内蒙古贝加尔针茅草原3种主要植物凋落物分解特征

以内蒙古贝加尔针茅（Stipa baicalensis）草原3种主要植物贝加尔针茅、羊草（Leymus chinensis）和冷蒿（Artemisia frigida）为对象,采用分解袋法研究了3种不同植物器官（叶、茎、根）凋落物的分解速率和养分动态变化过程。结果表明：3种主要植物凋落物分解速率存在明显差异,贝加尔针茅叶凋落物分解快于根凋落物,羊草和冷蒿根凋落物分解快于叶和茎凋落物;3种主要植物凋落物分解季节动态呈单峰型曲线,在8月份时达最大值;羊草和冷蒿茎凋落物初始N含量与其分解速率成显著正相关,3种主要植物凋落物C含量和C/N与各器官凋落物分解速率成显著负相关。     

增温对川西北高寒草甸草场植物凋落物分解的影响

凋落物分解是陆地生态系统碳循环的重要环节,温度是凋落物分解的关键影响因子。为探究增温对川西北高寒草甸草场植物凋落物分解的影响,使用开顶式生长箱(OTCs)研究模拟增温对分解袋法处理的凋落物样品的影响,以阐明增温对川西北高寒草甸生态系统中优势物种凋落物:禾本科的披碱草、发草,杂草类的野茴香、鹅绒委陵菜、蒙古蒿、星状雪兔子、长毛凤毛菊等群落混合地上凋落物和群落地下根系凋落物分解的影响。结果表明:1)模拟增温并未显著改变混合地上凋落物的分解周期,但是显著提高了根系凋落物的分解效率,地上、地下凋落物的分解周期对温度变化的响应存在差异。2)尽管不同种类植物凋落物的养分含量与化学计量特征存在显著差异(P<0.05),但与自然分解条件相比,模拟增温条件下单种植物地上凋落物的分解效率并未发生明显改变,模拟增温对单种植物地上凋落物的分解释放过程影响不显著。综上所述,川西北高寒草甸生态系统地上凋落物的分解过程对模拟增温响应不显著,而模拟增温显著影响地下凋落物分解过程。     

Yak Dung Deposition Affects Litter Mixing Effects on Mass Loss in Tibetan Alpine Grassland

Plant litter and livestock excreta are two important ways of carbon and nutrient input to soil in grassland grazing systems. Grazing livestock often deposit dung on plant litter, which may affect litter decomposition through a changing microenvironment. We assessed effects of yak dung on litter mixing effects on litter decomposition in a Tibetan alpine grassland. Six common species were selected, including low-quality litter species Kobresia capillifolia, Elymus nutans, and Ligularia virgaurea and high-quality litter species Anemone rivularis, Saussurea nigrescens, and Thermopsis lanceolata. Litter bags containing each species alone and all two-species combinations were allowed to decompose with and without experimental dung addition in the field. Mass loss of the leaf litter was measured after 6 and 12 mo. High-quality litter species had significantly greater mass loss than low-quality litter species. Dung significantly accelerated litter mass loss after both 6 and 12 mo for low-quality litter species, but only after 12 mo for high-quality litter species. Litter mixtures containing both high- and low-quality species showed positive nonadditive effects (NAEs) on mass loss after 6 mo but additive effects after 12 mo. Dung increased the strength of NAEs after 6 mo and shifted litter mixing effects from positive to negative NAEs after 12 mo. Our results support previous findings that litter mixing could produce NAEs on litter decomposition and that these NAEs could change with incubation time. Most importantly, we show that dung can modify NAEs, demonstrating that litter mixing effects are dependent on the microenvironment. Our findings also demonstrate that yak dung can influence soil processes by varying both single-species litter decomposition rates and litter interactions within mixtures. Furthermore, the results suggest yak dung is closely related to material and nutrient cycling, so we believe dung should remain and not be substantially removed from this grazing ecosystem.     

