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Salinization poses great threats to soil fungal communities that would cause the losses of ecosystems services. Soil fungal communities are composed of different functional guilds such as saprotrophic, symbiotrophic, and pathotrophic fungi, and each guild includes many rare taxa and a few abundant taxa. Despite of low abundance, rare taxa may be crucial in determining the responses of entire soil fungal communities to salinization. However, it remains poorly understood how rare taxa mediate the impacts of soil salinization on soil fungal community structure. Here, we took advantage of a salinity gradient in a desert ecosystem ranging from 0.60 to 31.09 g kg−1 that was created by a 12-year saline-water irrigation and assessed how the rare vs. abundant taxa of soil saprotrophic, symbiotrophic, and pathotrophic fungi respond to soil salinization through changes in the community biodiversity and composition. We found that the rare taxa of soil saprotrophic, symbiotrophic, and pathographic fungi were more sensitive to changes in soil salinity compared to the abundant taxa. In addition, the community composition of rare taxa of the saprotrophic and pathotrophic fungi not the symbiotrophic fungi was positively associated with soil salinity change. However, the symbiotrophic fungi showed greater variations in the species richness along the salinity gradient. These findings highlight the importance to differentiate rare taxa in predicting how the biodiversity and functional groups of soil fungal communities respond to soil salinization. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC), as a crucial measure of soil quality, is typically low in arid regions due to salinization, which is a global issue. How soil organic carbon changes with salinization is not a simple concept, as high salinity simultaneously affects plant inputs and microbial decomposition, which exert opposite effects on SOC accumulation. Meanwhile, salinization could affect SOC by altering soil Ca2+ (a salt component), which stabilizes organic matter via cation bridging, but this process is often overlooked. Here, we aimed to explore i) how soil organic carbon changes with salinization induced by saline-water irrigation and ii) which process drives soil organic carbon content with salinization, plant inputs, microbial decomposition, or soil Ca2+ level. To this end, we assessed SOC content, plant inputs represented by aboveground biomass, microbial decomposition revealed by extracellular enzyme activity, and soil Ca2+ along a salinity gradient (0.60–31.09 g kg−1) in the Taklamakan Desert. We found that, in contrast to our prediction, SOC in the topsoil (0–20 cm) increased with soil salinity, but it did not change with the aboveground biomass of the dominant species (Haloxylon ammodendron) or the activity of three carbon-cycling relevant enzymes (β-glucosidase, cellulosidase, and N-acetyl-beta-glucosaminidase) along the salinity gradient. Instead, SOC changed positively with soil exchangeable Ca2+, which increased linearly with salinity. These results suggest that soil organic carbon accumulation could be driven by increases in soil exchangeable Ca2+ under salinization in salt-adapted ecosystems. Our study provides empirical evidence for the beneficial impact of soil Ca2+ on organic carbon accumulation in the field under salinization, which is apparent and should not be disregarded. In addition, the management of soil carbon sequestration in salt-affected areas should be taken into account by adjusting the soil exchangeable Ca2+ level. [Display omitted] • Soil organic carbon increased with salinity after 12 years of saline-water irrigation. • Soil organic carbon increased positively with soil exchangeable Ca2+. • Soil exchangeable Ca2+ increased with salinity. • Hydrolytic enzyme activities did not vary significantly with salinity. • Aboveground biomass of Haloxylon ammodendron did not change with salinity. [ABSTRACT FROM AUTHOR]

