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。     

Litter decomposition in Mediterranean pine forests is enhanced by reduced canopy cover

Plant and Soil - Compare bacterial communities in non-vegetated soils and in the rhizosphere of Ni-hyperaccumulating or Ni-excluding plants from four serpentine sites of the Iberian Peninsula....     

氮磷添加对红壤区城郊湿地松林凋落叶分解的影响

城市化易导致城市森林氮(N)沉降和磷(P)富集,进而对凋落物分解过程产生影响。以位于南昌市郊的湿地松(Pinuse lliottii Engelm.)林为研究对象,采用尼龙网袋分解法,模拟N沉降(10g N·m-2·a-1,[N])、P积累(2.5g P·m-2·a-1,[P])和N沉降+P积累(10N·m-2·a-1+2.5g P·m-2·a-1,[N+P])对凋落叶分解速率与C、N、P含量及其化学计量比动态变化的影响。结果表明:与对照(CK)相比,[N]、[P]和[N+P]均促进凋落叶的前期(0～180d)分解速率,抑制中期(180～360d)、后期(360～540d)的分解速率;至540d时分解速率表现为[N]、[P]和CK无差异,但均高于[N+P](P0.05)。[N]提高分解过程中凋落叶N浓度,N含量表现为分解前期积累、后期释放;[P]提高分解过程中凋落叶P浓度,P含量持续积累;[N+P]提高N和P浓度,分解前期N、P含量积累,后期释放;而不同处理的C含量均表现为释放。凋落物基质C/N/P比与分解速率的相关性随分解阶段而表现各异。综合来看,城市化导致的N沉降和P富集叠加效应具有抑制城市森林凋落物分解过程的潜在性。     

Controls on decomposition and soil nitrogen availability at high latitudes

At high latitudes, decomposition rates and soil nitrogen (N) availability are pivotal in determining ecosystem responses to climate change. The effects of temperature, soil moisture content, resource quality, and saprotrophic fungi as an example of soil organisms, on carbon (C) and N mineralisation are reviewed. The controls on N availability are less well characterised than those on decomposition, and C and net N mineralisation sometimes do not respond to these controls in a parallel manner. Increases in mean summer temperatures of 2–4°C predicted for high latitudes may not necessarily cause greater rates of decomposition and N mineralisation because of concomitant small rises in soil temperature together with interactions between the controls, including interactions of the temperature and moisture content of the substrata with the diversity and function of decomposer fungi. Research on decomposition and soil N availability has been carried out at several scales, at all of which future research remains necessary. It is not clear whether species diversity of decomposer fungi influences decomposition and C and N release above the microscale.     

The effect of soil drying on humus decomposition and nitrogen availability

Summary Respirometer experiments show that when a dry soil is moistened a characteristic pattern of decomposition occurs in which an initial period of relatively rapid decomposition (Stage 1) falls, during a few days, to a slow steady rate (Stage 2). This pattern is repetitive with successive dryings and rewettings and is common to all soils so far investigated. The magnitude of decomposition depends in the percent carbon in the soil and on the drying conditions, air-drying being less effective than oven-drying. Decomposition during Stage 1 conforms approximately to a first-order reaction and proportionate amounts of nitrogen are mineralised. A similar pattern of decomposition occurs under field conditions throughout successive wet and dry seasons.Evidence is presented to show that decomposition involves direct microbial attack of the solid organic substrate and that the recurrent pattern of decomposition is due to the state in which the microbial population is left after drying and its subsequent behaviour on rewetting. The rapid decline in the rate of decomposition on rewetting (Stage 1) appears not to involve (1) the development of toxic conditions, (b) physical changes in the soil (since similar patterns of decomposition also occur with organic material alone or in sand) or (c) rapid decomposition of organic material made soluble by drying.The operation and repetition of this pattern of decomposition in the field has important consequences in the rundown of soil carbon and the mineralisation of soil nitrogen particularly where well-defined wet and dry seasons occur. These consequences are discussed in relation to climate and certain agricultural practices.     

