Effects of organic amendment on soil aggregation and microbial community composition during drying-rewetting alternation

Dynamic responses of soil microbial communities to seasonal freeze-thaw cycles in a temperate agroecosystem

Soil freeze-thaw cycles (FTCs) are common in temperate agricultural ecosystems during the non-growing season and are progressively influenced by climate change. The impact of these cycles on soil microbial communities, crucial for ecosystem functioning, varies under different agricultural management practices. Here, we investigated the dynamic changes in soil microbial communities in a Mollisol during seasonal FTCs and examined the effects of stover mulching and nitrogen fertilization. We revealed distinct responses between bacterial and fungal communities. The dominant bacterial phyla reacted differently to FTCs: for example, Proteobacteria responded opportunistically, Actinobacteria, Acidobacteria, Choroflexi and Gemmatimonadetes responded sensitively, and Saccharibacteria exhibited a tolerance response. In contrast, the fungal community composition remained relatively stable during FTCs, except for a decline in Glomeromycota. Certain bacterial OTUs acted as sensitive indicators of FTCs, forming keystone modules in the network that are closely linked to soil carbon, nitrogen content and potential functions. Additionally, neither stover mulching nor nitrogen fertilization significantly influenced microbial richness, diversity and potential functions. However, over time, more indicator species specific to these agricultural practices began to emerge within the networks and gradually occupied the central positions. Furthermore, our findings suggest that farming practices, by introducing keystone microbes and changing interspecies interactions (even without changing microbial richness and diversity), can enhance microbial community stability against FTC disturbances. Specifically, higher nitrogen input with stover removal promotes fungal stability during soil freezing, while lower nitrogen levels increase bacterial stability during soil thawing. Considering the fungal tolerance to FTCs, we recommend reducing nitrogen input for manipulating bacterial interactions, thereby enhancing overall microbial resilience to seasonal FTCs. In summary, our research reveals that microbial responses to seasonal FTCs are reshaped through land management to support ecosystem functions under environmental stress amid climate change.

Straw application improved soil biological properties and the growth of rice plant under low water irrigation

The application of organic amendments is one way to manage low water irrigation in paddy soils. In this 60-day greenhouse pot experiment involving paddy soil undergoing drying-rewetting cycles, we examined the effects of two organic amendments: azo-compost with a low carbon to phosphorus ratio (C:P) of 40 and rice straw with a high C:P ratio of 202. Both were applied at rates of 1.5% of soil weight (w/w). The investigation focused on changes in certain soil biochemical characteristics related to C and P in the rice rhizosphere, as well as rice plant characteristics. The irrigation regimes applied in this study included constant soil moisture in a waterlogged state (130% water holding capacity (WHC)), mild drying-rewetting (from 130 to 100% WHC), and severe drying-rewetting (from 130 to 70% WHC). The results indicated that the application of amendments was effective in severe drying-rewetting irrigation regimes on soil characteristics. Drying-rewetting decreased soil respiration rate (by 60%), microbial biomass carbon (by 70%), C:P ratio (by 12%), soil organic P (by 16%), shoot P concentration (by 7%), and rice shoot biomass (by 30%). However, organic amendments increased soil respiration rate (by 8 times), soil microbial biomass C (51%), total C (TC) (53%), dissolved organic carbon (3 times), soil available P (AP) (100%), soil organic P (63%), microbial biomass P (4.5 times), and shoot P concentration (21%). The highest significant correlation was observed between dissolved organic carbon and total C (r= 0.89**). Organic amendments also increased P uptake by the rice plant in the order: azo-compost > rice straw > control treatments, respectively, and eliminated the undesirable effect of mild drying-rewetting irrigation regime on rice plant biomass. Overall, using suitable organic amendments proves promising for enhancing soil properties and rice growth under drying-rewetting conditions, highlighting the interdependence of P and C biochemical changes in the rhizosphere during the rice vegetative stage.

Soil microbial functional profiles of P-cycling reveal drought-induced constraints on P-transformation in a hyper-arid desert ecosystem

Soil water conditions are known to influence soil nutrient availability, but the specific impact of different conditions on soil phosphorus (P) availability through the modulation of P-cycling functional microbial communities in hyper-arid desert ecosystems remains largely unexplored. To address this knowledge gap, we conducted a 3-year pot experiment using a typical desert plant species (Alhagi sparsifolia Shap.) subjected to two water supply levels (25 %-35 % and 65 %-75 % of maximum field capacity, MFC) and four P-supply levels (0, 1, 3, and 5 g P m-2 y-1). Our investigation focused on the soil Hedley-P pool and the four major microbial groups involved in the critical phases of soil microbial P-cycling. The results revealed that the drought (25 %-35 % MFC) and no P-supply treatments reduced soil resin-P and NaHCO3-Pi concentrations by 87.03 % and 93.22 %, respectively, compared to the well-watered (65 %-75 % MFC) and high P-supply (5 g P m-2 y-1) treatments. However, the P-supply treatment resulted in a 12 %-22 % decrease in the soil NH4+-N concentration preferred by microbes compared to the no P-supply treatment. Moreover, the abundance of genes engaged in microbial P-cycling (e.g. gcd and phoD) increased under the drought and no P-supply treatments (p < 0.05), suggesting that increased NH4+-N accumulation under these conditions may stimulate P-solubilizing microbes, thereby promoting the microbial community's investment in resources to enhance the P-cycling potential. Furthermore, the communities of Steroidobacter cummioxidans, Mesorhizobium alhagi, Devosia geojensis, and Ensifer sojae, associated with the major P-cycling genes, were enriched in drought and no or low-P soils. Overall, the drought and no or low-P treatments stimulated microbial communities and gene abundances involved in P-cycling. However, this increase was insufficient to maintain soil P-bioavailability. These findings shed light on the responses and feedback of microbial-mediated P-cycling behaviors in desert ecosystems under three-year drought and soil P-deficiency.

