for forest management. This study investigated the differences in ecosystem total C and N storage and flux between a low-density stand (LD) and a high-density stand (HD) and examined the effects of stand density on aboveground net primary productivity (ANPP), total belowground C allocation (TBCA) and net ecosystem production (NEP) in a naturally regenerated, 65- to 75-year-old Pinus densiflora S. et Z. forest.Methods LD (450 trees ha^-1) and HD (842 trees ha^-1) were established in an even-aged, mature P. densiflora forest in September 2006. The forest had been naturally regenerated following harvesting, and the stand density was naturally maintained without any artificial management such as thinning. The diameter at breast height (DBH ≥ 5.0cm) of all live stems within the stands was measured yearly from 2007 to 2011. To compare C and N storage and fluxes in LD and HD, C and N pools in aboveground and belowground biomass, the forest floor, coarse woody debris (CWD) and soil; soil CO₂ efflux (R S); autotrophic respiration (R A); litter production; and soil N availability were measured. Further, ANPP, TBCA and NEP were estimated from plot-based measurement data.Important findings Ecosystem C (Mg C ha^-1) and N (Mg N ha^-1) storage was, respectively, 173.0±7.3 (mean ± SE) and 4.69±0.30 for LD and 162±11.8 and 4.08±0.18 for HD. There were no significant differences in C and N storage in the ecosystem components, except for soils, between the two stands. In contrast, there were significant differences in aboveground ANPP and TBCA between the two stands (P < 0.05). Litterfall, biomass increment and R S were major C flux components with values of, respectively, 3.89, 3.74 and 9.07 Mg C ha^-1 year^-1 in LD and 3.15, 2.94 and 7.06 Mg C ha^-1 year^-1 in HD. Biometric-based NEP (Mg C ha^-1 year^-1) was 4.18 in LD and 5.50 in HD. Although the even-aged, mature P. densiflora forest had similar C and N allocation patterns, it showed different C and N dynamics depending on stand density. The results of the current study will be useful for elucidating the effects of stand density on C and N storage and fluxes, which are important issues in managing natural mature forest ecosystems.     

Microbial enzyme activity at the watershed scale: response to chronic nitrogen deposition and acute phosphorus enrichment

Microbial enzymes play a critical role in organic matter decomposition and enzyme activity can dynamically respond to shifts in inorganic nutrient and substrate availability, reflecting the nutrient and energy limitation of the microbial community. We characterized microbial enzyme response to shifting nitrogen (N) and phosphorus (P) availability across terrestrial and aquatic environments at the Bear Brook Watershed in Maine, the site of a whole-watershed N enrichment experiment. We compared activity of β-1,4-glucosidase (BG); β-1,4-N-acetylglucosaminidase (NAG); acid phosphatase (AP) in soil, leaf litter in terrestrial and stream habitats and stream biofilms in a reference and N enriched watershed, representing whole-ecosystem response to chronic N enrichment. In addition, we used shorter, experimental P enrichments to address potential P limitation under ambient and elevated N availability. We found that BG and NAG activity were not affected by the long-term N enrichment in either habitat. Enhanced P limitation due to N enrichment was evident only in the aquatic habitats with 5- and 8-fold higher treated watershed AP activity in stream biofilms and stream litter, respectively. Acute P additions reduced AP activity and increased BG activity and these effects were also most pronounced in the streams. The stoichiometry of enzyme activity was constrained across ecosystem compartments with regression slopes for lnBG:lnNAG, lnBG:lnAP, and lnNAG:lnAP close to 1, ranging 1.142–1.241. We found that microbial enzyme response to shifting N and P availability varied among watershed compartments, typically with stronger effects in aquatic habitats. This suggests that understanding the response of ecosystem function to disturbance at the watershed scale requires simultaneous consideration of all compartments.     

