Transformation of organic phosphorus compounds during 1500 years of organic soil formation in Bavarian Alpine forests – A 31P NMR study

Abstract

In Bavarian alpine forests, long-term input of organic matter from conifer species has built up thick organic surface layers, thus immobilized substantial soil organic phosphorus upon calcareous parent materials. The composition of these phosphorus stocks, however, remained unrevealed. Therefore, we analyzed organic phosphorus species in three so called Tangelhumus (Histosol according to World Reference Base) profiles with different radiocarbon ages using alkaline extraction followed by one-dimension and two-dimension 31P nuclear magnetic spectroscopy in order to understand changes in organic phosphorus composition with soil organic matter genesis. A diverse phosphorus pool composed of monoesters, diesters, and phosphonates was detected. However, in these Tangelhumus soils only monoesters accumulated overtime and eventually dominated the organic phosphorus pool after 1558 years. Diesters and phosphonates did not show any significant accumulation pattern. The origin of monoesters varied with soil depth and soil age: monoesters in young surface soils were actually hydrolysis products of diesters released by plant and microbial cells during alkaline extraction; whereas in older soils monoesters originated largely from Ca-bound precipitates formed after long-term in situ decomposition of organic matter and stabilization by Ca2+. Myo-inositol hexakisphosphate was below detection limit in all soils. The presence of its scyllo and other isomers likely pointed to direct synthesis by microorganisms instead of microbial epimerization from myo-inositol hexakisphosphate. In general, the overall picture of phosphorus speciation manifested as the continuous degradation of organic matter and accumulation of monoesters. Therefore, the current data indicated no involvement of humification process but favored the soil continuum model.