Accelerated soil development due to seasonal water-saturation under hydric conditions

Abstract

Pedogenesis is considered a long-term environmental process; however, it can be accelerated by periodic water saturation (hydric conditions). The exact nature and effectiveness of hydric conditions in the intensification of soil development are not clear yet. Our understanding of the timescales of the mineralogical changes occurring in soils is limited; however, the timeframe of soil organic matter accumulation is broadly known. Here, we described soil development over a period of 50 years.The studied soils Calcaric Mollic Gleysol (Endoarenic, Epiloamic, Hyperhumic) and a Calcaric Calcic Histic Gleysol (Endoarenic, Epiloamic) are located in a swampy area in Hungary, Central Europe. The start of the soil formation process is well documented here; the parent material was deposited during a major flood event in 1963. Therefore, the examined soil profile represents development over the last 50 years. We also studied the parent material of an adjacent dune as a reference. We used a CN elemental analyser to determine soil organic carbon and total bound nitrogen content. Selective extractions were also used to determine amorphous and crystalline Fe and Mn content alongside X-ray phase analysis (XRD) and transmission electron microscopy (TEM) for mineralogical analysis, X-ray fluorescence spectroscopy (XRF) for elemental analysis, and laser diffraction for particle size analysis of the bulk soil samples. Simple chemical tests were also performed in the field. Redox potential (Eh) and pH were measured by a field monitoring station.Fifty years under hydric conditions resulted in rapid vertical differentiation within the soil profile including remarkable soil organic matter enrichment in the topsoil, and the formation of smectite and Fe accumulation in the zone of groundwater fluctuation. A high proportion of amorphous and colloidal phases indicated that very intense processes had taken place in the zone of the most intensive redox oscillation. The presence of more crystalline goethite in the bulk soil reflects frequent Eh changes, associated with chemical or microbial processes, while the presence of amorphous ferrihydrite indicates the effect of plant roots. Permanently reductive subsoil conditions also facilitated intense carbonate precipitation. These findings demonstrate that soil formation can proceed faster than previously assumed under hydric conditions.