Nitrate losses from forest during snowmelt: An underestimated source in mid-high latitude watershed.

The forest nitrate cycle is a crucial part of the watershed nitrate load but has received limited attention compared to that of agricultural and residential land. Here, we analyzed the status and sources of riverine nitrate fluxes and identified the characteristics and contribution of forest nitrate loss to the riverine system in a mid-high latitude forested watershed using monthly field sampling and a modified Soil and Water Assessment Tool (SWAT) with enhanced forest nutrient cycle representation. The results indicate that nitrate losses in the headwater stream and downstream exhibit different seasonal characteristics. The nitrate losses in the headwater stream show a bimodal pattern due to lower temperatures and snowmelt runoff. Redundancy analysis (RDA) revealed that, unlike nitrogen (N) fertilizer-induced nitrate loss in the rainy season, forest loss has a positive effect on headwater stream nitrate concentration during the snowmelt season. The modified SWAT was then utilized to simulate nitrate losses in forest lands. The forest nitrate export per unit area of the headwater stream (1.58 ± 1.78 kg/ha/yr) was observed to be higher than that of the downstream (0.67 ± 0.74 kg/ha/yr) due to high snowmelt and mineralization of active organic N. At watershed scale, forest lands contributed 8.18 ± 3.94 % of the total nitrate losses to the water system in the headwater watersheds during the snowmelt season, representing the highest level within the entire basin. A comparison with forest streams in similar low-temperature conditions worldwide revealed that increasing nitrate loss occurred after extreme cold weather or soil freezing events, with an average increment of 6.32 kg/ha/yr. Therefore, forest nitrate losses should be better characterized and included in future watershed N budgets in low-temperature regions, which might help to reduce the N budget uncertainty and improve watershed management.

Occurrence, distribution, and potential ecological risks of antibiotics in a seasonal freeze-thaw basin.

Antibiotics have gained increasing attention as pharmaceuticals widely existing in human society. Under low temperature conditions, antibiotics tend to have higher environmental persistence, which poses a potential threat to ecological environment, but research on antibiotics in low-temperature basins is still lacking. Therefore, for investigating occurrence, spatio-temporal distributions, and ecological risks of antibiotics in a seasonal freeze-thaw basin, rivers in Tumen River basin were selected and sampled, including 25 samples during the river-freezing season and 27 samples during the non-freezing season. Overall, climate characteristics of different latitudes and renewal frequency of antibiotics are important factors that lead to diversity of antibiotics in basins. Eleven target antibiotics were detected and their average concentrations during the river-freezing season (0.83-27.5 ng L-1) were lower than that during the non-freezing season (2.80-45.30 ng L-1), severely impacted by river flow, ice sealed-melting, and local feeding practices. In addition, total antibiotic concentrations are usually highest in downstream areas of human settlements, receiving input from husbandry and sewage, respectively. Through ecological risk assessment, norfloxacin and amoxicillin posed high risks to algae, which were identified as high-risk pollutants in basin.

Industrial impacts on vanadium contamination in sediments of Chinese rivers and bays.

Vanadium, like many trace metals, is persistent and detrimental to ecosystems at elevated concentrations. Likewise, it is versatile, functional, and used in many industries. Jiaozhou Bay (JZB) and Laizhou Bay (LZB) are valuable coastal ecosystems in China coexisting with several of these vanadium-related industries; however, limited studies have been conducted regarding vanadium occurrence, distribution, sources and risks in sediments. 208 surface sediment samples were collected from rivers and bays over two years and analyzed using inductively coupled plasma optical emission spectrometry. Overall, sediments near vanadium-related industries have significantly higher vanadium concentrations than those near traditional industries, with 30.3% and 22.9% higher average concentrations of vanadium in sediments of JZB and LZB, respectively. Vanadium accumulation at LZB is positively correlated with fine sediment, oxides (e.g., Fe, Ti, Mn), and organic matter content, while temporal changes in parts of JZB highlight the impacts of oxides, pH, and redox conditions on its accumulation. After geochemical normalization, the concentrations in marine samples from LZB showed slightly polluted sediments under the Modified Nemerow pollution index. Likewise, the elevated concentrations of vanadium in JZB, rivers and bay, were classified as slightly polluted and correlated with anthropogenic activities, such as the coal and petrochemical industries. Temporal changes indicated higher enrichments in 2019. Last, humans could be responsible for up to 46.8% and 16.2% of the vanadium accumulation in JZB and LZB, respectively, yet risks to species remain limited.

Forest leaf litter nutrient discharge patterns in snowmelt surface runoff and watershed scale remote sensed simulation.

