Responses of soil dissolved organic carbon properties to the desertification of desert wetlands in the Mu Us Sandy Land.

In desert wetlands, the decline in ground water table results in desertification, triggering soil carbon and nutrient loss. However, the impacts of desertification on soil dissolved organic carbon (DOC) properties which determine the turnover of soil carbon and nutrients are unclear. Here, the desertification gradient was represented by the distance from the wetland center (0∼240 m) traversing reed marshes, desert shrubs and bare sandy land in the Hongjian Nur Basin, north China. Soil DOC properties were determined by ultraviolet and fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). Results showed that soil DOC content decreased significantly from 107.23 mg kg-1 to 8.44 mg kg-1 by desertification (p < 0.05). However, the proportion of DOC to soil organic carbon (SOC) was gradually significantly increased. According to spectral parameters, microbial-derived DOC decreased from 0 to 120 m (reed marshes to desert shrubs) but increased from 120 to 240 m (desert shrubs to bare sandy lands), with a reverse hump-shaped distribution pattern. The molecular weight and aromaticity of DOC increased from 0 to 120 m but decreased from 120 to 240 m, with a hump-shaped distribution pattern. For the DOC composition, although the relative abundances of humic-acid components remained stable (p > 0.05), they were ultimately decreased by serious desertification and the amino acids became the dominant component. A similar change pattern was also found for humification index. Additionally, MBC and C:N were the two most important variables in determining the content and spectral properties, respectively. Together, these findings relationships between the soil DOC properties and desertification degree, especially the increase in DOC proportion and the decrease in humification degree, which may reduce soil C stabilization in the Hongjian Nur Basin.

Oil degradation and variation of microbial communities in contaminated soils induced by different bacterivorous nematodes species.

Oil pollution poses a great threat to environments and makes the remediation of oil-contaminated soils an urgent task. Microorganisms are the main biological factor for oil removal in the environment but microbial remediation is greatly affected by environmental factors. For our research, we inoculated three species of bacterivorous nematodes into oil-contaminated soil to explore how bacterivorous nematodes affect soil microbial activities and community structure in contaminated soil, as well as how efficiently different nematodes remove oil pollution from the soil. Six treatments were set in this experiment: sterilized oil-contaminated soil (SOC); nematode-free soil (S); oil-contaminated soil (OC); oil-contaminated soil + Caenorhabditis elegans (OCN1); oil-contaminated soil + Cephalobus persegnis (OCN2); oil-contaminated soil + Rhabditis marina (OCN3) for a 168-day incubation experiment. After the experiment was done, the oil contents in SOC, OC, OCN1, OCN2, and OCN3 were reduced by 6.5%, 32.3%, 38.2%, 42.8%, and 40.2%, respectively, compared with the beginning of the experiment. The amount of phospholipid fatty acids (PLFAs) of Gram-negative bacteria in OC, OCN1, OCN2, and OCN3 was increased by 50.9%, 43.4%, 37.7%, and 47.9%, respectively, compared with that of S. During the 168-day incubation period, the maximum growth of the number of nematodes in OCN1, OCN2, and OCN3 compared with the initial number of the nematodes were 2.25-, 1.52-, and 1.65-fold, respectively. The amount of oil residue in the contaminated soil negatively correlated with the populations of nematodes, total microorganisms, Gram-negative bacteria, actinomycetes, and eukaryotes. Thus, oil pollution increased the number of Gram-negative bacteria, decreased the ratio of Gram-positive bacteria/Gram-negative bacteria and Fungi/Bacteria significantly, and altered the community structure of soil microorganisms. Each species of bacterivorous nematodes has got its unique effect on the microbial activity and community structure in oil contaminated soils, but those tested can promote oil degradation and thus improve the environment of oil contaminated soils.

Changes in microbial metabolic C- and N- limitations in the rhizosphere and bulk soils along afforestation chronosequence in desertified ecosystems.