Re-yellowing of chromium-contaminated soil after reduction-based remediation: Effects and mechanisms of extreme natural conditions

Chromium (VI) in soil poses a significant threat to the environment and human health. Despite efforts to remediate Cr contaminated soil (Cr-soil), instances of re-yellowing have been observed over time. To understand the causes of re-yellowing as well as the influence of overdosed chemical reductant in remediating Cr-soil, experiments on excess reducing agent interference and soil re-yellowing mechanisms under different extreme conditions were conducted. The results show that the USEPA method 3060A & 7196A combined with K2S2O8 oxidation is an effective approach to eliminate interference from excess FeSO4 reducing agents. The main causes of re-yellowing include the failure of reducing agents, disruption of soil lattice, and interactions between manganese oxides and microorganisms. Under various extreme conditions simulated across the four seasons, high temperature and drought significantly accelerated the failure of reducing agents, resulting in the poorest remediation effectiveness for Cr-soil (91.75 %). Dry-wet cycles promoted the formation of soil aggregates, negatively affecting Cr(VI) removal. While these extreme conditions caused relatively mild re-yellowing (9.46 %-16.79 %) due to minimal soil lattice damage, the potential risk of re-yellowing increases with the failure of reducing agents and the release of Cr(VI) within the lattice. Prolonged exposure to acid rain leaching and freeze-thaw cycles disrupted soil structure, leading to substantial leaching and reduction of insoluble Cr, resulting in optimal remediation effectiveness (94.37 %-97.73 %). As reducing agents gradually and the involvement of the water medium, significant re-yellowing occurred in the remediated soil (51.52 %). Mn(II) in soil enriched relevant microorganisms, and the Mn(IV)-mediated biological oxidation process was also one of the reasons for soil re-yellowing.

The role of edaphic variables and management practices in regulating soil microbial resilience to drought - A meta-analysis

Environmental disturbances such as drought can impact soil health and the resistance (ability to withstand environmental stress) and resilience (ability to recover functional and structural integrity after stress) of soil microbial functional activities. A paucity of information exists on the impact of drought on soil microbiome and how soil biological systems respond to and demonstrate resilience to drought stress. To address this, we conducted a systematic review and meta-analysis (using only laboratory studies) to assess the response of soil microbial biomass and respiration to drought stress across agriculture, forest, and grassland ecosystems. The meta-analysis revealed an overall negative response of microbial biomass in resistance (-31.6 %) and resilience (-0.3 %) to drought, suggesting a decrease in soil microbial biomass content. Soil microbial respiration also showed a negative response in resistance to drought stress indicating a decrease in soil microbial respiration in agriculture (-17.5 %), forest (-64.0 %), and grassland (-65.5 %) ecosystems. However, it showed a positive response in resilience to drought, suggesting an effective recovery in microbial respiration post-drought. Soil organic carbon (SOC), clay content, and pH were the main regulating factors of the responses of soil microbial biomass and respiration to drought. In agriculture ecosystem, soil pH was primarily correlated with soil microbial respiration resistance and resilience to drought, potentially influenced by frequent land preparation and fertilizer applications, while in forest ecosystem SOC, clay content, and pH significantly impacted microbial biomass and respiration resistance and resilience. In grassland ecosystem, SOC was strongly associated with biomass resilience to drought. The impact of drought stress on soil microbiome showed different patterns in natural and agriculture ecosystems, and the magnitude of microbial functional responses regulated by soil intrinsic properties. This study highlighted the importance of understanding the role of soil properties in shaping microbial responses to drought stress for better ecosystem management.

