In recent years, excessive accumulations of iron (Fe), manganese (Mn), and nitrogen (N) have been observed in the groundwater of agricultural regions, particularly in flood irrigation areas. Nevertheless, the causes of this phenomenon and the associated hydrobiogeochemical processes remain elusive. This study demonstrated that redox fluctuations instigated by flood irrigation triggered a synergistic interaction between the N cycles and the activation of Fe and Mn oxides, thereby resulting in elevated concentrations of Fe, Mn, and N simultaneously. Static experiments revealed that the properties of the topsoil exerted a profound influence on the N induced release of Fe and Mn. The black soil (TFe: 1.5-2.3 times, Mn(II): 1.1-1.5 times, nitrate: 1.3-1.4 times) had greater release potential than meadow and dark brown soils due to higher electron donors/acceptors and substrates. Dynamic column experiments further elucidated that the wet-dry cycles induced by agricultural cultivation regulated the release process through the formation of zonal redox gradients and the structuring of microbial community. Organic nitrogen mineralization, chemolithotrophic nitrification, and Feammox/Mnammox were identified as the primary mechanisms responsible for the reductive dissolution of Fe-Mn oxides. On the other hand, autotrophic denitrification, with nitrate serving as the electron acceptor, constituted the main process for the reoxidation of Fe and Mn. Furthermore, the agricultural activities exerted a significant impact on the nitrate attenuation process, ultimately resulting in the recurrence of TFe (black soil: 1.5-6.3 times) and nitrate (black soil: 1.4-1.6 times) pollution during the phase after harvesting of rice (days 40-45) in saturated zone. The findings of this study not only deepened the understanding of the intricate interactions and coupled cycles between primary geochemical compositions and anthropogenic pollutants, but also provided a scientific foundation for the effective management and prevention of groundwater pollution stemming from agricultural cultivation processes.

Styrofoams are widely used as floats and are likely to be damaged by burrowing animals in mariculture. However, the fate of fragmented Styrofoams and their interaction with organisms are not clear in coastal environments. In the present study, field investigations were conducted in 14 sites along the coast of China from July 2023 to October 2024. Results showed that Styrofoams were buried by sand or soil at a depth of up to 50-60 cm in patchy and belted distribution patterns. The abundances of Styrofoams in the deep layer of sediments ranged from 94 to 3042 items/kg. The buried Styrofoams were bunched by roots of six plant species in three ways, i.e., wrapping, crossing, and clinging. The abundance of bunched foams ranged from 1 to 495 items/plant. Simulation experiments in the laboratory showed that plant roots could interact with Styrofoams after 14 days of exposure and tended to cross through the gaps of foam materials. Our study indicates that the fragmented Styrofoams coming from mariculture floats could remain in the sediments due to the physical and biological factors, providing new insight into the biogeochemical cycle of plastic debris.

Peatland is a key component of terrestrial ecosystems in permafrost regions and have important effects on climate warming. Soil enzymes are involved in biogeochemical cycle of soil carbon (C), nitrogen (N) and phosphorus (P), which can be used as early sensitive indicators of soil nutrient changes caused by climate change. To predict the possible effects of permafrost degradation on soil enzymes in peatlands, ten peatlands from three types of permafrost regions along the permafrost degradation sequence (predominantly continuous permafrost region-predominantly continuous and island permafrost region-sparsely island permafrost region) in northeast China were selected to examine the activities of soil invertase, beta-glucosidase, urease and acid phosphatase and their relationships with soil physicochemical properties. The results demonstrated that permafrost type had significant effect on soil enzyme activities. Soil enzyme activities in predominantly continuous and island permafrost region were significantly higher than those in sparsely island permafrost region and predominantly continuous permafrost region. The activities of four soil enzymes were higher in 0-15 cm than 15-30 cm soil layer. Soil enzymes activities were positively correlated with soil ammonia nitrogen (NH4+-N), soil moisture content (SMC), total phosphorus (TP) and total nitrogen (TN), but negatively correlated with soil nitrate nitrogen (NO3--N). Soil inorganic nitrogen and moisture contents were the main factors affecting soil enzyme activities, with NH4+-N accounted for 41.6% of the variance, SMC 29.6%, and NO3--N 11.0%. These results suggested that permafrost degradation may change soil enzyme activities by changing soil physicochemical properties. In this study, only 0-30 cm peat soil in permafrost regions was collected during the complete thawing period of permafrost active layer, further studies should be placed on the change of soil enzyme activities in active layer and permafrost layer during freezing and thawing process in the southernmost location of northeast China in the Eurasia permafrost body and boreal forest belt.

Recent global warming models project a significant change in winter climate over the next few decades. The decrease in snowpack in the winter will decrease the heat insulation function of the snowpack, resulting in increased soil freeze-thaw cycles. Here, we examined the impact of winter freeze-thaw cycles on year-round dissolved nitrogen (N) and carbon (C) dynamics and their relationship with dissolved organic matter and microbial biomass in soil by conducting an in situ experimental reduction in snowpack. We investigated dissolved inorganic N (NH4+ and NO3-), dissolved organic N (DON), dissolved organic carbon (DOC), inorganic N leaching, soil microbial biomass, and microbial activities (mineralization and nitrification) in the surface soil of a northern hardwood forest located in Japan. Experimental snowpack reduction significantly increased the number of soil freeze-thaw cycles and soil frost depth. The NH4+ content of the surface soil was significantly increased by the amplified soil freeze-thaw cycles due to decreased snowpack, while the soil NO3- content was unchanged or decreased slightly. The gravimetric soil moisture, DON and DOC contents in soil and soil microbial biomass significantly increased by the snowpack removal in winter. Our results suggest that the amplified freeze-thaw cycles in soil increase the availability of DON and DOC for soil microbes due to an increase in soil freezing. The increases in both DON and DOC in winter contributed to the enhanced growth of soil microbes, resulting in the increased availability of NH4+ in winter from net mineralization following an increase in soil freeze-thaw cycles. Our study clearly indicated that snow reduction significantly increased the availability of dissolved nitrogen and carbon during winter, caused by increased soil water content due to freeze-thaw cycles in winter.