The resource acquisition strategy of soil microorganisms can be reflected by soil extracellular enzyme activity (EEA). However, there are few reports on the application of extracellular enzyme stoichiometry (EES) method to study the difference in microbial metabolic nutrient limitation between rhizosphere and bulk soil. Here, we choose the rhizosphere and bulk soils of Pinus sylvestris var. mongolica (P. sylvestris) plantations with five stand ages in the Mu Us sandy land, and analyzed the variation and differences of microbial metabolic limitation between rhizosphere and bulk soils with stand age by EES method. The results showed that the microbial metabolic C-limitation in the rhizosphere and bulk soil gradually increased with stand age. Almost all the vector angles were less than 45°, which indicated that the soil microbial metabolism was relatively limited by N rather than P. Furthermore, the microbial C- and N-limitation in rhizosphere soils were generally stronger than bulk soils. Soil physical properties (59.73%) explained most of the variations in soil EES based on the variation-partitioning analysis, followed by total nutrients (43.00%). The partial least squares path model suggested that the main driving factor for the variation of soil microbial metabolic C-limitation in the rhizosphere and bulk soils was physical properties, while the microbial N-limitation was for total nutrients. In general, the study emphasized the application of EES methods to assess the dynamic equilibrium between soil microbial resource acquisition and nutrient availability in desert ecosystems. These insights provide guidance for formulating afforestation strategies, such as nutrient management of sandy plantations.

Dynamic changes of soil microbial community in Pinus sylvestris var. mongolica plantations in the Mu Us Sandy Land.

Soil microbial communities maintain multiple ecosystem functions in terrestrial ecosystems. The response of soil microbial communities to vegetation restoration in desertification environments is still poorly understood. Therefore, the purpose of our study was to evaluate the dynamic changes of the soil microbial community during the growth of Pinus sylvestris var. mongolic (P. sylvestris) plantations. We collected soil samples from five P. sylvestris plantations with different stand age. High-throughput sequencing was performed to determine the microbial community structure. The dynamic relationship between soil microbial community and edaphic factors was analyzed using the co-occurrence network, mantel test and partial least squares path modeling. The results showed that the soil microbial alpha diversity and community structure were significantly various among the plantations (P < 0.001). The number of nodes and edges in microbial co-occurrence network gradually decreased and the interrelationships between species became weak with stand age. The Available phosphorus was the most significant factor affecting the structure of bacterial community (R2 = 0.952), while the total phosphorus was the most significant factor affecting the structure of fungal community (R2 = 0.745). However, soil moisture had no significant effect on the microbial community. pH (0.73) and available nitrogen (0.91) had the largest positive total effects on bacterial and fungal community, respectively. Stand age (-0.65) was an indirect factor with the largest negative total effects on the bacterial community. Therefore, we concluded that the soil microbial community was not limited by soil moisture during the natural restoration process of P. sylvestris plantations in the desertification environment and the phosphorus utilization efficiency played a leading role in shaping the soil microbial community.

Yield, Quality, and Nitrogen Leaching of Open-Field Tomato in Response to Different Nitrogen Application Measures in Northwestern China.

The overuse of fertilizers in open-field tomato leads to soil deterioration through nutrient leaching and increases the risk of agricultural non-point source contamination. Currently, the combined effects of different fertilization methods on soil nitrogen leaching and tomato production are still unclear. Therefore, the most effective fertilization method for open-field tomato should be discovered by examining how different fertilization methods affected tomato yield and quality, nitrogen use efficiency (NUE), and soil nitrogen leaching. Compared with CK (no fertilization), fertilization significantly increased the yield, total sugar (TS), total soluble solids (TSS), and vitamin C (vC) contents of fruits (p < 0.05), and OPT (optimal fertilization, controlled release nitrogen application, 240 kg ha-1) had the largest effect on increasing yield, quality, and net profit. However, when the fertilizer application rate reached 375 kg ha-1, these indices decreased. Nitrogen leaching concentrations, leaching amount, and titratable acids (TAs) increased with increased nitrogen application rates. Compared with other treatments, OPT reduced the total leaching amounts of total nitrogen (TN), nitrate nitrogen (NO3--N), and ammonia nitrogen (NH4+-N) by 30.09-51.79%, 24.89-50.03%, and 30-65%, respectively. Principal component analysis (PCA) showed that OPT achieved the highest overall score in terms of yield, quality, and nitrogen leaching conditions. The partial least squares path modeling (PLS-PM) further reveals that applications of high amounts of nitorigen have a positive effect on soil nitrogen leaching. The amount of nitrogen leaching vegetatively affects tomato yield and quality, while plant uptake of nitrogen positively affects tomato production. These findings confirm the importance of using controlled-release fertilizers and reducing nitrogen inputs to control nitrogen leaching and enhance open-field tomato yields.

