Grazing intensity significantly affects belowground carbon and nitrogen cycling in grassland ecosystems: a meta‐analysis

Livestock grazing activities potentially alter ecosystem carbon (C) and nitrogen (N) cycles in grassland ecosystems. Despite the fact that numerous individual studies and a few meta‐analyses had been conducted, how grazing, especially its intensity, affects belowground C and N cycling in grasslands remains unclear. In this study, we performed a comprehensive meta‐analysis of 115 published studies to examine the responses of 19 variables associated with belowground C and N cycling to livestock grazing in global grasslands. Our results showed that, on average, grazing significantly decreased belowground C and N pools in grassland ecosystems, with the largest decreases in microbial biomass C and N (21.62% and 24.40%, respectively). In contrast, belowground fluxes, including soil respiration, soil net N mineralization and soil N nitrification increased by 4.25%, 34.67% and 25.87%, respectively, in grazed grasslands compared to ungrazed ones. More importantly, grazing intensity significantly affected the magnitude (even direction) of changes in the majority of the assessed belowground C and N pools and fluxes, and C : N ratio as well as soil moisture. Specifically,light grazing contributed to soil C and N sequestration whereas moderate and heavy grazing significantly increased C and N losses. In addition, soil depth, livestock type and climatic conditions influenced the responses of selected variables to livestock grazing to some degree. Our findings highlight the importance of the effects of grazing intensity on belowground C and N cycling, which may need to be incorporated into regional and global models for predicting effects of human disturbance on global grasslands and assessing the climate‐biosphere feedbacks.     

The importance of grasslands for animal production and other functions: a review on management and methodological progress in the tropics

The global importance of grasslands is indicated by their extent; they comprise some 26% of total land area and 80% of agriculturally productive land. The majority of grasslands are located in tropical developing countries where they are particularly important to the livelihoods of some one billion poor peoples. Grasslands clearly provide the feed base for grazing livestock and thus numerous high-quality foods, but such livestock also provide products such as fertilizer, transport, traction, fibre and leather. In addition, grasslands provide important services and roles including as water catchments, biodiversity reserves, for cultural and recreational needs, and potentially a carbon sink to alleviate greenhouse gas emissions. Inevitably, such functions may conflict with management for production of livestock products. Much of the increasing global demand for meat and milk, particularly from developing countries, will have to be supplied from grassland ecosystems, and this will provide difficult challenges. Increased production of meat and milk generally requires increased intake of metabolizable energy, and thus increased voluntary intake and/or digestibility of diets selected by grazing animals. These will require more widespread and effective application of improved management. Strategies to improve productivity include fertilizer application, grazing management, greater use of crop by-products, legumes and supplements and manipulation of stocking rate and herbage allowance. However, it is often difficult to predict the efficiency and cost-effectiveness of such strategies, particularly in tropical developing country production systems. Evaluation and on-going adjustment of grazing systems require appropriate and reliable assessment criteria, but these are often lacking. A number of emerging technologies may contribute to timely low-cost acquisition of quantitative information to better understand the soil-pasture-animal interactions and animal management in grassland systems. Development of remote imaging of vegetation, global positioning technology, improved diet markers, near IR spectroscopy and modelling provide improved tools for knowledge-based decisions on the productivity constraints of grazing animals. Individual electronic identification of animals offers opportunities for precision management on an individual animal basis for improved productivity. Improved outcomes in the form of livestock products, services and/or other outcomes from grasslands should be possible, but clearly a diversity of solutions are needed for the vast range of environments and social circumstances of global grasslands.     

Dung beetle diversity and functions suggest no major impacts of cattle grazing in the Brazilian Pantanal wetlands

1. Dung beetles perform relevant ecological functions in pastures, such as dung removal and parasite control. Livestock farming is the main economic activity in the Brazilian Pantanal. However, the impact of cattle grazing on the Pantanal's native dung beetle community, and functions performed by them, is still unknown. 2. This study evaluated the effects of cattle activity on dung beetle community attributes (richness, abundance, biomass, composition, and functional group) as well as their ecological functions (dung removal and soil bioturbation) in the Pantanal. In January/February 2016, dung beetles were sampled and their ecological functions measured in 16 sites of native grasslands in Aquidauana, Mato Grosso do Sul, Brazil, 10 areas regularly grazed by cattle and six control ungrazed areas (> 20 years of abandonment). 3. In all, 1169 individuals from 30 species of dung beetles were collected. Although abundance, species richness, and biomass did not differ between grasslands with and without cattle activity, species composition and functional groups differed among systems. Large roller beetles were absent from non‐cattle grasslands, and the abundance, richness, and biomass of medium roller beetles was higher in those systems. 4. Despite causing changes in species/functional group composition, the results of this study show that a density compensation of functional groups in cattle‐grazed natural grasslands seems to have conserved the ecological functions (dung removal and soil bioturbation), with no significant differences between systems. 5. Therefore, these results provide evidence that cattle breeding in natural grasslands of the Brazilian Pantanal can integrate livestock production with the conservation of the dung beetle community and its ecological functions.     

Potential long-term impacts of livestock introduction on carbon and nitrogen cycling in grasslands of Southern South America

Empirical evidence based on grazing exclusion at the scale of years to decades shows that grazing modifies carbon (C) and nitrogen (N) cycling. However, long‐term effects at the scale of centuries are less known, yet highly relevant to understand local and global impacts of grazing. Additionally, most studies have focused on the isolated response of C and N, with little understanding of their interactions. Using CENTURY, a process‐based biogeochemical model, we analyzed the impacts of 370 years of livestock grazing (i.e. long term, from early European colonization to present) in 11 sites across the Río de la Plata grasslands and compared them with those resulting from two decades of grazing (i.e. mid‐term, typical exclosure experiment). In the long term, livestock grazing primarily altered the N cycle through faster N returns to the soil via urine and dung, which were offset by uninterrupted N outputs by volatilization and leaching. As a result, soil organic N decreased by ?880 kg ha^?1 or ?19%. Higher N outputs (mainly as NH₃) opened the N cycle, potentially decreasing N₂O and NO_x emissions and increasing N depositions over the region. These greater outputs of N constrained C accumulation in soils, reducing soil organic C by ?21 200 kg ha^?1 (?22%, a reduction of ?1.5 Pg of C for the whole region) and net primary production by ?2192 kg ha^?1 yr^?1 (?24%). Mid‐term simulations showed that the effects of livestock introduction in a decadal time scale were substantially different both in magnitude and direction from long‐term responses. Long‐term results were not substantially affected when atmospheric CO₂ content, species composition and fire regime were changed within plausible ranges, but highlighted fire‐grazing interactions as a major constraint of long‐term C and N dynamics in these grasslands.     

Grazing simplifies soil micro‐food webs and decouples their relationships with ecosystem functions in grasslands

Livestock grazing often alters aboveground and belowground communities of grasslands and their mediated carbon (C) and nitrogen (N) cycling processes at the local scale. Yet, few have examined whether grazing‐induced changes in soil food webs and their ecosystem functions can be extrapolated to a regional scale. We investigated how large herbivore grazing affects soil micro‐food webs (microbes and nematodes) and ecosystem functions (soil C and N mineralization), using paired grazed and ungrazed plots at 10 locations across the Mongolian Plateau. Our results showed that grazing not only affected plant variables (e.g., biomass and C and N concentrations), but also altered soil substrates (e.g., C and N contents) and soil environment (e.g., soil pH and bulk density). Grazing had strong bottom‐up effects on soil micro‐food webs, leading to more pronounced decreases at higher trophic levels (nematodes) than at lower trophic levels (microbes). Structural equation modeling showed that changes in plant biomass and soil environment dominated grazing effects on microbes, while nematodes were mainly influenced by changes in plant biomass and soil C and N contents; the grazing effects, however, differed greatly among functional groups in the soil micro‐food webs. Grazing reduced soil C and N mineralization rates via changes in plant biomass, soil C and N contents, and soil environment across grasslands on the Mongolian Plateau. Spearman's rank correlation analysis also showed that grazing reduced the correlations between functional groups in soil micro‐food webs and then weakened the correlation between soil micro‐food webs and soil C and N mineralization. These results suggest that changes in soil micro‐food webs resulting from livestock grazing are poor predictors of soil C and N processes at regional scale, and that the relationships between soil food webs and ecosystem functions depend on spatial scales and land‐use changes.     

Predicting the Effects of Woody Encroachment on Mammal Communities,Grazing Biomass and Fire Frequency in African Savannas

With grasslands and savannas covering 20% of the world’s land surface, accounting for 30–35% of worldwide Net Primary Productivity and supporting hundreds of millions of people, predicting changes in tree/grass systems is priority. Inappropriate land management and rising atmospheric CO₂ levels result in increased woody cover in savannas. Although woody encroachment occurs world-wide, Africa’s tourism and livestock grazing industries may be particularly vulnerable. Forecasts of responses of African wildlife and available grazing biomass to increases in woody cover are thus urgently needed. These predictions are hard to make due to non-linear responses and poorly understood feedback mechanisms between woody cover and other ecological responders, problems further amplified by the lack of long-term and large-scale datasets. We propose that a space-for-time analysis along an existing woody cover gradient overcomes some of these forecasting problems. Here we show, using an existing woody cover gradient (0–65%) across the Kruger National Park, South Africa, that increased woody cover is associated with (i) changed herbivore assemblage composition, (ii) reduced grass biomass, and (iii) reduced fire frequency. Furthermore, although increased woody cover is associated with reduced livestock production, we found indigenous herbivore biomass (excluding elephants) remains unchanged between 20–65% woody cover. This is due to a significant reorganization in the herbivore assemblage composition, mostly as a result of meso-grazers being substituted by browsers at increasing woody cover. Our results suggest that woody encroachment will have cascading consequences for Africa’s grazing systems, fire regimes and iconic wildlife. These effects will pose challenges and require adaptation of livelihoods and industries dependent on conditions currently prevailing.     

Temporary grazing exclusion promotes rapid recovery of species richness and productivity in a long‐term overgrazed Campos grassland

Moderate grazing intensity is considered the basic requirement to enhance ecosystem function in grasslands. Yet, deterioration by overgrazing is common in many biomes, including Campos grasslands in South America. Understanding how grazing management can lead to recovery of ecosystem function is essential to design and implement effective strategies for sustainable use of this resource. In a long‐term field experiment carried out in Southern Brazil, we studied the effects of temporal grazing exclusions (spring or fall) at moderate and severe livestock grazing intensities (maintained by adjusting contrasting forage allowances) on the species richness, botanical composition, forage mass, sward height, and photosynthetic active radiation intercepted. The experiment was arranged in a completely randomized design with three replications of grazing exclusions, applied simultaneously at moderate and severe grazing intensities. Moderate grazing intensity showed a bimodal structure of shorter and taller canopies, and high species richness. Severe grazing created a shorter and homogeneous sward structure characterized by less standing biomass and species loss. In response to grazing exclusions, sward height, standing biomass, and light interception recovered almost to the levels of moderate grazing. Further, within 2 years grass species richness increased and botanical composition changed toward grasses with erect habit prevailing in moderate grazing intensity. Our study confirms that (1) moderate grazing intensities allow the coexistence of high number of species and (2) spring grazing exclusions of long‐term overgrazed grasslands can lead to a quick start to recover the grass species richness, primary productivity, and species composition like that prevailing in well‐managed grasslands.     

Recovery of plant species composition and ecosystem function after cessation of grazing in a Mediterranean grassland

Short- and long-term changes in species composition, plant biomass production, and litter decomposition after cessation of grazing were examined in a Mediterranean grassland with high dominance of annual species and strong seasonality in biomass production. Short-term changes were assessed during three consecutive years in plots previously exposed to different grazing pressures and compared to plots in long-term (30–40 years) exclosures. Short-term cessation of grazing led in the short-term to an increase in relative biomass of annual crucifers and tall annual and perennial grasses, while biomass of annual legumes, annual thistles and short annual grasses decreased. Consequently, similarity increased between vegetation recently excluded from grazing and vegetation in long-term protected plots. Our research showed that in systems with high dominance of grasses and annual species, the rapid changes in plant species composition that occur after grazing cessation were associated with a fast recovery of the potential for biomass production to levels found in long-term protected plots, while litter decomposition rate did not change even after long-term cessation of grazing. Moreover, previous history of grazing did not affect plant litter decomposition, despite higher litter quality in grazed treatments. This study provides new insights about the processes involved in the diverse responses of ecosystem functions resulting from shifts in species composition associated with grazing cessation and land use change in Mediterranean grasslands.     

