Riparian plant-soil-microbial C:N:P stoichiometry: are they conserved at plant functional group level?

As a consequence of the tight linkages between plants, soil, and microorganisms, we hypothesized the variations in plant species would change soil and microbial stoichiometry. Here, we examined the plant leaf carbon (C):nitrogen (N):phosphorus (P) ratios of nine species coming from three plant functional groups (PFGs) in the riparian zones of Hulunbuir steppe during near-peak biomass. The soil C:N:P, microbial biomass carbon (MBC):microbial biomass nitrogen (MBN), and extracellular enzyme's C:N:P were also assessed using the soils from each species. We found that plant tissue, soil nutrient, microbial, and enzyme activity stoichiometry significantly differed among different PFGs. Plant leaf and soil nutrient ratios tended to be similar (p > 0.05) between different species within the same PFGs. The variations in leaf C:N:P significantly correlated with the changes in soil C:N:P and MBC:MBN ratios. The homeostatic coefficients (H) < 1 suggested the relationships between plants and their resources C:N:P ratios might be non-homeostatic in the examined riparian zone. By assessing plant tissue and its soil nutrient stoichiometry, this study provided a perspective to understand the linkages of plant community, soil nutrient, and microbial characteristics.

Partial root-zone drying subsurface drip irrigation increased the alfalfa quality yield but decreased the alfalfa quality content.

Water shortage seriously restricts the development of grassland agriculture in arid land and dramatically impacts alfalfa (Medicago sativa L.) quality content and hay yield. Reasonable irrigation methods have the potential to enhance the alfalfa quality content, hay yield, and thus quality yield. Whether partial root-zone drying subsurface drip irrigation (PRDSDI) improves the alfalfa quality yield, quality content, and hay yield is still unknown compared with conventional subsurface drip irrigation (CSDI). The effects of PRDSDI compared with that of CSDI and the interaction with irrigation volume (10 mm/week, 20 mm/week, and 30 mm/week) on the alfalfa quality yield were investigated in 2017-2018 and explained the change in quality yield with the alfalfa quality content and hay yield. Here, the results showed that PRDSDI did not increase the alfalfa quality yield in 2 years. PRDSDI significantly increased acid detergent fiber by 13.3% and 12.2% in 2018 with 10-mm and 20-mm irrigation volumes and neutral detergent fiber by 16.2%, 13.2%, and 12.6% in 2017 with 10-mm, 20-mm, and 30-mm irrigation volumes, respectively. PRDSDI significantly decreased the crude protein by 5.4% and 8.4% in 2018 with 10-mm and 20-mm irrigation volumes and relative feed value by 15.0% with 20-mm irrigation volume in 2017 and 9.8% with 10-mm irrigation volume in 2018, respectively. In addition, PRDSDI significantly increased the alfalfa average hay yield by 49.5% and 59.6% with 10-mm and 20-mm irrigation volumes in 2018, respectively. Our results provide a counterexample for PRDSDI to improve crop quality. Although there was no significant improvement in average quality yield by PRDSDI, the positive impact of average hay yield on quality yield outweighed the negative impact of quality content. Thus, it has the potential to improve quality yields. The novel findings regarding the effects of PRDSDI on quality yield are potentially favorable for the forage feed value in water-limited areas.

Biotic interactions contribute more than environmental factors and geographic distance to biogeographic patterns of soil prokaryotic and fungal communities.

