Modeling the vertical soil organic matter profile using ²¹⁰Pb_ex measurements and Bayesian inversion

Abstract. In view of its potential significance for soil organic matter (SOM) cycling, the vertical SOM distribution in the profile should be considered in models. To mechanistically predict the SOM profile, three additional processes should be represented compared to bulk SOM models: (vertically distributed) rhizodeposition, mixing due to bioturbation, and movement with the liquid phase as dissolved organic matter. However, the convolution of these processes complicates parameter estimation based on the vertical SOM distribution alone. Measurements of the atmospherically produced isotope ²¹⁰Pb_ex may provide the additional information needed to constrain the processes. Since ²¹⁰Pb_ex enters the soil at the surface and bind strongly to organic matter it is an effective tracer for SOM transport. In order to study the importance of root input, bioturbation, and liquid phase transport for SOM profile formation we performed Bayesian parameter estimation of the previously developed mechanistic SOM profile model SOMPROF. 13 parameters, related to decomposition and transport of organic matter, were estimated for the soils of two temperate forests with strongly contrasting SOM profiles: Loobos (the Netherlands) and Hainich (Germany). Measurements of organic carbon stocks and concentrations, decomposition rates, and ²¹⁰Pb_ex profiles were used in the optimization. For both sites, 3 optimizations were performed in which stepwise ²¹⁰Pb_ex data and prior knowledge were added. The optimizations yielded posterior distributions with several cases (modes) which were characterized by the dominant organic matter (OM) pool: non-leachable slow OM, leachable slow OM, or root litter. For Loobos, the addition of ²¹⁰Pb_ex data to the optimization clearly indicated which case was most likely. For Hainich, there is more uncertainty, but the most likely case produced by the optimization agrees well with other measurements. For both sites the most likely case of the final optimization was one where leachable slow OM dominates, suggesting that most organic matter is adsorbed to the mineral phase. Liquid phase transport (advection) of OM was responsible for virtually all organic matter transport for Loobos, while for Hainich bioturbation (diffusion) and liquid phase transport were of comparable magnitude. These results are in good agreement with the differences between the two sites in terms of soil texture and biological activity.