中亚热带不同森林更新方式生态酶化学计量特征

了解土壤生态化学计量特征对预测不同生态系统养分变化、功能以及植物生产力具有重要意义。森林更新是维持中亚热带森林生态系统可持续发展的重要手段。选取福建省三明市陈大林业采育场3种不同森林更新方式进行研究,包括米槠次生林（SF）、米槠人工促进天然更新林（AR）和杉木人工林（CF）,测定其土壤理化性质及土壤3种酶,计算酶化学计量。结果显示：1）AR的土壤总氮、全磷、铵态氮含量以及含水量最高（P < 0.05）,土壤有效磷的含量则是CF最高（P < 0.05）;2）生态酶化学计量结果表明AR的土壤微生物处于氮限制状态,CF的土壤微生物处于磷限制状态;3）冗余分析表明,土壤含水量和铵态氮是驱动不同森林更新方式土壤生态酶化学计量变异的重要环境因子。研究表明,人促更新方式更有利于土壤有效氮的积累,而人工林则有利于土壤有效磷积累,这可能与造林树种有关。土壤生态酶化学计量更易受到土壤含水量以及有效性养分的驱动,而与土壤化学计量未呈现良好的耦合性。     

Livestock exclusion reduces the spillover effects of pastoral agriculture on soil bacterial communities in adjacent forest fragments

Forest-to-pasture conversion is known to cause global losses in plant and animal diversity, yet impacts of livestock management after such conversion on vital microbial communities in adjoining natural ecosystems remain poorly understood. We examined how pastoral land management practices impact soil microorganisms in adjacent native forest fragments, by comparing bacterial communities sampled along 21 transects bisecting pasture–forest boundaries. Our results revealed greater bacterial taxon richness in grazed pasture soils and the reduced dispersal of pasture-associated taxa into adjacent forest soils when land uses were separated by a boundary fence. Relative abundance distributions of forest-associated taxa (i.e., Proteobacteria and Nitrospirae) and a pasture-associated taxon (i.e., Firmicutes) also suggest a greater impact of pastoral land uses on forest fragment soil bacterial communities when no fence is present. Bacterial community richness and composition were most related to changes in soil physicochemical variables commonly associated with agricultural fertilization, including concentrations of Olsen P, total P, total Cd, delta ¹⁵N and the ratio of C:P and N:P. Overall, our findings demonstrate clear, and potentially detrimental effects of agricultural disturbance on bacterial communities in forest soils adjacent to pastoral land. We provide evidence that simple land management decisions, such as livestock exclusion, can mitigate the effects of agriculture on adjacent soil microbial communities.     

中亚热带森林转换对土壤磷积累的影响

磷是植物生长的必需元素之一,是维持亚热带森林生态系统生产力的关键因子。研究森林转换后土壤因素对磷素的影响,对生态系统的稳定和森林经营具有重要意义。选取由亚热带常绿阔叶林转换而成的米槠次生林(SF)、米槠人促林(AR)和杉木人工林(CF)为研究对象,测定了土壤理化性质、铁铝氧化物、各形态磷含量以及酸性磷酸酶活性,旨在探究土壤磷对森林转换的响应和驱动土壤磷变化的影响因子。结果显示:米槠人促林土壤的全磷、有机磷和微生物生物量磷显著高于米槠次生林和杉木人工林;冗余分析(RDA)发现,土壤含水量、总氮和无定型铁是影响淋溶层土壤磷的主要因子,而在淀积层,则是酸性磷酸酶、游离型铁和总氮起主要作用;土壤生物化学属性和微生物特性都会影响着不同形态土壤P的积累,其中土壤中的水分和酸性磷酸酶活性是调控土壤磷的关键因子。研究表明,中亚热带地区天然林转换为人促更新林更有利于森林土壤磷的储存和供应,有助于维持本区域森林生态系统的稳定。     

Effects of nitrogen: phosphorus supply ratios on nitrogen fixation in agricultural and pastoral ecosystems

An analysis of data compiled from the literature confirms a strong inverse relationship between annual rates of nitrogen fixation and the soil nitrogen content in agricultural and pastoral ecosystems. However, this inverse relationship is strongly modified by the rate of application of phosphorus fertilizer, which strongly influences the activities of both symbiotic and non-symbiotic nitrogen fixing organisms. In the case of symbiotic legumes, the response of N-fixation to N and P is in part a result of changes in legume dominance within the plant community. These results, as well as supporting data presented from a review of experiments on nitrogen fixation in a variety of other terrestrial and aquatic ecosystems, provide important support for the hypothesis that phosphorus availability is a key regulator of nitrogen biogeochemistry. Published as Paper No. 9950, Journal Series, Nebraska Agricultural Research Division, University of Nebraska, Lincoln, NE, USA.     

