凋落物添加和移除对杉木人工林土壤水解酶活性及其化学计量比的影响

土壤生态酶化学计量比可用于评估微生物碳(C)、氮(N)和磷(P)养分的需求状况。以往从凋落物输入量变化对生态酶化学计量比的角度来探讨杉木人工林土壤养分状况的研究较少。以亚热带杉木人工林为研究对象,采用随机区组实验设计,对12块杉木人工林样地进行3种凋落物处理(凋落物添加(LA);凋落物移除(LR);对照(CK)),通过测定土壤C、N和P水解酶(β-葡糖苷酶(BG)、半纤维素酶(CB)、β-乙酰葡糖胺糖苷酶(NAG)、亮氨酸氨基肽酶(LAP)和酸性磷酸酶(AP))的活性及土壤理化性质,探讨凋落物添加和移除对杉木人工林土壤养分状况的影响。结果表明:LR显著抑制了AP、BG、CB、NAG和LAP活性,同时降低了土壤含水量(SMC)、有机碳(SOC)、总氮(TN)、总磷(TP)和有效氮含量;而LA对以上指标均存在显著的正效应,表明凋落物输入量变化主要是通过改变土壤水分和养分状况来影响水解酶的活性。LR和CK处理下土壤酶活性比ln(BG+CB):ln(AP)和ln(NAG+LAP):ln(AP)均低于全球尺度上土壤酶活性比ln(BG+CB):ln(AP)(0.62)和ln(NAG+LAP):ln(AP)(0.44),表明亚热带地区杉木人工林土壤微生物生长受磷限制;LA处理的土壤的ln(BG+CB):ln(AP)和ln(NAG+LAP):ln(AP)均高于全球尺度,表明LA在一定程度上缓解了土壤磷限制,生态酶化学计量比响应的差异表明磷可能是驱动亚热带杉木人工林土壤生态酶化学计量比内在联系的关键因子。相关分析表明SMC、SOC和有效氮含量与土壤酶活性及其化学计量比呈极显著正相关关系,因此,生态酶化学计量比可作为表征土壤当前养分有效性状况的重要指标,该研究可为亚热带杉木人工林土壤养分管理和可持续性经营提供科学的基础理论。     

Relationships of reproductive performance indicators in black rhinoceros (Diceros bicornis michaeli) with plant available moisture,plant available nutrients and woody cover

Plant available moisture and plant available nutrients in soils influence forage quality and availability and subsequently affect reproductive performance in herbivores. However, the relationship of soil moisture, soil nutrients and woody forage with reproductive performance indicators is not well understood in mega‐browsers yet these three are important in selecting suitable areas for conservation of mega‐browsers. Here, the eastern black rhinoceros (Diceros bicornis michaeli), a mega‐browser, was studied in seven geographically distinct populations in Kenya to understand the relationships between its reproductive performance indicators and plant available moisture, plant available nutrients and woody cover. Reproductive parameters showed a complex relationship with plant available moisture and plant available nutrients. We found an increase in the predicted yearly percentage of females calving as plant available nutrients decreased in areas of high levels of plant available moisture but no relationship with plant available nutrients in areas of low plant available moisture. Age at first calving was earlier, inter‐calving interval was longer and yearly percentage of females calving was higher at higher woody cover. Woody plant cover contributes positively to black rhino reproduction performance indicators, whereas plant available moisture and plant available nutrients add to the selection of conservation areas, in more subtle ways.     

Cyclic environmental changes during the Early Toarcian at the Mochras Farm Borehole (Wales): a variable response of the foraminiferal community

Cyclostratigraphical analysis of the foraminiferal assemblages from the Early Toarcian at the Mochras Farm Borehole (Wales) was conducted in order to evaluate the incidence of cyclic palaeoenvironmental changes on the foraminiferal community. Different variables such as type of morphogroup, evolutionary strategy, habitat, particular taxa, diversity and abundance were studied using the Lomb–Scargle periodogram implemented in the computer program SLOMBS. A well‐developed cyclostratigraphical pattern is recognized, with the presence of several cycles (in metres) at 3.4–4/7.2–7.5/10.1–10.6/32.1–33.3/104.2–111.2/128.2/166.7, belonging to the high‐, middle‐, middle‐/low‐, and low‐frequency bands. The incidence and relevance of the cycles is found to be related to particular variables marking the global and local character of the involved processes. Cyclic changes in the organic matter input are found to be the most relevant palaeoenvironmental factor, oxygenation being secondary. A correspondence with specific Milankovitch cycles is, at present, difficult to determine.     

30 years of free‐air carbon dioxide enrichment (FACE): What have we learned about future crop productivity and its potential for adaptation?

