Characteristics and influencing mechanisms of soil nitrous oxide (N₂O) emissions from forestlands at different slope positions in karst regions

Greenhouse gases reflect long-wave infrared radiation from the Earth’s surface, causing the rise in global temperatures. Nitrous oxide (N₂O) is one of the three major greenhouse gases, with a century-long warming potential 296 times greater than that of carbon dioxide (CO₂) and an atmospheric lifetime of about 116 years. In addition to its greenhouse effect, N₂O also depletes stratospheric ozone, further intensifying the impacts of global climate change. Studies have shown that soil N₂O emissions from subtropical and tropical forests account for about 18% of global atmospheric N₂O sources. However, these studies have primarily focused on non-karst regions, with relatively few investigations conducted in karst regions.Karst landscapes are widely distributed across the globe, and their soils exhibit high rates of mineralization and nitrification, which theoretically may make them hotspots for global soil N₂O emissions. Forestlands are important parts of karst regions, accounting for about one-third of their total area. Therefore, the emission of N₂O from forest soils in karst regions may have a substantial impact on global greenhouse gas emissions. Furthermore, karst regions feature complex topography, and differences in soil properties, such as soil mineralization and nitrification processes at different slope positions, may lead to significant spatial variability in N₂O emissions.

In this study, soil N₂O emissions were continuously monitored for 154 days from natural forestlands at different slope positions (upper slope, middle slope, foot slope) in Guilin, a typical karst region in southwest China. Soil N₂O emission fluxes were measured with the use of a static chamber method. Soil mineralization (M_Norg) and nitrification (O_NH4) rates were assessed with the ¹⁵N tracing technique. Soil temperature and soil Water-Filled Pore Space (WFPS) were recorded through a soil temperature and moisture logger. Additionally, soil physical and chemical indicators were analyzed. The results of this study show that the soil mineralization and nitrification rates varied significantly among different slope positions in karst regions. The rate of soil mineralization was maximum at the foot slope, reaching 5.98 mg N·kg⁻¹·d⁻¹, which was significantly higher than those at the middle slope (3.65 mg N·kg⁻¹·d⁻¹) and upper slope (2.27 mg N·kg⁻¹·d⁻¹). The rate of soil nitrification at foot slope was 8.85 mg N·kg⁻¹·d⁻¹, which was higher than those at the middle slope (6.57 mg N·kg⁻¹·d⁻¹) and upper slope (5.11 mg N·kg⁻¹·d⁻¹). These changes were primarily closely related to changes in Soil Organic Carbon (SOC) and Total Nitrogen (TN) contents, and soil pH. As slope positions decreased, SOC and TN contents and pH increased significantly, contributing to higher rates of soil mineralization and nitrification. Further analysis of soil N₂O emission fluxes from forestlands in karst regions revealed significant differences in soil N₂O emission fluxes at different slope positions. The N₂O emission fluxes of forest soils in karst regions ranged from 5.21 μg N·m⁻²·h⁻¹ to 39.6 μg N·m⁻²·h⁻¹, with an average emission flux of 14.1 μg N·m⁻²·h⁻¹. The N₂O emission fluxes of soils at the foot slopes were higher than those at the upper and middle slopes. The analysis showed a significant increase in soil WFPS and a rising trend in soil temperature as the slope position decreased. The soil N₂O emission fluxes were significantly and positively correlated with WFPS and temperature, indicating that soil temperature and moisture have important influences on soil N₂O emissions. Additionally, the cumulative N₂O emissions from forest soils in karst regions ranged from 0.43 kg N·hm⁻² to 0.71 kg N·hm⁻². The highest value was observed at the foot slope (0.71 kg N·hm⁻²), followed by the middle slope (0.53 kg N·hm⁻²), and lowest at the upper slope (0.43 kg N·hm⁻²). Correlation analysis showed that cumulative soil N₂O emissions were significantly and positively correlated with mineralization and nitrification rates, SOC and TN contents, as well as pH. The PLS-PM analysis also suggested that slope positions regulated the rates of soil mineralization and nitrification by influencing the physicochemical properties of soil pH, TN, and SOC. These changes, in turn, indirectly affected the cumulative emissions of soil N₂O. Therefore, when evaluating soil N₂O emissions in karst regions, it is important to fully consider topographic factors to more accurately estimate greenhouse gas emissions in these regions.

These findings are of great significance for advancing our understanding of the mechanisms underlying soil N₂O emissions in karst regions and their potential impact on global climate change. At the same time, they provide a theoretical basis for developing more precise strategies for the management of carbon and nitrogen emissions, which could aid in climate change mitigation.