Effects of grazing on seasonal variation of aboveground biomass quality in calcareous grasslands

Low-intensive grazing is a widely used management tool to conserve the outstanding biodiversity of calcareous grasslands. As conservation management is cost-intensive and often hampered by limited financial resources, combining adequate management for biodiversity conservation with feasible livestock production may be relevant for both conservationists and land managers. However, profound knowledge of the effect of grazing in non-intensively used grasslands on seasonal variation in biomass quality is scarce. We analyzed the floristic composition, abiotic soil properties and the chemical composition of aboveground biomass in a grazed calcareous grassland in NW Germany. Sampling took place in monthly intervals during one growing season. To separate the impact of grazing and non-grazing on biomass quality, an exclosure experiment was performed. Floristic composition of the studied calcareous grasslands was mainly related to two gradients representing the trophic status and the long-term management intensity. Differences in abiotic site conditions were hardly reflected by nutrient concentrations in the biomass. Irrespectively of abiotic site conditions, the chemical composition of the biomass showed a clear seasonal trend. Nutrient concentrations strongly declined from May to July but increased again in August, probably because of favourable current-year weather conditions. Sheep grazing improved biomass quality indicating that grazing modifies the environment beneficially for the animals. We conclude that early spring and late summer grazing is an appropriate management scheme to balance requirements of both feasible livestock production and biodiversity conservation, which is promising for sustainable and long-term conservation management.     

The effects of livestock grazing on biodiversity are multi‐trophic: a meta‐analysis

Anthropogenic disturbance has generated a significant loss of biodiversity worldwide and grazing by domestic herbivores is a contributing disturbance. Although the effects of grazing on plants are commonly explored, here we address the potential multi‐trophic effects on animal biodiversity (e.g. herbivores, pollinators and predators). We conducted a meta‐analysis on 109 independent studies that tested the response of animals or plants to livestock grazing relative to livestock excluded. Across all animals, livestock exclusion increased abundance and diversity, but these effects were greatest for trophic levels directly dependent on plants, such as herbivores and pollinators. Detritivores were the only trophic level whose abundance decreased with livestock exclusion. We also found that the number of years since livestock was excluded influenced the community and that the effects of grazer exclusion on animal diversity were strongest in temperate climates. These findings synthesise the effects of livestock grazing beyond plants and demonstrate the indirect impacts of livestock grazing on multiple trophic levels in the animal community. We identified the potentially long‐term impacts that livestock grazing can have on lower trophic levels and consequences for biological conservation. We also highlight the potentially inevitable cost to global biodiversity from livestock grazing that must be balanced against socio‐economic benefits.     

Intensity-dependent effects of cattle and sheep grazing in sand grasslands — Does livestock type really matter?

Aims

By analysing cattle- and sheep-grazed sand grasslands, we tested the following hypotheses: (i) livestock type has a stronger effect on the vegetation characteristics than grazing intensity; (ii) sheep grazing results in lower biomass and species and functional diversity than cattle grazing, regardless of intensity; and (iii) increased grazing intensity causes a shift of the trait composition in grasslands.

Location

Sand grasslands in the Nyírség region, East Hungary.

Methods

We selected 26 sand grassland sites grazed by cattle or sheep and classified them into four intensity levels. Vegetation composition was surveyed in 2 m × 2 m plots. We harvested the above-ground biomass from 20 cm × 20 cm plots; then dried and sorted it to live biomass, litter, moss, and lichen. We compared Rao dissimilarity index, species richness, Shannon diversity, evenness, and the community-weighted means of nine vegetative and generative traits along a grazing intensity gradient. We calculated functional richness, evenness, and divergence for comparison.

Results

We found that some diversity metrics and community-weighted means of most studied traits were significantly affected by grazing intensity. Several characteristics were also affected by the interaction of grazing intensity and livestock type, but none of the studied characteristics was affected by livestock type in itself. Increasing Rao dissimilarity index peaking at the fourth grazing intensity level was detected, but for other multitrait indices, no such changes were proven, except for functional divergence, which was the lowest at the first intensity level. Graminoid, forb, and litter biomass were significantly affected by intensity, but none of the biomass fractions was affected by livestock type.

Conclusions

We suggest that for the management of sand grasslands, grazing intensity should be carefully adjusted, considering not only livestock units per hectare. For practical recommendations, well-defined, long-term experiments studying different livestock and habitat types along an intensity gradient would be essential.     

Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands

Soil carbon sequestration (enhanced sinks) is the mechanism responsible for most of the greenhouse gas (GHG) mitigation potential in the agriculture sector. Carbon sequestration in grasslands can be determined directly by measuring changes in soil organic carbon (SOC) stocks and indirectly by measuring the net balance of C fluxes. A literature search shows that grassland C sequestration reaches on average 5 ± 30 g C/m2 per year according to inventories of SOC stocks and -231 and 77 g C/m2 per year for drained organic and mineral soils, respectively, according to C flux balance. Off-site C sequestration occurs whenever more manure C is produced by than returned to a grassland plot. The sum of on- and off-site C sequestration reaches 129, 98 and 71 g C/m2 per year for grazed, cut and mixed European grasslands on mineral soils, respectively, however with high uncertainty. A range of management practices reduce C losses and increase C sequestration: (i) avoiding soil tillage and the conversion of grasslands to arable use, (ii) moderately intensifying nutrient-poor permanent grasslands, (iii) using light grazing instead of heavy grazing, (iv) increasing the duration of grass leys; (v) converting grass leys to grass-legume mixtures or to permanent grasslands. With nine European sites, direct emissions of N2O from soil and of CH4 from enteric fermentation at grazing, expressed in CO2 equivalents, compensated 10% and 34% of the on-site grassland C sequestration, respectively. Digestion inside the barn of the harvested herbage leads to further emissions of CH4 and N2O by the production systems, which were estimated at 130 g CO2 equivalents/m2 per year. The net balance of on- and off-site C sequestration, CH4 and N2O emissions reached 38 g CO2 equivalents/m2 per year, indicating a non-significant net sink activity. This net balance was, however, negative for intensively managed cut sites indicating a source to the atmosphere. In conclusion, this review confirms that grassland C sequestration has a strong potential to partly mitigate the GHG balance of ruminant production systems. However, as soil C sequestration is both reversible and vulnerable to disturbance, biodiversity loss and climate change, CH4 and N2O emissions from the livestock sector need to be reduced and current SOC stocks preserved.     

Livestock greenhouse gas emissions and mitigation potential in Europe

The livestock sector contributes considerably to global greenhouse gas emissions (GHG). Here, for the year 2007 we examined GHG emissions in the EU27 livestock sector and estimated GHG emissions from production and consumption of livestock products; including imports, exports and wastage. We also reviewed available mitigation options and estimated their potential. The focus of this review is on the beef and dairy sector since these contribute 60% of all livestock production emissions. Particular attention is paid to the role of land use and land use change (LULUC) and carbon sequestration in grasslands. GHG emissions of all livestock products amount to between 630 and 863 Mt CO₂e, or 12–17% of total EU27 GHG emissions in 2007. The highest emissions aside from production, originate from LULUC, followed by emissions from wasted food. The total GHG mitigation potential from the livestock sector in Europe is between 101 and 377 Mt CO₂e equivalent to between 12 and 61% of total EU27 livestock sector emissions in 2007. A reduction in food waste and consumption of livestock products linked with reduced production, are the most effective mitigation options, and if encouraged, would also deliver environmental and human health benefits. Production of beef and dairy on grassland, as opposed to intensive grain fed production, can be associated with a reduction in GHG emissions depending on actual LULUC emissions. This could be promoted on rough grazing land where appropriate.     

Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems

Abstract The Chihuahuan desert of New Mexico, USA, has changed in historical times from semiarid grassland to desert shrublands dominated by Larrea tridentata and Prosopis glandulosa. Similar displacement of perennial grasslands by shrubs typifies desertification in many regions. Such structural vegetation change could alter average values of net primary productivity, as well as spatial and temporal patterns of production. We investigated patterns of aboveground plant biomass and net primary production in five ecosystem types of the Jornada Basin Long‐Term Ecological Research (LTER) site. Comparisons of shrub‐dominated desertified systems and remnant grass‐dominated systems allowed us to test the prediction that shrublands are more heterogeneous spatially, but less variable over time, than grasslands. We measured aboveground plant biomass and aboveground net primary productivity (ANPP) by species, three times per year for 10 years, in 15 sites of five ecosystem types (three each in Larrea shrubland, Bouteloua eriopoda grassland, Prosopis dune systems, Flourensia cernua alluvial flats, and grass‐dominated dry lakes or playas). Spatial heterogeneity of biomass at the scale of our measurements was significantly greater in shrub‐dominated systems than in grass‐dominated vegetation. ANPP was homogeneous across space in grass‐dominated systems, and in most growing seasons was significantly more patchy in shrub vegetation. Substantial interannual variability in ANPP complicates comparison of mean values across ecosystem types, but grasslands tended to support higher ANPP values than did shrub‐dominated systems. There were significant interactions between ecosystem type and season. Grasslands demonstrated higher interannual variation than did shrub systems. Desertification has apparently altered the seasonality of productivity in these systems; grasslands were dominated by summer growth, while sites dominated by Larrea or Prosopis tended to have higher spring ANPP. Production was frequently uncorrelated across sites of an ecosystem type, suggesting that factors other than season, regional climate, or dominant vegetation may be significant determinants of actual NPP.     

Can switches in disturbance type improve habitat for grassland birds in semi-arid grasslands of South-Eastern Australia?

Grasslands are one of the most endangered and degraded ecosystems globally. Switches in disturbance type can restore grassland function and improve conservation outcomes for fauna, but land-use legacies can limit the capacity of biota to respond positively to shifts in disturbance type, making it difficult to predict ecological outcomes. In semi-arid grasslands of South-Eastern Australia, habitat for grassland birds is managed using livestock grazing, a practice that has continued for >150 years. It is unknown if outcomes for birds can be improved by a switch in disturbance type as alternatives to livestock grazing have not been explored. We compare the effects of status-quo livestock grazing with alternative biomass management tools (no management, ‘crash’ grazing, planned fire) on birds, vegetation structure, and food resources using a BACI design across 3 years (2017–2019). We found crash grazing and fire produced more open, shorter grassy swards, with less exotic grass cover, compared to status-quo grazing or disturbance exclusion. Preferred habitat structure for the critically endangered plains-wanderer (Pedionomus torquatus) was maximized by status-quo grazing. Grassland birds responded in opposing ways to a disturbance switch. Brown songlarks (Megalurus cruralis) and Horsfield's bushlarks (Mirafra javanica) responded positively to disturbance exclusion, while stubble quail (Coturnix pectoralis) responded negatively to crash grazing. Australasian pipits (Anthus novaeseelandiae) were more frequent in response to status-quo grazing. Our findings suggest that multiple disturbance types should be used if the aim is to promote the spectrum of vegetation structures and food sources required to support a diverse grassland bird community in semi-arid grasslands of Australia.     

The role of grasslands in food security and climate change

Background

Grasslands are a major part of the global ecosystem, covering 37 % of the earth''s terrestrial area. For a variety of reasons, mostly related to overgrazing and the resulting problems of soil erosion and weed encroachment, many of the world''s natural grasslands are in poor condition and showing signs of degradation. This review examines their contribution to global food supply and to combating climate change.

Scope

Grasslands make a significant contribution to food security through providing part of the feed requirements of ruminants used for meat and milk production. Globally, this is more important in food energy terms than pig meat and poultry meat. Grasslands are considered to have the potential to play a key role in greenhouse gas mitigation, particularly in terms of global carbon storage and further carbon sequestration. It is estimated that grazing land management and pasture improvement (e.g. through managing grazing intensity, improved productivity, etc) have a global technical mitigation potential of almost 1·5 Gt CO₂ equivalent in 2030, with additional mitigation possible from restoration of degraded lands. Milk and meat production from grassland systems in temperate regions has similar emissions of carbon dioxide per kilogram of product as mixed farming systems in temperate regions, and, if carbon sinks in grasslands are taken into account, grassland-based production systems can be as efficient as high-input systems from a greenhouse gas perspective.

Conclusions

Grasslands are important for global food supply, contributing to ruminant milk and meat production. Extra food will need to come from the world''s existing agricultural land base (including grasslands) as the total area of agricultural land has remained static since 1991. Ruminants are efficient converters of grass into humanly edible energy and protein and grassland-based food production can produce food with a comparable carbon footprint as mixed systems. Grasslands are a very important store of carbon, and they are continuing to sequester carbon with considerable potential to increase this further. Grassland adaptation to climate change will be variable, with possible increases or decreases in productivity and increases or decreases in soil carbon stores.     

