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SOIL & WATER MANAGEMENT & CONSERVATION

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

Integrated Soil Fertility Management Res. Team, National Agricultural Research Center, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-8666, Japan

^* Corresponding author (mun{at}affrc.go.jp).

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 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 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 SO₄^2– 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.

Abbreviations: AEC, anion exchange capacity • AN, ammonium-nitrogen • CEC, cation exchange capacity • CU, coated urea • NF, no fertilizer • SC, swine compost