鄱阳湖湿地优势植物叶片-凋落物-土壤碳氮磷化学计量特征

2013年11月初在鄱阳湖南矶湿地国家级自然保护区,采集芦苇(Phragmites australis)、南荻(Triarrhena lutarioriparia)、菰(Zizania latifolia(Griseb.))、灰化苔草(Carex cinerascens)、红穗苔草(Carex argyi)和水蓼(Polygonum hydropiper)等6种优势植物新鲜叶片、凋落物及表层0—15cm土壤样品测定了碳(C)、氮(N)、磷(P)含量,以阐明不同物种、不同生活型间C、N、P化学计量差异,探讨化学计量垂直分异。结果表明:1)C、N、P含量变化范围分别为:叶片380.6—432.2 mg/g,15.3—32.6 mg/g和1.3—2.0 mg/g;凋落物345.4—416.1 mg/g,10.8—20.8 mg/g和1.1—1.7 mg/g;土壤15.0—38.1 mg/g,1.2—3.1 mg/g和0.7—1.1mg/g,不同物种间叶片、凋落物及土壤C、N、P含量差异显著,且叶片C、N、P含量显著高于凋落物与土壤。2)土壤C∶N、C∶P及N∶P值显著低于叶片与凋落物,且土壤C、N、P化学计量关系与凋落物更为密切,凋落物的C∶N、N∶P分别能解释土壤C∶N、N∶P变异的35%、18%。3)挺水植物与湿生植物之间叶片C∶N、N∶P值差异显著,C∶P则差异不显著,凋落物C∶N、C∶P与N∶P均未达到显著性差异。     

Isotopic constraints on plant nitrogen acquisition strategies during ecosystem retrogression

Plant root associations with microbes such as mycorrhizal fungi or N-fixing bacteria enable ecosystems to tap pools of nitrogen (N) that might otherwise be inaccessible, including atmospheric N or N in large soil organic molecules. Such microbially assisted N-foraging strategies may be particularly important in late-successional retrogressive ecosystems where productivity is low and soil nutrients are scarce. Here, we use natural N-stable isotopic composition to constrain pathways of N supplies to different plant functional groups across a well-studied natural soil fertility gradient that includes a highly retrogressive stage. We demonstrate that ectomycorrhizal fungi, ericoid mycorrhizal fungi, and N-fixing bacteria support forest N supplies at all stages of ecosystem succession, from relatively young, N-rich/phosphorus (P)-rich sites, to ancient sites (ca. 500 ky) where both N supplies and P supplies are exceedingly low. Microbially mediated N sources are most important in older ecosystems with very low soil nutrient availability, accounting for 75–96% of foliar N at the oldest, least fertile sites. These isotopically ground findings point to the key role of plant–microbe associations in shaping ecosystem processes and functioning, particularly in retrogressive-phase forest ecosystems.     

帽儿山3种森林生态系统土壤动物与土壤呼吸及其相互关系研究

土壤呼吸是森林生态系统碳循环的关键过程,土壤动物可通过自身代谢及影响微生物活动调控土壤呼吸,因此研究土壤动物与土壤呼吸的相互关系对进一步揭示生态系统碳循环的规律和机理具有重要意义。通过野外定点,以帽儿山3种森林生态系统的土壤呼吸及土壤动物为研究对象,探讨不同森林生态系统的土壤呼吸、土壤动物个体密度和生物量的时间变化规律及二者相互关系。结果表明:(1)3种森林生态系统土壤总呼吸速率与土壤异养呼吸速率均呈现先增强后减弱的时间动态变化(P<0.05),且不同森林生态系统土壤异养呼吸速率差异显著(P<0.05),表现为硬阔叶林最高,红松人工林最低;(2)3种森林生态系统土壤动物生物量也具有显著的时间动态变化(P<0.05),均在9月份达到最大,且不同森林生态系统土壤动物个体密度显著不同(P<0.05),蒙古栎林土壤动物个体密度显著小于红松人工林与硬阔叶林;(3)通过回归分析可得,土壤动物数量及生物量的增加抑制了土壤呼吸速率,尤其在生长季初期、末期。研究表明土壤动物可通过抑制微生物生命活动和降低根系呼吸从而对土壤总呼吸及异养呼吸产生负反馈作用,三者是不可分割的整体,与土壤温度、水分等环境因子共同调控着土壤呼吸。     

