Masters Thesis

Predicting phosphorus retention in two Haplohumult forest soils of northern California

Plant available phosphorus (P) occurs in anionic forms which become unavailable (retained) when iron and aluminum oxides form insoluble phosphate complexes. P-retention is especially likely to occur under acid conditions in soils containing short range order (SRO; poorly crystalline) materials, namely allophane and imogolite. This set of characteristics is common in volcanic-derived forest soils in Pacific Northwest timber regions. P-retention was investigated in Powellton (Fine-loamy, parasesquic, mesic Andic Haplohumults) and Aiken (Fine, parasesquic, mesic Xeric Haplohumults) soil series from Feather Falls and Whitmore “Garden of Eden” sites, respectively. Some soils received heavy fertilization over multiple years; another set of unfertilized soils were compared as a control. Results show that pHNaF (indicator of SRO materials), soil carbon, and New Zealand P-retention decreased with soil depth in both soils. In near-surface horizons, Feather Falls soil had higher pHNaF, carbon content, and P-retention than the Whitmore soil. Unexpected high P-retention at Feather Falls is probably due to the presence of P-fixing organo-metal complexes in Feather Falls surface horizons. TheWhitmore soil shows higher pHNaF and P-retention than Feather Falls in deep horizons. At Whitmore, the drop in P-retention with depth is attributed to decreasing SRO minerals deeper in the profile; at Feather Falls, a simultaneous drop in carbon and SRO minerals contributes to the pattern of P-retention with depth. Fertilized surface soils at Feather Falls showed ten percent less P-retention than unfertilized soils at that site. Fertilization did not affect P-retention at Whitmore. pHNaF was the single best predictor of P-retention, with other important factors including depth, soil carbon, and soil “redness” (a quantitative measure representing iron content) in a mixed-effects regression of P-retention for these soils.