Mechanism of groundwater recharge in the thick loess deposits by multiple environmental tracers.

The loess-covered region accounts for ∼10 % of global land surface. Because of dry climate and thick vadose zones, the subsurface water flux is low but the water storage is relatively large . As a result, the groundwater recharge mechanism is complicated and currently controversial (e.g., piston flow or dual mode with piston and preferential flow). Taking typical tablelands in China's Loess Plateau as example study area, this study aims to qualitatively and quantitively evaluate the forms/rates and controls of groundwater recharge considering space and time. We collected 498 precipitation, soil water and groundwater samples in 2014-2021 for hydrochemical and isotopic analysis (Cl-, NO3-, δ18O, δ2H, 3H and 14C). A graphical method was employed to determine appropriate model to correct 14C age. Dual model exhibited in the recharge: regional-scale piston flow and local-scale preferential flow. Piston flow dominated groundwater recharge with a proportion of 77 %-89 %. Preferential flow gradually declined with increasing water table depths, and the upper depth limit may be <40 m. The dynamics of tracers proved that mixing and dispersion effects of aquifers limited the ability of tracers to capture preferential flow at short-time scales. Long-term average potential recharge (79 ± 49 mm/year) was close to actual recharge (85 ± 41 mm/year) at the regional scale, indicating the hydraulic equilibrium between unsaturated and saturated zones. The thickness of vadose zone controlled recharge forms, and precipitation dominated the potential and actual recharge rates. Land-use change can also affect the potential recharge rates at point and field scales but maintain the dominance of piston flow. The revealed spatially-varied recharge mechanism is useful for groundwater modeling and the method can be referred for studying recharge mechanism in thick aquifers.

Changes in soil prokaryotic communities and nitrogen cycling functions along a groundwater table drawdown gradient in desert wetlands.

Desert wetlands are evolving into deserts by groundwater table (GWT) drawdown. However, the changes in microbial communities and functions during the GWT drawdown are unclear, which hinders the predictive power of biogeochemical processes across the desertification. Here, 16S rRNA gene sequencing, PICRUSt2 and qPCR were used to investigate soil prokaryotic diversity, composition and nitrogen cycling gene abundance at four vegetation types [flooded swamp (FS), drained swamp (DS), desert grassland (DG), and bare sandy land (BS)] along a GWT decline gradient in the Mu Us Desert, northern China. Results showed that prokaryotic Shannon and Chao1 indexes were significantly reduced at BS than those at FS (p < 0.05). Whereas no significant difference was observed between FS, DS and DG (p > 0.05). Distinct shifts in community composition were found along the GWT decline gradient. The dominant taxa gradually changed from obligate anaerobes and eutrophic microbes to facultative anaerobes, and finally to aerobic, oligotrophic and drought-tolerant microbes. Soil moisture was the most important factor in regulating the communities. In addition, GWT drawdown inhibited the relative abundance of genes involved in nitrogen fixation, assimilatory nitrite reduction, and nitrate oxidation, but enhanced the relative abundance of genes related to denitrification, assimilated nitrate reduction, ammonia oxidation and ammonification. Thus, GWT drawdown inhibits nitrogen input potential and exacerbates nitrogen loss potential. These results help in understanding the succession characteristics of desert wetland desertification.

