Effects of grazing on C : N:P stoichiometry attenuate from soils to plants and insect herbivores in a semi-arid grassland

Understanding the processing of limiting nutrients among organisms is an important goal of community ecology. Less known is how human disturbances may alter the stoichiometric patterns among organisms from different trophic levels within communities. Here, we investigated how livestock grazing affects the C:N:P ecological stoichiometry of soils, plants (Leymus chinensis), and grasshoppers (Euchorthippus spp.) in a semi-arid grassland in northeastern China. We found that grazing significantly enhanced soil available N and leaf N content of the dominant L. chinensis grass by 15% and 20%, respectively. Grazing also reduced (soluble) C:N of L. chinensis leaves by 22%. However, grazing did not affect total C, N, or P contents nor their ratios in Euchorthippus grasshoppers. Our results reveal that the effects of grazing disturbances on elemental composition attenuated from lower to higher trophic levels. These findings support the theory that organisms from higher trophic levels have relatively stronger stoichiometric homeostasis compared to those from lower trophic levels. Moreover, grasshopper abundance dropped by 66% in the grazed areas, and they reduced the feeding time on their host L. chinensis grass by 43%, presumably to limit the intake of excess nitrogen from host plants. The energetic costs associated with the maintenance of elemental homeostasis likely reduced grasshopper individual performance and population abundance in the grazed areas. A comprehensive investigation of stoichiometric properties of organisms across trophic levels may enable a better understanding of the nature of species interactions, and facilitate predictions of the consequences of future environmental changes for a community organization.Peer reviewe

Cattle grazing mitigates the negative impacts of nitrogen addition on soil nematode communities

Livestock grazing and atmospheric nitrogen (N) deposition have been reported as important factors affecting soil communities. However, how different large herbivore grazing and N addition may interact to affect soil biota in grassland ecosystems is unclear. Nematodes are the most abundant metazoan in soil ecosystems, play critical roles in regulating carbon and nutrient dynamics, and are valuable bioindicators. We examined the independent and interactive effects of grazing regimes (no grazing; sheep grazing; cattle grazing; mixed grazing of sheep and cattle) and N addition (ambient N; N addition) on soil nematodes in a meadow steppe. We found that grazing and N addition interacted to influence total nematode abundance, trophic group abundance, generic richness, diversity and several nematode-based indices (maturity index, channel ratio, enrichment index). In cattle grazing treatment, N addition significantly increased total nematode abundance, and the abundance of bacterial feeders, plant feeders, and omnivore-predators, and generic richness. By contrast, in the sheep and mixed grazing treatments, N addition had a negative effect on the same variables. Moreover, N addition reduced nematode maturity, enrichment and structure indices, and enhanced nematode channel ratio, in most grazing treatments, except mixed grazing where N addition had no effect on these variables. Structural equation modeling (SEM) revealed that N addition indirectly reduced nematode abundance and richness through increased soil NO3−-N content, whereas the effects of grazing were associated with increased relative biomass of grasses. Our results suggested that the response of soil nematodes to N addition strongly depended on herbivore assemblages. Nitrogen addition enhanced soil nematode diversity and maintained a relatively complex and mature soil food web in the presence of cattle rather than sheep grazing. Furthermore, our study highlighted that under N deposition, cattle grazing could benefit the soil nematode community

Livestock grazing impacts on plateau pika (Ochotona curzoniae) vary by species identity

Livestock and small burrowing mammals are potential ecosystem engineers and major drivers of ecosystem processes. Small mammals, often considered competitors with livestock, have been reported to increase in recent years and have been identified for their negative impacts on ecosystem services such as plant production and soil carbon sequestration. However, these increases and impacts by small mammals have rarely been studied with respect to how they may be influenced by livestock grazing. Here we experimentally manipulated large enclosures on the alpine grassland of Qinghai-Tibetan plateau, each with different livestock assemblages: yak (Bos grunniens), Tibetan sheep (Ovis aries), mixed populations of yak and sheep, and a control plot containing no livestock to study the potential impacts of livestock on the dominant small mammalian herbivore in the region, the plateau pikas (Ochotona curzoniae). We found that during the period of peak vegetative growth all pika populations in livestock experimental plots increased compared with those in the control plots. Additionally, pika populations and active pika burrow density increased more in experimental plots containing yaks compared with the sheep-only experimental plots. These results provide evidence that livestock grazing can increase the risk of pika outbreaks, which has led to this species being classified as a pest and subject to widespread control measures. We suggest explicitly incorporating livestock species identity in addition to grazing intensity and duration into management considerations to ensure maintenance of native biodiversity and long-term sustainability on the alpine grasslands of the Qinghai-Tibetan plateau.</p

