Deep‐Soil Adsorption of Nitrate in a Japanese Andisol in Response to Different Nitrogen Sources

Abstract

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 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 yr−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 content profiles: from the two types of slow‐release N additions (SC and CU) remained within a depth of 60 cm, whereas 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 content was Al content derived from allophane and imogolite, followed by soil pH and with negative impacts. In Andisols of Ibaraki, Japan, 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.