Comparing microbial transformation of maize residue‐N and fertilizer‐N in soil using amino sugar‐specific 15N analysis

Abstract

AbstractIn agroecosystems, both fertilizer‐nitrogen (N) and crop residue‐N make important contributions to soil‐N retention. However, how different N sources are dynamically involved in microbial processes and how they govern soil‐N transformations is poorly understood. A field experiment with half‐yield maize residue mulching was conducted in an Alfisol. The 15N‐labelled fertilizer and maize (Zea mays L.) residue were applied in the first year and then unlabelled ones were used in the following 4 years. The origins of newly formed fungal and bacterial residues were investigated by amino sugar‐specific 15N analysis. The aim was to gain insights into the microbial transformation dynamics of fertilizer‐N and maize residue‐N, as well as the maintenance of these exogenous N sources in soil. The initial transformation of fertilizer‐N into amino sugars was much more rapid than that of maize residue‐N. However, the eventual accumulation of maize residue‐N in amino sugars was significantly greater than that of fertilizer‐N. The transformation of maize residue‐15N into amino sugars was 3.5–6.7 times higher than that of fertilizer‐15N over the course of 5 years, implying a higher immobilization of maize residue‐derived N than fertilizer‐derived N in the soil. The effective transformation of maize residue‐N into microbial residues and the low level of maize residue‐derived mineral N accumulation suggested that a direct route probably dominated the microbial process. Moreover, the maize residue‐derived fungal glucosamine was more apt to accumulate than muramic acid, compared with that derived from fertilizer, suggesting that fungi contributed more to the direct assimilation of maize residue‐derived N. Such a pattern was mostly attributed to fungal decomposition of less bioavailable components of maize residue. Thus, maize residue‐N played a significant role in building soil organic N reserves, being a sustaining foundation for the effective utilization of fertilizer‐N by crops.Highlights Fungal and bacterial residues derived from maize residue‐N and fertilizer‐N were investigated. Fungi dominated maize residue‐N immobilization mostly via direct route. Maize residue‐N had higher microbial immobilization efficiency than fertilizer‐N. Maize residue‐N contributed dominantly to the building of soil organic N pool.