西北地区草地生态系统生态安全评价初探

草地生态系统是我国西北生态脆弱地区的重要组成部分,开展其生态安全的评价对于西部大开发战略的实施有着重要的指导作用,对维护西部地区的政治稳定、经济可持续发展、军事安全等有着不可估量的价值.通过对西北生态脆弱地区草地生态系统现状的剖析,阐述了开展草地生态系统生态安全评价的意义,确立了草地生态系统生态安全评价的原则,分析影响草地生态系统生态安全的6个主要因子:人类活动、草地生态系统功能、植被群落动态、气候变化、土壤条件、有害生物等.在此基础上,初步构建草地生态安全评价的理论指标体系.     

Are patterns in nutrient limitation belowground consistent with those aboveground: results from a 4 million year chronosequence

Accurately predicting the effects of global change on net carbon (C) exchange between terrestrial ecosystems and the atmosphere requires a more complete understanding of how nutrient availability regulates both plant growth and heterotrophic soil respiration. Models of soil development suggest that the nature of nutrient limitation changes over the course of ecosystem development, transitioning from nitrogen (N) limitation in ‘young’ sites to phosphorus (P) limitation in ‘old’ sites. However, previous research has focused primarily on plant responses to added nutrients, and the applicability of nutrient limitation-soil development models to belowground processes has not been thoroughly investigated. Here, we assessed the effects of nutrients on soil C cycling in three different forests that occupy a 4 million year substrate age chronosequence where tree growth is N limited at the youngest site, co-limited by N and P at the intermediate-aged site, and P limited at the oldest site. Our goal was to use short-term laboratory soil C manipulations (using ¹⁴C-labeled substrates) and longer-term intact soil core incubations to compare belowground responses to fertilization with aboveground patterns. When nutrients were applied with labile C (sucrose), patterns of microbial nutrient limitation were similar to plant patterns: microbial activity was limited more by N than by P in the young site, and P was more limiting than N in the old site. However, in the absence of C additions, increased respiration of native soil organic matter only occurred with simultaneous additions of N and P. Taken together, these data suggest that altered nutrient inputs into ecosystems could have dissimilar effects on C cycling above- and belowground, that nutrients may differentially affect of the fate of different soil C pools, and that future changes to the net C balance of terrestrial ecosystems will be partially regulated by soil nutrient status.     

克氏针茅草原土壤生态化学计量特征对放牧强度的响应

生态化学计量学是研究生态系统中各元素间平衡的科学,对于探究土壤有机碳（C）、全氮（N）、全磷（P）元素循环以及土壤养分限制状况具有重要意义。为了解克氏针茅（Stipa kirschnii）草原土壤生态化学计量特征,以内蒙古克氏针茅草原区为研究对象,在不同放牧强度草场进行采样分析。结果表明：随着放牧强度的增加,C、P含量先降低后升高,N含量先升高后降低;土壤C/N先降低后升高、C/P逐渐升高、N/P先升高后降低。C/N随着放牧强度的增加与有机碳相关性逐渐增大、与全氮的相关性逐渐减小;轻牧区的C/P受有机碳作用有所增强;中牧区N/P受全氮作用增强。C/N主要限制因素是有机碳含量,C/P和N/P主要限制因素是全磷含量。不同放牧强度会影响土壤化学计量比与其他物质的关系,也会改变土壤化学计量比各制约因素的强度。     

