Nitrogen fertilizer application in the rice-growing season can stimulate methane emissions during the subsequent flooded fallow period

Highlights

• N fertilization increased CH₄ emissions during the fallow period.

• CH₄ emission from N-fertilized plot increased with elevated dissolved organic carbon.

• Soil dissolved carbon to nitrogen ratio positively affected CH₄ emission.

• Soil temperature exhibited the main effect on CH₄ emission in both treatments.

Abstract

Winter-flooded rice paddy field (FR), characterized by water conserved in the field during the fallow period, is a typical cropping system in southwest China, leading to considerable methane (CH₄) emissions. The effect of nitrogen (N) fertilization on CH₄ emissions during rice-growing seasons is well studied in FR, further studies covering N fertilizer applied in the rice-growing seasons affects CH₄ emissions during the subsequent fallow period is needed. Therefore, a field experiment was conducted in an FR of Sichuan province, China, with conventional N fertilized (CN) and N unfertilized (NN) treatments. The cumulative CH₄ emission from CN treatment during the rice-growing season and the subsequent fallow period was 389 ± 29.4 and 158 ± 31.2 kg C ha⁻¹, which were increased by 29.5% and 395% in comparison with the NN treatment, indicting N applied during the rice growing-season significantly facilitated CH₄ emission during the subsequent fallow period. During the rice-growing season, higher CH₄ emission from CN treatment could be attributed to elevated soil dissolved organic carbon (DOC) content that might have provided sufficient substrates for CH₄ production. During the fallow period, as compared to NN treatment, higher CH₄ emissions from CN treatment could be explained by greater linear regression slopes between CH₄ fluxes, soil temperature and DOC to dissolved inorganic N (DIN) (DOC/DIN) ratio. Moreover, the structural equation model (SEM) described that the soil temperature exhibited the most significant effects on CH₄ emissions for both treatments during the rice-growing season and subsequent fallow period. These findings are a major step forward to showing that N fertilizer applied in the rice-growing season could also affect CH₄ emission during the subsequent fallow period, accompanying other soil parameters controlling CH₄ emission.

Introduction

Methane (CH₄), a potent greenhouse gas, has caused much attention and research interests all over the world (IPCC, 2014). Agriculture management and production practices contribute to elevated CH₄ concentrations in the atmosphere, with 11% of CH₄ emissions linked to rice production (IPCC, 2014). This is mainly due to the reduction of terminal electron acceptors like oxygen (O₂) and nitrate (NO₃^-) in paddy soils during flooding, eventually facilitating the production of CH₄ (Tang et al., 2016; Khan et al., 2019), a greenhouse gas with a global warming potential 28 times higher than carbon dioxide (CO₂) (IPCC, 2018). Globally, rice paddy accounts for more than 12% of the total cropland, including 19% of the world's rice planting area belonging to China (FAO, 2014).

The winter-flooded rice paddy field (FR) has been reported as a typical cropping production system in southwest China and includes more than 2.7 million cultivated hectares (Cai et al., 2003). Besides other, one of the most characteristic features of the FR system is flooding the field during winter fallow periods after rice harvest to conserve water for the following season (Chen, 2013). The flooded field soil after rice harvest could be in favor of substantial CH₄ emission (Cai et al., 1997; Dong et al., 2011; Qin et al., 2018), which depends on the balance of two processes, namely, CH₄ production and consumption (Cai et al., 2007; Shrestha et al., 2010; Huang et al., 2020).

Nitrogen (N) fertilization is an essential agricultural management practice for crop production which, may cause a profound impact on rice growth. It has also long been known that N fertilization is one of the most important variables regulating CH₄ production and emission (Cai et al., 1997; Tang et al., 2016). With paddy management practices, as reported by Liu and Greaver (2010), N fertilizer increased CH₄ emission and reduced CH₄ uptake in paddy soil. Similarly, Bodelier (2011) found that N fertilizer not only stimulated soil CH₄ production, but also inhibited CH₄ oxidation in the soil. These phenomena are likely due to enhanced methanogenic archaea activity (Cai et al., 2007), and can also be attributed to the partial inhibition of methanotrophic activity after ammonium (NH₄⁺)-based N fertilization (Dunfield and Knowles, 1995). Conversely, several researches recently observed that N fertilization decreased soil CH₄ emission (Zou et al., 2005; Su et al., 2017), and could be attributed to the growth of methanotrophic bacteria due to the application of NH₄⁺-based N fertilizer, resulting in increased CH₄ uptake in soils (Prasanna et al., 2002; Zou et al., 2005; Shrestha et al., 2010). Moreover, N fertilizer application could influence the stoichiometric ratios of carbon (C) to N, particularly, soil dissolved organic C (DOC) and inorganic N (DIN, including both NH₄⁺ and NO₃⁻), as essential substrates, thus affecting CH₄ emission related to microbial activities (Liu et al., 2016). However, previous studies typically mirrored the influence of N fertilization on CH₄ emission during the rice cultivation seasons (Cai et al., 1997, Cai et al., 2003, Cai et al., 2007; Zou et al., 2005; Ma et al., 2013; Zhou et al., 2018), and ignored preceding N fertilization affects stoichiometric ratios of soil available C to N, consequently affecting CH₄ emissions during the subsequent fallow period under a FR system.

It has been well stated that CH₄ emissions from soils are tightly linked to the change in soil temperature (Parashar et al., 1993; Yvon-Durocher et al., 2014). Given this, soil temperature, especially, at 5-cm depth, appeared to be another key variable having profound effects on CH₄ emission from paddy soils (Watanabe et al., 2005; Zhou et al., 2015; Zhou et al., 2017a). This is because soil anaerobic microbial activities related to CH₄ production and consumption (i.e. Methanogenic and Methanotrophic) mostly relied on soil temperature thereafter, influencing the intensity of CH₄ emission from paddy soils (King and Adamsen, 1992; Dong et al., 2011; Ho and Frenzel, 2012). Temperature-dependent interplay and CH₄ emission from paddy soils have long been associated. However, previous studies have rarely compared the response of CH₄ emissions to soil temperature at 5-cm depth linking the rice-growing season and fallow period in the FR system. The response of CH₄ emissions to soil temperature during the fallow period after N fertilization in rice-growing seasons is needed to evaluate.

In the present study, we aimed to compare variations in soil CH₄ emission during rice-growing season and subsequent fallow period and determine key factors (e.g. N fertilization, soil temperature, and dissolved C and N contents) affecting CH₄ emission under a typical FR system. For this purpose, a field experiment with N fertilization and unfertilized treatments was conducted to observe relevant soil factors driving changes in soil CH₄ emissions (i.e. DOC, NH₄⁺, NO₃^-, DOC/DIN ratio) and to investigate the relationships between CH₄ emissions and the crucial factors mentioned above.