Climate Change in the Intermountain West
Marie-Anne de Graaff
Climate change may affect soil carbon (C) storage in the Intermountain West by altering soil microbial activity. Specifically, an increase in precipitation may reduce soil C stocks. Water is a limiting resource for microbial decomposition so an increase in precipitation should promote microbial decomposition of soil C, leading to a net flux of CO2 from soil back to the atmosphere. However, it is currently uncertain to what extent altered precipitation rates will impact soil C and if different plant species mediate the impacts of precipitation on soil C differently. This study aims to asses: how precipitation will affect soil C storage, and if plant species mediate impacts of changed precipitation rates on soil C. Soil samples were collected from a long-term ecohydrology study located in the cold desert of the Idaho National Lab, USA. Precipitation treatments delivered during the previous 18 years consist of three regimes: (1) a control (ambient precipitation), (2) 200 mm irrigation added during the growing season, and (3) 200 mm irrigation added during the cold dormant season. Experimental plots were planted with a diverse native mix of big sagebrush (Artemisia tridentata) and associated shrubs, grasses, and forbs, but were invaded by crested wheatgrass (Agropyron cristatum). Soils were collected in February 2011 with a 4.8 cm diameter soil corer to a depth of 15 cm. Across all precipitation treatments, we sampled directly beneath sagebrush, crested wheatgrass, and from relatively bare plant interspaces. Subsamples (100 g) were sieved (4.75 mm) and air dried. The soils were fractionated into macroaggregates (> 250 µm), free microaggregates (53-250 µm) and free silt and clay fractions (<53 µm), using a wet sieving protocol. Further, macroaggregates were separated into particulate organic matter (POM), microaggregates and silt and clay fractions using a microaggregate isolator. Soil fractions were analyzed for soil organic carbon and nitrogen contents after removal of soil carbonates using sulfuric acid. Preliminary data shows a decline in soil C across all treatments relative to the control. This decline is smaller under sagebrush relative to crested wheatgrass, which appears to result from greater protection of C in stable microaggregates in sagebrush relative to crested wheatgrass. These preliminary data suggest that precipitation changes may result in a greater loss of soil C in degraded ecosystems relative to native ecosystems.