氮添加对川西高寒灌丛凋落枝化学计量特征及养分归还的影响

氮(N)是陆地生态系统初级生产力的重要限制因子,大气N沉降的增加将会对植物的化学元素含量和生物量产生重要影响,进而影响凋落物的化学计量特征及其养分归还。高寒灌丛是陆地生态系统的重要组成部分,但有关N沉降对高寒灌丛凋落物尤其是凋落枝的化学元素和生物量的研究还较为缺乏,难以深入揭示N沉降对高寒灌丛土壤碳(C)和养分循环的影响机理。基于此,以青藏高原东部地区的优势高寒灌丛类型—窄叶鲜卑花(Sibiraea angustata(Rehd.) Hand.-Mazz.)灌丛为研究对象,连续4年人工模拟N沉降,分析了凋落枝C、N、磷(P)、木质素和纤维素化学计量特征及其归还量对不同N添加浓度(0、20、50、100 kg hm^-2 a^-1)的响应趋势。结果表明：(1)N添加对凋落枝C、N含量无显著性影响(P>0.05),而对P、木质素和纤维素含量有显著性影响(P<0.05),但不同年份间的影响趋势不一致;(2)4年的N添加并未改变凋落枝的C/N、N/P,但显著降低了凋落枝的木质素/N(第3年)、C/P(第1年和第4年)和C/N/P(第1年);(...     

神农架常绿落叶阔叶混交林凋落物养分特征

凋落物是联结陆地生态系统植物与土壤养分的重要媒介,了解凋落物养分特征有助于理解陆地生态系统物质循环的机理。该研究于2015年收集了神农架地区常绿落叶阔叶混交林的新鲜凋落物及现存凋落物,测定其不同器官中大量元素(C、N、P、K、Ca、Mg)的含量,据此分析其养分含量、养分归还量、养分储量及化学计量比的特征。结果发现:该常绿落叶阔叶混交林新鲜凋落物的C、K养分含量显著高于现存凋落物,N、P、Ca、Mg养分含量显著低于现存凋落物;其凋落物大量元素的养分归还量及养分储量大小顺序均为C Ca N Mg K P,分别为1569.84、52.44、34.82、6.24、5.24、1.30 kg hm~(-2) a~(-1)及1835.29、87.87、51.17、12.12、3.90、1.95 kg hm~(-2) a~(-1);其新鲜凋落物及现存凋落物的C∶N∶P分别为1307.33∶27.73∶1及976.48∶26.77∶1,新鲜凋落物的C∶N、C∶P显著高于现存凋落物,N∶P无显著区别。研究表明,新鲜凋落物与现存凋落物养分含量之间的差异与不同元素在分解过程中的可淋溶性及生物固持等因素有关。该地区常绿落叶阔叶混交林凋落物养分归还量及养分储量相对于亚热带阔叶林平均水平较低;且显著低于喀斯特地区同类型森林,主要与其凋落物产量、降水量及植被类型有关。该森林生态系统新鲜凋落叶与中国及全球范围内阔叶树种凋落叶相比C∶N较低,C∶P、N∶P较高,这可能是由于该地区N沉降及P限制现象较为严重所致。     

Throughfall exclusion and leaf litter addition drive higher rates of soil nitrous oxide emissions from a lowland wet tropical forest

Tropical forests are a significant global source of the greenhouse gas nitrous oxide (N₂O). Predicted environmental changes for this biome highlight the need to understand how simultaneous changes in precipitation and labile carbon (C) availability may affect soil N₂O production. We conducted a small‐scale throughfall and leaf litter manipulation in a lowland tropical forest in southwestern Costa Rica to test how potential changes in both water and litter derived labile C inputs to soils may alter N₂O emissions. Experimentally reducing throughfall in this wet tropical forest significantly increased soil emissions of N₂O, and our data suggest that at least part of this response was driven by an increase in the concentration of dissolved organic carbon [DOC] inputs delivered from litter to soil under the drier conditions. Furthermore, [DOC] was significantly correlated with N₂O emissions across both throughfall and litterfall manipulation plots, despite the fact that native NO₃^? pools in this site were generally small. Our results highlight the importance of understanding not only the potential direct effects of changing precipitation on soil biogeochemistry, but also the indirect effects resulting from interactions between the hydrologic, C and N cycles. Finally, over all sampling events we observed lower mean N₂O emissions (<1 ng N₂O‐N cm^?2 h^?1) than reported for many other lowland tropical forests, perhaps reflecting a more general pattern of increasing relative N constraints to biological activity as one moves from drier to wetter portions of the lowland tropical forest biome.     

