Bedrock lithology and tree species type influence soil nitrogen dynamics in a temperate forest

Despite significant progress in studying soil organic carbon (SOC) and nitrogen (N) cycling in temperate forest soils, understanding of how bedrock lithology and tree species type influence these parameters remains tentative. To address this, we collected soil samples from three depth intervals and plant materials from two distinct tree species, beech, and lime, from sites within the Hyrcanian Forests (Iran) underlain by carbonate and intermediate volcanic bedrock. C and N elemental concentrations and their stable isotope compositions (δ¹³C and δ¹⁵N) were determined for bulk soil and four SOM fractions, including free particulate organic matter (FPOM), macroaggregates, microaggregates, silt + clay-sized fractions, as well as leaf litter and fine roots.

Results indicated that lithology and tree species had no significant relationship with SOC content and δ¹³C of various soil fractions. Along with their δ¹⁵N values, TN contents of bulk soil, FPOM, macro- and microaggregates covaried with tree species and lithology. Total N content in bulk soils underneath lime trees exceeded that found beneath beech trees (0.43 % vs. 0.36 %). In terms of N turnover, volcanic soils showed significantly higher mean ¹⁵N enrichment relative to that observed for carbonate soils. The C and N fluxes observed for different tree species and lithologies revealed a ¹³C and ¹⁵N enrichment trend in the following order: macroaggregates< microaggregates< silt and clay-sized particles. Our results showed that underlying lithology influences C and N dynamics in forest soils, and the analysis of the natural abundance of ¹³C and ¹⁵N provides detailed information on C and N cycling and stabilization pathways in soil aggregates. Our findings demonstrate the importance of lithology as a factor in nutrient cycle estimates for terrestrial ecosystems.