| In the past decades, nitrogen( N) deposition has become a hot environmental problem globally. Increasing N deposition has an important impact on ecological processes of terrestrial ecosystems, especially on the growth and nutrient cycling of forest ecosystems. The growth and death(i.e. fine root turnover) of tree fine roots(≤2mm) is an important part of the above- and below-ground distribution of carbon(C) and N, and nutrient cycling processes of forest ecosystem. Investigating the effects of N deposition on the dynamics of tree fine roots will help us to understand the response and feedback of underground physiological and ecological processes of forest(especially root effect) to increasing N deposition, which is beneficial for improving scientific management of plantations and constructive for improving forest productivity under the global change.In this study, three stand ages(6-,10-,16-year old) of poplar(Populus deltoides) was selected to investigate the effects of simulated N deposition on the fine roots of poplar in fields, four N levels: control( 0 g N·m-2·a-1, N0), low nitrogen( 5 g N·m-2·a-1, N1), medium nitrogen( 15 g N·m-2·a-1, N2), high nitrogen( 30 g N·m-2·a-1,N 3)) in a randomized block design, we tested the effects of morphology(diameter, order), soil layers, seasons of birth and N treatment levels on the longevity of fine roots in poplar. The results showed that:(1)The longevity of fine root s varied with the stand age. With the increase of stand age, the longevity of fine roots increased: 6-year old <10-year old <16-year old. With the extended time of birth, the cumulative survival rate of fine roots decreased, mortality increased.(2) The longevity of fine roots prolonged with diameter and order increasing, the cumulative survival rate of fine roots increased. In three plantations, the cumulative survival rate of finer fine roots(<0.3 mm, 0.3~0.6 mm) was lower than that of fine roots( >0.6 mm), and the former was short-lived. The cumulative survival rate of advanced root order(second and higher order) in 6-year old and 10-year old was higher than first order, however, the longevity of fine root 16-year old was no significant difference between orders.(3) The longevity of fine roots had obvious seasonal regularity and soil hierarchy.The cumulative survival rates of fine roots in three forest age showed: spring < summer < autumn, the shortest longevity of fine roots was found in spring-born, while longest in autumn. The lowest cumulative survival rates with shortest longevity was found in top soil(0 ~15cm) of three forest age, longevity of fine roots was the shortest, t he cumulative survival rate of fine roots in deep soil was larger than that in topper layer. Except for the median root lifespan of root in 6-year old topper soil was greater than that in 15~30cm, the MRL extended with soil depth.(4) The impacts of different N treatment on the longevity of fine roots were inconsistent. Overall, low N treatment extended the longevity of fine roots of three aged plantations, while high N treatment shortened the longevity(except for young plantation).N treatment affected the longevity of fine root with different diameter and root order. Low N treatment significantly affected the diameter of fine root and increased the longevity, but high N(N2, N3) treatment reduced the survival rate of fine roots, shortened longevity. Low N treatment increased the cumulative survival rates of fine roots in the first order of 6 and 16-year old, no significant difference in 10-year old. While high N treatment decreased the cumulative survival rates of fine roots in the second and higher order of 10 and 16-year old, no significant difference in young plantation. And the death trends of second and higher root order was only 82.4% and 82.7% of first root order, N treatment minimized the branch of fine roots.N treatment affected the longevity of fine root with different soil layers and season of birth. N treatment improved the cumulative survival rate of fine root of 15~30cm in 6-year old and 15~30cm, 45~60cm in 10-year old, and improved the cumulative survival of 0~15cm, 30~45cm layer in 16-year old, while reduced the cumulative survival rate of fine root in depth soil(30~45cm, 45~60cm) of 6-year old and 0~15cm, 30~45cm soil layer of 10-year old. N treatment reduced the longev ity of surface layer of soil in 10-year old, but increased 16-year old. At the same time, N treatment significantly affected spring- and summer-born fine roots in three plantations, but did not influence autumn-birth fine roots. |
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