黄土高原典型草原优势植物凋落物分解及养分释放对氮添加的响应

利用原位分解袋法研究了黄土高原典型草原优势植物长芒草(Stipa bungeana)和阿尔泰狗娃花(Heteropappus altaicus)凋落物的养分释放过程对氮添加的响应,试验周期为1 a。设置6个氮添加水平,分别为N0(0)、N1(1.15 g N m~(-2)a~(-1))、N2(2.3 g N m~(-2)a~(-1))、N3(4.6 g N m~(-2)a~(-1))、N4(9.2 g N m~(-2)a~(-1))和N5(13.8 g N m~(-2)a~(-1)),氮素类型为尿素((NH_2)_2CO)。结果表明:(1)氮添加处理两年显著改变了长芒草和阿尔泰狗娃花凋落物的初始化学性质。随着氮梯度的增加,凋落物的N(氮)含量逐渐增加,木质素含量先增加后下降,C/N(碳氮比)和木质素/N降低,C(碳)、P(磷)和C/P(碳磷比)没有显著的差异。(2)氮处理对长芒草和阿尔泰狗娃花凋落物的分解速率的影响不显著。长芒草和阿尔泰狗娃花凋落物C含量随分解时间整体为降低过程,N和P含量总体上为增加过程,且整个分解过程中N含量各处理间差异显著。(3)氮处理对长芒草和阿尔泰狗娃花凋落物C和P的分解基本无影响,两种元素都呈现释放过程。氮处理对凋落物的N残留率有显著的影响,在N1—N3(1.15—4.6 g/m~2)处理下的长芒草凋落物N残留率高于其他处理,且呈现富集过程;而阿尔泰狗娃花凋落物中的N呈现富集-释放过程。在土壤养分贫瘠的黄土高原典型草原,适量的氮输入可以促进系统的固氮。     

Ecosystem Level Impacts of Invasive Acacia saligna in the South African Fynbos

Recent efforts to clear invasive plants from the fynbos of South Africa forces managers to think about how N₂‐fixing invasives have altered ecosystem processes and the implications of these changes for community development. This study investigated the changes in nitrogen (N) cycling regimes in fynbos with the invasion of Acacia saligna, the effects of clear‐cutting acacia stands on soil microclimate and N cycling, and how altered N resources affected the growth of a weedy grass species. Litterfall, litter quality, soil nutrient pools, and ion exchange resin (IER)‐available soil N were measured in uninvaded fynbos, intact acacia, and cleared acacia stands. In addition, a bioassay experiment was used to ascertain whether the changes in soil nutrient availability associated with acacia would enhance the success of a weedy grass species. Acacia plots had greater amounts of litterfall, which had higher concentrations of N. This led to larger quantities of organic matter, total N, and IER‐available N in the soil. Clearing acacia stands caused changes in soil moisture and temperature, but did not result in differences in IER‐available N. The alteration of N availability by acacias was shown to increase growth rates of the weedy grass Ehrharta calycina, suggesting that secondary invasions by nitrophilous weedy species may occur after clearing N₂‐fixing alien species in the fynbos. It is suggested that managers use controlled burns, the addition of mulch, and the addition of fynbos seed after clearing to lower the levels of available N in the soil and initiate the return of native vegetation.     

Interactions between leaf litter and soil organic matter on carbon and nitrogen mineralization in six forest litter-soil systems

Background and aims

Leaf litter decomposes on the surface of soil in natural systems and element transfers between litter and soil are commonly found. However, how litter and soil organic matter (SOM) interact to influence decomposition rate and nitrogen (N) release remains unclear.

Methods

Leaf litter and mineral soil of top 0–5 cm from six forests were incubated separately, or together with litter on soil surface at 25 °C for 346 days. Litter N remaining and soil respiration rate were repeatedly measured during incubation. Litter carbon (C) and mass losses and mineral N concentrations in litter and soil were measured at the end of incubation.

