放牧对内蒙古典型草原大针茅凋落物中土壤动物组成及其分解功能的影响

放牧是影响草地生态系统中土壤动物组成和凋落物分解的重要因素.2010—2012年,选择内蒙古锡林郭勒盟白音锡勒牧场境内的禁牧草地、放牧草地和沙地为研究样地,以凋落物袋法研究了大针茅凋落物分解过程中主要理化特性以及其中土壤动物群落的变化特征.共采集到土壤动物67056头52类,隶属于5门8纲,其中螨类23科,昆虫19科.大针茅凋落物的初始有机质含量为92.5%,分解780 d后分别降至40.0%(禁牧草地)和41.3%(放牧草地),差异不显著;凋落物残留率分别降低至50.0%(禁牧草地)和23.0%(放牧草地),差异显著.放牧影响下,大针茅残留凋落物中土壤动物多度显著降低.将大针茅凋落物置入沙地环境,有机质分解速率无显著变化,但凋落物残留率显著降低,螨类群落组成发生显著变化.在内蒙古典型草原环境下,放牧显著改变了植物凋落物中的土壤动物群落组成和多度,但凋落物中有机质分解速率未发生显著变化;半干旱地区土壤动物的凋落物分解功能较为微弱.     

Grazing-induced changes in plant composition affect litter quality and nutrient cycling in flooding Pampa grasslands

Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G−) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G− grasses. While no consistent differences between G+ and G− grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species’ “responses” to grazing may only partially overlap with functional groups based on species “effects” on C and nutrient cycling.     

The effect of reindeer grazing on decomposition, mineralization and soil biota in a dry oligotrophic Scots pine forest

Reindeer grazing in the Fennoscandian area has a considerable influence on the ground vegetation, and this is likely in turn to have important consequences for the soil biota and decomposition processes. The effects of reindeer grazing on soil biota, decomposition and mineralization processes, and ecosystem properties in a lichen‐dominated forest in Finnish Lapland were studied inside and outside a large long term fenced reindeer exclosure area. Decomposition rates of Vaccinium myrtillus leaves in litter bags were retarded in the grazed area relative to the ungrazed area, as well as in subplots from which lichens had been artificially removed to simulate grazing. The effect of reindeer grazing on soil respiration and microbial C was positive in the lichen and litter layers of the soil profile, but retarded in the humus layer. There was no effect of grazing on gross N mineralization and microbial biomass N in the humus and upper mineral soil layer, but net N mineralization was increased by grazing. In these layers soil respiration was reduced by grazing, indicating that reindeer effects reduce the ratio of C to N mineralized by soil microorganisms. Grazing stimulated populations of all trophic groupings of nematodes in the lichen layer and microbe feeding nematodes in the litter layer, indicating that grazing by reindeer has multitrophic effects on the decomposer food‐web. Grazing decreased lichen and dwarf shrub biomasses and increased the mass of litter present in the litter layer on an areal basis, but did not significantly alter total C storage per unit area in the humus and mineral soil layers. The N concentration of lichens was increased by grazing, but the N concentrations of both living and dead Pinus sylvestris needles and Empetrum hermaphroditum leaves were not affected.
There was some evidence for each of three mechanisms which could account for the grazing effects that we observed in our study. Firstly, reindeer may have changed the composition and quality of litter input by affecting plant species composition and through addition of N from urine and faeces, resulting in a lack of available C relative to N for decomposer organisms. Secondly, the organic matter in the soil may be older in the grazed area, because of reduction of recent production of lichen litter relative to the ungrazed area. The organic matter in the grazed area may have been in a different phase of decomposition from that in the exclosure. Thirdly, the soil microclimate is likely to be affected by reindeer grazing through physical removal of lichen cover on the ground, and this can have a significant influence on soil microbial processes. This is supported by the strong observed effects of experimental removal of lichens on decomposer processes. The impact of reindeer grazing on soil processes may be a result of complex interactions between different mechanisms, and this could help to explain why the below‐ground effects of reindeer grazing have different consequences to those which have been observed in recent investigations on other grazing systems.     

