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Rhizoliths are the products of mineralization, petrification, or fossilization around and/or within plant roots. Among them, carbonate rhizoliths are the most common. Pristine carbonate rhizoliths with co-existing plant root relicts in the Tengeri Desert, NW China were studied, with a combination of intensive field observations and laboratory methods such as microscopy, scanning electronic microscopy, energy dispersive X-ray spectra, radiocarbon dating, and isotope mass spectrometer. The field observations revealed that the pristine rhizoliths are only present at the sites where Artemisia sphaerocephala Krasch are growing i.e. in swales among sand dunes. Soil moisture of the swales is the main controlling factor of rhizoliths formation. It is in turn affected by the soil physical properties, landscape position, and climate variability, consistent with the locations of sampled rhizoliths in the swales where Artemisia plants are mostly distributed. The 14C AMS dating indicated that the rhizoliths are much older (4000–5000 years) than their co-existing modern plant root relicts in agreement with field observations. Morphological, mineralogical and isotopic analyses revealed that carbon sources used for the rhizoliths formation were partially derived from decomposing plant roots but with significant contribution from dissolution of lithogenic carbonates. The calcium sources were suggested to be the in situ weathering of minerals (mostly lithogenic carbonates) and the pressure-dissolution of carbonates. Enough CO2 from the root decomposition have triggered carbonate accumulation around the root to form rhizoliths. Other minor chemical components of the root are S, N, P, which produce acidic water with the negative ions of SO42−, NO3–, PO43-, have also favored acidic soil environment and enhanced carbonate dissolution and mineral weathering. Redox environment around Artemisia roots were also observed to be a key factor for the pristine rhizolith formation. The pristine rhizoliths were preferentially formed in semi-closed redox condition with water nearly always available at intermediate depths. In addition, they were formed through carbonate epidiagenesis in shallow soils of the desert. Altogether, our results showed that the formation of the pristine rhizoliths was affected by the combination of several environmental factors. This led us to propose a conceptual model of rhizolith formation in desert soils.

Carbonate precipitation by soil microbes has generated major concern recently because of the potential for mitigating the challenge of increasing CO2 level; however, while desert soil microbes have the capacity of carbonate formation, it is unknown if they can deposit atmospheric carbon dioxide. We isolated soil bacteria from the Mu Us Desert in northern China to perform an experiment of carbonate formation by bacteria isolated from the soil. We detected the precipitates produced by the bacteria, and identified the carbon source of the carbonate precipitates using stable carbon isotopic tracing. Six bacterial strains, designated as strains 1–6, were isolated from the desert soil, including Arthrobacter sp., Rhodococcus sp., Planococcus sp., Streptomyces sp., Arthrobacter sp., and Microbacterium sp. We confirmed that all six species precipitated carbonate (vaterite and calcite) using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray. 13CO2 labeling showed that the abundance of 13C was significantly higher in carbonate precipitates, of which 1.03% was represented by atmospheric carbon; that is, the atmospheric CO2 participates in the formation of carbonate by desert soil bacteria. To our knowledge, this is the first study to demonstrate the occurrence of carbonate precipitation by these desert soil bacterial strains (strains 1, 2, 4, 5, and 6), which expands the current knowledge on carbonate-depositing bacteria in drylands. The carbon transformation process, from atmospheric CO2 to carbonate precipitates by soil bacteria, indicates a process of atmospheric CO2 deposition through microbial carbonate precipitation. Although under controlled conditions, this study provided an important insight supporting that drylands probably contribute strongly to global carbon processes via soil microbial activity.

