Soil organic matter turnover: Global implications from δ¹³C and δ¹⁵N signatures

The turnover and residence time of carbon (C) and nitrogen (N) in soil is a fundamental parameter reflecting the rates of soil organic matter (SOM) transformation and the contribution of soils to greenhouse gases fluxes. Based on the global database of the stable isotope composition of C (δ¹³C) and N (δ¹⁵N) depending on soil depth (171 profiles), we assessed С and N turnover and related them to climate, biome types and soil properties. The ¹³C and ¹⁵N discrimination between the litter horizon and mineral soil was evaluated to explain the key processes of litter transformation. The ¹³C and ¹⁵N discrimination by microbial utilization of litter and SOM, as well as the continuous increase of δ¹³C and δ¹⁵N with depth, enabled to assess C and N turnover within SOM. N turnover was two times faster than that of C, which reflects i) repeated N recycling by microorganisms accelerating N turnover, ii) C loss as CO₂ and input of new C atoms to cycling, which reduces the C turnover within soil, and iii) generally slower turnover of N free persistent organic compounds (e.g. lignin, suberin, cellulose) compared to the N containing compounds (e.g. amino acids, ribonucleic acids). An increase in temperature and precipitation accelerated C and N turnover because: i) higher microbial activity and SOM decomposition rate, ii) larger soil moisture and fast diffusion of dissolved organics towards exoenzymes, iii) downward transport of ¹³C-enriched organic matter (e.g. sugars, amino acids), and iii) leaching of ¹⁵N-depleted nitrates from the topsoil into subsoil and losses from the whole soil profile. Temperature accelerates SOM turnover stronger than precipitation. The temperature increase by 10 °C accelerates the C and N turnover for 40 %. SOM turnover is boosted by decreasing C/N ratio because: i) SOM with a high C/N ratio originated from litter is converted to microbially-derived SOM in mineral soil characterized by a low C/N ratio; ii) litter with a low C/N ratio is decomposed faster than litter with a high C/N; iii) microbial carbon-use efficiency increases with N availability. The biome type affects SOM decomposition by i) climate: slower turnover under wet and cold conditions, and ii) by litter quality: faster utilization of leaves than needles. Thus, the fastest C turnover is common under evergreen forests and the lowest under mixed and coniferous ones, whereas temperature and C/N ratio are the main factors controlling SOM turnover. Concluding, the assessment of SOM turnover by δ¹³C and δ¹⁵N approach showed two times faster N turnover compared to C, and specifics of SOM turnover depending on the biomes as well as climate conditions.