Decreased soil pH weakens the positive rhizosphere effect on denitrification capacity

Abstract

The stimulating effect of rhizosphere on denitrification is considered to be an unavoidable loss of soil nitrogen (N) and detrimental for crop N use efficiency, which is regulated by crop growth and soil properties. Soil acidification, occurring rapidly in many intensive farming lands, affects both crop growth and soil properties, therefore altering rhizosphere effect on denitrification. However, the mechanism by which soil acidification regulates rhizosphere denitrification still remains unclear. Here, we determined the denitrification capacity (DC) and associated community compositions of nirK- and nirS-type denitrifiers in maize rhizosphere and bulk soil with four acidity gradients (pH = 6.8, 6.1, 5.2, and 4.2). Results showed that the stimulating effect of rhizosphere on DC strongly depended on soil pH. Compared to the bulk soil, the rhizosphere significantly stimulated DC in the soils with pH above 5.2, but not in the soil with pH of 4.2. With increasing soil acidity, the stimulation of rhizosphere on DC (calculated by the difference in DC between rhizosphere and bulk soil) decreased from 8.01 to 0.01 mg N kg−1 d−1. Moreover, soil acidification significantly reduced the differences in dissolved organic carbon (DOC) and key nirK-type denitrifier taxa abundance between rhizosphere and bulk soil, both of which were positively related to the stimulation of rhizosphere on DC. These findings demonstrated that soil acidification could weaken the positive rhizosphere effect on denitrification via regulated C availability and associated nirK-type denitrifier community, potentially reducing N loss risk in the rhizosphere. The independent role of soil pH should be fully considered in future modelling N behaviour in plant-soil systems.