Atmospheric carbon dioxide (CO2) absorption by desert soils has received increasing interests in recent years; however, the underlying physical and chemical mechanisms are not commonly acceptable. Here, we hypothesised that autotrophic carbon fixation of soil microbes contributes to this process. To test this postulate, we investigated the genomic and biochemical potential of autotrophic carbon fixation and traced atmospheric autotrophic carbon absorption using metagenomics and 13CO2 labelling approaches in the Mu Us Desert in northern China. More than 30000 genes involved in the six carbon fixation pathways (approximately 2% of the assembled metagenomes, in relative abundance) were found in the metagenome of the desert soil, and the relative abundance for genes encoding for the reductive citrate cycle was the highest among the six pathways. The main autotrophic microbes employing the six pathways belong to Actinobacteria, Proteobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes, Firmicutes, Thaumarchaeota, Nitrospirae, Planctomycetes, and Bacteroidetes, respectively. 13CO2 labelling revealed that the contents of microbially incorporated soil organic carbon (13C-SOC) and dissolved organic carbon were 0.572–1.45 mg kg−1 and 0.290–0.914 mg kg−1, respectively. Further, the 13C-SOC correlated with the relative abundance of genes of the total six pathways, reductive citrate cycle, 3-hydroxypropionate bi-cycle, and reductive acetyl-CoA pathway. Another in situ labelling experiment showed a significant increase in δ13C of SOC, and the incorporated carbon (13C) in SOC accounted for 3.85% of total atmospheric carbon absorption. These results showed that desert soil microbes containing genetic potential for autotrophic carbon fixation spread over a broad taxonomic range, and incorporated atmospheric carbon into organic components, which contributed to atmospheric carbon absorption. Although more research is required to accurately evaluate the portions of autotrophic carbon in the amount of atmospheric carbon absorption, the biotransformation of carbon from the atmosphere to soil via autotrophic carbon fixation represents a microbial pathway for persistent atmospheric CO2 absorption in desert soils, and further implicates an important carbon biochemical cycle for carbon accumulation in oligotrophic desert soils.

Enhanced atmospheric nitrogen deposition can greatly influence the soil nitrogen content and soil microorganisms in terrestrial ecosystems. However, there is a lack of knowledge about the response of soil microbes to changes in soil nitrogen, particularly in desert ecosystems. To address this, we used long-term artificial nitrogen addition to simulate atmospheric nitrogen deposition in a desert ecosystem in Ningxia, China. We adopted a high-throughput sequencing method (16S rRNA and ITS) to explore the effects of different amounts of atmospheric nitrogen deposition on soil microbial diversity and community structure. Six years of nitrogen addition (6 g N m−2 y−1) significantly increased the soil organic carbon content and the total soil nitrogen content (p 0.05). Further analysis revealed that nitrogen addition substantially reduced the number of positive correlations between soil microbial species and the number of core microbial species. Our results suggest that for desert ecosystems, at present, atmospheric nitrogen deposition may increase soil nitrogen use by some vegetation but it does not alleviate the competition for soil nitrogen between soil microbes and vegetation. Moreover, although enhanced atmospheric nitrogen deposition did not significantly change the soil microbial community structure, it may negatively affect the interactions among soil microbial species. This would hinder soil microbial adaptation to the changing environment and the development of desert ecosystems.

Modifications of rainfall patterns are expected to accompany global climate changes. It has been suggested that in dry regions changes in soil carbon emission induced by precipitation will affect soil carbon storage and atmospheric CO 2 concentration. However, our understanding of the responses of soil carbon emission [often as soil respiration ( R S )] to rainfall pulses is still limited regarding changes in soil respiration components [heterotrophic respiration ( R H ) and autotrophic respiration ( R A )] and under different precipitation patterns in arid and semiarid ecosystems. To evaluate the variations in soil carbon emission in response to rainfall pulses, we measured R S and its components in situ before/after precipitation in the Mu Us Desert, China. Rates of R S and its components were significantly enhanced by rainfall pulses, but gradually reverted thereafter. Moreover, the magnitudes of diel hysteresis for R S , R H , and R A with respect to soil temperature ( T S ) were modified by precipitation, and the effects of rainfall pulses on R S were influenced by antecedent soil water availability. In addition, the ratio of respiration components was changed by individual precipitation events, with an increase in the amount of each rainfall pulse causing a decrease in the proportion of R H to R S . Our results indicate that rainfall pulses in desert ecosystems have a major impact on soil carbon emission via changes in the magnitude and ratio of respiration components. We accordingly suggest that greater carbon emission and alterations in respiration components may occur with more extreme precipitation in desert ecosystems.

