Study On Fine Root Distribution Of Two Artificial Robinia Pseudoacacia Forest In Yangou Watershed In The Hill Loess Plateau Region

The fine root spatial distribution can reflect the functions of the trees that adapt and improve the environment. In this paper, the 10a and 25a artificial robinia pseudoacacia root samples were taken in the growing season in Yangou watershed of hilly Loess Plateau region by drilling the soil core. The spatial distribution characteristics of the root simple (fine root and coarse root) biomass, specific root length(SRL), root length density(RLD), the root surface area index and the relationship between these root parameters and the soil physical factors(e.g. soil moisture content, soil temperature and soil bulk density) and soil nutrients were studied. The results showed that:(1) The standing biomass characteristic of the the artificial robinia pseudoacacia root was studied: the standing biomass of the two stand age robinia pseudoacacia root were 11316.54 g.m^-2and 17621.10g.m^-2respectively, in which the living fine root biomass accounted for 16.59% and 11.73% respectively, the living coarse foot biomass accounted for 81.52% and 86.16% respectively, the diving fine root biomass accounted for 1.89% and 2.11% respectively. Meanwhile, the bigger the stand age was and the larger the diameter of root was, the greater the root spatial distribution heterogeneity was.(2) The seasonal dynamics of the artificial robinia pseudoacacia root was studied: in the whole growing season, both stand ages roots (fine root and coarse root) biomass had two significant peaks in summer (July) and autumn (October) respectively and the seasonal variation difference reached the significant level ( except the 25a coarse root biomass) (p<0.05); the seasonal variations of the root length density (RLD) and the root area index were the same as the biomass's, which showed"double peak", the seasonal variation of the 10a root had no significant difference (p>0.05) while the 25a roots'was significant (p<0.05); the specific root length was the type of"single peak", which appeared in summer, and the values were 293.15 m·g^-1and 289.47 m·g^-1respectively, whose seasonal variation reached the most significant or significant level.(3) The spatial distribution of the artificial robinia pseudoacacia fine root was analyzed: the quantity of both stand age robinia pseudoacacia fine roots decreased layer by layer within 0²00 cm soil layer (where soil can be divided into five layers: a, b, c, d and e layer). The changes of the fine root biomass, RLD and the root area index with the depth could be described by the negative exponential function. The root distributed densely in the 0⁶0 cm soil layer and all peaks appeared in the 0²0 cm, accounted for 42.56﹪and 46.66﹪, 47.12﹪and 50.00﹪, 43.44﹪and 47.60﹪of the 0⁶0 cm soil layer respectively. For the SRL, the downtrend with the depth was not obvious and appeared repeatly in the 60¹40 cm soil layer. The relationship between the SRL and the soil depth can be described by the cubic polynomial. With the increase of the horizontal distance to the tree trunk base, the fine root biomass, RLD and the root area index decreased gradually, while the SRL increased firstly and then decreased, and the maximum appeared at 80cm away from the tree trunk base, but variation parameter changed smaller in the plant spacing.(4) The root distribution parameterβof the artificial robinia pseudoacacia forest was researched: the both stand age fine root extinction coefficient decreased with the increase of the horizontal distance to the tree trunk, and the maximum appeared at 40cm, which was 0.989 and 0.990 respectively, and the minimun appeared at 160cm, which was 0.983 and 0.986 respectively. The change of coarse root extinction coefficient was the same with the fine root's, and the maximum appeared at 40cm, which was 0.984 and 0.985 respectively, and the minimun appeared at 120cm, which was both 0.982. For the two stand age robinia pseudoacacia, the extinction coefficientβof fine root was greater than the coarse root, the 25a root extinction coefficient was greater than the 10a root, meaning that the distribution depth of fine root was greater that the coarse root, root depth increased with the stand age increasing, which beneficial for robinia pseudoacacia to the absorption and utilization of deep soil water and nutrient.(5) The relationship between fine root of artificial robinia pseudoacacia and the soil environment factors was analyzed: the degree order of soil physical factors influence on fine root of artificial robinia pseudoacacia was: Soil moisture content>soil bulk density>Soil temperature, and the order of soil chemical factors influence degree was: Soil total N> organic matter> Soil Ammonium> nitrate. The multivariate linear regression model between root parameters and environmental factors was established and the model achieved 95% significant level.