Spatial and seasonal variability, control factors and health risk of fluoride in natural water in the Loess Plateau of China.

Hundreds of millions of people around the world are currently exposed to excessive amounts of fluoride (F-) in drinking water. Although the factors controlling the spatiotemporal distribution of F- contents have been analyzed, their contributions have rarely been quantified. In this study, 510 water samples were collected in the dry and wet seasons in China's Loess Plateau to investigate the spatial and seasonal distribution, controlling factors, and potential health risks of F- in natural water. High-F- waters were mainly distributed in valley areas of the Loess Plateau, and more severe fluoride pollution of streamwater and groundwater was found in the wet and dry seasons, respectively. Mineral dissolution, competitive adsorption, adsorption/desorption and cation exchange jointly controlled F- enrichment. Spatiotemporal distribution of high-F- levels was mainly determined by climate and streamwater-groundwater connectivity in the dry season, with contribution rates of 41.7% and 37.6%, and by terrain and anthropogenic activities in the wet season, with contribution rates and 49.9-55.6% and 30.7%, respectively. Fluoride in groundwater through oral intake posed the greatest health risks to infants, followed by children, teenagers and adults in the dry and wet seasons. This study provides a scientific basis for the effective management of high-F- water in arid regions.

Distribution, Origins and Hazardous Effects of Polycyclic Aromatic Hydrocarbons in Topsoil Surrounding Oil Fields: A Case Study on the Loess Plateau, China.

The Loess Plateau has one of the most vulnerable ecological environments in the world, but it also contains abundant oil and gas resources that are regularly exploited, which has resulted in serious environmental problems. Therefore, it is important to analyze the polycyclic aromatic hydrocarbons (PAHs) present in the topsoil of this region. The ∑16PAHs concentrations between 1980-1999 and 2000-2019 ranged from 1134.20-15871.04 and 1010.67-18,068.80 µg kg-1, with average values of 5021.30 and 5662.82 µg kg-1. All samples displayed heavy pollution levels according to European soil quality standards. In addition, among the measured physicochemical properties, the soil organic carbon (SOC) had the greatest influence on PAHs, while soil particle size distribution had the smallest effect. Source apportionment indicated that the two main sources were petroleum source (37.57%) and vehicular traffic source (25.88%). Lastly, an assessment of the carcinogenic risks illustrated that more focus should be placed on the dermal pathway in which the human body is exposed to soil PAHs. Overall, the carcinogenic risks in different populations did not exceed 10-4, but there was still a potential carcinogenic risk in some age groups, especially in adult women.

The WEPP Model Application in a Small Watershed in the Loess Plateau.

In the Loess Plateau, soil erosion has not only caused serious ecological and environmental problems but has also impacted downstream areas. Therefore, a model is needed to guide the comprehensive control of soil erosion. In this study, we introduced the WEPP model to simulate soil erosion both at the slope and watershed scales. Our analyses showed that: the simulated values at the slope scale were very close to the measured. However, both the runoff and soil erosion simulated values at the watershed scale were higher than the measured. At the slope scale, under different coverage, the simulated erosion was slightly higher than the measured. When the coverage is 40%, the simulated results of both runoff and erosion are the best. At the watershed scale, the actual annual runoff of the Liudaogou watershed is 83 m(3); sediment content is 0.097 t/m(3), annual erosion sediment 8.057 t and erosion intensity 0.288 t ha(-1) yr(-1). Both the simulated values of soil erosion and runoff are higher than the measured, especially the runoff. But the simulated erosion trend is relatively accurate after the farmland is returned to grassland. We concluded that the WEPP model can be used to establish a reasonable vegetation restoration model and guide the vegetation restoration of the Loess Plateau.

Effects of Pisha sandstone content on solute transport in a sandy soil.