黄土丘陵区不同植物凋落叶片的分解及养分释放特性

叶片凋落物的分解对生态系统的养分循环具有重要意义。刺槐、油松和柠条是豫西黄土丘陵区退耕还林时普遍采用的树种,高羊茅是该区分布广泛的建种草本植物。采用分解网袋法,对该区 4 种植物叶片凋落物一年内的分解特征及营养元素的释放规律进行研究。结果表明:1)柠条叶的分解率分别比刺槐叶、油松和高羊茅高8.86%、19.87%和13.18%,分解规律符合Olson 指数模型,分解速率为柠条叶>刺槐叶>高羊茅叶>油松叶。2)分解过程中 4 种凋落叶片的C/N、P含量持续降低,木质素含量缓慢增加,纤维素含量呈先增加后降低的变化趋势;不同凋落叶片分解过程中全C和全N含量的变化趋势存在差异。4 种凋落叶片的C、P元素属于直接释放模式,N元素属于富集—释放模式。3)凋落叶片的分解速率与总C含量、木质素含量和C/N均呈显著负相关关系(P<0.01),因此该地区凋落叶片的总C、木质素含量和C/N可以作为预测该地区凋落物分解的指标,凋落物叶片功能特征(比叶面积、厚度、硬度等)可能是影响其分解速率的重要因素之一。     

Managing Grazing Intensity Linked to Forage Quantity and Quality Trade-off in Semiarid Rangelands

Knowledge of herbivory tolerance of dominant plant species and their responses to varying grazing intensity is required for sustainable grazing management in semiarid rangeland ecosystems. In a field experiment, we studied the effects of simulated grazing 0%, 30%, 50%, and 80% leaf removal (control, partial, intermediate, and heavy leaf removal) of three dominant perennial legumes: Astragalus cyclophyllon, Astragalus effuses, and Hedysarum criniferum in semiarid rangelands of southwestern Iran for 2 yr and tested 1) differential herbivory tolerance in terms of regrowth ability of belowground and aboveground biomass (AGB), fecundity, and nonstructural carbohydrate reserves; and 2) trade-off between forage quantity and quality—less defoliated plants produce larger amount of biomass with less forage quality and vice versa. The results showed a reduction in AGB and reproduction in each of the three plants. However, three forbs exhibited some sort of variations in shoot and root growth, reproduction and forage quality under intermediate and heavy leaf removal. Leaf removal in individuals of A. effusus and H. criniferum are more likely to compensate for tissue loss by allocating the available resources to the expansion of root biomass, at the cost of AGB and reproductive effort. Leaf removal also led to an increase in forage quality in each of the three forb species, depending on the level of height removal. The control plant had a higher amount of biomass production and lower amount of crude protein compared with heavy leaf removal treatments (i.e., 80%), by contrasting responses of acid detergent fiber and neutral detergent fiber, indicating a trade-off between forage quality and forage quantity. The results indicate that there is a balance among forage quantity, quality, and regrowth ability of belowground and aboveground tissues in intermediate leaf removal treatments of these forb species. This balance is used to develop principles for grazing management of steppe rangelands because such a regime meets different criteria including forage quantity, quality, and nonstructural carbohydrate reserves.     

Grazing Intensity on Vegetation Dynamics of a Typical Steppe in Northeast Inner Mongolia

Vegetation features radiating from residential areas in response to livestock grazing were quantified for an arid steppe rangeland in the Keshiketeng Banner, Chifeng Prefecture, in northeastern Inner Mongolia in 2004 and 2006. The aim of this study was to estimate grazing impacts on the vegetation dynamics of these historical grazed ecosystems. Grazing intensities were classified as reference area (RA), light (LG), moderate (MG), and heavy (HG) according to the vegetation utilization across the study area. Rangelands were studied along a grazing gradient, where characteristics of plant communities, heights of dominant species, aboveground vertical structures, and belowground biomass were investigated. Along this grazing gradient, vegetation changed from the original dominant plant species Leymus chinensis (Trin.) Tzvel. to a semi-subshrub species Artemisia frigida Willd. when moving from the reference area (RA) to the region around the settlement. Canopy coverage, aboveground productivity, and the number of perennial species declined as one moved toward the residential area. Heights of five dominant species, except for Stipa grandis P. Smirn., declined with increased grazing intensity. Aboveground vertical structure in the RA treatment showed more resilience than the other treatments. There was no difference in root biomass in the top 1 m of soil (P > 0.05) between the RA treatment and the area immediately around settlement (HG treatment). Generally, we found that the intensity of grazing disturbance did not exceed the tolerance of the rangeland ecosystem within LG treatment. However, vegetative conditions in HG treatment became worse with increased grazing pressure. Rangelands in this arid steppe are under tremendous threat due to excessive forage utilization, which cannot be considered a sustainable practice.     

