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Biochar application to amend acidified tobacco-soils can enhance tobacco quality and reduce nitrous oxide (N2O) emissions. Microplastics from agricultural mulch are commonly found in cash-crop farmland soils and, together with biochar, affect soil N2O emissions. In this study, we applied three types of microplastics (polyethylene, PE; polylactic acid, PLA; polybutylene adipate terephthalate, PBAT) and rice biochar alone or in combination to acidified tobacco planting soil in central China to investigate their effects on soil N2O emissions, soil chemical properties, nitrogen-cycle-related functional genes, and microbial functional diversity during a 35-day laboratory incubation period. Significant increases in N2O emissions were observed with PE and PLA, which raised emissions by 15.96 % and 21.52 %, respectively. Additionally, different microplastics affected soil N2O emissions through distinct regulatory pathways. Co-application of microplastics and biochar suppressed N2O emissions compared to microplastics alone. Biochar mitigates N2O emissions mainly by increasing the abundance of the nosZ gene. It can remediate soil contaminated by microplastics and reduce their negative impacts on the soil environment. This study provides deeper insight into the effects of microplastics on soil nitrogen cycling and biochar-mitigated remediation of microplastic-contaminated soil.

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Straw return, as an important measure for soil fertility improvement in farmland, significantly affects the emissions of greenhouse gases N2O and CO2. Thus, the collected soil samples from five long-term (30-year) fertilization treatments (no fertilization, CK; recommended chemical fertilizer, F; 200 % of recommended chemical fertilizer, 2F; pig manure, M; and chemical fertilizer combined with pig manure, FM) were amended with and without straw and incubated under constant temperature and humidity conditions (25 ℃ and 65 % maximum field water holding capacity) for 20 days so as to investigate the key factors influencing N2O and CO2 emissions in response to straw addition in long-term fertilization treatments. The results showed that fertilization significantly increased N2O emissions. Compared to those under the unfertilized treatment[(22.05 ±2.09) µg·kg-1, calculated as nitrogen, the same as below], cumulative N2O emissions from the chemical fertilizer treatments significantly increased by 119 %-195 %[(48.38 ±20.81) µg·kg-1 and (65.13 ±12.55) µg·kg-1 from the F and 2F treatments, respectively], and those from the manure treatments increased by 275 %-399 %[(82.72 ±12.73) µg·kg-1 and (1 101.99 ±425.71) µg·kg-1 from the M and FM treatments, respectively]. Soil NO3--N, DOC, and DTN were the main factors influencing N2O emissions from fertilized treatments in the absence of straw addition. Straw addition significantly increased cumulative N2O emissions by 345 % and 247 % in the 2F and M treatments, respectively, compared to those in the corresponding fertilized treatments without straw addition, with no significant effect on N2O emissions in the CK, F, and FM treatments. Straw addition increased DOC content and microbial activity and decreased soil NO3--N and DTN contents, thereby increasing N2O emissions. Fertilization also significantly increased CO2 emissions. Compared to those from the unfertilized treatment, cumulative CO2 emissions from the manure treatments significantly increased by 120 %-130 %[(122.11 ±4.3) mg·kg-1 (calculated as carbon, the same as below) and (116.47 ±4.55) mg·kg-1 from the M and FM treatments, respectively], and those in the 2F treatment increased by 28 %[(65.13 ±12.55) mg·kg-1]. In the absence of straw addition, soil MBC, DOC, and DTN were the main factors influencing CO2 emissions. Compared to those in the treatments without straw addition, straw addition significantly increased cumulative CO2 emissions by 660 %-1132 % among fertilization treatments, due to increased DOC and MBC contents and enhanced microbial activity. In conclusion, straw addition significantly increased N2O and CO2 emissions through increased soil DTN consumption and DOC content among fertilization treatments. In soils treated with manure amendment, straw return should be rationally considered for the purpose of balancing the comprehensive trade-offs between fertility improvement and greenhouse gas emissions.