In sandy soil, water, nutrients and even pollutants are easily leaching to deeper layers. The objective of this study was to assess the effects of Pisha sandstone on soil solute transport in a sandy soil. The miscible displacement technique was used to obtain breakthrough curves (BTCs) of Br(-) as an inert non-adsorbed tracer and Na(+) as an adsorbed tracer. The incorporation of Pisha sandstone into sandy soil was able to prevent the early breakthrough of both tracers by decreasing the saturated hydraulic conductivity compared to the controlled sandy soil column, and the impeding effects increased with Pisha sandstone content. The BTCs of Br(-) were accurately described by both the convection-dispersion equation (CDE) and the two-region model (T-R), and the T-R model fitted the experimental data slightly better than the CDE. The two-site nonequilibrium model (T-S) accurately fit the Na(+) transport data. Pisha sandstone impeded the breakthrough of Na(+) not only by decreasing the saturated hydraulic conductivity but also by increasing the adsorption capacity of the soil. The measured CEC values of Pisha sandstone were up to 11 times larger than those of the sandy soil. The retardation factors (R) determined by the T-S model increased with increasing Pisha sandstone content, and the partition coefficient (K(d)) showed a similar trend to R. According to the results of this study, Pisha sandstone can successfully impede solute transport in a sandy soil column.

Increasing aridity, temperature and soil pH induce soil C-N-P imbalance in grasslands.

Due to the different degrees of controls exerted by biological and geochemical processes, climate changes are suggested to uncouple biogeochemical C, N and P cycles, influencing biomass accumulation, decomposition and storage in terrestrial ecosystems. However, the possible extent of such disruption in grassland ecosystems remains unclear, especially in China's steppes which have undergone rapid climate changes with increasing drought and warming predicted moving forward in these dryland ecosystems. Here, we assess how soil C-N-P stoichiometry is affected by climatic change along a 3500-km temperate climate transect in Inner Mongolia, China. Our results reveal that the soil from more arid and warmer sites are associated with lower soil organic C, total N and P. The ratios of both soil C:P and N:P decrease, but soil C:N increases with increasing aridity and temperature, indicating the predicted decreases in precipitation and warming for most of the temperate grassland region could lead to a soil C-N-P decoupling that may reduce plant growth and production in arid ecosystems. Soil pH, mainly reflecting long-term climate change in our sites, also contributes to the changing soil C-N-P stoichiometry, indicating the collective influences of climate and soil type on the shape of soil C-N-P balance.

The combined effects of moss-dominated biocrusts and vegetation on erosion and soil moisture and implications for disturbance on the Loess Plateau, China.

Biological soil crusts (BSCs, or biocrusts) have important positive ecological functions such as erosion control and soil fertility improvement, and they may also have negative effects on soil moisture in some cases. Simultaneous discussions of the two-sided impacts of BSCs are key to the rational use of this resource. This study focused on the contribution of BSCs while combining with specific types of vegetation to erosion reduction and their effects on soil moisture, and it addressed the feasibility of removal or raking disturbance. Twelve plots measuring 4 m × 2 m and six treatments (two plots for each) were established on a 15° slope in a small watershed in the Loess Plateau using BSCs, bare land (as a control, BL), Stipa bungeana Trin. (STBU), Caragana korshinskii Kom. (CAKO), STBU planted with BSCs (STBU+BSCs) and CAKO planted with BSCs (CAKO+BSCs). The runoff, soil loss and soil moisture to a depth of 3 m were measured throughout the rainy season (from June to September) of 2010. The results showed that BSCs significantly reduced runoff by 37.3% and soil loss by 81.0% and increased infiltration by 12.4% in comparison with BL. However, when combined with STBU or CAKO, BSCs only made negligible contributions to erosion control (a runoff reduction of 7.4% and 5.7% and a soil loss reduction of 0.7% and 0.3%). Generally, the soil moisture of the vegetation plots was lower in the upper layer than that of the BL plots, although when accompanied with a higher amount of infiltration, this soil moisture consumption phenomenon was much clearer when combining vegetation with BSCs. Because of the trivial contributions from BSCs to erosion control and the remaining exacerbated consumption of soil water, moderate disturbance by BSCs should be considered in plots with adequate vegetation cover to improve soil moisture levels without a significant erosion increase, which was implied to be necessary and feasible.