Annual nutrient budget for an alpine grassland in the Garhwal Himalaya

Abstract. N, P and K dynamics were investigated in grazed and ungrazed alpine forb and grassy meadows in the Garhwal Himalaya. The growth forms examined were dwarf shrubs, forbs and graminoides. N, P and K contents were determined for various plant components and soil. The contribution of plant parts to the total vegetation capital of N, P and K was 20–33% (live shoot), 6–8% (dead shoot), 2–3% (litter) and 56–71% (root) in ungrazed plots, and 16–27, 6–7, 1–2, and 64–76% respectively in grazed plots. Grazing removed between 41–69% of total uptake of nutrients from the grassland. In protected areas, however, 65 to 81% of all nutrients were retained by the vegetation. This retention of nutrients is due to translocation to roots and rhizomes and is considered beneficial during grazing as it aids resprouting of the vegetation.     

Soil nutrient patchiness and plant genotypes interact on the production potential and decomposition of root and shoot litter: evidence from short-term laboratory experiments with Triticum aestivum

Aims

The purpose of this study was to test the hypotheses that soil nutrient patchiness can differentially benefit the decomposition of root and shoot litters and that this facilitation depends on plant genotypes.

Methods

We grew 15 cultivars (i.e. genotypes) of winter wheat (Triticum aestivum L.) under uniform and patchy soil nutrients, and contrasted their biomass and the subsequent mass, carbon (C) and nitrogen (N) dynamics of their root and shoot litters.

Results

Under equal amounts of nutrients, patchy distribution increased root biomass and had no effects on shoot biomass and C:N ratios of roots and shoots. Roots and shoots decomposed more rapidly in patchy nutrients than in uniform nutrients, and reductions in root and shoot C:N ratios with decomposition were greater in patchy nutrients than uniform nutrients. Soil nutrient patchiness facilitated shoot decomposition more than root decomposition. The changes in C:N ratios with decomposition were correlated with initial C:N ratios of litter, regardless of roots or shoots. Litter potential yield, quality and decomposition were also affected by T. aestivum cultivars and their interactions with nutrient patchiness.

Conclusions

Soil nutrient patchiness can enhance C and N cycling and this effect depends strongly on genotypes of T. aestivum. Soil nutrient heterogeneity in plant communities also can enhance diversity in litter decomposition and associated biochemical and biological dynamics in the soil.     

放牧对青藏高原东部两种典型高寒草地类型凋落物分解的影响

为认识放牧对青藏高原东部中生性的高寒草甸草地和半湿生的沼泽草地凋落物分解的影响,在这两种草地上分别设置了围栏和放牧样地,研究了其各自的混合凋落物样品和4个优势物种(发草Deschampsiacaespitos、鹅绒委陵菜Potentilla anserine、木里苔草Carexmuliensis、藏嵩草Kobresiatibetica)凋落物的分解和养分释放动态,这4个优势物种也大致代表了当地沼泽草地生态系统在放牧和气候变暖驱动下逆行演替不同阶段的优势物种类群。结果表明,各优势物种凋落物的分解速率有显著差异;放牧在总体上促进了凋落物的分解,但不同物种的响应有所不同;放牧对凋落物C的释放影响不显著或有抑制作用,但对N、P的释放具有一定促进作用。对各优势物种凋落物分解和养分释放模式的分析表明,群落逆行演替过程中,凋落物分解和C释放加速,可能促进沼泽湿地退化的正反馈效应。草甸草地的退化标志物种鹅绒委陵菜具有较高的凋落物质量和分解速度,反映了中生条件下植物应对牲畜啃食采用"逃避"而非"抵抗"策略的趋向。     

放牧对呼伦贝尔草地植物和土壤生态化学计量学特征的影响

放牧通过畜体采食、践踏和排泄物归还影响草地群落组成、植物形态和土壤养分,植物通过改变养分利用策略适应环境变化。通过分析呼伦贝尔草原放牧和围封样地中的群落植物和土壤的碳氮磷养分及化学计量比,探讨放牧对生态系统化学计量学特征和养分循环速率的影响机制。结果如下:(1)群落尺度上,放牧和围封草地植物叶片C、N和P的含量没有显著差异;但是在种群尺度上,放牧草地植物叶片N含量显著高于围封草地;(2)放牧草地土壤全C、全N、有机C、速效P含量,低于围封草地,硝态N含量高于围封草地;土壤全P和铵态N指标没有显著差异;(3)放牧草地植物C∶N比显著低于围封草地,植物残体分解速率较快,提高了生态系统养分循环速率。     

