Stoichiometry of peatlands and plants: Mutual interactions in biogeochemical cycles

• The stoichiometry of peat soils and conifer needles varies by histosol type. • Fibric histosols show highest C and C/N, sapric have the lowest values. • Sapric histosols are enriched in base cations and Al, showing decomposition. • Cation ratios are significantly higher in plant material than in soil. • Stoichiometric indices are sensitive markers of peatland degradation. Although peatlands cover only a small fraction of the Earth’s surface, they store large amounts of carbon and play a key role in global biogeochemical cycles. A joint assessment of the stoichiometry of biogenic elements (C, N and P) and nutrients ratios may improve our understanding of soil–plant interactions across different types of Histosol. The aim of this study was to analyse the stoichiometry of C:N:P and nutrients ratios in peat soils and conifer needles ( Picea abies and Pinus sylvestris ) across three different histosol types: fibric, hemic and sapric. Particular emphasis was placed on soil–plant relationships and evaluating the potential of stoichiometric indicators to diagnose peatland condition. Samples were collected from 255 sites across Poland. Composite soil samples (0–15 cm depth) and conifer needles were analysed for C and N using an elemental analyser, and for Ca, Mg, K, Al and P using ICP-OES. Ratios (C/N, C/P, N/P, Ca/Mg, Ca/Al and Ca·K·Mg/Al) were then calculated. Statistical procedures included descriptive statistics (median and quartile deviation), a Kruskal–Wallis test (p < 0.05), a principal component analysis (PCA), generalised linear models (GLM), Spearman’s rank correlations and simple linear regressions. Soil carbon content, as well as the C/N, C/P and N/P ratios, decreased systematically from fibric to sapric histosols. Meanwhile, the contents of Ca, Mg, K and Al were significantly higher in sapric soils. In plants, the contents of C, N and P, as well as the C/N, C/P and N/P ratios, did not differ between histosol types. However, the ratios (particularly Ca/Al and Ca·K·Mg/Al) were highest in sapric histosols. Soil–plant correlations were weak for C/N, C/P, and N/P, but slightly stronger for Ca/Al and Ca·K·Mg/Al. PCA (PC1 = 35.52 %; PC2 = 19.41 %) revealed a clear separation, with plants associated with phosphorus and nutrients ratios and soils grouped with C/N and C/P. GLM confirmed the significant effects of both sample and histosol types, with an interaction observed for most indices (except Ca/Mg). Both classical ratios (C/N, C/P and N/P) and ratios (Ca/Al, Ca/Mg and Ca·K·Mg/Al) provide complementary insights into peatland functioning. The decline of C/N and C/P ratios with increasing humification, combined with higher nutrient ratios, indicates their potential for diagnosing peatland degradation and monitoring nutrient cycling dynamics.