Rocky desertification succession alters soil microbial communities and survival strategies in the karst context.

Rocky desertification is one of the most ecological problems in the karst context. Although extensive research has been conducted to explore how to restore and protect, the responses of soil fungi and archaea to rocky desertification succession remain limited. Here, four grades of rocky desertification in a karst ecosystem were selected, amplicon sequencing analysis was conducted to investigate fungal and archaeal community adaptation in response to rocky desertification succession. Our findings revealed that the diversity and community structure of fungi and archaea in soils declined with the aggravation of rocky desertification. As the rocky desertification succession intensified, microbial interactions shifted from cooperation to competition. Microbial survival strategies were K-strategist and r-strategist dominated in the early and late stages of succession, respectively. Additionally, the driving factors affecting microorganisms have shifted from vegetation diversity to soil properties as the intensification of rocky desertification. Collectively, our study highlighted that plant diversity and soil properties play important roles on soil microbiomes in fragile karst ecosystems and that environmental factors induced by human activities might still be the dominant factor exacerbating rocky desertification, which could significantly enrich our understanding of microbial ecology within karst ecosystems.

Response of bacterial and micro-eukaryotic communities to spatio-temporal fluctuations of wastewater in full scale constructed wetlands.

How microbial communities respond to wastewater fluctuations is poorly understood. Full-scale surface flow constructed wetlands (SFCWs) were constructed for investigating microbial communities. Results showed that influent wastewater changed sediment bacterial community composition seasonally, indicating that a single bacterial taxonomic group had low resistance (especially, Actinobacteriota and Gammaproteobacteria). However, copy numbers of 16S rRNA, ammonia oxidizing archaea, ammonia oxidizing bacteria, nirS and nirK in the first stage SFCWs were 2.49 × 1010, 3.48 × 109, 5.76 × 106, 8.77 × 108 and 9.06 × 108 g-1 dry sediment, respectively, which remained stable between seasons. Moreover, decreases in the nitrogen concentration in wastewater, changed microbial system state from heterotrophic to autotrophic. Micro-eukaryotic communities were more sensitive to wastewater fluctuations than bacterial communities. Overall, results revealed that microbial communities responded to spatio-temporal fluctuations in wastewater through state changes and species asynchrony. This highlighted complex processes of wastewater treatment by microbial components in SFCWs.

Intensive N2 fixation accelerates microbial turnover in cropland soils.

Biological nitrogen fixation (BNF) is strongly affected by the carbon (C) and nitrogen (N) stoichiometry in soil and depends on the input of organic C. Due to the high metabolic costs of nitrogenase activity, however, the response of BNF to organic C input and its impact on microbial turnover remain unclear. To address this knowledge gap, we combined 15N2 tracing with high-throughput sequencing by adding glucose or glucose plus mineral N fertilizer for a 12-day incubation in three cropland soils. Glucose addition alone strongly changed the BNF activity (0.76-2.51 mg N kg-1 d-1), while BNF was completely absent after mineral N fertilization. This switch-on of BNF by glucose addition supported equally high rates of microbial growth and organic C mineralization compared with the direct mineral N assimilation by microorganisms. Glucose-induced BNF was predominantly catalyzed by Azotobacter-affiliated free-living diazotrophs (>50 % of the total nifH genes), which increased with diverse nondiazotrophs such as Nitrososphaera, Bacillus and Pseudoxanthomonas. Structural equation models (SEMs) and random forest (RF) analyses consistently revealed that the soil C:N ratio and Azotobacter-affiliated diazotrophic abundances were the key factors affecting glucose-induced BNF. Our findings emphasize the importance of free-living diazotrophs for microbial turnover of organic C in soil.

[Response of Organic Carbon Mineralization to Nitrogen Addition in Micro-aerobic and Anaerobic Layers of Paddy Soil].

