Stability and composition of different soluble soil organic matter fractions – evidence from δ13C and FTIR signatures

Carbon turnover models are used to estimate changes in soil organic matter (SOM) content caused by management or land use. A limitation of most of these models is that their conceptual C-pools generally do not correspond to experimentally verifiable organic matter (OM) fractions. Our intention was to isolate OM fractions different in solubility and to identify their composition and their stability. The latter was done to find out if they could be used to evaluate the conceptual C-pools. NPK fertilized soils with maize and wheat cropping from two long-term field experiments (Halle and Rotthalmünster) were selected. Water (W) and sodium-pyrophosphate soluble (PYP) OM fractions were extracted sequentially from the soils using water in the first and 0.1 M sodium-pyrophosphate solution (pH=10) in the second step. The amounts of W and PYP fractions isolated from Rotthalmünster soils are somewhat lower than those obtained from Halle soils caused by higher Fe and Mn content in Rotthalmünster soil, which hinder the OM extraction. However, based on the natural abundance trace technique, for both sites, the PYP fraction was found to be older than the W fraction and hence to be more stable than W. Therefore, it could be assumed that sequential extraction seems to be useful to isolate OM fraction different in stability. PYp represents up to 40% of total soil organic matter; therefore, it may be a useful tool to evaluate a stable C-pool in C turnover models. According to FTIR spectra of the isolated OM fractions (especially PYp), their composition was influenced by growing crop and site conditions. This is of importance as composition of OM is related to its sorption properties, which may be relevant for transport processes.