Effects of drying-rewetting cycles on colloidal phosphorus composition in paddy and vegetable soils

The impact of drying-rewetting (DRW) cycles on soil phosphorus (P) behavior is well-established; however, its impact on the different-sized colloidal P (CP) in agricultural soils is still unclear. To investigate the effect of DRW events on the mobilization of CP in agricultural soils, and to understand how this impact varies with different DRW cycles and drought intensities, the study explored the role of soil type, CP fractions, and compositions. The concentration of CP was measured in paddy soil and vegetable soil after 3, 6, and 9 DRW cycles of varying intensities. The CP was then fractionated into fine-sized colloids (FC-P; 1-220 nm), medium-sized colloids (MC-P; 220-450 nm), and coarse-sized colloids (CC-P; 450-1000 nm) through soil supernatant filtration. CP accounted for 71.1 % and 55.6 % of water-dispersible colloidal P (<1000 nm) in paddy and vegetable soils, with FC-P constituting the greatest proportion at 50 % and 44 % of CP respectively. The colloidal fraction correlated with organic carbon, aluminum, and iron. DRW cycles did not change the overall distribution of the three CP size fractions. However, they affected the concentration and composition of CP. This study concluded that DRW can have significant implications for nutrient release and water quality in agricultural soils and that maintaining soil moisture at 50 % to 70 % of water-holding capacity could alleviate CP accumulation resulting from DRW cycles.

Responses of bacterial community and N-cycling functions stability to different wetting-drying alternation frequencies in a riparian zone

Wetting-drying alternation (WD) of the soil is one of the key characteristics of riparian zones shaped by dam construction, profoundly impacting the soil microenvironment that determines the bacterial community. Knowledge concerning the stability of bacterial community and N-cycling functions in response to different frequencies of WD remains unclear. In this study, samples were taken from a riparian zone in the Three Gorges Reservoir (TGR) and an incubation experiment was conducted including four treatments: constant flooding (W), varied wetting-drying alternation frequencies (WD1 and WD2), and constant drying (D) (simulating water level of 145 m, 155 m, 165 m, and 175 m in the riparian zone respectively). The results revealed that there was no significant difference in the diversity among the four treatments. Following the WD1 and WD2 treatments, the relative abundances of Proteobacteria increased, while those of Chloroflexi and Acidobacteriota decreased compared to the W treatment. However, the stability of bacterial community was not affected by WD. Relative to the W treatment, the stability of N-cycling functions estimated by resistance, which refers to the ability of functional genes to adapt to changes in the environment, decreased following the WD1 treatment, but showed no significant change following the WD2 treatment. Random forest analysis showed that the resistances of the nirS and hzo genes were core contributors to the stability of N-cycling functions. This study provides a new perspective for investigating the impacts of wetting-drying alternation on soil microbes.

A Review on Remediation of Iron Ore Mine Tailings via Organic Amendments Coupled with Phytoremediation

Mining operations degrade natural ecosystems by generating a large quantity of mine tailings. Mine tailings remain in dams/open ponds without further treatment after valuable metals such as iron ore have been extracted. Therefore, rehabilitation of tailings to mitigate the negative environmental impacts is of the utmost necessity. This review compares existing physical, chemical and amendment-assisted phytoremediation methods in the rehabilitation of mine tailings from the perspective of cost, reliability and durability. After review and discussion, it is concluded that amendment-assisted phytoremediation has received comparatively great attention; however, the selection of an appropriate phytoremediator is the critical step in the process. Moreover, the efficiency of phytoremediation is solely dependent on the amendment type and rate. Further, the application of advanced plant improvement technologies, such as genetically engineered plants produced for this purpose, would be an alternative solution. Further research is needed to determine the suitability of this method for the particular environment.

The older, the better: Ageing improves the efficiency of biochar-compost mixture to alleviate drought stress in plant and soil

Due to increased drought frequency following climate change, practices improving water use efficiency and reducing water-stress are needed. The efficiency of organic amendments to improve plant growth conditions under drought is poorly known. Our aim was to investigate if organic amendments can attenuate plant water-stress due to their effect on the plant-soil system and if this effect may increase upon ageing. To this end we determined plant and soil responses to water shortage and organic amendments added to soil. We compared fresh biochar/compost mixtures to similar amendments after ageing in soil. Results indicated that amendment application induced few plant physiological responses under water-stress. The reduction of leaf gas exchange under watershortage was alleviated when plants were grown with biochar and compost amendments: stomatal conductance was least reduced with aged mixture aged mixture (-79 % compared to -87 % in control), similarly to transpiration (-69 % in control and not affected with aged mixture). Belowground biomass production (0.25 times) and nodules formation (6.5 times) were enhanced under water-stress by amendment addition. This effect was improved when grown on soil containing the aged as compared to fresh amendments. Plants grown with aged mixtures also showed reduced leaf proline concentrations (two to five times) compared to fresh mixtures indicating stress reduction. Soil enzyme activities were less affected by water-stress in soil with aged amendments. We conclude that the application of biochar-compost mixtures may be a solution to reduce the effect of water-stress to plants. Our findings revealed that this beneficial effect is expected to increase with aged mixtures, leading to a better water-stress resistance over time. However, while being beneficial for plant growth under water-stress, the use of amendments may not be suited to increase water use efficiency.

