Storage and drivers of soil organic carbon and nitrogen in a rangeland ecosystem across a lithosequence in western Iran

Abstract

Identifying the drivers of soil organic carbon (SOC) and nitrogen (N) dynamics is profoundly needed to get insight into the nutrient cycling in the terrestrial ecosystems. Soil parent material is one of the important factors affecting SOC and N dynamics. The aim of this research was to explore C and N dynamics of a soil lithosequence with parent materials ranging from sedimentary to metamorphic and plutonic bedrock in the eastern Kurdistan province, in western Iran.We examined the hypothesis that SOC and N accumulation potential and turnover vary significantly with soil bedrock lithology. To test our hypothesis, we collected soil samples at three depth intervals (0–5, 5–25 and 25–50 cm) as well as aboveground plant materials in six geological contexts: clastic carbonate sedimentary (CCS), foliated metamorphic (FM), non-foliated metamorphic (NFM), intermediate plutonic (IP), felsic plutonic (FP) and intermediate felsic plutonic (IFP). Soil characteristics (SOC, total N, δ13C, δ15N, soil texture, pH, EC, carbonates, available P) and plant material properties (C and N contents and their stable isotopic signatures) were determined in 45 soil samples and 18 plant materials, respectively.Results showed that both geology and soil depth strongly influenced the SOC and total N concentrations as well as their stable C and N isotopic composition. Both SOC and total N were highest per site/lithology in the soils over FM geology (5.42 and 0.51 g kg−1 soil for SOC and total N, respectively) and least in the soils forming on FP parent material (0.97 and 0.12 g kg−1 soil for SOC and total N, respectively). In the case of stable C and N isotopic composition, the highest mean value per lithology was found in the NFM soils (−23.68‰ for δ13C) and FP soils (13.74‰ for δ15N), while the lowest mean signature was observed in the soils developed over FM geology (−25.41‰ and 7.88‰ for δ13C and δ15N, respectively). Vertical distribution of both δ13C and δ15N signatures in the studied soils showed an increasing trend by depth. The main controlling factors for organic C and N concentrations across the lithosequence were explored using PLSR and PCA analyses.Results indicated that different soil properties controlled the SOC and total N contents at the different depths. Soil stable C and N isotopic compositions and pH were the factors that influenced negatively the organic C and N contents across the studied lithosequence. Differently, soil C/N ratio was strongly positively associated with SOC and TN contents. Clay- and silt-size fractions had no substantial influence on the organic C and N concentrations, although importance of silt particles for SOC and TN storage increased with increasing soil depth. These findings provide evidence that geology, as soil parent material, can control both SOC and N accumulation and turnover. However, a more in-depth study is required to understand the mechanisms by which parent lithology affects the organic C and N dynamics.