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.