Drying and rewetting cycles increased soil carbon dioxide rather than nitrous oxide emissions: A meta-analysis

The increased frequency of extreme weather variations worldwide has resulted in dramatic changes in the soil water content via pronounced drying and rewetting cycles (DWCs). A comprehensive exploration of carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions in response to DWCs can help summarize the existing results and better estimate terrestrial greenhouse gas emissions under the intensified drought and precipitation variations. This meta-analysis based on soil emissions of CO₂ (868 observations, 29 studies) and N₂O (52 observations, 19 studies) at the global scale investigated the direction and intensity of the changes in soil CO₂ and N₂O emissions in response to DWCs as controlled by experimental variables including land use type, soil texture, soil nutrients, and frequency and duration of DWCs. The results showed that, compared to the constant soil water content, DWCs led to the increase in CO₂ emissions by 35.7% (95% confidence intervals ranging from 0.300 to 0.415), whereas it had no significant effect on N₂O emissions (-0.2638 to 1.4751). The random-effects model indicated that soil water-filled pore space during wetting, soil clay content, days of drying and wetting, and frequency of DWCs significantly affected CO₂ and N₂O emissions in response to DWCs. Furthermore, potential biotic and abiotic factors affecting soil CO₂ and N₂O emissions under DWCs are also summarized, and it was proposed that mobility and availability of carbon substrate as well as enhanced microbial activity and abundance are the main drivers facilitating soil CO₂ and N₂O emissions in response to DWCs. However, soil gas diffusion or oxygen availability also dominated soil N₂O emissions under DWCs. Overall, this study improves our understanding of soil CO₂ and N₂O emissions in response to various DWC scenarios and facilitates the development of better greenhouse gas mitigation strategies against the background of a rapidly changing climate.

Linkages of nitrogen-cycling microbial resistance and resilience to soil nutrient stoichiometry under dry-rewetting cycles with different fertilizations and temperatures in a vegetable field

Multiple dry-rewetting (DRW) cycles occur in intensively managed vegetable fields due to frequent tillage and irrigation. Soil nitrogen (N) cycling depends on the resistance and resilience of related microbial populations to DRW cycles, which could be closely related to soil nutrient status. However, the linkage of N-cycling microbial resistance and resilience and soil nutrient stoichiometry remains unknown in vegetable field. Here, we established four fertilization treatments in a four-year greenhouse vegetable field: no N fertilization, synthesized N fertilization, substituting 50% of chemical N with organic fertilizer or biofertilizer. Then, we set up an 85-day DRW-cycling incubation at 15, 25 and 35 °C including a 55-day fluctuating moisture for microbial resistance and then a 30-day constant moisture for microbial resilience. The results showed that microbial resistance was high (resistance index = 0.87- 0.99) in response to DRW cycles, but microbial resilience was generally low (resilience index = -0.36- 0.76), especially in 50% organic substitution or 15 °C. N-cycling microbes showed an important trade-off between their resistance and resilience to DRW cycles. Furthermore, most treatments showed microbial carbon limitation and N abundance during DRW cycles and recovered gradually to the undisturbed state. Microbial resistance was significantly related to the soil nutrient stoichiometry of carbon, N and phosphorus, while microbial resilience was mainly correlated with carbon-related indicators. In conclusion, N-cycling microbes presented good stability with oligotrophic strategy to frequent DRW cycles, which was linked to not only the historical legacy effect of DRW cycles but also soil nutrient stoichiometry in the vegetable field.

Macroaggregates Serve as Micro-Hotspots Enriched With Functional and Networked Microbial Communities and Enhanced Under Organic/Inorganic Fertilization in a Paddy Topsoil From Southeastern China

Microbial communities of soil aggregate-size fractions were explored with molecular and networking assays for topsoil samples from a clayey rice paddy under long-term fertilization treatments. The treatments included no fertilizer (NF) as control, chemical fertilizer only (CF), chemical fertilizer with swine manure (CFM), and chemical fertilizer with rice straw return (CFS). Following a wet-sieving protocol, water-stable aggregates were separated into size fractions of large macroaggregates (L-MacA, >2,000 μm), macroaggregates (MacA, 2,000-250 μm), microaggregates (MicA, 250-53 μm), fine microaggregates (F-MicA, 53-2 μm), and fine clay (F-Clay, <2 μm). Mass proportion was 32.3-38.2% for F-MicA, 23.0-31.5% for MacA, 19.0-23.1% for MicA, 9.1-12.0% for L-MacA, and 4.9-7.5% for F-Clay, respectively. The proportion of MacA was increased, but F-Clay was reduced by fertilization, whereas the mean weight diameter was increased by 8.0-16.2% from 534.8 μm under NF to 621.5 μm under CFM. Fertilization affected bacterial 16S rRNA and fungal 18S rRNA gene abundance in F-MicA and F-Clay but not in aggregates in size larger than 53 μm. However, bacterial and fungal community α-diversities and community structures were quite more divergent among the fertilization treatments in all size fractions. Organic carbon and gene abundance of bacteria and fungi were enriched in both L-MacA and MacA but depleted in F-Clay, whereas microbial Shannon diversity was rarely changed by fraction size under the four treatments. L-MacA and MacA contained more bacteria of r-strategists and copiotrophs, whereas F-MicA and F-Clay were demonstrated with a higher abundance of K-strategists and oligotrophs. Guilds of parasitic and litter saprotrophic fungi were enriched in F-MicA but depleted in L-MacA. Furthermore, most of bacterial and fungal operational taxonomic units were strongly interacted in L-MacA and MacA rather than in MicA and F-Clay. Thus, MacA acted as micro-hotspots enriched with functional and networked microbial communities, which were enhanced with organic/inorganic fertilization in the rice paddy.

