The Role of Soil Characteristics on Temperature Sensitivity of Soil Organic Matter

Abstract

T emperature is an important factor controlling SOM turnover and understanding how temperature aff ects SOM decomposition will allow us to better predict how global climate change will aff ect SOM stocks. Understanding the temperature sensitivity of SOM decomposition is challenging because SOM is composed of many diff erent organic C compounds, with diff ering inherent kinetic properties (Davidson and Janssens, 2006). To simplify the process of modeling SOM decomposition, this range of compounds is usually classifi ed into a small number of discrete, kinetically defi ned pools with some portion of SOM being easily decomposable and the rest comprising one or more other pools decomposing more slowly.In most decomposition models, temperature eff ects are modeled as a decomposition rate multiplier for fi xed SOM pools (Lloyd and Taylor, 1994;Del Grosso et al., 2005), but some recent studies have hypothesized that temperature may actually alter the amount of substrate that would be considered easily decomposable (Zogg et al., 1997;Zak et al., 1999;Dalias et al., 2003;Rasmussen et al., 2006). Th e most broadly used terrestrial C models typically have a fi xed Q 10 of 1.5 to 2.0 or an Arrhenius-type function with the same respiration-temperature relationship to each of the diff erent SOM pools (Melillo et al., 1995;Burke et al., 2003;Friedlingstein et al., 2006 Th e uncertainty associated with how projected climate change will aff ect global C cycling could have a large impact on predictions of soil C stocks. Th e purpose of our study was to determine how various soil decomposition and chemistry characteristics relate to soil organic matter (SOM) temperature sensitivity. We accomplished this objective using long-term soil incubations at three temperatures (15, 25, and 35°C) and pyrolysis molecular beam mass spectrometry (py-MBMS) on 12 soils from 6 sites along a mean annual temperature (MAT) gradient (2-25.6°C). Th e Q 10 values calculated from the CO 2 respired during a long-term incubation using the Q 10-q method showed decomposition of the more resistant fraction to be more temperature sensitive with a Q 10-q of 1.95 ± 0.08 for the labile fraction and a Q 10-q of 3.33 ± 0.04 for the more resistant fraction. We compared the fi t of soil respiration data using a two-pool model (active and slow) with fi rst-order kinetics with a three-pool model and found that the two and three-pool models statistically fi t the data equally well. Th e three-pool model changed the size and rate constant for the more resistant pool. Th e size of the active pool in these soils, calculated using the two-pool model, increased with incubation temperature and ranged from 0.1 to 14.0% of initial soil organic C. Sites with an intermediate MAT and lowest C/N ratio had the largest active pool. Pyrolysis molecular beam mass spectrometry showed declines in carbohydrates with conversion from grassland to wheat cultivation and a greater amount of protected carbohydrates in allophanic soils which may have lead to diff erences found between the total...

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