Effects and mechanisms of addition of different types of exogenous organic materials on priming effect of organic carbon in arable black soils

The addition of exogenous organic matter (EOM) affects the mineralization and stabilization of soil organic carbon (SOC) via priming effects (PE). However, few studies have considered the effects of different EOM additions on PE in the same soil type with a gradient in SOC content. The underlying mechanisms have rarely been revealed, and related studies can provide in-depth insights into the microbial mechanisms that regulate carbon accumulation and stability in agricultural soils. It is crucial to predict dynamic changes in SOC and carbon pool stability in response to EOM inputs from different sources. Therefore, this study focused on topsoil with four SOC contents, which ranged from 10 g·kg⁻¹ to nearly 70 g·kg⁻¹, in the typical black-soil region of Northeast China, and aimed to investigate the effects and microbial mechanisms involving different types of EOM addition on PEs by adding ¹³C-labeled maize straw, glucose, and alanine to the soil. Compared to a control treatment without EOM, the addition of EOM promoted the mineralization of SOC in the four soils with different SOC contents. Specifically, glucose, alanine, and straw addition increased the cumulative mineralization of SOC by 50.88%–419.65%, 69.54%–409.48%, and 13.14%–321.43%, respectively. The addition of the three types of EOM also induced a positive PE in soils with different SOC contents. During the initial 30 days of incubation, the cumulative PEs in soils with different SOC contents under glucose and alanine addition treatments were considerably higher than those under straw addition treatment. Soils with higher SOC content exhibited greater cumulative mineralization and PEs with the addition of glucose and alanine, whereas their relative cumulative PEs were lower. SOC mineralization and PEs decreased and reached a stable state with incubation time in soils with different SOC contents. Fourier-Transform Infrared spectroscopy revealed a slight increase in the relative peak area of aliphatic carbon functional group and a slight decrease in the peak area of aromatic carbon group in soils with different contents of SOC after addition of EOM. The effects of SOC content on aliphatic and aromatic carbon functional groups were greater than those of the EOM type. Correlation analysis revealed that cumulative SOC mineralization and PEs were significantly positively correlated with total phospholipid fatty acids, biomass of total bacteria, gram-positive bacteria, gram-negative bacteria, and actinomycetes, with a peak area of aliphatic carbon at 1420 cm⁻¹ (P<0.05). In addition, cumulative SOC mineralization and cumulative PEs were significantly negatively correlated with the biomass of fungi and anaerobic bacteria, with a peak area of aromatic carbon at 1630 cm⁻¹ (P<0.05). Structural equation modeling indicated that the positive PE resulting from EOM addition was primarily influenced by bacterial and actinomycete phospholipid fatty acids in the soil, regardless of the SOC content of the four soil samples. These results demonstrated that EOM addition significantly increased the growth and metabolism of k-type microorganisms, such as gram-negative bacteria and actinomycetes, in arable black soil and promoted the decomposition of recalcitrant components in SOC. The “co-metabolism” theory, namely the co-decomposition of EOM and SOC, is considered as the primary mechanism behind the positive PE in black soil.