Fire and grazing impacts on silica production and storage in grass dominated ecosystems

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54% greater biogenic silica totals (kg ha^?1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon.     

放牧对草地生态系统影响的研究进展

全球草地占据30%左右的陆地面积, 在全球气候变化、碳氮及养分循环、保持水土、调节畜牧业生产等方面具有重要的作用。目前草地的主要利用方式之一就是放牧, 不同的牲畜种类、放牧强度、年限、历史和制度等, 会影响草地植物群落、生物多样性及土壤微生物, 进而影响草地生态系统结构、功能和过程。该文围绕放牧对草地生态系统结构、功能和过程的影响, 1)回顾了20世纪50年代到现在各个历史阶段放牧对草地生态系统影响的研究; 2)利用文献计量分析的方法, 剖析了放牧对草地影响研究的热点内容、重要区域和关键词等; 3)阐明了放牧对草地植物生长、群落特征、碳氮及养分循环、生产力及土壤质量等的各方面影响的研究进展及国内相关研究的优势及存在的主要问题和不足; 4)基于上述分析, 从草地放牧精准管理、经典假说验证、放牧和全球变化研究相结合等方面, 提出未来研究的前沿方向和优先领域。该文在系统总结放牧对草地生态系统影响的研究进展、研究优势及存在问题的基础上, 提出未来的研究应与全球变化相结合, 为我国的草地放牧生态学研究、适应性管理和可持续利用等提供科学基础。     

Research progress on the effects of grazing on grassland ecosystem

全球草地占据30%左右的陆地面积, 在全球气候变化、碳氮及养分循环、保持水土、调节畜牧业生产等方面具有重要的作用。目前草地的主要利用方式之一就是放牧, 不同的牲畜种类、放牧强度、年限、历史和制度等, 会影响草地植物群落、生物多样性及土壤微生物, 进而影响草地生态系统结构、功能和过程。该文围绕放牧对草地生态系统结构、功能和过程的影响, 1)回顾了20世纪50年代到现在各个历史阶段放牧对草地生态系统影响的研究; 2)利用文献计量分析的方法, 剖析了放牧对草地影响研究的热点内容、重要区域和关键词等; 3)阐明了放牧对草地植物生长、群落特征、碳氮及养分循环、生产力及土壤质量等的各方面影响的研究进展及国内相关研究的优势及存在的主要问题和不足; 4)基于上述分析, 从草地放牧精准管理、经典假说验证、放牧和全球变化研究相结合等方面, 提出未来研究的前沿方向和优先领域。该文在系统总结放牧对草地生态系统影响的研究进展、研究优势及存在问题的基础上, 提出未来的研究应与全球变化相结合, 为我国的草地放牧生态学研究、适应性管理和可持续利用等提供科学基础。     

Response of Polygonum viviparum species and community level to long-term livestock grazing in alpine shrub meadow in Qinghai-Tibet Plateau

Grazing by domestic herbivores is generally recognized as a major ecological factor and an important evolutionary force in grasslands. Grazing has both extensive and profound effects on individual plants and communities. We investigated the response patterns of Polygonum viviparum species and the species diversity of an alpine shrub meadow in response to long-term livestock grazing by a field manipulative experiment controlling livestock numbers on the Qinghai-Tibet Plateau in China. Here, we hypothesize that within a range of grazing pressure, grazing can alter relative allocation to different plant parts without changing total biomass for some plant species if there is life history trade-offs between plant traits. The same type of communities exposed to different grazing pressures may only alter relative species' abundances or species composition and not vary species diversity because plant species differ in resistant capability to herbivory. The results show that plant height and biomass of different organs differed among grazing treatments but total biomass remained constant. Biomass allocation and absolute investments to both reproduction and growth decreased and to belowground storage increased with increased grazing pressure, indicating the increasing in storage function was attained at a cost of reducing reproduction of bulbils and represented an optimal allocation and an adaptive response of the species to long-term aboveground damage. Moreover, our results showed multiform response types for either species groups or single species along the gradient of grazing Intensity. Heavy grazing caused a 13.2％ increase in species richness. There was difference in species composition of about 18％--20％ among grazing treatment. Shannon-Wiener (H') diversity index and species evenness (E) index did not differ among grazing treatments. These results support our hypothesis.