The roles of soil fungal diversity and community composition in regulating soil respiration when above‐ and below‐ground plant carbon (C) inputs are excluded remain unclear. In the present study, we aimed to examine the following. (a) How does the exclusion of above‐ and below‐ground plant C inputs affect soil respiration and soil fungi singly and in combination? (b) Are changes in soil fungal diversity aligned with changes in soil respiration? A field experiment with manipulation of plant C inputs was established in a subtropical forest in southwest China in 2004 with litter removal and tree stem‐girdling to exclude inputs of the above‐ and below‐ground plant C, respectively. In 2009, we measured the rates of soil respiration with an infrared gas analyser and soil fungal community structure using Illumina sequencing. We found that the rates of soil respiration were reduced significantly by litter removal and girdling, by similar magnitudes. However, they were not decreased further by the combination of these two treatments compared to either treatment alone. In contrast, litter removal increased the diversity of soil fungal communities, whereas girdling decreased the abundance of symbiotrophic fungi but increased the abundance of saptrotrophic and pathotrophic fungi. These changes in soil fungal community might initiate CO2 emission from soil C decomposition, offsetting further decline in soil respiration when plant C inputs are excluded. These results revealed that the exclusion of the above‐ and below‐ground plant C inputs led to contrasting soil fungal communities but similar soil function. Our findings suggest that both above‐ and below‐ground plant C are important in regulating soil respiration in subtropical forests, by limiting substrates for soil fungal growth and altering the diversity and composition of the soil fungal community. Highlights: Litter removal and girdling decreased soil respiration by similar magnitudesThe combination of litter removal and girdling did not further decrease soil respirationLitter removal significantly increased species richness of soil fungal communitiesGirdling changed the abundance of functional guilds of soil fungal communities [ABSTRACT FROM AUTHOR]

Soil organic carbon ( SOC) dynamics are regulated by the complex interplay of climatic, edaphic and biotic conditions. However, the interrelation of SOC and these drivers and their potential connection networks are rarely assessed quantitatively. Using observations of SOC dynamics with detailed soil properties from 90 field trials at 28 sites under different agroecosystems across the Australian cropping regions, we investigated the direct and indirect effects of climate, soil properties, carbon (C) inputs and soil C pools (a total of 17 variables) on SOC change rate ( r C, Mg C ha−1 yr−1). Among these variables, we found that the most influential variables on r C were the average C input amount and annual precipitation, and the total SOC stock at the beginning of the trials. Overall, C inputs (including C input amount and pasture frequency in the crop rotation system) accounted for 27% of the relative influence on r C, followed by climate 25% (including precipitation and temperature), soil C pools 24% (including pool size and composition) and soil properties (such as cation exchange capacity, clay content, bulk density) 24%. Path analysis identified a network of intercorrelations of climate, soil properties, C inputs and soil C pools in determining r C. The direct correlation of r C with climate was significantly weakened if removing the effects of soil properties and C pools, and vice versa. These results reveal the relative importance of climate, soil properties, C inputs and C pools and their complex interconnections in regulating SOC dynamics. Ignorance of the impact of changes in soil properties, C pool composition and C input (quantity and quality) on SOC dynamics is likely one of the main sources of uncertainty in SOC predictions from the process-based SOC models. [ABSTRACT FROM AUTHOR]

Improving predictions of soil organic carbon (SOC) dynamics by multi-compartment models requires validation of turnover times of different SOC pools. Techniques such as laboratory incubation and isotope analysis have been adopted to estimate C turnover times, yet no studies have systematically compared these techniques and assessed the uncertainties associated with them. Here, we tested whether C turnover times of soil fractions were biased by methodology, and how this changed across soil particle sizes and ecosystems. We identified 52 studies that quantified C turnover times in different soil particles fractionated either according to aggregate size ( e.g. , macro- versus micro-aggregates) or according to soil texture ( e.g. , sand versus silt versus clay). C turnover times of these soil fractions were estimated by one of three methods: laboratory incubation (16 studies), δ 13 C shift due to C 3 –C 4 vegetation change (25 studies), and 14 C dating (19 studies). All methods showed that C turnover times of soil fractions generally increase with decreasing soil particle size. However, estimates of C turnover times within soil fractions differed significantly among methods, with incubation estimating the shortest turnover times and 14 C the longest. The short C turnover times estimated by incubation are likely due to optimal environmental conditions for microbial decomposition existing in these studies, which is often a poor representation of field conditions. The 13 C method can only be used when documenting a successive C 3 versus C 4 vegetation shift. C turnover times estimated by 14 C were systematically higher than those estimated by 13 C, especially for fine soil fractions ( i.e. , silt and clay). Overall, our findings highlight methodological uncertainties in estimating C turnover times of soil fractions, and correction factors should be explored to account for methodological bias when C turnover times estimated from different methods are used to parameterize soil C models. [ABSTRACT FROM AUTHOR]