In field conditions, a micro-aerobic layer with 1 cm thickness exists on the surface layer of paddy soil owing to the diffusion of dissolved oxygen via flooding water. However, the particularity of carbon and nitrogen transformation in this specific soil layer is not clear. A typical subtropical paddy soil was collected and incubated with13C-labelled rice straw for 100 days. The responses of exogenous fresh organic carbon(13C-rice straw) and original soil organic carbon mineralization to nitrogen fertilizer addition[(NH4)2SO4]in the micro-aerobic layer(0-1 cm) and anaerobic layer(1-5 cm) of paddy soil and their microbial processes were analyzed based on the analysis of 13C incorporation into phospholipid fatty acid(13C-PLFAs). Nitrogen addition promoted the total CO2 and 13C-CO2 emission from paddy soil by 11.4% and 12.3%, respectively. At the end of incubation, with the addition of nitrogen, the total soil organic carbon (SOC) and13C-recovery rate from rice straw in the anaerobic layer were 2.4% and 9.2% lower than those in the corresponding micro-aerobic layer, respectively. At the early stage(5 days), nitrogen addition increased the total microbial PLFAs in the anaerobic layer with a consistent response of bacterial and fungal PLFAs. However, there was no significant effect from nitrogen on microbial abundance in the micro-aerobic layer. Nitrogen addition had no significant impact on the abundance of total 13C-PLFAs in the micro-aerobic and anaerobic layers, but the abundance of 13C-PLFAs for bacteria and fungi in the micro-aerobic layer was decreased dramatically. At the late stage(100 days), the effect of nitrogen addition on microbial PLFAs was consistent with that at the early stage. The abundances of total, bacterial, and fungal 13C-PLFAs were remarkably increased in the anaerobic layer. However, the abundance of 13C-PLFAs in the micro-aerobic layer showed no significant response to nitrogen addition. During the incubation, the content of NH4+-N in the anaerobic soil layer was higher than that in the micro-aerobic soil layer. This indicates that nitrogen addition increased microbial activity in the anaerobic soil layer caused by the higher NH4+-N concentration, as majority of microorganisms preferred to use NH4+-N. Consequently, the microbial utilization and decomposition of organic carbon in the anaerobic soil layer were accelerated. By contrast, richer available N existed in the form of NO3--N in the micro-aerobic soil layer owing to the ammoxidation process. Thus, the shortage of NO3--N preference microorganisms in the paddy soil environment prohibited the microbial metabolism of organic carbon in the micro-aerobic layer. As a whole, nitrogen fertilization enhanced organic carbon loss via microbial mineralization in paddy soil with a weaker effect in the micro-aerobic layer than that in the anaerobic layer, indicating the limited microbial metabolic activity in the surface micro-aerobic layer could protect the organic carbon stabilization in paddy soil. This study emphasizes the heterogeneity of paddy soil and its significant particularity of carbon and nitrogen transformation in micro-aerobic layers. Consequently, this study has implications for optimizing the forms and method for the application of nitrogen fertilizer in paddy cropping systems.

Comparative Effects of Heat Stress at Booting and Grain-Filling Stage on Yield and Grain Quality of High-Quality Hybrid Rice.

Rice plants are highly sensitive to high-temperature stress, posing challenges to grain yield and quality. However, the impact of high temperatures on the quality of high-quality hybrid rice during the booting stage, as well as the differing effects of the booting and grain-filling stages on grain quality, are currently not well-known. Therefore, four high-quality hybrid rice were subjected to control (CK) and high-temperature stress during the booting (HT1) and grain-filling stages (HT2). Compared to the control, HT1 significantly reduced the spikelets panicle-1 (16.1%), seed setting rate (67.5%), and grain weight (7.4%), while HT2 significantly reduced the seed setting rate (6.0%) and grain weight (7.4%). In terms of quality, both HT1 and HT2 significantly increased chalkiness, chalky grain rate, gelatinization temperature, peak viscosity (PV), trough viscosity (TV), final viscosity (FV), and protein content in most varieties, and significantly decreased grain length, grain width, total starch content, and amylose content. However, a comparison between HT1 and HT2 revealed that the increase in chalkiness, chalky grain rate, PV, TV, and FV was greater under HT2. HT1 resulted in a greater decrease in grain length, grain width, total starch content, and amylose content, as well as an increase in protein content. Additionally, HT1 led to a significant decrease in amylopectin content, which was not observed under HT2. Therefore, future efforts in breeding and cultivating high-quality hybrid rice should carefully account for the effects of high temperatures at different stages on both yield and quality.

Different in root exudates and rhizosphere microorganisms effect on nitrogen removal between three emergent aquatic plants in surface flow constructed wetlands.

