Seasonal Phosphorus Dynamics in a Volcanic Soil of Northern California

Soil Fertility & Plant Nutrition Seasonal Phosphorus Dynamics in a Volcanic Soil of Northern California Stewart G. Wilson* Land, Air, and Water Resources Dep. Univ. of California–Davis 1 Shields Ave. Davis, CA 95616 Jean-Jacques Lambert Dep. of Viticulture and Enology Univ. of California–Davis 1 Shields Ave. Davis CA 95616 Randy A. Dahlgren Land, Air, and Water Resources Dep. Univ. of California–Davis 1 Shields Ave. Davis, CA 95616 Phosphorus dynamics and management strategies were investigated in a strong- ly weathered rhyolitic soil in the northern Coast Ranges of California. Recent conversion of upland, native oak (Quercus spp.) woodland ecosystems to wine- grape production has led to a P deficiency in these soils. We investigated the growing season behavior of Hedley phosphorus fractions, the P sorption index (PSI), microbial biomass C (MBC), and dissolved organic C (DOC) in response to a single P application from three sources: composted steer manure (CSM), triple superphosphate (TSP), and mixed compost and triple superphosphate (MIX). Phosphorous sources, equal in P application rate (39 kg ha −1 ), were applied in the winter of 2012, and soils were sampled in spring, summer, and fall. Labile inorganic P (Pi) (resin Pi + NaHCO 3 Pi), Fe/Al-Pi (NaOH Pi), and total P were significantly (p < 0.01) increased by P application, regardless of source. Calcium-Pi (HCl Pi), DOC, MBC, and pH were increased, and PSI was reduced, by compost addition. Net extractable Fe/Al-Pi increased during the study period. Moderately labile organic P (Po) (NaOH Po) declined, indepen- dent of fertilization, reflecting background soil P cycling. Calcium-Pi totals were low (mean 9 mg kg −1 for untreated plots), and other indicators of chemi- cal weathering were high, suggesting an advanced stage of pedogenesis with respect to Walker–Syers P availability. In spite of advanced pedogenesis, labile Pi increased significantly from fertilization and remained elevated through- out the growing season. Results of this study inform P management in highly weathered, P-fixing soils experiencing P deficiencies. Abbreviations: CSM, composted steer manure; DOC, dissolved organic C; Fe d , pedogenic Fe oxides; Fe t , total Fe; G–G, Greenhouse–Geisser; MBC, microbial biomass C; MIX, mixed compost and triple superphosphate; Pi, inorganic P; Po, organic P; PSI, P sorption index; SOC, soil organic C; TSP, triple superphosphate. P Core Ideas • Phosphorous fertility was evaluated in weathered volcanic soil in winegrape production. • Seasonal Hedley P fractionations, P sorption, microbial biomass carbon, and DOC were investigated. • Application of compost, alone or with TSP, reduced sorption and increased MBC. • Labile Pi responded significantly to fertilization despite advanced pedogenesis. • Significant labile Pi response in weathered soil may be due to low- Fe lithology. Soil Science Society of America Journal hosphorus availability in soils is regulated by interactions between plant, microbial, and geochemical pools (Tiessen et al., 1984; Frossard et al., 2000, 2011; Oberson and Joner, 2005). As soils undergo pedogenesis, pri- mary minerals deplete (including apatite) and Fe and Al (hydr)oxides increase, resulting in low P availability in highly weathered soils (Cross and Schlesinger, 1995; Frossard et al., 2000). Similarly, P is a common limiting nutrient in many volcanic soils due to retention by active Fe and Al compounds (Dahlgren et al., 2004; Simpson et al., 2011). In these P-fixing soils, P application requirements can be as much as five times plant demand to overcome the strong sorption properties (Simpson et al., 2011). Phosphorus deficiency in arable soils is overcome princi- pally by the application of organic or inorganic fertilizers. Given the limited fu- ture availability of P fertilizer from global rock phosphate deposits (Cordell et al., 2009; Van Vuuren et al., 2010), understanding P cycling in diverse soils and strate- gies to improve P availability in P-fixing soils are critical research needs. This study investigates growing season P dynamics and P management strategies in a strongly weathered volcanic soil in the Coast Ranges of northern Soil Sci. Soc. Am. J. doi:10.2136/sssaj2016.02.0028 Received 3 Feb. 2016. Accepted 23 May 2016 *Corresponding author (stuwilson@ucdavis.edu). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All Rights reserved.