Positive feedbacks between decomposition and soil nitrogen availability along fertility gradients

Background and aims

We determined the relationship between site N supply and decomposition rates with respect to controls exerted by environment, litter chemistry, and fungal colonization.

Methods

Two reciprocal transplant decomposition experiments were established, one in each of two long-term experiments in oak woodlands in Minnesota, USA: a fire frequency/vegetation gradient, along which soil N availability varies markedly, and a long-term N fertilization experiment. Both experiments used native Quercus ellipsoidalis E.J. Hill and Andropogon gerardii Vitman leaf litter and either root litter or wooden dowels.

Results

Leaf litter decay rates generally increased with soil N availability in both experiments while belowground litter decayed more slowly with increasing soil N. Litter chemistry differed among litter types, and these differences had significant effects on belowground (but not aboveground) decay rates and on aboveground litter N dynamics during decomposition. Fungal colonization of detritus was positively correlated with soil fertility and decay rates.

Conclusions

Higher soil fertility associated with low fire frequency was associated with greater leaf litter production, higher rates of fungal colonization of detritus, more rapid leaf litter decomposition rates, and greater N release in the root litter, all of which likely enhance soil fertility. During decomposition, both greater mass loss and litter N release provide mechanisms through which the plant and decomposer communities provide positive feedbacks to soil fertility as ultimately driven by decreasing fire frequency in N-limited soils and vice versa.     

Litter addition and understory removal influenced soil organic carbon quality and mineral nitrogen supply in a subtropical plantation forest

Plant and Soil - Aboveground litter inputs have been modified by global changes in plantation forests, where understory management is also prevalent, which may alter soil fertility and stand...     

Community food web, decomposition and nitrogen mineralisation in a stratified Scots pine forest soil

A soil community food web model was used to improve the understanding of what factors govern the mineralisation of nutrients and carbon and the decay of dead organic matter. The model derives the rates of C and N mineralisation by organisms by splitting their uptake rate of food resources into a rate at which faeces or prey remains are added to detritus, a rate at which elements are incorporated into biomass, and a rate at which elements are released by organisms as inorganic compounds. The functioning of soil organisms in the mineralisation of C and N was modelled in the soil horizon of a Scots pine forest. The organic horizon was divided into three distinct layers, representing successive stages of decay, i.e. litter, fragmented litter, and humus. Each of the layers had a different, quantitative, biota composition. For each layer the annual C and N mineralisation rates were simulated and compared to observed C and N mineralisation rates from organic matter in stratified litterbags. Simulated C and N mineralisation was relatively close to measured losses of C and N, but the fit was not perfect. Discrepancies between the observed and predicted mineralisation rates are discussed in terms of variation in model parameter values of those organisms that showed the highest contribution to mineralisation rates. The measured, and by the model predicted, significant decrease in mineralisation rates down the profile was not explained by the biomass of the primary decomposers and only partly by the total food web biomass. Modelling results indicated that indirect effects of soil fauna, due to trophic interactions with their resources, are an important explanatory factor. In addition, the analyses suggest that community food web structure is an important factor in the regulation of nutrient mineralisation. The model provided the means to evaluate the contribution of functionally defined groups of organisms, structured in a detrital food web, to losses of C and N from successive decay stages.     

Rhizosphere soil bacterial communities and nitrogen cycling affected by deciduous and evergreen tree species

