1. Introduction
Soil biodiversity loss induced by agricultural practices threatened the provision of soil ecosystem functions (De valença et al. , 2017; Huang et al. , 2019). One of the functions is soil organic carbon (SOC) storage (Chen et al. , 2017; Novara et al. , 2017), which is crucial to the determination of carbon (C) cycling in ecological systems. The C stock is susceptible to microbial carbon use efficiency (CUE) that is the fraction of C taken up by microbial cells and retained in biomass as opposed to being respired (Li et al., 2019; Li et al., 2014; Zhou et al., 2020). Furthermore, tillage practices could influence soil microbial CUE by changing some soil properties (e.g., temperature and moisture) (Domeignoz-Hortaet al. , 2020; Manzoni et al. , 2012). Soil microbial CUE can also be regulated by nitrogen (N) addition (Kallenbach et al. , 2019; Widdig et al. , 2020). Previous studies further found that microbial community structure and compositions are critical factors influencing microbial CUE (Nunes et al. , 2020; Sinsabaughet al. , 2016; Wang et al. , 2020). Therefore, it is essential to study the soil microbial mechanism responsible for the effect of N application on microbial CUE to better understanding carbon sequestration under tillage management.
No-tillage is one of the main conservation tillage practices and numerous studies have investigated its effect on microbial CUE (Kallenbach et al. , 2019; Mo et al. , 2021; Yang et al. , 2020a). Some studies have shown that no-tillage increased microbial CUE compared with conventional tillage (Kallenbach et al. , 2019; Mo et al. , 2021; Sauvadet et al. , 2018), but no effect was also found (Van Groenigen et al. , 2013). A possible reason for the different effects is that N application could influence microbial CUE (Kallenbach et al. , 2019; Mo et al. , 2021; Van Groenigen et al. , 2013) and its application rate is different among these studies. N application can also affect microbial growth and respiration by changing soil nutrient availability, particularly for N, because decomposer cells need to maintain balanced compositions of C and N (Manzoni et al. , 2012). In addition, the limitation of N increases over-flow respiration or C excretion rather than microbial growth, which further decreases microbial CUE (Qiao et al. , 2019). Previous studies showed that no-tillage with straw retention could decrease soil N availability (Gentile et al. , 2011; Thierfelder et al. , 2018). These findings indicate that N application is a promising way to induce no-tillage systems to increase microbial CUE.
Microbial CUE can be influenced by microbial populations that have different rates of organic matter decomposition and absorption (Waldrop & Firestone, 2004). Adu and Oades (1978) found that fungi played a more important role than bacteria on microbial CUE. The main reason is that the C:N variation range of fungi is generally wider than that of bacteria and fungi have a higher demand for C element than bacteria (Keiblinger et al. , 2010). However, other studies showed no significant difference in the effect of microbial CUE induced by fungi and bacteria (Six et al. , 2006; Thiet et al. , 2006). One reason for these conflicting results is that N application could also influence microbial CUE by stimulating microbial activity and decreasing microbial respiration metabolism (Lee & Schmidt, 2014; Liu et al. , 2018; Thiet et al. , 2006) and the difference N application rates under these studies could contribute to the discrepancy. Another reason is that these studies only focused on the influence of microbial populations and biomass on microbial CUE (Keiblinger et al. , 2010; Waldrop & Firestone, 2004) and ignore the key role of microbial diversity on microbial CUE (Domeignoz-Horta et al. , 2020). Hence, studying the impact of N application on microbial CUE based on its effects on microbial diversity and community structure could provide a comprehensive perspective to reveal the effect of N addition on C cycling.
Furthermore, the increase of microbial CUE is an effective means of increasing SOC sequestration (Bradford et al. , 2013; Haddixet al. , 2016). SOC fractions, especially for particulate organic matter carbon (POC) and mineral-associated organic matter carbon (MAOC), are more sensitive to microbial CUE than total SOC (Averill & Waring, 2018; Chenet al. , 2018; Ye et al. , 2018). Averill and Waring (2018) found that substrate use efficiency can also directly affect C cycling through changing POC and MAOC. In addition, N addition significantly influenced on soil POC and MAOC (Chenet al. , 2020b, 2019; Ye et al. , 2018). However, it remains unclear how N application regulates the effect of soil microbial CUE on POC and MAOC under tillage management. Therefore, studying the effects of N application is essential to understanding the role of soil microbial CUE on carbon sequestration potential.
This study was conducted to investigate the influence of N application on microbial CUE under tillage practices from a microbiological perspective. We hypothesized that: (i) the responses of soil microbial diversity, community structure, biomass, and CUE to N application under CT and NT were different, and (ii) microbial diversity plays a more important role than microbial biomass in microbial CUE. The objectives of this study were to (i) evaluate the effects of tillage management and N application on soil microbial diversity, community compositions, and soil microbial CUE, (ii) reveal how N application influences soil microbial CUE by regulating microbial diversity, community structure, and biomass, and (iii) assess the influence of microbial CUE on soil POC and MAOC under tillage management with different N application rates.