Correction: Spatial Analysis of Soil Organic Carbon in Zhifanggou Catchment of the Loess Plateau.

[This corrects the article DOI: 10.1371/journal.pone.0083061.].

Spatial analysis of soil organic carbon in Zhifanggou catchment of the Loess Plateau.

Soil organic carbon (SOC) reflects soil quality and plays a critical role in soil protection, food safety, and global climate changes. This study involved grid sampling at different depths (6 layers) between 0 and 100 cm in a catchment. A total of 1282 soil samples were collected from 215 plots over 8.27 km(2). A combination of conventional analytical methods and geostatistical methods were used to analyze the data for spatial variability and soil carbon content patterns. The mean SOC content in the 1282 samples from the study field was 3.08 g · kg(-1). The SOC content of each layer decreased with increasing soil depth by a power function relationship. The SOC content of each layer was moderately variable and followed a lognormal distribution. The semi-variograms of the SOC contents of the six different layers were fit with the following models: exponential, spherical, exponential, Gaussian, exponential, and exponential, respectively. A moderate spatial dependence was observed in the 0-10 and 10-20 cm layers, which resulted from stochastic and structural factors. The spatial distribution of SOC content in the four layers between 20 and 100 cm exhibit were mainly restricted by structural factors. Correlations within each layer were observed between 234 and 562 m. A classical Kriging interpolation was used to directly visualize the spatial distribution of SOC in the catchment. The variability in spatial distribution was related to topography, land use type, and human activity. Finally, the vertical distribution of SOC decreased. Our results suggest that the ordinary Kriging interpolation can directly reveal the spatial distribution of SOC and the sample distance about this study is sufficient for interpolation or plotting. More research is needed, however, to clarify the spatial variability on the bigger scale and better understand the factors controlling spatial variability of soil carbon in the Loess Plateau region.

[Profile variability of soil properties in check dam on the Loess Plateau and its functions].

To understand the profile variability of soil properties of check dam and its possibility of engineering control over non-point source pollution, we used classical statistics to characterize the profile change of soil properties of a 5.20 m depth soil profile in the typical check dam on the Loess Plateau. The roles of check dam as organic carbon storage and available nutrients storage were discussed. The results showed that: 1) The bulk density and sand content of dam-head were lower than dam-tail, while, soil water content, silt, loam, organic carbon, available P, NO3(-) -N and NH4+ -N were higher than dam-tail. The bulk density for both dam-head and dam-tail showed weak variability while other properties showed moderate variability. All variables followed a normal distribution except sand in dam-head and soil moisture in dam-tail. 2) The change pattern of soil moisture on the soil profile for both dam-head and dam-tail was saw-tooth type. The change trends of soil organic carbon, available P and NH4+ -N were comparable to that of soil moisture. 3) The correlations among soil water content, organic carbon, bulk density, silt, loam, sand, available P, NO3(-) -N and NH4+ -N were significant (p < 0.05) except the relationship between bulk density and NO3(-) -N, NH4+ -N and relationship between available P and NH4+ -N in dam-tail. The positive or negative correlation of soil properties both in dam-head and dam-tail were coincident. 4) The check dam can be an important carbon storage on the Loess Plateau, and the organic carbon storage in dam-head was higher than dam-tail. The storage of organic carbon in 400-520 cm depth was the biggest for dam-head, in 0-100 cm depths for dam-tail. 5) The check dam is an enrichment sink of available nutrients. The storage of available P, NO3(-) -N and NH4+ -N in dam-head were higher than dam-tail, and the range of storage was: NH4+ -N > available P > NO3(-) -N. The coefficient of enrichment for NH4+ -N and NO3(-) -N were 1.132 and 1.956, respectively. 6) As the sink of soil nutrients, check dam has an important theoretical value for region carbon balance, ecological environment reconstruction and the effective control over non-point source pollution.