Food and habitat provisions jointly determine competitive and facilitative interactions among distantly related herbivores

Interactions between distantly related herbivores exert powerful influences on ecosystems, but most studies to date have only considered unidirectional effects. Few have simultaneously examined the mutual effects that vertebrate herbivores and insect herbivores have on one another. We conducted a set of manipulative experiments to evaluate the potential competition and facilitation between two pairs of distantly related herbivore taxa: an insect caterpillar (Gynaephora alpherakii) and two large vertebrate herbivores, yak (Bos grunniens) and Tibetan sheep (Ovis aries tibetica). We found that these large herbivores consistently increased the density of caterpillars likely by improving the habitat for caterpillars. The caterpillars, in turn, decreased yak but increased Tibetan sheep foraging time and intake bites by differently changing available food resources of the two large herbivores. Diet preferences of herbivores modified the habitat and food resources, thereby causing a diet‐mediated competition between yak and caterpillars, and facilitation between sheep and caterpillars. The vertebrate herbivores non‐feeding upon Lamiophlomis rotata and Gentiana straminea, the caterpillars preferred habitat, increased densities of the two plant species, thus favouring the caterpillars. In turn, the caterpillar preference for Kobresia pygmaea significantly reduced food resources for yak, while promoting food resources (multiple forbs) for sheep. Synthesis. Our study indicates that two different mechanisms (the changes in habitat and food availability) induced by herbivore foraging jointly determine competitive and facilitative interactions between distantly related herbivore species. We also suggest that examining the bi‐directional effects between herbivores offers a better understanding of competition and facilitation in terrestrial animal communities. </ol

Smooth bromegrass seed yield and yield component responses to seeding rates and row spacings in two climates

Successful grass seed production depends on identifying a suitable environment for the species and proper agronomic practices. Previous research on many species has addressed identifying appropriate agronomic practices for grass seed production, but these studies have not evaluated the effects of environment. By conducting the same experiments in Jiuquan, China (a desert climate) and Tongliao, China (a semiarid continental monsoon climate), the effects of environment, seeding rate, row spacing and their interactions were determined for smooth bromegrass (Bromus inermis Leyss) seed production. Three seeding rates (.3, .5, and .7 g m−1 pure live seed) and four row spacings (30, 50, 70, and 90 cm) were evaluated over three years. Jiuquan had comparable seed yield (SY) and greater thousand-seed weight (TSW) than Tongliao. Three-year average SY decreased with increased row spacings at both sites. Results suggest that in both climates, successful smooth bromegrass seed production was possible, but greater TSW is predicted for desert climates with good irrigation conditions than in semiarid continental monsoon climates due to greater sunshine duration (574 h compared with 527 h) and low relative humidity during seed development (48% vs. 66%). A seeding rate of .3 g m−1 and a row spacing of no wider than 30 cm appears to be adequate for smooth bromegrass seed production in these research locations and in similar ecological regions around the world

Data from: Grazer effects on soil carbon storage vary by herbivore assemblage in a semi-arid grassland