Saline-sodic soils cover ∼10% of the global land surface and deliver various ecosystem services to human society in the arid/semiarid regions. Flue gas desulfurization gypsum (FGDG), a byproduct from coal-fired power plants, is widely used to ameliorate saline-sodic soils. Here, we aimed to quantify the impacts of FGDG application on multiple soil functions across climatic conditions, management practices, and soil types, and to explore how FGDG application affects plant productivity. We conducted a meta-analysis by compiling 2658 pairs of data points with and without FGDG application from 59 locations across China. We found that FGDG application significantly increased crop yield by 91.2% ± 22.5% (mean ± 95% CI) regardless of local climate and soil type, and improved soil quality by reducing soil exchangeable sodium percentage (ESP) by 37.4% ± 9.6% and pH by 8.1% ± 1.7%. Increases in soil productivity were strongly correlated with decreases in soil ESP and pH, suggesting that increases in soil productivity were due to alleviated stress for plant growth. Meanwhile, some heavy elements (e.g., Hg and Ni) increased after FGDG application, likely imposing threats to soil health. Overall, the FGDG application is effective in improving the quality and productivity of saline-sodic soils across China. Our findings suggest that simultaneous assessment of changes in soil water (e.g., water holding capacity and infiltration), nutrient transformation, soil organic matter dynamics, and microbial communities helps disentangle mechanisms that are responsible for optimizing ecosystem service provided by saline-sodic soils after FGDG amendment application. • FGDG application significantly reduces soil pH and salinity in different magnitudes. • FGDG application significantly increases crop yield and plant seedling emergence. • Enhanced plant productivity after FGDG application is due to reduced soil salinity. • FGDG application may posit negative environmental impacts of saline-sodic soils. [ABSTRACT FROM AUTHOR]

Small changes in soil organic carbon (SOC) may have great influences on the climate-carbon (C) cycling feedback. However, there are large uncertainties in predicting the dynamics of SOC in soil profiles at the global scale, especially on the role of soil microbial biomass in regulating the vertical distribution of SOC. Here, we developed a database of soil microbial biomass carbon (SMBC), SOC, and soil microbial quotient (SMQ = SMBC/SOC) from 289 soil profiles globally, as well as climate, ecosystem types, and edaphic factors associated with these soil profiles. We assessed the vertical distribution patterns of SMBC and SMQ and the contributions of climate, ecosystem type, and edaphic condition to their vertical patterns. Our results showed that SMBC and SMQ decreased exponentially with soil depth, especially within the 0–40 cm soil depth. SOC also decreased exponentially with depth but in different magnitudes compared to SMBC and SMQ. Edaphic factors (e.g. , soil clay content and C/N ratio) had the strongest control on the vertical distributions of SMBC and SMQ, probably by mediating substrate and nutrient supplies for microbial growth in soils. Mean annual temperature and ecosystem types (i.e. , forests, grasslands, and croplands) had weak influences on SMBC and SMQ. In contrast, the vertical distribution of SOC was significantly affected by climate and edaphic factors. Climate and ecosystem types likely simultaneously affected multiple factors that control SMBC, such as the distribution of soil clay and nutrients along soil profiles. Overall, our data synthesis provides quantitative information of how SMBC, SMQ, and SOC changed along soil profiles at large spatial scales and identifies important factors that influence their vertical distributions. The findings can help improve the prediction of C cycling in terrestrial ecosystems by incorporating the contribution of soil microbes in Earth system models. Image 1 • A comprehensive dataset was collected for soil organic C and microbial biomass along soil profiles. • Vertical distributions and main driving factors were assessed. • Soil microbial biomass and its ratio to soil organic C decreased with soil depth at large spatial scales. • Edaphic not climatic conditions drove vertical distributions of soil microbial biomass. • This synthesis highlights the importance of microbial roles in soil C cycling. [ABSTRACT FROM AUTHOR]