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.