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Topography can influence nitrous oxide (N2O) emission via its influences on soil nutrient availability, moisture, and microbial communities. Nevertheless, it is still unclear whether topography modulates the responses of soil N2O emissions to elevated N deposition. Here the N addition experiment was conducted in the valley and on the slope of a subtropical karst forest in southwest China. Nitrogen was applied as NH4NO3 in two levels, that is, 50 (moderate N) and 100 (high N) kg N ha−1 yr−1 with no N addition plots as the control. Nitrogen addition consistently increased N2O emission in the valley, but only high N addition significantly increased N2O emission on the slope in 2017. The cumulative N2O fluxes across the 3 years were 1.16 ± 0.24 kg N ha−1 in the valley and 1.50 ± 0.06 kg N ha−1 on the slope under the control. Nitrogen addition stimulated N2O emission by 88.7%–113.3% in the valley due to increased ammonium, nitrate and dissolved organic carbon availabilities and ammonia‐oxidizing bacteria (AOB) amoA abundance. High N addition stimulated N2O emission by 84.3% on the slope owing to increased nitrate and carbon availabilities, AOB amoA, and nirK abundances. The stimulation of N2O emission by moderate N addition was more pronounced in the valley than on the slope largely owing to the lower N status in the valley. This work highlights the importance of N status in regulating the responses of soil N2O emissions to elevated N deposition. Plain Language Summary: Atmospheric N deposition is identified as one of the strongest driving factors responsible for the increase of forest soil N2O emission, and topography can influence N2O emission via its influences on soil nutrient availability, moisture, and microbial communities. However, there is uncertainty about whether topography modulates the responses of soil N2O emissions to elevated N deposition. We investigated soil N2O emission and related microbial functional gene abundances at two topographic positions under three N addition levels in a subtropical region of China. We find that the N addition consistently increased N2O emission in the valley, but only stimulated N2O emission at the early stage on the slope. Stimulation of soil N2O emission by moderate N addition was more pronounced in the valley than on the slope. Our findings highlight the importance of N status in regulating the response of soil N2O emissions to elevated N deposition. Key Points: N addition increased soil N2O emission in the valley across the 3 yearsHigh N addition stimulated N2O emission on the slope at the early stageModerate N stimulated greater N2O emission in the valley than on the slope [ABSTRACT FROM AUTHOR]

The influences of atmospheric nitrogen (N) deposition and topographical position on soil methane (CH 4) uptake have been intensively investigated; however, it remains unclear whether topographic position regulates the responses of soil CH 4 uptake to N deposition. Here, we examined the effects of N addition on soil CH 4 uptake in the valley and on the slope of a subtropical karst forest. Control (0 kg N ha−1 yr−1), moderate N addition (50 kg N ha−1 yr−1), and high N addition (100 kg N ha−1 yr−1) treatments were included at each topographic position. CH 4 fluxes were determined from 2017 to 2019 along with potential methane production rate (PMPR), potential methane oxidation rate (PMOR), mcrA and pmoA gene abundances, and plant and soil properties. The averaged annual CH 4 uptake under the control was 0.67 ± 0.07 kg CH 4 ha−1 yr−1 in the valley and 0.99 ± 0.32 kg CH 4 ha−1 yr−1 on the slope across the three years. High N addition consistently increased CH 4 uptake in the valley in 2017 and 2019 and on the slope in 2017 and 2018. The annual CH 4 uptake was stimulated by high N addition by 81.27 % in the valley owing to inhibited PMPR caused by increased NO 3 −, and to stimulated PMOR caused by increased NO 3 − and soil organic carbon. On the slope, annual CH 4 uptake was enhanced by high N addition by 75.83 % attributed to inhibited PMPR caused by increased fine root biomass and to stimulated PMOR induced by increased soil phosphorus availability. Our study highlights the importance of topography in regulating soil CH 4 uptake under elevated N addition. • N addition significantly increased soil CH 4 uptake in the valley and on the slope. • N addition stimulated CH 4 uptake due to increased NO 3 − and SOC in the valley. • N addition increased CH 4 uptake via raising available P and fine root on the slope. [ABSTRACT FROM AUTHOR]

• N addition increased the rates of most N transformation processes in the valley. • N addition decreased the rates of most N transformation processes on the slope. • Topography modulates effects of N deposition on soil N transformations. Soil nitrogen (N) transformations play key roles in ecosystem productivity and other functions of terrestrial ecosystems via regulating soil N availability. Although the effects of elevated atmospheric N deposition on soil N transformations have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, we conducted an N addition experiment consisting of three treatments: control (0 kg N ha−1 yr−1), moderate N addition (50 kg N ha−1 yr−1), and high N addition (100 kg N ha−1 yr−1) in the valley and on the slope, respectively, of a subtropical karst forest. Under the control, protein depolymerization, amino acid uptake, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates were significantly higher on the slope than in the valley attributed to the higher soil dissolved organic carbon, total dissolved N and pH on the slope. Nitrogen addition significantly increased the rates of protein depolymerization, amino acid uptake, mineralization, nitrification and DNRA through alleviating microbial carbon limitation in the valley, but decreased the rates of depolymerization, amino acid uptake, mineralization and DRNA by enhancing microbial carbon and phosphorus limitations on the slope. The increase in soil N transformations, microbial N use efficiency (NUE) and microbial biomass N but lowered microbial N turnover time resulted in a 94 % increase of total microbial necromass N in the valley under N addition. However, increase in NUE and microbial biomass N led to a 33 % decrease of necromass N due to enhanced microbial necromass destabilization on the slope under N addition. Our results suggest that the responses of soil N transformations to N addition may be mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict soil N dynamics in forest in the context of elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]

