Suppression of methane uptake by precipitation pulses and long-term nitrogen addition in a semi-arid meadow steppe in northeast China

In the context of global change, the frequency of precipitation pulses is expected to decrease while nitrogen (N) addition is expected to increase, which will have a crucial effect on soil C cycling processes as well as methane (CH(4)) fluxes. The interactive effects of precipitation pulses and N addition on ecosystem CH(4) fluxes, however, remain largely unknown in grassland. In this study, a series of precipitation pulses (0, 5, 10, 20, and 50 mm) and long-term N addition (0 and 10 g N m(-2) yr(-1), 10 years) was simulated to investigate their effects on CH(4) fluxes in a semi-arid grassland. The results showed that large precipitation pulses (10 mm, 20 mm, and 50 mm) had a negative pulsing effect on CH(4) fluxes and relatively decreased the peak CH(4) fluxes by 203-362% compared with 0 mm precipitation pulse. The large precipitation pulses significantly inhibited CH(4) absorption and decreased the cumulative CH(4) fluxes by 68-88%, but small precipitation pulses (5 mm) did not significantly alter it. For the first time, we found that precipitation pulse size increased cumulative CH(4) fluxes quadratically in both control and N addition treatments. The increased soil moisture caused by precipitation pulses inhibited CH(4) absorption by suppressing CH(4) uptake and promoting CH(4) release. Nitrogen addition significantly decreased the absorption of CH(4) by increasing NH(4) (+)-N content and NO(3) (–)-N content and increased the production of CH(4) by increasing aboveground biomass, ultimately suppressing CH(4) uptake. Surprisingly, precipitation pulses and N addition did not interact to affect CH(4) uptake because precipitation pulses and N addition had an offset effect on pH and affected CH(4) fluxes through different pathways. In summary, precipitation pulses and N addition were able to suppress the absorption of CH(4) from the atmosphere by soil, reducing the CH(4) sink capacity of grassland ecosystems.