Mechanisms behind the inhibition of autotrophic nitrification following rice-straw incorporation in a subtropical acid soil
Graphical abstract
Introduction
To meet increasing grain demand, chemical fertilizers are used to improve productivity in agricultural ecosystems (Wang et al., 2018). However, overuse of chemical fertilizers can lead to multiple environmental issues, such as soil acidification, low fertilizer use efficiency, and groundwater contamination (Ju et al., 2006; Wang et al., 2018). To alleviate environmental deterioration and improve soil fertility, incorporating crop straw to cultivated soils has been widely adopted in China in recent years (Gai et al., 2019). This is due to a new policy of reducing chemical fertilizers by partial replacement with organic fertilizers. Previous studies have reported the effects of straw incorporation on soil organic N mineralization (Takahashi et al., 2003; Thomsen and Sørensen, 2006), NH4+ immobilization (Corbeels et al., 2000; Cao et al., 2018), nitrification (Liu, 2002; Thomsen and Sørensen, 2006), dissimilatory NO3− reduction to NH4+ (Lu et al., 2015), or NO3− immobilization (Shindo and Nishio, 2005). However, few studies have been conducted to investigate the simultaneous N transformations which drive soil N availability following straw incorporation, which are required to understand the mechanisms behind straw incorporation effects on N availability and loss risk.
Straw incorporation may change microbial community composition and influence N transformation dynamics and N availability (Bird et al., 2003; Ryals et al., 2014). As a main pathway of NH4+ production, N mineralization would be improved by the incorporation of exogenous materials (Thomsen and Sørensen, 2006). Additionally, because of the positive effect of straw incorporation, the mineralization of native soil organic matter would be promoted (Muhammad et al., 2007). Soil N availability are governed by soil N mineralization and immobilization processes (Huygens et al., 2007). Generally, incorporation of straw with a large C:N ratio would improve N immobilization and cause N limitation (Bhogal et al., 1997; Beaudoin et al., 2005; Zhao et al., 2018a). However, most studies focus on the net rates of soil N transformation after straw incorporation (Khalil et al., 2005; Muhammad et al., 2007; Cao et al., 2018; Gai et al., 2019), which does not provide a good understanding about the gross rates of N transformation. The gross rates of N transformation, associated with individual processes, can elucidate the mechanisms involved in the soil N cycling (Whitehead, 2000; Müller et al., 2007).
In subtropical regions of China, which are characterized by high precipitation, NO3− leaching losses occur readily, while NH4+ is less mobile due to inhibition of ammonia volatilization caused by low soil pH (Zhang et al., 2013) and it is charged making it more susceptible to soil adsorption processes. Thus, NO3− is more difficult to immobilize than NH4+ (Nishio et al., 2001). However, crop straw incorporation can accelerate the assimilation of NO3− at soil water holding capacity (WHC) of 35–46% (Nishio et al., 2001). Additionally, as a main pathway of NO3− production in agricultural soil, autotrophic nitrification may be inhibited by straw incorporation (Zhang et al., 2012; Wang et al., 2015). Thus, adjusting the soil C:N ratio and increasing the soil microbial biomass C via straw incorporation could help reduce NO3− losses (Said-Pullicino et al., 2014). Others have reported that straw incorporation could reduce NO3− leaching losses from soil (Yang et al., 2018; Wang et al., 2019). However, it has been reported that organic matter has a positive stimulatory effect on the community and function of ammonia-oxidizers (Kalvelage et al., 2013; He et al., 2007), and that the increased dissolved organic C concentration associated with straw incorporation could increase the potential nitrification rates (Liu et al., 2016), which could improve the risk of NO3− loss. Therefore, to explain this phenomenon, the effects of straw incorporation on soil NO3− production and consumption should be considered.
We hypothesized that rice-straw incorporation influences soil inorganic N availability and its loss risk by affecting soil N transformation rates. Thus, a 15N incubation experiment was conducted with rice-straw incorporation at different rates, and a 15N tracing method was applied to quantify gross N transformation rates. The aims of this study were: (1) to investigate the effects of rice-straw incorporation on inorganic N production and consumption; and (2) to identify the effects of rice-straw incorporation on nitrification for risk of NO3− loss estimations. The results will help guide the recommendations of suitable practices for straw incorporation in subtropical acid soils.
Section snippets
Soil samples
Soils were collected from a paddy site of the Fujian Academy of Agricultural Sciences, Fuzhou China (26°13′31″N, 119°04′10″E). The mean annual temperature and precipitation (thirty years average) in this region is 19.5 °C and 1350 mm. According to World Reference Base for Soil Taxonomy (Nachtergaele et al., 2000), the soil is an Anthrosol developed from granite. Topsoil (to a depth of 15 cm) was collected before fertilization in May 2018. Plant residues and roots in soil were removed after
Soil properties
Following incorporation of rice-straw by the tenth week, there was a positive relationship between increased pH and greater rice-straw incorporation, particularly for RS2 and RS3, with significantly greater pH than the other treatments (Table 1). Compared to RS0, SOC was increased by 3.2, 5.7 and 9.4% in RS1, RS2 and RS3, respectively. Although a general trend for increased TN concentration with greater rates of rice-straw incorporation was observed, this was not significant (Table 1). Soil NO3−
Mechanisms of soil N available affected by rice straw incorporation
The concentration of inorganic N decreased with increasing rates of rice-straw incorporation (Table 1), in accordance with previous studies of Zhao et al. (2018b), who suggested that organic material incorporation reduced net mineralization rates. The reduction of soil inorganic N concentration following rice-straw incorporation indicates that soil N supply may be reduced, which may have a negative impact on crop growth in the early stages (Chapman, 1997). However, the two main pathways of
Conclusion
Rice-straw incorporation stimulated N mineralization, NH4+ immobilization, oxidization of organic N to NO3−, NO3− immobilization, and dissimilatory NO3− reduction to NH4+ within the first week. Inorganic N concentration in soil was driven by soil inorganic N supply capacity, which decreased with increasing rates of rice-straw incorporation, because inorganic N consumption was greater than production. Autotrophic nitrification was inhibited by rice-straw incorporation in the first week, with no
Acknowledgements
This work was supported by the following grants: the National Natural Science Foundation of China (41771330, 41401339, 41907077) and of Fujian Province (2018J01058, 2019J01104, 2019J01105); the public welfare project of Fujian Province (2019R1025-1); the project of China Scholarship Council (201809350003), and Foundation of Fujian Academic of Agricultural Sciences (YC2015-6, AB2017-2, SIIT2017-1-9), the Newton Fund through the UK-China Virtual Joint Centre for Improved Nitrogen Agronomy (CINAg,
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