The sensitivity of temperate steppe CO2 exchange to the quantity and timing of natural interannual rainfall

Precipitation quantity and timing have been shown to fundamentally influence grassland ecosystem CO2 exchange by both model simulation and field observation. Carbon dioxide exchange and patterns of variation (magnitude and timing) in precipitation were observed to determine their relationship in typical steppe in Inner Mongolia Plateau during four years (2003–2006) with different rainfall patterns. The response of ecosystem-level CO2 fluxes to rainfall pulse events was also examined. The four years studied were classified into three levels of rainfall amount (100%, 2004; reduced by 70% in 2005 and by 45% in 2006 compared with 2004) and two temporal patterns of rain events (average temporal patterns in 2004 and altered precipitation timing in 2003, a year with precipitation fell during the first half of the growing season, compared with 2004). The grassland ecosystem switched from a carbon sink (2004) to carbon sources (2005 and 2006) under reduced rainfall amounts. Carbon sink reduced by 34% under variational rainfall timing conditions (2003). There was a significant positive correlation between annual rainfall and above-ground NPP across the study (P = 0.05, r2 = 0.9). Antecedent soil water before rainfall pulse and rainfall pulse size are important in controlling the response of CO2 exchange to rainfall in this steppe. After rainfall pulse events, CO2 fluxes had 4 to 5 days of system-level hysteresis and the increase degree of CO2 exchange was different among the four years. Precipitation pulse size not only affected the duration of CO2 fluxes, but also instant flux rate. Our results suggest that CO2 exchange in temperate steppe will be more sensitive to altered rainfall timing and more affected by changes in rainfall quantity, with a longer dry season resulting in significant C losing from Eurasia grassland in Inner Mongolia Plateau.