Reindeer influence on ecosystem processes in the tundra

Reindeer have been recorded to increase nutrient cycling rate and primary production in studies from fences almost 40 years old that separate areas with different grazing regimes in northern Fennoscandia. To further understand the mechanism behind the effects of herbivores on primary production, we measured the size of the major C and N pools, soil temperature, litter decomposition rate and N mineralization rate in lightly, moderately and heavily grazed areas along two of these fences.
Plant N found in new biomass, indicative of plant N assimilation, was significantly higher in moderately and heavily grazed areas than in lightly grazed areas, which corresponded to a decreased amount of N in old plant parts. The amount of N found in plant litter or organic soil layer did not differ between the grazing treatments. Together with soil N concentrations and litter decomposition rates, soil temperatures were significantly higher in moderately and heavily grazed areas.
We conclude that the changes in soil temperature are important for the litter decomposition rate and thus on the nutrient availability for plant uptake. However, the changes in plant community composition appear to be more important for the altered N pools and thus the enhanced primary production. The results provide some support for the keystone herbivore hypothesis, which states that intensive grazing can promote a transition from moss-rich tundra heath to productive grasslands. Grazing altered N fluxes and pools, but the total N pools were similar in all grazing treatments. Our study thus indicates that grazing can increase the primary production through enhancing the soil nutrient cycling rate, even in a long term perspective on an ecological timescale.     

Leaf litterfall, fine-root production, and decomposition in shrublands with different canopy structure induced by grazing in the Patagonian Monte, Argentina

Selective sheep grazing in the Patagonian Monte induces the reduction of total and perennial grass cover, species replacement within life forms, and the increase in dominance of long-lived evergreen woody plants with slow growth rates and high concentration of secondary compounds in leaves. We hypothesized that these changes in the canopy structure induced by sheep grazing will affect the mass, chemistry and decomposability of leaf litter and fine roots. We selected two sites in the Patagonian Monte, representative of ungrazed and grazed vegetation states. At each site, we assessed canopy structure (total cover and absolute and relative grass and shrub cover), monthly leaf litterfall, and fine-root biomass and production in the upper soil (15 cm). We also estimated the rates of mass, C, soluble phenolics, lignin and N decay in litterbags containing both leaf litter and fine roots of each site under field conditions during two consecutive years. The ungrazed site exhibited higher total plant cover, absolute and relative grass- and shrub-cover than the grazed one. Leaf litterfall was lower at the grazed site than at the ungrazed site. Fine-root production did not vary between sites. Leaf litter and fine root tissues had higher concentration of secondary compounds at the grazed than at the ungrazed site. However, fine roots showed lower mass and C decay than leaf litter, attributable to the predominant secondary compound (lignin and soluble phenolics, respectively). Leaf litter decomposed slower but released more N during decay at the ungrazed than at the grazed site, probably due to its low concentration of secondary compounds. We concluded that changes in canopy structure induced by grazing disturbance such as those explored in our study could reduce leaf litterfall mass and increase the concentration of secondary compounds of both leaf litter and fine roots leading to slow N release to soil during decay.     

Ecosystem changes associated with grazing in subhumid South American grasslands

Question: What are the changes in vegetation structure, soil attributes and mesofauna associated with grazing in mesic grasslands? Location: Southern Campos of the Río de la Plata grasslands, in south‐central Uruguay. Methods: We surveyed seven continuously grazed and ungrazed paired plots. Plant and litter cover were recorded on three 5‐m interception lines placed parallel to the fence in each plot. We extracted soil fauna from a 10 cm deep composite sample and analysed the oribatids. Soil attributes included bulk density, water content, organic carbon (in particulate and mineral associated organic matter) and nitrogen content and root biomass at different depths. Changes in floristic, Plant Functional Types and mesofauna composition were analysed by Non‐metric Multidimensional Scaling. Results: Species number was lower in ungrazed than in grazed plots. Of 105 species in grazed plots only three were exotics. Shrub and litter cover were significantly higher inside the exclosures, while the cover of Cyperaceae‐Juncaceae was lower. Grazing treatments differed significantly in plant and oribatid species composition. Grazing exclusion significantly reduced soil bulk density and increased soil water content. Carbon content in particulate organic matter was lower in the upper soil of ungrazed sites, but deeper in the profile, grazing exclosures had 8% more carbon in the mineral associated organic matter. Conclusions Our results generally agree with previous studies but deviate from the results of previous analyses in (1) the increase of shrub cover in ungrazed sites; (2) the redistribution of the soil organic carbon in the profile and (3) the low invasibility of the prairies regardless of grazing regime.     