The leaf litter decomposition is the important chain for the nutrient cycle in forest ecosystem, but its degradation dynamics and pulse discharge patterns in freeze-thawing watershed needed complete understanding. By integrating field observations and MODIS data, the temporal-spatial distributions of snow coverage and forest leaf litter biomass were analyzed. The critical period for snowmelt runoff under warming temperature and the relatively slow degradation patterns were identified. The on-site observations snowmelt runoff showed discharge concentration and fraction dynamics of typical forest leaf litter nutrients (carbon, nitrogen, and phosphorus) in thawing period. The snowmelt runoff flow and nutrient flux observed the linear regressions with the increased temperature from -8 °C to 6 °C (r2 = 0.443-0.987). The concentration of TOC, TN, and TP reached summit value around 50.0, 6.0, and 0.5 mg L-1 in the snowmelt runoff, respectively. The fraction analysis proved that the much high composition of dissolved organic fraction and the biggest organic phosphorus percentage was 94%. The comparison experiments of forest soil with or without leaf litter cover demonstrated that the leaf litter caused a lower discharge load in the snowmelt flow, and the leaf litter cover can decease the potential transport capability of the snowmelt runoff. Coupled with remote sensing data, the watershed leaf litter nutrient discharge model was developed with snowmelt hydrological process mode. The watershed averaged discharge of TOC, TN, and TP from deciduous broad-leaved forest leaf litter was around 851.99, 75.05, and 9.78 mg·m-2, respectively. The yearly simulation showed the spatial distribution variance of the nutrient discharge loads were held by different forest types, elevations, and slopes. The critical loss area identification provided new mitigations solution. The findings suggested that seasonal discharge of forest leaf litter nutrient in thawing period acted as a key contributor to watershed water pollution.

Enhanced release, export, and transport of diffuse nutrients from litter in forested watersheds with climate warming.

Variations in litter decomposition and nutrient migration are constraints to accurately estimate watershed diffuse forest pollution under the combined effects of topographic heterogeneity and climate change. In this study, remote sensing data, decomposition and leaching experiments, and the Soil and Water Assessment Tool (SWAT) were used to quantify the release, export, and transport characteristics of diffuse nutrients from forest litter under two climate scenarios (the current climate condition [S1] and the future warming and drying climate condition [S2]), and the impacts on aquatic environment were identified. The annual litter decomposition was 27.80 × 106 t in S2, which was 1.39 times that of S1. Additionally, the annual litter nutrient release in S2 (C, N, and P was 8.65 × 106, 3.31 × 105, and 1.57 × 104 t, respectively) also increased by 31.16%-45.62% compared with that of S1. The spatial patterns of nutrient export showed that the annual exports of C, N, and P in S1 were 109.77, 46.85, and 0.43 kg/ha, respectively. The annual nutrient export in S2 increased by 1.44 times, and S2 also had higher values of nutrient transport. In addition, variation trends of temperature and precipitation increased significantly with increasing altitude, which promoted differences in nutrient transport between S1 and S2 in the high-altitude areas. The response analysis of the diffuse nutrient in surface water also indicated that forest nutrient discharge load were critical factors affecting the aquatic environmental quality. This study indicated that climate warming accelerated litter decomposition and made litter a potential source of diffuse forest pollution, and watershed discharge load varied intensively with the terrestrial conditions. The combination of experiments and modeling can improve the accuracy of diffuse forest pollution simulation and provide valuable information for formulating watershed climate change adaptation strategies.

Diffuse nutrient export dynamics from accumulated litterfall in forested watersheds with remote sensing data coupled model.

Nutrients exported from forest litterfall significantly contribute to the global cycling of elements and the water quality in watersheds. Simulating the watershed discharge load is challenging because of the combined effects of the decomposing litterfall and topographic heterogeneity. We quantified the contribution of diffuse nutrient export from forest litterfall in a low temperature watershed using artificial rainfall experiments and watershed territorial modeling with remote sensing data, and therefore, the critical spatial factors and corresponding nutrient export dynamics were identified. Rainfall intensity and terrain slope were found to be the key factors for nutrient export under different litterfall decomposition conditions. Based on the moderate resolution imaging spectroradiometer data and field observations, the temporal patterns of litterfall biomass of two types of dominant forests (broad-leaved and mixed) were interpreted. The spatial patterns of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) exports from watershed litterfall were simulated by coupling the observed discharge parameters under artificial rainfall conditions and watershed surface flow modeling with the hydrological characteristics of the forested areas. The average watershed TOC, TN, and TP loads exported from the litterfall were approximately 58.22, 7.89, and 0.37 kg ha-1 a-1, respectively. The exported loads of TOC, TN, and TP varied with the forest types, and the loads from the litterfall of deciduous broad-leaved forest were found to be ∼50-70% of loads from the litterfall of mixed forest. A comparison with similar studies worldwide also indicated that low temperature decreased the litterfall decomposition rate and diffuse nutrient export. This study indicated that litterfall nutrients were a key contributor to watershed water pollution, and their spatial discharge trend varied intensively with the terrestrial conditions. The modified simulation methods were found to accurately assess the cycling of nutrients from the forest litterfall on a watershed scale.