| Fine roots,usually considered to be roots ≤ 2 mm,are important functional organs of plants and the most vigorous part of plants.They are responsible for obtaining necessary soil resources.Fine roots usually have a short life span and strong absorption capacity.Nowadays,most related researches to assess the impact of biodiversity loss on ecosystem functions focus on grassland ecosystems or forest above-ground ecosystems.It is still unclear how species diversity affects the dynamic changing process in the below-ground ecosystem.This study relies on 27 plots of the China Subtropical Forest Biodiversity and Ecosystem Function Research Platform(BEF-China)to monitor the production,standing volume,and turnover of fine roots of 14 tree species(Castanea henryi,Nyssa sinensis,Liquidambar formosana,Sapindus saponaria,Triadica sebifera,Choerospondias axillaris,Quercus serrata,Castanopsis sclerophylla,Cyclobalanopsis glauca,Quercus fabri,Schima superba,Rhus chinensis,Lithocarpus glaber,Koelreuteria bipinnata)through minirhizotrons,and calculate the net effect(NE),compensation effect(CE)and selection effect of fine root dynamics.The measurement of soil factors and root and leaf traits helps to discover their change pattern in different species richness(1,2,4,8),different root distribution combinations(deep root-deep root,deep root-shallow root,shallow root-shallow root),and different soil layers(0-10 cm,10-20 cm,20-30 cm).This study also explores how tree species diversity and soil factors affect root and leaf functional traits,and further affect root system dynamics,with a view to revealing the impact mechanism of tree species diversity on the dynamic variation of below-ground root systems in subtropical forests.The main results are as follows:(1)There were significant differences(P < 0.05)in soil factors across different species richness,root distribution combinations,and soil layers.8-species mixtures had the lowest nutrient and water content and storage.The deep root-deep root combination had the most soil water,while the shallow root-shallow root combination had the most soil nutrients.Soil water and nutrient content decreased with soil depth,whereas soil water storage increased with soil depth;Soil C and N nutrient storages decreased with soil depth.In communities with high species richness,soil water and nutrients were more evenly distributed.In monoculture which the species richness is 1,the distribution of soil water is the shallowest,while the distribution of soil C and N nutrients was the deepest in mixtures which the species richness is 8.(2)In monoculture,due to different characteristics of tree species,there were usually significant differences in specific root length,root diameter,root N content,specific leaf area,leaf dry matter content,and leaf N and P nutrients among different tree species.Specific root length and root N content decreased with the increase of root order,while root diameter increased with the increase of root order.In terms of root and leaf traits at the community level,monoculture had the highest specific root length and root nutrient content,while 2-species mixtures had the highest root diameter.The leaf traits of 4-species mixtures were the smallest.The redundant analysis showed that there was a certain trade-off and synergy between root and leaf traits.Root traits had the highest explanation for variations in soil factors.In the 0-10 cm soil layer,the interaction of root and leaf traits could explain 3.62% of the variation in soil factor.In the 10-20 cm soil layer,the interaction of root and leaf traits could explain 6.1% of the variations in soil factor.In the20-30 cm soil layer,the interaction of root and leaf traits could explain 10.26% of the variations in soil factor.As the soil depth increased,the degree of explanation of the interaction of root and leaf traits increased.(3)There were usually significant differences in fine root dynamics and biodiversity effects across different tree species.In monoculture,the shallow root-shallow root combination in the same soil layer had the highest fine root production and fine root standing volume and the fastest fine root turnover,which gradually decreased with the increase of soil depth.In mixtures,there were also significant differences in fine root production,alive fine root amount and fine root turnover among different species richness,different root distribution combinations and different soil layers.Compared with monoculture,the deep root-deep root combinations in mixtures accelerated the fine root production,fine root standing volume,and fine root turnover,while the deep root-shallow root combinations slowed down the fine root production and turnover.(4)Root and leaf traits and soil factors had different effects on fine root dynamics and biodiversity effects across different soil layers.In the 0-10 cm soil layer,the specific root length of absorbing roots was positively correlated,and the specific leaf area was negatively correlated,with NE and CE of fine root production and fine root standing volume.Leaf dry matter content and leaf N content were positively correlated with NE and CE of fine root turnover,while specific root length of absorption root was negatively correlated with NE and CE of fine root turnover.In the 10-20 cm soil layer,soil C storage was negatively correlated with NE and CE of fine root production and fine root standing volume,and specific leaf area was positively correlated with NE and CE of fine root turnover,while specific root length of absorbing root was negatively correlated with NE and CE of fine root turnover.In the 20-30 cm soil layer,the soil water storage was negatively correlated with NE and CE of fine root standing volume.The increase of soil C content increased NE and CE of fine root production and turnover,while leaf dry matter content was negatively correlated with NE and CE of fine production and turnover.In the0-30 cm soil layer,the heterogeneity of soil C and N storage were positively correlated with NE and CE of fine root production and NE of fine root standing volume,indicating that the heterogeneity of soil nutrient resources is conducive to the fine roots production and helps to speed up root turnover in mixtures with different niches.The higher the soil N storage,the slower the fine root turnover in mixtures became.RDA analysis showed that root trait had higher explanation degree on fine root dynamics and biodiversity effects compared with that on soil factors.In summary,soil factors and root and leaf traits usually have significant differences among different species diversity levels,and the root traits have higher explanation compared with leaf traits on soil factors in all soil layers.With the increase of soil depth,the synergy of root and leaf traits gradually strengthen.In terms of fine root dynamics,the deep root-deep root combination promotes fine root production,standing volume and turnover due to the competition of the root system for soil resources,and the deep root-shallow root combination reduces the fine root production and extends the fine root lifespan,due to the niche complementation caused by the vertical root distribution.Root and leaf traits and soil factors have different effects on fine root dynamics and biodiversity effects across different soil layers,but root trait is the main driver for fine root dynamics and biodiversity effects.Therefore,as the species richness increases,tree species with different root system combinations in mixtures can produce morphological adaptations through the heterogeneous distribution of water and nutrient resources in different soil layers,and achieve the complementary effects of resources through the trade-off and synergy of root and leaf traits to drive the fine root production and turnover.In view of the important role of fine root production and turnover in the carbon sink and nutrient cycle of forest ecosystem,this study is expected to provide scientific basis and theoretical guidance for the improvement of subtropical forest underground productivity and carbon sink management under the background of global change. |
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