Rainfall-associated chronic N deposition induces higher soil N2O emissions than acute N inputs in a semi-arid grassland
2021
Higher nitrogen (N) deposition and altered precipitation patterns are two important global change factors contributing to increasing atmospheric concentrations of greenhouse gas nitrous oxide (N₂O). In most experiments manipulating global change factors, wet N deposition is simulated by acute N addition once or a few times per year, rather than chronic low N inputs associated to rainfalls as they occur in the field. Whether N deposition pattern (acute or chronic) matters for soil N₂O emissions, and whether altered rainfall frequency and modified N deposition pattern have an interactive effect on soil N₂O emissions, are still knowledge gaps. Here we used a full factorial design for a mesocosm experiment in a semi-arid grassland. The experimental factors included rainfall frequency (ambient number of rainfall events; +50% number of rainfall events; and -50% number of rainfall events), and N deposition pattern (no N deposition; acute N deposition with two N inputs; and chronic N deposition co-occurring with rainfall events). Our results showed that altered rainfall frequency had no effect on soil N₂O emissions irrespective of N deposition pattern. In contrast, N deposition pattern matters for soil N₂O emissions. As compared to no N addition, cumulative N₂O emissions during the vegetation growing season were significantly increased by 36% for chronic N deposition, but not significantly for acute N deposition. The temporal dynamics of N₂O fluxes also varied between acute and chronic N deposition. These findings suggest that acute N addition may underestimate the actual N deposition effect on N₂O emissions in grasslands, and that N deposition pattern should be considered when building models of ecosystem-climate feedback.
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