Research On Soil Nitrogen Availability For Different Forest Types

Forest soil nitrogen usually accounted for more than 90% of the entire ecosystem nitrogen pool; but in which only the nitrate (NO₃^--N) and ammonium(NH₄⁺-N) derived from soil soil nitrogen mineralization ,which make up 1-5% of soil total nitrogen , are the main forms that plant can directly absorb and/or may cause entironmental pollution. This study, using the Close-Top Tube Incubation method and monthly sampling from May, 2007 to January, 2008, investigated the characteristcs of soil inorganic nitrogen contents and soil net rates of ammonification, nitrifacation and mineralization for three different forest types of 17-?89-yr-old Chinese fir plantations and a mixed broad-leaved forest located at Nanping, Fujian Province (117°57'E, 26°28'N). We expected this study could provide foundmental data for further research into nitrogen cycling process of the local major forests and withby offer references for improveing the soil fertility and productivity in forests sustainable management.The results showed that the averaged contents of soil inorganic nitrogen (mg·kg^-1) followed the order of the mixed broad-leaved stand (58.93)>the 17-aged stand (50.20)>the 89-aged stand (49.44), indicating that the mixed broad-leaved stand had higher soil nitrogen availibility than the two 89- and 17-aged Chinese fir plantations.; the net rates of soil nitrogen mineralization and ammonification (mg·kg^-1·30d^-1) presented the rank of mixed broad-leaved stand (9.71)>the 89-aged stand(7.59)>the 17-aged stand (6.41) and, the net rates of nitrifacation (mg·kg^-1·30d^-1), the 89-aged stand (7.42)>the 17-aged stand (5.57)>the mixed broad-leaved stand (-0.66). But the ratio of nitrifacation to mineralization was 49.14% for 89-aged stand, higher than that of 17-aged stand (46.36%), and the mixed broad-leaved stand was negative (-6.92%), which indicated that the the 89-aged stand had little higher soil nitrogen supply capacity than 17-aged stand, but had lower potential of nitrogen preservation than the latter and, the mixed broad-leaved stand could prevent more NO₃^--N from losing while supplying more NH₄⁺-N to forests. Obviously, with the planting time extended of the Chinese fir plantations, the soil availibility was improved, but the risk of lossing nitrogen was meanwhile increased. After the mixed broad-leaved replaced by the Chinese fir plantations, its soil availibility would decrease, demonstrating that there existed a certain nitrogen preserving mechanism formed in the course of succession.In this study we found soil temperature and moisture could affect soil nitrogen availability and, in most cases, they was positively correlated with soil nitrogen minerzliazaton rates, especially with the net nitrification rates of Chinese fir plantations; soil bulk density, C/N ratio, pH value, the lignin content of litter falls, lignin /nitrogen ratio also had effect on soil nitrogen availability. We also found that soil nitrogen availability was, sometimes, positively correlated with LDOC measured one or two months ago and, the net rates of ammonification and mineralization were significantly negatively correlated with initial NH₄⁺-N content of an incubation period and AMBC. In addition, DOC, MBC, ADOC and AMBC had also effect on NH₄⁺-N and NO₃^--N content, which related to the effcets of soil organic carbon (energy substances) on soil microbes and to the microbial preference for NH₄⁺-N in soil.