半干旱草地植物枯落物碳、氮和磷元素释放对放牧和封育管理的响应

在半干旱草地,关于放牧和封育管理对草地植物枯落物分解及其与土壤动物互作关系的研究一直是该区域生态系统物质循环与生态恢复过程研究的重要科学问题。以放牧和封育样地中赖草(Leymus secalinus)、牛枝子(Lespedeza potaninii)及其混合枯落物为研究对象。于2017年9月、2018年5月和9月、2019年5月和9月,采用2种不同孔径(30目和250目)网袋量化中小型土壤动物的作用,调查了宁夏半干旱草地枯落物碳、氮和磷元素分解与土壤动物群落分布及其对放牧和封育样地的响应规律。结果显示：(1)封育样地中,枯落物碳元素最终残留率均表现为3种枯落物处理间无显著差异;但250目网孔中枯落物氮、磷元素最终残留率和30目网孔中氮元素最终残留率均表现为牛枝子显著高于赖草和混合物,而后两者间无显著差异;仅30目网孔中磷元素最终残留率表现为牛枝子和混合物显著高于赖草,而前两者间无显著差异。放牧样地中,250目网孔的碳、磷元素最终残留率和30目网孔中的氮、磷元素最终残留率均表现为牛枝子显著高于混合物,而赖草居中;氮元素最终残留率表现为牛枝子显著高于赖草和混合物,后两者间无显著差异。(2)...     

Grazing enhances belowground carbon allocation,microbial biomass,and soil carbon in a subtropical grassland

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant‐derived SOC following microbial decomposition. Here, we used in situ isotope pulse‐chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long‐term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4‐dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.     

Grazing-induced effects on soil properties modify plant competitive interactions in semi-natural mountain grasslands

Plant-soil feedbacks are widely recognized as playing a significant role in structuring plant communities through their effects on plant-plant interactions. However, the question of whether plant-soil feedbacks can be indirectly driven by other ecological agents, such as large herbivores, which are known to strongly modify plant community structure and soil properties, remains poorly explored. We tested in a glasshouse experiment how changes in soil properties resulting from long-term sheep grazing affect competitive interactions (intra- and inter-specific) of two graminoid species: Nardus stricta, which is typically abundant under high sheep grazing pressure in British mountain grasslands; and Eriophorum vaginatum, whose abundance is typically diminished under grazing. Both species were grown in monocultures and mixtures at different densities in soils taken from adjacent grazed and ungrazed mountain grassland in the Yorkshire Dales, northern England. Nardus stricta performed better (shoot and root biomass) when grown in grazing-conditioned soil, independent of whether or not it grew under inter-specific competition. Eriophorum vaginatum also grew better when planted in soil from the grazed site, but this occurred only when it did not experience inter-specific competition with N. stricta. This indicates that plant-soil feedback for E. vaginatum is dependent on the presence of an inter-specific competitor. A yield density model showed that indirect effects of grazing increased the intensity of intra-specific competition in both species in comparison with ungrazed-conditioned soil. However, indirect effects of grazing on the intensity of inter-specific competition were species-specific favouring N. stricta. We explain these asymmetric grazing-induced effects on competition on the basis of traits of the superior competitor and grazing effects on soil nutrients. Finally, we discuss the relevance of our findings for plant community dynamics in grazed, semi-natural grasslands.     