Swine wastewater contains high concentration of nitrogen (N), causing pollution of surrounding water bodies. Constructed wetlands (CWs) are considered as an effective ecological treatment measure to remove nitrogen. Some emergent aquatic plants could tolerate high ammonia, and play a crucial part in CWs to treat high concentration N wastewater. However, the mechanism of root exudates and rhizosphere microorganisms of emergent plants on nitrogen removal is still unclear. Effects of organic and amino acids on rhizosphere N cycle microorganisms and environmental factors across three emergent plants were investigated in this study. The highest TN removal efficiency were 81.20% in surface flow constructed wetlands (SFCWs) plant with Pontederia cordata. The root exudation rates results showed that organic and amino acids were higher in 56 d than that in 0 d in SFCWs plants with Iris pseudacorus and P. cordata. The highest ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) gene copy numbers were found in I. pseudacorus rhizosphere soil, while the highest nirS, nirK, hzsB and 16S rRNA gene copy numbers were detected in P. cordata rhizosphere soil. Regression analysis results demonstrated that organic and amino acids exudation rates were positive related to rhizosphere microorganisms. These results indicated that organic and amino acids secretion could stimulate growth of emergent plants rhizosphere microorganisms in SFCWs for swine wastewater treatment. In addition, the EC, TN, NH4+-N and NO3--N were negatively correlated with organic and amino acids exudation rates, and abundances of rhizosphere microorganisms via Pearson correlation analysis. These results imply that organic and amino acids, and rhizosphere microorganisms synergically affected on the nitrogen removal in SFCWs.

Effect of plant-self debris on nitrogen removal, transformation and microbial community in mesocosm constructed wetlands planted with Myriophyllum aquaticum.

Aquatic macrophytes debris decomposition could release pollutants and nutrients into the water of constructed wetlands (CWs), but their role in nitrogen removal and transformation remains poorly understood. The present study investigated the effects of plant-self debris on nitrogen removal and microbial communities in mesocosm CWs planted with Myriophyllum aquaticum. During the 68-day operation, the plant debris addition did not change the mean removal efficiency of ammonium (NH4+-N) and total nitrogen (TN) of CWs but showed significant differences over the operation time. The NH4+-N and organic nitrogen released from the plant debris decomposition affected the nitrogen removal. The plant debris decreased the effluent nitrate concentration and N2O emission fluxes of the CWs with the increased abundance of denitrifying bacterial genera, indicating that plant debris decomposition increased the denitrification activities via dissolved organic carbon release. High-throughput sequencing indicated that the plant debris altered the distribution and composition of the microbial community in the sediments. Proteobacteria was the dominant phylum (28-52%), and the relative abundance of denitrifying bacteria genera was significantly higher in the sediments with debris addition (37-40%) than in the non-addition (6.6-7.7%). The present study provided new insights into the role of macrophytes in pollutant removal and the plant management strategy of CWs.

Distinct effects of biochar addition on soil macropore characteristics at different depths in a double-rice paddy field.

Soil macropores largely control the water and nutrients transport as well as runoff processes in the soil. Biochar is frequently applied to soils to improve the macropore structure, but the effects remain controversial. To clarify depth-dependent soil macropore characteristics affected by biochar addition, the intact soil cores with a depth of 200 mm were collected from biochar-amended paddy field at addition rates of 0, 24, and 48 t ha-1 (CK, BC1, and BC2, respectively). The two biochar treatments did not change the overall soil pore indices (e.g., macroporosity, pore number, fractal dimension, and circularity), but showed distinct effects at different soil depths. At a soil depth of 0-50 mm, the biochar treatments had higher macroporosity (8.59-8.85 %) than CK (4.94 %) (p < 0.05), but relatively lower pore circularity (0.83-0.84) than CK (0.88) (p < 0.05). The connectivity of biochar treatments (88-97) was 9.5-10.4 times higher than that of CK (9.3). At a soil depth of 100-200 mm, the biochar treatments exhibited lower macroporosity, macropore number, connectivity, and fractal dimension than CK (p < 0.05). The macropore indices (except circularity) of BC1 were relatively higher than those of BC2 in the most soil depths. Whether biochar altered the soil macropore indices depended on the addition rate of biochar and soil depth. The expansion and occupying effects of biochar were dominant at soil depths of 0-50 and 100-200 mm, respectively; and the two effects were stronger in BC1 than in BC2. A combination of the expansion and occupying effects occurred at a soil depth of 50-100 mm. The distinct effects of biochar on soil pore structure at different depths could mitigate methane emission and nutrient runoff loss from the double-rice paddy. Therefore, soil depth-dependent macropore structure should be considered when assessing the influence of biochar on soil properties and the associated environmental effects.

