| Fine root play important roles in forest ecosystem. Root biology and ecology have become the focus in the field of ecology over the past 20 years. In this study, soil core method, in Manchurian ash (Fraxinus mandshurica) plantation (17a) located in Maoershan Experiment Station, was used in estimating root indexes biomass, root length density (RLD) and specific root length (SRL) within a growing season. Soil N availability and other resources were analyzed. Main contents and results on this research are as follows:1, Plenty of samples were acquired in different season by soil core method. Sampled and statistic the coefficient variation (CV) of root indexes (biomass, RLD and SRL) in different sample sizes, and further determined the proper sample size of different root diameter size in different seasons. The results showed that coarse root spatial CV increased with root diameter, so it is reasonable for us sampling more samples in order to determine demography indexes mentioned above. As for fine root, when we sampling for RLD data and sample in autumn, more sample size were needed.2, Standing biomass, RLD and SRL were researched. The results showed that annual fine root (<1mm) standing biomass of Manchurian ash plantation was 363g.m-2. Living fine root biomass accounted for 57% and dead biomass for 48% of the total respectively. Living coarse root (>5mm) biomass was the highest (976 g.m-2, 70%) of total living root,and the second was living fine root (187 g.m-2, 13%). Inversely, dead coarse root biomass was the lowest (16 g.m-2, 7%) and dead fine root was the highest (176 g.m-2, 75%) of total dead root. Suggesting fine root has higher mortality than coarse root in growing season. Living fine root RLD accounted for more than 90% of total living root, and SRL of fine root was 10 to 1000 times of coarse root, and this means fine root has great surface area; it is the most active part in water and nutrient absorption.3. Vertical distribution of living fine root biomass was identical with that of RLD in 0~20cm depth soil layer, accounting for 86.6% and 86.7% of the total that living in 0~30cm soil layer, respectively. All kinds of soil resources decreased with soil depth, especially available N and organic matter, which may be mainly account for fine root vertical distribution. Fine root SRL in different soil layers were almost similar, it was due to much more soil nutrients and water availability in the upper soil layer.4, Analyzed fine root parameters (biomass, RLD and SRL) of spatial distributionand their relationships with the patches distribution of N availability. Results showed that NH4-N was positive correlated with fine root, while NO3-N was positive or negative correlated to fine root parameters in different seasons. In fast growing season, fine root proliferation in NH4-N rich patches but no response to NO3-N. In autumn, NO3-N may accelerate mortality of fine root. Fine root parameters are related to soil N availability in some degree, most of the case, this relationships are not significant. We supposed that it may be due to higher soil resources availability in study stand or due to the physiological plasticity of fine root is more important than morphological plasticity, which need further studies.5, In accordance with the estimation of fine root biomass monthly, seasonal variation of fine root biomass was significant (P<0.05). The peaks of fine root biomass were observed in spring (May) and autumn (September), but SRL and RLD were highest only in spring, and lowest in autumn. The fact that SRL and RLD showing higher pattern in spring and summer means that the diameter of fine root was thinner and more fine roots were developed in soil. In autumn, SRL and RLD parameters decreased significantly due to secondary thickening of fine root diameter or non-structure carbohydrates (such as soluble sugar and starch) storage.6, Seasonal dynamics of fine root were associated with soil N availability, soil temperature and monthly precipitations. Fine root biomass was strongly correlated with NH4-N avail...
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