Ammonium transformation in paddy soils affected by the presence of nitrate

Coupled nitrification and denitrification is considered as one of the main pathways of nitrogen losses in paddy soils. The effect of NO₃^– on NH₄⁺ transformation was investigated by using the ¹⁵N technique. The paddy soils were collected from Wuxi (soil pH 5.84) and Yingtan (soil pH 5.02), China. The soils were added with either urea (18.57 mol urea-N enriched with 60 atom% ¹⁵N excess) plus 2.14 mol KNO₃-N (natural abundance) per gram soil (U+NO₃) or urea alone (U). The KNO₃ was added 6 days after urea addition. The incubation was carried out under flooded condition in either air or N₂ gas headspace at 25°C. The results showed that in air headspace, ¹⁵NH₄⁺ oxidization was so fast that about 10% and 8% of added ¹⁵N in the treatment U could be oxidized during the incubation period of 73 hours after KNO₃ addition in Wuxi and Yingtan soil, respectively. The addition of KNO₃ significantly inhibited ¹⁵NH₄⁺ oxidation (p<0.01) in air headspace, while it stimulated ¹⁵NH₄⁺ oxidation in N₂ gas headspace, although the oxidation was depressed by the N₂ gas headspace itself. Therefore, the accumulation of NO₃^– would inhibit further nitrification of NH₄⁺ at micro-aerobic sites in paddy soils, especially in paddy soils with a low denitrification rate. On the other hand, NO₃^– would lead to oxidation of NH₄⁺in anaerobic bulk soils.