The deterioration of starch physiochemical and minerals in high-quality indica rice under low-temperature stress during grain filling.

Low temperatures during the grain-filling phase have a detrimental effect on both the yield and quality of rice grains. However, the specific repercussions of low temperatures during this critical growth stage on grain quality and mineral nutrient composition in high-quality hybrid indica rice varieties have remained largely unexplored. The present study address this knowledge gap by subjecting eight high-quality indica rice varieties to two distinct temperature regimes: low temperature (19°C/15°C, day/night) and control temperature (28°C/22°C) during their grain-filling phase, and a comprehensive analysis of various quality traits, with a particular focus on mineral nutrients and their interrelationships were explored. Exposure of rice plants to low temperatures during early grain filling significantly impacts the physicochemical and nutritional properties. Specifically, low temperature increases the chalkiness rate and chalkiness degree, while decreases starch and amylopectin content, with varying effects on amylose, protein, and gelatinization temperature among rice varieties. Furthermore, crucial parameters like gelatinization enthalpy (ΔH), gelatinization temperature range (R), and peak height index (PHI) all significantly declined in response to low temperature. These detrimental effects extend to rice flour pasting properties, resulting in reduced breakdown, peak, trough, and final viscosities, along with increased setback. Notably, low temperature also had a significant impact on the mineral nutrient contents of brown rice, although the extent of this impact varied among different elements and rice varieties. A positive correlation is observed between brown rice mineral nutrient content and factors such as chalkiness, gelatinization temperature, peak viscosity, and breakdown, while a negative correlation is established with amylose content and setback. Moreover, positive correlations emerge among the mineral nutrient contents themselves, and these relationships are further accentuated in the context of low-temperature conditions. Therefore, enhancing mineral nutrient content and increasing rice plant resistance to chilling stress should be the focus of breeding efforts to improve rice quality.

[Effects of Chemical Fertilizer Reduction Combined with Straw Application on Diazotrophic Communities in a Double Rice Cropping System].

Based on a three-year field experiment, the effects of reduced chemical fertilizer combined with straw application on paddy yield, soil fertility properties, and community structure of diazotrophs in a double-rice cropping field three years after straw application were examined. Three treatments were applied:conventional fertilizer application (CF), chemical fertilizer reduction combined with a low straw application rate (CFLS, 3 t·hm-2), and a high straw application rate (CFHS, 6 t·hm-2). The results showed that CFLS and CFHS did not significantly reduce rice grain yield (P>0.05); significantly neutralized soil acidification; increased soil microbial biomass carbon and nitrogen, dissolved organic carbon, and organic carbon content (P<0.05); and significantly reduced soil redox potential, ammonium nitrogen, and nitrate nitrogen contents (P<0.05). This was more conducive to improve soil nitrogen use efficiency. Compared with those under the CF treatment, the natural nitrogen fixation functional communities of CFLS and CFHS increased the Shannon, PD, and Evenness indexes (P<0.05) due to the improvement of conditions such as the increase in soil carbon storage and the decrease in acidification degree. The relative abundance of microbial communities with nitrogen fixation, carbon fixation, and plant growth promotion functions such as Ferrigenium, Sulfurivermis, Methylomonas, Methylovulum, Ectothiorhodospira, and Nostoc increased significantly (P<0.05). In conclusion, the reduction in chemical fertilizer combined with 3 t·hm-2 and 6 t·hm-2 straw application was an effective measure to improve the community structure of soil diazotrophs and the potential of soil nitrogen fixation.

[Annual Nitrogen Removal Efficiency and Change in Abundance of Nitrogen Cycling Microorganisms in Swine Wastewater Treated by Crop Straw Materials].