Results

Net N transfer from soil to litter was found in all litters when incubated with soil. Litter incubated with soil lost more C than litter incubated alone after 346 days. For litters with initial C: N ratios lower than 52, net N_min after 346 days was 100 % higher when incubated with soil than when incubated alone. Litter net N_min rate was negatively related to initial C: N ratio when incubated with soil but not when incubated alone. Soil respiration rate and net N_min rate did not differ between soil incubated with litter and soil incubated alone.

Conclusions

We conclude that soils may enhance litter decomposition rate by net N transfer from soil to litter. Our results together with studies on litter mixture decomposition suggest that net N transfer between decomposing organic matter with different N status may be common and may significantly influence decomposition and N release. The low net N_min rate during litter decomposition along with the small size of litter N pool compared to soil N pool suggest that SOM rather than decomposing litter is the major contributor to plant mineral N supply.     

Typha latifolia and Cladium jamaicense litter decay in response to exogenous nutrient enrichment

The role of nutrient availability in the decay of Typha latifolia and Cladium jamaicense litter and associated microbial responses were studied under controlled experimental conditions. The experimental setup consisted of three 14 m² mesocosms: (i) an experimentally enriched (N&P) mesocosm containing organic soil, (ii) a mesocosm with organic soil but no external enrichment, and (iii) a mesocosm with no external nutrient inputs and a mineral soil, each equally divided into two areas predominated by T. latifolia and C. jamaicense. Air dried senesced material of each plant species from the three units were placed in litterbags and were introduced back into their respective communities on the soil and water interface. Litter from T. latifolia degraded significantly faster than that of C. jamaicense. The half life of T. latifolia litter averaged approximately 274 days, C. jamaicense litter half life was extrapolated to approximately 377 days. Nutrient enrichment significantly increased the decay rates of T. latifolia, the nutrient effect on C. jamaicense decomposition was less apparent. The microbial biomass carbon in T. latifolia and C. jamaicense litter increased significantly as the litter decomposed. No significant differences between the litter types or amongst mesocosms were found. The relative activities of the extracellular enzymes acid phosphatase and β-glucosidase were significantly (P < 0.001 and P = 0.0284, respectively) affected by litter type and mesocosm over time. Litter associated alkaline phosphatase activity was largest in the mineral mesocosm, followed by the organic control and then organic enriched irrespective of litter type, β-glucosidase activity showed an inverse effect, enriched organic > organic control > mineral. The litter CO₂ and CH₄ microbial production rates showed a significant litter type and mesocosm effect (P = 0.0003 and 0.001, respectively). T. latifolia litter had larger associated methanogenic and microbial respiration rates than C. jamaicense litter. Nutrient enrichment enhanced both forms of microbial metabolic activities (CO₂ and CH₄ production). The effect of nutrient enrichment was primarily evident in the initial (3–6 months) period of decay, extracellular enzyme activities and the litter associated microbial metabolic activities showed most response during this decay stage.     

模拟干旱和磷添加对热带低地雨林叶凋落物分解的影响

凋落物是植物在其生长发育过程中新陈代谢的产物,是土壤有机质输入的重要途径,凋落物分解是生态系统养分循环的关键过程之一。在全球气候变化背景下,热带地区干旱事件发生的频率和强度均在增加,同时,普遍认为热带地区受磷(P)限制,所以探讨干旱胁迫和土壤磷可用性对热带地区叶凋落物分解的影响及两者是否存在交互效应十分必要,有助于了解干旱对该区叶凋落物分解的影响机制以及是否受土壤磷调控。依据植物多度、碳固持类型、叶质地,以海南三亚甘什岭热带低地雨林的4个树种叶凋落物(铁凌 Hopea exalata、白茶树 Koilodepas bainanense、黑叶谷木 Memecylon nigrescens、山油柑 Acronychia pedunculata)为实验材料,依托2019年在该区建成的热带低地雨林模拟穿透雨减少、磷(P)添加双因素交互控制实验平台,包括干旱(D -50%穿透雨)、P添加(P +50Kg P hm^-2a^-1)、模拟干旱×P添加(DP -50%穿透雨×+50Kg P hm^-2a^-1)、对照(CK)4个处理,且4种处理随机分布于3个区组,即设置了3个重复。使用常规的凋落物分解袋法探究实验处理对4个树种叶凋落物的分解系数、碳(C)、氮(N)元素动态变化的影响。结果表明:不同树种的叶凋落物因基质质量不同分解存在差异。模拟干旱处理对叶凋落物C、N损失产生抑制作用,但是对不同树种叶凋落物的抑制作用不同,原因是干旱处理通过抑制土壤分解者活动、减弱凋落物的物理破碎作用,间接抑制凋落物分解,并且由于高质量(含N量高)凋落物受微生物分解者影响较大,所以该凋落物分解受干旱抑制程度较大;P添加处理对叶凋落物C损失存在促进作用、N损失存在抑制作用,原因是土壤中P含量的升高,提高了微生物分解高C物质的能力,以及当土壤中P含量较高时,间接抑制微生物通过分解凋落物获取养分或者促进微生物优先完成自身生长代谢需要而不是合成分解凋落物所需要的酶,导致叶凋落物N损失下降;模拟干旱与P添加处理存在显著交互效应,P添加处理缓解或反转了干旱胁迫对叶凋落物分解的抑制作用。以上结果表明,不同基质质量的凋落物分解存在差异,对干旱胁迫的响应不同;在叶凋落物分解过程中,P添加促进C损失、抑制N损失;此外,在热带低地雨林,土壤中P可用性变化可调节干旱对凋落物分解的影响。     

