Seasonal Dynamics In Phosphorus Management In a Volcanic Soil In California.

Seasonal dynamics in phosphorus (P) management were investigated in volcanic soils formed in obsidian and dacitic ash and planted to wine grapes in northern California. The response of phosphorus fractions, P-sorption, microbial biomass carbon (MBC), dissolved organic carbon (DOC) and pH were examined throughout the growing season for three phosphorus sources: composted steer manure (CSM), triple super phosphate (TSP), and mixed compost and triple super phosphate (MIX). P-sources, equal in P application rate (39 kg/ha), were applied in January of 2012 in a randomized complete block design. Soils were sampled in spring, summer and fall, and sequentially fractionated using a modified Hedley procedure. P-sorption was assessed by single point sorption and MBC determined by chloroform fumigation direct extraction. Compost containing treatments significantly (p<0.05) increased resin inorganic P (resin-Pi) in spring and summer, and mixed treatments significantly reduced P-sorption overall. Compost application significantly increased DOC, MBC, pH, and weak acid-extractable Pi (1M HCL). Fertilizer application, regardless of source, significantly increased labile Pi (resin-Pi and NaHCO3-Pi), moderately labile Pi (NaOH-Pi), and total P. Occluded Pi (12M HCL) and moderately labile organic P (NaOH-Po) were unaffected by treatment. Resin-Pi and DOC significantly decreased throughout the growing season, moderately labile Po declined from spring to summer and MBC declined sharply in fall. Occluded Pi and moderately labile Pi were seasonally stable. There were no significant correlations between MBC and any P fraction, suggesting that phosphorus and carbon cycling are decoupled. Seasonally, occluded and moderately labile Pi fractions do not fluctuate, while labile Pi, moderately labile Po, DOC, pH and MBC are seasonally dynamic. Improved P availability through reduced sorption and increased labile Pi, coupled with increases to the size of the microbial biomass, indicate that mixed compost/TSP fertilization may be a preferred P management strategy in P-fixing volcanic soils in northern California.