Based on previous research, using straw material to treat swine wastewater can effectively reduce the concentration of nitrogen (N); however, the annual N-removal efficiency and change in the abundance of N-cycling functional genes remain unclear. In this study, four treatments (wheat straw, rice straw, corn stalk, and CK) were set up, with the aim of studying the annual N-removal efficiency and change in the abundance of functional genes. Our results showed that:① the total nitrogen (TN) removal and NH4+-N removal efficiency were the best in the first six months and were significantly reduced in the following six months. In addition, the TN removal and NH4+-N efficiency in straw and wheat straw were better than those in corn straw. The TN-removal efficiency in straw and wheat straw were 32.81%±11.34% and 32.99%±9.60%, respectively. The NH4+-N removal efficiency in straw and wheat straw were 35.3%±13.23% and 34.97%±12.00%, respectively. ② The abundance of N-cycling functional genes significantly increased by the addition of straw materials, compared with that of the CK (P<0.05). The average abundances of nirK, nirS, and hzsB were 6.45×109 copies·L-1, 6.18×109 copies·L-1, and 2.31×109 copies·L-1, respectively. The average abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were 6.12×1010 copies·L-1 and 4.93×109 copies·L-1, respectively. The average hzsB gene abundance was 2.31×109 copies·L-1. The average abundance of 16S rRNA in the treatment was 8.90×1010 copies·L-1. The abundances of hzsB and nirS genes in the straw and wheat straw were higher than those in the other treatment, indicating that the activities of anaerobic ammonia oxidation and denitrifying microorganisms were significantly increased by the addition of straw and wheat straw (P<0.05). In addition, the abundance of AOA and AOB genes were increased in wheat straw, suggesting that wheat straw could promote nitrification. The results provided data supporting the molecular mechanism of nitrogen removal in swine wastewater treatment with straw materials.

Visualization and quantification of carbon "rusty sink" by rice root iron plaque: Mechanisms, functions, and global implications.

Paddies contain 78% higher organic carbon (C) stocks than adjacent upland soils, and iron (Fe) plaque formation on rice roots is one of the mechanisms that traps C. The process sequence, extent and global relevance of this C stabilization mechanism under oxic/anoxic conditions remains unclear. We quantified and localized the contribution of Fe plaque to organic matter stabilization in a microoxic area (rice rhizosphere) and evaluated roles of this C trap for global C sequestration in paddy soils. Visualization and localization of pH by imaging with planar optodes, enzyme activities by zymography, and root exudation by 14 C imaging, as well as upscale modeling enabled linkage of three groups of rhizosphere processes that are responsible for C stabilization from the micro- (root) to the macro- (ecosystem) levels. The 14 C activity in soil (reflecting stabilization of rhizodeposits) with Fe2+ addition was 1.4-1.5 times higher than that in the control and phosphate addition soils. Perfect co-localization of the hotspots of ß-glucosidase activity (by zymography) with root exudation (14 C) showed that labile C and high enzyme activities were localized within Fe plaques. Fe2+ addition to soil and its microbial oxidation to Fe3+ by radial oxygen release from rice roots increased Fe plaque (Fe3+ ) formation by 1.7-2.5 times. The C amounts trapped by Fe plaque increased by 1.1 times after Fe2+ addition. Therefore, Fe plaque formed from amorphous and complex Fe (oxyhydr)oxides on the root surface act as a "rusty sink" for organic matter. Considering the area of coverage of paddy soils globally, upscaling by model revealed the radial oxygen loss from roots and bacterial Fe oxidation may trap up to 130 Mg C in Fe plaques per rice season. This represents an important annual surplus of new and stable C to the existing C pool under long-term rice cropping.

Multi-spatial scale effects of multidimensional landscape pattern on stream water nitrogen pollution in a subtropical agricultural watershed.

Multidimensional (coupled land use, soil properties, and topography) landscape effects on stream water nitrogen (N) are complex and scale-dependent. However, studies that identify critical buffer zones that explain large variations in riverine N, and estimate specific thresholds of multidimensional landscape patterns at the class level, result in a sudden changes in riverine N pollution, are still limited. Here, a new multidimensional landscape metric that combined land use, soil properties, and topography effects was applied to various riparian buffer zones and sub-watershed scales, and their relationships to riverine N levels were investigated. We used stream water ammonium-N, nitrate-N, and total-N concentrations datasets, from 2010 to 2017, in the nine subtropical sub-watersheds in China. The results of model selection and model averaging in ordinary least squares regressions, indicated that the riparian buffer zone with widths of 400 m, had more pronounced influence on water NH4-N and TN levels than at other scales. Within the 400 m buffer zone, the key landscape metrics for NH4-N, NO3-N and TN concentrations in stream water were different, and explained up to 43.35%-76.55% (adjusted R2) of the total variation in river N levels. When ENN_MNClass17 below 39-56 m, PDClass8 above 4.63-6.55 n/km2, PLANDClass27 above 23-29%, and CONTIG_MNClass42 below 0.35-0.37% within the 400 m buffer zone, riverine NH4-N and TN would be abruptly increased. This study provided practical ideas for regulation regarding landscape management linked to watershed structure, and identified reference thresholds for multidimensional landscape metrics, which should help reduce riverine N pollution in subtropical China.

