Soil temperature control of rhizosphere effects on soil organic matter decomposition

While it is well known that soil temperature directly affects microbial activity and soil organic matter (SOM) decomposition, it is unclear if the presence of plant roots alters the temperature effect through rhizosphere processes. We studied soil temperature effects on soil C decomposition and N mineralization with and without sunflower roots in a growth chamber experiment. The two soil temperature treatments were ambient soil temperature (18-28°C) and 3°C above ambient. Soil warming (3±0.2°C above ambient) was achieved by using automatically controlled electrical heating cables buried in the soil. Sunflower (Helianthus annuus) plants were grown in an organically farmed soil for 51 days and a no-plant control was included for each temperature treatment. Soil moisture contents were maintained at near water holding capacity by weighing and daily watering. We continuously labeled plants with ¹³C-depleted CO₂, which allowed us to separate plant-derived CO₂ from soil-derived CO₂. We measured shoot and root biomass, soil microbial biomass and turnover rate, extracellular enzyme activities, soil ammonium and nitrate concentrations, and net and gross N mineralization and immobilization. We hypothesize that soil warming intensifies rhizosphere effects on SOM decomposition because the higher soil microbial biomass turnover rates and the enhanced extracellular enzyme activities under warmer conditions form a positive feedback with plant growth and substrate input from roots. Preliminary data show that warming significantly enhanced extracellular enzyme activities and increased plant biomass.