The Native Arbuscular Mycorrhizal Fungi and Vermicompost-Based Organic Amendments Enhance Soil Fertility, Growth Performance, and the Drought Stress Tolerance of Quinoa

The present study aimed to determine the effects of biostimulants on the physicochemical parameters of the agricultural soil of quinoa under two water regimes and to understand the mode of action of the biostimulants on quinoa for drought adaptation. We investigated the impact of two doses of vermicompost (5 and 10 t/ha) and arbuscular mycorrhizal fungi applied individually, or in joint application, on attenuating the negative impacts of water shortage and improving the agro-physiological and biochemical traits of quinoa, as well as soil fertility, under two water regimes (well-watered and drought stress) in open field conditions. Exposure to drought decreased biomass, leaf water potential, and stomatal conductance, and increased malondialdehyde and hydrogen peroxide content. Mycorrhiza and/or vermicompost promoted plant growth by activating photosynthesis machinery and nutrient assimilation, leading to increased total soluble sugars, proteins, and antioxidant enzyme activities in the leaf and root. After the experiment, the soil's total organic matter, phosphorus, nitrogen, calcium, and soil glomalin content improved by the single or combined application of mycorrhiza and vermicompost. This knowledge suggests that the combination of mycorrhiza and vermicompost regulates the physiological and biochemical processes employed by quinoa in coping with drought and improves the understanding of soil-plant interaction.

Shift of lakeshore cropland to buffer zones greatly reduced nitrogen loss from the soil profile caused by the interaction of lake water and shallow groundwater

The interaction of lake water (LW) and shallow groundwater (SGW) accelerates nitrogen (N) loss from the soil profile in the lakeshore cropland, and cropland buffer zone (CBZ) significantly inhibits N loss in this area. Here, characteristics of N loss and transformations driven by SGW and LW interactions were explored using microcosmic experiments, and N loss was estimated using in situ monitoring data before and after the construction of the CBZ along the west bank of Erhai Lake. The results indicated that NO₃^--N, dissolved organic N and total dissolved N sustained the main N losses in the soil, and the organic N was responsible for the main N loss in the effluent. The lower total nitrogen (TN) concentrations of SGW in this area, the greater the soil N loss. Moreover, N total loss from the 100 cm soil profile in the control check was 1.8 times that in the simulated SGW treatment. We found that nitrification, denitrification and anammox driven by the microbial community and N functional genes were the key processes leading to N loss. The effluent N (3.64%) and gaseous N (0.32%) loss ratios in the cropland for continuously growing vegetables (CGV) were much higher than that in the CBZ (1.07% of effluent N and 0.25% of gaseous N loss ratios). If a 100 m wide and 48 km long area of lakeshore cropland is CGV, an increase by 47% is projected by 2030 compared with the N loss in 2020. But this region was built as a 100 m wide CBZ or 50 m wide CBZ + 50 m wide CGV after 2019, N loss will be reduced by 87% and 44% in 2030 compared with the N loss in CGV. The results implied that restoring a suitable width of CBZ can significantly reduce N loss.

Rice straw application with different water regimes stimulate enzymes activity and improve aggregates and their organic carbon contents in a paddy soil

Soil organic carbon plays considerable roles in binding soil particles together forming aggregates. Carbon (C) incorporated within these aggregates is thought to be microbially processed; thus, investigating changes in microbial activities i.e. dehydrogenase, urease, catalase and phosphatase enzymes may explain, to some extent, the dynamics and probably mechanisms responsible of formation of these aggregates. Since, soil water content (SWC) may take part in stimulating/lessening activities of organic matter decomposers; thus, this study aimed at investigating the effects of rice straw as a source of organic C in combination with variable SWC on bioaccumulation of C within different soil aggregate size fractions (2000-250, 250-53 and < 53 μm) and hence formation of these aggregates. To achieve these objectives, a pot experiment was conducted for 90 days, including five water levels i.e. maintaining a water head 1 cm above the soil surface (W1), 100% of the saturation percentage, SP (W2), 80% of SP (W3), 65% of SP (W4) and 50% of SP (W5), beside of two rates of applied rice straw i.e. 0 and 15 g kg^-1 (w/w). Results revealed that application of rice straw at a rate of 15 g kg^-1 increased the activities of dehydrogenase, urease, neutral phosphatase and catalase enzymes within the first 60 days after application; thereafter, activities of the first three enzymes decreased considerably. Likewise, formation of soil macro- (2000-250 μm) and micro-aggregates (250-53 μm) increased by the end of the experimental period. The highest concentrations of soil carbon were incorporated within soil macro-aggregate, whereas the least C content was found within the "silt + clay" fraction. Increasing SWC resulted in significant reductions in activities of the aforementioned enzymes and consequent reductions occurred in soil aggregation. Carbon content within aggregates sized <250 μm were significantly correlated with the percentage of these aggregates in soil. Thus, soil aggregation is thought to be the byproduct of an aerobic biosynthetic microbial process in which more stable hydrophobic organic C existed mainly in macropores. This process probably occurred within the first 60 days after RS application.

