Seasonal Variations in N₂O Emissions in a Subtropical Forest With Exogenous Nitrogen Enrichment are Predominately Influenced by the Abundances of Soil Nitrifiers and Denitrifiers

Abstract

Elevated nitrogen (N) deposition has induced substantial impacts on the emissions of nitrous oxide (N 2 O) from forest ecosystems, but how soil microbes regulate the production/consumption of N 2 O under elevated N deposition remains poorly understood, particularly in high N deposition subtropical forests that are characterized by distinct wet-dry seasonality. We established a field N addition experiment in a subtropical forest in southern China to explore the influences of low, medium and high (35, 70, and 105 kg N ha -1 yr -1 , respectively) N addition on N 2 O efflux and its associated microbial functional genes [amoA for nitrifiers (ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA)) and nirK and nosZ for denitrifiers]. The results showed the following: (1) The N 2 O emissions were stimulated by N addition in the dry season but were depressed in the wet season. (2) The nirK and nosZ abundances were generally stimulated by N addition, whereas the AOB-amoA and AOA-amoA abundances showed divergent responses to N addition. (3) Based on the results of principal component and Pearson correlation analyses, N 2 O effluxes were associated with microbial biomass in the wet season but with nirK and nosZ abundances in the dry season. Structural equation modeling analyses further indicated that both nitrifiers and denitrifiers under N addition contributed to the generation of N 2 O in the dry season, whereas the decreased production of N 2 O in the wet season was primarily caused by denitrifiers. Therefore, seasonally specific strategies should be developed to mitigate the emissions of N 2 O from subtropical forests with distinct seasonal precipitation patterns.Plain Language Summary Tropical and subtropical forests are considerable sources of nitrous oxide (N 2 O), one of the greenhouse gases mainly produced via microbial nitrification and denitrification in soil. In recent decades, the emissions of N 2 O have been influenced by increasing nitrogen (N) deposition due to enhanced atmospheric reactive N derived primarily from human activities. Moreover, among the types of N deposition, wet N deposition depends greatly on precipitation. This dependence makes predicting future change trends in N 2 O emissions challenging, especially in tropical and subtropical forests in southern China due to the large amount of natural N deposition and the distinct wet-dry seasonality caused by unevenly distributed annual precipitation in this region. We studied the responses of N 2 O emissions to increasing N deposition by establishing a field N addition experiment in a subtropical forest in southern China. We explored the responsive differences in N 2 O emissions between wet and dry seasons from microbial regulation aspects. The N addition depressed N 2 O emissions in the wet season but stimulated N 2 O emissions in the dry season. Moreover, microbial biomass in the wet season and denitrifier abundance in the dry season were the indicators of N 2 O emissions in response to elevated N deposition. Key Points:• N 2 O emissi...

Responses of Soil Nitrous Oxide Emission to Nitrogen Addition at Two Topographic Positions of a Subtropical Forest

“…The annual N 2 O emissions (0.39 ± 0.08–0.50 ± 0.02 kg N ha −1 yr −1 ) in the control plots of the current study are comparable to some previously reported values (0.39–0.52 kg N ha −1 yr −1 ) observed in some subtropical/tropical forests (Fan et al., 2017; Rowlings et al., 2012; Soper et al., 2018), but are much lower than those from other subtropical/tropical forests (0.70–3.50 kg N ha −1 yr −1 ; Han et al., 2019; Wieder et al., 2011; Xie et al., 2018; M. H. Zheng et al., 2016). Several potential mechanisms are responsible for the relatively low N 2 O emission in the current study.…”

“…For instance, Han et al. (2019) found that experimental N addition (N addition hereafter) increased N 2 O emission under N poor conditions via stimulating the abundances of nitrifiers and denitrifiers, but decreased N 2 O emission in N rich conditions by depressing the abundances of denitrifiers in a subtropical forest. M. H. Zheng et al.…”

“…However, the results from individual studies in the subtropical/tropical region are inconsistent. For instance, Han et al (2019) found that experimental N addition (N addition hereafter) increased N 2 O emission under N poor conditions via stimulating the abundances of nitrifiers and denitrifiers, but decreased N 2 O emission in N rich conditions by depressing the abundances of denitrifiers in a subtropical forest. M. H. Zheng et al (2016) observed significant increases in soil N 2 O emission under N addition in the N rich but not in the N limited tropical forests.…”

Nitrogen-Induced Changes in Soil Environmental Factors Are More Important Than Nitrification and Denitrification Gene Abundance in Regulating N2O Emissions in Subtropical Forest Soils

“…Soil pH can exert a direct and an indirect effect on biological properties (e.g., reductase activities, the abundance, and expression of functional genes, etc.) ( Liu et al, 2010 ; Hu et al, 2015 ; Han et al, 2019 ). The reductases for nitrate, nitrite, and nitric oxide are more active at pH <7 ( Gillam et al, 2008 ), while the N 2 O reductase activities are inhibited in soil with low pH ( Levy-Booth et al, 2014 ).…”