Grasslands in arid and semiarid areas are experiencing vegetation transitions characterized by shrub encroachment associated with increasing shrub cover and density. Yet the detailed effects on fungal communities across varying shrub cover and soil depths remain unclear. Here, we investigated 11 shrub-encroached grasslands with three shrub coverage levels (low, moderate, high), and evaluate changes in soil nutrients and fungal communities between shrub patches and grass matrices across four soil layers (0–10 cm, 10–20 cm, 20–30 cm, 30–50 cm). Our study found that shrub encroachment is associated with changes in soil properties at the 0–10 cm depth in the grass matrix. Specifically, the contents of soil dissolved nitrogen (DTN), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) all increased with increasing shrub cover. In contrast, nitrate (NO3−) showed a decreasing trend. The fungal operational taxonomic unit (OTU) richness was higher in the shrub patches than in the grass matrices across all layers at high shrub cover, and a clear decreasing pattern with increasing shrub cover at the 0–10 cm soil layer. Additionally, fungal community structure exhibits greater variation within grass matrices compared to shrub patches, influenced by both shrub cover and soil depth. Importantly, key fungal taxa enriched in shrub patches, including Magnaporthales, Togniniales, Cantharellales, Corticales, Geastrales, Malasseziales, and Diversisporales, were highly related to wood litter decomposition. Besides, wood saprotrophs occupied a considerable proportion (2.0–22.1

Precipitation changes exert a fundamental effect on the nitrogen (N) cycle in water-limited grasslands. Soil microbes are essential drivers of N cycle, and the rates and their stabilities of interrelated N-cycling processes are reflected by the abundance and diversity of N-cycling genes. Yet, little is known about how altered precipitation affects the genes involved in the entire N-cycling pathways.By combining a 6-year precipitation manipulation experiment (-30%, ambient, +30%, +50%) with metagenomic sequencing, we investigated the responses of N-cycling gene abundance and diversity to altered precipitation at two soil depths (0-10 and 30-50 cm).We found that increased precipitation enhanced the abundance of numerous key genes, leading to an acceleration of N turnover, but decreased the diversity of ammonium assimilation genes. Decreased precipitation did not reduce abundance or diversity of N-cycling genes. Most N-cycling genes showed generally consistent responses to altered precipitation in the topsoil (0-10 cm) and subsoil (30-50 cm), albeit with clear distinctions in both abundance and diversity by soil depth. These precipitation-specific responses and depth-dependent variabilities of functional genes were attributed to the distinct taxonomic composition of each N-cycling gene. Furthermore, we quantified gross N transformation rates and found that they were well predicted by the abundance of most N-cycling genes (e.g. genes involved in ammonium assimilation and nitrification).Our study sheds new light on the soil N cycle under precipitation alterations from the perspective of individual gene abundance and diversity and shows that future increases in precipitation could accelerate soil N turnover in arid and semi-arid lands.Read the free Plain Language Summary for this article on the Journal blog. Read the free Plain Language Summary for this article on the Journal blog.image

Widespread shrub encroachment is profoundly impacting the structures and functions of global drylands, and precipitation change is assumed to be one of the most critical factors affecting this phenomenon. However, there is little evidence to show how precipitation changes will affect the process. In this study, we conducted a 6-year precipitation manipulation experiment (-30%, ambient, +30% and +50%) to investigate the effects of precipitation changes on the growth of shrubs and herbaceous plants in a shrub-encroached grassland in Inner Mongolia. We found that the increasing precipitation significantly increased the mean height, coverage and above-ground biomass of herbaceous species, while the growth of shrub species did not exhibit a significant response to precipitation changes. With increasing precipitation, the relative coverage of shrubs decreased, while that of herbs increased. The native dominant herbaceous plant (Leymus chinensis), with more sensitive maximum photosynthetic rate to the precipitation change, showed higher photosynthetic nitrogen use efficiency and water use efficiency than those of the encroached shrub species (Caragana microphylla) at high soil moisture contents, reflecting that the ecophysiological characteristics of L. chinensis might provide it a competitive advantage under increased precipitation. Our findings suggest that increasing precipitation may slow down shrub encroachment by facilitating herbaceous growth in Mongolian grasslands, and consequently affect the forage value and carbon budget in these ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Abstract Widespread shrub encroachment in grasslands can lead to changes in soil aggregates and soil organic carbon (SOC), especially in deep soils. Soil aggregates physicochemically protect SOC and thus affect soil carbon sequestration. Characterizing the changes in soil aggregates and their organic carbon associated with shrub encroachment is critical for evaluating the ecological consequences of shrub encroachment in different grasslands. In this study, we investigated soil aggregates of various sizes and their associated organic carbon at maximum soil depths of 5 m in shrub patches and neighboring grass matrix in two types (desert and typical) of grasslands in Inner Mongolia, China. The mean weight diameter (MWD) of the soil aggregates was similar between the shrub patches and grass matrix in both grasslands. However, the proportion of each aggregate size fraction to whole soil in the shrub patches was significantly lower than that in the grass matrix in the desert grasslands, while there was no significant difference between the shrub patches and grass matrix in the typical grasslands. In addition, the organic carbon content of aggregates in the shrub patches was greater than that in the grass matrix in deep soil layers (>50 cm) but was lower in the topsoil (0–10 cm depth) in the desert grasslands. However, the organic carbon content of aggregates at all soil depths was greater in shrub patches than in the grass matrix in typical grasslands. Our results suggest that the effects of shrub encroachment on the soil aggregate proportion and associated organic carbon vary at different soil depths and in different grassland types. The results of this study also suggest that it is necessary to determine the size structure and its relation to carbon content for soil aggregates to accurately predict SOC dynamics with shrub encroachment.

