Successional dynamics of a semiarid grassland: effects of soil texture and disturbance size

We evaluated effects of soil texture and disturbance size on the successional dynamics of a semiarid grassland dominated by the perennial bunchgrass, Bouteloua gracilis (H.B.K.) Lag. ex Griffiths. A spatially-explicit gap dynamics simulation model was used to evaluate recovery patterns. The model simulates establishment, growth, and mortality of individual plants on an array of small plots through time at an annual time step. Each simulated disturbance consisted of a grid of plots of the same soil texture interconnected by processes associated with dispersal of B. gracilis seeds. Soil texture was incorporated into the model as effects on seed germination, seedling establishment, and subsequent growth of B. gracilis. Five soil texture classes and five disturbance sizes were simulated.Soil texture was more important to recovery of B. gracilis than either size of a disturbance or location of plots within a disturbance. Constraints on recruitment of seedlings had a greater effect on recovery than constraints associated with plant growth. Fastest recovery occurred on soils with the largest silt content, the variable that affects seedling establishment. Disturbances with slowest recovery were on soils with low silt contents, and either high or low water-holding capacity, the variable that affects plant growth. Biomass and recovery decreased as disturbance size increased, and as distance from a disturbed plot to the edge of the disturbance increased. In most cases, important interactions between soil texture and disturbance size on recovery were not found.     

Bare soil cover and arbuscular mycorrhizal community in the first montane forest restoration in Central Argentina

Soil erosion affects extensive areas worldwide and must be urgently reduced promoting plant cover and beneficial microorganisms associated with plants, including arbuscular mycorrhizal fungi (AMF). In mountain environments, plant cover is difficult to enhance due to harsh conditions during the dry season and steep slopes. Our objective was to evaluate the percentage of the soil surface covered by plants and the AMF community associated with trees 12.5 years after planting during forest restoration efforts in microsites at different levels of soil degradation. The study was performed in the first montane forest restoration initiative of Central Argentina, where one of the trials consisted of planting Polylepis australis saplings at microsites with different levels of soil degradation: high, intermediate, and low. After 12.5 years, percentage of bare soil cover was significantly reduced by 36 and 37% in the high and intermediate degradation microsites, respectively. Low degradation microsites were initially very low in bare soil and did not significantly change. Mycorrhizal colonization, hyphae, vesicles, arbuscules, AMF diversity, and community structure were similar among microsite types. Percentage of hyphal entry points was higher at microsites with low degradation, number of spores was higher in high and intermediate degradation, and species richness was higher in high degradation. Acaulospora and Glomus were the most abundant genera in all microsites. We conclude that even in the most degraded microsites around 2.8% of the bare soil is covered by vegetation each year and that the arbuscular mycorrhizal community is highly tolerant and adapted to soils with different disturbance types.     

青藏高原典型草地植被退化与土壤退化研究

采用野外样方调查和室内分析法,探讨了青藏高原不同退化程度高寒草原和高寒草甸植被群落结构、植物多样性、地上-地下生物量、根系分配及土壤理化特性差异。研究表明:(1)随着退化程度加剧,高寒草原禾草优势地位未改变,高寒草甸优势种莎草逐渐被杂类草取代。(2)随着退化程度加剧,高寒草原地上生物量显著降低(P<0.05),高寒草甸地上生物量先保持稳定再下降。高寒草甸地下生物量较高寒草原地下生物量对退化响应更敏感。(3)高寒草原退化过程中,莎草地上物生量变化不明显(P>0.05),禾草地上生物量贡献率由88.12%减少至53.54%,杂类草地上生物量贡献率由0.08%增加至42.81%;高寒草甸退化过程中,禾草和杂类草地上生物量先增加后减小,莎草地上生物量占比由69.15%减少至0.04%,杂类草地上生物量占比由12.56%增加至92.61%。(4)随着退化程度加剧,高寒草原根系向浅层迁移,高寒草甸根系向深层迁移。(5)退化对高寒草甸土壤含水量(θ)、土壤有机碳(SOC)、总氮(TN)及土壤容重(BD)影响均比高寒草原更强烈。本研究对青藏高原退化草地恢复治理具有重要的参考价值。     

