Effects Of Precipitation Regime Changes On Carbon Fluxes In A Temperate Grassland In Northern China

Precipitation regime changes may influence terrestrial biosphere through changing inter-annual mean precipitation amounts and intra-annual precipitation timings,especially for the arid and semi-arid grasslands where water is the most limited factor.Previous studies of precipitation regime changes on ecosystem and soil carbon fluxes have been mainly focused on the changes of chronic mean precipitation amount rather than intra-annual precipitation timing.Effects of precipitation timing changes on ecosystem and soil carbon fluxes cannot be ignored with the ongoing of studies on changing precipitation timings in recent years.However,few studies have addressed the influence of changing precipitation amounts in different periods of growing-season.This study examined the impacts of experimentally imposed changing rainfall regimes(increased/reduced 60% mean annual precipitation of the early growing-season(April-June)or of the late growing-season(June-September)on a temperate grassland in Northern China.The primary objectives were(1)to assess the responses of net ecosystem carbon exchange,gross ecosystem productivity,ecosystem respiration,and soil respiration to the individual and combined effects of early and late growing-season precipitation changes,and(2)to exploit the underlying mechanisms driving these responses.Precipitation reduction in the early growing-season decreased gross ecosystem productivity by 12%,ecosystem respiration by 11%,net ecosystem carbon exchange by 11%,and soil respiration by 13%.Precipitation reduction in the late growing-season reduced gross ecosystem productivity,ecosystem respiration,net ecosystem carbon exchange,and soil respiration by 32,21,44,and 26%,respectively.However,neither precipitation addition in the early nor in the late growing-season affected gross ecosystem productivity,net ecosystem carbon exchange,ecosystem respiration,and soil respiration.Reduced both early and late growing-season precipitation significantly suppressed gross ecosystem productivity and net ecosystem carbon exchange by decreasing soil moisture of the late period and normalized difference vegetation index.Reductions of ecosystem respiration triggered by reduced both early and late growing-season precipitation were closely related to decreased soil moisture,normalized difference vegetation index and net ecosystem gas exchange.Compared to the effects of decreased precipitation in the early growing-season,the indexes of ecosystem carbon fluxes were more sensitive to precipitation reduction in the late growing-season.The direction and magnitude of soil respiration responses to decreased precipitation in the early growing-season was similar to those of ecosystem respiration responses,but the negative effect of decreased precipitation in the late growing-season on soil respiration was twice greater than that of decreased precipitation in the early growing-season.Changed soil respiration triggered by reduced both early and late growing-season precipitation may be caused by reducing soil moisture during the early and late growing-season,normalized difference vegetation index and gross ecosystem productivity.These observations suggest that carbon cycles will be more vulnerable to reduced than increased precipitation in different periods of growing-season in temperate grassland.Decreased soil moisture and net ecosystem gas exchange triggered by precipitation reduction may decline grassland production and vegetation cover in this region,and consequently grassland degeneration.The findings improve the understandings of grassland carbon cycle under changing precipitation scenarios,and provide data support and theoretical basis to develop the adaptive management strategies for maintaining grassland ecosystem functioning.