Vegetation restoration altered the soil organic carbon composition and favoured its stability in a Robinia pseudoacacia plantation.

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration.

Environmental stress-discriminatory taxa are associated with high C and N cycling functional potentials in dryland grasslands.

Increasing environmental stress strongly affects soil microbial communities, but the responses of the microbial assembly and the functional potential of the dominant microbial community in the presence of environmental stress in drylands are still poorly understood. Here, we undertook a broad appraisal of the abundance, diversity, similarity, community assembly, network properties and functions of soil microbiomes in 82 dryland grasslands along environmental gradients. We found that the bacterial and fungal diversity and community similarity showed different sensitivities to environmental stress (decreased mean annual precipitation (MAP) and soil nutrient levels and increased soil pH), and MAP was the most important factor influencing microbial community patterns. In addition, the dominant subcommunity of both bacteria and fungi was more sensitive to environmental stress than the nondominant subcommunity. Although increasing environmental stress decreased microbial phylogenetic clustering, it had no effects on the stochastic and deterministic assembly process balance. Moreover, we identified 101 bacterial and 34 fungal environmental stress-discriminatory taxa that were sensitive to environmental stress, and these bacterial markers showed a high correlation with the abundance of carbon (C) and nitrogen (N) cycling-related genes, whereas the taxa classified as connectors in the network were mainly correlated with C degradation genes. Our study shows that the different responses of bacteria and fungi to environmental stress bring challenges to predicting microbial function, but a relatively small number of taxa play an important role in driving C and N cycling-related functional genes, indicating that identifying an organism's phenotypic characteristics or traits of key taxa may improve our knowledge of the microbial response to ongoing global changes.

Aromatic compounds releases aroused by sediment resuspension alter nitrate transformation rates and pathways during aerobic-anoxic transition.

Aromatic compounds (ACs) releases aroused by sediment resuspension would certainly change the concentrations of suspended sediment (SPS) and organic carbon, which may alter nitrate-N transformation during aerobic-anoxic transition. To prove this, three typical ACs (aniline, nitrobenzene, and methylbenzene) with different octanol-water partition coefficients (Kow) were selected to investigate the effects of ACs releases aroused by sediment resuspension on nitrate-N transformation during aerobic-anoxic transition. ACs releases aroused by sediment resuspension accelerated nitrate-N transformation and enhanced the potential for dissimilatory nitrate reduction to ammonium (DNRA), compared to that without sediment resuspension. With sediment resuspension, methylbenzene releases affected nitrate-N transformation rates and pathways more significantly than aniline and nitrobenzene releases. Microbial analysis indicated that sediment resuspension created complicated microbial co-occurrence networks and changed the associations among bacteria; dominant bacteria abundance varied with different ACs releases. Further analysis revealed that ACs distributed in SPS, which increased with logKow, indirectly affected nitrate-N transformation rates and pathways via altering dominant bacteria abundance and electron transport system activity (ETSA). Especially, ETSA, which was positively associated with ACs distributed in SPS, affected nitrate-N transformation most directly. Overall, ACs release fate played important roles in nitrate-N transformation, causing ammonia-N retention and alterations in nitrogen cycle during aerobic-anoxic transition.

Differential microbial assembly processes and co-occurrence networks in the soil-root continuum along an environmental gradient.

Microorganisms of the soil-root continuum play key roles in ecosystem function. The Loess Plateau is well known for its severe soil erosion and thick loess worldwide, where mean annual precipitation (MAP) and soil nutrients decrease from the southeast to the northwest. However, the relative influence of environmental factors on the microbial community in four microhabitats (bulk soil, rhizosphere, rhizoplane, and endosphere) in the soil-root continuum along the environmental gradient in the Loess Plateau remains unclear. In this study, we investigated 82 field sites from warm-temperate to desert grasslands across the Loess Plateau, China, to assess the bacterial diversity, composition, community assembly, and co-occurrence networks in the soil-root continuum along an environmental gradient using bacterial 16S recombinant DNA amplicon sequencing. We discovered that the microhabitats explained the largest source of variations in the bacterial diversity and community composition in this region. Environmental factors (e.g., MAP, soil organic carbon, and pH) impacted the soil, rhizosphere, and rhizoplane bacterial communities, but their effects on the bacterial community decreased with increased proximity to roots from the soil to the rhizoplane, and the MAP enlarged the dissimilarity of microbial communities from the rhizosphere and rhizoplane to bulk soil. Additionally, stochastic assembly processes drove the endosphere communities, whereas the soil, rhizosphere, and rhizoplane communities were governed primarily by the variable selection of deterministic processes, which showed increased importance from warm-temperate to desert grasslands. Moreover, the properties of the microbial networks in the rhizoplane community indicate more stable networks in desert grasslands, likely conferring the resistance of microbial communities in higher stress environments. Collectively, our results showed that the bacterial communities in the soil-root continuum had different sensitivities and assembly mechanisms along an environmental gradient. These patterns are shaped simultaneously by the intertwined dimensions of proximity to roots and environmental stress change in the Loess Plateau.