Impact of sulfate and iron oxide on bacterial community dynamics in paddy soil under alternate watering conditions

Sulfate and iron oxides are often used as amendments in paddy soil, but their ecological risks for soil microbiomes are not well understood. Paddy soil amended with gypsum or hematite was incubated in laboratory microcosms under submerged (56 d) and subsequent dry (35 d) conditions. The soil bacterial community composition stabilized after 15-21 d of submergence and remained largely unchanged after redrying. The presence of OTUs related to facultative anaerobic bacteria (mainly Acidobacteria groups 7 and 16, Gemmatimonas, and unclassified bacteria) probably accounted for the limited variation in community composition after redrying, as suggested by random forest regressions. Redrying caused remarkable changes in the relative abundance of many bacteria putatively involved in soil reduction and oxidation. Gypsum and hematite did not stimulate sulfate or iron reduction after soil submergence. Although gypsum and hematite largely preserved the bacterial community composition, they significantly reduced the abundance and diversity of the total bacteria (by 3-12%), as well as the relative abundance of many putative bacterial reducers and oxidizers (by 17-100%), compared to the control. The results suggested the potential hazardous effects of sulfate and iron oxide on the bacteria in paddy soil, which should be considered before applying these amendments.

Short-term soil drying-rewetting effects on respiration rate and microbial biomass carbon and phosphorus in a 60-year paddy soil

Paddy soils represent the largest anthropogenic wetlands on earth. Soil drying and rewetting that occurs annually inflict significant stress on soil microbial activities in paddy soils. An incubation experiment of 60 years of paddy soil was conducted to simulate the conditions of paddy fields (25 °C and 75% air humidity) during a 16-day incubation time. The effect of drying-rewetting [DRW, with 4 levels: (1) constant soil moisture (CSM), (2) one-stage drought stress (DRW1), (3) two-stage drought stress (DRW2), and (4) three-stage drought stress (DRW3)] and how it evolves over 0, 4, 8. 12, and 16 days after incubation on the concentration of available phosphorus (AP), microbial biomass P (MBP) and microbial biomass C (MBC), and respiration rate (RES) was determined using repeated measures analysis (RMA). The results revealed that an increase in the number of drying-rewetting increases MBC and RES. Compared to CSM, frequent drying and rewetting caused an increase in RES, MBC and MBP by 88%, 38%, and 11%, respectively. Drying-rewetting increased microbial biomass C (MBC) and P (MBP) by 24-38% and 11-54%, respectively, during 8-16 days of incubation. Increasing the number of DRW cycles reduced AP concentration (except in DRW1). The decrease in available phosphorus is due to the increase in the intensity of immobilization under these conditions. Positive correlations were also observed between AP and MBP (r = 0.52), and between RES and MBC (r = 0.91). In general, the frequency of moisture in the paddy soil is favorable for increasing microbial activity.

Rice root exudates enhance desorption and bioavailability of phthalic acid esters (PAEs) in soil associating with cultivar variation in PAE accumulation

Phthalic acid esters (PAEs) is a class of prevalent pollutants in agricultural soil, threating food safety through crop uptake and accumulation of PAEs. Accumulation of PAEs varies largely among crop species and cultivars. Nevertheless, how root exudates affect PAE bioavailability, dissipation, uptake and accumulation is still not well understood. In the present study, desorption and pot experiments were designed to investigate how root exudates from high-(Peizataifeng) and low-(Fengyousimiao) PAE accumulating rice cultivars affect soil PAE bioavailability, dissipation, and accumulation variation. Rice root exudates including low molecular weight organic acids (LMWOAs) of Peizataifeng and Fengyousimiao could enhance desorption of two typical PAE compounds, di-n-butyl phthalate (DBP) and di-(2-ethylhexyl) phthalate (DEHP), from aged soil to their available fractions by increasing soil dissolved organic carbon (DOC), thus improving their bioavailability in soil. Peizataifeng produced twice higher amounts of oxalic acid, critic acid and malonic acid in root exudates, and exhibited stronger effects on enhancing desorption and bioavailability of DBP and DEHP than Fengyousimiao. Higher (by about 50%) total organic carbon contents of root exudates from Peizataifeng led to higher (by 10-30%) soil microbial biomass carbon and nitrogen than Fengyousimiao, and thus promoted more PAE dissipation from soil than Fengyousimiao. Nevertheless, higher (by 20-50%) soil DOC and significantly higher PAE bioavailability in the soils planted Peizataifeng resulted in greater (by 53-93%) PAE accumulation in roots and shoots of Peizataifeng than Fengyousimiao, confirming by higher (by 1.82-3.48 folds) shoot and root bioconcentration factors of Peizataifeng than Fengyousimiao. This study reveals that the difference in root exudate extent and LMWOAs between Peizataifeng and Fengyousimiao differentiates PAE accumulation.

