Driving Factors of Carbon Distribution in Soils as Determined by z*

Abstract:

Terrestrial soils store approximately three times the amount of carbon(C) stored in the atmosphere. Understanding the mechanisms resulting in soil organic carbon stabilization is necessary for predicting the fate of this carbon and potential feedbacks to climate change. Here, we explore how soil carbon depth gradients are influenced by factors such as age, parent material, mean annual temperature (MAT) and a modeled moisture/leaching index (LI). Specifically, we calculate a quantitative metric reflecting the depth gradient of organic carbon, z*, described by Rosenbloom et al. 2006, which describes the depth attenuation of C turnover by fitting the %C by depth relationship with an exponential decay function. We compare z* across several soil chronosequences (n=33) spanning a broad array of ecosystems and climates.

The compiled chronosequence data were collected from previously published studies and ongoing USGS work. Each sequence consisted of at least two soils spanning a developmental age gradient and each soil included depth resolved carbon concentrations, with at least four different depths sampled in vertical profile. When the soil profile data were integrated across all depths, we found only weak relationships between total C in these profiles with various soil forming factors. Comparing depth resolved concentrations provided an opportunity to determine the significance of depth resolved gradients and particularly the importance of deep soil C.

We found a significant positive relationship between z* and age, (R²=0.57, p=4.0*10^-13), a significant negative relationship between z* and LI (R²=0.02, p=0.02), a significant relationship between (age x LI) and z* (R²=0.02, p=0.02, a positive relationship between z* and MAT (R²=0.1 p=0.01), and no relationship between z* and general parent material type (felsic, mafic, calcareous ; R²=0.04, p=0.01). These findings aid in determining the drivers of soil carbon storage across sites.