Identification of the mechanisms contributing to nitrogen (N) fertilizer-induced changes in CO2 efflux from soil under agricultural crops has been extremely challenging because of difficulties in separating root and microbial contribution to total CO2 efflux. In this study we present the evidence that high costs of nitrate reduction result in a strong increase of root-derived respiration and the magnitude of an increase differs between the species with various contribution of shoots and roots to the nitrate reduction process.
Fertilization of Lupinus albus and Zea mays with nitrate or ammonium and pulse labeling of plants in 14CO2 atmosphere allowed evaluation of the effect of N type on total and recently assimilated CO2 efflux from soil. Addition of nitrate to planted soil increased recently assimilated CO2 efflux by 168% in Lupinus albus (nitrate reduction site – in roots) and by 121% in Zea mays (nitrate reduction site both, in shoots and roots) in comparison with control. Ammonium-induced CO2 increase amounted for 82% in Lupinus albus and for 73% in Zea mays. Clear diurnal changes in CO2 efflux from planted soil at constant day/night temperature showed fast response of below-ground processes to photosynthesis. Both approaches for root-derived CO2 assessment: 14C pulse labeling and difference of CO2 from planted and unplanted soil showed similar results: the form of N supply and the location of the nitrate reduction site have a strong significant effect on the amount of root-derived CO2 respiration.
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