Abstract
Retardation of nitrate transport due to adsorption and its effects on nitrate leaching in the fields were investigated in different Andosols in Hokkaido. Seventy-five agricultural soils differing in the soil horizon and source ejecta were repacked into columns, and one-dimensional unsaturated flow experiments were conducted by supplying 2.3-11.5mM Ca(NO_3)_2. Retardation of nitrate transport was determined from the water and nitrate content profiles in the columns, and correlated with routinely-determined soil properties. The velocity of nitrate relative to water, averaged for soils grouped with respect to the soil horizon and source ejecta, ranged from 0.58 to 1.00, with the general order of B, C≤buried A<Ap horizons. A highly negative correlation was observed between the relative velocity of nitrate and the acid oxalate-extractable Si content (Sio). Except for the Ap horizons, the relative velocity was negatively correlated (significant at 1% level) with the adsorbed SO_4^<2-> content, which could stand proxy for the anion exchange capacity of the soils. Nitrate velocity in the B, C horizons was also affected by the type and age of source ejectas, with the retardation increasing with age for the source ejectas younger than 2000-3000y. Truog-P and total carbon contents were found to increase the relative velocity of nitrate in the Ap and buried A horizons, respectively. Based on these results, multiple regression equations were developed for predicting the relative velocity of nitrate from Sio, clay and Truog-P contents for the Ap horizons, from Sio, total carbon and adsorbed SO_4^<2-> contents for the buried A horizons, and from Sio and adsorbed SO_4^<2-> contents for the B, C horizons. Nitrate leaching observed in two Andosol fields was in accord with the laboratory assessments, and demonstrated that nitrate adsorption can effectively retard the downward transport of nitrate in the Andosol fields.