天山北坡改良草地凋落物混合分解特征研究

研究凋落物分解过程及其调控机制对于预测生态系统凋落物分解、揭示营养元素循环规律具有重要意义。本试验采用野外凋落物网袋分解法,设置了紫花苜蓿（Medicago sativa）和无芒雀麦（Bromus inermis）不同混合比例的叶凋落物处理,在分解1个月、3个月和12个月后取出测定凋落物剩余量及碳氮含量。试验结果表明：单独紫花苜蓿凋落物的分解和碳、氮释放量均高于无芒雀麦凋落物单独分解,混合凋落物剩余率介于两种单一凋落物分解剩余率之间;混合凋落物紫花苜蓿和无芒雀麦凋落物比例为1：3的组合表现为显著的协同效应,其他组合为加和效应;凋落物剩余率及碳氮元素的剩余率与其初始氮含量呈负相关关系,与初始碳氮比呈正相关关系。研究结果表明,在新疆山地草原生态系统,凋落物混合分解效应与其组合比例密切相关,在豆禾凋落物适宜比例下混合凋落物分解可产生协同效应。     

Production and decomposition of plant litter in an arid rangeland of Kenya

Data on litter production and decomposition in an arid rangeland in Kenya was collected over a two-year period. Utter sampling was carried out at monthly intervals using a rectangular 0.25m^?2 quadrat frame. Utter within the quadrats was handpicked and washed with running water to get rid of soli particles, dried, and weighed. Weights were expressed on organic matter basis. Monthly litter production ranged from 31.4g m^?2 to 130.0g m^?2. Mean monthly yield was 92.5 ± 26g m^?2, with a 28% coefficient of variation. There was no significant difference (p>0.01) in litter yield between 1992 and 1993. Rate of decomposition for aboveground material ranged from 0.005g g^?1 day^?1 to 0.084g g^?1 day^?1. The mean annual rate of decomposition was 0.026g g^?1 day^?1. Belowground plant material rates of decomposition spread from 0.009g g^?1 day^?1 to 0.062g g^?1 day^?1. with a mean annual rate of 0.041g g^?1 day^&minus1. Belowground material consistently decomposed faster than aboveground material. Peaks in both aboveground and belowground material decomposition rates coincided with rainfall peaks. Overall, in this arid environment, litter production and decomposition is pursed in nature, and trends are closely related to rainfall occurrence. Moisture is thus a limiting factor both to the production and decomposition of litter. Belowground litter plays a significant role in nutrient cycling.     

Seasonal Losses of Surface Litter in Northern Great Plains Mixed-Grass Prairies

Surface litter protects rangeland soils against wind and water erosion and provides food and nesting materials for wildlife and insects. However, the ability of grassland systems to provide these services depends on the little studied topic of seasonal surface litter decomposition. Seasonal and annual surface litter decomposition rates were determined between 2014 and 2015 in central and western South Dakota at three mixed-grass prairie locations. Residue bags containing surface litter were placed in the field in late fall (1 November) of 2014 and removed after the winter (1 April), spring (1 July), and summer + fall seasons (1 November) of 2015. The litter was analyzed for total C, total N, acid detergent fiber (ADF), and acid detergent lignin (ADL). Average winter temperatures ranged from −5^oC to −15^oC, while summer temperatures ranged from 10^oC to 35^oC. Litter decomposition was lowest during the winter (0.57−0.86 g [kg × day]⁻¹) and greatest during the summer + fall (2.12−2.69 g [kg × day]⁻¹). Over the entire season, 40.8−62% of the surface litter decomposed. Winter litter decomposition was positively correlated with air temperature (r = 0.62, P < 0.01) and snow depth (r = 0.61, P < 0.01), and negatively correlated with C/N ratio (r = −0.65, P < 0.01), ADF (r = −0.35, P < 0.05), and ADL (r = −0.25, P < 0.05) concentrations. These findings indicate that winter decomposition cannot be ignored and that winter surface litter decomposition increases with snow depth.     

