To investigate the nitrification potential of phyllospheric microbes, we incubated throughfall samples collected under the canopies of Japanese cedar (Cryptomeria japonica) and analyzed the transformation of inorganic nitrogen in the samples. Nitrate concentration increased in the unfiltered throughfall after 4 weeks of incubation, but remained nearly constant in the filtered samples (pore size: 0.2 and 0.4 µm). In the unfiltered samples, δ18O and δ15N values of nitrate decreased during incubation. In addition, archaeal ammonia monooxygenase subunit A (amoA) genes, which participate in the oxidation of ammonia, were found in the throughfall samples, although betaproteobacterial amoA genes were not detected. The amoA genes recovered from the leaf surface of C. japonica were also from archaea. Conversely, nitrate production, decreased isotope ratios of nitrate, and the presence of amoA genes was not observed in rainfall samples collected from an open area. Thus, the microbial nitrification that occurred in the incubated throughfall is likely due to ammonia-oxidizing archaea that were washed off the tree canopy by precipitation.

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha⁻¹ yr⁻¹. We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

We assessed how warm air temperatures, high solar radiation, and weak wind speeds might induce hypoxia in a shallow lake during a heat wave. We simulated bottom‐water dissolved oxygen concentrations and compared concentrations in 2022 with the average for the previous 30 years. We found that hypoxia was most sensitive to wind speeds. When the wind speed was low, convection was insufficient to prevent hypoxia, but there was no hypoxia if the wind speed equaled the average speed during the previous 30 years. However, if solar radiation and air temperatures equaled the respective averages during the previous 30 years, hypoxia did not occur, even if wind speeds were low. We conclude that the combined effects of weak winds and either high solar radiation or air temperatures induced hypoxia during the heat wave of 2022.

Ecosystems of suburban landscapes (i.e., forest, inland water ecosystem) are threatened by high nitrogen (N) loadings derived from urban air pollutants. Forest ecosystems under high chronic N loadings tend to leach more N via streams. In the northern suburbs of Tokyo, N deposition loading on terrestrial ecosystems has increased over the past 30 years. In this region, we investigated nitrate concentrations in 608 independent small forested catchment water samples from northeastern suburbs of Tokyo. The nitrate concentrations varied from 0.07 to 3.31 mg-N L⁻¹ in this region. We evaluated the effects of N deposition and catchment properties (e.g., meteorological and topographic factors, vegetation and soil types) on nitrate concentrations. In the random forest model, simulated N deposition rates from an atmospheric chemistry transportation model explained most of the variance of nitrate concentration. To evaluate the effects of afforestation management in the catchment, we followed a model-based recursive partitioning method (MOB). MOB succeeded in data-driven identification of subgroups with varying sensitivities to N deposition rate by vegetation composition in the catchment. According to MOB, the catchment with dominant coniferous coverage that mostly consisted of plantation with old tree age tended to have strong sensitivity of nitrate concentrations to N deposition loading.