Soil alkaline phosphatase and phosphodiesterase activities in relation to phosphorus content in a greenhouse organic tomato crop in Almería (SE Spain)

Abstract

The aim of this work is to assess, in a greenhouse organic tomato crop, the effects of two soil organic amendments (T1:11.0 kg dry manure m-2, and T2: 4.5 kg dry manure m-2 + 6.5 kg dry vermicompost m-2) on levels of alkaline phosphatase and phosphodiesterase activities, in relation to available (Pa), soluble (Ps) and inorganic phosphorus content (Pi). Four plots were compared: P1 and P2 had loamy sandy soil (79% illite clay) with saturated hydraulic conductivity (Kfs) of 17.07 mm h-1; P3 and P4 had loamy soil (45 % smectite clay), and Kfs of 3.13 mm h-1. Gravel mulch (5-10 mm) was added to plots P1 and P4, and sand mulch (0.05-2 mm) to P2 and P3. Three replicates per plot of each organic treatment were sampled on a weekly basis over seventeen weeks. There were no statistical differences among treatments for inorganic phosphorus content and phosphodiesterase activity. Available and soluble phosphorus were greater in T2 (901.6 ± 343.27 mg Pa kg-1; 6.93 ± 4.09 mg Ps kg-1) than in T1 (312.2 ± 137.69 mg Pa kg-1; 2.72 ± 1.66 mg Ps kg-1), while alkaline phosphatase activity was greater in T1 (856.0 ± 406.4 mg PNP kg-1 h-1) than in T2 (684.9 ± 324.5 mg PNP kg-1 h-1). Mean values for all analyzed variables were significantly higher (p<0.01) in plots with gravel mulch (P1 and P4). There were positive and significant correlations (P< 0.01) between enzyme activities and forms of phosphorus, except for alkaline phosphatase and available phosphorus content, where no correlation was detected. At plot scale, therefore, phosphorus availability did not inhibit enzyme activity. All values decrease with time in all plots, except in P4 which is the poorest drained.