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

Abstract

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 NH4+ 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 NH4+ immobilization rates (0.01, and 0.27 mg N kg−1 d−1, respectively), suggesting that NH4+-preferring plants outcompeted microbial NH4+ acquisition. The gross rates of NO3− immobilization increased with decreasing gross NH4+ immobilization rates, indicating a switch towards microbial NO3− uptake under high plant NH4+ 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.