Characterization of phosphorus availability in selected New Zealand grassland soils

Abstract

Appropriate evaluation of phosphorus (P) availability in soil is aprerequisite for ensuring the productivity and long-term sustainable managementof agroecosystems. Fifteen soils presently under grassland were collected fromdifferent areas of New Zealand and soil P availability was assessed by isotopicexchange kinetics (IEK) and related to P forms obtained by chemicalfractionation (sequential extraction). Concentrations of total P determined inthe 15 soils ranged from 375 to 2607 mg kg-1(mean1104 mg kg-1). Mean concentrations of inorganic P(Pi) extracted by sequential extraction with ammonium chloride, sodiumbicarbonate, 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 sodiumbicarbonate, 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/r1 and n), and quantity (E value,isotopically exchangeable P pools (E1 min,E1 min-24 h,E24 h-3 m,E>3 m)) factors varied markedlyamongst soils. Thus Cp concentrations ranged from 0.02-1.90 mgL-1, while concentrations of Pi determined in theE1 min, E1 min-24,E24 h-3 m,E>3 m pools were 2-29 (mean 10), 10-321(76), 11-745 (152), and 8-498 (177) mgkg-1, respectively. The corresponding values forR/r1 and n were 1.0-17.7 (mean 4.5) and0.10-0.50 (mean 0.37), respectively. Regression analysis revealed that Cpconcentrations were exponentially and inversely proportional toR/r1,n and P sorption index (PSI)(R2=0.806(P<0.01), 0.852 (P<0.01) and 0.660(P<0.01), respectively). Cluster analysis identified twobroad groups of soils, namely those with low P availability (mean Cp0.11 mg L-1, E1 min Pi 5mg kg-1, R/r1 3.9,n 0.44), and those with high P availability (mean Cp 1.33mg L-1, E1 min Pi 20mg kg-1, R/r1 1.21,n 0.16). Correlation analysis indicated thatE1 min P i was significantly correlated with bicarbonateextractable Pi (BPi, R2=0.37,P<0.05) and thesum of ammonium chloride extractable Pi (APi) and BPi(R2=0.38,P<0.05). However, the concentration of Pi in theE1 min pool was generally lower than the sum of APi andBPi. Sodium hydroxide extractable Pi (N1Pi) was significantlycorrelated with the sum of the E1 min,E1 min-24 h,E24 h-3 m Pi pools(R2=0.974, P<0.01),indicating that N1Pi fractioncould be considered as representing potentially available soil P for pasturespecies over a growing season.