[Effect of Long-term Straw Returning on the Mineralization and Priming Effect of Rice Root-carbon].

Long-term straw returning to the field changes the environmental conditions of rice paddy soil, which affects the mineralization and priming effect of residual rice roots in the soil, but the direction and intensity of its influence is not clear. Therefore, based on a long-term fertilization field experiment, 13C-CO2 isotopic labeling technology and laboratorial incubation were used to analyze the characteristics of mineralization of rice roots and native soil organic carbon, the intensity and direction of the priming effect, and the source partitioning of CO2 emissions in three treatments, consisting of no fertilization (CK), chemical fertilizer (CF), and straw returning with chemical fertilizer (CFS). The results showed that after 120 days of flooding incubation, the root residue (R) increased the cumulative CO2 emissions by 617.41-726.27 mg·kg-1. The cumulative CO2 emissions from roots and root mineralized proportions in the CFS+R and CF+R treatments were 470.82 and 444.04 mg·kg-1, respectively, and 18.8% and 17.8%, respectively. These were significantly higher than those in the CK+R treatment (384.19 mg·kg-1, 15.4%). There was no significant difference in the cumulative CO2 emissions from native soil organic carbon among the three treatments. However, the mineralized proportion of native soil organic carbon in the CFS+R treatment (4.2%) was significantly lower than that in the CF+R and CK+R treatments (5.4% and 5.8%). The priming effect in the CFS+R treatment was 29.6%, which was significantly lower than that in the CK+R treatment (42.5%) and higher than that in the CF+R treatment (14.4%). A total of 23.47% to 27.59% of the cumulative CO2 emission of the flooded paddy soil was from the roots, and the remainder was from the soil. In addition, the proportion of CO2 emission caused by the priming effect was smaller in the CFS+R treatment than that in the CK+R treatment and larger than that in the CF+R treatment. In summary, the long-term straw returning in the flooded paddy soil will increase the mineralization potential of rice roots, but it is more conducive to the stability of the native soil organic carbon.

[Phosphorus Adsorption Characteristics of Different Biochar Types and Its Influencing Factors].

In order to understand the resource utilization of plant biomass, five types of biomass materials were used to produce biochar to treat wastewater containing phosphorus. The phosphorus adsorption capacity of five materials was preliminarily compared through laboratory experiments, and two materials with strong phosphorus adsorption capacity were screened out. The physicochemical characteristics of the selected biochar were analyzed using scanning electron microscopy and a BET specific surface area analyzer, and the effects of different pH values on phosphorus adsorption of the biochar were investigated. Furthermore, the phosphorus adsorption characteristics of the selected biochar were analyzed via isothermal adsorption and adsorption kinetics models. The results showed that among the five biochar materials, only rice straw and corn straw biochar had the ability to adsorb phosphorus. The Langmuir isothermal adsorption curve showed that the adsorption capacity of rice straw biochar for phosphorus in wastewater was stronger than that of corn straw biochar, and the theoretical maximum adsorption capacity was as follows:rice straw biochar (9.78 mg·g-1)>corn straw biochar (0.39 mg·g-1). The specific surface area (148.30 m2·g-1) and total pore volume (0.11 cm3·g-1) of rice straw biochar were much higher than those of corn straw biochar (8.26 m2·g-1 and 0.03 cm3·g-1, respectively), and the contents of Mg, Ca, Fe, and Al were higher in rice straw biochar. The best pH for phosphorus adsorption of rice straw biochar and corn straw biochar was acidic. In different pH ranges (3.0-11.0), the phosphorus adsorption capacity of rice straw and corn straw biochar decreased with the increase in pH. These results indicated that rice straw biochar has strong phosphorus adsorption capacity and has a better application prospect in wastewater treatment.

Glutamine synthetase plays an important role in ammonium tolerance of Myriophyllum aquaticum.