Deciduous and evergreen trees differ in their responses to drought and nitrogen (N) demand. Whether or not these functional types affect the role of the bacterial community in the N cycle during drought remains uncertain. Two deciduous tree species (Alnus cremastogyne, an N₂‐fixing species, and Liquidambar formosana) and two evergreen trees (Cunninghamia lanceolata and Pinus massoniana) were used to assess factors in controlling rhizosphere soil bacterial community and N cycling functions. Photosynthetic rates and biomass production of plants, 16S rRNA sequencing and N‐cycling‐related genes of rhizosphere soil were measured. The relative abundance of the phyla Actinobacteria and Firmicutes was higher, and that of Proteobacteria, Acidobacteria, and Gemmatimondaetes was lower in rhizosphere soil of deciduous trees than that of evergreen. Beta‐diversity of bacterial community also significantly differed between the two types of trees. Deciduous trees showed significantly higher net photosynthetic rates and biomass production than evergreen species both at well water condition and short‐term drought. Root biomass was the most important factor in driving soil bacterial community and N‐cycling functions than total biomass and aboveground biomass. Furthermore, 44 bacteria genera with a decreasing response and 46 taxa showed an increased response along the root biomass gradient. Regarding N‐cycle‐related functional genes, copy numbers of ammonia‐oxidizing bacteria (AOB) and autotrophic ammonia‐oxidizing archaea (AOA), N₂ fixation gene (nifH), and denitrification genes (nirK, nirS) were significantly higher in the soil of deciduous trees than in that of the evergreen. Structural equation models explained 50.2%, 47.6%, 48.6%, 49.4%, and 37.3% of the variability in copy numbers of nifH, AOB, AOA, nirK, and nirS, respectively, and revealed that root biomass had significant positive effects on copy numbers of all N‐cycle functional genes. In conclusion, root biomass played key roles in affecting bacterial community structure and soil N cycling. Our findings have important implications for our understanding of plants control over bacterial community and N‐cycling function in artificial forest ecosystems.     

氮添加和采脂对湿地松林土壤酶活性及酶化学计量比的影响

大气氮沉降和采脂会引起树木生长和代谢的变化,从而影响土壤养分循环和酶活性.土壤酶和酶化学计量可以揭示土壤碳、氮循环和微生物生长代谢过程的养分限制,但目前亚热带湿地松人工林土壤酶和酶化学计量对氮添加和采脂的响应还不清楚.以亚热带北缘的湿地松人工林为研究对象,设置采脂(resin tapping,RT)和未采脂(no re...     

Inter- and intra-species intercropping of barley cultivars and legume species,as affected by soil phosphorus availability

Aims

Intercropping can improve plant yields and soil phosphorus (P) use efficiency. This study compares inter- and intra-species intercropping, and determines whether P uptake and shoot biomass accumulation in intercrops are affected by soil P availability.

Methods

Four barley cultivars (Hordeum vulgare L.) and three legume species (Trifolium subterreneum, Ornithopus sativus and Medicago truncatula) were selected on the basis of their contrasting root exudation and morphological responses to P deficiency. Monocultures and barley-barley and barley-legume intercrops were grown for 6 weeks in a pot trial at very limiting, slightly limiting and excess available soil P. Above-ground biomass and shoot P were measured.

Results

Barley-legume intercrops had 10–70% greater P accumulation and 0–40% greater biomass than monocultures, with the greatest gains occurring at or below the sub-critical P requirement for barley. No benefit of barley-barley intercropping was observed. The plant combination had no significant effect on biomass and P uptake observed in intercropped treatments.

Conclusions

Barley-legume intercropping shows promise for sustainable production systems, especially at low soil P. Gains in biomass and P uptake come from inter- rather than intra-species intercropping, indicating that plant diversity resulted in decreased competition between plants for P.

     

Litter quality and nutrient cycling affected by grazing-induced species replacements along a precipitation gradient

