Factors affecting soil P dynamics in temperate volcanic soils of southern Argentina

During volcanic soil development, factors controlling plant-available P shift from hydrolysis of P bound to primary Ca compounds to mineralization of organic P, and an increasing role of amorphous materials in P retention. Monospecific forests of the Andean–Patagonian region are distributed in a narrow strip characterized by young volcanic soils and decreasing west–east precipitation. We analyzed sequential P fractionation, P retention, active Al and Fe, and acid phosphatase activity in the superficial soil (0–15 cm) of ten dominant woody species, and related these data to previous results on soil chemical and biological fertility, and green leaf P concentration. Higher plant-labile P (water–Pi + NaHCO3–Pi) corresponded to soils with higher total P, and this was directly related to the increase of organic P (NaHCO3–Po + NaOH–Po). Phosphatase activity was mainly explained by the increase of organic C and the decrease of primary minerals (P–HCl). The strongest determinant of P retention was the formation of active Al, which increased with acidification and losses of cations and primary minerals. In the drier extreme of the precipitation gradient, we found the lowest values of active Al and P retention coupled with high exchangeable cations and neutral pH. The effect of plant species increased in wetter areas, and was reflected in differential patterns of SOM accumulation, base cation cycling and active Al formation. When comparing plant functional groups, potential net N mineralization and nitrification, phosphatase activity, organic C, organic P (NaHCO3–Po) and green leaf P were significantly higher in broad-leaf deciduous species than in conifers.