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

Abstract

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.

The Effect of Soil Microbial Residues-Mediated Nitrogen Conservation and Supply during the Growing Season on Nitrogen Uptake by Wheat

“…In this study, long-term nitrogen application was beneficial to the accumulation of soil microbial residues, but the conversion and utilization nitrogen by microorganisms did not continue to increase with the increase in the nitrogen application level (Figure 3b). This is consistent with the results of Hu et al in the rice-wheat system and Anning et al in the farmland soil of the Loess Plateau [25,26]. Zhang et al's research results in tropical forest ecosystems showed that high-level nitrogen addition decreased the accumulation of soil microbial residues compared to medium-level nitrogen addition [35].…”

“…As commonly found in some long-term field location experiments, nitrogen addition can increase soil microbial residue content, and there is a positive correlation between soil microbial residue accumulation and crop yield; however, soil microbial residue accumulation does not continue to increase with nitrogen fertilizer input, indicating that high nitrogen fertilization triggers the upper limit of the "microbial carrying capacity" [25][26][27][28][29]. These studies focus on the construction of soil organic matter pools using microbial residue, while there are few studies on the mineralization of microbial residue on crop growth, especially in the short term to verify the impact of microbial residue mineralization on crop nutrient absorption.…”