One of the potential mechanisms for the impact of herbivores on nutrient cycling is the effect of selective grazing on litter quality through changes in species composition. However, the scarce evidence collected on this mechanism is controversial and seemingly influenced by site-specific variables. In this paper, we explored the consequences of grazing-induced changes in species composition on litter quality and nitrogen cycling with a regional perspective. Along a 900-mm of mean annual rainfall gradient, we selected species promoted and diminished by grazing from three natural rangelands of Argentina, analyzed their litter quality, and determined their decomposition and nutrient release kinetics under common greenhouse conditions. Litter quality and decomposition rates were strongly associated with plant response to grazing. However, the magnitude and direction of these differences depended on the ecosystem considered. In the wettest site, the species promoted by grazing (forbs) had higher nitrogen and phosphorus contents, faster decomposition rates, and higher release of nitrogen to the soil than species diminished by grazing (C₃ and C₄ grasses). In the intermediate and dry sites, species promoted by grazing had lower nitrogen and phosphorus contents, and slower decomposition rates than those diminished by grazing (C₃ grasses in both cases). Decomposition of the entire group of species was not correlated with mean annual rainfall, but when litter of the species diminished by grazing was analyzed, it was negatively correlated with precipitation. Nitrogen was immobilized more often than mineralized, even after one year of incubation. Immobilization was negatively correlated with precipitation. All these results indicate that grazing may significantly alter nutrient cycling by affecting litter quality through changes in species composition. These effects seem to be larger when species replacements induced by grazing either involve functional groups, as it was the case in our wettest site, or change root to shoot ratios. Therefore, the functional groups involved in the replacement of species as well as shifts between belowground and aboveground allocation should play a key role in grazing-induced changes on nitrogen cycling.     

Assessment of soil nitrogen and phosphorous availability under elevated CO2 and N-fertilization in a short rotation poplar plantation

Photosynthetic stimulation by elevated [CO₂] is largely regulated by nitrogen and phosphorus availability in the soil. During a 6 year Free Air CO₂ Enrichment (FACE) experiment with poplar trees in two short rotations, inorganic forms of soil nitrogen, extractable phosphorus, microbial and total nitrogen were assessed. Moreover, in situ and potential nitrogen mineralization, as well as enzymatic activities, were determined as measures of nutrient cycling. The aim of this study was to evaluate the effects of elevated [CO₂] and fertilization on: (1) N mineralization and immobilization processes; (2) soil nutrient availability; and (3) soil enzyme activity, as an indication of microbial and plant nutrient acquisition activity. Independent of any treatment, total soil N increased by 23% in the plantation after 6 years due to afforestation. Nitrification was the main process influencing inorganic N availability in soil, while ammonification being null or even negative. Ammonium was mostly affected by microbial immobilization and positively related to total N and microbial biomass N. Elevated [CO₂] negatively influenced nitrification under unfertilised treatment by 44% and consequently nitrate availability by 30% on average. Microbial N immobilization was stimulated by [CO₂] enrichment and probably enhanced the transformation of large amounts of N into organic forms less accessible to plants. The significant enhancement of enzyme activities under elevated [CO₂] reflected an increase in nutrient acquisition activity in the soil, as well as an increase of fungal population. Nitrogen fertilization did not influence N availability and cycling, but acted as a negative feed-back on phosphorus availability under elevated CO₂.     

Plant uptake of nitrogen and phosphorus among grassland species affected by drought along a soil available phosphorus gradient

Plant and Soil - Environmental factors controlling nitrous oxide (N2O) uptake in forest soils are poorly known, and the atmospheric impact of the forest N2O sink is not well constrained compared to...     

The effect of hydraulic lift on organic matter decomposition,soil nitrogen cycling,and nitrogen acquisition by a grass species

Hydraulic lift (HL) is the passive movement of water through plant roots, driven by gradients in water potential. The greater soil–water availability resulting from HL may in principle lead to higher plant nutrient uptake, but the evidence for this hypothesis is not universally supported by current experiments. We grew a grass species common in North America in two-layer pots with three treatments: (1) the lower layer watered, the upper one unwatered (HL), (2) both layers watered (W), and (3) the lower layer watered, the upper one unwatered, but with continuous light 24 h a day to limit HL (no-HL). We inserted ingrowth cores filled with enriched-nitrogen organic matter (¹⁵N-OM) in the upper layer and tested whether decomposition, mineralization and uptake of ¹⁵N were higher in plants performing HL than in plants without HL. Soils in the upper layer were significantly wetter in the HL treatment than in the no-HL treatment. Decomposition rates were similar in the W and HL treatments and lower in no-HL. On average, the concentration of NH₄ ⁺-N in ingrowth cores was highest in the W treatment, and NO₃ ⁻-N concentrations were highest in the no-HL treatment, with HL having intermediate values for both, suggesting differential mineralization of organic N among treatments. Aboveground biomass, leaf ¹⁵N contents and the ¹⁵N uptake in aboveground tissues were higher in W and HL than in no-HL, indicating higher nutrient uptake and improved N status of plants performing HL. However, there were no differences in total root nitrogen content or ¹⁵N uptake by roots, indicating that HL affected plant allocation of acquired N to photosynthetic tissues. Our evidence for the role of HL in organic matter decomposition and nutrient cycling suggests that HL could have positive effects on plant nutrient dynamics and nutrient turnover.     

