High initial soil organic matter level combined with aboveground plant residues increased microbial carbon use efficiency but accelerated soil priming effect

Input of plant residue carbon (C) stimulates microbial growth and activity, and thus may alter native soil organic matter (SOM) mineralization. The partition of plant residue C between microbial growth and respiration, and priming effect on soil organic C (SOC) are affected by initial SOM levels and plant residue types. However, how the interaction between SOM level and plant residue on microbial C use efficiency (CUE) and soil priming effect remains not very clear. Here, we quantified the ratio of plant residue C converted to microbial biomass production (as MBC) over that uptake by microorganism (MBC + respiration) and the priming effect on native SOC in two soils (with low and high initial SOM levels, abbreviated as LSOM and HSOM, respectively) added with C-13-labeled maize residues (root, stem and leaf) through a 180-day incubation. Microbial CUE of maize residue was the highest in the HSOM soil with leaf residue addition, and was the lowest in LSOM soil with stem and leaf residues addition. About 37%similar to 47% of maize residue C was remained in the soil after 180 days. At the end of incubation, the positive cumulative priming effects on native SOC mineralization induced by stem and leaf residues were 23% and 30% stronger (P < 0.05) in the HSOM soil than those of the LSOM soil, respectively. In contrast the root residue addition induced the negative priming effect on native SOC in the two SOM levels of soils. Overall, microbial CUE of maize residue was higher in soil with high initial SOM level, which is likely to promote SOM formation via microbial biomass, although there are many other factors that influence SOM formation. The interactive effect between initial SOM level and plant residue quality should be considered when understanding long-term SOM storage.

Automated summary

In soils with high initial SOM levels, plant residue carbon is more efficiently utilized by microorganisms, potentially promoting soil organic matter formation. However, the addition of root residues may have negative impacts on soil organic matter in both low and high initial SOM level soils.