Microbial carbon-use efficiency and straw-induced priming effect within soil aggregates are regulated by tillage history and balanced nutrient supply

Soil samples from an Alfisols with long-term (> 12 years) contrasting tillage intensities (i.e., no-till versus plow-till) were first divided into three distinct dry aggregate-size classes (i.e., mega-, macro-, and microaggregates) and then incubated with 13 C-labeled wheat residue at three input levels of balanced nutrients supply under controlled laboratory conditions. Across all the treatments, from 55.1 to 83.4% of total straw-C was mineralized within the incubation period. The microaggregates significantly increased the straw-C mineralization rate by 3.6 and 3.1 mg C g −1 straw-C day −1 compared to the mega- and macroaggregates respectively, mainly due to the higher microbial biomass. The high input of balanced nutrients enhanced the straw-C mineralization. Moreover, the microbial C-use efficiency (CUE) of straw residue in the microaggregates was 25.1% and 7.3% higher than that in the mega- and macroaggregates, respectively. Legacy effect of the long-term no-till increased microbial CUE, mainly by reducing the C-nutrient stoichiometric imbalance. In contrast, straw addition to the microaggregates resulted in the lowest positive priming effect (PE) of SOC compared to that added to the mega- and macroaggregates. Both the nutrient input and no-till treatments positively contributed to the decrease in positive PE. The negative correlation between the microbial CUE of the straw residue and the positive PE was reported for the first time, and the inputs of straw residue and high-level nutrients to the microaggregates from the no-till treatment increased CUE but decreased positive PE.