Duration and frequency of drainage and flooding events interactively affect soil biogeochemistry and N flux in subtropical peat soils

Interaction between root exudates and PFOS mobility: Effects on rhizosphere microbial health in wetland ecosystems

Perfluorooctanesulfonate (PFOS), a persistent organic pollutant, poses significant ecological risks. This study investigates the effects of PFOS on rhizosphere microbial communities of two wetland plants, Lythrum salicaria (LS) and Phragmites communis (PC). We conducted microcosm experiments to analyze the physiological status of soil microbes under varying PFOS concentrations and examined the role of root exudates in modulating PFOS mobility. Flow cytometry and soil respiration measurements revealed that PFOS exposure increased microbial mortality, with differential impacts observed between LS and PC rhizospheres. LS root exudates intensified microbial stress, whereas PC exudates mitigated PFOS toxicity. Thin-layer chromatography indicated that LS exudates decreased PFOS mobility, leading to higher local concentrations and increased microbial toxicity, while PC exudates enhanced PFOS mobility, reducing its local impact. Fourier-transform infrared spectroscopy and excitation-emission matrix fluorescence spectroscopy of root exudates identified compositional shifts under PFOS stress, highlighting distinct defense strategies in LS and PC. These findings underscore the importance of plant-microbe interactions and root exudate composition in determining microbial resilience to PFOS contamination.

Responses of depth-dependence of C:N:P stoichiometry to check dam in mangrove wetlands

Carbon (C) sequestration and nitrogen (N) and phosphorus (P) deposition in mangrove wetlands are significant for global climate regulation and the removal of marine nutrient pollutant. To protect and restore mangroves, numerous check dams have been implemented within mangrove wetlands; however, the influence of these check dams on C, N, and P content, stoichiometry, and their depth-dependence remains largely unexplored. To establish a causal relationship between the check dams and C and nutrient variations, we first determined soil deposition depth post-check dam construction in both a natural mangrove and a neighboring check dam-affected mangrove. We then analyzed the C, N, and P content, stoichiometry, and their depth-dependence in soil systems-including bulk soil, microbial biomass, and extracellular enzymes-at depths of 0-100 cm across three vegetation types. Our results indicated that check dam significantly increased the contents of C, N, and P within the soil-microorganism-enzyme system by15.42%-851.15%, and enhanced the stoichiometry of soil C:P and N:P by 79.81%-94.48%. Check dams also enhanced the depth-dependence of soil C, with C varying more slowly than N and P in the natural mangrove but faster in the check dam-affected mangrove. These findings demonstrate that check dams can significantly promote C accumulation in mangroves. Structural equation models revealed that check dam-induced hydrological processes primarily affected C, N, and P content by regulating soil physical properties, while influenced C:N:P stoichiometry through basic chemical properties. Overall, our research emphasizes the implications of costal management for C accumulation and nutrient pollution deposition in mangrove wetlands.

Nitrogen dynamics as a function of soil types, compaction, and moisture

In this study, the complex interactions between soil types, compaction, and moisture on nitrogen (N) transformation processes such as ammonia (NH3) volatilization, ammonification, nitrification, and denitrification were examined over a 30-day period using a simulated column approach. Two soil types: loam, and sandy loam, were subjected to three compaction treatments-control, surface, and sub-surface compaction-and two moisture regimes, dry and wet. Liquid urea ammonium nitrate (32-0-0) was used as the N fertilizer source at a rate of 200 kg N ha-1. Key indicators of N transformations were measured, including residual concentrations of ammonium (NH4-N) and nitrate (NO3-N), NO3-N leaching, NH3 volatilization, and nitrous oxide (N2O) emissions. Findings revealed that compaction significantly increased residual NH4-N concentrations in deeper soil profiles, with the highest 190.80 mg kg-1 recorded in loam soil under sub-surface compaction and dry conditions. Nitrification rates decreased across both soil types due to compaction, evidenced by elevated residual NH4-N levels. Increased NO3-N leaching was observed in loam soil (178.06 mg L-1), greater than sandy loam (81.11 mg L-1), due to initial higher residual NO3- in loam soil. The interaction of compaction and moisture most affected N2O emissions, with the highest emissions in control treatments during dry weather at 2.88 kg ha -1. Additionally, higher NH3 volatilization was noted in moist sandy loam soil under control conditions at 19.64 kg ha -1. These results highlight the necessity of considering soil texture, moisture, and compaction in implementing sustainable N management strategies in agriculture and suggest recommendations such as avoiding broadcast application in moist sandy loam and loam soil to mitigate NH3 volatilization and enhance N use efficiency, as well as advocating for readjustment of fertilizer rate based on organic matter content to reduce potential NO3-N leaching and N2O emissions, particularly in loam soil.