Substitution of manure for chemical fertilizer affects soil microbial community diversity, structure and function in greenhouse vegetable production systems

Soil microbial communities and enzyme activities together affect various ecosystem functions of soils. Fertilization, an important agricultural management practice, is known to modify soil microbial characteristics; however, inconsistent results have been reported. The aim of this research was to make a comparative study of the effects of different nitrogen (N) fertilizer rates and types (organic and inorganic) on soil physicochemical properties, enzyme activities and microbial attributes in a greenhouse vegetable production (GVP) system of Tianjin, China. Results showed that manure substitution of chemical fertilizer, especially at a higher substitution rate, improved soil physicochemical properties (higher soil organic C (SOC) and nutrient (available N and P) contents; lower bulk densities), promoted microbial growth (higher total phospholipid fatty acids and microbial biomass C contents) and activity (higher soil hydrolase activities). Manure application induced a higher fungi/bacteria ratio due to a lower response in bacterial than fungal growth. Also, manure application greatly increased bacterial stress indices, as well as microbial communities and functional diversity. The principal component analysis showed that the impact of manure on microbial communities and enzyme activities were more significant than those of chemical fertilizer. Furthermore, redundancy analysis indicated that SOC and total N strongly influenced the microbial composition, while SOC and ammonium-N strongly influenced the microbial activity. In conclusion, manure substitution of inorganic fertilizer, especially at a higher substitution rate, was more efficient for improving soil quality and biological functions.

Assessment of 14C-prosulfocarb dissipation mechanism in soil after amendment and its impact on the microbial community

Adding organic amendments to soil could modify the bioavailability of herbicides and lead to changes in the microbial community's activity and structure. The objective here was to study the dissipation and total mass balance of ¹⁴C-labeled prosulfocarb applied at two rates (4 and 10 mg kg^-1) in unamended and green compost (GC)-amended soil. Soil dehydrogenase activity (DHA) and phospholipid fatty acid (PLFA) profile analysis were determined to evaluate the effect of herbicide residues on microbial community's activity and structure over the dissipation period. The dissipation rate of prosulfocarb decreased after soil amendment due to higher herbicide adsorption by the amended soil. The 50% dissipation time (DT₅₀) increased 1.7 times in the unamended soil when the concentration of prosulfocarb increased 2.5 times. The mass balance results indicate that the sum of water and organic extractable fractions represented the highest amounts up to the dissipation of 50% ¹⁴C-prosulfocarb. The ¹⁴C-herbicide was then mainly mineralized (up to 11%-31%) or formed non-extractable residues (up to 35%-44%). The amount of ¹⁴C-prosulfocarb residues extracted with methanol was slightly higher in amended soils than in unamended ones. ¹⁴C-prosulfocarb mineralization was higher in unamended soils than in amended ones. The formation of non-extractable residues was continuous, and increased over time. Soil DHA decreased in the unamended soil and was maintained in the GC-amended soil at the end of the assay. The microbial structure was barely disturbed over the prosulfocarb degradation process, although it was clearly influenced by the application of GC. The results obtained reveal the influence organic amendment has on herbicide bioavailability to decrease its biodegradation and buffer its impact on the soil microbial structure.

Changes in distribution and microstructure of bauxite residue aggregates following amendments addition

Bauxite residue is a highly alkaline byproduct which is routinely discarded at residue disposal areas. Improving soil formation process to revegetate the special degraded lands is a promising strategy for sustainable management of the refining industry. A laboratory incubation experiment was used to evaluate the effects of gypsum and vermicompost on stable aggregate formation of bauxite residue. Aggregate size distribution was quantified by fractal theory, whilst residue microstructure was determined by scanning electron microscopy and synchrotron-based X-ray micro-computed tomography. Amendments addition increased the content of macro-aggregates (>250 μm) and enhanced aggregate stability of bauxite residue. Following gypsum and vermicompost addition, fractal dimension decreased from 2.84 to 2.77, which indicated a more homogeneous distribution of aggregate particles. Images from scanning electron microscopy and three-dimensional microstructure demonstrated that amendments stimulate the formation of improved structure in residue aggregates. Pore parameters including porosity, pore throat surface area, path length, and path tortuosity increased under amendment additions. Changes in aggregate size distribution and microstructure of bauxite residue indicated that additions of gypsum and vermicompost were beneficial to physical condition of bauxite residue which may enhance the ease of vegetation.