Shrub encroachment has caused a vegetation shift in arid and semiarid grassland ecosystems around the world, leading to marked changes in ecosystem structures and functions. Soil extracellular enzyme activity (EEA) is an informative indicator of microbial decomposition and plays an important role in soil biogeochemical cycles, but it is unclear how shrub encroachment affects soil EEA, especially in the future precipitation pattern under climate change. In this study, we measured soil EEAs in a shrub-encroached grassland in Inner Mongolia, China, after four years of in situ experimental precipitation manipulation. Soil samples were collected from grass patches and shrub patches in four precipitation manipulation treatments (−30%, control, +30%, and +50%). Four soil enzymes involved in carbon cycling (α-glucosidase, AG; β-1,4-glucosidase, BG; β-D-cellobiosidase, CB; and β-xylosidase, XS), two nitrogen-acquiring enzymes (β-1,4-N-acetyl-glucosaminidase, NAG; and leucine amino peptidase, LAP), and one phosphorus-acquiring enzyme (acid phosphatase, AP) were investigated. The results showed that BG, XS, and LAP activities were significantly enhanced in shrub patches than grass patches (p < 0.05). BG, LAP, and AP were significantly different in precipitation treatments (p < 0.05) and were highest in +30% and shrub patches. Shrub encroachment and increased precipitation (+30%) interactively enhanced LAP (p < 0.05). Our study also showed that soil temperature and soil moisture were major factors for the variations in EEA under different precipitation treatments in the grass patches, while soil temperature, NH4+-N and NO3−-N were major causes for those in the shrub patches. Overall, our study highlights that shrub encroachment and altered precipitation can influence soil EEA, which in turn may have consequential effects on nutrients availability and biochemical cycling.

Aims Planting density, as one of the most important factors affecting crop yield and quality, will result in plant competition for light, water and nutrients. The objective of this study is to explore the effect of planting density on growth and yield of Medicago sativa population. Methods The population density experiments of M. sativa was conducted in a greenhouse with six planting densities, i.e. 25, 100, 400, 800, 1 500 and 2 000 plants·m. At each plot, we measured plant height, basal diameter, branch number, biomass, and number of survivors. Important findings The results showed that the average planting density was 25, 100, 373, 745, 1 255 and 1 938 plants·m in the 15 days after sowing. With the growth of M. sativa, except for the low density treatments (25 and 100 plants·m), the number of plant individuals under other density treatments decreased, and self-thinning occurred at some degrees. At the second harvest (187 days after sowing), the number of surviving plants decreased to 297, 571, 759 and 839 plants·m, respectively. The plant height, basal diameter and branch number of individual plants decreased exponentially with the increase of existing density. The relationships between individual biomass and existing density followed the competitive density effect law, that is, individual biomass decreased with the increase of density. The results also showed that the aboveground biomass of M. sativa per unit area has no significant differences among different densities, but the underground biomass tended to increase first and then decrease with the increase of planting density.

Widespread shrub encroachment in global drylands may increase plant biomass and change soil organic carbon stocks of grassland ecosystems. However, the response of soil inorganic carbon (SIC), which is a major component of dryland carbon pools, to this vegetation shift remains unknown. Here, we conducted a systematic field survey in 75 pairs of shrub-encroached grassland and control plots at 25 sites in the grasslands of the Inner Mongolia Plateau to evaluate how shrub encroachment affects SIC density (SICD) in these ecosystems. We found that shrub encroachment significantly reduced SICD in the upper 100 cm, especially in the subsurface soil (20-50 cm layer). The magnitude of SICD changes was related to the change in soil pH, shrub patch size, and initial SICD, reflecting that the reduction in SICD might be attributed to the shrub encroachment-related soil acidification. Our results also revealed that the lost SIC was mainly released into the atmosphere rather than redistributed into deeper soil layers. Overall, we provide the first evidence for the soil acidification-induced SIC loss caused by shrub encroachment. Our findings highlight the non-negligible role of SIC dynamics in the C budget of shrub-encroached grassland ecosystems and the need to consider these dynamics in terrestrial C cycle research.