三峡消落带几种一年生优势草本枯落物分解及养分释放特征

基于三峡消落带夏汛期水淹特征,设置两种水分处理[无水淹对照组(CK)和水淹组(F)],采用分解袋法,对三峡库区消落带3种具有典型代表性的一年生草本植物狼杷草(Bidens tripartita)、稗(Echinochloa crus-galli)和水蓼(Polygonum hydropiper)地上枯落物干质量损失率和养分释放特征进行研究。结果表明:1)3种一年生草本枯落物的初始化学组成及其化学计量比差异显著(P<0.05),干质量损失率和元素释放率与初始N、P、K含量及N/P比值显著正相关,而与初始C、木质素含量以及木质素/N和C/N比值显著负相关;2)水淹加速了3种植物枯落物的干质量损失率和元素释放速率,对N、P、K释放率的影响最大;3)除对照组的P元素释放速率和水淹组的K元素释放率外,狼杷草和稗的干质量损失率和元素释放率均显著高于水蓼(P<0.05)。这表明狼杷草和稗对三峡库区的水体和底泥的营养状态具有较大影响,其枯落物的快速分解可能造成水体和底泥N、P含量升高,而水蓼对其的影响较小。因此,建议优先考虑对狼杷草和稗在夏汛期前适时回收、刈割,从而减少其对库区水体及底泥的养分释放。     

Semiarid Rangeland Is Resilient to Summer Fire and Postfire Grazing Utilization

Most wildfires occur during summer in the northern hemisphere, the area burned annually is increasing, and fire effects during this season are least understood. Understanding plant response to grazing following summer fire is required to reduce ecological and financial risks associated with wildfire. Forty 0.75-ha plots were assigned to summer fire then 0, 17, 34 or 50% biomass removal by grazing the following growing season, or no fire and no grazing. Root, litter, and aboveground biomass were measured before fire, immediately after grazing, and 1 yr after grazing with the experiment repeated during 2 yr to evaluate weather effects. Fire years were followed by the second driest and fifth wettest springs in 70 yr. Biomass was more responsive to weather than fire and grazing, with a 452% increase from a dry to wet year and 31% reduction from a wet to average spring. Fire reduced litter 53% and had no first-year effect on productivity for any biomass component. Grazing after fire reduced postgrazing grass biomass along the prescribed utilization gradient. Fire and grazing had no effect on total aboveground productivity the year after grazing compared to nonburned, nongrazed sites (1 327 vs. 1 249 ± 65 kg · ha^-1). Fire and grazing increased grass productivity 16%, particularly for Pascopyrum smithii. The combined disturbances reduced forbs (51%), annual grasses (49%), and litter (46%). Results indicate grazing with up to 50% biomass removal the first growing season after summer fire was not detrimental to productivity of semiarid rangeland plant communities. Livestock exclusion the year after summer fire did not increase productivity or shift species composition compared to grazed sites. Reduction of previous years' standing dead material was the only indication that fire may temporarily reduce forage availability. The consistent responses among dry, wet, and near-average years suggest plant response is species-specific rather than climatically controlled.     

