Phosphorus Fractionation in Managed and Natural Wetlands in Northern California.

Phosphorus (P) cycling and availability in managed and natural wetlands in the Sacramento Valley have both agronomic and environmental implications.  Quantifying progressively extracted P fractions provides insight into how agricultural management has altered P cycling in this region, how much P may be available for rice uptake during the growing season, and the potential for P losses to surface waterways.  The objective of this study was to compare contents of P fractions in soils among conventional rice systems, organic rice fields, and natural wetlands in the Sacramento Valley.  Ninety-three soils were collected over three years (2005, 2006, and 2007).  The P from each soil was fractionated into exchangeable P (sodium bicarbonate extraction), iron-bound P (sodium hydroxide extraction), calcium-bound P (dilute hydrochloric acid extraction) and residual P (sulfuric acid digestion).  Results indicate that geographic location within the Sacramento Valley (and subsequent soil properties) had a large effect on P fraction contents.  Soils in or near the Sacramento River flood bypass (southern valley) had the highest total P contents.  Soils in the western valley had much greater calcium-bound P compared to soils from the eastern valley.  There did not appear to be major differences between managed wetland soils and agricultural soils with respect to total P content.  Overall, these results provide a clear indication that P fertilizer recommendations need to be developed separately for each geographic area within the Sacramento Valley.