Molecular and Dual-Isotopic Profiling of the Microbial Controls on Nitrogen Leaching in Agricultural Soils under Managed Aquifer Recharge

Nitrate (NO3-) leaching is a serious health and ecological concern in global agroecosystems, particularly those under the application of agricultural-managed aquifer recharge (Ag-MAR); however, there is an absence of information on microbial controls affecting NO3- leaching outcomes. We combine natural dual isotopes of NO3- (15N/14N and 18O/16O) with metagenomics, quantitative polymerase chain reaction (PCR), and a threshold indicator taxa analysis (TITAN) to investigate the activities, taxon profiles, and environmental controls of soil microbiome associated with NO3- leaching at different depths from Californian vineyards under Ag-MAR application. The isotopic signatures demonstrated a significant priming effect (P < 0.01) of Ag-MAR on denitrification activities in the topsoil (0-10 cm), with a 12-25-fold increase of 15N-NO3- and 18O-NO3- after the first 24 h of flooding, followed by a sharp decrease in the enrichment of both isotopes with ∼80% decline in denitrification activities thereafter. In contrast, deeper soils (60-100 cm) showed minimal or no denitrification activities over the course of Ag-MAR application, thus resulting in 10-20-fold of residual NO3- being leached. Metagenomic profiling and laboratory microcosm demonstrated that both nitrifying and denitrifying groups, responsible for controlling NO3- leaching, decreased in abundance and potential activity rates with soil depth. TITAN suggested that Nitrosocosmicus and Bradyrhizobium, as the major nitrifier and denitrifier, had the highest and lowest tipping points with regard to the NO3- changes (P < 0.05), respectively. Overall, our study provides new insight into specific depth limitations of microbial controls on soil NO3- leaching in agroecosystems.

Effects of periodic drying-wetting on microbial dynamics and activity of nitrite/nitrate-dependent anaerobic methane oxidizers in intertidal wetland sediments

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays an important role in methane (CH₄) consumption in intertidal wetlands. However, little is known about the responses of n-DAMO in intertidal wetlands to periodic drying-wetting caused by tidal cycling. Here, comparative experiments (waterlogged, desiccated, reflooded) with the Yangtze estuarine intertidal sediments were performed to examine the effects of periodic tidal changes on n-DAMO microbial communities, abundances, and potential activities. Functional gene sequencing indicated the coexistence of n-DAMO bacteria and archaea in the tide-fluctuating environments and generally higher biodiversity under reflooded conditions than consecutive inundation or emersion. The n-DAMO microbial abundance and associated activity varied significantly during alternative exposure and inundation, with higher abundance and activity under the waterlogged than desiccated conditions. Reflooding of intertidal wetlands might intensify n-DAMO activities, indicating the resilience of n-DAMO microbial metabolisms to the wetting-drying events. Structural equation modeling and correlation analysis showed that n-DAMO activity was highly related to n-DAMO microbial abundance and substrate availability under inundation, whereas salt accumulation in sediment was the primary factor restraining n-DAMO activity under the desiccation. Overall, this study reveals tidal-induced shifts of n-DAMO activity and associated contribution to mitigating CH₄, which may help accurately project CH₄ emission from intertidal wetlands under different tidal scenarios.