围栏封育对高寒草原植被−土壤养分含量的影响

围栏封育是青藏高原高寒退化草地最常见的恢复措施,开展围栏内外土壤理化性质与植物养分变化及其相互关系研究是评估草地恢复效益的基础。本研究选取了西藏那曲市班戈县围封时间分别为6、11和12年的3块样地,分析了围封对土壤理化特征和植物养分的影响,并探究了植物?土壤之间的内在联系。结果显示:1)围栏封育显著提高了高寒草原地上生物量和植被盖度(P<0.05),但是显著降低了Shannon-Wiener多样性指数和Margalef丰富度指数(P<0.05);2)不同深度土壤容重、全碳(total carbon,TC)、全氮(total nitrogen,TN)含量在围栏内外差异不显著(P>0.05);3)围栏封育对植物相对重要值的影响因物种而异,种间TC、TN、氮素利用效率(nitrogen-use efficiency,NUE)差异显著(P<0.05);4)植被与表层土壤主要养分含量之间的关联性仅存在于部分物种与部分营养元素之间。研究结果表明,围栏封育可以显著改变植被生产力和群落组成,但是土壤理化性质和植物养分利用却具有相对的稳定性。围封对青藏高原退化高寒草原生态系统的恢复效益,需要系统的研究和综合的评价。     

Precipitation and Nitrogen Deposition Alter Litter Decomposition Dynamics in Semiarid Temperate Steppe in Inner Mongolia,China

Plant litter decomposition is one of the most important links connecting plants to the soil through the carbon (C) and nitrogen (N) cycles. Climate change scenarios predict changes in precipitation and N deposition, and previous studies have demonstrated that increases in the availability of water and N affect the litter decomposition rate and nutrient release. We studied the effects of increased N deposition and precipitation on changes in the remaining mass and the C and N contents of shoot litter after decomposition in a typical steppe in Inner Mongolia, China. The treatments included the addition of NH₄NO₃ at rates equivalent to 0, 25, 50, and 100 kg ? N ? ha^? 2yr^? 1 with and without added water. The addition of water proved to be a more effective practice than amendment with NH₄NO₃ for improving the litter decomposition rate; the addition of water significantly increased the rate of litter decomposition (P < 0.001), whereas the addition of N alone had no apparent effect on litter mass loss. However, a repeated measures analysis of variance (ANOVA) showed that the interaction of water and N significantly affected both mass loss and litter N content (P < 0.05), and a linear relationship was identified between litter mass loss and litter decomposition time (P < 0.001). No correlation was found between litter mass loss and organic C content, but a significant positive correlation was found between residual litter mass and N content (P < 0.01). Although the study was conducted over a relatively short period, our results indicate that increased precipitation could potentially promote litter decomposition, whereas increased N input has little effect. The effects of time on litter mass loss and residual C and N concentrations indicate the need for long-term trials that measure the complete process of litter decomposition and the peaks of C and N release.     

Prescribed Fire,Soil, and Plants: Burn Effects and Interactions in the Central Great Basin

Pinyon and juniper expansion into sagebrush ecosystems results in decreased cover and biomass of perennial grasses and forbs. We examine the effectiveness of spring prescribed fire on restoration of sagebrush ecosystems by documenting burn effects on soil nutrients, herbaceous aboveground biomass, and tissue nutrient concentrations. This study was conducted in a central Nevada woodland and included control and burn treatment plots sampled before and after a prescribed fire. Six native understory plant species (Crepis acuminata, Eriogonum umbellatum, Eriogonum elatum, Poa secunda secunda, Festuca idahoensis, and Lupinus argenteus) important for native sagebrush obligate foragers were chosen to represent the understory plant community. L. argenteus is also important for system nutrient cycling and nitrogen fixation. Plants were collected from three microsites (under tree canopy, under shrub canopy, and interspace) common in transitional woodlands during peak growth the summer before a spring prescribed burn and each of two summers following the burn. Soils were collected from corresponding locations at two depth intervals (0–8 and 8–52 cm) to determine the relationships between soil and plant nutrients following fire. Microsite affected soil nutrients but did not influence plant tissue concentrations with the exception of F. idahoensis. Burning resulted in increases in soil surface NH+4, NO₃⁻, inorganic N, Ca²⁺, Mn²⁺, and Zn²⁺. Increases in NO₃⁻, inorganic N, and Zn²⁺ were also observed in deeper horizons. Burning did not affect aboveground plant biomass or nutrient concentrations in the first year with the exception of F. idahoensis, which had increased tissue P. By the second year, all species had statistically significant responses to burning. The most common response was for increased aboveground plant weight and tissue N concentrations. Plant response to burning appeared to be related to the burn treatment and the soil variables surface K⁺, NO₃⁻, and inorganic N.     