Recent studies have shown distinct soil microbial assembly patterns across taxonomic types, habitat types and regions, but little is known about which factors play a dominant role in soil microbial communities. To bridge this gap, we compared the differences in microbial diversity and community composition across two taxonomic types (prokaryotes and fungi), two habitat types (Artemisia and Poaceae) and three geographic regions in the arid ecosystem of northwest China. To determine the main driving factors shaping the prokaryotic and fungal community assembly, we carried out diverse analyses including null model, partial mantel test and variance partitioning analysis etc. The findings suggested that the processes of community assembly were more diverse among taxonomic categories in comparison to habitats or geographical regions. The predominant driving factor of soil microbial community assembly in arid ecosystem was biotic interactions between microorganisms, followed by environmental filtering and dispersal limitation. Network vertex, positive cohesion and negative cohesion showed the most significant correlations with prokaryotic and fungal diversity and community dissimilarity. Salinity was the major environmental variable structuring the prokaryotic community. Although prokaryotic and fungal communities were jointly regulated by the three factors, the effects of biotic interactions and environmental variables (both are deterministic processes) on the community structure of prokaryotes were stronger than that of fungi. The null model revealed that prokaryotic community assembly was more deterministic, whereas fungal community assembly was structured by stochastic processes. Taken together, these findings unravel the predominant drivers governing microbial community assembly across taxonomic types, habitat types and geographic regions and highlight the impacts of biotic interactions on disentangling soil microbial assembly mechanisms.

Effect of temperature variation on phosphorus flux at the sediment-water interface of the steppe wetlands.

Environmental factors are generally considered to be important factors affecting the release process of phosphorus (P) in sediments. However, little is known about the effect of temperature increased at first and then decreased with the season change on the P flux rate and flux amount at the sediment-water interface in the steppe wetlands. The effects of the temperature variation on P flux at the sediment-water interface in the steppe wetlands during the vegetation growing season under simulated wetland habitat were studied. The results showed that the release of P from sediments to overlying water was greatly affected by temperature changes. When the temperature rose, P was released from the sediment into the overlying water, while P was precipitated from the water into the sediment with the temperature dropped. During simulation period, the total P in water flux rates between sediment and overlying water (FP) was ranged from - 4.51 to 4.99 mg·m-2·day-1, while the dissolved reactive P in water flux rates between sediment and overlying water (FDP) was changed from - 5.37 to 5.14 mg·m-2·day-1. The FP and FDP were negatively correlated with the content of total P in water (WTP), dissolved reactive P in water (WDRP), pH of sediment (pH), and microbial biomass P (MBP), but increased with temperature (T), aluminum phosphate (Al.P), and occluded phosphate (Oc.P). The P flux rates were affected by temperature variation both directly and indirectly; the mechanism of how temperature influenced the fate of P in the wetland is still not clear. Therefore, the physicochemical properties and kinetic, thermodynamic, and microbiology characteristics should be combined together to clarify the mechanism in future research.

Meta-Analysis of the Effect of Saline-Alkali Land Improvement and Utilization on Soil Organic Carbon.

There is a large amount of saline-alkali land in China. Through the improvement and utilization of saline-alkali land to improve the carbon content in soil, it can not only become a reserve resource of cultivated land or grazing grassland, but also become an important land "carbon sink". In this study, we performed a comprehensive meta-analysis to identify the impact of improvement and utilization of saline-alkali soil on soil organic carbon (SOC) in China. Our results showed that the soil salt and alkali content in Heilongjiang Province and Jilin Province in China was the highest, with an SOC content between 3.05 and 17.8 g/kg and pH between 8.84 and 9.94. Among the five methods of reclamation, halophyte planting, fertilization, biochar and modifier application, only biochar and modifier application significantly increased the SOC content (p < 0.05). The content of SOC in saline-alkali soil was 2.9−6.3 g/kg before biochar application, and significantly increased to 6.2−13.05 g/kg after biochar application (p < 0.01). The SOC content was 3.05−8.12 g/kg before the application of the modifier, and significantly increased to 3.68−9 g/kg (p < 0.05) after the application of the modifier. After utilization and improvement of saline-alkali land, the total nitrogen, available phosphorus and available potassium also increased significantly (p < 0.05). This study provides a scientific basis for further understanding the improvement and utilization of saline-alkali land in China and its potential for increasing carbon sinks.

Solar park promoted microbial nitrogen and phosphorus cycle potentials but reduced soil prokaryotic diversity and network stability in alpine desert ecosystem.

