Wenju Liang

Nitrogen deposition weakens plant–microbe interactions in grassland ecosystems

Soil carbon (C) and nitrogen (N) stoichiometry is a main driver of ecosystem functioning. Global N enrichment has greatly changed soil C : N ratios, but how altered resource stoichiometry influences the complexity of direct and indirect interactions among plants, soils, and microbial communities has rarely been explored. Here, we investigated the responses of the plant-soil-microbe system to multi-level N additions and the role of dissolved organic carbon (DOC) and inorganic N stoichiometry in regulating microbial biomass in semiarid grassland in northern China. We documented a significant positive correlation between DOC and inorganic N across the N addition gradient, which contradicts the negative nonlinear correlation between nitrate accrual and DOC availability commonly observed in natural ecosystems. Using hierarchical structural equation modeling, we found that soil acidification resulting from N addition, rather than changes in the plant community, was most closely related to shifts in soil microbial community composition and decline of microbial respiration. These findings indicate a down-regulating effect of high N availability on plant-microbe interactions. That is, with the limiting factor for microbial biomass shifting from resource stoichiometry to soil acidity, N enrichment weakens the bottom-up control of soil microorganisms by plant-derived C sources. These results highlight the importance of integratively studying the plant-soil-microbe system in improving our understanding of ecosystem functioning under conditions of global N enrichment.

Contrasting elevational diversity patterns between eukaryotic soil microbes and plants

The diversity of eukaryotic macroorganisms such as animals and plants usually declines with increasing elevation and latitude. By contrast, the community structure of prokaryotes such as soil bacteria does not generally correlate with elevation or latitude, suggesting that differences in fundamental cell biology and/or body size strongly influence diversity patterns. To distinguish the influences of these two factors, soil eukaryotic microorganism community structure was investigated in six representative vegetation sites along an elevational gradient from forest to alpine tundra on Changbai Mountain in Northeast China, and compared with our previous determination of soil bacterial community structure along the same gradient. Using bar-coded pyrosequencing, we found strong site differences in eukaryotic microbial community composition. However, diversity of the total eukaryotic microorganism community (or just the fungi or protists alone) did not correlate with elevation. Instead, the patterns of diversity and composition in the total eukaryotic microbial community (and in the protist community alone) were closely correlated with soil pH, suggesting that just as for bacteria, acidity is a particularly important determinant of eukaryotic microbial distributions. By contrast, as expected, plant diversity at the same sites declined along our elevational gradient. These results together suggest that elevational diversity patterns exhibited by eukaryotic microorganisms are fundamentally different from those of plants.

Distribution of Soil Enzyme Activities and Microbial Biomass Along a Latitudinal Gradient in Farmlands of Songliao Plain, Northeast China

Soil enzymes activities and microbial biomass have an important influence on nutrient cycling. The spatial distribution of soil enzymes activities and microbial biomass were examined along a latitudinal gradient in farmlands of Songliao Plain, Northeast China to assess the impact of climatic changes along the latitudinal transect on nutrient cycling in agroecosystems. Top soils (0–20 cm depth) were sampled in fields at 7 locations from north (Hailun) to south (Dashiqiao) in the end of October 2005 after maize harvest. The contents of total C, N, and P, C/N, available N, and available P increased with the latitude. The activities of invertase and acid phosphatase, microbial biomass (MB) C and N, and MBC/MBN were significantly correlated with latitude ( P< 0.05, r 2 =0 .198, 0.635, 0.558, 0.211 and 0.317, respectively), that is, increasing with the latitude. Significant positive correlations ( P< 0.05) were observed between invertase activity and the total N and available P, and between acid phosphatase activity and the total C, C/N, available N, total P and available P. The urease, acid phosphatase, and dehydrogenase activities were significantly correlated with the soil pH and electrical conductivity (EC) ( P< 0.05). MBC and MBN were positively correlated with the total C, C/N, and available P( P< 0.05). The MBC/MBN ratio was positively correlated with the total C, total N, C/N, and available N ( P< 0.05). The spatial distribution of soil enzyme activities and microbial biomass resulted from the changes in soil properties such as soil organic matter, soil pH, and EC, partially owing to variations in temperature and rainfall along the latitudinal gradient.