Microcystis aeruginosa is one of the most common algae found in eutrophicated water bodies. Alkaline phosphatase (AKP) can be produced by Microcystis aeruginosa to utilize organic phosphates under phosphorus deficiency stress, thereby AKP can be regarded as an important indicator for algal growth. Sulfur compounds are ubiquitous in waters, while investigation on the interactions between sulfur compounds and Microcystis aeruginosa is limited. In this work, we introduced 33 types of sulfur compounds to culture Microcystis aeruginosa, and the results demonstrated that algal growth is positively related to AKP activities. Toxicity of organic sulfur compounds was further evaluated using Toxicity Estimation Software Tool based on quantitative structure-activity relationship prediction. The algal growth results exhibited strong correlation to the toxicity endpoints suggesting the organic sulfur compounds inhibits the algal growth as toxic matters. K-means cluster analyses have been carried out subsequently via Python based on the results of algal growth and AKP activities of each sample and statistically, the sulfur compounds can be adequately clustered into 2 groups. According to clustering results, sulfonic acids exhibit low toxicity while sulfur amino acids can be considered as more toxic compounds. Graphical abstract Varied sulfur compounds (33 types) were investigated to find out the interactions between them and Microcystis aeruginosa, a common alga. K-means cluster and correlation analyses demonstrate that algal growth and alkaline phosphatase activities exhibited strong correlation to the predicted toxicity endpoints.

Anomalous CO2 absorption by soils in drylands has been reported worldwide. It has been reported that atmospheric carbon can first be converted into soil dissolved inorganic carbon, and then be carried and rapidly sequestered in groundwater through irrigation (artificial water input). Nevertheless, the deposition rate may be inapplicable in drylands without artificial water input, and the fate of the atmospheric carbon may also be different. Here, we assessed the amount of the local rainfall, evaluated the elapsed time of the natural water infiltration and the feldspar weathering-induced carbon consumption rate, and analysed the characteristics of soil inorganic carbon in the Mu Us Desert, northwest China, where we have previously studied abiotic atmospheric carbon absorption and the soil water input mainly derives from precipitation. The results showed that, even with extreme precipitation, the inputted water could not rapidly transport carbon to the first aquiclude, indicating that natural water input cannot deposit the majority of the atmospheric carbon absorbed by soil into groundwater. Further analysis indicated that feldspar weathering might be able to rapidly consume most of the atmospheric carbon. The consumed carbon would be converted into pedogenic carbonate and conserved in subsoil due to the transportation of inputted water. These results can provide a basis for determining the fate of atmospheric carbon absorbed by soils in drylands.

This study aimed at assessing whether patch type (i.e., under-shrub soil patch and inter-shrub soil patch) has an effect on soil microbes and how different shrub species altered the soil microbes through understanding soil microbial activity, biomass, and community structure. We characterized the soil microbes in under-shrub and inter-shrub soil patches in three shrublands (Artemisia ordosica, Salix psammophila, and Caragana microphylla), respectively, in the Mu Us Desert, China, using microbial activity indicators, chloroform fumigation-extraction analysis, and high-throughput 16S rRNA gene sequencing. Members of the phyla Proteobacteria, Actinobacteria, Acidobacteria, Planctomycetes, Bacteroidetes, Chloroflexi, Firmicutes, and Gemmatimonadetes were dominant. Inter-shrub soil patch differed from under-shrub soil patch in soil bacterial composition, microbial enzyme activity, and biomass, but not in diversity. Soil collected in A. ordosica shrubland exhibited the highest microbial enzyme activity, biomass, and diversity. Shrub species had significant effects on community structure, primarily the relative abundance of Proteobacteria, Actinobacteria, and Bacteroidetes. The results indicated that both shrub species and patch type had effects on soil microbial communities. In shrub-dominated desert ecosystems, spatial heterogeneity of soil nutrients and moisture might not be the main factors underlying variations in bacterial diversity. The different compositions of microbial communities in various shrublands provide a foundation for further research into the mechanisms of soil organic carbon accumulation.