High-strength ammonium (NH4+), the main characteristic of swine wastewater, poses a significant threat to the rural ecological environment. As a novel phytoremediation technology, Myriophyllum aquaticum wetlands have high tolerance and removal rate of NH4+. Glutamine synthetase (GS), a pivotal enzyme in nitrogen (N) metabolism, is hypothesized to play an important role in the tolerance of M. aquaticum to high NH4+. Herein, the responses of M. aquaticum to GS inhibition by 0.1 mM methionine sulfoximine (MSX) under 15 mM NH4+ were investigated. After 5 days, visible NH4+ toxicity symptoms were observed in MSX-treated plants. Compared with the control, the NH4+ accumulation in the leaves increased by 20.99 times, while that of stems and roots increased by 3.27 times and 47.76 %, suggesting that GS inhibition had a greater impact on the leaves. GS inhibition decreased pigments in the leaves by 8.64 %-41.06 %, triggered oxidative stress, and affected ions concentrations in M. aquaticum. The concentrations of glutamine (Gln) and asparagine decreased by 63.46 %-97.43 % and 12.37 %-76.41 %, respectively, while the concentrations of most other amino acids increased after 5 days of MSX treatment, showing that GS inhibition reprogrammed the amino acids synthesis. A decrease in Gln explains the regulations of N-related genes, including increased expression of AMT in roots and decreased expression of GS, GOGAT, GDH, and AS, which would cause further NH4+ accumulation via promoting NH4+ uptake and decreasing NH4+ assimilation in M. aquaticum. This study revealed for the first time that GS inhibition under high NH4+ condition can lead to phytotoxicity in M. aquaticum due to NH4+ accumulation. The physiological and molecular responses of the leaves, stems, and roots confirmed the importance of GS in the high NH4+ tolerance of M. aquaticum. These findings provide new insights into NH4+ tolerance mechanisms in M. aquaticum and a theoretical foundation for the phytoremediation of high NH4+-loaded swine wastewater.

Physiological response and tolerance of Myriophyllum aquaticum to a wide range of ammonium concentrations.

Myriophyllum aquaticum (M. aquaticum) can be used in constructed wetlands (CWs) to effectively purify swine wastewater with high-ammonia nitrogen (NH3-N and NH4+-N) concentrations. However, the understanding of its tolerance mechanism to ammonia nitrogen is limited. The physiological response and tolerance mechanism of M. aquaticum to a wide range of NH4+ concentrations (0-35 mM) were investigated in the present study. The results indicated that M. aquaticum can tolerate NH4+ concentrations of up to 30 mM for 21 days and grow well with high nutrient (N, P) uptake. A suitable concentration of NH4+ for a better growth of M. aquaticum was 0.5-20 mM. The free NH4+ content was no obviously increase at NH4+ concentration below 15 mM, indicated there was no obviously ammonium accumulation. Exogenous NH4+ inhibited K+ absorption and improved Ca2+ absorption, indicating mineral cation could mediate NH4+ homeostasis under NH4+ stress. Moreover, comparison with those in the control group, the activities of glutamine synthetase (GS), glutamate synthetase (GOGAT) in M. aquaticum increased by 52.7%-115% at 1-20 mM NH4+, and superoxide dismutase (SOD) increased by 29.2-143% at 1-35 mM NH4+. This indicated that the high NH4+ tolerance of M. aquaticum was mainly due to the balance of free NH4+ content in tissues, as well as improved nitrogen metabolism and antioxidant system. This could be attributed to the role of the GS-GOGAT cycle and SOD. In conclusion, M. aquaticum, which tolerates high NH4+ concentration and has a high N uptake ability, can be used as a good candidate specie to help develop more efficient management strategies for treating high-NH4+ wastewater in CW systems.

Influence path identification of topography, soil, hydrology and landscape on phosphorus buffering capacity in typical agricultural catchments in central subtropical China.

