Effect of nitrogen supply on rhizobacterial propagation and soil enzyme activity in pot soil cultivated with Phytophthora infestans-infected potato plants

We previously demonstrated that providing potato plants with an optimum nitrogen supply (in the form of urea; 135 kg hm-2) produces high crop yields and increased resistance to late blight. However, the mechanisms underlying these responses have not been well characterized. In this study, we examined the effects of various levels of nitrogen fertilization (four levels: N1, 45 kg hm-2, N2: 90 kg hm-2, N3: 135 kg hm-2 and N4: 180 kg hm-2) on soil bacterial growth, community diversity and soil enzyme activity. We found that application of the optimum amount of nitrogen promoted maximum levels of bacterial growth and community diversity development compared with the other treatments. In addition, the highest activities were detected for soil enzymes such as urease, invertase and acid phosphatase (but not catalase) under N3 conditions but not under N1, N2 or N4 conditions. These results suggest that proper nitrogen application provides soil microbes with optimum conditions for development. Thus, the optimum growth of rhizobacteria conferred by N3 treatment appears to be responsible for achieving the highest yields and strongest pathogen resistance in potato plants exposed to Phytophthora infestans.