Soil Nematode Response to Biochar Addition in a Chinese Wheat Field

While studies have focused on the use of biochar as soil amendment, little attention has been paid to its effect on soil fauna. The biochar was produced from slow pyrolysis of wheat straw in the present study. Four treatments, no addition (CK) and three rates of biochar addition at 2 400 (B1), 12 000 (B5) and 48 000 kg ha −1 (B20), were investigated to assess the effect of biochar addition to soil on nematode abundance and diversity in a microcosm trial in China. The B5 and B20 application significantly increased the total organic carbon and the C/N ratio. No significant difference in total nematode abundance was found among the treatments. The biochar addition to the soil significantly increased the abundance of fungivores, and decreased that of plant parasites. The diversity of soil nematodes was significantly increased by B1 compared to CK. Nematode trophic groups were more effectively indicative to biochar addition than total abundance.

Distinct soil bacterial communities along a small-scale elevational gradient in alpine tundra.

The elevational diversity pattern for microorganisms has received great attention recently but is still understudied, and phylogenetic relatedness is rarely studied for microbial elevational distributions. Using a bar-coded pyrosequencing technique, we examined the biodiversity patterns for soil bacterial communities of tundra ecosystem along 2000–2500 m elevations on Changbai Mountain in China. Bacterial taxonomic richness displayed a linear decreasing trend with increasing elevation. Phylogenetic diversity and mean nearest taxon distance (MNTD) exhibited a unimodal pattern with elevation. Bacterial communities were more phylogenetically clustered than expected by chance at all elevations based on the standardized effect size of MNTD metric. The bacterial communities differed dramatically among elevations, and the community composition was significantly correlated with soil total carbon (TC), total nitrogen, C:N ratio, and dissolved organic carbon. Multiple ordinary least squares regression analysis showed that the observed biodiversity patterns strongly correlated with soil TC and C:N ratio. Taken together, this is the first time that a significant bacterial diversity pattern has been observed across a small-scale elevational gradient. Our results indicated that soil carbon and nitrogen contents were the critical environmental factors affecting bacterial elevational distribution in Changbai Mountain tundra. This suggested that ecological niche-based environmental filtering processes related to soil carbon and nitrogen contents could play a dominant role in structuring bacterial communities along the elevational gradient.

Nitrogen Addition and Warming Independently Influence the Belowground Micro-Food Web in a Temperate Steppe

Climate warming and atmospheric nitrogen (N) deposition are known to influence ecosystem structure and functioning. However, our understanding of the interactive effect of these global changes on ecosystem functioning is relatively limited, especially when it concerns the responses of soils and soil organisms. We conducted a field experiment to study the interactive effects of warming and N addition on soil food web. The experiment was established in 2006 in a temperate steppe in northern China. After three to four years (2009–2010), we found that N addition positively affected microbial biomass and negatively influenced trophic group and ecological indices of soil nematodes. However, the warming effects were less obvious, only fungal PLFA showed a decreasing trend under warming. Interestingly, the influence of N addition did not depend on warming. Structural equation modeling analysis suggested that the direct pathway between N addition and soil food web components were more important than the indirect connections through alterations in soil abiotic characters or plant growth. Nitrogen enrichment also affected the soil nematode community indirectly through changes in soil pH and PLFA. We conclude that experimental warming influenced soil food web components of the temperate steppe less than N addition, and there was little influence of warming on N addition effects under these experimental conditions.