Plant life on gypsum: a review of its multiple facets

The adaptation of plants to particular soil types has long intrigued biologists. Gypsum soils occupy large areas in many regions of the world and host a striking biological diversity, but their vegetation has been much less studied than that developing over serpentine or saline soils. Herein, we review all aspects of plant life on gypsum ecosystems, discuss the main processes driving their structure and functioning, and highlight the main conservation threats that they face. Plant communities in gypsum habitats typically show distinctive bands at very small spatial scales, which are mainly determined by topography. Plants living on gypsum soils can be classified into three categories: (i) wide gypsophiles are specialists that can penetrate the physical soil crust during early life stages and have physiological adjustments to cope with the chemical limitations imposed by gypsum soils; (ii) narrow gypsophiles are refugee plants which successfully deal with the physical soil crust and can tolerate these chemical limitations but do not show specific adaptations for this type of soils; and (iii) gypsovags are non‐specialist gypsum plants that can only thrive in gypsum soils when the physical crust is absent or reduced. Their ability to survive in gypsum soils may also be mediated by below‐ground interactions with soil microorganisms. Gypsophiles and gypsovags show efficient germination at low temperatures, seed and fruit heteromorphism within and among populations, and variation in seed dormancy among plants and populations. In gypsum ecosystems, spatio‐temporal changes in the composition and structure of above‐ground vegetation are closely related to those of the soil seed bank. Biological soil crusts (BSCs) dominated by cyanobacteria, lichens and mosses are conspicuous in gypsum environments worldwide, and are important drivers of ecosystem processes such as carbon and nitrogen cycling, water infiltration and run‐off and soil stability. These organisms are also important determinants of the structure of annual plant communities living on gypsum soils. The short‐distance seed dispersal of gypsophiles is responsible for the high number of very narrow endemisms typically found in gypsum outcrops, and suggests that these species are evolutionarily old taxa due to the time they need to colonize isolated gypsum outcrops by chance. Climate change and habitat fragmentation negatively affect both plants and BSCs in gypsum habitats, and are among the major threats to these ecosystems. Gypsum habitats and specialists offer the chance to advance our knowledge on restrictive soils, and are ideal models not only to test important evolutionary questions such as tolerance to low Ca/Mg proportions in soils, but also to improve the theoretical framework of community ecology and ecosystem functioning.     

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.     

陆地生态系统磷素循环及其影响因素

 磷是生命系统的重要组成成分,其在生态系统内的迁移转化是生态系统结构和功能的决定性因素之一。近20年来,磷在陆地生态系统内的重要性受到越来越多的关注。该文总结了国内外磷循环研究的成果,从磷的来源、在土壤中的存在形态和固定特性、影响因素的复杂性等方面分析了磷素循环的特点;系统阐述了磷在陆地生态系统各库之间及其内部,主要是植被-土壤亚系统内的迁移转化规律及影响因素。陆地生态系统磷素循环主要是系统内部的生物化学循环,由植物自身的遗传特性和土壤的生物、理化性质共同控制,不同控制因素的相对重要性因生态系统类型、时间和空间尺度而异。文章简述了磷循环研究方法的发展及存在的局限性;另外,分析了干旱、半干旱地区磷循环研究的重要性和意义;干旱区生态系统的脆弱性及其植被、土壤特性决定了其磷素循环有其自身的特点及研究的必要性。最后指出了当前陆地生态系统磷循环研究的发展趋势。     

Integration of ecosystem engineering and trophic effects of herbivores

Herbivores affect vegetation in a variety of ways, involving both trophic and ecosystem engineering interactions, but the study of these different interaction types has rarely been integrated. The aim of this study was to investigate both the trophic and engineering effects of herbivores on plant communities in the Negev desert, Israel, and to promote an integrative approach to the study of herbivore effects in ecosystems. First, we summarise previous studies of the Indian crested porcupine (Hystrix indica), which show that in digging for food, porcupines excavate soil pits, which accumulate resources and seeds resulting in marked changes in plant species richness, density and biomass. By contrast, their trophic effect, via consumption of bulbs, has little impact on populations of perennial plants. Second, we present an empirical study of the trophic and ecosystem engineering effects of harvester ants (Messor spp.). An exclusion experiment, using barriers to restrict ant access, failed to reveal any significant effect of seed collection by harvester ants on plant species incidence (proportional occurrence in samples) or abundance (number of individuals). However, we show that vegetation on nest mounds of M. ebeninus differs in plant density, species richness and biomass from that on undisturbed soil. An analysis of incidence and abundance responses of individual plant species suggests that the observed differences in vegetation resulted from multiple interacting mechanisms.
The case studies highlight that many interactions between herbivores and plant communities can occur simultaneously, and that ecosystem engineering and trophic processes can be closely associated, resulting from single actions of herbivores. We propose a conceptual framework that classifies the range of possible trophic and engineering interactions between herbivores and plant communities with respect to the level of association between trophic and engineering effects. The framework is presented as an aid to the design and interpretation of studies of interactions between herbivores and plant communities, and promotes integrative research into the roles of herbivores in ecosystems.     

