Effects of nitrogen and water addition on N2O emissions in temperate grasslands, northern China

Temperate grasslands are considered an important natural source of the trace greenhouse gas N₂O, which has attracted much attention in recent years. Increased annual precipitation and N deposition have been observed in temperate grasslands in northern China owing to climate change and anthropogenic activities. Although the individual effects of increased precipitation and N deposition on N₂O emissions have been extensively reported, their interactive effects remain unclear. An in situ experiment with two levels of water supply (ambient and +15% precipitation levels) and four levels of N application (0, 25, 50, and 100 kg N ha⁻¹ yr⁻¹) was conducted in a semi-arid temperate grassland from 2016 to 2018 to quantify the effects of additional precipitation and N on N₂O emissions in temperate grasslands. The results showed that temperate grasslands are a small source of N₂O with negative N₂O fluxes during the growing seasons. Water addition amplified the variability in N₂O fluxes by lowering the mean N₂O fluxes and strengthening N₂O emission pulses triggered by precipitation. The trade-off between increased N₂O emission pulses and decreased mean fluxes resulted in decreased growing season cumulative N₂O emissions. The stimulated N₂O emissions by N addition were linearly related to N application rates. However, this linear correlation shifted to similar N₂O emissions among the N application rates by water addition. In addition, N₂O emissions were negatively correlated with soil moisture and microbial biomass carbon (MBC) but positively correlated with soil dissolved organic carbon (DOC), NH₄⁺, and NO₃⁻. Our results suggest that (1) water addition increases the sensitivity of N₂O emissions to N addition and limits the loss of additional N as N₂O. (2) In addition to directly increasing N₂O emissions, N addition increases N₂O emissions indirectly by increasing soil DOC, NH₄⁺, and NO₃⁻ levels, whereas water addition decreases N₂O emissions indirectly by increasing MBC and soil moisture. (3) The ability of soil properties to explain the variability in N₂O fluxes was limited, whereas the ability of soil properties to explain the interannual variability in cumulative N₂O emissions was highly reliable.