Understanding the stimulation of microbial oxidation of organic N to nitrate in plant soil systems

In the soil N cycle, heterotrophic nitrification is poorly understood. Our understanding of the factors controlling soil heterotrophic nitrification requires support from investigations in the presence of plants. In this study, a series of 15N tracing pot experiments using maize (Zea mays L.) was conducted and the heterotrophic nitrification rate (ONrec) and maize N uptake rate were estimated using the NtracePlant tool to explore the mechanisms that stimulate heterotrophic nitrification by plants. The results showed that the ONrec (0.79–3.67 mg N kg−1 d−1) was much higher in the presence of maize than in the control (CK, no plants, <0.10 mg N kg−1 d−1). After the maize was removed, the ONrec decreased significantly, becoming similar to that of CK. These results indicated that the ONrec was stimulated by the presence of plants. The ONrec declined rapidly to 0.16 and 0.13 mg N kg−1 d−1 after the maize was covered with a black box for 2 and 4 days (preventing photosynthesis), respectively. Meanwhile, the soil dissolved organic carbon (DOC) concentration decreased significantly after photosynthesis was prevented. Moreover, the ONrec correlated significantly with the soil DOC content (P < 0.05). These results revealed that root exudates derived from plant photosynthesis were the key factors that altered soil organic matter, thereby accelerating heterotrophic nitrification. We also found that the maize NO3− uptake rate correlated significantly and positively with the ONrec (P < 0.01), suggesting that the stimulation of heterotrophic nitrification by plants played an important role in the supply of NO3− to meet the N requirements of maize and microorganisms.