Intraspecific changes in plant morphology,associated with grazing,and effects on litter quality,carbon and nutrient dynamics during decomposition

Abstract: Continuous biomass removal by grazing usually changes the resource allocation pattern of plants. These changes often increase resistance to tissue removal and produce individuals with different morphometric traits, such as root to shoot or blade to sheath ratios. Shifts in morphometric traits, in turn, may alter nutrient cycling through changes in the average quality of litter that decomposes in soil. Previous work has shown that Paspalum dilatatum, a native grass from the Pampas grasslands, which inhabits a vast area and supports a wide range of grazing conditions, increases its blade to sheath ratio under continuous grazing with respect to ungrazed conditions. Here, we explored the consequences of these changes apparently associated with grazing regime on litter quality and nutrient dynamics during litter breakdown in soil. We separately analysed litter quality of blades and sheaths of P. dilatatum and determined under controlled conditions their decomposition and nutrient release kinetics over a maximum period of 1 year. We also studied the mineral nitrogen contents in soil amended with each litter type. Blade quality was significantly higher than sheath quality, nitrogen concentrations of blades and sheaths were approximately 1% and 0.6%, respectively, and lignin to nitrogen ratios were approximately 5 and 11 for blades and sheaths, respectively. Phosphorus concentration, however, was similar in both litter types. Blades decomposed 10% faster than sheaths, released 20% more nitrogen and released 15% more phosphorus than sheaths during the last half of the incubation period. During the first 3 months, the soil nitrogen content of litter‐amended incubations indicated immobilization with respect to non‐amended control; however, later blade incubations mineralized nitrogen, whereas sheath incubations continued immobilizing it. Results revealed that grazing potentially accelerates nutrient cycling during decomposition by increasing the blade to sheath ratio of P. dilatatum individuals, and suggest that this may be an important mechanism underlying grazing impact on nutrient cycling.     

Grazing‐induced changes in plant–soil feedback alter plant biomass allocation

Large vertebrate herbivores, as well as plant–soil feedback interactions are important drivers of plant performance, plant community composition and vegetation dynamics in terrestrial ecosystems. However, it is poorly understood whether and how large vertebrate herbivores and plant–soil feedback effects interact. Here, we study the response of grassland plant species to grazing‐induced legacy effects in the soil and we explore whether these plant responses can help us to understand long‐term vegetation dynamics in the field. In a greenhouse experiment we tested the response of four grassland plant species, Agrostis capillaris, Festuca rubra, Holcus lanatus and Rumex acetosa, to field‐conditioned soils from grazed and ungrazed grassland. We relate these responses to long‐term vegetation data from a grassland exclosure experiment in the field. In the greenhouse experiment, we found that total biomass production and biomass allocation to roots was higher in soils from grazed than from ungrazed plots. There were only few relationships between plant production in the greenhouse and the abundance of conspecifics in the field. Spatiotemporal patterns in plant community composition were more stable in grazed than ungrazed grassland plots, but were not related to plant–soil feedbacks effects and biomass allocation patterns. We conclude that grazing‐induced soil legacy effects mainly influenced plant biomass allocation patterns, but could not explain altered vegetation dynamics in grazed grasslands. Consequently, the direct effects of grazing on plant community composition (e.g. through modifying light competition or differences in grazing tolerance) appear to overrule indirect effects through changes in plant–soil feedback.     

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

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

Effects of external and internal nutrient supplies on decomposition of wild rice, Zizania palustris

We examined effects of external supplies of nitrogen (N) and phosphorus (P) from the environment and internal supplies of N and P from within litter tissue on wild rice shoot and root litter decomposition and N and P dynamics. To investigate the effects of external supplies, wild rice shoot and root litterbags were decayed in mesocosms in the field over 115 days with either added N or P or a control in ambient conditions. To investigate the effects of the internal nutrient supply, wild rice plants were grown with added N, P, both N and P, or no supplemental nutrient, to produce enriched litters, which were then decayed for 168 days under controlled temperature in the laboratory. Both external and internal N and P supplies affected shoot litter decay more than decay of root litter. Increased external P supply significantly increased the rate of wild rice shoot decay and P mineralization but adding N had no effect on decay rates through time. Neither adding N nor P influenced root decay. Enrichment of P internally in the litter through fertilization increased the concentration of P (0.16%) and water-soluble compounds (28.7% WS) in shoot litter compared to control shoot litter (0.11% P, 19.8% WS), which likely caused the significant increase in shoot decay rates, particularly in the labile pool. In contrast, N enrichment not only increased plant growth but also increased lignin concentrations (7.5%) compared to control shoot litter (2.7% lignin) for added structural support. This significantly inhibited decay and nearly doubled the amount of mass remaining after 168 days (42.1% OM) when compared to control shoots (22.4% OM). Increased lignin likely overrides a concomitant increase in nitrogen concentration in shoot litter and appears to control wild rice decomposition. Lignin and phosphorus appear to play a key role in driving wild rice decay through the effects on litter quality.     