不同林龄马尾松凋落物基质质量与土壤养分的关系

凋落物的质量、数量及分解速率在一定程度上代表了土壤的营养状况。为了精确估算凋落物分解对土壤碳库的年净归还量及凋落物-土壤生物化学连续体的深层理解,从凋落物基质质量的角度分析了三峡库区不同林龄马尾松凋落物基质质量与土壤养分的作用关系,结果表明:中龄林、近熟林、成熟林马尾松凋落物基质质量中的C、C/N比、C/P比、木质素/N比、木质素/P比差异显著,其中近熟林凋落物叶木质素/N分别比中龄林和成熟林的高33.65%、39.24%,N、P、K、木质素含量差异不显著;但各组织器官的N、P、K含量差异显著,均是皮<枝<叶<杂物,C/N比、C/P比的变化则相反。不同林龄马尾松0-20 cm(0-5 cm、5-10 cm、10-20 cm)土壤有机质、总氮、有效磷含量均表现出近熟林<中龄林<成熟林,0-5 cm最大,10-20 cm最小,且随着土壤深度的增加而明显降低,总磷则是中林龄最低,成熟林最大,pH值则各土层均表现为中龄林<成熟林<近熟林,平均pH值为4.55-5.51。凋落物基质质量指标与土壤养分之间冗余分析(RDA)表明:马尾松凋落物基质质量和土壤养分之间关系紧密,N、P、纤维素、半纤维素、木质素、木质素/N比、C/N比对土壤养分影响比较大;凋落物中木质素/N比、C/N比与土壤有机质呈显著负相关,其含量越高越不利于土壤有机质的形成,土壤养分积累的越慢;凋落物基质质量氮含量与土壤氮含量呈显著正相关;土壤pH值、容重与N含量呈显著负相关,与凋落物C/N比、木质素/N比呈显著正相关。马尾松土壤表面有机质、N、P养分含量与凋落物基质质量对应养分含量变化规律一致,土壤养分高,凋落物基质质量相对较高,土壤贫瘠,凋落物基质质量相对较低。     

Litter decomposition contrasts in second- and old-growth Douglas-fir forests of the Pacific Northwest, USA

Litterfall and its subsequent decomposition are important feedback mechanisms in the intrasystem cycling of nutrients in forest ecosystems. The amount of litterfall and the rate of decomposition are expected to vary with stand age and climate. Over a 2-year period, decomposition of five litter types were measured in two second-growth forest stands and one old-growth stand in the Cascade Mountains of southern Washington state, USA. Both second-growth stands were dominated by Douglas-fir [Pseudotsuga menziesii (Mirb.,) Franco] but one had a significant proportion of red alder (Alnus rubra Bong.), a nitrogen (N) fixer. The old-growth stand was dominated by Douglas-fir and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. All stands had a relatively shallow layer of forest floor mass. The five litter types were placed in each stand to evaluate decomposition patterns. Despite significant differences in stand age, microclimate and mean residence times for carbon (C) and N, the rates of litter mass loss varied only slightly between sites. The relative order of species litter mass loss was: vine maple ≫ salal = western hemlock > Douglas-fir (from the youngest stand) > Douglas-fir (from the N rich stand with red alder). The initial litter lignin concentration, not lignin:N, was the primary determinant of decomposition rates, although the initial N concentration was the predictor for mass loss after 2 years in the N rich Douglas-fir-alder stand. All litter types showed immobilization of N for nearly 2 years. Data for Douglas-fir litter suggest that higher levels of N may retard decomposition of tissues with greater amounts of lignified material. The retention of N by the litter appeared influenced by the nutrient capital of the stands as well as the forest floor C:N ratio. Decomposition was minimal during the cold winter months, but displayed a definitive peak period during early Fall with wet weather, warm soils, and fungal activity. Thus, long-term climatic change effects on forest floor C storage may depend more on changes in seasonality of precipitation changes than just temperature changes.     

Carbon : nitrogen stoichiometry in forest ecosystems during stand development

Aim Carbon (C) and nitrogen (N) stoichiometry is a critical indicator of biogeochemical coupling in terrestrial ecosystems. However, our current understanding of C : N stoichiometry is mainly derived from observations across space, and little is known about its dynamics through time. Location Global secondary forests. Methods We examined temporal variations in C : N ratios and scaling relationships between N and C for various ecosystem components (i.e. plant tissue, litter, forest floor and mineral soil) using data extracted from 39 chronosequences in forest ecosystems around the world. Results The C : N ratio in plant tissue, litter, forest floor and mineral soil exhibited large variation across various sequences, with an average of 145.8 ± 9.4 (mean ± SE), 49.9 ± 3.0, 38.2 ± 3.1 and 18.5 ± 0.9, respectively. In most sequences, the plant tissue C : N ratio increased significantly with stand age, while the C : N ratio in litter, forest floor and mineral soil remained relatively constant over the age sequence. N and C scaled isometrically (i.e. the slope of the relationship between log‐transformed N and C is not significantly different from 1.0) in litter, forest floor and mineral soil both within and across sequences, but not in plant tissue either within or across sequences. The C : N ratio was larger in coniferous forests than in broadleaf forests and in temperate forests than in tropical forests. In contrast, the N–C scaling slope did not reveal significant differences either between coniferous and broadleaf forests or between temperate and tropical forests. Main conclusions These results suggest that C and N become decoupled in plants but remain coupled in other ecosystem components during stand development.