Direct and Legacy Effects of Varying Cool‐Season Precipitation Totals on Ecosystem Carbon Flux in a Semi‐Arid Mixed Grassland

ABSTRACT In the semi‐arid grasslands of the southwest United States, annual precipitation is divided between warm‐season (July–September) convective precipitation and cool‐season (December–March) frontal storms. While evidence suggests shifts in precipitation seasonal distribution, there is a poor understanding of the ecosystem carbon flux responses to cool‐season precipitation and the potential legacy effects on subsequent warm‐season carbon fluxes. Results from a two‐year experiment with three cool‐season precipitation treatments (dry, received 5th percentile cool‐season total precipitation; normal, 50th; wet, 95th) and constant warm‐season precipitation illustrate the direct and legacy effects on carbon fluxes, but in opposing ways. In wet cool‐season plots, gross primary productivity (GPP) and ecosystem respiration (ER) were 103% and 127% higher than in normal cool‐season plots. In dry cool‐season plots, GPP and ER were 47% and 85% lower compared to normal cool‐season plots. Unexpectedly, we found a positive legacy effect of the dry cool‐season treatment on warm‐season carbon flux, resulting in a significant increase in both GPP and ER in the subsequent warm season, compared to normal cool‐season plots. Our results reveal positive legacy effects of cool‐season drought on warm‐season carbon fluxes and highlight the importance of the relatively under‐studied cool‐growing season and its direct/indirect impact on the ecosystem carbon budget. Summary statement Through a field manipulation experiment, we found that carbon fluxes decreased during a dry winter but increased in the subsequent warm season when no treatment was applied. This suggests the positive legacy effects of a dry winter and emphasizes the significance of the cool growing season's impacts.