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Soil contains a substantial amount of organic carbon, and its feedback to global warming has garnered widespread attention due to its potential to modulate atmospheric carbon (C) storage. Temperature sensitivity (Q10) has been widely utilized as a measure of the temperature-induced enhancement in soil organic carbon (SOC) decomposition. It is currently rare to incorporate Q10 of CO2 and CH4 into the study of waterlogged soil profiles and explore the possibility of artificially reducing Q10 in rice fields. To investigate the key drivers of Q10, we collected 0-1 m paddy soil profiles, and stratified the soil for submerged anaerobic incubation. The relationship between SOC availability, microbial activity, and the Q10 of CO2 and CH4 emissions was examined. Our findings indicate that as the soil layer deepens, soil C availability and microbial activity declined, and the Q10 of anaerobic degradation increased. Warming increased C availability and microbial activity, accompanied by weakened temperature sensitivity. The Q10 of CO2 correlated strongly with soil resistant C components, while the Q10 of CH4 was significantly influenced by labile substrates. The temperature sensitivity of CH4 (Q10 = 3.99) was higher than CO2 emissions (Q10 = 1.78), indicating the need for greater attention of CH4 in predicting warming's impact on anaerobic degradation in rice fields. Comprehensively assessing CO2 and CH4 emissions, the 20-40 cm subsurface soil is the most temperature-sensitive. Despite being a high-risk area for C loss and CH4 emissions, management of this soil layer in agriculture has the potential to reduce the threat of global warming. This study underscores the importance of subsurface soil in paddy fields, advocating greater attention in scientific simulations and predictions of climate change.

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The sudden increase in water nutrients caused by environmental factors have always been a focus of attention for ecologists. Fertilizer inputs with spatio-temporal characteristics are the main contributors to water pollution in agricultural watersheds. However, there are few studies on the thresholds of nitrogen (N) and phosphorus (P) fertilization rates that affect the abrupt deterioration of water quality. This study aims to investigate 28 ponds in Central China in 2019 to reveal the relationships of basal and topdressing fertilization intensities in surrounding agricultural land with pond water N and P concentrations, including total N (TN), nitrate (NO3--N), ammonium (NH4+-N), total P (TP), and dissolved P (DP). Abrupt change analysis was used to determine the thresholds of fertilization intensities causing sharp increases in the pond water N and P concentrations. Generally, the observed pond water N and P concentrations during the high-runoff period were higher than those during the low-runoff period. The TN, NO3--N, TP, DP concentrations showed stronger positive correlations with topdressing intensities, while the NH4+-N concentrations exhibited a higher positive correlation with basal intensities. On the other hand, the NO3--N concentrations had a significant positive correlation with the topdressing N, basal N, and catchment slope interactions. Significant negative correlations were observed between all water quality parameters and pond area. Spatial scale analysis indicated that fertilization practices at the 50 m and 100 m buffer zone scales exhibited greater independent effects on the variations in the N and P concentrations than those at the catchment scale. The thresholds analysis results of fertilization intensities indicated that pond water N concentrations increased sharply when topdressing and basal N intensities exceeded 163 and 115 kg/ha at the 100 and 50 m buffer zone scales, respectively. Similarly, pond water P concentrations rose significantly when topdressing and basal P intensities exceeded 117 and 78 kg/ha at the 50 m buffer zone scale, respectively. These findings suggest that fertilization management should incorporate thresholds and spatio-temporal scales to effectively mitigate pond water pollution.

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Nitrate-nitrogen (NO3--N) loss is a significant contributor to water quality degradation in agricultural catchments. The amount of nitrogen (N) fertilizer input in citrus orchard is relatively large and results in significant NO3--N loss, compared to cropland. To promote sustainable N fertilizer management, it is crucial to identify the sources of runoff NO3--N loss in citrus orchards catchments. Particularly, we poorly know the sources of NO3--N and the mitigation mechanisms in these areas, which are highly polluted with NO3--N in water bodies. In this study conducted in central China, we conducted a field experiment with four treatments (CK: no N fertilizer; CF: conventional N fertilizer, 371.3kg N ha-1 yr-1 urea; OM: CF with organic manure; GM: CF with legume green manure) and a catchment-scale experiment in two citrus orchards (34.3%; 51.6%) catchments. To determine the source of runoff NO3--N loss, we used the dual isotope tracer method (δ15N and δ18O of NO3-) to identify the sources of NO3--N, and a 15-day incubation experiment to determine the potential and rate of soil N mineralization. Our findings revealed that soil organic nitrogen (SON) mineralization was the primary contributor to runoff NO3--N loss, and soil N mineralization potential (0.65⁎⁎⁎) and rate (0.54⁎⁎⁎) were the key factors impacting NO3--N loss. Interestingly, organic manure significantly increased 29.0% of NO3--N loss derived from SON in the runoff by enhancing soil N mineralization potential (+36.6%) and rate (+77.1%). But green manure mulching significantly reduced the soil N mineralization rate (-18.6%) compared to organic manure application, making it the most effective measure to reduce NO3--N loss (-12.4%). Our study highlights the critical role of regulating SON mineralization in controlling NO3--N pollution in surface waters in citrus orchard catchments.

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