Contrasting dynamics of leaf potential and gas exchange during progressive drought cycles and recovery in Amorpha fruticosa and Robinia pseudoacacia.

Leaf gas exchange is closely associated with water relations; however, less attention has been given to this relationship over successive drought events. Dynamic changes in gas exchange and water potential in the seedlings of two woody species, Amorpha fruticosa and Robinia pseudoacacia, were monitored during recurrent drought. The pre-dawn leaf water potential declined in parallel with gas exchange in both species, and sharp declines in gas exchange occurred with decreasing water potential. A significant correlation between pre-dawn water potential and gas exchange was observed in both species and showed a right shift in R. pseudoacacia in the second drought. The results suggested that stomatal closure in early drought was mediated mainly by elevated foliar abscisic acid (ABA) in R. pseudoacacia, while a shift from ABA-regulated to leaf-water-potential-driven stomatal closure was observed in A. fruticosa. After re-watering, the pre-dawn water potential recovered quickly, whereas stomatal conductance did not fully recover from drought in R. pseudoacacia, which affected the ability to tightly control transpiration post-drought. The dynamics of recovery from drought suggest that stomatal behavior post-drought may be restricted mainly by hydraulic factors, but non-hydraulic factors may also be involved in R. pseudoacacia.

Contrasting responses of leaf stomatal characteristics to climate change: a considerable challenge to predict carbon and water cycles.

Stomata control the cycling of water and carbon between plants and the atmosphere; however, no consistent conclusions have been drawn regarding the response of stomatal frequency to climate change. Here, we conducted a meta-analysis of 1854 globally obtained data series to determine the response of stomatal frequency to climate change, which including four plant life forms (over 900 species), at altitudes ranging from 0 to 4500 m and over a time span of more than one hundred thousand years. Stomatal frequency decreased with increasing CO2 concentration and increased with elevated temperature and drought stress; it was also dependent on the species and experimental conditions. The response of stomatal frequency to climate change showed a trade-off between stomatal control strategies and environmental factors, such as the CO2 concentration, temperature, and soil water availability. Moreover, threshold effects of elevated CO2 and temperature on stomatal frequency were detected, indicating that the response of stomatal density to increasing CO2 concentration will decrease over the next few years. The results also suggested that the stomatal index may be more reliable than stomatal density for determination of the historic CO2 concentration. Our findings indicate that the contrasting responses of stomata to climate change bring a considerable challenge in predicting future water and carbon cycles.

A meta-analysis of leaf gas exchange and water status responses to drought.

Drought is considered to be one of the most devastating natural hazards, and it is predicted to become increasingly frequent and severe in the future. Understanding the plant gas exchange and water status response to drought is very important with regard to future climate change. We conducted a meta-analysis based on studies of plants worldwide and aimed to determine the changes in gas exchange and water status under different drought intensities (mild, moderate and severe), different photosynthetic pathways (C3 and C4) and growth forms (herbs, shrubs, trees and lianas). Our results were as follows: 1) drought negatively impacted gas exchange and water status, and stomatal conductance (gs) decreased more than other physiological traits and declined to the greatest extent in shrubs and C3 plants. Furthermore, C4 plants had an advantage compared to C3 plants under the same drought conditions. 2) The decrease in gs mainly reduced the transpiration rate (Tr), and gs could explain 55% of the decrease in the photosynthesis (A) and 74% of the decline in Tr. 3). Finally, gas exchange showed a close relationship with the leaf water status. Our study provides comprehensive information about the changes in plant gas exchange and water status under drought.

The Characters of Dry Soil Layer on the Loess Plateau in China and Their Influencing Factors.

