Basal organic phosphorus mineralization in soils under different farming systems

Soil organic P (P-o) mineralization plays an important role in soil P cycling. Quantitative information on the release of available inorganic P (P-i) by this process is difficult to obtain because any mineralized P-i gets rapidly sorbed. We applied a new approach to quantify basal soil P-o mineralization, based on (PO4)-P-33 isotopic dilution during 10 days of incubation, in soils differing in microbiological activity. The soils originated from a 20 years old field experiment, including a conventional system receiving exclusively mineral fertilizers (MIN), a bio-organic (ORG) and bio-dynamic (DYN) system. Indicators of soil microbiological activity, such as size and activity of the soil microbial biomass and phosphatase activity, were highest in DYN and lowest in MIN. In order to assess P-o hydrolysis driven by phosphatase in sterile soils, a set of soil samples was gamma-irradiated. Basal P-o mineralization rates in non-irradiated samples were between 1.4 and 2.5 mg P kg(-1) day(-1) and decreased in the order DYN > ORG greater than or equal to MIN. This is an amount lower, approximately equivalent to, or higher than water soluble P-i of MIN, ORG and DYN soils, respectively, but in every soil was less than 10% of the amount of P isotopically exchangeable during one day. This shows that physico-chemical processes are more important than basal mineralization in releasing plant available P-i. Organic P mineralization rates were higher, and differences between soils were more pronounced in gamma-irradiated than in non-irradiated soils, with mineralization rates ranging from 2.2 to 4.6 mg P kg(-1) day(-1). These rates of hydrolysis, however, cannot be compared to those in non-sterile soils as they are affected by the release of cellular compounds, e.g. easily mineralizable P-o, derived from microbial cells killed by gamma-irradiation. (C) 2004 Elsevier Ltd. All rights reserved.