Yanbin Hao

Responses of soil respiration and its components to drought stress

PurposeClimate change is likely to increase both intensity and frequency of drought stress. The responses of soil respiration (Rs) and its components (root respiration, Rr; mycorrhizal respiration, Rm; and heterotrophic respiration, Rh) to drought stress can be different. This work aims to review the recent and current literature about the variations in Rs during the period of drought stress, to explore potential coupling processes and mechanisms between Rs and driving factors in the context of global climate change.Results and discussionThe sensitivity of soil respiration and its components to drought stress depended on the ecosystems and seasonality. Drought stress depressed Rs in mesic and xeric ecosystems, while it stimulated Rs in hydric ecosystems. The reductions in supply and availability of substrate decreased both auto- and heterotrophic respirations, leading to the temporal decoupling of root and mycorrhizal respiration from canopy photosynthesis as well as C allocation. Drought stress also reduced the diffusion of soluble C substrate, and activities of extracellular enzymes, consequently, limited microbial activity and reduced soil organic matter decomposition. Drought stress altered Q10 values and broke the coupling between temperature and soil respiration. Under drought stress conditions, Rm is generally less sensitive to temperature than Rr and Rh. Elevated CO2 concentration alleviated the negative effect of drought stress on soil respiration, principally due to the promotion of plant C assimilation subsequently, which increased substrate supply for respiration in both roots and soil microorganisms. Additionally, rewetting stimulated soil respiration dramatically in most cases, except for soil that experienced extreme drought stress periods. The legacy of drought stress can also regulate the response of soil respiration rate to rewetting events in terrestrial ecosystems through changing abiotic drivers and microbial community structure.Conclusions and perspectivesThere is increasing evidence that drought stress can result in the decoupling of the above- and belowground processes, which are associated with soil respiration. However, studies on the variation in rates of soil respiration and its components under different intensities and frequencies of drought stress over the ecosystems should be reinforced. Meanwhile, molecular phylogenetics and functional genomics should be applied to link microbial ecology to the process of Rs. In addition, we should quantify the relationship between soil respiration and global change parameters (such as warming and elevated [CO2]) under drought stress. Models simulating the rates of soil respiration and its components under global climate change and drought stress should also be developed.

Intermediate grazing intensities by sheep increase soil bacterial diversities in an Inner Mongolian steppe

Ungulate grazing is known to play a crucial role in regulating nutrient cycling and controlling plant community structure and productivity in grassland ecosystems. However, little is known about the effects of grazing intensities on soil bacterial community structure and diversity, particularly at the long-term scale. In this study, we measured plant biomass and diversity, soil characteristics and bacterial community structure, and diversity in a 16-year field experiment that had four grazing intensity treatments (non-grazed, CK; low-intensity grazing (LG), moderate-intensity grazing (MG), and high-intensity grazing (HG)) in an Inner Mongolian typical grassland. The CK, LG, MG, and HG sites were grazed by 0.00, 1.33, 4.00, and 6.67 sheep ha−1, respectively. Bacterial community structure and diversity under grazing intensity treatments were assessed with PCR amplification of DNAs extracted from soils and denaturing gradient gel electrophoresis (DGGE) separation. The results showed that the CK soil had higher moisture, organic C, NH4+–N, and NO3−–N concentrations than grazed soils, and the HG treatment had the lowest plant biomass and diversity across all the treatments. Principal component analysis of DGGE patterns showed that the LG and MG treatments were different from the CK and HG treatments. In addition, soil bacterial diversities in the LG and MG treatments were significantly higher than those in the other treatments. The relationships between environmental variables and soil bacterial community structure were assessed using redundancy analysis, and we found that soil moisture content, Artemisia frigida biomass, and pH were the best indicator of the changes in soil bacterial community structure among all the treatments. Overall, our results indicated that intermediate grazing intensities (LG and MG) increased soil bacterial diversities, and along with previous studies in this area, we suggested the MG treatment was the most suitable management practice in the Inner Mongolian steppe, not only supporting greater livestock amounts but also harboring greater bacterial diversity.

