| Fine root decomposition is an important process of nutrient cycling in forest ecosystems. Due to global climate and in-depth researches on carbon cycling, more attention has been paid to study fine root decomposition. However, fine root decomposition is a complex process and is mainly affected by substrate quality, environmental factors, and soil microorganisms. At present, the studies of fine root decomposition have been conducted mostly in temperate and tropical forest, while fewer studies have been found in subtropical forest. In this study, we selected six subtropical tree species(Cunninghamia lanceolata, Pinus massoniana, Choerospondias axillaris, Liquidambar formosana, Lithocarpus glaber, and Cyclobalanopsis glauca), to investigate the differences in substrate quality among the six species, fine root decomposition process and constants, to examine the relationship of decomposition rate between leaf litters and fine roots, and to analyze the effects of substrate quality on fine root decomposition. Our results could provide scientific basis for understanding the roles of fine root decomposition in nutrient cycling and its controlling factors in forest ecosystems. The main results are presented as follows.(1) Nutrient concentrations in fine roots varied with nutrient elements and tree species. For example, C concentrations were between 37.1% and 40.6% with the highest value for C. glauca and the lowest value for L. formosana, while other four species were ranked as order from high to lower as P. massoniana, C. axillaris, L. glaber and C. lanceolata. Fine roots of six species had the N concentration between 1.2% and 1.7%. The highest was found in C. lanceolata, while the lowest was found in P. massoniana. The order from high to low was as C. axillaris, L. formosana, L. glaber and C. glauca. The concentration of Pin fine root ranged from 0.26 g.kg-1 to 0.81 g.kg-1, with the largest value for L. formosana and the lowest for C. glauca. Other four species was ranked from hight to low as C. axillaris, C. lanceolata, P. massoniana, and C. glauca. The Ca concentration in fine roots was between 1.65 g.kg-1 and 6.14 g.kg-1. The highest was for L. formosana and the lowest for C. lanceolata. Other four speceis were ranked as order C. axillaris, L. glaber. P. massoniana, and C. glauca. The C/N ratio ranged between 22.12 and 33.08, with the highest value for P. massoniana and the lowest for C. lanceolata. The C/P ratio varied from 456.25 to 1473.88, with L. formosana having the highest value and L. glaber having the lowest values. The N/P ratio was between 17.22 and 49.38, with the highest value for L. glaber and the lowedt for L. formosana.The cellusose concentration in fine roots of six species ranged from 16.85% to 27.93%, with the highest for C. axillaries and the lowest for C. glauca. Other four species were ranked as order from high to low as L. formosana, L. glaber, C. lanceolata, and P. massoniana. Significant difference was found between C. axillaries and P. massonian, C. glauca. The lignin concentration in fine roots were between 30.87% and 47.34%. C. glauca had the highest value while L. formosana had the lowest value. Other four species were in the decreasing order as P. massoniana, C. lanceolata, C. axillaries and L. glaber. They had significant different values from C. glauca and L. formosana. The ratio of lignin to N was between 22.05 and 36.42, with the highest value for C. glauca and the lowest for C. lanceolata and L. formosana. The ratio of lignin to P was between 381.1 and 1578.0. C. glauca had the highest value and L. formosana had the lowest value.(2) Fine root decomposition of the six species exhibited a pattern of rapid mass loss at early stage and then a slow mass loss process at late stage. After 180 days of decomposition, the percentage of remaining mass was high for C. lanceolata (80.68%), L. glaber (81.72%), and C. glauca (81.12%), while was low for P. massoniana (73.76%), L. formosana (73.32%), and C. axillaries (79.25%). However, the differences in the percentage of remaining mass among the six species were not significant (p=0.4829, F=0.9332). Olson formulation was used to fit the percentage of remaining mass and a significant level was achieved. The square determined coefficients were low with range from 0.02 to 0.16. The fitted decomposition constants were higher for P. massoniana (0.00247), L. formosana (0.00278), and C. axillaries (0.00294), compared with C. lanceolata (0.00186), L. glaber (0.00151), and C. glauca (0.00176). This result indicated that fine roots of early successional tress species (P. massoniana, L. formosana, and C. axillaries) decomposed faster, while late successional tree species (L. glaber and C. glauca) and C. lanceolata decomposed slower.(3) During the fine root decomposition process, C concentration increased between 0 to 30 days and then decreased after 60 days. The tendency for N concentration in fine root was that an increase occurred at early stage and nutrient release took place. C. lanceolata and C. axillaries increased their N concentration in fine roots at early stage and remained stable, other four species decreased N concentration in fine root after 60 days. This phenomenon could be attributed to N accumulation by microbial absorption from soil. The changes in P concentrations in fine roots during decompostion varied with species, with low changes for C. lanceolata and continunous decrease for L. formosana and C. axillaries. P. massoniana, L. glaber and C. glauca showed an increase in P concentration in fine roots during 30 days or 60 days and then a decrease was found. There was an increase for Ca concentration in fine roots and then a low variation for Ca concentration.(4) Leaf litters of P. massoniana, L. formosana and C. axillaries decomposed faster while that of C. lanceolata, L. glaber and C. glauca decomposed slower. The fitted Olson equation showed a significant level with determined coefficient r2 ranging from 0.10 to 0.67. The relationship of decomposition constants between leaf litter and fine root was significant (p=0.0012) and the determined coefficient r2 was 0.978. This result indicate that fine root decomposition had a good relationship with leaf litter decomposition, but deomposition rate of fine root was about half time of leaf litter. This low decomposition rate could be due to low nutrient concentration in fine root and infavoriable soil condition belowground.(5) Fine root deomposition constants showed a parabolic relationship with initial N concentration in fine root and negative linear relationship with C/N ratio. But the relationships were not significant and the determined coefficients were low. Decomposition rate of fine root increased with increase in P concentration and decreased with increase in C/P and N/P ratio. The relationship was significant and the determined coefficients r2 were 0.48,0.46 and 0.64, respectively. Decomposition rate of fine roots also increased with increase in Ca concentration. The relationship was significant and the determined coefficient r2 was 0.55. Our results indicated that P and Ca concentrations were the main factors for fine root decomposition but N and lignin concentration had low effects on fine root decomposition at early stage. |
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