雪被和土壤水分对典型半干旱草原土壤冻融过程中CO_2和N_2O排放的影响

土壤冻融期间的温室气体排放量会显著增加,并在全年总排放量中占有重要的份额。但目前开展的土壤冻融循环模拟实验大多是在土壤冻结之前调节土壤水分含量,而忽视了雪被在整个土壤冻融过程中的作用,因此导致室内模拟研究的结果与野外原位观测的结果差异较大。为探索开展室内模拟土壤冻融实验的优化方案,采用人工浇水和覆雪两种方式调节土壤水分含量,研究了雪被和土壤水分对内蒙古典型半干旱草原土壤冻融过程中CO2和N2O排放的影响。结果表明,浇水和覆雪两种处理对冻融循环过程中土壤CO2排放影响的差异不显著,CO2排放量在消融期都会明显增加并随着冻融循环次数的增加而逐渐减小。当土壤孔隙含水率达50%左右时,浇水处理中的N2O排放量在第1次土壤冻融循环中最高并随冻融循环次数增加而降低,但在覆雪处理中,N2O在第1次冻融循环中的排放较小,而在后两次冻融循环中的排放量更为显著。造成两种处理N2O排放规律出现显著不同的原因可能是土壤剖面水分动态变化过程和微生物性状等方面的差异。土壤冻融过程中CO2和N2O排放量随土壤含水量升高而增加,但N2O在土壤含水量较低时排放不明显,这表明可能只有当土壤含水量达到一定阈值时,冻融作用才会对N2O的排放产生显著影响。这些结果显示,雪被和土壤水分显著影响土壤冻融过程中的CO2和N2O排放,室内模拟土壤冻融实验应进一步优化。     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

植物、土壤及土壤管理对土壤微生物群落结构的影响

土壤微生物是土壤生态系统的重要组成部分,对土壤微生物群落结构多样性的研究是近年来土壤生态学研究的热点。本文综述了有关植物、土壤类型以及土壤管理措施对土壤微生物群落结构影响的最新研究结果,指出植物的作用因植物群落结构多样性、植物种类、同种植物不同的基因型,甚至同一植物不同根的区域而异;而土壤的作用与土壤质地和有机质含量等因素有关;植物和土壤类型在对土壤微生物群落结构影响上的作用存在互作关系。不同的土壤管理措施对土壤微生物群落结构影响较大,长期连作、大量的外援化学物质的应用降低了土壤微生物的多样性;而施用有机肥、免耕可以增加土壤微生物群落结构多样性,有利于维持土壤生态系统的功能。     

干旱半干旱草地生态系统与土壤水分关系研究进展

研究干旱半干旱草地生态系统与土壤水分关系和相互作用机理对于揭示草地生态系统稳定性及其水土关键要素的变化过程具有重要意义。从不同界面、不同尺度综述了草地生态系统对土壤水分的影响及草地生态系统的响应与适应机制,总结了草地生态系统与土壤水分关系模型研究的相关进展,并分析了气候变化对草地生态系统和土壤水分关系的影响。草地生态系统通过影响水文过程和生态过程来影响土壤水分,土壤水分在植物生长发育、形态、生理生态过程、种间关系、群落组成和结构以及草地生态系统功能等方面对草地生态系统产生影响;充分揭示草地生态系统-土壤水分相互作用机理是模型研究的关键;气候变化对草地生态系统植物与土壤水分关系具有重要影响。今后应加强以下研究:1)开展草地不同优势种和植物功能型与土壤水分关系的研究,找出能反映植物对土壤水分响应的性状指标,阈值响应点及适应机制;2)注重对不同时间和空间尺度上的转换和比较;3)加强个体、群体和生态系统尺度草地植物生长模型的研究及其与土壤-植被-大气水分传输模型的耦合;4)加强草地生态系统与土壤水分关系对气候变化响应的研究。     

