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 N-15 tracing pot experiments using maize (Zea mays L.) was conducted and the heterotrophic nitrification rate (O-Nrec) and maize N uptake rate were estimated using the Ntrace(Plant) tool to explore the mechanisms that stimulate heterotrophic nitrification by plants. The results showed that the O-Nrec (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 O-Nrec decreased significantly, becoming similar to that of CK. These results indicated that the O-Nrec was stimulated by the presence of plants. The O-Nrec 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 O-Nrec 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 O-Nrec (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.