Soil and litter respiration in rainforests of contrasting nutrient status and physiognomic structure near Lake Eacham,north-east Queensland

Soil and litter respiration and nutrient concentrations (N, P, Ca, Mg and K) were measured in two adjacent rainforests near Lake Eacham on the Atherton Tableland in north-east Queensland. One forest had soil formed on basalt and, in physiognomic-structural classification of Webb (1968, 1978), was structurally complex. The other had soil formed on metamorphic rock and was structurally simple. Respiration was measured by the alkali trap method on 16 monthly occasions in 1986 and 1987. Soil and litter nutrient concentrations were higher in the complex forest (with the exception of soil N). The relative differences were greater for soil than litter and more pronounced for P and Ca (and also soil Mg) than other nutrients. Litter polyphenol concentrations were lower in the complex forest. Rates of litter respiration in the complex forest were, on average, nearly twice those of the simple forest. Soil respiration rates were occasionally slightly lower in the complex forest during the wet season but did not differ between the forests during the dry season. Highest rates of respiration were measured during the wet season although high rates for litter occasionally occurred during the dry season. Cumulative CO₂ release from the soil and overlying litter did not differ between forests and averaged 5134 ± 96 g CO₂ m^-2 per year (mean ± s.e.m.) (1400 g C m^-2 per year). Litter respiration accounted for 14% of the annual release in the complex forest and 11% in the simple forest. The association between site nutrient status and forest physiognomic structure at Lake Eacham represents a more general pattern in rainforests of north-east Queensland. Further study is needed to ascertain whether the results from this study apply more generally in both primary and secondary rainforests.     

Litter production, leaf litter decomposition and nutrient return in Cunninghamia lanceolata plantations in south China: effect of planting conifers with broadleaved species

This study compared litter production, litter decomposition and nutrient return in pure and mixed species plantations. Dry weight and N, P, K, Ca, Mg quantities in the litterfall were measured in one pure Cunninghamia lanceolata plantation (PC) and two mixed-species plantations of C. lanceolata with Alnus cremastogyne (MCA) and Kalopanax septemlobus (MCK) in subtropical China. Covering 6 years of observations, mean annual litter production of MCA (4.97 Mg·ha⁻¹) and MCK (3.97 Mg·ha⁻¹) was significantly higher than that of PC (3.46 Mg·ha⁻¹). Broadleaved trees contributed 42% of the total litter production in MCA and 31% in MCK. Introduction of broadleaved tree species had no significant effect on litterfall pattern. Total litterfall was greatest in the dry season from November to March. Nutrient returns to the forest floor through leaf litter were significantly higher in both MCA and MCK than in PC (P < 0.05). The amounts of N, K, and Mg returned to the forest floor through leaf litter were highest in the MCA, and P and Ca returns were highest in the MCK. Percent contribution of broadleaf litter to total nutrient returns ranged from 41.7% to 86.9% in MCA and from 49.3% to 74.8% in MCK. The decomposition rate of individual leaf litter increased in the order: C. lanceolata < K. septemlobus < A. cremastogyne. Litter mixing had a positive effect on decomposition rate of the more recalcitrant litter and promoted nutrient return. Relative to mass loss of A. cremastogyne decomposing alone, higher mass loss of the mixture of C. lanceolata and A. cremastogyne was observed after 330 days of decomposition. These results indicate that mixed plantations of different tree species have advantages over monospecific plantations with regards to nutrient fluxes and these advantages have relevance to restoration of degraded sites. Responsible Editor: Alfonso Escudero.     

Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE)

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO₂, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO₂ did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO₂ lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO₂ attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO₂, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.     

Variability of indices of macronutrient availability in soils at different spatial scales along an elevation transect in tropical moist forests (NE Ecuador)

The availability of key plant nutrients may change with elevation in tropical mountains due to altitudinal gradients in temperature and moisture which affect pedogenesis and nutrient cycling. In a transect from upper lowland to montane forests in NE Ecuador, we tested the hypotheses that (1) the availability of P is low in low-elevation forests but increases upslope, while the availability of N is relatively high at low elevations but decreases with elevation, and (2) increasing amounts of calcium, magnesium and potassium are stored on top of the soil with progressive humus accumulation toward higher elevations, likely to improve nutrient availability. In each 20 plots in undisturbed natural forest at 500, 1000, 1500 and 2000 m?a.s.l., we measured in situ N net mineralization and nitrification rate (N_NM and N_NI, buried bag method), plant-available phosphorus (P_a, resin-bag method), and salt-exchangeable calcium, potassium and magnesium concentrations (Ca_ex, K_ex, Mg_ex) in the organic and mineral topsoils. N_NM and N_NI, and the Ca_ex, K_ex and Mg_ex concentrations were much more variable at the plot level than across the four elevations, while P_a varied equally at small and large spatial scales. P_a increased more than 10fold from 500 to 2000 m. The net release of nitrate dominated over ammonium at all elevations. While mass-related N_NM and N_NI rates and also organic matter C/N ratio in the topsoil remained invariant along the slope, N_NM and N_NI rates per ground area decreased by about 40% from 500 to 2000 m. Thus, the N_NM/P_a ratio decreased markedly with elevation proving our first hypothesis. In support of the second hypothesis, the pools of Ca_ex, Mg_ex and K_ex in the organic layers increased with elevation, demonstrating the key role that organic topsoil horizons are playing for forest nutrition at high elevations. We suggest that the large difference in N versus P availability of tropical (upper) lowland and montane forests is likely to be a key factor influencing the species composition and productivity along tropical mountain slopes.     

Phytoplankton community responses to nutrient and iron enrichment under different nitrogen to phosphorus ratios in the northern Baltic Sea

The impact of nutrient enrichment on the phytoplankton community structure, and particularly cyanobacteria, was studied in a 3-week mesocosm experiment conducted in August 2001 in the Archipelago Sea, a part of the northern Baltic Sea. The factorial design experiment included daily additions of nitrogen (N) and phosphorus (P) at two mass ratios, 1N:1P and 7N:1P, respectively, additions of iron (Fe) and a synthetic chelator, ethylenediaminetetraacetic acid (EDTA). The floating enclosures (400 l) were sampled for analyses of phytoplankton biomass and community structure, phytoplankton primary production, chlorophyll a, nutrients, and hepatotoxins. Chlorophyll a concentration, phytoplankton biomass and primary production increased most in the 7N:1P treatment. The increase was mainly due to an abundant growth of chlorophytes (Dictyosphaerium subsolitarium, Kirchneriella spp., Monoraphidium contortum, and Oocystis spp.), pennate diatoms (especially Nitzschia spp.), dinophytes and the chroococcalean cyanobacterium Synechococcus sp. The nutrient enrichments had no effect on the total biomass of N₂-fixing cyanobacteria. Nevertheless, the biomass of Anabaena spp. was highest in the enrichments with a low N/P ratio. Chlorophyll a concentration and total phytoplankton biomass were not affected by Fe or EDTA, but Fe alone had a positive effect on the chlorophyte Kirchneriella sp. The N₂-fixing cyanobacteria Aphanizomenon sp. responded positively to Fe alone and to both Fe and EDTA added together. The hepatotoxin concentration increased during the experiment, but no clear responses to nutrient enrichments were found. Our study showed species-specific responses to nutrient enrichments among the N₂-fixing cyanobacteria. Although the total phytoplankton production was not Fe-limited; the availability of Fe clearly affected the phytoplankton community structure.     