Effect of grazing intensities on the activity and community structure of methane-oxidizing bacteria of grassland soil in Inner Mongolia

The effects of different grazing intensities on in situ methane flux and the structure and diversity of the methanotrophic community are measured in the typical grassland of Inner Mongolia. Four grazing intensity sites founded in 1989, control (CK), low-intensity grazing (LG), middle-intensity grazing (MG) and heavy-intensity grazing (HG), were selected. Group-specific PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) of 16S rRNA genes for the type I and type II methanotrophs was used to characterize the composition of the methanotrophic community. DGGE patterns were further analyzed using the method of the Shannon-wiener index H and non-metric multi-dimensional scaling (MDS). The results showed that there were no significant differences in methane flux among different sites, yet methanotrophic communities showed significant differences. MDS analysis showed that type I methanotroph community composition at the CK site were significantly different from the three other sites. For type II methanotrophic community composition, it was similar between CK and HG site, and between LG and MG site, while that at the former two sites were significantly different from latter two ones. Additionally Shannon indices of type II methanotrophs were higher at the LG and MG sites than two other sites. Though grazing intensities had an impact on the structure of the methanotrophic community, management-induced changes in the structure of methanotrophic community did not reflect methane consumption capacity across sites. These results suggest that methane consumption is a complex process in soil, and we should be cautious when speculating on the change of methane consumption rates based on a change of methanotrophic community structure.

The fluxes of CO2 from grazed and fenced temperate steppe during two drought years on the Inner Mongolia Plateau, China

The CO(2) flux was measured by the eddy covariance method on a temperate Leymus chinensis steppe over a period of 17 months spanning two consecutive growing seasons. The amount of precipitation was nearly normal, but it was low in the early and high in the late growing period in 2006. In the 2007 growing season, the amount of precipitation was about 45% less than the multi-year average and more evenly distributed. Comparisons were made between a moderately grazed site and a 28-year-old fenced site. The maximum instantaneous CO(2) release and uptake rates were 0.12 (May) and -0.11mg CO(2)m(-2)s(-1) (July) at the fenced site, and 0.11 and -0.16mg CO(2)m(-2)s(-1) (both in July) at the grazed site. In both growing seasons, the grazed site always had a higher daily uptake rate or lower release rate than the fenced site. The grazed site was a CO(2) sink during the growing season of 2007 and a CO(2) source in the growing season of 2006, whereas the fenced site was a CO(2) source in both seasons. Lower precipitation decreased CO(2) loss during the growing season more in the grazed site than in the fenced site, mainly because of depression of total ecosystem respiration (R(e)) in the former and stimulation in the latter. During the dormant season (from October to April), the fenced and grazed sites released 60.0 and 32.4g of C per m(2), respectively. Path analysis showed that temperature had the greatest effect on daily variation of ecosystem CO(2) exchange during the growing seasons at the two study sites. The results suggest that decrease of precipitation and/or increase of temperature will likely promote C loss from L. chinensis steppes, whether fenced or grazed, and that a grazed site is more sensitive.

Variability and Changes in Climate, Phenology, and Gross Primary Production of an Alpine Wetland Ecosystem

