干旱区生态环境知识资源中心

Shifts in the seasonal timing of rainfall have the potential to substantially affect the immense terrestrial stores of soil organic carbon (C, SOC). It remains unclear, however, how changes in the timing of rainfall are influencing SOC storage. We hypothesized that a sustained shift in rainfall timing from winter to a spring-summer regime would reduce desert SOC stores by creating moist and warm soil conditions, thus promoting decomposition. To investigate this, we evaluated how an 11-year seasonal shift in rainfall (winter to spring-summer regime) affected SOC storage (that is, dissolved organic C, light SOC, and heavy SOC) in soils beneath dominant shrub and perennial grass species in a cold desert sagebrush-steppe ecosystem. We also measured the soil C to nitrogen (N) ratios, standing litter stocks, and root biomass C to help interpret the long-term changes in SOC stores. As predicted, a seasonal shift in rainfall caused heavy SOC to decline beneath Artemisia tridentata ssp. wyomingensis by 14%, from 3.1 to 2.7 kg C m(-2), and Pseudoroegneria spicata by 19%, from 3.0 to 2.4 kg C m(-2). Neither dissolved organic C, nor the light fraction, responded to changes in rainfall. The C to N ratio of heavy SOC beneath Artemisia declined by at least 6% under the warmer and moister conditions of the spring-summer regime, suggesting that alterations in decomposition dynamics contributed to the loss of SOC. Unexpectedly, coarse litter and root C in Artemisia soils were lower under the spring-summer than winter rainfall regime, suggesting that a decline in litter inputs may also have contributed to the loss of SOC. The C to N ratio of heavy SOC, litter stores (that is, coarse litter and thatch), and root C in Pseudoroegneria soils demonstrated similar responses as in Artemisia soils, but these variables were at best only marginally significant. Our results suggest that a sustained seasonal shift in rainfall from winter to spring-summer will reduce heavy SOC across cold deserts, and that this reduction will stem from alterations in decomposition dynamics and net primary production by plants. Further, as global temperatures rise we may see more overlap of moist and warm soil conditions, especially in ecosystems with winter rainfall regimes (for example, Mediterranean-climate ecosystems and temperate forests), that may reduce SOC in the absence of rainfall changes.