PARENT SESSION
Contributed Oral Session 60: Biogeochemistry: Soil Nutrient Dynamics; Decomposition
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 516 D, Level 5, Palais des congrès de Montréal

Plant biomass and soil type influence rhizosphere effects on soil organic matter decomposition.

Dijkstra, Feike^*,1, Cheng, Weixin¹, ¹ University of California, Santa Cruz, Santa Cruz, CA

ABSTRACT- We used a novel continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition for two C3 plant species (soybean, Glycine max, and sunflower, Helianthus annus) and two differently managed soils (organically farmed soil and soil from an annual grassland, both dominated by C3 plants). This labeling method allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration under ambient atmospheric CO₂ conditions. Rhizosphere priming effects on SOM decomposition (i.e. differences in soil-derived CO₂-C between planted and non-planted treatments) were significantly different between the two soil types, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite a greater total soil C and overall efflux of soil-derived CO₂-C both in planted and no-plant control pots in the annual grassland soil. Differences in rhizosphere priming effects on SOM decomposition between the two soil types could be explained by differences in plant biomass, and in particular leaf biomass. Greater production of leaf biomass in the organically farmed soil resulted in greater rhizosphere priming effects on SOM decomposition, which in return may have promoted plant growth. Therefore, in soils with high potential for rhizosphere priming effects on SOM decomposition, plant growth and rhizosphere SOM decomposition may stimulate each other posing a positive feedback.

Key words: soil respiration, continuous ¹³C labeling, SOM priming effects, rhizosphere