Free‐air CO₂ enrichment (FACE) allows open‐air elevation of [CO₂] without altering the microclimate. Its scale uniquely supports simultaneous study from physiology and yield to soil processes and disease. In 2005 we summarized results of then 28 published observations by meta‐analysis. Subsequent studies have combined FACE with temperature, drought, ozone, and nitrogen treatments. Here, we summarize the results of now almost 250 observations, spanning 14 sites and five continents. Across 186 independent studies of 18 C₃ crops, elevation of [CO₂] by ca. 200 ppm caused a ca. 18% increase in yield under non‐stress conditions. Legumes and root crops showed a greater increase and cereals less. Nitrogen deficiency reduced the average increase to 10%, as did warming by ca. 2°C. Two conclusions of the 2005 analysis were that C₄ crops would not be more productive in elevated [CO₂], except under drought, and that yield responses of C₃ crops were diminished by nitrogen deficiency and wet conditions. Both stand the test of time. Further studies of maize and sorghum showed no yield increase, except in drought, while soybean productivity was negatively affected by early growing season wet conditions. Subsequent study showed reduced levels of nutrients, notably Zn and Fe in most crops, and lower nitrogen and protein in the seeds of non‐leguminous crops. Testing across crop germplasm revealed sufficient variation to maintain nutrient content under rising [CO₂]. A strong correlation of yield response under elevated [CO₂] to genetic yield potential in both rice and soybean was observed. Rice cultivars with the highest yield potential showed a 35% yield increase in elevated [CO₂] compared to an average of 14%. Future FACE experiments have the potential to develop cultivars and management strategies for co‐promoting sustainability and productivity under future elevated [CO₂].     

冬季增温和减雪对黄土高原典型草原土壤养分和细菌群落组成的影响

冬季增温和积雪变化可改变土壤-微生物系统结构和功能。微生物作为陆地生态系统关键生物因子, 发挥着调控土壤养分循环的重要作用, 并对环境扰动, 特别是冬季气候变化十分敏感。开展半干旱区典型草原土壤养分和微生物特性对冬季气候变化的响应研究, 对预测未来气候变化情景下草地生态过程和功能变化意义重大。该研究以宁夏云雾山国家级自然保护区半干旱草原为研究对象, 于冬季布设增温、减雪、增温减雪互作及对照4种处理, 探究了黄土高原典型草原0-5 cm土层土壤养分、酶活性、土壤细菌群落组成对冬季温度和积雪变化的响应规律。结果表明: (1)冬季增温、减雪及互作均提高了0-5 cm土壤温度, 降低了土壤相对湿度, 但却显著增加了土壤冻融循环次数; (2)与对照相比, 不同处理整体上降低了微生物生物量及其多样性, 降低了土壤β-1,4-葡萄糖苷酶(BG)、β-1,4-N-乙酰基氨基葡萄糖苷酶(NAG)、碱性磷酸酶(AKP)活性, 增加了土壤有机碳、全氮、速效磷及铵态氮含量, 硝态氮含量有所下降; (3)研究区土壤细菌以酸杆菌门、变形菌门、放线菌门、芽单胞菌门为主, 优势菌纲以酸杆菌纲、γ-变形杆菌纲、嗜热油菌纲及σ-变形菌纲为主。冗余分析显示, 速效磷含量对细菌群落构成影响最显著, 对群落变异的解释度为21.3%。总之, 冬季气候变化可通过影响土壤温湿度, 特别是冻融循环进而作用于土壤养分循环、酶活性和土壤细菌多样性变化, 这些结果对丰富和拓展气候变化对草地生态系统影响过程与机制的认识, 准确预测典型草原中长期动态变化具有重要意义。     

Responses of primary producers to mouth closure in the temporarily open/closed Great Brak Estuary in the warm-temperate region of South Africa

Low river inflow conditions during 2009/2010 resulted in the mouth of the Great Brak Estuary remaining closed for almost two years. The low water level in Wolwedans Dam resulted in no annual environmental flow releases being made, causing mouth closure. The response of primary producers to this prolonged period of mouth closure was investigated in 2010/2011. Urban and agricultural development in the river catchment and along the estuary banks had increased the nutrient inputs into the estuary. Mouth closure, combined with elevated nutrient concentrations, increased the growth of both macroalgae and microalgae, but little change was observed in the submerged macrophytes. Macroalgal mats covered large areas of open-water surface, smothering the salt marsh and causing a decrease in its cover. These results have important implications for the management of temporarily open/closed estuaries, as increased development, freshwater abstraction and reduced river flow will result in prolonged periods of closure and reduced tidal exchange, which is likely to lead to eutrophication.     

Did the evolution of the phytoplankton fuel the diversification of the marine biosphere?

