Divergent Accumulation of Microbial Residues and Amino Sugars in Loess Soil after Six Years of Different Inorganic Nitrogen Enrichment Scenarios

Amino sugars are key microbial biomarkers for determining the contribution of microbial residues in soil organic matter (SOM). However, it remains largely unclear as to what extent inorganic nitrogen (N) fertilization can lead to the significant degradation of SOM in alkaline agricultural soils. A six-year field experiment was conducted from 2013 to 2018 to evaluate the effects of chronic N enrichment on microbial residues, amino sugars, and soil biochemical properties under four nitrogen (urea, 46% N) fertilization scenarios: 0 (no-N, control), 75 (low-N), 225 (medium-N), and 375 (high-N) kg N ha(-1). The results showed that chronic N enrichment stimulated microbial residues and amino sugar accumulation over time. The medium-N treatment increased the concentration of muramic acid (15.77%), glucosamine (13.55%), galactosamine (18.84%), bacterial residues (16.88%), fungal residues (11.31%), and total microbial residues (12.57%) compared to the control in 2018; however, these concentrations were comparable to the high-N treatment concentrations. The ratio of glucosamine to galactosamine and of glucosamine to muramic acid decreased over time due to a larger increase in bacterial residues as compared to fungal residues. Microbial biomass, soil organic carbon, and aboveground plant biomass positively correlated with microbial residues and amino sugar components. Chronic N enrichment improved the soil biochemical properties and aboveground plant biomass, which stimulated microbial residues and amino sugar accumulation over time.

Automated summary

The study found that chronic N enrichment stimulated the accumulation of microbial residues and amino sugars over time. In the medium-N treatment, concentrations of various amino sugars and microbial residues increased compared to the control in 2018, and were comparable to the high-N treatment concentrations. Changes in the ratios of amino sugars over time were attributed to a larger increase in bacterial residues compared to fungal residues. Additionally, microbial residues and amino sugar components were positively correlated with microbial biomass, soil organic carbon, and aboveground plant biomass.