Solar park (SP) is rapidly growing throughout the planet due to the increasing demand for low-carbon energy, which represents a remarkable global land-use change with implications for the hosting ecosystems. Despite dozens of studies estimating the environmental impacts of SP based on local microclimate and vegetation, responses of soil microbial interactions and nutrient cycle potentials remain poorly understood. To bridge this gap, we investigated the diversity, community structure, complexity, and stability of co-occurrence network and soil enzyme activities of soil prokaryotes and fungi in habitats of ambient, the first, and sixth year since solar park establishment. Results revealed different response patterns of prokaryotes and fungi. SP led to significant differences in both prokaryotic and fungal community structures but only reduced prokaryotic alpha diversity significantly. Co-occurrence network analysis revealed a unimodal pattern of prokaryotic network features and more resistance of fungal networks to environmental variations. Microbial nitrogen and phosphorus cycle potentials were higher in SP and their variances were more explained by network features than by diversity and environmental characteristics. Our findings revealed for the first time the significant impacts of SP on soil prokaryotic and fungal stability and functional potentials, which provides a microbial insight for impact evaluation and evidence for the optimization of solar park management to maximize the delivery of ecosystem services from this growing land use.

Organic carbon storage and its influencing factors under climate warming of sediments in steppe wetland, China.

The effect of climate warming on soil organic carbon (SOC) of sediment in wetlands is important for accurately projecting SOC content. Thus, understanding the mechanism influencing SOC content under climate warming is necessary. Field investigation and a laboratory incubation experiment were conducted in Hulunbeier steppe wetland during 2016 and 2017. Four types of wetland were selected to incubate with ambient temperature and temperature increased by 2.5 °C. The results showed that SOC content was negatively affected by temperature warming. The SOC content reduction in sediment caused by increasing temperature was ranged from - 2.34 to 39.52%. In addition, the content of sand, silt, total phosphorus (TP), calcium phosphate tribasic (Ca-P), total nitrogen (TN), and sediment moisture (MC) should be considered in models of SOC content in steppe wetland. However, it requires further validation, in particular how SOC content varies with warming temperatures, the duration of incubation, and other abiotic and biotic factors. These findings provide evidence that both climate warming and original characteristics of sediment can control the SOC storage dynamics in the steppe wetland. Graphical abstract.

Laboratory investigation of phosphorus loss with snowmelt and rainfall runoff from a Steppe wetland catchment.

Phosphorus (P) losses from terrestrial soils contribute to eutrophication of surface waters. As priority non-point source pollution ways, rainfall runoff (RS1) and snowmelt runoff (RS2) are the main carrier of P loss from terrestrial ecosystem. The aim of this study was to investigate the similarities and differences between P loss with RS1 and RS2 of the same soil type. Six types of soil were used in this experiment. Results have shown that 1), Different types of soil have different P loss with RS1 and RS2 under different slope, and the changes ranged from 0.003 to 0.370 mg L-1. 2), The effects of soil type, slope and runoff type on P loss with surface runoff was not independent, both individual effects of all factors and their interaction with the other two factors effected the P loss with runoff. 3), In our experiment, some soils showed no significant difference between P content in RS1 and RS2. In some soils, P loss with RS1 was higher than that with RS2 while the opposite conclusion was showed in Bog soil (BS) which with higher soil water content. 4), The P loss with RS1 and RS2 of different soils were both mainly affected by soil water content (SW), Olsen-P content (OP) and soil organic matter content (OM). These results can help us understand the P loss with different patterns of surface runoff better and are expected to provide pertinent opinions on the analysis of P loss with runoff and its influencing factors of grassland soils.

Models of turfgrass seed germination related to water content.