氮、磷添加对贝加尔针茅草原凋落物分解的影响

凋落物分解是草原生态系统养分循环和能量流动的主要途径。养分是温性草原生产力主要限制因素之一。本文主要研究了养分添加对草原凋落物分解的影响,为草原生态系统养分管理和天然草地的恢复建设提供理论依据。试验以内蒙古贝加尔针茅草原为研究对象,使用网袋分解法,实验材料为贝加尔针茅(Sb)、羊草(Lc)以及冷蒿(Af)3种植物的根、茎、叶组织,设置4个处理:CK、N、P和NP(CK:不施任何肥料;N:硝酸铵;P:重过磷酸钙;NP:硝酸铵和重过磷酸钙)。结果表明,1) N、P、NP添加下,3种植物的残留率显著低于无养分添加,氮磷添加对凋落物分解有显著的促进作用。2)分解过程中,叶和根凋落物N元素前期释放,后期有轻微的富集现象,而茎凋落物呈现富集-释放-富集的模式。P元素和C元素基本呈现持续释放的模式。3)磷元素的残留率在N、P以及NP处理条件下均呈下降的趋势,与植物器官残留率的变化趋势相同。4)羊草各器官凋落物分解95%所用的时间为2.95~3.96年,贝加尔针茅为3.32~3.77年,冷蒿为2.64~4.89年。5)不同植物以及植物的不同器官凋落物分解速率对氮、磷添加的响应不同。     

Plant Species Composition and Forage Production 14 Yr After Biosolids Application and Grazing Exclusion

This paper examines the effects of a single surface application of biosolids (at 20 dry Mg ha^{? 1}) on plant species composition, forage quality and quantity, and C stocks after 14 yr of rest in rangelands of the Central Interior of British Columbia. More than two times the aboveground biomass of grasses and the percent cover of plant litter were found in the biosolids treatment relative to the control, along with reductions in bare soil and microbiotic crust cover. Significantly greater plant uptake of all macronutrients (C, N, P, K, S, Ca, and Mg); most micronutrients (B, Cu, Mn, Mo, and Zn); and Al occurred in the biosolids treatment. P and Cu were the only two nutrients to be more concentrated in the biosolids-treated forage relative to the control forage, while N, Mg, and protein were more concentrated in the control forage. No significant difference in forage digestibility was found between biosolids and control treatments. Bluebunch wheatgrass, the late-seral native grass species, had significantly increased cover and aboveground biomass in the biosolids treatment relative to the control; however, between 2006 and 2016, non-native Kentucky bluegrass had reached > 25% cover in the biosolids plots, perhaps restricting the full recovery of bluebunch wheatgrass. Our findings indicate that biosolids application to ungrazed rangeland can increase long-term forage production and reduce bare soil. However, at our study site biosolids application also led to a long-term shift in the plant community composition away from the late-seral (i.e., bluebunch wheatgrass) trajectory, and the effects of this shift on rangeland health and productivity require further investigation.     

Introduced and Invasive Species in Novel Rangeland Ecosystems: Friends or Foes?