Comprehensive improvement of soil quality and rice yield by flooding-midseason drying-flooding

Many water-saving technologies have been developed to reduce water input and the associated irrigation costs. However, the influence of water management technologies on soil quality is unclear. Soil quality is fundamental to rice yield and sustainable productivity of ecosystems. Therefore, it is important to understand the effect of water management on soil quality and its linkage with rice yield. In this work, a field experiment was conducted to assess the influence of water management on soil physico-chemical properties, microbial biomass, bacterial community, and rice yield in paddy fields. Three water treatments were selected for the study, including flooding-rain-fed (F-RF), flooding-midseason drying-flooding (F-D-F), and continuous flooding (CF). Total nitrogen (TN), total phosphorus (TP), dissolved carbon content (DOC), available phosphorus (AP), nitrate nitrogen (NO₃^-), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) contents were 11%, 20%, 29%, 30%, 11%, 183%, and 215% higher in F-D-F, respectively, than those in the CF (p < 0.05). Additionally, the bacterial diversity in F-D-F and CF was significantly higher compared to the F-RF (p < 0.05). Correspondingly, soil quality index (SQI) was higher in the F-D-F (0.8) than that of F-RF (0.53) and CF (0.5). Compared with the F-RF, water management remarkably altered bacterial community composition, with higher enrichment of anaerobic bacteria (such as Firmicutes and Chloroflexi) in flooding treatments (CF and F-D-F). Differences in the bacterial community were closely related to key soil quality indicators, such as AP. Parallel increases in soil quality and bacterial diversity resulted in increased rice yield in the F-D-F, which was 53% and 12% higher than that in F-RF and CF, respectively. Therefore, F-D-F is the suggested water management method because it can comprehensively improve soil microbial diversity, soil quality, and rice yield. KEY POINTS: • Water management changed bacterial community mainly via SMC (soil moisture content), TP, AP, and NO₃^-contents. • The F-D-F had greater SQI and higher rice yield in comparison with F-RF and CF.

Nitrogen process in stormwater bioretention: effect of the antecedent dry days on the relative abundance of nitrogen functional genes

In this study, we evaluated the relative abundance of nitrogen functional genes (amoA, nirK and nirS) involved in ammonia oxidation and denitrification bacteria in laboratory-scale bioretention columns in response to environmental factors (e.g., moisture content, pH, soil organic matter, soil nitrogen) under different antecedent dry days (ADDs). We observed a decrease tendency of the relative abundance of ammonia-oxidizing bacteria at first and then increased when increasing ADDs from 1 to 22 day, while the relative abundance of denitrifying bacteria showed a downward trend. The abundance of bacteria gene amoA was positively associated with soil ammonia nitrogen concentration (r² = 0.389, p < 0.05) and soil organic matter concentration (r² = 0.334, p < 0.05), while the abundance of bacteria gene nirS was positively correlated with soil ammonia nitrogen (r² = 0.730, p < 0.01), soil organic matter (r² = 0.901, p < 0.01) and soil total nitrogen (r² = 0.779, p < 0.01). Furthermore, gene counts for bacteria gene nirS were correlated negatively with plant root length (r² = 0.364, p < 0.05) and plant biomass (r² = 0.381, p < 0.05). Taken together, these results suggest that both nitrification and denitrification can occur in bioretention systems, which can be affected by environmental factors.

Variation in the content and fluorescent composition of dissolved organic matter in soil water during rainfall-induced wetting and extract of dried soil

Dissolved organic matter (DOM) is present in all soils, providing a readily available carbon source for microorganisms, which influences microbially mediated biogeochemical processes. Rainfall-induced wetting can alter the content and composition of soil DOM. However, conventional methods commonly used to extract DOM from soils involve air- or oven-drying followed by extraction with water, and the results vary considerably in terms of indexes used. Therefore, this study aimed to determine the variation in DOM content and composition of soil gravity water and capillary water during wetting, and establish a better method to obtain real soil DOM information. Following simulated rainfall (50 mm h^-1, 0-24 h), gravity water and capillary water in fresh soil samples were separated using a high-speed refrigerated centrifuge. Additionally, DOM in dried soil samples was extracted using various soil/water ratios after drying by different methods. The DOM data obtained by conventional methods were compared with capillary water data. The results showed that DOM degradation occurred mainly in capillary water close to the surface of soil particles. Among the six fluorescent components of DOM identified, a tryptophan-like component (E_x/E_m = 295/335 nm and 230/335 nm) was possibly derived from terrestrial plants, and a tyrosine-like component (E_x/E_m = 265/305 nm) was likely derived from microbial secretion. Except for little variation in the fluorescence index, dissolved organic carbon concentrations in capillary water were double those in dried soil extracted by conventional methods. The humification index and spectral slope ratio of DOM extracted by conventional methods also markedly varied, and no clear patterns were observed for the variation in specific UV absorbance at 254 nm. These findings allow real information to be obtained regarding soil DOM during wetting, and better selection of the extraction method and indexes when studying soil DOM.

