Grazing weakens competitive interactions between active methanotrophs and nitrifiers modulating greenhouse-gas emissions in grassland soils

Grassland soils serve as a biological sink and source of the potent greenhouse gases (GHG) methane (CH4) and nitrous oxide (N2O). The underlying mechanisms responsible for those GHG emissions, specifically, the relationships between methane- and ammonia-oxidizing microorganisms in grazed grassland soils are still poorly understood. Here, we characterized the effects of grazing on in situ GHG emissions and elucidated the putative relations between the active microbes involving in methane oxidation and nitrification activity in grassland soils. Grazing significantly decreases CH4 emissions while it increases N2O emissions basing on 14-month in situ measurement. DNA-based stable isotope probing (SIP) incubation experiment shows that grazing decreases both methane oxidation and nitrification processes and decreases the diversity of active methanotrophs and nitrifiers, and subsequently weakens the putative competition between active methanotrophs and nitrifiers in grassland soils. These results constitute a major advance in our understanding of putative relationships between methane- and ammonia-oxidizing microorganisms and subsequent effects on nitrification and methane oxidation, which contribute to a better prediction and modeling of future balance of GHG emissions and active microbial communities in grazed grassland ecosystems.

Soil Organic Carbon Stocks and Its Driving Factors Under Different Land-Use Patterns in Semiarid Grasslands of the Loess Plateau, China

Fencing for grazing exclusion and grazing are common land-use methods in the semi-arid areas of the Loess Plateau in China, which have been widely found to change grassland soil organic carbon (SOC); however empirical studies that evaluated driving factors of soil carbon (C) stocks under the different land use are still weak. In this study, we investigated soil physicochemical and soil respiration (Rs) in the fenced and grazed grassland, to study the soil C stock variations and the main driving mechanism of soil C accumulation. The results showed that bulk density (BD), soil moisture content (SMC), and soil porosity (SP) had no significant difference between fenced and grazed grassland. Fencing increased the SOC, total nitrogen (TN), and C/N ratio, and significantly increased the aboveground biomass (AGB), belowground biomass (BGB), and the amount of soil large macro-aggregates in the topsoil layer (0-10 cm), and the soil stability was improved. Meanwhile, grazing increased soil temperature (ST) and Rs. The soil C stock in the topsoil layer (0-10 cm) of fenced grassland was significantly higher than that of grazed grassland. The soil C/N ratio, BD, and MWD explained large proportions of the variations in soil C stocks. Our results indicate that fencing can improve the stability of soil structure, and reduce Rs, then increase soil C stocks, which is an effective way to improve soil C stocks of grassland ecological in semi-arid areas of northwest China.

Unpalatable plants induce a species-specific associational effect on neighboring communities

In grazing conditions, unpalatable species may induce either associational defense or neighbor contrast susceptibility in neighboring communities. Using surveys from eight grasslands, we tested whether various unpalatable species have the same impacts on neighboring communities in response to grazing. The studied unpalatable species were: Phlomis cancellata (an unpalatable nonpoisonous plant), Euphorbia boissieriana, E.microsciadia (poisonous plants), and Seseli transcaucasicum (a highly poisonous plant). Our results showed that, in the ungrazed grasslands, communities containing P. cancellata had lower biodiversity than communities without it. In the moderately- and heavily grazed grasslands, P. cancellata induced associational defense in the neighboring communities. In heavily grazed grasslands, both Euphorbia species promoted neighbor contrast susceptibility in the neighboring communities. Similarly, S. transcaucasicum in a heavily grazed grassland, induced neighbor contrast susceptibility. Different responses of plant community vulnerability among the studied unpalatable plants might be due to herbivore different foraging decisions. Accordingly, grazers selectively choose from other patches when facing P. cancellata and other plant individuals when there is a poisonous plant in a patch. Our results suggested that grazing intensity may not substantially affect the foraging decisions of sheep and goats in response to unpalatable species. We recommend monitoring the abundance of poisonous species to maintain the sustainable use of grasslands.

Fire frequency influenced grazed grasslands' resistance and resilience to extreme drought

<p>Grasslands provide critical ecosystem functions and services globally, including forage production for livestock and other animals. As frequency and intensity of disturbances, including fire and drought, are increasing globally, grasslands and the services they provide are particularly vulnerable. In this changing environment, resistance, the capacity to withstand disturbance, and resilience, the capability to recover from disturbance, are important for the stability of grassland ecosystems during and after extreme climate events. Quantifying how grazed grassland’s resistance and resilience respond to these disturbances provides important information of stability of grassland function under forecast climate change.</p><p>In this study, we focus on fire experiments in grasslands located in the Kruger National Park in South Africa (tropical savanna grassland) and the Konza Prairie Biological Station in the US (mesic temperate grassland). Both sites experienced extreme drought (SPEI <-2) this past decade, in 2015 and 2012, respectively. Further, both sites have long-term fire frequency treatments (annually burned, burned every 3-4 years and unburned) that are grazed by large native herbivores (~14 species at Kruger and bison at Konza), which allows us to explore influences of fire frequency on grazed grassland’s resistance and resilience to extreme drought. Using Landsat remote sensing data, we generated 30 m x 30 m NDVI monthly time series for each fire frequency treatment and conducted repeated measures ANOVA to compare the vegetation productivity two years before, during, and two years after the extreme one-year drought events.</p><p>Although large reductions in productivity occurred during the extreme drought at both sites and across the grazed fire frequency treatments, full recovery of production was observed the following year, consistent with trends observed in ungrazed grasslands at the study sites. These results suggest that grazed grasslands show high resilience, but low resistance to extreme drought. However, the degree of resistance and resilience was influenced by fire frequency. At Konza, during and after extreme drought in 2012, unburned grassland showed the lowest resistance but higher resilience, while grassland burned every four years and annually had higher resistance but relatively lower resilience. The anomaly of NDVI at Kruger exhibited an opposite pattern. These differences in resistance and resilience of production to extreme drought across the fire frequency treatments are likely due to changes in species composition or ecosystem structure (i.e., increased density of woody species in the absence of fire). Ultimately, these results suggest that fire frequency plays an important role in grazed grassland ecosystems’ vulnerability to extreme drought.</p>

Only fungal migration runs through the biotopes of soil, phyllosphere and faeces

Background: Understanding the microbial linkages among the soil, plants and animals is crucial for maintaining the balance of thet ecosystem in grazed grasslands. However, previous study always focused on the biotopes of soil, phyllosphere and faeces respectively and little has been known about the microbial distribution and migration among these biotopes. Here, a systematic survey to investigate the alteration and overlap among the various microbiotas of biotopes and how the microbial cycle served for the ecosystem was conducted on the molecular ecological level.Results: Our findings revealed that the biotopes drived the distinct microbial community assemblages with various richness, β-diversity and composition. The substantial overlaps between soil and phyllosphere in fungi, bacteria and archaea indicated that soil played the role of the microbial source for phyllosphere. Nevertheless, Ascomycota were the only microorganisms definitely migrating among all the biotopes. After the long-term impact of faeces via grazing, the soil and phyllosphere microbiota were altered significantly.Conclusions: Biotopes driving leads the discrepancy of microbiota distribution among soil, phyllosphere and faeces. Soil could potentially perform as the microbial reservoir for phyllosphere. However, there was only fungal migration running through the ecosystem to link all the biotopes. These findings improved our understanding of microbial linkages among the biotopes in the grazed grassland ecosystem and better managing the soil, plants and animals for the pasture ecosystem service.