Sources and transformations of N in reclaimed coastal tidelands: Evidence from soil δ¹⁵N data

Electrical conductivity of saturated soil extracts (EC_e) in three reclaimed tideland (RTL) soils on the west coast of Korea decreased with time since reclamation, indicating natural desalinization through leaching of salts by precipitation water. Soil N concentration increased with decreasing EC_e. With the increase in soil N concentration, the δ¹⁵N decreased, likely caused by the input of ¹⁵N-depleted N sources. As N₂-fixing plant species were found in the oldest RTL, atmospheric N₂ fixation likely contributed to the increase in soil N concentration in the oldest RTL. Negative δ¹⁵N (-7.1 to -2.0‰) of total inorganic N (NH ₄ ⁺+NO₃ ^-) and published data on N deposition near the study area indicate that atmospheric N deposition might be another source of N in the RTLs. Meanwhile, the consistently negative δ¹⁵N of soil NO₃ ^- excluded N input from chemical fertilizer through groundwater flow as a potential N source, since NO₃ ^- in groundwater generally have a positive δ¹⁵N. The patterns of δ¹⁵N of NH₄ ⁺ (+2.3 to +5.1‰) and NO₃ ^- (-9.2 to -5.0‰) suggested that nitrification was an active process that caused ¹⁵N enrichment in NH₄ ⁺ but denitrification was probably minimal which would otherwise have caused ¹⁵N enrichment in NO₃ ^-. A quantitative approach on N budget would provide a better understanding of soil N dynamics in the studied RTLs.