Photosynthesis controls of rhizosphere respiration and organic matter decomposition

The effects of shading wheat plants on rhizosphere respiration and rhizosphere priming of soil organic matter decomposition were investigated by using a natural abundance C-13 tracer method and C-14 Pulse labeling simultaneously. Seven days with strongly reduced photosynthesis (12/60 h day/night period) resulted in only half of the total CO2 efflux from soil compared to the treatment with a 12/12 h day/night period. The CO2 efflux from unplanted soil amounted to only 12 and 20% of the total CO2 efflux from the soil with non-shaded and shaded plants, respectively. On average 75% of total CO2 efflux. from the planted soil with prolonged night periods was root-derived. Rhizosphere respiration was tightly coupled with plant photosynthetic activity. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO2 efflux, and thereby, total CO2 efflux from soils. Clear diurnal dynamics of the total CO2 efflux. intensity indicate the existence of an endogenous control mechanism of rhizosphere respiration. The light-on events after prolonged dark periods lead to strong increases of root-derived and therefore of total CO2 efflux. from soil. After C-14 pulse labeling, two maxima of the root-derived (CO3)-C-14 efflux were measured (6 and 24 h). This result demonstrated the diurnal dynamics of the rhizosphere respiration of recently-assimilated C in both the normal light conditions and shaded plants as well. The total amount of root-derived C respired in the rhizosphere was 17.3 and 20.6% of the total assimilated C for non-shaded and shaded plants, respectively. Both methods used, C-13 natural abundance and C-14 pulse labeling, gave similar estimates of root-derived CO2 during the whole observation period: 1.80 +/-0.27 and 1.67 +/-0.37 mg C kg(-1) h(-1) (+/- SD), respectively. Both tracer methods show that the cultivation of wheat led to the increasing decomposition intensity of soil organic matter (priming effect). Additionally, C-13 natural abundance allows tracing of the dynamics of the priming effect depending on the light-on events. (C) 2 001 Elsevier Science Ltd. All rights reserved.