Contrasting effects of rabbit exclusion on nutrient availability and primary production in grasslands at different time scales

Herbivores influence nutrient cycling and primary production in terrestrial plant communities. However, both empirical and theoretical studies have indicated that the mechanisms by which herbivores influence nutrient availability, and thus their net effects on primary production, might differ between time scales. For a grassland in southeast England, we show that the effects of rabbits on primary production change over time in a set of grazed plots paired with exclosures ranging from 0 to 14 years in age. Herbivore exclusion decreased net aboveground primary production (APP) in the short term, but increased APP in the long term. APP was closely correlated with N mineralization rates in both grazed and ungrazed treatments, and accumulation of litter within the grazing exclosures led to higher N mineralization rates in exclosures in the long run. Rabbit grazing did not influence litter quality. The low contrast in palatability between species and the presence of grazing-tolerant plants might prevent rabbits from favoring unpalatable plant species that decompose slowly, in contrast to results from other ecosystems. Our results indicate that it is essential to understand the effects on N cycling in order to predict the effect of rabbit grazing on APP. Rabbits might decrease N mineralization and APP in the long term by increasing losses of N from grasslands.     

Effect of grazing on carbon stocks and assimilate partitioning in a Tibetan montane pasture revealed by 13CO2 pulse labeling

Since the late 1950s, governmental rangeland policies have changed the grazing management on the Tibetan Plateau (TP). Increasing grazing pressure and, since the 1980s, the privatization and fencing of pastures near villages has led to land degradation, whereas remote pastures have recovered from stronger overgrazing. To clarify the effect of moderate grazing on the carbon (C) cycle of the TP, we investigated differences in below‐ground C stocks and C allocation using in situ ¹³CO₂ pulse labeling of (i) a montane Kobresia winter pasture of yaks, with moderate grazing regime and (ii) a 7‐year‐old grazing exclosure plot, both in 3440 m asl. Twenty‐seven days after the labeling, ¹³C incorporated into shoots did not differ between the grazed (43% of recovered ¹³C) and ungrazed (38%) plots. In the grazed plots, however, less C was lost by shoot respiration (17% vs. 42%), and more was translocated below‐ground (40% vs. 20%). Within the below‐ground pools, <2% of ¹³C was incorporated into living root tissue of both land use types. In the grazed plots about twice the amount of ¹³C remained in soil (18%) and was mineralized to CO₂ (20%) as compared to the ungrazed plots (soil 10%; CO₂ 9%). Despite the higher contribution of root‐derived C to CO₂ efflux, total CO₂ efflux did not differ between the two land use types. C stocks in the soil layers 0–5 and 5–15 cm under grazed grassland were significantly larger than in the ungrazed grassland. However, C stocks below 15 cm were not affected after 7 years without grazing. We conclude that the larger below‐ground C allocation of plants, the larger amount of recently assimilated C remaining in the soil, and less soil organic matter‐derived CO₂ efflux create a positive effect of moderate grazing on soil C input and C sequestration.     

放牧对全球草地生态系统碳氮磷化学计量特征影响的Meta分析

为明确全球尺度下放牧管理措施对草地生态系统碳(C)、氮(N)、磷(P)化学计量特征的影响,提高草地生态系统管理水平,本研究选取国内外83篇中英文文献进行Meta分析,并通过亚组分析探讨了放牧家畜组合(羊单牧、牛单牧和牛羊混牧)和放牧强度(轻度、中度、重度)对草地生态系统叶片、凋落物、根系,以及土壤C、N、P化学计量特征的影响。结果表明: 放牧会显著降低叶片和凋落物C含量、C/N、C/P,增加N、P含量及N/P;显著降低根系和土壤C、N含量,C/P和N/P,增加P含量和C/N。叶片、凋落物化学计量特征变化对牛、羊单独放牧响应更为明显,而根系、土壤化学计量特征变化则对混牧响应更为明显,重度放牧会对草地生态系统化学计量特征产生更大的影响。放牧会降低土壤N含量,增加P含量,表明放牧对草地N、P含量的影响路径不同。进一步研究N、P含量变化对放牧活动不平衡响应机制,将放牧方式、强度的影响纳入草地生态系统预测、管理模型,能够有效提高草地生态系统管理水平。