We discuss the possible links between the fossil record of marine biodiversity, nutrient availability and primary productivity. The parallelism of the fossil records of marine phytoplankton and faunal biodiversity implicates the quantity (primary productivity) and quality (stoichiometry) of phytoplankton as being critical to the diversification of the marine biosphere through the Phanerozoic. The relatively subdued marine biodiversity of the Palaeozoic corresponds to a time of relatively low macronutrient availability and poor food quality of the phytoplankton as opposed to the diversification of the Modern Fauna through the Mesozoic–Cenozoic. Increasing nutrient runoff to the oceans through the Phanerozoic resulted from orogeny, the emplacement of Large Igneous Provinces (LIPs), the evolution of deep-rooting forests and the appearance of more easily decomposable terrestrial organic matter that enhanced weathering. Positive feedback by bioturbation of an expanding benthos played a critical role in evolving biogeochemical cycles by linking the oxidation of dead organic matter and the recycling of nutrients back to the water column where they could be re-utilized. We assess our conclusions against a recently published biogeochemical model for geological time-scales. Major peaks of marine diversity often occur near rising or peak fluxes of silica, phosphorus and dissolved reactive oceanic phosphorus; either major or minor ⁸⁷Sr/⁸⁶Sr peaks; and frequently in the vicinity of major (Circum-Atlantic Magmatic Province) and minor volcanic events, some of which are associated with Oceanic Anoxic Events. These processes appear to be scale-dependent in that they lie on a continuum between biodiversification on macroevolutionary scales of geological time and mass extinction.     

Release from sheep-grazing appears to put some heart back into upland vegetation: A comparison of nutritional properties of plant species in long-term grazing experiments

Rewilding or wilding is a popularised means for enhancing the conservation value of marginal land. In the British uplands, it will involve a reduction, or complete removal, of livestock grazing (sheep), based on the belief that grazing has reduced plant species diversity, the ‘Wet Desert’ hypothesis. The hope is that if livestock is removed, diversity will recover. If true, we hypothesise that the species extirpated/reduced by grazing and then recover on its removal would more nutritious compared to those that persisted. We test this hypothesis at Moor House National Nature Reserve (North-Pennines), where seven sets of paired plots were established between 1953 and 1967 to compare ungrazed/sheep-grazed vegetation. Within these plot-pairs, we compared leaf properties of seven focal species that occurred only, or were present in much greater abundance, in the absence of grazing to those of 10 common species that were common in both grazed and ungrazed vegetation. Each sample was analysed for macro-nutrients, micro-nutrients, digestibility, palatability and decomposability. We ranked the species with respect to 22 variables based on effect size derived from Generalised Linear Modelling (GLM) and compared species using a Principal Components Analysis. We also assessed changes in abundance of the focal species through time using GLMs. Our results support the ‘Wet Desert’ hypothesis, that is, that long-term sheep grazing has selectively removed/reduced species like our focal ones and on recovery, they were more nutritious (macro-nutrients, some micro-nutrients) palatable, digestible and decomposable than common species. Measured changes in abundance of the focal species suggest that their recovery will take 10–20 years in blanket bog and 60 years in high-altitude grasslands. Collectively, these results suggest that sheep grazing has brought about biotic homogenization, and its removal in (re)wilding schemes will reverse this process eventually! The ‘white woolly maggots’ have eaten at least part of the heart out of the highlands/uplands, and it will take some time for recovery.     

Edaphic Factors are a More Important Control on Surface Fine Roots than Stand Age in Secondary Tropical Dry Forests

Although there are generalized conceptual models that predict how above and belowground biomass increase during secondary succession after abandonment from agriculture, there are few data to test these models for fine roots (defined as ≤2 mm diameter) in tropical forests. We measured live and dead fine roots (0–10 cm depth) in 18 plots of regenerating tropical dry forest in Costa Rica that varied in age from 5 to 60 yrs, as well as in soil properties. We predicted that both stand age and soil fertility would affect fine roots, with greater values in older forests on low fertility soils. Across two sampling dates and locations, live fine roots varied from 0.35 to 3.53 Mg/ha and dead roots varied from 0.15 to 0.93 Mg/ha. Surprisingly, there was little evidence that surface fine roots varied between sampling dates or in relation to stand age. By contrast, total, live, and dead fine roots averaged across sampling dates within plots were negatively correlated with a multivariate index of soil fertility (Pearson correlations coefficients were ?0.64, ?0.58, and ?0.68, respectively; P < 0.01) and other individual edaphic variables including pH, silt, calcium, magnesium, nitrogen, and phosphorus. These results suggest that soil fertility is a more important determinant of fine roots than forest age in tropical dry forests in Costa Rica, and that one‐way these plant communities respond to low soil fertility is by increasing fine roots. Thus, simple conceptual models of forest responses to abandonment from agriculture may not be appropriate for surface fine roots.