Quantifying the variability and changes in phenology and gross primary production (GPP) of alpine wetlands in the Qinghai–Tibetan Plateau under climate change is essential for assessing carbon (C) balance dynamics at regional and global scales. In this study, in situ eddy covariance (EC) flux tower observations and remote sensing data were integrated with a modified, satellite-based vegetation photosynthesis model (VPM) to investigate the variability in climate change, phenology, and GPP of an alpine wetland ecosystem, located in Zoige, southwestern China. Two-year EC data and remote sensing vegetation indices showed that warmer temperatures corresponded to an earlier start date of the growing season, increased GPP, and ecosystem respiration, and hence increased the C sink strength of the alpine wetlands. Twelve-year long-term simulations (2000–2011) showed that: (1) there were significantly increasing trends for the mean annual enhanced vegetation index (EVI), land surface water index (LSWI), and growing season GPP (R2 ≥ 0.59, p < 0.01) at rates of 0.002, 0.11 year−1 and 16.32 g·C·m−2·year−1, respectively, which was in line with the observed warming trend (R2 = 0.54, p = 0.006); (2) the start and end of the vegetation growing season (SOS and EOS) experienced a continuous advancing trend at a rate of 1.61 days·year−1 and a delaying trend at a rate of 1.57 days·year−1 from 2000 to 2011 (p ≤ 0.04), respectively; and (3) with increasing temperature, the advanced SOS and delayed EOS prolonged the wetland’s phenological and photosynthetically active period and, thereby, increased wetland productivity by about 3.7–4.2 g·C·m−2·year−1 per day. Furthermore, our results indicated that warming and the extension of the growing season had positive effects on carbon uptake in this alpine wetland ecosystem.

Effects of Reed Straw, Zeolite, and Superphosphate Amendments on Ammonia and Greenhouse Gas Emissions from Stored Duck Manure

Stored poultry manure can be a significant source of ammonia (NH) and greenhouse gases (GHGs), including nitrous oxide (NO), methane (CH), and carbon dioxide (CO) emissions. Amendments can be used to modify physiochemical properties of manure, thus having the potential to reduce gas emissions. Here, we lab-tested the single and combined effects of addition of reed straw, zeolite, and superphosphate on gas emissions from stored duck manure. We showed that, over a period of 46 d, cumulative NH emissions were reduced by 61 to 70% with superphosphate additions, whereas cumulative NO emissions were increased by up to 23% compared with the control treatment. Reed straw addition reduced cumulative NH, NO, and CH emissions relative to the control by 12, 27, and 47%, respectively, and zeolite addition reduced cumulative NH and NO emissions by 36 and 20%, respectively. Total GHG emissions (as CO-equivalents) were reduced by up to 27% with the additions of reed straw and/or zeolite. Our results indicate that reed straw or zeolite can be recommended as amendments to reduce GHG emissions from duck manure; however, superphosphate is more effective in reducing NH emissions.

Increase in ammonia-oxidizing microbe abundance during degradation of alpine meadows may lead to greater soil nitrogen loss

Alpine meadows on the Tibetan Plateau have experienced severe degradation in recent decades. Although the effects of alpine meadow degradation on soil properties have been well documented, there is still a paucity of knowledge regarding the responses of nitrogen-cycling microbes (NCMs) to degradation and their links to the changes in soil properties. Here, we systematically determined the effects of degraded patch formation on soil properties (i.e., total carbon, total nitrogen, ammonium nitrogen, nitrate nitrogen, available phosphorus, dissolved organic carbon, moisture, δ15N, δ13C, and pH) and NCMs (based on nifH, amoA, narG, nirK, and nirS genes and their transcripts) across three Tibetan alpine meadows at different degradation stages. Results showed that compared to the original grassed patches, the contents of most soil nutrients (e.g., carbon, nitrogen, and phosphorus) were significantly decreased in the degraded patches across the study sites. Degraded patches also tended to have higher soil δ15N values and nitrate contents. Among the aforementioned NCMs, soil diazotrophs and denitrifiers only showed weak responses to the patch formation, while ammonia-oxidizing microbes showed the highest consistency and sensitivity in response to the patch formation across the study sites. The abundance of amoA gene and archaeal amoA mRNA significantly increased in the degraded patches, and they were positively correlated with soil δ15N values and nitrate nitrogen contents, but negatively correlated with soil total nitrogen and inorganic nitrogen contents. These results suggest that the increased ammonia-oxidizing microbial abundance may be an important driver of soil nitrogen loss during degraded patch formation in alpine meadows.

