Effects of N deposition on the quality and quantity of soil organic matter in a boreal forest: Contrasting roles of ammonium and nitrate

Abstract

Elevated nitrogen (N) deposition plays a critical role in soil organic matter (SOM) transformation, but the responses of the quantity and quality of SOM to ammonium and nitrate fertilization remain unclear. In this work, a 6-year field experiment amended with ammonium chloride (NH4Cl) and potassium nitrate (KNO3) fertilizers at four rates of 0, 10, 20, and 40 kg N ha−1 yr−1 was conducted in a boreal forest. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis and elemental analysis were used to investigate the chemical composition and soil organic carbon (SOC) contents in bulk soils and two density fractions (light fraction < 1.70 g cm−3 and heavy fraction > 1.70 g cm−3). Soil microbial community composition was determined based on phospholipid fatty acids (PLFAs) analysis. Results showed that the SOC contents in the organic and mineral layers under nitrate addition were significantly increased by 27–87%. By contrast, ammonium addition significantly decreased the SOC contents in the organic layer by 28–54%. Nitrate addition increased relative proportions of acetic acid in the light fraction and toluene in bulk soil, but decreased relative abundance of phenol in bulk soil, indicating that more plant-derived input contributed to the increase of SOC. Contrastingly, ammonium addition increased the proportions of benzene in bulk soil and pyrrole as well as the mineralization index (pyrrole/phenol) in the heavy fraction, suggesting more advanced mineralization degree of SOM. Moreover, ammonium addition altered microbial community structure with a higher ratio of fungi/bacteria (F/B) while nitrate addition had little effect on soil microbial community. Overall, our findings highlight the contrasting roles of ammonium and nitrate in SOM dynamics, which should be distinguished to accurately predict the responses of SOC sequestration to N deposition especially in N-limited boreal forests.