A dry soil layer (DSL) is a common soil desiccation phenomenon that generally forms at a particular depth in the soil profile because of climatic factors and poor land management, and this phenomenon can influence the water cycle and has been observed on the Loess Plateau of China and other similar regions around the world. Therefore, an investigation of the DSL formation depth (DSLFD), thickness (DSLT) and mean water content (MWDSL) on the Loess Plateau can provide valuable information. This paper synthesized 69 recent publications (1,149 observations of DSLs from 73 sites) that focused on DSLs in this region, and the results indicated that DSLs are significantly affected by climatic and vegetation factors. The mean annual precipitation had a significant positive relationship with DSLFD (p = 0.0003) and MWDSL (p<0.0001) and a negative relationship with DSLT (p = 0.0071). Crops had the lowest DSLT and highest MWDSL values compared with other vegetation types. A significant correlation was observed between the occurrence of DSLs and the years since planting for grasses, shrubs, trees and orchards, and the severity of DSLs increased with increasing planting years and wheat yield. Our results suggest that optimizing land-use management can mitigate DSL formation and development on the Loess Plateau. Understanding the dominant factors affecting DSLs will provide information for use in guidelines for the sustainable development of economies and restoration of natural environments experiencing water deficiencies.

Net ammonium and nitrate fluxes in wheat roots under different environmental conditions as assessed by scanning ion-selective electrode technique.

Wheat is one of the most important food crops in the world, its availability affects global food security. In this study, we investigated variations in NH4(+) and NO3(-) fluxes in the fine roots of wheat using a scanning ion-selective electrode technique in the presence of different nitrogen (N) forms, N concentrations, and pH levels as well as under water stress. Our results show that the fine roots of wheat demonstrated maximum NH4(+) and NO3(-) influxes at 20 mm and 25 mm from the root tip, respectively. The maximal net NH4(+) and NO3(-) influxes were observed at pH 6.2 in the presence of a 1/4 N solution. We observed N efflux in two different cultivars following the exposure of roots to a 10% PEG-6000 solution. Furthermore, the drought-tolerant cultivar generally performed better than the drought-intolerant cultivar. Net NH4(+) and NO3(-) fluxes may be determined by plant growth status, but environmental conditions can also affect the magnitude and direction of N flux. Interestingly, we found that NO3(-) was more sensitive to environmental changes than NH4(+). Our results may be used to guide future hydroponic experiments in wheat as well as to aid in the development of effective fertilisation protocols for this crop.

Water consumption characteristics and water use efficiency of winter wheat under long-term nitrogen fertilization regimes in northwest China.

Water shortage and nitrogen (N) deficiency are the key factors limiting agricultural production in arid and semi-arid regions, and increasing agricultural productivity under rain-fed conditions often requires N management strategies. A field experiment on winter wheat (Triticum aestivum L.) was begun in 2004 to investigate effects of long-term N fertilization in the traditional pattern used for wheat in China. Using data collected over three consecutive years, commencing five years after the experiment began, the effects of N fertilization on wheat yield, evapotranspiration (ET) and water use efficiency (WUE, i.e. the ratio of grain yield to total ET in the crop growing season) were examined. In 2010, 2011 and 2012, N increased the yield of wheat cultivar Zhengmai No. 9023 by up to 61.1, 117.9 and 34.7%, respectively, and correspondingly in cultivar Changhan No. 58 by 58.4, 100.8 and 51.7%. N-applied treatments increased water consumption in different layers of 0-200 cm of soil and thus ET was significantly higher in N-applied than in non-N treatments. WUE was in the range of 1.0-2.09 kg/m3 for 2010, 2011 and 2012. N fertilization significantly increased WUE in 2010 and 2011, but not in 2012. The results indicated the following: (1) in this dryland farming system, increased N fertilization could raise wheat yield, and the drought-tolerant Changhan No. 58 showed a yield advantage in drought environments with high N fertilizer rates; (2) N application affected water consumption in different soil layers, and promoted wheat absorbing deeper soil water and so increased utilization of soil water; and (3) comprehensive consideration of yield and WUE of wheat indicated that the N rate of 270 kg/ha for Changhan No. 58 was better to avoid the risk of reduced production reduction due to lack of precipitation; however, under conditions of better soil moisture, the N rate of 180 kg/ha was more economic.