氮添加对红松人工林草本层植物多样性的影响

氮沉降是驱动生物多样性变化的重要因素之一。为了探索氮添加对红松（Pinus koraiensis）人工林草本层植物多样性的影响及其驱动机制,以黑龙江凉水国家级自然保护区红松人工林为研究对象,设置N0（对照处理,0 kg hm^-2 a^-1）、N20（低氮处理,20 kg hm^-2 a^-1）、N40（中氮处理,40 kg hm^-2 a^-1）和N80（高氮处理,80 kg hm^-2 a^-1）4个施氮水平,进行6年的氮添加实验。结果表明：（1）氮添加显著降低草本层3个功能群的密度和盖度,而对高度无显著影响;（2）6年氮添加使对照与施氮处理间群落相似度随施氮水平的增加而减小;（3）氮添加显著降低草本植物的丰富度和Shannon-Wiener多样性指数,而未对蕨类和木本植物的丰富度和Shannon-Wiener多样性指数产生显著影响,对草本层3个功能群的Pielou均匀度指数均无显著影响;（4）氮添加对草本植物的C、N、P含量、N：P、C：P产生显著影响,对木本植物的P含量、N：P、C：P产生显著影响,对蕨类植物的C：N：P生态化学计量均无显著影响;（5）草本植物多样性与土壤化学性质无显著的相关关系,草本植物丰富度、Shannon-Wiener多样性指数与植物盖度、密度呈显著的正相关关系,丰富度与植物N含量呈显著的负相关关系,Shannon-Wiener多样性指数与植物N：P呈显著的负相关关系。研究表明6年氮添加改变植物草本层中物种组成和群落结构,3个功能群密度和盖度显著降低,高度未产生显著变化,仅降低草本植物的丰富度和多样性。造成该现象的原因可能是,不同物种对于氮的利用特性和耐受程度存在差异,氮添加引起草本植物养分失衡,改变物种组成和群落结构,从而影响草本植物多样性。研究结果可为我国温带森林生态系统持续性管理提供数据和理论基础。     

Litter mixture dominated by leaf litter of the invasive species,Flaveria bidentis,accelerates decomposition and favors nitrogen release

Journal of Plant Research - In natural ecosystems, invasive plant litter is often mixed with that of native species, yet few studies have examined the decomposition dynamics of such mixtures,...     

Edge effect of a pine plantation reduces dry grassland invertebrate species richness

Natural steppes in European agricultural landscapes are characterized by high biotic richness but are subject to fragmentation and associated edge effects. Edge effects on species richness were investigated at an ecotone from a pine plantation to a short-grass steppe in Eastern Austria for eleven invertebrate taxa differentiated into habitat guilds based on known live-history strategies of individual species (grassland species, forest species, generalist species), including Red-Listed and non-threatened grassland species. The large size of the studied grassland site provided an opportunity to test edge effects in the absence of confounding factors and to a gradient length of 208 m into the grassland habitat along a clear-cut border to a pine plantation. All sampling was done by pitfall trapping. Species richness of habitat guilds, but not total richness, was effectively explained by biotic variables reflecting the influence of shading in particular (i.e. soil temperature sums). Total species richness showed a bimodal response pattern, with increases towards the habitat edge and interior grassland habitat. Habitat guilds showed diverging responses to distances from the edge, but no saturation in species richness, with a continuum of edge effects across the entire distance of the grassland samples. Our findings contrast those of previous investigations based on samples taken from smaller patches and across shorter distances from the edge. Methodological and conservation implications are discussed.