Distribution characters of fine root of artificial Pinus tabulaeformis and natural Betula platyphylla forests and their relation to soil nutrients in Hilly Loess Regions

Fine root turnover plays an important role in below-ground carbon budget and affects the nutrient cycling in the terrestrial ecosystem.The vertical distribution characteristic of fine root biomass,specific root length(SRL),root lengthy density(RLD) and root surface area of artificial Pinus tabulaeformis and natural Betula platyphylla forests,and their relationship with soil moisture,bulk density,nitrogen and content of organic matter in Ziwuling of Loess Plateau were studied.The results indicated that vertical distribution of artifical Pinus tabulaeformis and natural Betula platyphylla forests showed different trends: fine root biomass of artifical Pinus tabulaeformis forests showed a single hump-shaped curve with soil depth increased,but fine root biomass of Betula platyphylla forests decreased with soil depth increased.Fine root biomass of Pinus tabulaeformis mostly distributed in the soil layer(040 cm),among which 37% of fine root biomass was distributed in the upper soil layer(0-20 cm),and 41% of fine root biomass was distributed in the soil layer(20-40 cm);root surface area,SRL and RLD of artificial Pinus tabulaeformis in the upper soil layer(0-20 cm) were greater than these in the lower two layers(20-40 cm,40-60 cm).Root surface area,SRL and RLD in the deep layer(40-60 cm) were the lowest compared to the upper two soil layers(0-20 cm and 20-40 cm).Total soil nitrogen and content of organic matter of Pinus tabulaeformis forests among different soil layer decreased with soil depths increased.Nitrate(NO3--N) showed a single hump-shaped curve with soil depth increased,but ammonia(NH4+-N) showed a parabola curve,with lowest value found in 20-40 cm soil layer and highest value in 0-20 cm and 40-60 cm soil layer.75% of fine root biomass of Betula platyphylla was distributed in the upper soil layer(0-20 cm),vertical changes of root surface area,SRL and RLD of Betula platyphylla forests showed a similar trend regardless soil depths with artificial Pinus tabulaeformis forest.Root surface area of Betula platyphylla in upper soil depth(0-20 cm) was 3.91 times higher than that of artifical Pinus tabulaeformis,but root surface area of Betula platyphylla in the soil layer(20-40 cm) was 33% lower than that of artifical Pinus tabulaeformis.Vertical changes of total soil nitrogen,content of organic matter, NO3--N and NH4+-N of Betula platyphylla among different layers presented a similar trend with artificial Pinus tabulaeformis forest.Distribution characteristics of fine root of artificial Pinus tabulaeformis and Betula platyphylla forests more significantly related to the changes of soil moisture,bulk density,total nitrogen and content of organic matter than ammonia(NH4+-N) and nitrate(NO3--N).The fine root biomass was significantly correlated to the content of organic matter of Betula platyphylla forests in upper soil layer(020 cm).The correlation coefficient of root surface area of Pinus tabulaeformis and Betula platyphylla forests with total soil nitrogen decreased with the soil depth increased(0-40 cm),but the correlation coefficient of SRL and RLD with total soil nitrogen increased with the increasing of soil depth.Fine root biomass,root surface area and RLD increased with the decreasing content of organic matter in soil layer (20-60 cm),but SRL presented a reversed trend.