Distribution of Soil Organic Carbon Fractions Along the Altitudinal Gradient in Changbai Mountain, China

Understanding the responses of soil organic carbon (SOC) fractions to altitudinal gradient variation is important for understanding changes in the carbon balance of forest ecosystems. In our study the SOC and its fractions of readily oxidizable carbon (ROC), water-soluble carbon (WSC) and microbial biomass carbon (MBC) in the soil organic and mineral horizons were investigated for four typical forest types, including mixed coniferous broad-leaved forest (MCB), dark coniferous spruce-fir forest (DCSF), dark coniferous spruce forest (DCS), and Ermans birch forest (EB), along an altitudinal gradient in the Changbai Mountain Nature Reserve in Northeast China. The results showed that there was no obvious altitudinal pattern in the SOC. Similar variation trends of SOC with altitude were observed between the organic and mineral horizons. Significant differences in the contents of SOC, WSC, MBC and ROC were found among the four forest types and between horizons. The contents of ROC in the mineral horizon, WSC in the organic horizon and MBC in both horizons in the MCB and EB forests were significantly greater than those in either DCSF or DCS forest. The proportion of soil WSC to SOC was the lowest among the three main fractions. The contents of WSC, MBC and ROC were significantly correlated (P < 0.05) with SOC content. It can be concluded that vegetation types and climate were crucial factors in regulating the distribution of soil organic carbon fractions in Changbai Mountain.

Soil Nematode Responses to Increases in Nitrogen Deposition and Precipitation in a Temperate Forest

The environmental changes arising from nitrogen (N) deposition and precipitation influence soil ecological processes in forest ecosystems. However, the corresponding effects of environmental changes on soil biota are poorly known. Soil nematodes are the important bioindicator of soil environmental change, and their responses play a key role in the feedbacks of terrestrial ecosystems to climate change. Therefore, to explore the responsive mechanisms of soil biota to N deposition and precipitation, soil nematode communities were studied after 3 years of environmental changes by water and/or N addition in a temperate forest of Changbai Mountain, Northeast China. The results showed that water combined with N addition treatment decreased the total nematode abundance in the organic horizon (O), while the opposite trend was found in the mineral horizon (A). Significant reductions in the abundances of fungivores, plant-parasites and omnivores-predators were also found in the water combined with N addition treatment. The significant effect of water interacted with N on the total nematode abundance and trophic groups indicated that the impacts of N on soil nematode communities were mediated by water availability. The synergistic effect of precipitation and N deposition on soil nematode communities was stronger than each effect alone. Structural equation modeling suggested water and N additions had direct effects on soil nematode communities. The feedback of soil nematodes to water and nitrogen addition was highly sensitive and our results indicate that minimal variations in soil properties such as those caused by climate changes can lead to severe changes in soil nematode communities.

Effects of heavy metal pollution of highway origin on soil nematode guilds in North Shenyang, China

Soil samples were collected with distance at 5, 20, 40, 80, 160, and 320 m from the Shen-Ha (Shenyang-Harbin) Highway, Northeast China, to investigate the effect of heavy metals of highway origin on soil nematode guilds. The contents of soil Pb, Cu, Zn, and the nematode community structure were analyzed. The results showed that the contents of total and available Pb, Cu, Zn varied significantly with the different distances from the highway. Pb was the main pollutant in the soils in the vicinity of Shen-Ha Highway. The zone from 20 to 40 m away from the highway was the most polluted area. The highest abundance of soil nematodes was found at 5 m while the lowest at 20 m away from the highway. Thirty six genera of nematodes belonging to 23 families were identified. Nematode guilds having different responses to soil heavy metals were classified into four types. Soil nematode guilds may act as a prominent indicator to heavy metal pollution of highway origin.

Soil nematode responses to heavy metal stress

SummaryThe effect of Cu and Zn on soil nematode communities at the depth of 0–20 cm was investigated along a pollution gradient with increasing distance from a copper smelter in Northeast China. The results showed that the abundance of total nematodes significantly increased with increasing distance from the smelter, and nematode taxon richness also exhibited an increasing trend. Twenty genera were observed in our study, and Paratylenchus was the dominant genus at all the sampling sites. The numbers of total nematodes, bacterivores, plant parasites, omnivores-predators and the values of NCR (Nematode Channel Ratio) negatively correlated with total Cu (P < 0.01), total and available Zn (P < 0.01). The values of SI (Structure Index) negatively correlated with total Cu (P < 0.05). Among the nematode ecological indices examined, NCR and SI were found to be sensitive indicators for assessing the effect of heavy metals on soil nematode communities in this study.