马尾松人工纯林凋落松针数量及基质质量动态

凋落物是森林生态系统的重要组成部分,其产量与基质质量是影响植物-土壤间养分循环的重要因素。以3种密度马尾松人工纯林为研究对象,分析松针凋落模式及基质质量变化规律,比较不同林分密度间松针凋落特性差异,探讨松针基质质量、产量及气候因素间的关系。结果表明:松针凋落数量动态变化属双峰型,在2月呈现一个小高峰,在10/11月达最高峰。3种密度林分中各月凋落松针的N、P浓度均差异显著(P0.05),全年最高值出现在4—6月,8—12月的松针N、P浓度显著低于其他时段;不同月份凋落松针木质素浓度也差异显著(P0.05),在2月木质素含量达峰值,7、8月含量较低;3种密度林分中各月凋落松针的C/N、C/P、L/N和L/P比存在显著差异(P0.05),4—6月呈现最低值,9/11/12月则出现最高值,相差达2—3倍,且均高于养分释放临界值,不利于松针N素和P素的释放。林分密度对松针凋落数量和N浓度影响显著(P0.05),中等密度林分松针凋落数量高于低、高密度林分,中、高密度林分凋落松针N浓度显著高于低密度林分。凋落松针基质质量与自身凋落数量密切相关,并且受气温和降水量的影响。凋落松针N、P浓度与凋落数量呈显著负相关(P0.05),C/N、C/P、L/N、L/P比与凋落数量呈显著正相关(P0.05),说明更多凋落物产量将伴随着更低的基质质量,将有更慢的分解速度。     

Response of cyanobacterial mats to nutrient and salinity changes

Cyanobacterial mats (CBM), complex assemblages of cyanobacteria, bacteria and algae, are important ecosystem components of oligotrophic marshes in limestone-based regions of the Caribbean. We conducted a mesocosm experiment and evaluated the response of CBM to factorial combinations of low, medium and high phosphorus, nitrogen and salinity. Changes in composition of the main species groups of cyanobacteria and algae, primary production, cellular nutrients and enzymatic activities were recorded as response variables. The redundancy analysis with concentrations of P, N and salinity as explanatory variables showed that the primary production of CBM and the amount of phytoplankton expressed as Chl a were best explained by concentration of P, with less significant positive effect of N and a negative effect of salinity. Abundance of green algae and Chroococcales was positively correlated with increasing concentrations of P and N and reached 27.6% and 21.9%, respectively, in high P and high N treatment at the end of experiment. N₂-fixation averaged 75 and 175 nmol C₂H₄ cm⁻² min⁻¹, at low nitrogen and medium or high P, respectively, and it was negatively correlated with nitrogen concentration and positively correlated with abundance of a group of heterocytous cyanobacteria from genus Nostoc. At low N concentrations, increasing P concentrations supported higher N₂-fixation. Activity of the alkaline phosphatase, APA, was negatively correlated with P and salinity and positively with N. We also found a significant negative correlation between the APA activity and the P content of the mat. At high P and N concentrations, the mats were impacted by grazing, had a tendency to disintegrate and become shaded out by a massive growth of phytoplankton. We confirmed an overall negative effect of nutrient increase on CBM.     

