Mitigating the effects of global climate change caused by increasing atmospheric greenhouse gas (GHG) emissions has become one of society’s most important challenges (Trivedi, 2013). Nitrous oxide (N2O) is a potent GHG greatly contributing to global climate change, with a 265-fold higher global warming potential (GWP) than carbon dioxide (CO2) (IPCC, 2014), and is responsible for destruction of protective ozone layer (Dickinson and Cicerone, 1986; Ravishankara et al., 2009). Globally, natural and anthropogenic N2O sources are primarily dominated by emissions from soils (Tian et al., 2020), particularly acidic soils (Liu et al., 2017). The excessive use of nitrogen (N)-based fertilizers for enhancing agricultural productivity (Erisman et al., 2018), has induced serious soil acidification recently (Guo et al., 2010; Tian and Niu, 2015). Guo et al. (2010) reported a reduced pH of 0.50 in Chinese agricultural systems from the 1980s to the 2000s. Soil acidification decreases soil pH and increases N2O emissions (Wang et al., 2018), in turn, contributes to adverse effects on soil microorganisms, plants growth and environmental quality (Kunhikrishnan et al., 2016). Therefore, amelioration of soil acidification is critical to offset global warming by mitigating N2O emissions.
Application of alkaline amendments to acidic soils is a common approach to ameliorate soil acidification. Phosphorus tailing (PT), an alkaline waste product of phosphorus mineral flotation process, which mainly consists of dolomite (CaMg(CO3)2), magnesium oxide (MgO) and silicon dioxide (SiO2) (Zhou et al., 2020). It has the potential to increase total alkalinity and soil pH (Chadwick and Chorover, 2001; Shen et al., 2018) and is a source of base cations, such as calcium (Ca) and magnesium (Mg) (Guo et al., 2010). However, currently only 7% of PT produced in the world is recycled (Li et al., 2019). A large amount of PT is stacked in tailings pond, which can result in severe environmental, ecological, and economic consequences (Zhou et al., 2019). Therefore, the use of alkaline waste materials for ameliorating soil acidification presents a desirable and economically attractive alternative as well as assisting in the disposal of otherwise troublesome wastes.
Soil pH-increase by applying alkaline amendments ultimately affects N2O emissions (Shaaban et al., 2020). Recent studies suggest that N2O emissions can be reduced by increasing soil pH (Shaaban et al., 2018; Shaaban et al., 2019). However, there are also cases where N2O emissions were even increased in response to elevated soil pH (Baggs et al., 2010). Given the major influence of soil pH on N transformation, soil pH-increase could affect N2O emissions by trigging virtually every N cycling process involved in N2O formation and consumption in soils. For example, there is increasing evidence that pH-increase could enhance nitrification by increasing nitrifier population (Nicol et al., 2008), which would enhance nitrification-related N2O emission (Hink et al., 2017). Higher soil pH also can stimulate denitrification (ŠImek and Cooper, 2002) through affecting synthesis of functional N2O reductase (Liu et al., 2014). Robinson et al. (2014) reported that addition of lime to acidic pasture soils greatly influenced the completion of denitrification and lead to reduced N2O emissions. Therefore, soil pH-increase effects are complex and it is necessary to improve the understanding of how alkaline amendments affect N transformation and N2O emissions.
Traditionally, bacteria were presumed to be the exclusive contributors to soil N2O-producing metabolic pathway (Zumft, 1997). However, recent evidence from several ecosystems suggests that both bacteria and fungi have the genetic potential to produce N2O (Xu et al., 2019; Wang et al., 2021). Fungi generally show better adaptability to acidified soil than bacteria due to the stronger cell walls and unique mycelial networks of fungi (Ma et al., 2017). Chen et al. (2015) reported that the relative contribution of fungi to N2O production was greater than that of bacteria under acidic conditions. Little information is available concerning the differential susceptibility of bacteria and fungi to soil pH-increase. Contrasting observations have been reported for soil pH on the relative role of fungi and bacteria in N2O production (Chen et al., 2015). Mitigating N2O emissions by alkaline amendment addition is strongly hindered by lack of knowledge on microbial mechanisms underpinning the N2O producing processes.
Thus, this study is aimed to explore the effect of using an alkaline industrial waste on soil pH as well as N2O emissions. We hypothesized that i) the application of such industrial waste would effectively reduce N2O emissions via influencing N transformation by pH-increase effect directly and indirectly; ii) the amendment decreasing N2O emissions mainly through affecting fungal function.