The high levels of nitrate (NO3-) in the surface water have contributed to eutrophication and other eco-environmental damages worldwide. Although the excessive NO3- concentrations in rivers were often attributed to anthropogenic activities, some undisturbed or slightly disturbed rivers also had high NO3- levels. This study utilized multi-pronged approaches (i.e., river natural abundance isotopes, N-15-labeling techniques, and qPCR) to provide a comprehensive explanation of the reason for the high NO3- levels in a river draining forest-dominated terrene. The river natural abundance isotopes (delta N-15/delta O-18-NO3-) indicated that the soil source (i.e., soil organic nitrogen-SON and chemical fertilizer-CF) were the primary contributors to the NO3-, and the NO3- removal was probably prevalent in the basin scale. The N-15-labeling techniques quantitatively showed that denitrification and anammox were stronger than nitrification in the soils and sediments. Structural equation models suggested that nitrification in the soils was regulated by NH4+-N contents, which, in turn, were closely related to fertilization in spring. Denitrification and anammox were largely controlled by elevation and functional gene abundances (i.e., nirK and hzsB, respectively). The hydrological isotopes (i.e., delta D/delta O-18-H2O) indicated that the transport of NO3- from soil to the river was related to the intensity of runoff leaching to the soil, In contrast, the riverine NH4+ was largely from point sources; thus, increasing runoff led to a dilution effect. This study clearly showed that soil biogeochemistry and hydrological condition of a river basin jointly shaped the high NO3- levels in the almost undisturbed river.

Soil salinity typically exhibits non-uniform distribution in the natural environment. However, how vertically non-uniform distribution of soil salinity in the root zone (VNDSR) regulated plant nitrogen metabolism is still largely elusive. This study aimed to investigate the impact of VNDSR on leaf Malondialdehyde (MDA) content, upper and lower root activity, leaf Na+/Ca2+ and Na+/K+, various tomato organs' nitrogen concentration (%) and natural abundance of nitrogen isotopes (delta N-15,parts per thousand), and nitrogen utilization efficiency of tomato plants. Four treatments were established, including the upper layer of the root zone having soil salinity levels of 1 parts per thousand, 1 parts per thousand, 2 parts per thousand, and 3 parts per thousand, while the corresponding lower layer of the root zone had soil salinity levels of 1 parts per thousand, 5 parts per thousand, 4 parts per thousand, and 3 parts per thousand, respectively, denoted as T-1:1, T-1:5, T-2:4, and T-3:3. The results showed that under the same average soil salinity conditions and compared to the treatment with uniform soil salinity distribution in the root zone (T-3:3), the VNDSR treatment (T-1:5) significantly reduced leaf MDA content (p < 0.01), Na+/Ca2+ (p < 0.01) and Na+/K+ (p < 0.01), and stem delta N-15 values (p < 0.05). Moreover, the VNDSR treatment (T-1:5) significantly increased the ratio of upper and lower root biomass-weighted root activity (p < 0.01), tomato fruit yield (p < 0.01), and nitrogen partial factor productivity (PFP, gg(-1), p < 0.01) compared to uniform salt distribution treatment (T-3:3). There were significant positive correlations (p < 0.05) between leaf delta N-15 values and Nitrogen Absorption Ratio (NAR, %, p < 0.05) and PFP (p < 0.05), indicating that under VNDSR, delta N-15 values can serve as an indicator that comprehensively reflects the information of plant nitrogen utilization efficiency. In conclusion, the VNDSR could mitigate the damage of salt stress to tomatoes, enhance plant nitrogen uptake and utilization efficiency, and promote the growth and development of tomatoes.

The study aims to evaluate the difference of nitrogen (N) utilization in peanut varieties with different nodulation efficiency and the contribution of different N sources to yield formation. Based on an outdoor pot experiment, N-15 isotope-labeled urea was used as a N source to investigate the effects of different N fertilization levels (N rates with 45, 75, 105, 135, 165 kg N ha(-1), defined as N45, N75, N105, N135 and N165 in the study, respectively) on peanut photosynthesis, photosynthate accumulation, yield, and N distribution and transport. The results showed that N application can improve peanut yield by creating photosynthesis, dry matter weight, and N accumulation, and the N105 treatment had the most significant effect. However, higher N applications inhibited the number of peanut root nodules. The ratio of N supply for peanuts from nodules, soil, and fertilizer at the pod setting stage was about 5:3:2, and the ratio of fertilizer distribution for low nodulation peanut variety of reproductive organ (pod) to nutrient organs (root, stem, leaf) was about 3:2, while the high nodulation variety was about 1:1. Biological N fixation is an important N source during peanut growth and development. Appropriate N fertilizer can further promote peanut growth and yield formation without inhibiting nodulation and N fixation. In agricultural production, optimizing N fertilizer management (105 similar to 135 kg N ha(-1)) in combination with using nodulation efficient peanut varieties not only promotes the N-cycling in agriculture, but also effectively reduce the waste of N fertilizer as well as environmental damage.

