The Belowground Productivity And Ecological Stoichiometry In Pinus Tabulaeformis And Robinia Pseudoacacia Plantation Forests On The Loess Plateau

Fine roots(? 2 mm in diameter)are main organs for water and nutrient uptakes in plants,and can be major contributors to belowground carbon allocation and nutrient cycling in forest ecosystems.With the deepening of research on global carbon cycle and as the main source of soil carbon in forests,the study of fine roots has attached considerable attention.The ecosystem of Loess Plateau of China plays an important role in regulating the global carbon cycle and climate change.However,the field of fine root productivity and ecological stoichiometry remains less understand in this area.This study was based on the issues of global change ecology and biogeochemical cycle research.By the methods of soil cores and ingrowth soil cores,we studied the fine root productivity and ecological stoichiometry characteristics of Pinus tabulaeformis and Robinia pseudoacacia plantation forests on the Loess Plateau.The factors that are associated with fine root dynamics and distribution were analyzed.These results can provide scientific basis for the adaptive management of plantation forests,and also provide data support for evaluating the carbon sink capacity of forests in the study area.The main results are as follows:(1)The fine root biomass of P.tabulaeformis and R.pseudoacacia varied significantly with season and soil depthThe maximum value of fine root biomass appeared in May and the minimum value occurred in July due to the variation of temperature and precipitation.Fine roots decreased with soil depth in the soil layer of 0–180 cm.Fine root biomass was mainly distributed in the 0–60 cm soil layer and was the highest in the 0–20 cm soil layer.Fine root biomass was positively correlated with soil temperature,indicating that the soil temperature had significant influence on distribution of fine roots.The amount of live fine roots accounted for more than 70 percent of the total.The proportion of ?1 mm fine root biomass was 57%–65% in the 0–60 cm soil layer,suggesting that fine roots that smaller than 1 mm in diameter were of great importance.(2)Fine root biomass and production of P.tabulaeformis decreased with stand age,but those of R.pseudoacacia increased with stand ageThe fine root biomass of P.tabulaeformis forests with different ages was 10 a(653 g m-2)> 25 a(589 g m-2)> 40 a(433 g m-2),and that of R.pseudoacacia forests was 10 a(107 g m-2)< 25 a(278 g m-2)< 40 a(478 g m-2).The fine root productivity of P.tabulaeformis forests was in the order of 10 a(564 g m-2 a-1)> 25 a(362 g m-2 a-1)> 40 a(165 g m-2 a-1),and that of R.pseudoacacia forests was 10 a(83 g m-2 a-1)< 25 a(112 g m-2 a-1)< 40 a(257 g m-2 a-1).The average fine root biomass and productivity of P.tabulaeformis were greater than those of R.pseudoacacia.These results indicated that the stand age and tree species were important factors affecting the fine root productivity.The fine root turnover rate of two tree species was greater in 10-year-old(0.86 and 0.78 a-1)stands than in 25-(0.62 and 0.40 a-1)and 40-year-old(0.38 and 0.54 a-1)stands,suggesting that fine root turnover is faster in younger stands.Soil organic carbon content and total nitrogen showed a significant and positive correlation to the fine root biomass of both tree species,indicating that fine root biomass is closely related to soil nutrient conditions.The root growth was usually limited when the soil bulk density increased,leading to the decrease of fine root biomass.The fine root biomass of R.pseudoacacia was negatively correlated with soil water content.(3)There were significant differences in carbon: nitrogen: phosphorus ecological stoichiometry of fine roots between different soil layers,stand ages and tree speciesThe carbon(C),nitrogen(N)and phosphorus(P)concentrations of fine roots were usually greater in the upper soil layer than in the subsoil layer.The fine root C,N and N:P ratio of R.pseudoacacia increased with stand age.The fine root P and C:P ratio of both tree species were significantly influenced by the stand age.The C,N and C:N ratio of fine roots showed nearly the same age-related patterns to those of green leaves.The fine root N of R.pseudoacacia stands was 24.69–30.26 g kg-1,which was greater than that of P.tabulaeformis stands(3.55–4.39 g kg-1).It indicated that the N-fixing tree of R.pseudoacacia could obtain more N resource than the non N-fixing tree of P.tabulaeformis.R.pseudoacacia had lower average fine root C:N(16.0 vs.123.4)and C:P(605.1 vs.806.3)but greater average fine root N:P(38.1 vs.6.8),when compared to those of P.tabulaeformis.The fine root stoichiometry and soil nutrients were tightly coupled.The fine root P of P.tabulaeformis was positively correlated with soil total phosphorus and available nitrogen.The fine root C of R.pseudoacacia had significant positive correlation with soil total and available nitrogen.The fine root C:P of both tree species increased linearly with soil C:P.The fine root N:P of R.pseudoacacia was positively associated with soil total and available N:P.(4)The fertilization treatment had no significant influence on fine root productivity of P.tabulaeformis or R.pseudoacacia forests,while the N and P concentrations and their ratios of fine roots changed significantly following fertilizationThe fine root biomass,productivity and turnover rate had no significant changes with three fertilization treatments(N,P and N+P additions)in P.tabulaeformis or R.pseudoacacia forests.Compared to the control,the fine root N concentration was respectively increased by 47%,44% and 55% with the N,P and N+P additions in 10-year-old P.tabulaeformis,while it was respectively decreased by 20%,16% and 17% with the N,P and N+P additions in 40-year-old R.pseudoacacia.The fine root P concentrations of 10-and 40-year-old P.tabulaeformis were reduced 9% and 13% with N addition compared to the control.The fine root P concentration was respectively enhanced by 58% and 74% with P and N+P additions in 10-year-old R.pseudoacacia,and it was respectively enhanced by 29% and 43% with P and N+P additions in 40-year-old R.pseudoacacia.The fine root N:P ratios of P.tabulaeformis stands increased significantly with N addition(P<0.05),while those of R.pseudoacacia stands decreased significantly with P addition(P<0.05).