Biochar and crushed straw additions affect cadmium absorption in cassava-peanut intercropping system

Cassava (Manihot esculenta Crantz) intercropped with peanut (Arachis hypogaea) has good complementary effects in time and space. In the field plot test, the land equivalent ratio (LER) of cassava-peanut intercropping system was 1.43, showing obvious intercropping yield advantage. Compared with monocropping, Cd contents in the roots of cassava and seeds of peanut were significantly reduced by 20.00% and 31.67%, respectively (p < 0.05). Under the unit area of hectare, compared with monocropping of cassava and peanut, the bioconcentration amount (BCA) of Cd in the intercropping system increased significantly by 24.98% and 25.59%, respectively (p < 0.05), and the metal removal equivalent ratio (MRER) of Cd was 1.25, indicating that the intercropping pattern had advantage in Cd removal. In the cement pool plot test, compared with the control, cassava intercropped with peanut under biochar and crushed straw additions did not only enhance the available nutrients and organic matter contents in rhizosphere soil but also promoted the crop growth and increased the content of chlorophyll (SPAD values) of plant leaves. The peanut seeds biomass under biochar and straw additions were significantly increased by 112.34% and 59.38% (p < 0.05), respectively, while the cassava roots biomass under biochar addition was significantly increased by 63.54% (p < 0.05). Applying biochar significantly decreased the content of Cd which extracted by diethylenetriaminepentaacetic acid (DTPA-Cd) in soil and reduced Cd uptake as well as translocation into plant tissues. The BCA of Cd of cassava under biochar addition decreased significantly by 53.87% in maturity stage (p < 0.05), thus reduced the ecological risk of Cd to crops and was of great significance to produce high quality and safe agricultural products. Besides, the crushed straw enhanced the biomass of crops, reduced Cd content in all tissues and maintained Cd uptake in the intercropping system. Therefore, it can realize the integration of ecological remediation and economic benefit of two energy plants in Cd contaminated soil after applied crushed straw in cassava-peanut intercropping system.

Influence of different agricultural management practices on soil microbial community over dissipation time of two herbicides

Soil microbiology could be affected by the presence of pesticide residues during intensive farming, potentially threatening the soil environment. The aim here was to assess the dissipation of the herbicides triasulfuron and prosulfocarb, applied as a combined commercial formulation, and the changes in soil microbial communities (through the profile of phospholipid fatty acids (PLFAs) extracted from the soil) during the dissipation time of the herbicides under field conditions. The dissipation of herbicides and the soil microbial structure were assessed under different agricultural practices, such as the repeated application of herbicides (twice), in unamended and amended soils with two organic amendments derived from green compost (GC1 and GC2) and with non-irrigation and irrigation regimes. The results obtained indicate slower dissipation for triasulfuron than for prosulfocarb. The 50% dissipation time (DT₅₀) decreased under all conditions for the second application of triasulfuron, although not for prosulfocarb. The DT₅₀ values for both herbicides increased in the GC2 amended soil with the highest organic carbon (OC) content. The DT₅₀ values decreased for prosulfocarb with irrigation, but not for triasulfuron, despite its higher water solubility. The herbicides did not have any significant effects on the relative population of Gram-negative and Gram-positive bacteria during the assay, but the relative abundance of Actinobacteria increased in all the soils with herbicides. At the end of the assay (215 days), the negative effects of herbicides on fungi abundance were significant (p < 0.05) for all the treatments. These microbiological changes were detected in non-irrigated and irrigated soils, and were more noticeable after the second application of herbicides. Actinobacteria could be responsible for the modification of herbicide degradation rates, which tend to be faster after the second application. This study makes a useful contribution to the evaluation of the soil environment and microbiological risks due to the long-term repeated application of herbicides under different agricultural management practices.

Soybean supplementation increases the resilience of microbial and nematode communities in soil to extreme rainfall in an agroforestry system

A current challenge for ecological research in agriculture is to identify ways in which to improve the resilience of the soil food web to extreme climate events, such as severe rainfall. Plant species composition influence soil biota communities differently, which might affect the recovery of soil food web after extreme rainfall. We compared the effects of rainfall stress up on the soil microbial food web in three planting systems: a monoculture of the focal species Zanthoxylum bungeanum and mixed cultures of Z. bungeanum and Medicago sativa or Z. bungeanum and Glycine max. We tested the effect of the presence of a legume on the recovery of trophic interactions between microorganisms and nematodes after extreme rainfall. Our results indicated that all chemical properties of the soil recovered to control levels (normal rainfall) in the three planting systems 45 days after exposure to extreme rain. However, on day 45, the bulk microbial community differed from controls in the monoculture treatment, but not in the two mixed planting treatments. The nematode community did not fully recover in the monoculture or Z. bungeanum and M. sativa treatments, while nematode populations in the combined Z. bungeanum and G. max treatment were indistinguishable from controls. G. max performed better than M. sativa in terms of increasing the resilience of microbial and nematode communities to extreme rainfall. Soil microbial biomass and nematode density were positively correlated with the available carbon and nitrogen content in soil, demonstrating a link between soil health and biological properties. This study demonstrated that certain leguminous plants can stabilize the soil food web via interactions with soil biota communities after extreme rainfall.