Deep-soil adsorption of nitrate in a Japanese Andisol in response to different nitrogen sources

Nitrate adsorption in a deep Andisol with a high anion exchange capacity (AEC) needs to be studied for groundwater management. We evaluated N balances and NO3- content profiles under cropping in Andisol fields after 10 yr of repeated N additions of different N sources-swine compost (SC, N application rate of 800 kg ha(-1) yr(-1)), coated urea (CU, 400 kg ha(-1) yr(-1)), or NH4-N (AN, 400 kg ha(-1) y(r-1))-and an unfertilized control (NF). The N losses from the 0- to 20-cm layer of three N-treated soils during the experiment were 2300 to 2700 kg N ha(-1), which were not significantly different (P > 0.05). Nitrate-N retention in deep-soil profiles up to 450 cm was 1800 to 2300 kg N.ha(-1) irrespective of N source (P > 0.05), and was 788 kg ha(-1) in the NF treatment. On the other hand, the type of N source affected soil NO3- content profiles: NO3- from the two types of slow-release N additions (SC and CU) remained within a depth of 60 cm, whereas NO3- in the 180- to 280-cm layer was significantly higher in the AN treatment (P < 0.05). Stepwise multiple regression analysis showed that the primary contributor to soil NO3- content was Al content derived from allophane and imogolite, followed by soil pH and SO42- with negative impacts. In Andisols of Ibaraki, Japan, NO3- leached from the root zones is retained by adsorption in deep layers, which may reduce the risk of groundwater contamination at least for 10 yr.