半干旱区沙质退化草地造林对土壤质量的影响

采用野外调查与室内培养相结合的方法,研究了我国北方半干旱区科尔沁沙地退化草地营造樟子松人工林32年后0～10 cm表层土壤理化性状、土壤碳氮矿化量、土壤微生物量以及土壤酶活性等的变化. 结果表明32年生樟子松人工林土壤有机碳、全氮和全磷等养分含量分别下降了21%、42%和45%;5月和11月樟子松人工林土壤NH4 -N显著高于草地(P=0.001;P=0.019),而5、8和11月草地土壤NO3--N含量显著高于樟子松人工林(P＜0.001;P=0.048;P=0.031);5、8和11月樟子松人工林土壤有机碳日矿化释放的CO2-C量均大于草地,而二者土壤氮矿化率差异不显著(P＞0.05);5和8月樟子松人工林土壤微生物量碳含量与草地相比差异不显著,11月则显著高于草地;土壤养分和水分含量是影响土壤微生物量碳含量的重要因素;与草地相比,樟子松人工林土壤脲酶和蔗糖酶活性降低,而土壤过氧化氢酶活性升高. 上述结果说明半干旱区沙质退化草地营造樟子松人工林32年后土壤质量出现一定程度的下降;由于植被的改变,樟子松人工林土壤理化性状和生物学性状等表现出与草地不同的季节动态特征.造林作为我国北方半干旱区沙地退化生态系统的一种恢复手段具有一定的局限性.     

降水格局对北方温带草原土壤微食物网结构的影响

全球气候变化背景下, 降水格局发生改变, 呈现降水总量不变, 但降水强度增加、降水频率降低的趋势, 影响了地下生态系统的结构和功能。土壤微食物网作为地下生态系统的重要组成部分, 在驱动生态系统多功能性方面起着重要作用。降水格局的变化能够通过土壤微食物网的改变对生态系统产生影响。然而, 以往研究多关注于降水量的变化对微食物网的影响, 降水格局变化对其影响的研究较少。因此, 本研究在内蒙古温带草原开展连续8年的降水添加控制试验(控制降水总量不变, 降水频率及强度改变), 包括5个降水强度处理(2 mm、5 mm、10 mm、20 mm和40 mm), 通过磷脂脂肪酸法(PLFA)确定微生物含量, 高通量测序法(16S和ITS)确定微生物多样性及群落结构, 线虫形态学鉴定确定线虫群落组成及结构。结果表明在降水总量不变降水强度改变的背景下, 高降水强度(20 mm)促进了北方温带草原真菌含量的增长, 适度降水强度(10 mm)促进了微生物的多样性。而线虫的多度随着降水强度的增加而增大, 中高降水强度下线虫多样性最高。土壤微食物网的变化进一步影响了生态系统多功能性, 主要通过提高真菌生物量、食真菌线虫多度和线虫多样性, 从而提高了生态系统多功能性。     

Disturbance intensity and above- and belowground herbivory effects on long-term (14 y) recovery of a semiarid grassland

