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Summary: Conifers are considered to prefer to take up ammonium (NH4+) over nitrate (NO3−). However, this conclusion is mainly based on hydroponic experiments that separate roots from soils. It remains unclear to what extent mature conifers can use nitrate compared to ammonium under field conditions where both roots and soil microbes compete for nitrogen (N).We conducted an in situ whole mature tree nitrogen‐15 (15N) labeling experiment (15NH4+ vs 15NO3−) over 15 d to quantify ammonium and nitrate uptake and assimilation rates in four 40‐yr‐old monoculture coniferous plantations (Pinus koraiensis, Pinus sylvestris, Picea koraiensis and Larix olgensis, respectively).For the whole tree, 15NO3− contributed 39% to 90% to total 15N tracer uptake among four plantations during the study period. At day 3, the 15NO3− accounted for 77%, 64%, 62% and 59% by Larix olgensis, Pinus koraiensis, Pinus sylvestris and Picea koraiensis, respectively.Our study indicates that mature coniferous trees assimilated nitrate as efficiently as ammonium from soils even at low soil nitrate concentration, in contrast to the results from hydroponic experiments showing that ammonium uptake dominated over nitrate. This implies that mature conifers can adapt to increasing availability of nitrate in soil, for example, under the context of globalization of N deposition and global warming. [ABSTRACT FROM AUTHOR]

Denitrification is a major process contributing to the removal of nitrogen (N) from ecosystems, but its rate is difficult to quantify. The natural abundance of isotopes can be used to identify the occurrence of denitrification and has recently been used to quantify denitrification rates at the ecosystem level. However, the technique requires an understanding of the isotopic enrichment factor associated with denitrification, which few studies have investigated in forest soils. Here, soils collected from two tropical and two temperate forests in China were incubated under anaerobic or aerobic laboratory conditions for two weeks to determine the N and oxygen (O) isotope enrichment factors during denitrification. We found that at room temperature (20 °C), NO 3 − was reduced at a rate of 0.17 to 0.35 μg N g −1 h −1 , accompanied by the isotope fractionation of N ( 15 ε ) and O ( 18 ε ) of 31‰ to 65‰ (48.3 ± 2.0‰ on average) and 11‰ to 39‰ (18.9 ± 1.7‰ on average), respectively. The N isotope effects were, unexpectedly, much higher than reported in the literature for heterotrophic denitrification (typically ranging from 5‰ to 30‰) and in other environmental settings (e.g., groundwater, marine sediments and agricultural soils). In addition, the ratios of Δδ 18 O:Δδ 15 N ranged from 0.28 to 0.60 (0.38 ± 0.02 on average), which were lower than the canonical ratios of 0.5 to 1 for denitrification reported in other terrestrial and freshwater systems. We suggest that the isotope effects of denitrification for soils may vary greatly among regions and soil types and that gaseous N losses may have been overestimated for terrestrial ecosystems in previous studies in which lower fractionation factors were applied. [ABSTRACT FROM AUTHOR]

• First long-term daily soil NO flux measurement with an automated system from a temperate forest in Northeast China. • Soil temperature explained 90–92% of the seasonal variation in NO emission, with a Q 10 of 3.67. • The optimum soil moisture for NO emission was 37% WFPS during the growing season. • Annual soil NO emission was 0.42±0.04 kg N ha−1, being 31% of the global forest average. • Soil NO emission during the non-growing season was only 7.7% of the annual flux. Nitric oxide (NO) is a highly reactive trace gas affecting atmospheric chemistry and air quality. Although forest soils have been recognized as an important source of atmospheric NO, there are large uncertainties in global forest soil NO emission inventories (ranged from 0.03 to 8.00 kg N ha−1, averaged 1.34 ± 0.28 kg N ha−1), partly due to the paucity of high-frequency monitoring from unmanaged forests. In this study, we used an automated sampling system to measure NO fluxes with a high temporal resolution over two years (daily measurements from January 2019 to December 2020) in a mixed forest in Northeast China. We found that the mean annual NO emission was 0.42 ± 0.04 kg N ha−1, being 31% of the global forest average. The contribution of NO emission during the growing season was 92% of the yearly NO flux. Soil temperature was the most important edaphic factor in regulating NO emission, explaining 90–92% of the seasonal variation. The apparent temperature sensitivity (Q 10) of NO flux was 3.67. In the growing season, NO emission was also influenced by soil moisture, with optimum soil moisture of 37% WFPS. By providing a detailed measurement of diurnal, seasonal, and annual dynamics of NO emissions and their environmental controls from forest soils, our data are useful to develop more accurate biogeochemical models that will improve upscaled global NO budgets. [ABSTRACT FROM AUTHOR]