| The temperature sensitivity of soil organic matter (SOM) decomposition has been a crucial topic in global change research, yet remains highly uncertain. In Chapter 1, I investigated the impact of rhizosphere priming effect (RPE) on the temperature sensitivity of SOM decomposition. Based on data from five measurements in three growth chamber experiments with two plant species ( Helianthus annuus and Glycine max), two growth stages and two warming methods, I found that RPE significantly and consistently increased the temperature sensitivity of SOM decomposition (Q10 values increased by 0.3-0.9). This result clearly indicates that root-soil interactions play an important role in shaping the temperature sensitivity of SOM decomposition. In Chapter 2, I studied the effect of soil temperature regimes on the temperature sensitivity of SOM decomposition. In a 122-day soil incubation experiment, with two soils exposed to four temperature levels, I found that the Q 10-q values (calculated temperature sensitivity of SOM decomposition after accounting for changes in substrate availability and quality among treatments over time) were consistently higher by 6-23% under constant temperature regime than under diurnally-varying temperature regime, particularly in the later stages of decomposition (up to 32%). This result suggests that different temperature regime is one of the important confounding factors causing the current controversy about the temperature sensitivity of SOM decomposition in published reports. In Chapter 3, I examined the apparent 13C isotope fractionation during rhizosphere respiration. In a 62-day greenhouse experiment, I found a consistent 13C-depletion (0.9-1.7%0) in rhizosphere respiration relative to root biomass in three C3 species (Helianthus annuus, Glycine max and Triticum aestivum), but a relatively large 13C-depletion (3.7-7.0%0) in three C4 species (Zee mays, Sorghum bicolor and Amaranthus tricolor). This result shows that CO2 from rhizosphere respiration is 13C-depleted than root biomass and accounting for this apparent 13C fractionation may modify the results of partitioning total soil CO2 efflux into root-derived and soil-derived components. In Chapter 4, I discussed policy constraints and options to implementing agricultural soil carbon sequestration projects in the U.S. These specific policy approaches may create a role for agriculture in the emerging U.S. carbon markets and provide a competitive, low-cost, and near-term option to meet U.S. greenhouse gas reduction targets. |