The importance of disturbance intensity and herbivory by cattle and white grubs, or the larvae of June beetles (including Phyllophaga fimbripes), to recovery of shortgrass steppe ecosystems in Colorado, U.S.A. were evaluated over a fourteen year time period. Disturbance intensity was defined by survival of the dominant grass species (Bouteloua gracilis) after an outbreak of root feeding activity by white grubs. Sixteen patches of vegetation consisting of four pairs of adjacent ungrazed-grazed by cattle locations with two replicates that were recently affected by white grubs were selected in 1977. Disturbance intensity was determined in 1977 by the area in each patch that contained live tillers of B. gracilis. Permanent plots were located both within and outside of each patch. Plant basal cover and density by species were estimated at time of peak aboveground biomass in six different years on each plot.Successional dynamics on patches was similar to areas affected by other types of disturbances, however, rate of recovery was faster for patches affected by grubs. Grazing by cattle was infrequently important to plant recovery, a result similar to effects of grazing on other aspects of shortgrass steppe ecosystems. Disturbance intensity was important to recovery of B. gracilis since tiller survival in 1977 was linearly related to cover in each year of sampling. For ungrazed patches, initial conditions were important to recovery of B. gracilis for as many as 14 years. For grazed patches, initial conditions decreased and grazing increased in importance through time. Changes in resource quality and a more uniform distribution of roots due to grazing likely resulted in more complete mortality of plants by grubs under grazed compared to ungrazed conditions.Persistence of shortgrass ecosystems in spite of disturbances with different intensities are determined at least in part by characteristics of disturbances interacting with the ability of plants to respond, and in part by the evolutionary history of the system. Although white grubs affect shortgrass communities infrequently, they have large and important effects on plant community structure through time, and represent an important class of disturbance defined by intensity.     

Disturbance is more important than seeding or grazing in determining soil microbial communities in a semiarid grassland

A primary goal of ecological restoration is often to return processes and functions to degraded ecosystems. Soil, while often ignored in restoration, supports diverse communities of organisms and is a fundamental actor in providing ecosystem processes and services. We investigated the impact of seeding and livestock grazing on plant communities, soil microorganisms, and soil fertility 3 years after the restoration of a disturbed pipeline corridor in southeastern Arizona. The initial soil disturbance and topsoil treatment, regardless of seeding or grazing, was the most influential factor in determining differences in both plant and microbial communities. Compared with the control, the disturbed and restored sites had greater plant species richness, greater total herbaceous plant cover, greater soil organic matter, higher pH, and differed in soil nutrients. Bacteria and fungi appeared to generally correlate with micro‐environment and soil physiochemical properties rather than specific plant species. The undisturbed control had a smaller proportion of bacterial functional groups associated with the breakdown of plant biomass (polysaccharide decomposition) and a smaller proportion of arbuscular mycorrhizal fungi (AMF) compared with disturbed and restored sites. The ability of the unseeded disturbed site to recover robust vegetation may be due in part to the high presence of AMF. These differences show selection for soil microorganisms that thrive in disturbed and restored sites and may contribute to increased plant productivity. Restoration of specific plant species or ecological processes and services would both benefit from better understanding of the impacts of disturbance on soil microorganisms and soil fertility.     

Effects of grazing on grassland soil carbon: a global review

Soils of grasslands represent a large potential reservoir for storing CO₂, but this potential likely depends on how grasslands are managed for large mammal grazing. Previous studies found both strong positive and negative grazing effects on soil organic carbon (SOC) but explanations for this variation are poorly developed. Expanding on previous reviews, we performed a multifactorial meta‐analysis of grazer effects on SOC density on 47 independent experimental contrasts from 17 studies. We explicitly tested hypotheses that grazer effects would shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, transition from dominant grass species with C₃ to C₄ photosynthesis, and decreasing grazing intensity, after controlling for study duration and sampling depth. The six variables of soil texture, precipitation, grass type, grazing intensity, study duration, and sampling depth explained 85% of a large variation (±150 g m^?2 yr^?1) in grazing effects, and the best model included significant interactions between precipitation and soil texture (P = 0.002), grass type, and grazing intensity (P = 0.012), and study duration and soil sampling depth (P = 0.020). Specifically, an increase in mean annual precipitation of 600 mm resulted in a 24% decrease in grazer effect size on finer textured soils, while on sandy soils the same increase in precipitation produced a 22% increase in grazer effect on SOC. Increasing grazing intensity increased SOC by 6–7% on C₄‐dominated and C₄–C₃ mixed grasslands, but decreased SOC by an average 18% in C₃‐dominated grasslands. We discovered these patterns despite a lack of studies in natural, wildlife‐dominated ecosystems, and tropical grasslands. Our results, which suggest a future focus on why C₃ vs. C₄‐dominated grasslands differ so strongly in their response of SOC to grazing, show that grazer effects on SOC are highly context‐specific and imply that grazers in different regions might be managed differently to help mitigate greenhouse gas emissions.     

Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N₂O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. −30%) influenced soil N₂O and carbon dioxide (CO₂) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N₂O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N₂O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N₂O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.     

放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应

为了揭示高寒小嵩草草甸群落在放牧扰动下,探讨土壤养分供给水平的变化对生态系统初级生产力和多样性影响,为高寒草地的退化演替机理研究提供依据,以野外样地调查和室内分析法研究了放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应.结果表明,放牧干扰不仅改变了高寒小蒿草草甸群落土壤根系和蕴育土壤根系的"载体"量及根土比例,改变了植物群落的结构和功能,而且使土壤的物理和化学特性发生了明显的改变.随着放牧强度的增加,蕴育土壤根系的基质量逐渐减少,根土比特别是0～10 cm土层的根土比例增加;"载体"量减少导致大部分地下根系由于营养供给水平的降低而死亡,归还土壤中有机质的数量逐渐减少,加之地上部分持续利用,土壤养分也在不断消耗,土壤基质量的减少和土壤资源持续供给能力的下降,草地发生逆向演替(退化),表现在:物种数减少、多样性下降、能量的分配转向地下等;土壤性状上的某些改变(土壤容重、土壤湿度等),也会引起植被组成、物种多样性变化;放牧主要通过影响土壤环境及其养分含量来改变草地群落生物量(地上、地下);土壤表面的适度干扰和原有植物的适度破坏为新成员提供了小生境,从而允许新的植物侵入群落,并提高了植物的丰富度.但是,在受到强度干扰时,草地植物群落的主要物种的优势地位发生明显的替代变化.     

黄土高原典型小流域草地群落功能性状对土壤水分的响应

植物功能性状可以响应生境的变化并决定生态系统的功能,探究植物功能性状间的关系及其随土壤有效水分梯度的变化规律,对认识不同水分条件下植被在群落水平碳水代谢关系和维持水分平衡的生理生态学机制具有重要意义。以甘肃定西典型半干旱黄土小流域草地群落为研究对象,采用排序分析和回归拟合方法,分析了30个代表性草地样地中7个植物功能性状加权均值对土壤有效水分的响应以及响应性状间的相关关系。结果显示:(1)7个性状中,除叶宽与土壤有效水分无明显相关外,叶长、株高、叶面积、比叶面积、叶厚和叶干物质含量均与土壤有效水分显著性相关,可识别为草地在群落水平对土壤水分的响应性状。(2)草地群落通过降低株高,减小叶长、叶面积和比叶面积,增加叶厚和叶干物质含量以适应土壤有效水分减少;其中叶干物质含量的解释度最大,是土壤水分的最优响应性状。(3)除叶厚与叶长无显著相关外,其余功能性状均存在显著相关,说明草地群落的功能性状在土壤水分梯度上已基本形成了一个相互权衡或协同变化的功能性状组合。     

内蒙古武川县农田退耕还草对大型土壤动物群落的影响

用手拣法采集大型土壤动物,应用个体数、类群数、多样性指数等指标分析了内蒙古武川县农田退耕还草管理下大型土壤动物群落的变化.共捕获大型土壤动物49类1479只,隶属于3纲、7目、41科;农田退耕后大型土壤动物的类群数、个体数和生物量均有增加趋势;Margalef丰富度指数(D_Ma)和密度-类群指数(DG)显示,农田退耕还草后大型土壤动物群落多样性明显增加,但由于退耕时间短,Shannon-Wiener多样性指数(H′)和Pielou均匀度指数(E)并不比农田高;不同动物类群对退耕还草的响应不同.退耕后采用紫花苜蓿或紫花苜蓿+蒿属植物的人工种植管理方式较为合适.