Characterization of phosphorus availability in selected New Zealand grassland soils

Appropriate evaluation of phosphorus (P) availability in soil is a prerequisite for ensuring the productivity and long-term sustainable management of agroecosystems. Fifteen soils presently under grassland were collected from different areas of New Zealand and soil P availability was assessed by isotopic exchange kinetics (IEK) and related to P forms obtained by chemical fractionation (sequential extraction). Concentrations of total P determined in the 15 soils ranged from 375 to 2607 mg kg(-1) (mean 1104 mg kg(-1)). Mean concentrations of inorganic P (Pi) extracted by sequential extraction with ammonium chloride, sodium bicarbonate, sodium hydroxide (first), hydrochloric acid and sodium hydroxide (second) were 1.2, 41, 205, 113 and 23 mg kg(-1), respectively. Mean concentrations of organic P (Po) extracted by sodium bicarbonate, sodium hydroxide (first) and sodium hydroxide (second) were 133, 417 and 105 mg kg(-1), respectively. Similarly, results from IEK analysis showed that the intensity (water soluble Pi (Cp)), capacity (R/r(1) and n), and quantity (E value, isotopically exchangeable P pools (E-1 (min), E-1 (min-24 h), E-24 (h-3) (m), E (>) (3 m))) factors varied markedly amongst soils. Thus Cp concentrations ranged from 0.02-1.90 mg L-1, while concentrations of Pi determined in the E-1 (min), E-1 (mm-24), E-24 (h-3 m), E->3 (m) pools were 2-29 (mean 10), 10-321 (76), 11-745 (152), and 8-498 (177) mg kg(-1), respectively. The corresponding values for R/r(1) and n were 1.0-17.7 (mean 4.5) and 0.10-0.50 (mean 0.37), respectively. Regression analysis revealed that Cp concentrations were exponentially and inversely proportional to R/r(1) n and P sorption index (PSI) (R-2 = 0.806 (P < 0.01), 0.852 (P < 0.01) and 0.660 (P < 0.01), respectively). Cluster analysis identified two broad groups of soils, namely those with low P availability (mean Cp 0.11 mg L-1, E-1 (min) Pi 5 mg kg(-1), R/r(1) 3.9, n 0.44), and those with high P availability (mean Cp 1.33 mg L-1, E, min Pi 20 mg kg(-1), R/r(1) 1.21, n 0.16). Correlation analysis indicated that E-1 (min) Pi was significantly correlated with bicarbonate extractable Pi (BPi, R-2 = 0.37, P < 0.05) and the sum of ammonium chloride extractable Pi (APi) and BPi (R-2 = 0.38, P < 0.05). However, the concentration of Pi in the E-1 (min) pool was generally lower than the sum of APi and BPi. Sodium hydroxide extractable Pi (N(1)Pi) was significantly correlated with the sum of the E-1 (min), E1 min-24 (h), E-24 (h-3) (m) Pi pools (R-2 = 0.974, P < 0.01), indicating that N(1)Pi fraction could be considered as representing potentially available soil P for pasture species over a growing season.