Determining the hot spots and hot moments of soil N2O emissions and mineral N leaching in a mixed landscape under subtropical monsoon climatic conditions
Introduction
Soil nitrous oxide (N2O) emissions and mineral nitrogen (N) leaching act as two main pathways of soil N losses (Fowler et al., 2013). Soil N2O emissions, which are mainly driven by microbial processes (e.g., nitrification and denitrification), could cause global warming and ozone depletion (Erisman et al., 2013, Ussiri and Lal, 2013). Excessive mineral N leaches into the ground and surface water, which could decrease the quality of water bodies and potentially promote the eutrophication (Sinha et al., 2017). Disproportionately high N2O emissions or mineral N leaching occur as localized areas (hot spots) relative to the surrounding matrix or during short periods (hot moments) relative to longer intervening time periods (Bernhardt et al., 2017), which have been observed at the field and watershed scales (Kurunc et al., 2011, Weitzman et al., 2021). Therefore, it is necessary to reveal the mechanisms of HSHMs on these two reactive N loss pathways to provide effective mitigation strategies (Krichels and Yang, 2019, Palta et al., 2014).
The hot moments of soil N2O emissions and mineral N leaching are influenced by multiple factors, e.g., climate factors, vegetation growth, and management practices (Molodovskaya et al., 2012, Wagner-Riddle et al., 2020, Zhu et al., 2012). For example, abundant precipitation increases water input, which may form a short-term anaerobic state, inhibit nitrification, promote denitrification, and even induce water flows that leach N (Blaen et al., 2017). Warming may promote soil respiration and microbial activity, strengthening microbial processes (Dai et al., 2020). As plants grow, plant uptake of N changes the availability of soil N for microbial or leaching processes (de Vries and Bardgett, 2012). Plant evapotranspiration rates affect soil water content (SWC), controlling the denitrification rate (Krichels and Yang, 2019). Land management practices, especially fertilization, aggravate mineral N input (Eagle et al., 2017). Meanwhile, tillage, irrigation or cover cropping influence the availability of organic carbon (C), N, oxygen and water, regulating soil N losses (Quemada et al., 2013, Thapa et al., 2018, Yangjin et al., 2021). However, controlling factors of hot moments are controversial among regions or land-use types. For instance, hot moments of N2O emissions have been observed during heavy precipitation under warm conditions in soybean field (Krichels and Yang, 2019), or during summer precipitation and spring thaw in corn field, enhanced by fertilization (Molodovskaya et al., 2012). Hot moments of N leaching in forestland or cropland have been during the non-growing season (Zhu et al. 2012). Influencing factors of hot moments of N losses are complex, so further research is needed to reveal them in mixed landscape areas.
The hot spots of soil N2O emissions and mineral N leaching are influenced by soil properties and vary with topography and land-use type (Kurunc et al., 2011, Parn et al., 2018, Wagner-Riddle et al., 2020). For instance, rock fragment content (RFC) changes the hydraulic conductivity and volume of fine-soil matrix, regulating soil environment and solute transportation (Hlaváčiková et al., 2015). Low relative gas diffusion rates in high bulk density (BD) soils diminish gas diffusion, which may generate anaerobic conditions (Balaine et al., 2013). The SWC controls the gas diffusion rate and microbial activity and changes redox conditions and thus N availability (Parn et al., 2018). Texture affects soil pore space and water-holding capacity, regulating subsurface flows and rates of microbial processes (Palta et al., 2014). Soil C/N ratio and organic matter content provide the C and N substrates for microbial processes and change the cation exchange capacity and soil structure (Figueiredo et al., 2016, Gundersen et al., 2012). Topographic factors, e.g., slope and elevation, influence soil hydrology and spatial distribution of soil properties (Lai et al., 2018a, Pennock et al., 2010). Land-use type regulates soil properties by plant attributes (e.g., plant N uptake and evapotranspiration) and affects N input with management practices (Abalos et al., 2016, Palmer et al., 2014). However, controlling factors of hot spots vary with the sampling period. For example, hot spots of N2O emissions have been located in areas of low slope and elevation after fertilization (Turner et al., 2016), while topographic depressions have not triggered them during the soybean-growing season (Krichels and Yang, 2019). Hot spots of N leaching in farmland during May to June have coincided with low water-holding capacity and heavy irrigation (Kurunc et al., 2011). Therefore, factors affecting hot spots of N losses have to be further identified at longer time scales (e.g., annual).
Tea plant (Camellia sinensis (L.) O. Kuntze) has rapidly expanded to 66 kha yr−1 by destroying forestlands (Han et al., 2021), aggravating the possibility of soil N2O emissions and eutrophication of water bodies (Chen et al., 2019a, Wang et al., 2020). Therefore, this paper used the DayCent model to simulate N2O emissions and mineral N leaching under different precipitation years in a mixed landscape. The objective of this study was to identify hot moments and hot spots of these two N losses and reveal their main controlling factors. We hypothesized that their hot moments were controlled by climate factors and land management (e.g., fertilization), while hot spots were primarily affected by land-use type, assisted by soil properties and topography.
Section snippets
Study site
Qing Hill (31°20.72′-31°23.33′N, 119°2.42′-119°4.48′E) has an area of 747.33 ha, and is located in the northwestern hilly region of Taihu Lake Basin, China (Fig. 1). It has a subtropical monsoon climate with the mean annual air temperature and precipitation of 15.9℃ and 1157 mm over 60 years, respectively. The primary land-use types are TG, bamboo forest or BF (Phyllostachys edulis (Carr.) H. de Lehaie), and coniferous and broad-leaved mixed forests (MFs). The MFs contain Masson pine (Pinus
Environmental conditions and model performances
The climate factors varied significantly (P < 0.05) among years, and the representatively spatial factors were significantly (P < 0.05) associated with N2O emissions and mineral N leaching (Figs. S1 and 2). Compared with the NY, the precipitation amount during the DY decreased in summer and fall, while that during the WY increased in spring and fall. The DY had higher mean annual air temperature (17.79℃) than the NY and WY. The representatively spatial factors included soil BD (ranging from
Controlling factors of hot moments
Temperature rather than precipitation was closely associated with soil N2O emissions, which was a positive relationship (Figs. S2 ∼ S3, Table 2). Hot moments of N2O emissions appeared in months or seasons with high air temperatures, which were similar to other ecosystems, e.g., tea fields and Masson pine forests (Chen et al., 2019a), grasslands and cornfields (Li et al., 2015), and cropland with alfalfa and corn (Molodovskaya et al., 2012). This may be attributed to the fact that high
Conclusions
Based on the DayCent simulation, the HSHMs of soil N2O emissions and mineral N leaching were identified using the quantification method in hilly mountain areas. Hot moments of N2O emissions at inter-annual, seasonal, and monthly time scales included the DY, the summer, and the months with high temperature (July to September) and months after fertilization (April or May), respectively. Hot moments of mineral N leaching across these time scales, were the NY or WY, spring or rainy season after
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (42125103 and 42171077), the China Postdoctoral Science Foundation (2021M692145), and the Youth Innovation Promotion Association, Chinese Academy of Sciences (2020317 and 2019313).
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