Background and aims Cover crops can increase nitrogen (N) retention in agroecosystems by taking up soil soluble N when the grain crop is absent. We examined how the combination of cover crops and variability in winter conditions can affect soil N retention and N transfer to the subsequent crop.Methods We used N-15 tracer to quantify how the presence of cover crops (both winter-hardy and winter-killed) modifies the recovery by a corn crop of soil soluble N added the previous fall, and we used snow removal to assess how increased freezing would alter N-15 recovery. We predicted snow removal would decrease N-15 recovery in corn, and this decrease would be highest for plots with winter-hardy cover crops, given they remain vulnerable to increased frost over winter.Results We used N-15 tracer to quantify how the presence of cover crops (both winter-hardy and winter-killed) modifies the recovery by a corn crop of soil soluble N added the previous fall, and we used snow removal to assess how increased freezing would alter N-15 recovery. We predicted snow removal would decrease N-15 recovery in corn, and this decrease would be highest for plots with winter-hardy cover crops, given they remain vulnerable to increased frost over winter.Conclusion Although increased soil freezing reduced grain N recovery, cover crops increased soil N retention, which indicates decreased N losses to the surrounding environment, and the potential for increased contributions to grain N in future years.

Despite the fact that winter lasts for a third of the year in the temperate grasslands, winter processes in these ecosystems have been inadequately represented in global climate change studies. While climate change increases the snow depth in the Mongolian Plateau, grasslands in this region are also simultaneously facing high pressure from land use changes, such as grazing, mowing, and agricultural cultivation. To elucidate how these changes affect the grasslands' winter nitrogen (N) budget, we manipulated snow depth under different land use practices and conducted a(15)NH(4)(15)NO(3)-labeling experiment. The change in(15)N recovery during winter time was assessed by measuring the(15)N/N-14 ratio of root, litter, and soils (0-5 cm and 5-20 cm). Soil microbial biomass carbon and N as well as N2O emission were also measured. Compared with ambient snow, the deepened snow treatment reduced total(15)N recovery on average by 21.7% and 19.2% during the first and second winter, respectively. The decrease in(15)N recovery was primarily attributed to deepened snow increasing the soil temperature and thus microbial biomass. The higher microbial activity under deepened snow then subsequently resulted in higher gaseous N loss. The N2O emission under deepened snow (0.144 kg N ha(-1)) was 6.26 times than that of under ambient snow (0.023 kg N ha(-1)) during the period of snow cover and spring thaw. Although deepened snow reduced soil(15)N recovery, the surface soil N concentration remained unchanged after five years' deepened snow treatment because deepened snow reduced soil N loss via wind erosion by 86%.

Warming in the Arctic accelerates top-soil decomposition and deep-soil permafrost thaw. This may lead to an increase in plant-available nutrients throughout the active layer soil and near the permafrost thaw front. For nitrogen (N) limited high arctic plants, increased N availability may enhance growth and alter community composition, importantly affecting the ecosystem carbon balance. However, the extent to which plants can take advantage of this newly available N may be constrained by the following three factors: vertical distribution of N within the soil profile, timing of N-release, and competition with other plants and microorganisms. Therefore, we investigated species- and depth-specific plant N uptake in a high arctic tundra, northeastern Greenland. Using stable isotopic labelling (N-15-NH4+), we simulated autumn N-release at three depths within the active layer: top (10 cm), mid (45 cm) and deep-soil near the permafrost thaw front (90 cm). We measured plant species-specific N uptake immediately after N-release (autumn) and after 1 year, and assessed depth-specific microbial N uptake and resource partitioning between above- and below-ground plant parts, microorganisms and soil. We found that high arctic plants actively foraged for N past the peak growing season, notably the graminoidKobresia myosuroides. While most plant species (Carex rupestris,Dryas octopetala,K. myosuroides) preferred top-soil N, the shrubSalix arcticaalso effectively acquired N from deeper soil layers. All plants were able to obtain N from the permafrost thaw front, both in autumn and during the following growing season, demonstrating the importance of permafrost-released N as a new N source for arctic plants. Finally, microbial N uptake markedly declined with depth, hence, plant access to deep-soil N pools is a competitive strength. In conclusion, plant species-specific competitive advantages with respect to both time- and depth-specific N-release may dictate short- and long-term plant community changes in the Arctic and consequently, larger-scale climate feedbacks.

Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. To evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during 5years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research (CiPEHR) project. Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the 5year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures, and higher levels of soil moisture. Within 5years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools.

Winter biogeochemical processes have received considerable attention. Biological processes (e.g., microbial respiration and plant photosynthesis) do not cease, even at sub-zero temperatures. However, our knowledge of plant nitrogen (N) uptake at sub-zero soil temperatures is particularly limited for deciduous plant species, which do not have leaves during winter. We investigated plant N uptake by evergreen and deciduous species and soil N processes during sub-zero soil temperatures in cool temperate forest soil. Isotopically labelled nitrate (NO3-N-15) was injected into soil as a tracer of plant uptake and soil N dynamics at sub-zero temperature soil at a cool temperate field site. Over a period of 41 days, 6-48 mg/kg DW-1 of N-15 accumulated in evergreen species and deciduous tree species. Furthermore, the N-15 content in ammonium increased, suggesting ammonium production at sub-zero soil temperatures. The increase in (NH4)-N-15 was positively correlated with soil moisture, indicating an important role for soil water in N dynamics at sub-zero soil temperatures. Our findings demonstrate that N uptake by plants and soil N transformation did not cease at sub-zero soil temperatures. Further studies are needed to understand the importance of N dynamics at sub-zero soil temperatures.