Turfgrasses have been widely utilized by humans to enhance the environment for more than several centuries. Seed germination is of great importance in the development of the turfgrass industry. In this study, the seed germination models of the responses of three types of turfgrasses to water were studied. The results indicated that irrigation water was mainly related to the variation of seed volume rather than a fixed value in this experiment. The maximum k value of tall fescue, Kentucky bluegrass and perennial ryegrass is 1.1548, 1.6946 and 4.0940, respectively. The optimal value of k is 1.0617 for tall fescue, 1.4610 for Kentucky bluegrass, and 1.6614 for perennial ryegrass. Perennial ryegrass seeds are more sensitive to water than those of tall fescue and Kentucky bluegrass, and this turfgrass is the fastest to reach the maximum value of the germination rate. Based on the results from the present experiment, the seed germination function can describe the response of turfgrass seed germination to external water content variation and their sensitivities. The function obtained could be used to perform quantitative studies on the dynamic changes of seed germination under different water conditions that will contribute to improved predictions of the optimal fitting curves of the germination over a range of water.

Dynamics of Phosphorus Content in the Water-Sediment Interface of Wetlands in a Mid-Temperature Steppe.

In order to reveal dynamics changes in phosphorus release from the sediments of wetlands in a mid-temperature steppe without external phosphorus input, the relationship between phosphorus content in the overlying water, sediment, and interstitial water was studied using the variables control method. The results showed that, during the incubation period, the content of total phosphorus in the overlying water and the content of total phosphorus in interstitial water both presented in the order of August > September > July. Furthermore, the correlation relationships between phosphorus content in the overlying water, total phosphorus in the interstitial water, and the ratios of sediment Olsen-P to total phosphorus in the interstitial water, indicated that Olsen-P might be a kind of release form of phosphorus - from the sediment to the overlying water of the wetlands in Hulunbeier steppe, without external phosphorus input.

Analysis of factors controlling soil phosphorus loss with surface runoff in Huihe National Nature Reserve by principal component and path analysis methods.

Phosphorus (P) loss with surface runoff accounts for the P input to and acceleration of eutrophication of the freshwater. Many studies have focused on factors affecting P loss with surface runoff from soils, but rarely on the relationship among these factors. In the present study, rainfall simulation on P loss with surface runoff was conducted in Huihe National Nature Reserve, in Hulunbeier grassland, China, and the relationships between P loss with surface runoff, soil properties, and rainfall conditions were examined. Principal component analysis and path analysis were used to analyze the direct and indirect effects on P loss with surface runoff. The results showed that P loss with surface runoff was closely correlated with soil electrical conductivity, soil pH, soil Olsen P, soil total nitrogen (TN), soil total phosphorus (TP), and soil organic carbon (SOC). The main driving factors which influenced P loss with surface runoff were soil TN, soil pH, soil Olsen P, and soil water content. Path analysis and determination coefficient analysis indicated that the standard multiple regression equation for P loss with surface runoff and each main factor was Y = 7.429 - 0.439 soil TN - 6.834 soil pH + 1.721 soil Olsen-P + 0.183 soil water content (r = 0.487, p < 0.01, n = 180). Soil TN, soil pH, soil Olsen P, and soil water content and the interactions between them were the main factors affecting P loss with surface runoff. The effect of physical and chemical properties of undisturbed soils on P loss with surface runoff was discussed, and the soil water content and soil Olsen P were strongly positive influences on the P loss with surface runoff.

Effects of Sediment Chemical Properties on Phosphorus Release Rates in the Sediment-Water Interface of the Steppe Wetlands.

Rising temperature causes a process of phosphorus release, which can be characterized well using phosphorus release rates (VP). The objective of the present study was to investigate the major factors affecting sediment phosphorus release rates through a wetland habitat simulation experiment. The results showed that the VP of different wetland sediments were different and changed with the order of W-R (river wetland) > W-L (lake wetland) > W-M (grassy marsh wetland) > W-A (reservoir wetland). The main driving factors which influenced sediment phosphorus flux velocity in the sediment-water interface were sediment B-SO42-, B-MBN and A-MBP content. Path analysis and determination coefficient analysis indicated the standard multiple regression equation for sediment phosphorus release rates in the sediment-water interface, and each main factor was Y = -0.105 + 0.096X1 + 0.275X2 - 0.010X3 (r = 0.416, p < 0.01, n = 144), where Y is sediment phosphorus release rates; X1 is sediment B-SO42- content; X2 is sediment B-MBN; and X3 is sediment A-MBP content. Sediment B-SO42-, B-MBN and A-MBP content and the interaction between them were the main factors affecting sediment phosphorus release rates in the sediment-water interface. Therefore, these results suggest that soil chemical properties and microbial activities likely play an important role in phosphorus release rates in the sediment-water interface. We hope to provide effective scientific management and control methods for relevant environmental protection departments.

