Introduction
Nitrous oxide (N2O), a non-carbon dioxide
(CO2) greenhouse gas, has a global warming potential
nearly 300-fold greater than that of CO2 over a 100-year
lifespan (Dijkstra et al., 2013). The accumulation of
N2O in the atmosphere will deplete stratospheric ozone
and contribute to global warming (Ravishankara et al., 2009). The main
sources of atmospheric N2O are closely associated with
soil nitrogen (N) cycling (i.e., nitrification and denitrification) of
terrestrial ecosystems, which contribute to ~56–70% of
global N2O emissions (Butterbach-Bahl et al., 2013).
Grasslands host one of the most widely distributed vegetation types on
earth, and grassland ecosystems are the main component of terrestrial
ecosystems (Scurlock et al., 2002). On the Qinghai-Tibetan Plateau
(QTP), alpine grassland ecosystems (e.g., alpine meadows and alpine
steppes) are huge nitrogen (N) reservoirs because of sluggish microbial
decomposition (Yang et al., 2018; Zhang et al., 2020). However, the
substantial labile N stored in alpine soils, which is a large source of
N2O, is often neglected (Mao et al., 2020). Global
change, particularly atmospheric N deposition and changing precipitation
regimes, has considerable consequences for storage and patterns of N in
alpine ecosystems (Fu et al., 2017; Lin et al., 2016). Given that alpine
grasslands may possess the capacity for N2O release and
are sensitive to global change (Xiao et al., 2020), understanding how
alpine soil N2O emissions respond to N deposition and
precipitation changes is crucial for predicting future atmospheric
N2O concentrations.
The main regulatory factors for plant communities and soil ecological
processes in grasslands are N and water. Field simulations of the impact
of atmospheric N deposition on N2O emissions are not
scarce, especially in the alpine grasslands of the QTP. However, reports
of the effects of N addition in these ecosystems are inconsistent. N
addition has been shown to significantly increase soil
N2O emissions, because N input elevates the
concentration of inorganic N and the abundance of functional microbes in
the soil (Geng et al., 2019; Peng et al., 2018; Wu et al., 2020; Yan et
al., 2018). In addition, a greater labile carbon (C) supply (e.g.,
litter decomposition or root exudation) under N enrichment provides
substrate C for heterotrophic denitrifiers, thereby stimulating
N2O emissions (Brown et al., 2012; Dijkstra et al.,
2013). However, Zhu et al. (2015) showed that N input did not affect
N2O emissions. A possible interpretation of this finding
is that low temperature and inadequate soil moisture limit the
activities of microorganisms associated with N cycling in cold
conditions (Banerjee et al., 2016; Curtis et al., 2006; Schaufler et
al., 2010). Despite this work on grasslands, the response of
N2O emissions to long-term N deposition on the QTP
remains understudied.
Soil N2O emissions are also susceptible to hydrologic
variations (Knapp et al., 2002). Generally, changes in soil water
content influence N mineralization and organic matter degradation, which
then affect the provision of N and C reactants for N cycling processes.
On a global scale, elevated precipitation in grassland ecosystems
accelerates N2O emissions while decreased precipitation
mitigates N2O emissions. These processes are
predominantly regulated by shifts in soil water availability (Li et al.,
2020). By contrast, Liu et al. (2014) showed that short-term water
increment did not affect N2O emissions from semiarid
steppes. Even increased precipitation decreased N2O
emission in arid grasslands (Cai et al., 2016). This finding may be
attributable to soil leaching and run-off events caused by the increased
rainfall, which intensified the loss of inorganic N in soil and thereby
limited soil N cycling (Cregger et al., 2014). Little is known about how
long-term precipitation changes impact N2O emissions on
the QTP. Both N and water affect soil biogeochemical cycles. N
deposition and variation in precipitation usually occur simultaneously;
thus, their effects are interdependent (Harpole et al., 2007). The
combined effect of N deposition and altered precipitation on
N2O emissions is also unknown. N-cycling microbiomes
play a crucial role in regulating soil N dynamics and global climate
stabilization. On the QTP it is also unclear how pivotal N-cycling
functional microorganisms respond to global change and which microbes
better explain N2O emissions.
Due to multifactorial climate change and intensive interventions
targeting anthropogenic activities, the environmental conditions of the
QTP have undergone dramatic changes in the past few decades (Gong et
al., 2017). The amount, frequency, and intensity of precipitation
increased from 1975 to 2014 (Ge et al., 2017). The QTP is also
confronting pronounced N deposition, with an average of
~8 kg N ha−1 year−1 (Lü et al., 2007). The alpine
steppes, the largest grassland ecosystem on the QTP, are extremely
sensitive to global change (Ding et al., 2016; Wang et al., 2011).
Therefore, understanding the effects of N enhancement and altered
precipitation on N2O emissions in the alpine steppes is
essential. This study consists of altered precipitation and N addition
manipulation experiments that were conducted in an alpine steppe on the
QTP in 2013. We monitored the N2O flux during the 2020
growing season (May to October) based on in-situ experiments. To
identify the key abiotic and biotic factors regulating
N2O emissions, we measured N2O flux on
six consecutive days in mid-August (during peak plant growth). Soils
were also collected to measure abiotic parameters and functional
microbes, including nitrifiers (ammonia-oxidizing bacteria: AOB;
ammonia-oxidizing archaea: AOA) and denitrifiers (nirS -,nirK -, and nosZ gene-containing microorganisms). The
objectives of the study were to (1) assess whether N2O
emissions were altered by long-term N addition, precipitation changes,
and their interaction; and (2) identify the mechanisms that regulated
N2O emissions under N addition and altered precipitation
patterns.