Plants with an ammonium preference affect soil N transformations to optimize their N acquisition

Our understanding of how plants influence the gross rates of specific soil N transformations in plant-soil systems is still rudimentary, providing the incentive for our current study. A 15N tracing study was carried out with plants known for their NH4+-preference to quantify the gross soil N transformation and gross plant N uptake rates. Significant interactions between plants and gross rates of soil N transformations were observed. The rates of NH(4)(+ )uptake by sugarcane (3.74 mg N kg(-1)-d(-1)) and tea (3.34 mg N kg(-1)-d(-1)) were much higher than microbial NH(4)(+ )immobilization rates (0.01, and 0.27 mg N kg(-1)-d(-1), respectively), suggesting that NH(4)(+)preferring plants outcompeted microbial NH(4)(+ )acquisition. The gross rates of NO3 immobilization increased with decreasing gross NH4+ immobilization rates, indicating a switch towards microbial NO3 uptake under high plant NH(4)(+ )demand. Moreover, the gross rates of autotrophic nitrification, the classical NO3 production pathway, was generally low in the studied acidic soil (average 0.40 mg N kg(-1)-d(-1) in plant treatments), and was insufficient to meet the total NO3 demand (average 2.37 mg N kg(-1)-d(-1)). Gross rates of heterotrophic nitrification, ranging from 0.31 to 0.57 mg N kg(-1)-d(-1), were stimulated by the presence of plants and were generally responsible for 49-69% of total NO3 production in the plant treatments, while this rate was negligible in the absence of plant. Heterotrophic nitrification might provide additional NO3 to meet N requirements of plants and microorganisms. This is supported by the positive correlation of gross heterotrophic nitrification coupled with gross NO3 immobilization and plant N uptake rates. Interactions between plant N acquisition and soil N transformations exist and plant-soil studies are key to identify feedbacks between plants and soil microbes.

Automated summary

Plants have significant interactions with gross rates of soil nitrogen transformations, with NH4+-preferring plants outcompeting microbial NH4+ acquisition. Heterotrophic nitrification stimulated by the presence of plants is a major contributor to total NO3 production, providing additional nitrogen for plant and microbial N requirements. This study highlights the importance of understanding the feedbacks between plant N acquisition and soil N transformations.