Effects of continuous mono-cropping of flue-cured tobacco on nitrogen relations in the soil and plant
In order to understand the effects of continuous mono-cropping on the forms of soil nitrogen and their effects on plant nutrition, we analyzed soil nitrogen, various soil microorganisms and the nature of nitrogen accumulation in the tobacco leaf in a potting experiment using different soil types. We found that total N in the soil, alkali-hydrolysable N, nitrate N and ammonium N increased with continuous mono-cropping but nitrifying bacteria, ammonium oxygenation bacteria and nitrification decreased. Continuous mono-cropping led to decreased chlorophyll and nitrate reductase levels and, increased proline content of the leaf. Mono-cropping also led to a gradual increase in total N, nicotine and protein in the leaf and their decline in the stem and the root. These changes were affected by soil type. A quaternary yellow soil suitable for tobacco had lower amounts of nitrate, nitrifying bacteria, ammonium oxygenation bacteria and nitrification than a less suitable yellow lime soil. Improved crop performance under continuous mono-cropping could be achieved by reducing the soil N content to induce higher levels of beneficial microorganisms.