Globally, new combinations of introduced and native plant and animal species have changed rangelands into novel ecosystems. Whereas many rangeland stakeholders (people who use or have an interest in rangelands) view intentional species introductions to improve forage and control erosion as beneficial, others focus on unintended costs, such as increased fire risk, loss of rangeland biodiversity, and threats to conservation efforts, specifically in nature reserves and parks. These conflicting views challenge all rangeland stakeholders, especially those making decisions on how best to manage novel ecosystems. To formulate a conceptual framework for decision making, we examined a wide range of novel ecosystems, created by intentional and unintentional introductions of nonnative species and land-use–facilitated spread of native ones. This framework simply divides decision making into two types: 1) straightforward–certain, and 2) complex–uncertain. We argue that management decisions to retain novel ecosystems are certain when goods and services provided by the system far outweigh the costs of restoration, for example in the case of intensively managed Cenchrus pastures. Decisions to return novel ecosystems to natural systems are also certain when the value of the system is low and restoration is easy and inexpensive as in the case of biocontrol of Opuntia infestations. In contrast, decisions whether to retain or restore novel ecosystems become complex and uncertain in cases where benefits are low and costs of control are high as, for example, in the case of stopping the expansion of Prosopis and Juniperus into semiarid rangelands. Decisions to retain or restore novel ecosystems are also complex and uncertain when, for example, nonnative Eucalyptus trees expand along natural streams, negatively affecting biodiversity, but also providing timber and honey. When decision making is complex and uncertain, we suggest that rangeland managers utilize cost–benefit analyses and hold stakeholder workshops to resolve conflicts.     

放牧对内蒙古不同类型草原植物和土壤总氮与稳定氮同位素的影响

为了探究过度放牧对不同草原生态系统氮(N)素的影响,选取内蒙古3种主要类型草原(草甸草原、典型草原和荒漠草原)为研究对象,分析了连续多年过度放牧对植物生物量与叶片、根系和土壤中总氮(TN)和稳定氮同位素(δ15N)值的影响.研究结果表明:过度放牧显著降低草原植被地上、地下生物量(P<0.05);无牧条件下,3种主要类型...     

Plant Community and Soil Environment Response to Summer Fire in the Northern Great Plains

Fire is an important process in many ecosystems, especially grasslands. However, documentation of plant community and soil environment responses to fire is limited for semiarid grasslands relative to that for mesic grasslands. Replicated summer fire research is lacking but necessary because summer is the natural fire season and the period of most wildfires in the western United States. We evaluated summer fire effects on soil temperature, soil moisture, aboveground biomass, root biomass, and functional group composition for 2 yr in semiarid C₃-dominated northern Great Plains. Following pre-treatment measures, four 0.75-ha sites were burned during August for comparison with nonburned sites, and the experiment was repeated the next year on adjacent sites to assess weather effects. Soils were about 0.5°C cooler on burned sites in the first experiment and similar in the second. Burned sites were consistently 1% drier than nonburned sites. Litter was reduced by fire but did not account for changes in soil moisture because differences occurred before the growing season. Current-year aboveground biomass and root biomass were similar between treatments, indicating productivity was resistant to summer fire. Perennial C₃ grasses increased in dominance because of positive biomass responses to fire for all but the bunchgrass, Hesperostipa comata, and a reduction of annual grasses. Perennial C₄ grasses were unaffected by summer fire. H. comata was resilient, with biomass on burned sites equaling nonburned sites the second growing season. Biomass was more responsive to precipitation than fire, and the fire-induced changes in species composition suggest exclusion of fire may be a greater disturbance than summer fire.     

羊草生物量和养分分配对养分和水分添加的响应

本研究通过3个盆栽控制试验（施氮肥试验,0、0.5、1、2、4、8、16、32、64和128 g·m-2 10个水平;施磷肥试验,梯度同施氮肥试验;控水试验,11.5、12.7、14.3、16.4、19.1、22.9、28.7、38.2、57.3和114.6 mL·cm-2 10个水平),探讨了养分和水分改变对羊草（Leymus chinensis）生物量和养分分配的影响。结果表明,随着氮肥和磷肥施用量的增加,羊草地上、地下生物量和养分库呈先增加后降低的趋势,根冠比呈降低的趋势;随着水分供给增加,羊草地上、地下生物量和养分库亦呈现出先增加后降低的趋势,根冠比无明显变化;生长旺期地上生物量和养分库高于地下部分,生长末期地下生物量和养分库高于地上部分。