Joint structural and physiological control on the interannual variation in productivity in a temperate grassland: A data-model comparison

Given the important contributions of semiarid region to global land carbon cycle, accurate modeling of the interannual variability (IAV) of terrestrial gross primary productivity (GPP) is important but remains challenging. By decomposing GPP into leaf area index (LAI) and photosynthesis per leaf area (i.e., GPP_leaf), we investigated the IAV of GPP and the mechanisms responsible in a temperate grassland of northwestern China. We further assessed six ecosystem models for their capabilities in reproducing the observed IAV of GPP in a temperate grassland from 2004 to 2011 in China. We observed that the responses to LAI and GPP_leaf to soil water significantly contributed to IAV of GPP at the grassland ecosystem. Two of six models with prescribed LAI simulated of the observed IAV of GPP quite well, but still underestimated the variance of GPP_leaf, therefore the variance of GPP. In comparison, simulated pattern by the other four models with prognostic LAI differed significantly from the observed IAV of GPP. Only some models with prognostic LAI can capture the observed sharp decline of GPP in drought years. Further analysis indicated that accurately representing the responses of GPP_leaf and leaf stomatal conductance to soil moisture are critical for the models to reproduce the observed IAV of GPP_leaf. Our framework also identified that the contributions of LAI and GPP_leaf to the observed IAV of GPP were relatively independent. We conclude that our framework of decomposing GPP into LAI and GPP_leaf has a significant potential for facilitating future model intercomparison, benchmarking and optimization should be adopted for future data-model comparisons.

Degradation and Management of Steppes in China

China has 400 million ha of grassland, 72% of which is temperate grassland, including meadow steppe, typical steppe, and desert steppe. Some 90% of the grassland is degraded to different degrees. We present an overview of the indicators of various stages of degradation. The degradation is caused by long-term inappropriate use of the grassland in interaction with climate change. The most important causes are the explosive growth of the human population and livestock numbers in the Chinese grassland domain, together with the growing demand for economic improvement. We briefly review the effects of overgrazing, frequent mowing, transformation into cropland, and overharvesting of useful plants, and discuss the laws installed and measures taken to address these problems and manage grassland properly. Though the past decade has shown promising improvements, it is still a long way for China to achieve the goal of sustainable economic development and grassland conservation.

Autotrophic and symbiotic diazotrophs dominate nitrogen-fixing communities in Tibetan grassland soils

Biological nitrogen fixation, conducted by soil diazotrophs, is the primary nitrogen source for natural grasslands. However, the diazotrophs in grassland soils are still far from fully investigated. Particularly, their regional-scale distribution patterns have never been systematically examined. Here, soils (0-5 cm) were sampled from 54 grasslands on the Tibetan Plateau to examine the diazotroph abundance, diversity, and community composition, as well as their distribution patterns and driving factors. The diazotroph abundance was expressed as nifH gene copies, measured using real-time PCR. The diversity and community composition of diazotrophs were analyzed through MiSeq sequencing of nifH genes. The results showed that Cyanobacteria (47.94%) and Proteobacteria (45.20%) dominated the soil diazotroph communities. Most Cyanobacteria were classified as Nostocales which are main components of biological crusts. Rhizobiales, most of which were identified as potential symbiotic diazotrophs, were also abundant in approximately half of the soil samples. The soil diazotroph abundance, diversity, and community composition followed the distribution patterns in line with mean annual precipitation. Moreover, they also showed significant correlations with prokaryotic abundance, plant biomass, vegetation cover, soil pH values, and soil nutrient contents. Among these environmental factors, the soil moisture, organic carbon, available phosphorus, and inorganic nitrogen contents could be the main drivers of diazotroph distribution due to their strong correlations with diazotroph indices. These findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil diazotroph distribution on the Tibetan Plateau.