Long-term influences of biochar, manure, or sawdust additions on soil water retention in silt loam

Additions of organic materials to eroded calcareous soils have been suggested as a means of increasing soil carbon storage and a way to increase soil water retention.  Due to its more recalcitrant nature, the organic C in hardwood-derived biochar (73% C, 15% ash) may produce different and more long-lasting effects on soil water retention than more degradable organic-C sources.  An outdoor, 6-y pot study compared the effect of a one-time application of four amendments (biochar, dairy manure, sawdust, or acidified sawdust, each applied at three rates: 0, 1, and 2% by mass) to a calcareous, silt loam soil.   Pot soils were tilled manually to 0.15-m depth each spring, seeded with crops typical for the region and placed outdoors, where pots were irrigated equally using flow emitters to meet evapotranspiration demand.  The soil water release curve was determined on soil from spring samples using repacked soil cores saturated with water and sequentially equilibrated at nine pressure heads using a pressure plate. Plant available water (PAW) was influenced by an interaction of treatment and year.  The pattern of annual PAW values for manure was more stable than that for biochar or sawdust treatments; however biochar produced a greater maximum PAW during the 6-y period compared to manure and control treatments. This report will include a discussion of treatment effects on the water retention at each pressure head tested.