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

Abstract

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.