Extreme rainfall events can alter inter-annual biomass responses to water and N enrichment

Water availability has profound effects on plant growth and productivity in temperate and semiarid grass- lands. However, it remains unclear how variation of inter- annual precipitation by extreme rainfall events will alter the aboveground and belowground responses of plants, and how these responses may be contingent on N availability. In this study, we examined the interactive effects of inter-annual precipitation variation and N addition on aboveground and live fine root biomass of a semiarid grassland in northern China for two consecutive years (2007 and 2008). Inter- annual variation in precipitation resulting mainly from the occurrence of extreme rainfall events in 2008 significantly affected above- and belowground plant biomass responses to water addition. In addition, variation of inter-annual pre- cipitation by this extreme rainfall event suppressed plant re- sponses to nitrogen addition and reduced the interaction ef- fects between water and nitrogen addition. These effects of inter-annual precipitation fluctuation could be attributed to the negative influence of the extreme rainfall event on soil N and water availability, ultimately reducing plant rainfall use efficiency and nitrogen use efficiency. In conclusion, our results suggest ecosystem responses to water and N enrich- ment could be altered by inter-annual variation of precipita- tion regime caused by the naturally occurring extreme rain- fall events.