Using the Spatial Pattern of Water Isotope Ratios to Understand Peatland Hydrology: Examples From Northeast China and a Model-Based Interpretive Framework

Hydrology is the fundamental control of peatland ecosystems. The spatial pattern of isotopic variability in peatland water provides insights into the site-scale heterogeneity of water sources, turnover and flow patterns. However, we still lack comprehensive documentation of spatial isotope datasets across different peatland types and climates, and no study has presented an interpretative framework for such spatial data or related them to quantitative models of isotopic fractionation. Here we investigate the spatial distribution of isotope ratios in water at the peatland surface within temperate peatlands of three different types (sloping poor fen, drainage-affected poor fen on a flat platform, steeply sloping bog with strong pool patterning) in Northeast China and develop a mechanistic model of isotopic fractionation for a generalised understanding of relevant mechanisms. Our data show an evaporation-induced increasing trend in peatland water δ 18 O towards the downslope direction in the sloping poor fen. This pattern can be reproduced by the model simulation but is not observed in the drainage-affected poor fen or in the patterned bog. The latter two sites show isotopic heterogeneities that directly indicate poor surface water connectivity due to topographic, hydraulic and hydrological factors, as supported by the model sensitivity analysis and constraints from river isotope data. Additionally, all three sites have notably narrow ranges of variability in peatland water δ 18 O compared to previously investigated European and North American peatlands. This may result from the high transpiration fraction of herbaceous vegetation and accelerated turnover of the surface water reservoir under a wet monsoon climate, which together prevent the development of strongly enriched isotopic signatures in regional peatlands. This study underscores the potential for using the spatial distribution of water isotope ratios as a simple and efficient method to distinguish surface hydrology among different hydrogenetic types of peatlands, with scope for making quantitative inferences by developing isotope-based hydrological models.