Asymmetric effects of grazing intensity on macroelements and microelements in grassland soil and plants in Inner Mongolia Grazing alters nutrient dynamics of grasslands

Grazing is a traditional grassland management technique and greatly alters ecosystem nutrient cycling. The effects of grazing intensity on the nutrient dynamics of soil and plants in grassland ecosystems remain uncertain, especially among microelements. A 2‐year field grazing experiment was conducted in a typical grassland with four grazing intensities (ungrazed control, light, moderate, and heavy grazing) in Inner Mongolia, China. Nutrient concentration was assessed in soil and three dominant plant species (Stipa krylovii, Leymus chinensis, and Cleistogenes squarrosa). Assessed quantities included four macroelements (carbon (C), nitrogen (N), phosphorus (P), and magnesium (Mg)) and four microelements (copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)). Soil total C, total N, total P, available N, and available P concentrations significantly increased with grazing intensity but soil Mg, Cu, Fe, Mn, Zn concentrations had no significant response. Plant C concentration decreased but plant N, P, Mg, Cu, Fe, Mn, and Zn concentrations significantly increased with grazing intensity. In soil, macroelement dynamics (i.e., C, N, and P) exhibited higher sensitivity with grazing intensity, conversely in plants, microelements were more sensitive. This result indicates macroelements and microelements in soil and plants had asymmetric responses with grazing intensity. The slopes of nutrient linear regression in C. squarrosa were higher than that of S. krylovii and L. chinensis, indicating that C. squarrosa had higher nutrient acquisition capacity and responded more rapidly to heavy grazing. These findings indicate that short‐term heavy grazing accelerates nutrient cycling of the soil–plant system in grassland ecosystems, elucidate the multiple nutrient dynamics of soil and plants with grazing intensity, and emphasize the important function of microelements in plant adaptation in grazing management.     

不同基因型甘蔗种质资源的表型遗传多样性

为探明甘蔗原种和地方种的遗传多样性和亲缘关系,以期筛选出优良甘蔗种质和优良杂交亲本.该研究对18份甘蔗原种和地方种的14个数量性状进行了表型遗传多样性分析.结果表明：通过14个数量性状的变异系数(coefficient of variance,CV)和性状之间的相关分析,18份甘蔗原种和地方种的数量性状遗传变异主要来自甘蔗蔗糖分、单茎重、叶宽、茎径和纤维分;对14个数量性状进行主成分分析提取获得了4个主成分因子,分别命名为“品质因子”、“生长因子”、“成熟度因子”和“光合因子”,主成分因子累积贡献率达83．482％;进一步通过对主成分因子开展综合评价分析,获得数量性状综合表型高于平均水平的10份材料,依次为 Sampana→甜圪塔→合庆草甘蔗→桂林竹蔗→坦桑尼亚→芒戈→古芝蔗→大岛再来→托江红→春尼;聚类分析基于不同的遗传距离可将18份种质聚为5个类别,潜在的优良杂交组合是 Sampana 和甜圪塔或 Sampana 和合庆草甘蔗,表明在甘蔗遗传育种亲本选择上既要考虑各性状主要因子的互补,又要保持一定的遗传距离.该研究认为,在甘蔗育种工作中,利用因子分析法进行表型遗传多样性分析,将更加有助于亲本和杂交组合的选择.     

Moderate changes in nutrient input alter tropical microbial and protist communities and belowground linkages

We investigated the response of soil microbial communities in tropical ecosystems to increased nutrient deposition, such as predicted by anthropogenic change scenarios. Moderate amounts of nitrogen and phosphorus and their combination were added along an altitudinal transect. We expected microorganisms and microbial grazers (testate amoebae) to significantly respond to nutrient additions with the effect increasing with increasing altitude and with duration of nutrient additions. Further, we expected nutrients to alter grazer–prey interrelationships. Indeed, nutrient additions strongly altered microbial biomass (MB) and community structure as well as the community structure of testate amoebae. The response of microorganisms varied with both altitude and duration of nutrient addition. The results indicate that microorganisms are generally limited by N, but saprotrophic fungi also by P. Also, arbuscular mycorrhizal fungi benefited from N and/or P addition. Parallel to MB, testate amoebae benefited from the addition of N but were detrimentally affected by P, with the addition of P negating the positive effect of N. Our data suggests that testate amoeba communities are predominantly structured by abiotic factors and by antagonistic interactions with other microorganisms, in particular mycorrhizal fungi, rather than by the availability of prey. Overall, the results suggest that the decomposer system of tropical montane rainforests significantly responds to even moderate changes in nutrient inputs with the potential to cause major ramifications of the whole ecosystem including litter decomposition and plant growth.     