Shrub encroachment has significant impacts on carbon (C) and nitrogen (N) dynamics of grassland ecosystems, but, how it influences soil organic C (SOC) and soil total N (STN) stocks of grasslands is not well documented, especially in deep soils. Here, we examined these effects by investigating the SOC density (SOCD) and the STN density (STND) at depths of up to 5m in shrub patches and grassy matrix at 18 grassland sites in Inner Mongolia, China. Our results revealed that 44.0-54.1% of SOC and 65.5-74.8% of STN were stored in soil layers of 100cm to 500cm, and that shrub encroachment resulted in a significant accumulation of SOC and STN in shrub patches. The mean SOCD at 0-500cm soil layers in shrub patches and grassy matrix was 11.34 and 8.39kgCm(-2), respectively, and the mean STND was 2.45 and 2.13kgNm(-2), respectively. These differences between the shrub patches and grassy matrix were equivalent to an increase in the mean SOC and STN stocks of 2.95kgCm(-2) and 0.32kgNm(-2), respectively. Increases in SOCD and STND mainly occurred in 30-300cm soil layers, and varied between typical grassland and desert grassland. The changes in SOCD and STND exhibited significant negative correlations with temperature and pH and positive correlations with precipitation and plant biomass. Our findings suggest that shrub encroachment can significantly accumulate SOC and STN in deep soils in temperate grasslands, which helps to understand soil C and N dynamics in shrub-encroached grasslands.

In recent decades, ecologists have investigated the effects of shrub encroachment on regional carbon cycling in semi-arid and arid regions. Although differences in carbon sequestration and stocks have been recognized in different soil layers, the vertical changes in soil organic carbon (SOC) at the molecular level following shrub encroachment remain unexplored. In this study, we used biomarkers to assess the impacts of shrub encroachment on SOC composition.

The ongoing expansion of shrub encroachment into grasslands represents a unique form of land cover change. How this process affects soil microbial communities is poorly understood. In this study, we aim to assess the effects of shrub encroachment on soil microbial biomass, abundance and composition by comparing data between shrub patches and neighboring herb patches in shrub-encroached grasslands (SEGs) in Inner Mongolia, China. Fourteen SEG sites from two ecosystem types (typical and desert grasslands) were investigated. The phospholipid fatty acid (PLFA) method was used to analyze the composition and biomass of the soil microbial community. Our results showed that the top-soil microbial biomass and abundances of gram-negative bacteria, arbuscular mycorrhizal fungi, and actinomycetes were significantly higher in shrub patches than in herb patches in both typical and desert grasslands (P < 0.05). The fungi to "bacteria ratio was significantly higher in shrub patches than in herb patches in desert grassland (P < 0.05). The microbial biomass was positively associated with mean annual precipitation, total nitrogen and available phosphorus, and negatively associated with mean annual "temperature. Our results also indicated that the variation in microbial composition was largely explained by edaphic factors, followed by climate factors. In conclusion, shrub encroachment in Inner Mongolia grasslands has significantly influenced the structure and abundance of soil microbial communities, which makes the microbial communities toward a fresh organic carbon-based structure. This study highlights the importance of edaphic and climate factors in microbial community shifts in SEGs. (C) 2017 Elsevier Masson SAS. All rights reserved.

Grasslands cover a large area of terrestrial China.However,the consensus has not been reached so far with regard to their distribution,biomass,productivity and other properties of the extensive ecosystem.A comprehensive assessment on the grassland ecosystem is needed not only for its integrated development and sustainable utilization,but also for understanding its ecological role in future climate change.We surveyed a large body of the literature accumulated during last few decades,combining with model estimation to clarify the grassland distribution,biomass and productivity.The grassland area varied largely among different studies with the range from 1.67×106-4.31×106 km2.We considered that an area from 2.80×106 km2 to 3.93×106 km2 would be reasonable depending on the vegetation map.The averaged biomass of China's grasslands varied greatly among previous study,ranging from 79-123 g m-2.Using remote sensing dataset of vegetation index (NOAA/AVHRR-NDVI) and corresponding climate data,our re-estimation indicated that the mean above-ground biomass of natural grassland from 1982-2011 is about 178 g m-2,with an increasing rate of 0.4 g m-2 a-1.The net primary productivity reported so far,ranging from 89-320 gC m-2 a-1 with an average of 176 gC m-2 a-1,showed also an increasing trend.The potential primary productivity based on the climate model was much higher and reached about 348 gC m-2 a-1.Comparing with nature grasslands,the sown pasture area is much smaller and is only about 2.09×107 hm2,but its productivity was 2.7-12.1 times higher than that of natural grasslands.Concerning with the livestock grazing in China's grassland,we found that the carrying capacity of natural grassland in China is very low,with the average overgrazing rate estimated to be about 20％.Precipitation is the most important factors affecting grassland biodiversity,biomass and productivity and their distribution pattern in China.Because of the extensive area and frangible environments,it is critical to further explore the effects of climate change and human activities such as livestock grazing on the vast grasslands in the East Asia.