• N addition increased POC via accelerating aboveground litter input in the valley. • N addition increased MAOC via increasing CUE and necromass formation in the valley. • N addition enhanced POC by increasing fine root biomass on the slope. • N addition inhibited MAOC by declining microbial necromass on the slope. While the responses of soil organic carbon (SOC) dynamics to atmospheric nitrogen (N) deposition have been widely investigated, little is known in terms of if, and how the responses are modulated by topography. Here, a N addition experiment with three N addition levels, i.e., 0, 50 and 100 kg N ha−1 yr−1, was carried out in the valley and on the slope of a subtropical karst forest. Soil type was Cambisols on the slope but Luvisols in the valley developed on a mixture of limestone and dolomite. Bulk SOC content in the valley was marginally (p < 0.1) enhanced attributed to the stimulation of both particulate organic C (POC) and mineral-associated organic C (MAOC), but that on the slope was not significantly altered owing to enhanced POC but suppressed MAOC under N addition. High N addition (100 kg N ha−1 yr−1) significantly increased POC through accelerating aboveground litter input, and increased MAOC via increasing carbon use efficiency (CUE) and necromass formation in the valley. On the slope, high N addition significantly stimulated POC owing to increase of fine root biomass, but inhibited MAOC due to decrease in microbial necromass accumulation. Our results suggest that the impacts of N addition on SOC formation and hence persistence likely modulated by topography, so that provide a novel view in underlying the differential responses of SOC pool to N addition. [ABSTRACT FROM AUTHOR]

• N addition enhanced soil respiration by increasing microbial activity in the valley. • N addition increased soil respiration via increasing fine root biomass on the slope. • Greater response of soil respiration in the valley than that on the slope under N50. Soil respiration is the largest terrestrial source of atmospheric carbon dioxide, and hence plays a vital role in the carbon cycle-climate feedback. Although the effects of elevated atmospheric nitrogen (N) deposition on soil respiration have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, a field N addition experiment with three rates, i.e., 0, 50 and 100 kg N ha−1 yr−1 was performed in the valley and on the slope of a subtropical karst forest in southwest China to explore whether and how topography mediates soil respiration. Nitrogen addition increased soil respiration by 55–63 % in the valley due to increased microbial activity. However, high N addition stimulated soil respiration by 56 % on the slope due to enhanced fine root biomass. The response of soil respiration to moderate N addition was significantly greater in the valley than on the slope, likely owing to the lower N availability in the valley. Our results demonstrate that the responses of soil respiration to N addition were mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict forest carbon dynamics in the context of elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]

• AOB are primarily responsible for soil N 2 O production. • N addition stimulated AOA–derived N 2 O in the organic horizon of the valley. • N addition suppressed AOA and AOB–derived N 2 O in the organic horizon of the slope. • N addition increased AOB–derived N 2 O in the mineral horizon of the valley. • N addition enhanced AOA and AOB–derived N 2 O in the mineral horizon of the slope. Anthropogenic nitrogen (N) deposition may substantially affect the contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to soil nitrous oxide (N 2 O) production. Nevertheless, it is still unclear whether topography modulates the responses of AOA– and AOB–derived N 2 O to elevated N deposition. We conducted an ex-situ experiment with organic and mineral soils collected from the valley and slope, respectively, of a subtropical karst forest with three N addition levels, i.e., 0 (control), 50 and 100 kg N ha−1 year−1 for each topographic position. Soil ecoenzymatic stoichiometry indexes were calculated as indicators of microbial resource limitation. AOA– and AOB–derived N 2 O were distinguished by inhibitors. AOB was primarily responsible for soil N 2 O production under the control regardless of topographic position. For the organic horizon, N addition stimulated AOA–derived N 2 O by 112.5–138.2% in the valley because of increased N mineralization due to alleviated microbial carbon limitation; but suppressed AOA and AOB–derived N 2 O by 40.2–53.5% and 35.6–46.8%, respectively, on the slope because of decreased N mineralization attributed to aggravated microbial phosphorus limitation. For the mineral horizon, N addition enhanced AOB–derived N 2 O by 104.5–143% in the valley because of increased ammonia availability, but stimulated AOA and AOB–derived N 2 O by 149.8–1162.5% and 26–64.5%, respectively, on the slope because of increased N mineralization and ammonia availability owing to aggravated microbial C limitation and alleviated phosphorus limitation. Our results indicate that the mechanisms underlying the impacts of N deposition on soil N 2 O production by ammonia oxidizers are topography–dependent, so that topography-specific niche specialization between AOA and AOB should be integrated into Earth system models in order to better predict soil N 2 O production under elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]