1. Accounting for 10-30% of global soil organic carbon, grassland soils potentially present a large reservoir for storing atmospheric CO2. Livestock grazing management can substantially affect grassland soil carbon (C) storage, but few controlled experiments have explored how herbivore assemblages (different herbivore species and combinations) affect soil C storage. 2. We examined effects of moderate grazing by different herbivore assemblages (no grazing; sheep grazing; cattle grazing; mixed grazing by sheep and cattle) on soil organic carbon storage in two types of grassland communities (high forbs/high diversity and low forbs/low diversity), within a semi-arid grassland with a five-year grazing history. 3. We found that herbivore assemblage generated varying effects on soil C storage and the effects were subject to grassland community types. In the low diversity community, none of three herbivore assemblages studied had obvious effects on soil C storage. In the high diversity community, however, sheep grazing significantly decreased soil C storage due to high selectivity for high quality forbs, and cattle grazing had no effects on soil C storage, while mixed grazing by sheep and cattle significantly increased soil C storage. Overall, soil C storage was highest in mixed-grazed grassland sites with high diversity. 4. Synthesis and applications. Our study suggests that explicitly incorporating grazer species and the combination of grazing livestock into grassland grazing management may help mitigate greenhouse gas emissions. Caution should be exercised when using grazer species with high food selectivity when grazing management is also aimed at climate mitigation, especially in grasslands with abundant high quality forbs and high plant diversity, as sheep grazing may reduce soil carbon (C) storage. Moreover, mixed grazing, including multiple herbivore species, may contribute to a reduction in foraging selectivity for a plant community by means of complementary foraging. It could therefore be considered as an optimal grazing management strategy to maintain and improve soil C storage

Herbivore phenology can predict response to changes in plant quality by livestock grazing

Livestock grazing can have a strong impact on herbivore abundance, distribution and community. However, not all species of herbivores respond the same way to livestock grazing, and we still have a poor understanding of the underlying mechanisms driving these differential responses. Here, we investigate the effect of light intensity cattle grazing on the abundance of two grasshoppers (Euchorthippus cheui and E. unicolor) that co-occur in the same grasslands and feed on the same food plant (the dominant grass Leymus chinensis). The two grasshopper species differ in phenology so that their peak abundances are separated into early- and late-growing seasons. We used an exclosure experiment to monitor grasshopper abundance and food quality in the field under grazed and ungrazed conditions, and performed feeding trials to examine grasshopper preference for grazed or ungrazed food plants in the laboratory. We found that the nitrogen content of L. chinensis leaves continuously declined in the ungrazed areas, but was significantly enhanced by cattle grazing over the growing season. Cattle grazing facilitated the early-season grasshopper E. cheui, whereas it suppressed the late-season grasshopper E. unicolor. Moreover, feeding trials showed that E. cheui preferred L. chinensis from grazed plots, while E. unicolor preferred the leaves from ungrazed plots. We conclude that livestock grazing has opposite effects on the two grasshopper species, and that these effects may be driven by grazing-induced changes in plant nutrient content and the unique nutritional niches of the grasshoppers. These results suggest that insects that belong to the same guild can have opposite nutrient requirements, related to their distinct phenologies, and that this can ultimately affect their response to cattle grazing. Our results show that phenology may link insect physiological needs to local resource availabilities, and should be given more attention in future work on interactions between large herbivores and insects

Genome-Wide Identification, Phylogeny, and Expression Analysis of ARF Genes Involved in Vegetative Organs Development in Switchgrass

Auxin response factors (ARFs) have been reported to play vital roles during plant growth and development. In order to reveal specific functions related to vegetative organs in grasses, an in-depth study of the ARF gene family was carried out in switchgrass (Panicum virgatum L.), a warm-season C4 perennial grass that is mostly used as bioenergy and animal feedstock. A total of 47 putative ARF genes (PvARFs) were identified in the switchgrass genome (2n = 4x = 36), 42 of which were anchored to the seven pairs of chromosomes and found to be unevenly distributed. Sixteen PvARFs were predicted to be potential targets of small RNAs (microRNA160 and 167). Phylogenetically speaking, PvARFs were divided into seven distinct subgroups based on the phylogeny, exon/intron arrangement, and conserved motif distribution. Moreover, 15 pairs of PvARFs have different temporal-spatial expression profiles in vegetative organs (2nd, 3rd, and 4th internode and leaves), which implies that different PvARFs have specific functions in switchgrass growth and development. In addition, at least 14 pairs of PvARFs respond to naphthylacetic acid (NAA) treatment, which might be helpful for us to study on auxin response in switchgrass. The comprehensive analysis, described here, will facilitate the future functional analysis of ARF genes in grasses