The potential change in soil CO₂ efflux rates in response to warming constitutes a crucial feedback between the global carbon cycle and the climate. Total soil CO₂ efflux originates from two main sources: (1) rhizosphere respiration of plant-derived recently assimilated carbon by roots and associated rhizosphere microbes, and (2) microbial respiration of SOC. The two sources also interact through a process known as the rhizosphere priming effect (RPE) in which rhizosphere micribial activities may enhance respiration of SOC. Although studies have shown that many biotic and abiotic factors (e.g. plant species, biomass and phenological stages, rhizodeposition, soil type, soil moisture, N, CO₂, and light) affect the magnitude and direction of RPE, little is known about how soil temperature controls RPE.

We carried out a continuous ¹³C-labeling experiment in a growth chamber and assessed responses of SOC decomposition and RPE to relatively short- (48 hours) and long-term soil warming (57 days). Sunflower was grown in an organically farmed sandy loam in PVC pots for 57 days with ambient (15-25^oC) and elevated soil temperature (+5^oC by buried heating cables). Soil moisture contents were maintained at 25% gravimetrically by daily watering. We continuously labeled plants with ¹³C-depleted CO₂, which allowed us to separately measure plant- and soil-derived CO₂ in soil respiration measurements. After 43 and 53 days of plant growth, we measured RPE at three consecutive times (24-hour period), the first at regular temperature setting, the second at elevated (+5^oC) temperature (both air and soil warming), and the third after returning to regular temperature, with one full day acclimation time in between. At the end of final measurement (57 days after planting), we measured plant biomass, soil extra-cellular enzyme activities, microbial biomass and community composition (PLFA).  

Results/Conclusions

We hypothesized: (1) both short- and long-term soil warming significantly increase RPE, due to higher extra-cellular enzyme activities and faster microbial turnover; and (2) because of acclimation, the response of RPE to long-term warming is lower than to short-term warming. Preliminary data show that: (1) soil warming does not significantly change plant biomass and allocation; (2) total soil respiration is more stimulated by short-term warming compared to long-term warming; and (3) extra-cellular enzyme activities were significantly enhanced by both warming and planting, while soil microbial biomass carbon was unresponsive to both treatments. Now we are analyzing ¹³C-PLFA and δ¹³C of total soil respiration to further test our hypotheses.