It has been reported that soils in drylands can absorb CO2, although the patterns and mechanisms of such a process remain under debate. To address this, we investigated the relationships between soil CO2 flux and meteorological factors and soil properties in Northwest China to reveal the reasons for "anomalous" soil CO2 flux in a desert ecosystem. Soil CO2 flux increased significantly and exponentially with surficial turbulence at the diel scale under dry conditions (P

Revegetation with xerophilous shrubs is an effective approach to combat desertification in northwestern China; however, evaluation studies on fine-root properties of shrubs and soil organic carbon are limited. To gain a better understanding of revegetation practices, we investigated the vertical distribution of fine-root biomass, necromass, production, and effect on soil organic carbon (SOC) content in three shrub species (Salix psammophila, Hedysarum mongolicum, and Artemisia ordosica). In addition, we also estimated the fine root decomposition rate with litterbag techniques. The results showed that revegetation practices resulted in a significant increase in SOC content. Over a 10 year period of revegetation, the SOC content in S. psammophila, H. mongolicum, and A. ordosica plots increased by 0.87, 1.07, and 1.82 times, respectively, more than that in bare-land plot. Increase in total SOC content was mainly due to increase in light-fraction SOC, except for the A. ordosica plot. Variations in the short-term increase of SOC content after revegetations with the three shrubs on sand land might be explained by fine root decomposition rates, at least in part. A. ordosica may be a better species for SOC accumulation and sequestration in the study site. Additionally, fine-root biomass and production were not associated with more SOC content increase in shrub plots. The results suggest the mechanism of SOC accumulation and sequestration differed among shrub plots and highlight the effectiveness of different shrub species as revegetation materials in terms of SOC accumulation and sequestration.

Fine root (

Deserts may show strong downward CO2 fluxes and thus could be a significant carbon sink. However, this hypothesis has been strongly challenged because of the failure to determine both the reliability of flux measurements and the exact location of fixed carbon. In this study, we added 13CO2 to natural (unsterilized) soil in the Mu Us Desert in northern China and quantified the partitioning of added 13CO2 into soil solid and vapor phases. Results show that natural desert soil absorbed 13CO2 at a mean rate of 0.28 g m−2 d−1. Of the absorbed 13CO2, 7.1% was released over a 48 h period after 13CO2 feeding, 72.8% was stored in the soil solid phase, 0.0007% was found in the vapor phase, while 20.0% of the absorbed 13CO2 was undetected. These results indicate that undisturbed desert soils can absorb CO2 from the atmosphere, with the majority of fixed carbon conserved in the soil solid phase.

Soil CO2 flux is strongly influenced by precipitation in many ecosystem types, yet knowledge of the effects of precipitation on soil CO2 flux in semi-arid desert ecosystems remains insufficient, particularly for sandy soils. To address this, we investigated the response of sandy soil CO2 flux to rainfall pulses in a desert ecosystem in northern China during August–September 2011. Significant changes (P

Although abiotic CO2 exchange between soil and atmosphere is often observed in deserts, its contribution to carbon balance and accurate response to soil temperature (T s (°C)) are still uncertain. Abiotic soil carbon flux (ASCF) and T s were continuously measured on sterilized soil at 2-day intervals in the Mu Us Desert, northwestern China from May to October in 2012. The results showed that negative ASCF (CO2 absorption) occurred at night while positive ASCF (CO2 emission) during the day. Net CO2 sequestration during the 6 months totaled 64.37 g CO2 m−2 (0.09 μmol m−2 s−1). On the seasonal scale, ASCF was weakly correlated with T s (γ = 0.15, p

Soil respiration (Rs) is a major pathway for carbon cycling and is a complex process involving abiotic and biotic factors. Biological soil crusts (BSCs) are a key biotic component of desert ecosystems worldwide. In desert ecosystems, soils are protected from surface disturbance by BSCs, but it is unknown whether Rs is affected by disturbance of this crust layer. We measured Rs in three types of disturbed and undisturbed crusted soils (algae, lichen, and moss), as well as bare land from April to August, 2010, in Mu Us desert, northwest China. Rs was similar among undisturbed soils but increased significantly in disturbed moss and algae crusted soils. The variation of Rs in undisturbed and disturbed soil was related to soil bulk density. Disturbance also led to changes in soil organic carbon and fine particles contents, including declines of 60–70% in surface soil C and N, relative to predisturbance values. Once BSCs were disturbed,Q10increased. Our findings indicate that a loss of BSCs cover will lead to greater soil C loss through respiration. Given these results, understanding the disturbance sensitivity impact on Rs could be helpful to modify soil management practices which promote carbon sequestration.

It is generally considered that desert ecosystems release CO2 to the atmosphere, but recent studies in drylands have shown that the soil can absorb CO2 abiotically. However, the mechanisms and exact location of abiotic carbon absorption remain unclear. Here, we used soil sterilization, 13CO2 addition, and detection methods to trace 13C in the soil of the Mu Us Desert, northern China. After 13CO2 addition, a large amount of 13CO2 was absorbed by the sterilised soil, and 13C was found enriched both in the soil gaseous phase and dissolved inorganic carbon (DIC). Further analysis indicated that about 79.45% of the total 13C absorbed by the soil was trapped in DIC, while the amount of 13C in the soil gaseous phase accounted for only 0.22% of the total absorbed 13C. However, about 20.33% of the total absorbed 13C remained undetected. Our results suggest that carbonate dissolution might occur predominately, and the soil liquid phase might trap the majority of abiotically absorbed carbon. It is possible that the trapped carbon in the soil liquid phase leaches into the groundwater; however, further studies are required to support this hypothesis.

Although vegetation rehabilitation on semi-arid and arid regions may enhance soil carbon sequestration, its effects on soil carbon fractions remain uncertain. We carried out a study after planting Artemisia ordosica (AO, 17 years), Astragalus mongolicum (AM, 5 years), and Salix psammophila (SP, 16 years) on shifting sand land (SL) in the Mu Us Desert, northwest China. We measured total soil carbon (TSC) and its components, soil inorganic carbon (SIC) and soil organic carbon (SOC), as well as the light and heavy fractions within soil organic carbon (LF-SOC and HF-SOC), under the SL and shrublands at depths of 100 cm. TSC stock under SL was 27.6 Mg ha(-1), and vegetation rehabilitation remarkably elevated it by 40.6 Mgha(-1), 4.5 Mgha(-1), and 14.1 Mgha(-1) under AO, AM and SP land, respectively. Among the newly formed TSC under the three shrublands, SIC, LF-SOC and HF-SOC accounted for 75.0%, 10.7% and 13.1% for AO, respectively; they made up 37.0%, 50.7% and 10.6% for AM, respectively; they occupied 68.6%, 18.8% and 10.0% for SP, respectively. The accumulation rates of TSC within 0-100 cm reached 238.6 g m(-2) y(-1), 89.9 g m(-2) y(-1) and 87.9 g m(-2) y(-1) under AO, AM and SP land, respectively. The present study proved that the accumulation of SIC considerably contributed to soil carbon sequestration, and vegetation rehabilitation on shifting sand land has a great potential for soil carbon sequestration.

The responses of soil respiration to environmental conditions have been studied extensively in various ecosystems. However, little is known about the impacts of temperature and moisture on soils respiration under biological soil crusts. In this study, CO2 efflux from biologically-crusted soils was measured continuously with an automated chamber system in Ningxia, northwest China, from June to October 2012. The highest soil respiration was observed in lichen-crusted soil (0.93 ± 0.43 µmol m-2 s-1) and the lowest values in algae-crusted soil (0.73 ± 0.31 µmol m-2 s-1). Over the diurnal scale, soil respiration was highest in the morning whereas soil temperature was highest in the midday, which resulted in diurnal hysteresis between the two variables. In addition, the lag time between soil respiration and soil temperature was negatively correlated with the soil volumetric water content and was reduced as soil water content increased. Over the seasonal scale, daily mean nighttime soil respiration was positively correlated with soil temperature when moisture exceeded 0.075 and 0.085 m3 m-3 in lichen- and moss-crusted soil, respectively. However, moisture did not affect on soil respiration in algae-crusted soil during the study period. Daily mean nighttime soil respiration normalized by soil temperature increased with water content in lichen- and moss-crusted soil. Our results indicated that different types of biological soil crusts could affect response of soil respiration to environmental factors. There is a need to consider the spatial distribution of different types of biological soil crusts and their relative contributions to the total C budgets at the ecosystem or landscape level.