不同基因型甘蔗种质资源的表型遗传多样性

植物的碳、氮、磷化学计量特征能反映植物对土壤营养元素的利用效率,岩溶区植物经过长期的进化形成了自身独特的生理生态和生态化学计量特征,通过岩溶区植物叶片碳、氮、磷化学计量可以揭示岩溶生态系统各组分之间的养分循环规律.该研究在桂林毛村岩溶区次生林中选择3个20 m×20 m 的样方,采用多元统计方法分析了岩溶区森林12种典型植物叶片共186个样品的碳、氮、磷的生态化学计量特征,研究它们之间的相互关系,探讨碳、氮、磷化学计量学在岩溶生态系统中的生态指示作用.结果表明：(1)虽然岩溶区石灰土氮和磷的含量较高,但由于其有效性低,植物对养分的吸收和利用缓慢,岩溶区石灰土植物的生长仍然受到 N 和 P 的共同限制;(2)由于岩溶区植物叶片中 N 和 P 的含量显著偏低导致较高的 C∶N 和 C∶P 值(C∶N 的平均值为80．86;C∶P 的平均值为639．65);(3)利用 N∶P＜14表明氮受限制,N∶P＞16表明磷受限制,14＜N∶P＜16表明 N 和 P 共同限制的标准判断植物叶片受 N 或 P 的限制在岩溶区不完全适合;(4)元素间相关性分析表明,叶片的 C 和 N 呈极显著负相关关系(P ＜0．01),C 和 P 呈显著负相关关系(P ＜0．05),N 和 P之间呈现极显著正相关关系(P ＜0．01).这体现了植物体内两营养元素含量需求变化的相对一致性.研究结果有助于了解岩溶区森林植物的适生机制及其生态地球化学过程,可为岩溶区生态治理提供理论依据.     

Soil microbial biomass and the fate of phosphorus during long-term ecosystem development

Background

Soil phosphorus availability declines during long-term ecosystem development on stable land surfaces due to a gradual loss of phosphorus in runoff and transformation of primary mineral phosphate into secondary minerals and organic compounds. These changes have been linked to a reduction in plant biomass as ecosystems age, but the implications for belowground organisms remain unknown.

Methods

We constructed a phosphorus budget for the well-studied 120,000 year temperate rainforest chronosequence at Franz Josef, New Zealand. The budget included the amounts of phosphorus in plant biomass, soil microbial biomass, and other soil pools.

Results

Soil microbes contained 68–78 % of the total biomass phosphorus (i.e. plant plus microbial) for the majority of the 120,000 year chronosequence. In contrast, plant phosphorus was a relatively small pool that occurred predominantly in wood. This points to the central role of the microbial biomass in determining phosphorus availability as ecosystems mature, yet also indicates the likelihood of strong competition between plants and saprotrophic microbes for soil phosphorus.

Conclusions

This novel perspective on terrestrial biogeochemistry challenges our understanding of phosphorus cycling by identifying soil microbes as the major biological phosphorus pool during long-term ecosystem development.     

Shifts in symbiotic associations in plants capable of forming multiple root symbioses across a long‐term soil chronosequence

Changes in soil nutrient availability during long‐term ecosystem development influence the relative abundances of plant species with different nutrient‐acquisition strategies. These changes in strategies are observed at the community level, but whether they also occur within individual species remains unknown. Plant species forming multiple root symbioses with arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, and nitrogen‐(N) fixing microorganisms provide valuable model systems to examine edaphic controls on symbioses related to nutrient acquisition, while simultaneously controlling for plant host identity. We grew two co‐occurring species, Acacia rostellifera (N₂‐fixing and dual AM and ECM symbioses) and Melaleuca systena (AM and ECM dual symbioses), in three soils of contrasting ages (c. 0.1, 1, and 120 ka) collected along a long‐term dune chronosequence in southwestern Australia. The soils differ in the type and strength of nutrient limitation, with primary productivity being limited by N (0.1 ka), co‐limited by N and phosphorus (P) (1 ka), and by P (120 ka). We hypothesized that (i) within‐species root colonization shifts from AM to ECM with increasing soil age, and that (ii) nodulation declines with increasing soil age, reflecting the shift from N to P limitation along the chronosequence. In both species, we observed a shift from AM to ECM root colonization with increasing soil age. In addition, nodulation in A. rostellifera declined with increasing soil age, consistent with a shift from N to P limitation. Shifts from AM to ECM root colonization reflect strengthening P limitation and an increasing proportion of total soil P in organic forms in older soils. This might occur because ECM fungi can access organic P via extracellular phosphatases, while AM fungi do not use organic P. Our results show that plants can shift their resource allocation to different root symbionts depending on nutrient availability during ecosystem development.