Analysis of factors controlling sediment phosphorus flux potential of wetlands in Hulun Buir grassland by principal component and path analysis method.

Phosphorus (P) flux potential can predict the trend of phosphorus release from wetland sediments to water and provide scientific parameters for further monitoring and management for phosphorus flux from wetland sediments to overlying water. Many studies have focused on factors affecting sediment P flux potential in sediment-water interface, but rarely on the relationship among these factors. In the present study, experiment on sediment P flux potential in sediment-water interface was conducted in six wetlands in Hulun Buir grassland, China and the relationships among sediment P flux potential in sediment-water interface, sediment physical properties, and sediment chemical characteristics were examined. Principal component analysis and path analysis were used to discuss these data in correlation coefficient, direct, and indirect effects on sediment P flux potential in sediment-water interface. Results indicated that the major factors affecting sediment P flux potential in sediment-water interface were amount of organophosphate-degradation bacterium in sediment, Ca-P content, and total phosphorus concentrations. The factors of direct influence sediment P flux potential were sediment Ca-P content, Olsen-P content, SOC content, and sediment Al-P content. The indirect influence sediment P flux potential in sediment-water interface was sediment Olsen-P content, sediment SOC content, sediment Ca-P content, and sediment Al-P content. And the standard multiple regression describing the relationship between sediment P flux potential in sediment-water interface and its major effect factors was Y = 5.849 - 1.025X 1 - 1.995X 2 + 0.188X 3 - 0.282X 4 (r = 0.9298, p < 0.01, n = 96), where Y is sediment P flux potential in sediment-water interface, X 1 is sediment Ca-P content, X 2 is sediment Olsen-P content, X 3 is sediment SOC content, and X 4 is sediment Al-P content. Therefore, future research will focus on these sediment properties to analyze the interrelation among sediment properties factors, main vegetable factors, and environment factors which influence the sediment P flux potential in sediment-water interface.

Responses of Biogeochemical Characteristics and Enzyme Activities in Sediment to Climate Warming under a Simulation Experiment in Geographically Isolated Wetlands of the Hulunbuir Grassland, China.

Climate warming generates a tremendous threat to the stability of geographically-isolated wetland (GIW) ecosystems and changes the type of evaporation and atmospheric precipitation in a region. The intrinsic balance of biogeochemical processes and enzyme activity in GIWs may be altered as well. In this paper, we sampled three types of GIWs exhibiting different kinds of flooding periods. With the participation of real-time temperature regulation measures, we assembled a computer-mediated wetland warming micro-system in June 2016 to simulate climate situation of ambient temperature (control group) and two experimental temperature differences (+2.5 °C and +5.0 °C) following a scientific climate change circumstance based on daily and monthly temperature monitoring at a two-minutes scale. Our results demonstrate that the contents of the total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) in the warmed showed, roughly, a balance or a slight decrease than the control treatment. Warming obstructed the natural subsidence of sediment, but reinforced the character of the ecological source, and reduced the activity of urease (URE), but promoted the activity of alkaline phosphatase (AKP) and sucrase (SUC). Redundancy analysis showed that sucrase, urease, available phosphorus (AP), and pH were the major correlating factors under warming conditions in our research scope. Total organic carbon, total nitrogen, sucrase, catalase (CAT), and alkaline phosphatase were the principal reference factors to reflect the ambient temperature variations. Nutrient compositions and enzyme activities in GIW ecosystems could be reconstructed under the warming influence.