UC Santa Cruz

Drying-wetting cycles influence both soil organic matter (SOM) decomposition and rhizosphere processes. Rhizosphere processes also affect SOM decomposition through rhizosphere priming. However, little is known about how drying-wetting cycles regulate SOM decomposition with rhizosphere priming, because most previous studies incubated root-free soils and omitted the rhizosphere effect. To investigate the effect of drying-wetting cycles on SOM decomposition in the presence of plants, we grew sunflower (Helianthus annuus) and soybean (Glycine max) in a sandy loam soil under the treatments of either constant moisture or 12 drying-wetting cycles, and used a continuous 13C-labeling method to partition soil respiration into rhizosphere respiration and SOM decomposition. We found that compared to the constantly-moist treatment, the severe drying-wetting cycles in soils planted with sunflower significantly reduced shoot biomass (32%), root biomass (52%), rhizosphere respiration (29%), and SOM decomposition (22%), while the moderate drying-wetting cycles in soils planted with soybean did not significantly affect these variables. Moreover, SOM decomposition rates in the planted treatment subjected to constantly-moist or drying-wetting conditions were significantly higher compared with the constantly-moist unplanted treatment, indicating a positive rhizosphere priming effect under both soil moisture regimes. However, drying-wetting reduced the rhizosphere priming of sunflower (69% versus 33%) likely due to lower plant biomass and rhizodeposition, but produced similar rhizosphere priming of soybean (82% versus 85%). Overall, drying-wetting cycles significantly modulated rhizosphere respiration and SOM decomposition, with the magnitude depending on soil drying intensity and plant growth performance. © 2013.