Effects of environmental stress on seedlings root growth and nodulation of leguminous shrubs in the dry valley of Minjiang River

A field investigation was made to understand the seedlings root nodulation, biomass accumulation, root length, and fine root percentage of Sophora davidii, Indigofera lenticellata and Campylotropis polyantha along an altitudinal gradient on two contrasting sloped hills (north Zongqu and south Jingzhoushan) in the dry valley of Minjiang River. In the meantime, a greenhouse experiment was conducted to study the adaptation responses of 2 month-old S. davidii and C. polyantha seedlings root nodulation to different soil moisture regimes (80%, 60%, 40%, and 20% WHC). For the three test shrub species, fewer nodules were observed at lower altitude (1600-1950 m) areas, the nodule number per plant of S. davidii, I. lenticellata, and C. polyantha being 0.1 +/- 0.1, 0.9 +/- 0.5, and 5.7 +/- 1.9, and the non-nodulation plant accounting for 65.1%, 12.3% and 17.6%, respectively. The nodule number of the three species increased with increasing altitude, and correlated positively with root length and fine root percentage. However, there were no significant differences in the plant growth and biomass at different altitudes. When the soil moisture content was lower than 60% WHC, the nodule number and the fresh and dry mass of both S. davidii and C. polyantha decreased markedly, and at 20% WHC, no nodule and only 9.8 +/- 3.6 nodules were observed for S. davidii and C. polyantha, respectively, indicating that in this dry valley, the root nodulation capability of endemic leguminous shrubs was very low. Comparing with S. davidii, C. polyantha had higher root nodulation capability and drought-resistance. Prior to introducing these shrub species in forestation practices, to keep the soil moisture content higher than 40% WHC was recommended for relatively efficient biological nitrogen fixation.