Nitrogen, phosphorus, and the ratio between them affect nodulation in Alnus incana and Trifolium pratense

Nodulation of Alnus incana by Frankia was studied at three levels of N (amm onium nitrate, 0.071-7.1 mM N) combined with three levels of P (0.01-1.0 mM ) in a factorial design. Nodulation of Trifolium pratense by Rhizobium legu minosarum by. trifolii was studied in a partial factorial design. Plants we re in growth pouches for 10.5 weeks. In general, the degree of N inhibition depended on the P level. In A. incana, high P level stimulated nodule numb er and nodule dry matter per plant and per plant dry matter or per root dry matter. High P also stimulated nodule size and nitrogenase activity. Effec ts on nodule number seemed to be largely explained by plant growth whereas P had more of a specific effect on nodule dry matter. The N/P ratio was imp ortant, and increased N levels inhibited nodulation at N/P ratios >7 but no t at N/P ratios less than or equal to7. In T, pratense, high P level counte racted the inhibition of high N on nodule number and nitrogenase activity. The fact that N effects on nodulation and nitrogenase activity depend on P level should encourage more detailed work on effects of nutrient interactio ns on nodulation, both in actinorhizal plants and in legumes.