Nitrate and Ammonium Leaching in Variable- and Permanent-Charge Paddy Soils

A variable-charge (VC) and a permanent-charge paddy soil (PC) were selected to study nitrate-N (NO−3-N) and ammonium-N (NH+4-N) leaching with N isotopes for one consecutive year. An irrigation and intermittent drainage pattern was adopted to mimic natural occurrence of rainfall during the upland crop season and drainage management during the flooded rice season. Treatments to each soil type were no-N controls (CK), 15N-labeled (NH4)2SO4 (NS), and milk vetch (NV) applied at a rate equivalent to 238 kg N ha−1 to unplanted lysimeters, totaling six treatments in triplicates. Results indicated that the soil type dominated N leaching characteristics. In the case of PC, NO−3-N accounted for 78% of the total leached inorganic N; NS was prone to leach three times more than the NV, being 8.2% and 2.4% of added 15N respectively; and > 85% of leached NO−3-N came from native N in the soil. In the case of VC, NH+4-N made up to 92% of the total inorganic N in leachate. Moreover, NH+4-N particularly high during the flooded season. NO−3-N leaching in VC occurred later at a lower rate compared to that in PC. The findings of this study indicate that NO−3-N leaching during the drained season in permanent-charge paddy soils and NH+4-N leaching in variable-charge soils deserve more attention for regional environmental control.