Quantifying NH4+ and NO3− uptake by wheat under microbial competition in acid and calcareous soils

Plant-microbial interactions for nitrogen (N) in the rhizosphere strongly influence plant productivity and N cycling. Soil pH plays a pivotal role in regulating N availability and shaping competition for N between plants and microorganisms. However, the effect of pH on N competition between microbes and plants with specific N preferences remains insufficiently understood. In this study, a short-term experiment combined with 15N isotope tracing was conducted to quantify ammonium (NH4+) and nitrate (NO3−) uptake by wheat (which preferentially takes up NO3−) and microorganisms in acid and calcareous agricultural soils. The study showed that wheat exhibited pH-dependent uptake patterns for different inorganic N forms and 15N uptake rates. In the calcareous soil, wheat preferentially absorbed NO3− over NH4+ during the first 24 h (NH4+ : NO3− = 0.61), whereas in the acid soil, wheat showed no significant preference for either NO3− or NH4+ during the first 24 h (NH4+ : NO3− = 0.81). Total 15N uptake rates of wheat were higher in the calcareous soil than in the acid soil, while 15N assimilation rates of microbes were lower in the calcareous soil. Wheat dominated N uptake over microorganisms in the calcareous soil rather than in the acid soil at 48 h after labelling. Consequently, the ratios of microbial inorganic N assimilation to plant N uptake were higher in the acid soil than in the calcareous soil (95.23% vs 41.3%). Overall, the findings demonstrate that wheat exhibits distinct acquisition strategies for NO3− and NH4+ in soils with contrasting pH, and dominates N uptake over microorganisms in a calcareous soil (with a higher nitrification rate).