Guimin Liu

How important are trophic state, macrophyte and fish population effects on cladoceran community? A study in Lake Erhai.

Environmental controls on cladoceran community structure in lake ecosystems are complex and may involve many environmental parameters including trophic state and fish populations. In Lake Erhai, a plateau lake located in southwest China, it was hypothesized that a combination of lake eutrophication and planktivorous fish introduction would increase the abundance of cladoceran, while also decreasing cladoceran size. To test this hypothesis, we examined temporal changes in cladoceran microfossils in the sediments of Lake Erhai over the past century. The influence of changing macrophyte coverage within the littoral region of the lake was also considered. Results demonstrated that cladoceran abundance (measured as flux of cladoceran fossils in the sediments) increased markedly accompanying eutrophication of the lake. In addition, there was a shift in the dominant cladoceran species from those species that prefer oligotrophic conditions to those that prefer mesotrophic and eutrophic conditions. A reduction in the ephippium length of Daphnia spp. was observed and attributed to the introduction of the planktivorous fish Neosalanx taihuensis. Our findings indicated that eutrophication and fish introduction were the main controls affecting cladoceran community structure during the recent decades, and predation by planktivorous fish had an important impact on Daphnia body size.

Response of the cladoceran community to eutrophication, fish introductions and degradation of the macrophyte vegetation in Lake Dianchi, a large, shallow plateau lake in southwestern China

A paleolimnological evaluation was made in order to analyze the effects of increasing nutrient load, macrophyte degradation and fish introductions on the cladoceran community of a large, shallow plateau lake in southwestern China. The trophic state of Lake Dianchi has increased rapidly during recent decades, its macrophyte vegetation has suffered severe degradation, and fish introductions in the late 1950s and early 1980s have had a marked effect on the structure of the fish community. Our results show an increase in abundance of cladoceran species with a preference for eutrophic conditions over the last few decades, while species preferring oligotrophic conditions have decreased or disappeared. These changes correspond to the eutrophication in Lake Dianchi. The loss of the cladocerans Kurzia latissima and Disparalonarostrata is likely to be a reflection of the degradation of the macrophyte community. An increase in Daphnia body size indicated by the ephippia length since the early 1990s is associated with the decline of planktivorous species.

Vertical patterns and controls of soil nutrients in alpine grassland: Implications for nutrient uptake

Vertical patterns and determinants of soil nutrients are critical to understand nutrient cycling in high-altitude ecosystems; however, they remain poorly understood in the alpine grassland due to lack of systematic field observations. In this study, we examined vertical distributions of soil nutrients and their influencing factors within the upper 1m of soil, using data of 68 soil profiles surveyed in the alpine grassland of the eastern Qinghai-Tibet Plateau. Soil organic carbon (SOC) and total nitrogen (TN) stocks decreased with depth in both alpine meadow (AM) and alpine steppe (AS), but remain constant along the soil profile in alpine swamp meadow (ASM). Total phosphorus, Ca2+, and Mg2+ stocks slightly increased with depth in ASM. K+ stock decreased with depth, while Na+ stock increased slightly with depth among different vegetation types; however, SO42- and Cl- stocks remained relatively uniform throughout different depth intervals in the alpine grassland. Except for SOC and TN, soil nutrient stocks in the top 20cm soils were significantly lower in ASM compared to those in AM and AS. Correlation analyses showed that SOC and TN stocks in the alpine grassland positively correlated with vegetation coverage, soil moisture, clay content, and silt content, while they negatively related to sand content and soil pH. However, base cation stocks revealed contrary relationships with those environmental variables compared to SOC and TN stocks. These correlations varied between vegetation types. In addition, no significant relationship was detected between topographic factors and soil nutrients. Our findings suggest that plant cycling and soil moisture primarily control vertical distributions of soil nutrients (e.g. K) in the alpine grassland and highlight that vegetation types in high-altitude permafrost regions significantly affect soil nutrients.

Spatial variability of soil salinity in Bohai Sea coastal wetlands, China: Partition into four management zones

Soil salinization constitutes an environmental hazard worldwide. The Bohai Sea coastal wetland area is experiencing dramatic soil salinization, which is affecting its economic development. This study focused on the spatial variation and distribution characteristics of soil salinity in this area using geostatistical analysis combined with the kriging interpolation method, based on a large-scale field investigation and layered soil sampling (0–30, 30–60 and 60–100 cm). The results revealed that soil salinity in these layers demonstrated strong variability, obvious spatial structure characteristics and strong spatial autocorrelation. Soil salinity displayed a significant zonal distribution, gradually decreasing with increasing distance from the coastline. Apart from the northern part of the study area, which appeared to be not affected by soil salinization, there were varying degrees of soil salinization in nearly 70% of the total area. With increasing soil depth, the areas of non-salinized and mild salinized soil gradually decreased, while those of moderate salinized and strong salinized soils increased. The area of saline soil first decreased and then increased. The study area could be divided into four management zones according to soil salinities in the top 1-m soil body, and utilization measures, adapted to local conditions, were proposed for each zone. The results of our study present an important theoretical basis for the improvement of saline soils, for wetland re-vegetation and for the sustainable utilization of soil resources in the Bohai Sea coastal wetland.