杉木人工林凋落物分解对氮沉降的响应

凋落物分解是陆地生态系统养分循环的关键过程,是全球碳（C）收支的一个重要主要组成部分,正受到全球大气氮（N）沉降的深刻影响。探讨大气氮沉降条件下森林凋落物的分解,有利于揭示森林生态系统C平衡和养分循环对全球变化的响应。选择福建沙县官庄林场1992年栽种的杉木（Cunninghamia lanceolata）人工林为研究对象,自2004年开始野外模拟氮沉降试验,至今12年。氮沉降处理分4个水平,N0、N1、N2和N3分别为0、60、120、240 kg N hm^-2 a^-1。2015年12月开展分解袋试验,对经过氮沉降处理12年的凋落物（叶、枝、果）进行模拟原位分解,每3个月收回一次分解袋样品,为期2年,同时测定凋落物干物质残留量及其C、N和磷（P）含量。结果表明,经2年分解后,氮沉降条件下凋落物叶、枝和果的干物质残留率平均值分别为27.68%、47.02%和43.18%,说明分解速率大小依次为叶 > 果 > 枝。凋落物叶、枝和果的分解系数平均为0.588、0.389和0.455,周转期（分解95%年限）分别为4-5年、6-8年和5-7年。低-中氮处理（N1和N2）均促进凋落物叶、枝和果的分解,以N1的效果更明显,而N3起到抑制作用。N1处理的凋落物叶、枝和果的周转期分别为：4.50年、6.09年和5.85年,N2处理的分别为4.95年、8.16年和6.19年。模拟氮沉降在一定程度上增加了凋落物叶、枝和果分解过程中的N和P含量,但降低了C含量。凋落物叶、枝和果分解过程中C元素呈现释放-富集-释放模式,N和P元素呈现释放与富集交替,除枝的N元素外,其他均表现为释放量大于富集量。     

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。     

Endogenous versus exogenous nutrient affects C, N, and P dynamics in decomposing litters in mid-subtropical forests of China

The effect of nutrient availability on litter decomposition has been a major focus of global change ecology. The relative impacts of endogenous (litter) and exogenous (soil) nutrient availability remain unclear. We studied the nutrient dynamics of decomposition in litter from two species with contrasting litter nutrient contents and stoichiometry: Pinus massoniana and Castanopsis sclerophylla. During a 540-day field incubation, we manipulated exogenous nutrient levels by adding microbially available C (+C), N (+N), P (+P), and all three (+CNP) at 90-day intervals. Relative to the no-nutrient control (CK), nutrient additions decreased organic C retention in C. sclerophylla, with the greatest effect observed in +CNP. Nitrogen content in P. massoniana litter similarly increased with nutrient addition, particularly +P and +CNP. The P addition treatments also increased P content in the litter of both species. Nitrogen content in C. sclerophylla and organic C content in P. massoniana were unaffected by nutrient additions. The C/N and C/P ratios in decomposing C. sclerophylla litter were significantly lower in the CK treatment, while those of P. massoniana litter were influenced by the interaction of nutrient addition and decomposition time. Increased availability of C, N, and P individually and collectively alters nutrient release dynamics in decomposing foliar litter. Litter quality, as determined by source species, is a key determinant of the impact of exogenous nutrient inputs. A stronger effect of P addition than N addition indicates a relatively N-rich and P-poor ecosystem.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.     

马尾松人工林林窗大小对四种凋落叶质量损失和养分释放的影响

马尾松人工林乔木层植物凋落物的分解对林地养分平衡和系统物质循环具有重要意义,并可能受不同大小林窗下微环境差异的影响。采用凋落物袋分解法,以马尾松(Pinus massoniana)人工林人为砍伐形成的7个不同大小林窗(G1:100 m~2、G2:225 m~2、G3:400 m~2、C4:625 m~2、G5:900 m~2、G6:1225m~2、G7:1600 m~2)为研究对象,林下(G0)为对照,研究林窗大小对红椿(Toona ciliata)、桢楠(Phoebe zhennan)、香樟(Cinnamomum camphora)和马尾松4种乡土树种凋落叶质量损失及养分释放的影响。结果显示:1)林窗大小(G0-G7)显著影响林窗中心放置的红椿和桢楠凋落叶N和P释放率、香樟凋落叶失重率和N、P、K释放率以及马尾松凋落叶P和K释放率。相对于林下,中小型林窗(G1-G4)的凋落叶失重率和N、P释放率明显较大,而大型林窗(G6-G7)的凋落叶K释放率明显较大。2)林窗内放置位置显著影响红椿、桢楠和马尾松凋落叶的K释放率及香樟凋落叶的P释放率。红椿和桢楠的凋落叶K释放率从林窗中心到边缘显著减少,而马尾松凋落叶K释放率及香樟P释放率从林窗中心到边缘显著增加。3)4种凋落叶类型中红椿凋落叶分解最快,其分解50%和95%所需时间分别为5.29和23.14个月。上述结果表明,林窗大小和林窗内位置对凋落物质量损失及其养分释放具有显著影响,但影响大小及趋势随物种初始基质质量的差异具有明显变化,研究结果为亚热带低山丘陵区马尾松人工低效林的科学经营及管理提高了一定的科学依据。