Photosynthesis controls of CO2 efflux from maize rhizosphere

The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a C-13 natural abundance and C-14 pulse labeling simultaneously. C-13 was a tracer for total C assimilated by maize during the whole growth period, and C-14 was a tracer for recently assimilated C. CO2 efflux. from bare soil was 4 times less than the total CO2 efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO2 efflux from soil, respectively. The analysis of C-13 natural abundance showed that root-derived CO2 efflux. accounted for 82%, 68% and 56% of total CO2 efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO2 efflux. from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO2 efflux from soil. 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. C-14 labeling of plants before the first light treatment did not show any significant differences in the (CO2)-C-14 respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (C-14) to the root-derived CO2 efflux by shaded plants. Results from C-13 natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.