Abstract
Abstract. The vertical distribution of soil organic matter (SOM) in the profile may constitute an important factor for soil carbon cycling. However, the formation of the SOM profile is currently poorly understood due to equifinality, caused by the entanglement of several processes: input from roots, mixing due to bioturbation, and organic matter leaching. In this study we quantified the contribution of these three processes using Bayesian parameter estimation for the mechanistic SOM profile model SOMPROF. Based on organic carbon measurements, 13 parameters related to decomposition and transport of organic matter were estimated for two temperate forest soils: an Arenosol with a mor humus form (Loobos, the Netherlands), and a Cambisol with mull-type humus (Hainich, Germany). Furthermore, the use of the radioisotope 210Pbex as tracer for vertical SOM transport was studied. For Loobos, the calibration results demonstrate the importance of organic matter transport with the liquid phase for shaping the vertical SOM profile, while the effects of bioturbation are generally negligible. These results are in good agreement with expectations given in situ conditions. For Hainich, the calibration offered three distinct explanations for the observations (three modes in the posterior distribution). With the addition of 210Pbex data and prior knowledge, as well as additional information about in situ conditions, we were able to identify the most likely explanation, which indicated that root litter input is a dominant process for the SOM profile. For both sites the organic matter appears to comprise mainly adsorbed but potentially leachable material, pointing to the importance of organo-mineral interactions. Furthermore, organic matter in the mineral soil appears to be mainly derived from root litter, supporting previous studies that highlighted the importance of root input for soil carbon sequestration. The 210Pbex measurements added only slight additional constraint on the estimated parameters. However, with sufficient replicate measurements and possibly in combination with other tracers, this isotope may still hold value as tracer for SOM transport.
Highlights
The current lack of understanding of the soil system forms an important contribution to the uncertainty of terrestrial carbon cycle predictions (Heimann and Reichstein, 2008; Trumbore, 2009)
In order to study the processes involved in soil organic matter (SOM) profile formation, we performed Bayesian estimation of SOMPROF model parameters for Loobos and Hainich, based on organic carbon and 210Pbex measurements as well as prior knowledge
The final calibration yielded a multi-modal posterior distribution for Hainich, with two dominant modes corresponding to two distinct explanations for the observations
Summary
The current lack of understanding of the soil system forms an important contribution to the uncertainty of terrestrial carbon cycle predictions (Heimann and Reichstein, 2008; Trumbore, 2009). To improve simulation of soil carbon cycling, it is necessary to move beyond the simple description of organic matter decomposition that is currently being applied in most large-scale models (Reichstein and Beer, 2008). Increasing evidence indicates that decomposition and stabilization are controlled by a range of mechanisms that depend on physical, chemical, and biological factors (von Lutzow et al, 2006). These factors vary laterally at landscape scale in relation to climate, vegetation and soil type. The conditions that determine soil carbon cycling are highly depth-dependent and different
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