1 Introduction
In cropland soil, the preservation or improvement of soil quality and
productivity is of major importance. Enhancing the yield and nutritional
quality of agricultural produce while maintaining the soil quality
including the soil carbon level is a challenge for sustainable
agriculture. Currently, intensive agriculture generally relies on
synthetic fertilizer application, which leaded to considerable
environmental risks and economic loess. For example, up to 50% of N
runs off from cropland and cause greenhouse gas emission and soil
degeneration (Cui et al., 2018; Ramesh et al., 2019). Additionally, soil
fertility declines with continuous application of inorganic fertilizers
without organic inputs due to soil acidification and degradation of soil
structure (Liang et al., 2013). Therefore, development of agriculture in
a low-input and suitable way is urgently needed for the transition to
sustainable agriculture while reducing the trade-offs (Ramesh et al.,
2019).
Soil organic C (SOC) is an important component playing key
multifunctional roles in soil quality and determining many soil physical
and biological properties (Lal, 2009). Various factors influence SOC
content, such as regional climate, soil conditions, aggregate stability
and agricultural management practices can elevate SOC content by
affecting the balance between C input and C output. Fertilization has
been widely used as a common management practice to increase SOC level.
In general, fertilization with organic fertilizers alone or plus
chemical fertilizers tend to increase SOC level (Zhang, Ding, Yu, & He,
2015; Zhang et al., 2017), while chemical fertilizer alone yielded in
inconsistent results (Bado, Lompo, Sedogo, & Cescas, 2010; Gong, Yan,
& Wang, 2012). Fertilizer N might positively affect SOC accumulation
and macro-aggregate formation due to increased residue biomass input
(Tang, Xiao, Li, Wang, & Pan, 2018), it could also decrease SOC via
increasing SOC mineralization which primed by N fertilizer addition (Su,
Wang, Suo, Zhang, & Du, 2006). However, Tripathi et al. (2014) showed
that no difference of soil aggregate distribution and SOC between soil
received fertilizer N and soil not received fertilizer N. Soil organic
matter can be the nucleus for aggregate formation and act as a soil
binding agent, and SOC sequestration is closely associated with the
stability and structure of soil aggregates (Bronick & Lal, 2005). The
aggregate-associated C and N are protected from mineralization because
of their being less vulnerable to degradation. Additionally, stable soil
aggregates can reduce soil erosion and surface runoff. Therefore,
regarding the detrimental impacts of chemical fertilizer alone on soil
aggregates and C level, a new pragmatic fertilization strategy is
urgently needed.
The application of bio-fertilizers that contains living microorganisms
is one of the management practices that can help to maintain or increase
the content of organic matter and improve soil fertility in arable
soils. More recently, it was reported that application of a chemical
fertilizer mixed with bio-fertilizer, contained living microorganisms,
was regarded as an efficient way to supplement soil nutrient level
(Vessey, 2003), improve soil physic-chemical properties and regulate
soil microbial community structure, which were benefit for crop yields
(Liu et al., 2017; Singh et al., 2019). Many studies have focused on the
impacts of bio-fertilizer and chemical fertilizer on soil microorganisms
(Zhang, Xu, Wang, Wu, & Xiao, 2020), such as bacterial community (Wu et
al., 2016), antagonistic bacteria abundance (Li, Tao, Ling, & Chu,
2017). Though studies have determined the effects of bio-fertilizer
supplement on soil aggregate stability and organic C content (Graf &
Frei, 2013; Yilmaz & Sönmez, 2017). However, changes of soil C
fractions and soil aggregation in soil applied with bio-fertilizer and
chemical fertilizer in North China Plain was not very unclear.
The North China Plain is an important food-production area in China. The
major agricultural planting system in this region, summer maize
(Zea mays L.)-winter wheat (Triticum aestivum L.) double
cropping system, is characterized by intensive farming with high
applications of mineral fertilizers (nitrogen and phosphorous) and no
extra organic materials application. This resulted in high
mineralization of native SOC due to increased microbial biomass and
cellulose-decomposing enzymes (Kumar, Shahid, Tripathi, Mohanty, &
Nayak, 2017; Liu et al., 2017). What’s more, high frequency of rotary
cultivators also aggregated the decomposition of soil native organic C.
Application of organic materials has been adopted by farmers to improve
soil quality. It has been showed that the soil microbial abundance
generally increased with bio-fertilizer application rates. However,
whether the soil C and soil aggregates would show a similar tendency
remains unknown. Therefore, to achieve a balance between soil
conservation and economic benefits, it is necessary to adopt appropriate
bio-fertilizer application rate for improving soil structure and
promoting sustainable crop production.
To fill this knowledge gap, based on a field experiment under different
bio-fertilizer application rates, the objectives of this study were to
1) investigate concentrations of SOC, soil labile organic C and basal
respiration after two years of bio-fertilzer addition; 2) determine soil
aggregate and distribution of organic C and N in different aggregates in
North China Plain. The results from this study can be useful to further
understand the potential of bio-fertilizer to change quality of soil
organic matter under field conditions. We hypotheses that bio-fertilizer
will increase SOC in soil and aggregates, and be benefit for the content
of macro-aggregate and the water stable aggregate stability.