Characteristics and influencing mechanisms of soil nitrous oxide (N2O) emissions from forestlands at different slope positions in karst regions
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摘要: 岩溶区地形复杂, 土壤氧化亚氮（N2O）排放差异较大，量化其排放有助于全球N2O排放清单的精准编制。目前岩溶区不同地形条件下土壤N2O的空间排放特征及其影响机制尚不清楚。本研究区位于西南典型岩溶区桂林，选择不同地形坡位（坡顶、坡中和坡脚）的自然林地连续监测154 d 土壤N2O排放，并采用15N同位素标记技术测定土壤矿化速率与硝化速率，从土壤氮供应角度揭示N2O排放的影响机制。结果表明，随坡位下降，土壤有机碳、全氮含量和pH显著提高。不同地形坡位下土壤N2O排放具有较大差异，排放通量介于5.21~39.6 μg N·m−2·h−1之间。坡脚土壤N2O累积排量（0.71 kg N·hm−2）最高，其次是坡中（0.53 kg N·hm−2），坡顶N2O最低（0.43 kg N·hm−2）。岩溶区土壤具有较高的矿化和硝化速率，且随着坡位降低，土壤矿化和硝化速率均显著提高。与坡顶（2.27 mg N·kg−1·d−1和5.11 mg N·kg−1·d−1）相比，坡脚土壤矿化和硝化速率分别提高到5.98 mg N·kg−1·d−1和8.85 mg N·kg−1·d−1。相关分析表明，土壤N2O累积排放量与矿化速率和硝化速率、有机碳、全氮、pH呈显著正相关，表明岩溶区不同坡位下, 土壤理化性质通过影响无机氮供应能力（矿化速率和硝化速率）来改变N2O排放。本研究结果强调了在评估岩溶区土壤N2O排放时，应充分重视地形的影响。Abstract:
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 (MNorg) and nitrification (ONH4) 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. -
Key words:
- karst region /
- N2O emissions /
- 15N labeling /
- mineralization rate /
- nitrification rate /
- terrain
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图 1 不同坡位森林采样地点示意图
Figure 1. Schematic representation of forestland sampling sites at different slope positions
图 2 不同坡位土壤的矿化速率与硝化速率
注：图中不同字母表示相同指标在不同坡位之间差异显著（P ＜0.05），下同。
Figure 2. Rates of mineralization and nitrification in soils at different slope positions
Note: Different letters indicate significant differences for the same indicator at different slope positions. (hereinafter the same)
图 3 不同坡位土壤的NH$_4^{+}$与${\rm{NO}}_3^{-}$平均滞留时间
Figure 3. Mean detention times of NH$_4^{+}$ and ${\rm{NO}}_3^{-}$ in soils at different slope positions
图 4 不同坡位土壤N2O排放通量与N2O累积排放量
Figure 4. N2O emission flux and the cumulative N2O emissions in soils under different slope positions
图 5 不同坡位土壤WFPS和土壤温度、降水量
Figure 5. Soil WFPS, soil temperatures, and precipitation at different slope positions
图 6 土壤N2O排放通量与土壤WFPS和土壤温度的相关性
Figure 6. Correlations of N2O emission fluxes with soil WFPS and temperatures
图 7 土壤基本理化性质与矿化速率、硝化速率、N2O累积排放量的相关性热图
Figure 7. Correlation heat map between soil physicochemical properties with soil mineralization and nitrification rates and the cumulative N2O emissions
图 8 偏最小二乘路径模型（PLS-PM）中土壤理化性质和土壤无机氮生产对N2O累积排放的影响
Figure 8. Effects of soil physicochemical properties and soil inorganic N production on the cumulative N2O emissions in a Partial Least Squares Path Model(PLS-PM)
表 1 不同坡位条件下土壤基本理化性质
Table 1. Soil physicochemical properties under different slope conditions
指标 坡顶 坡中 坡脚 有机碳/g·kg−1 43.2 ± 4.07b 58.8 ± 0.77a 59.6 ± 2.53a 全氮 /g·kg−1 4.25 ± 0.35b 5.65 ± 0.05a 5.75 ± 0.20a C: N 10.2 ± 0.12a 10.4 ± 0.23a 10.5 ± 0.81a WHC /% 107 ± 5.58b 128 ± 4.63a 114 ± 2.95b pH 6.75 ± 0.09c 7.47 ± 0.13b 7.74 ± 0.10a 无机氮 /mg·kg−1 15.9 ± 2.05b 21.9 ± 0.21a 20.2 ± 0.43a ${\rm{NO}}_3^{-}$: NH$_4^{+}$ 2.94 ± 0.20c 8.13 ± 0.70a 4.79 ± 0.47b NH$_4^{+}$/mg·kg−1 3.66 ± 0.22a 2.39 ± 0.17b 3.51 ± 0.36a ${\rm{NO}}_3^{-}$/mg·kg−1 12.2 ± 2.15c 19.4 ± 0.43a 16.7 ± 0.07b 注：表中同行不同字母表示不同坡位土壤各指标差异显著（P ＜0.05）。 Note: Different lowercase letters indicate significant differences in various indicators of soils under different slope positions (P < 0.05). 
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