Search

Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (fNO3-), ammonium (fNH4+), and organic N (fEON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of fNO3-, fNH4+, and fEON. The fNO3- increases with MAT, reaching 46% at 28.5 °C. The fNH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the fEON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems. Isotopic constraints reveal that soil nitrogen contribution to global plants is temperature-controlled, not by precipitation or nitrogen deposition. As temperatures rise, inorganic nitrogen becomes more important and preferred over organic nitrogen. [ABSTRACT FROM AUTHOR]

Epilithic mosses are early colonizers of the terrestrial biosphere, which constitute a special ecosystem regulating rock‐atmosphere interactions. Terrestrial mosses can take up nitrate (NO3−), a major form of bioavailable N, from soil substrates. However, the importance of substrate NO3− relative to atmospheric NO3− remains unclear in moss NO3− utilization. This has prevented the understanding of moss NO3− dynamics and their responses to environmental N loadings. This study investigated monthly concentrations, δ15N, and δ18O of NO3− in four epilithic moss species from August 2006 to August 2007 in Guiyang, southwestern China. We developed a non‐steady state isotope mass‐balance model to evaluate fractional contributions of atmospheric NO3− (Фatm) and soil NO3− (Фsoil), moss NO3− uptake flux (Finflux), moss NO3− reduction flux (Freduction), and the percentage of NO3− reduction in moss NO3− uptake (freduced). The monthly Фsoil values averaged 53 ± 13% and the monthly freduced values averaged 50 ± 35%. Both the monthly Freduction and freduced increased as the monthly Finflux increased, particularly when the Фsoil values were higher than Фatm values. However, the amount of annual NO3− reduction (219.7 ± 30.5 μg‐N/g, dw) accounted for only 1.0 ± 0.2% of the bulk N of the mosses. We conclude that half of the NO3− in epilithic mosses is derived from the soil NO3− and that NO3− uptake from the soil induces moss NO3− reduction, but the total NO3− assimilation contributed a low fraction of the total N in the studied mosses. These findings are important for understanding N sources and N dynamics in terrestrial mosses. Key Points: A non‐steady state isotope mass‐balance model was used to evaluate moss nitrate sources and reductionEpilithic mosses acquire about half of their nitrate from the underlying soil substratesNitrate contributed a low fraction of the total N in the studied mosses [ABSTRACT FROM AUTHOR]

Anthropogenic nitrogen (N) emission has been the major cause of rapidly increasing atmospheric N deposition in many regions of the world. However, fluxes, distributions, and physi-chemical compositions of regional N deposition and the response of N deposition levels to N emissions remain poorly characterized, particularly in high N-emission regions. To explore these issues, this study collated observations on N deposition fluxes in the Beijing-Tianjin-Hebei (BTH) region of northern China. We found that total inorganic N (TIN) deposition in the study region averaged 61.3 ± 13 kg-N/ha/yr based on observations from 1998 to 2017. Respectively, about 97%, 73%, 20%, 32%, and 59% of farmland, constructed land, forest, grassland, and water areas had TIN deposition above 50 kg-N/ha/yr. These results indicated substantial ecological risks of atmospheric N loading to these ecosystems. Dry deposition accounted for 59% of TIN deposition and gaseous N accounted for 73% in dry IN deposition. In gaseous N deposition, ammonia, N dioxides, and nitric acid accounted for 42%, 47%, and 11%, respectively. Ratios of NH 3 -N (N in ammonia) to (NO 2 -N + HNO 3 -N) (N in N dioxides, nitric acid) or NH 4 -N (N in ammonium) to NO 3 -N (N in nitrate) averaged 0.8, 2.0, 1.0, 1.9, and 1.3 in gaseous, particulate, dry, wet, and total deposition fluxes, respectively. These indicated different transformation and deposition mechanisms between NH 4 -N and NO 3 -N under high emissions. Moreover, we found a non-linear relationship between TIN deposition and anthropogenic IN emissions. All these results provide a systematic assessment of the N deposition and its responses to human activities in a high N-emission region, which is valuable for emission mitigation and effect evaluation of regional N pollution. Graphical abstract Unlabelled Image • Nitrogen (N) deposition was dissected in a hotspot region of N pollution. • Anthropogenic N emissions caused high N deposition to major land uses. • There was a non-linear relationship between regional N emission and deposition. [ABSTRACT FROM AUTHOR]