Salt dynamics in soil profiles during long-term evaporation under different groundwater conditions

To study salt dynamics in soil profiles under different groundwater conditions, a 3-year indoor experiment was carried out under conditions of open-air evaporation. Silt loam soil was treated under three groundwater table depths (0.85, 1.05, and 1.55 m) combined with three groundwater salinities: 0.40 dS m− 1 (2 g l− 1), 0.80 dS m− 1 (4 g l− 1), and 1.60 dS m− 1 (8 g l− 1). A total of nine soil columns (0.14 m internal diameter) were used to simulate different combinations of groundwater depths and salinities. The results obtained showed that salt first accumulated at the bottom of the soil column, and only when soil salinity in this layer had remained relatively stable with time, salt began to accumulate in the adjacent upper soil layers. When all subsoil layers had reached dynamic salinity equilibrium, electrical conductivity (EC) of soils in the surface layer began to increase drastically. With increasing salt accumulation in the surface soil, EC of the subsoil began to rise tardily. The further up the soil layer, the earlier EC started to increase, although the redistribution of salts in the soil profile tended to be homogenous. Groundwater depth did not significantly change subsoil EC values at the same depth; however, it distinctly affected the time needed for the subsoil to reach dynamic salinity equilibrium. Groundwater salinity, on the other hand, did not significantly alter the time point at which soil salinity at the same depth began to increase rapidly or the time period needed to reach dynamic salinity equilibrium. This study explored salt transport processes in the soil profile through a long-term experiment, enabling us to reveal some general laws governing salt dynamics that will be very important to understand the mechanism of soil salinization. The results could be further used to set up strategies to prevent salinization or to improve salt-affected soils.

Permafrost and land cover as controlling factors for light fraction organic matter on the southern Qinghai-Tibetan plateau

Permafrost degradation can stimulate the decomposition of organic soil matter and cause a large amount of greenhouse gas emissions into the atmosphere. The light fraction organic matter (LFOM) is a labile substrate for microbial decomposition and probably plays an important role in future permafrost carbon cycles. However, little is known about the distribution of LFOM and its relationship with permafrost and environmental factors. Here, we investigated the light fraction carbon (LFC) and nitrogen (LFN) contents and stocks under meadows and wet meadows with different permafrost conditions on the southern Qinghai-Tibetan Plateau. Our results showed that LFC and LFN were mainly distributed in the upper 30cm of soils, and the sites with permafrost had significantly higher contents of LFC and LFN than those from the sites without existing permafrost. The LFC and LFN decreased sharply with depth, suggesting that the soil organic matter (SOM) in this area was highly decomposed in deep soils. Soil moisture and bulk density explained approximately 50% of the variances in LFC and LFN for all the sampling sites, while soil moisture explained approximately 30% of the variance in permafrost sites. Both the C:N ratios and LFC:LFN ratios in the sites with permafrost were higher than those in the sites without permafrost. The results suggested that the permafrost and land cover types are the main factors controlling LFOM content and stock, and that permafrost degradation would lead to a decrease of LFOM and soil C:N ratios, thus accelerating the decomposition of SOM.

Influence of land cover on riverine dissolved organic carbon concentrations and export in the Three Rivers Headwater Region of the Qinghai-Tibetan Plateau

The Qinghai-Tibetan plateau (QTP) stores a large amount of soil organic carbon and is the headwater region for several large rivers in Asia. Therefore, it is important to understand the influence of environmental factors on river water quality and the dissolved organic carbon (DOC) export in this region. We examined the water physico-chemical characteristics, DOC concentrations and export rates of 7 rivers under typical land cover types in the Three Rivers Headwater Region during August 2016. The results showed that the highest DOC concentrations were recorded in the rivers within the catchment of alpine wet meadow and meadow. These same rivers had the lowest total suspended solids (TSS) concentrations. The rivers within steppe and desert had the lowest DOC concentrations and highest TSS concentrations. The discharge rates and catchment areas were negatively correlated with DOC concentrations. The SUVA254 values were significantly negatively correlated with DOC concentrations. The results suggest that the vegetation degradation, which may represent permafrost degradation, can lead to a decrease in DOC concentration, but increasing DOC export and soil erosion. In addition, some of the exported DOC will rapidly decompose in the river, and therefore affect the regional carbon cycle, as well as the water quality in the source water of many large Asian rivers.