Improved immobilization of soil cadmium by regulating soil characteristics and microbial community through reductive soil disinfestation

Cadmium (Cd) contamination arising from industrialization has attracted increasing attention in recent years. Reductive soil disinfestation (RSD) as an effective agricultural practice has been widely applied for soil sterilization. However, there is little research regarding RSD affecting Cd immobilization. Here, five treatments, namely untreated soil (CK), flooding-treated soil (FL), RSD with 2% ethyl alcohol (EA), 2% sugarcane bagasse (SB), and 2% bean dregs (BD) were designed to detect their performance for Cd immobilization in contaminated soils, and the change of soil properties and microbial communities were monitored. The results revealed that pH significantly increased in FL and RSD-treated soils, but was negatively correlated with the exchangeable fraction of Cd (EX-Cd), while Oxidation-Reduction Potential (Eh) significantly decreased in FL and RSD-treated soils, and was positively correlated with EX-Cd. BD treatment might contribute to the increase of CaCO₃ as shown by X-Ray Diffractomer analysis and strongly decreased the EX-Cd in the soil, but increased the relative abundances of Firmicutes, Planctomycetes, Acidobacteria, and Gemmatimonadetes, which may promote Fe (III) reduction or induce resistance to Cd. Bacterial communities at the phylum and genus levels were closely related to Cd fraction. The FL and RSD treatments moderately altered bacterial specific functions, including iron respiration, which may contribute to remediation of Cd-polluted soil by Fe (III) reduction. Field experiments were conducted to confirm that BD treatment resulted in a significant increase in pH whereas the content of total available Cd was reduced in soils. Compared to the control, concentration of total available Cd of red amaranth, sweet potato, towel gourd, and cowpeas were reduced by approximately 46%, 74%, 72%, and 76% in a BD-treated field, respectively. Our study highlights the potential of RSD as an effective method for Cd immobilization in contaminated soils by improving soil characteristics and altering the composition of the microbial community.

Top-down control of foundation species recovery during coastal wetland restoration

Restoration has been increasingly adopted to halt trends in coastal wetland loss globally. Existing restoration often assumes that once abiotic stress is relieved, disturbances are prevented, and invasive species are eradicated, coastal wetlands will recover if propagules of native species are supplied either through natural dispersal or planting. Whether other factors including consumers can help explain the often suboptimal performance of existing restoration remains poorly understood. In a series of field experiments in the Yangtze estuary, we examined the relative importance of abiotic stress and crab grazing in regulating the recovery of the native foundation plant species Scirpus mariqueter in salt marsh areas where exotic cordgrass was successfully eradicated. We found that grazing by herbivorous crabs, rather than abiotic stress, was the primary obstacle restricting the recovery of planted Scirpus. This negative effect of crab grazing varied predictably across elevation and was strongest at low elevations where abiotic conditions were positive for Scirpus. These findings highlight that i) measures to control crab grazing are needed to enhance the success of Scirpus restoration, even in areas where abiotic conditions are set to be optimal, and ii) restoration measures purely focused on reducing abiotic stress could be ineffective or suboptimal in field conditions, likely jeopardizing restoration investment and success. Since top-down control of foundation plant species is common in many coastal wetlands and can be especially important in degraded systems where herbivores are abundant, we urge that future coastal wetland restoration assesses for the impacts of grazers and, when present, apply intervention measures.