Effects of sewage-sludge and drought stress on eco-physiology of mongolian pine seedlings

Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) was the first tree specie to defend wind and retain sand in North china. However, a decline of forest growth has occurred. In order to expose the decline problem, we cultured 2-year old Mongolian pine seedlings in sand added different sludge for 3 months and followed by drought stresses experiment to examine the effect of sewage-sludge and drought on eco-physiology of seedlings. net photosynthesis rate (Pn), stomatal conductance (Gs), CO₂ concentration inside the cell (Ci), transpiration rate (Tr), leaf water potential and chlorophyll (Chl) of seedlings were measured. The results showed that seedlings applied sludge increased apparently in the height, stem diameter and root length than those grew in sands and increased the most when sludge volume got 30%. During the course of drought stresses, the soil relative water content added sludge was increasing higher than that of sand with the time extension. Values of Pn, Gs, Tr. Ci and leaf water potential increased significantly with the increasing of contents of sludge and reached the peak when the sludge was 30% in volume as the soil relative water content at the same level. 10% and 20% sludge had no significant effects on relief stresses on seedling. Added 30% to 50% sludge, the seedlings occurred heavily stressed at 20 % of field water capacity while seedlings in sands showed heavily stressed at 30 % of field water capacity. 80% sludge aroused serious metal stress of seedlings, values of Pn, Gs, Tr. Ci and leaf water potential were very low and had no significant difference with CK Proper sludge proved the drought resistance of seedlings due to the moisture capacity of sludge. For the sludge supplied more nutrients (e.g. nitrogen), values of Chl increased with the increasing of sludge and were highest ca. 50% sludge, then, decreased with more sludge. When sludge content was 80% of total volumes, the values of Chl were the lowest compared to others o- - wing to the soil was glued strongly and the air, water conductance in soil declined and heavy metal in sludge destroyed the structure of chloroplast.