Soil N/P and C/P ratio regulate the responses of soil microbial community composition and enzyme activities in a long-term nitrogen loaded Chinese fir forest

Abstract

Long-term nitrogen (N) fertilization has been shown to profoundly affect the soil microorganisms and strongly result in several imbalances in element concentrations. The objective of this study was to examine links among the soil microorganisms, enzyme activities, and soil carbon (C), N, and phosphorus (P) stoichiometry in a subtropical Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation after continuous N fertilization for 13 years. This study was performed in 25-year-old fir plantation along a fertilization gradient (0, 60, 120, and 240 kg N ha−1 yr.−1), designated as N0, N1, N2, and N3, respectively. Soil microbial properties, including the microbial community composition, as revealed by phospholipid fatty acids (PLFAs), and soil enzyme activities (i.e., sucrase, urease and catalase) were measured, and soil elemental stoichiometry was calculated based on soil C, N, and P concentrations. A redundancy analysis (RDA) was conducted to determine the relationship between soil C:N:P stoichiometry and soil microbial properties. Compared with the control (N0), N fertilization decreased the total PLFAs (−12.20%), bacteria (−14.33%), fungi (−12.97%), and actinomycetes (−17.11%) on average. Sucrase, urease and catalase activities were enhanced by low and middle levels of N (N1 and N2), but not with high level of N (N3). Long-term N fertilization decreased soil pH, C to N ratio (C/N), and C to P ratio (C/P), while increased soil C, N and N to P ratio (N/P). The RDA identified the first two axes of soil stoichiometry variation that explained 20.4% of the variation at the soil depth of 0–20 cm, 28.6% at 20–40 cm and 49.9% at 40–60 cm in PLFAs biomarkers and enzymes, respectively. Significant correlations between soil stoichiometry (soil N/P and C/P ratio) and soil microbial properties were found in this study. These observations suggested that long-term N fertilization influenced soil microbial community composition and enzyme activities by changing the soil C/P and N/P ratios. Future studies are needed to consider the coupling relationships between soil microbial community composition, enzyme activities and elemental stoichiometry in different ecosystems under future climatic change.