The catchment phosphorus buffering capacity (PBF) determines the pressure-state-response relationship between anthropogenic P inputs and aquatic ecosystems at a catchment scale, and is affected by biogeochemical, hydrological, and ecological catchment characteristics. However, the complex relationship between these catchment characteristic factors and their impact pathways on PBF remains ambiguous, leading to large uncertainty in balancing agricultural productivity and water conservation via improving BF through management practices. In this study, the short-term buffering index, calculated from net anthropogenic P input and riverine P exports, was used to quantify the spatiotemporal variations in PBF in source agricultural catchments in the Dongting Lake basin. Partial least squares structural equation modeling was used to investigate the relationship between the PBF and the catchment characteristics. The results indicate that catchment PBF was directly determined by soil properties and hydrological conditions, while landscape patterns significantly mediated the effects of topography on soil and hydrology. Considering the pathway preferences of the model, landscape patterns could be the priority for characterizing and regulating PBF. According to a change-point analysis, the probability of PBF weakening increases dramatically when the proportion of farmland (Farm%) > 24.6%, degree of patch interspersion (Contagion index) < 64.5%, and Perimeter-Area Ratio Distribution (PARA) > 348.7. These findings provide new insights into catchment buffering mechanisms and can be used to promote the simultaneous achievement of agricultural production and environmental conservation goals.

Impacts of organic materials amendment on the soil antibiotic resistome in subtropical paddy fields.

The organic material amendment has been proven to change the soil antibiotic resistance genes (ARGs) profile, which may threaten human health through the food chain, but the effects and mechanisms of different organic materials on ARGs in paddy soils are less explored. In this study, a field experiment was set up with the treatments of conventional chemical fertilization (NPK) and common organic material amendment [rice straw (RS), swine manure (SM), and biochar (BC)] to explore the effects and mechanisms. In total, 84 unique ARGs were found across the soil samples with different organic material amendments, and they conferred resistance to the major antibiotic classes. Compared with NPK, SM significantly increased the detected number and relative abundance of ARGs. A higher detected number of ARGs than NPK was observed in BC, whereas BC had a lower relative abundance of ARGs than NPK. Compared with NPK, a detected number decrease was observed in RS, although abundance showed no significant differences. Compared with other treatments, a higher detected number and relative abundance of mobile genetic elements (MGEs) were observed in BC, indicating a higher potential for horizontal gene transfer. There were significantly positive relationships between the relative abundances of total ARGs and MGEs and the bacterial abundance. The network analysis suggested the important role of MGEs and bacterial communities in shaping the ARGs profile. Mantel test and redundancy analysis (RDA) suggested that soil carbon, nitrogen, and C/N were the major chemical drivers of the ARGs profile. The risk of ARGs spreading to the food chain should be considered when applying SM and biochar, which shifted the ARGs and MGEs profiles, respectively. Pre-treatment measures need to be studied to reduce the dissemination of ARGs in paddy fields.

Anaerobic primed CO2 and CH4 in paddy soil are driven by Fe reduction and stimulated by biochar.

Soil C inputs and its priming effect (PE) are important in regulating soil C accumulation and mitigating climate change; however, the factors that control the direction and intensity of PE remains unclear. Soil C accumulation is strongly affected by the reductive iron status in paddy fields, while the addition of organic substances increases the emission of certain gases (CO2/CH4) under the PE, contributing to climate change. Here, we elucidated the mechanism by which Fe reduction, measured by Fe(II) production, regulates PE for CO2 and CH4 in paddy soils. Specifically, we quantified PE induced by 13C-labeled straw in anaerobic paddy soil, augmented by ferrihydrite and/or biochar, over 150 days in a laboratory experiment. The PE of CO2 was initially negative (-15.3 to -41.5 mg C kg-1) before 20 days of incubation and subsequently became positive. PE intensity for both gases depended on ferrihydrite or biochar application. Straw+biochar had the highest PEs (CO2, 116.5 mg C kg-1; CH4, 309.4 mg C kg-1), while straw+ferrihydrite produced the lowest PEs (CO2, 41.3 mg C kg-1; CH4, 107.8 mg C kg-1). Fe reduction was approximately three times higher with straw+ferrihydrite than with straw alone and was further stimulated by additional biochar. Thus, biochar appeared to accelerate Fe reduction, destabilize mineral-bound organic C, and increase nutrient availability to microbes. Enhanced microbial C and N mining led to a positive PE for CO2. Cumulative PE for CH4 was 2-3 times higher than that for CO2, indicating conversion via methanogenesis. Biochar acted as an electron shuttle, increasing Fe reduction and stimulating interspecies electron transfer, and increased CH4 production. Therefore, Fe reduction and biochar jointly increased PE intensity for CH4. In conclusion, water and fertilizer management of paddy soil could contribute toward mitigating climate change.