Functional characterization of soil organic matter fractions different in solubility originating from a long-term field experiment

Abstract

The functional composition of soil organic matter (SOM) is influenced by land use and cropping practices. We studied the ability to separate organic matter (OM) fractions different in stability by sequential extraction with water and sodium pyrophosphate solution. Additionally we investigated effects of crop rotation and fertilization on the content and composition of the isolated OM fractions. The control plot and three combinations of manurial treatments from a long-term field experiment were selected: mineral fertilization with P and K, mineral fertilizer with N, P and K, and cattle manure, only. SOM fractions were extracted sequentially from the soil samples using water in the first and 0.1 M sodium–pyrophosphate-solution (pH=10) in the second step. The obtained fractions were analyzed using Fourier Transform infrared (FTIR) spectroscopy. To get information on age and stability of the obtained OM fractions 14C-data were determined using accelerated mass spectrometry and the content of maize derived carbon was determined from δ 13C-analysis. Based on 14C-data and the content of maize derived carbon the pyrophosphate-soluble OM fraction [OM(PY)] was found to be older than the pyrophosphate and HCL-soluble OM [OM(PY*)] and the water-soluble fraction [OM(W)]. It can be assumed therefore that the OM(PY) fraction is more stable than OM(PY*) and OM(W). Additionally, the OM(PY) fraction represents a higher proportion of total SOM and contains more carboxylic groups than the water-soluble fraction. Furthermore, this fraction shows a greater variation in the C=O content depending on the kind of fertilizer treatment than OM(W) fraction.