Heavy rainfall in peak growing season had larger effects on soil nitrogen flux and pool than in the late season in a semiarid grassland
Introduction
One of the characteristics of global climate change is the increase in frequency, intensity, and extent of heavy rainfalls (Donat et al., 2016, Fischer and Knutti, 2016; Jian et al., 2020), which can have profound impacts on ecosystem nitrogen (N) cycling processes, such as nitrification, denitrification, and resultant nitrous oxide (N2O) emission (Greaver et al., 2016). Soil N2O flux variation would further influence the size of soil N pool (Fowler et al., 2013). N2O is the third most important anthropogenic greenhouse gas with a global warming potential 265 times that of carbon dioxide over a 100-year lifespan (Stocker, 2014). Soil N content was highly related to multiple ecosystem structure and functions, such as carbon uptake and microbial community (Yang et al., 2021, Widdig et al., 2020). As such, understanding the effects of heavy rainfalls on ecosystem N2O flux and soil N pool is particularly important for assessing global warming potential and ecosystem functions.
Ecosystem N cycling processes are mainly driven by microorganisms. For example, each process in nitrification and denitrification is performed by a specialized group of nitrifiers and denitrifiers (e.g., ammonium‐oxidizing and nitrite‐reducing microorganisms) which contain specific functional genes (e.g., amoA and nirK/nirS) encoding corresponding oxido-reductase (e.g., ammonia monooxygenase and Cu-containing/haem-containing cd1 nitrite reductase) (Kuypers et al., 2018). Changes in precipitation are expected to fundamentally affect N2O flux and/or soil N pool through directly altering the abundance and structure of nitrifiers and denitrifiers (Chen et al., 2017, Chen et al., 2017, Sun et al., 2018, Zhang et al., 2017, Zhang et al., 2013), in addition to indirectly changing soil conditions, such as soil moisture, aeration, and redox potential (Chen et al., 2017, Chen et al., 2017, Patil et al., 2010, Sun et al., 2018). However, the microbial mechanisms of N2O fluxes responses to precipitation variation are not well understood due to limited data of soil functional genes (gene markers for nitrifiers and denitrifiers) currently available (Li et al., 2020b, Li et al., 2020b).
Recently, several meta-analysis studies suggested that increased precipitation promoted terrestrial N2O emissions, but unchanged soil total N content based on current field manipulative studies at a global scale (Li et al., 2020b, Li et al., 2020b, Yan et al., 2018). Note that most of these manipulative studies focused on the scenario that every precipitation event proportionately increased in magnitude over the growing season or whole year, rather than the scenario that abrupt heavy rainfall events within few days. Nevertheless, the two precipitation scenarios may have different ecological impacts (Reyer et al., 2013). Chronic increases in precipitation at a seasonal or yearly scale would produce better soil moisture conditions for nitrifiers and denitrifiers activity (Yang et al., 2018, Zhang et al., 2018). However, heavy rainfall-induced inundation may lead to soil water saturation and inhibit microbial activity, especially for aerobic nitrifiers. Additionally, chronic increases in precipitation may promote N mineralization and N availability, providing substrate for nitrification and denitrification processes, thus ultimately accelerating N2O emissions (Liu et al., 2017). In contrast, dramatic heavy rainfalls are likely to cause flood and ecosystem N loss from topsoil via leaching and runoff (Greaver et al., 2016, Kasper et al., 2019), thereby potentially limiting production of N2O. On the other hand, large precipitation events possibly induce abrupt N2O emission pulse (Petrakis et al., 2017), in particular after a dry period (Groffman et al., 2009). In some cases, consistent high rainfalls can suppress N2O emission (Rowlings et al., 2015). Hence, speculating heavy rainfall effects on soil N2O emissions, based on past relatively long-term evenly increased precipitation manipulative experiments, may not be straightforward.
Previous limited studies have suggested that high intensity precipitation occurred in different periods of a season had discrepant impacts on multiple ecological processes (Craine et al., 2012). For example, experimental heavy rainfalls in late-growing season consistently reduced below-ground and total biomass while heavy rainfalls in mid-growing season had little effects in a semiarid grassland (Li et al., 2019). In another semiarid grassland, deluge at mid-season caused the greatest increase in soil respiration, aboveground net primary production and canopy greenness than those in early- and late-season (Post and Knapp, 2020). Jian et al. (2020) found that yield variability of early rice in China was negatively affected by the frequency of extreme precipitation from the end of flowering to doughty stages but not in other stages. Likewise, seasonal timing may also largely regulate heavy rainfall effects on soil N2O emissions and soil N content. However, we still lack an understanding of whether responses of soil N2O flux and N pool to heavy rainfalls depend on the seasonal timing.
Here, we conducted a 3-year field manipulative experiment in which heavy rainfall was respectively imposed in middle and late plant-growing seasons in a semiarid grassland of Inner Mongolia, China. The seasonal dynamic of N2O fluxes, soil inorganic and total N content, soil functional genes were measured. The specific questions addressed in this study were: (1) what are the effects of heavy rainfall on N2O fluxes and soil N pool? (2) what are the biological mechanisms for N2O fluxes following heavy rainfalls? (3) how does the seasonal timing of an event heavy rainfall regulate its effects on soil N2O fluxes and soil N pools?
Section snippets
Study site
This study was conducted in a long-term fenced (since 1979) semiarid grassland at the Inner Mongolia Grassland Ecosystem Research Station in the Xilin River Basin (43°20′ N, 116°40′ E, 1200 m a.s.l), Inner Mongolia, China. Mean annual air temperature (1953–2017) was 2.5 °C and mean annual precipitation was 281 mm, of which 86% (∼ 242 mm) falls during the growing season from May to September. The soil is classified as dark chestnut in Chinese soil classification or Calcis-orthic Aridisol in US
Soil water content and soil temperature
Heavy rainfall plots received nearly double the amount of total precipitation over the growing season (256, 481, and 495 mm in 2014; 242; 487, and 501 mm in 2015; 182, 425, 439 mm in 2016 for three treatments, respectively; Fig. 1i–l). As a result, heavy rainfall clearly increased soil water content during periods of treatment and for c. 2–3 weeks thereafter, increasing the seasonal mean SWC by 14–27% (Fig. 1d) regardless of seasonal timing (Fig. 1a–d). However, seasonal dynamic and
Discussion
This study examined the seasonal patterns and biotic mechanisms of N2O flux and N pool variation in response to heavy rainfalls applied near the middle and end of the growing season. Our results provided, to the best of our knowledge, the first experimental evidence that seasonal timing of heavy rainfalls strongly regulates total N2O fluxes over the plant growing season. This advance agrees generally with previous observational and manipulative studies suggesting that the effects of extreme
Conclusions
Projected increases in heavy rainfalls may alter soil N biogeochemical processes and N pools, including nitrification, denitrification and N2O fluxes. Based on the heavy rainfall × seasonal timing manipulative experiment in a semiarid grassland, our results revealed that N2O flux responses to heavy rainfall depended on the seasonal timing, with mid-season heavy rainfall promoting N2O emission while late heavy rainfall having no significant effects. Positive effects of mid-season heavy rainfall
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 project was funded by the National Natural Science Foundation of China (Grant nos. 42041005 and 32101313), the China Postdoctoral Science Foundation (Grant no. 2021M693138), the CAS Strategic Priority Research Programmer (A) (Grant no. XDA19030202), and the Fundamental Research Funds for the Central Universities (Grant no. E1E40511). J. Biederman’s contributions were supported by the US Department of Agriculture, Agricultural Research Service. USDA is an equal-opportunity employer.
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