Landscape-level variation in temperature sensitivity of soil organic carbon decomposition

We examined landscape-level variation in temperature sensitivity of labile SOC across 71 sites at a central North American grassland. The observed range in activation energy of decomposition (Ea), an index of temperature sensitivity, was as great at the landscape scale as has been observed at the continental scale. Ea was lower for soils with more labile C, consistent with the ‘Carbon qualitytemperature’ hypothesis. Soil pH explained 67% of the variation in Ea. Although there are strong environmental correlates with the Ea of SOC decomposition at landscape scales, the amount of variation within landscapes could confound regionalto global-scale predictions of the response of soil C to warming. 2009 Elsevier Ltd. All rights reserved. The fate of soil organic carbon (SOC) in a warmer world is one of the largest uncertainties in predicting future climate and terrestrial ecosystem function (Holland et al., 2000; Jones et al., 2003; Lenton and Huntingford, 2003). Although the ‘Carbon quality-temperature’ (CQT) hypothesis (Bosatta and Agren, 1999; Fierer et al., 2005; Davidson and Janssens, 2006) predicts that the temperature sensitivity of organic matter decomposition increases with biochemical recalcitrance and it has been tested on a variety of substrates and scales (Fierer et al., 2005, 2006b), we know little about landscape-level variation in SOC temperature sensitivity and whether the CQT hypothesis explains a significant proportion of the observed variation in it. To better understand the landscape-level variation in temperature sensitivity of SOC decomposition, we collected surface soils (0e20 cm) across 71 sites at Konza Prairie Biological Station, KS, USA in June, 2009. Sites were all located within 5.3 km of one another and distributed among 11 watersheds, each of which differed in the frequency at which they experience fire (annual, biennial, or greater than every 20 years). In 9watersheds, soils were acquired from a grassland topographic sequence that spanned uplands, slopes, and lowlands. Soils from an additional 9 sites were acquired from riparian areas, defined as having tree cover adjacent to permanent or intermittent streams. After collection, soils were passed through a 4-mm sieve. For each soil, we measured texture via the hydrometer method (Gee and Bauder, 1979), C and N concentrations on a Leco CHN analyzer, water holding capacity (WHC) (Elliott et al., 1999), and pH in a 1:1mixture of soil andwater (Robertson et al., 1999). After adjusting the soil moisture of each soil to 35% WHC, 7 replicates of each soil were incubated at 20 C. At two time points during this incubation, individual replicates of each soil were moved to different temperatures (10, 15, 20, 25, 30 C) for approximately 24 h with the last two replicates maintained at 20 C. Average date since acquisition for the first set of respiration measurements was 13.3 d and 35.7 d for second set. From these incubations for each soil, we used the Arrhenius equation to calculate the apparent activation energy of the chemical reactions that contributed to respiration (Ea) where the rate of respiration relative to total SOC (k) is described by: