End water content determines the magnitude of N2O pulse from nitrifier denitrification after rewetting a fluvo-aquic soil

Large nitrous oxide (N$_{2}$O) emissions pulses have been observed after rewetting dry soil. However, few studies have uncoupled the effects of drought severity from the degree to which the soil is saturated. In this study, we conducted three aerobic incubation experiments to investigate the effects of soil rewetting on N$_{2}$O emissions from a dryland soil. The results showed that, at constant soil moisture, total N$_{2}$O emissions in soil with 90% water-holding capacity (WHC) were significantly higher than those in 30%, 45%, 60% and 75% WHC treatments. In the dry–wet group, the soil moisture content was adjusted from 30%, 45% and 60% WHC to the end content of 75% and 90% WHC, respectively; the cumulative N$_{2}$O emissions in the 30–90%, 45–90% and 60–90% WHC nitrogen (N) treatments were significantly higher than those in the 30–75%, 45–75% and 60–75% WHC N treatments. Regarding fertilizer N types, there was no significant difference in N$_{2}$O emissions from soil at 90% WHC when (NH$_{4}$)$_{2}$SO$_{4}$ or urea was applied. Nitrification inhibitor significantly reduced N$_{2}$O emissions in soil applied with NH$_{4}$$^{+}$-N fertilizer, indicating that nitrification played a major role in N$_{2}$O emissions from soils. The contribution of denitrification was negligible, according to the low emission rate of soils with only NO$_{3}$$^{-}$ additions. High N$_{2}$O emissions occurred in soil treated with NO$_{2}$$^{-}$, accounting for about 83.6% of those of the NH$_{4}$$^{+}$ treatment. Therefore, in this study we concluded that the end water content of soil was more important than the role of drought severity in the dry-wet process and that nitrifier denitrification was probably the main pathway of N$_{2}$O production under the condition of 90% WHC moisture after rewetting soil.