Pool Size And Dynamics Of Soil Available Nitrogen And Plant Uptake Characteristics In Eight Forest Types In Northeast China

The availability of soil nitrogen(N)often limits the growth and development of plants and the productivity of many ecosystems.More and more plants are identified to be able to take up soil organic N,and therefore,the traditional N cycling system of plant-soil is necessary to be revised.Knowledge about soil organic N and the uptake characteristics of soil N by plants is limited,especially in the context of global change.Based on this,the study chose typical forest ecosystems in northeast China.and two serial experiments were conducted.With the use of dual-labeled stable isotope,ecologically important issues,e.g.the pool,component and variation of soil dissolved organic N and the uptake characteristics of various forms of N by plants in these forest ecosystems,were explored and illustrated.The first experiment chose six typical forest types in northeast China in 2016.Soil inorganic N,dissolved organic N(DON),components of amino acids,microbial biomass N and fundamental soil physicochemical properties were measured continuously in growing season for five months(May-September).This investigation aims to explore,in typical forest types of this region:a)the pool of soil dissolved N;b)the components of soil dominant amino acids;c)variation of soil dissovled N across plant communities and seasons,and the potential drivers.Based on the first experiment,the second experiments chose two typical forest types in the region,and investigated eight plant species.With the use of stable isotope tracer technique(13C and 15N).isotopic tracer solutions(NH4+,NO3-and glycine)were injected into soils in the field,and these experiments were conducted three times.The second experiments aim to explore:a)whether plants in this region can take up soil organic N;b)whether the uptake of soil N by plants results from preference or opportunistic response;c)Niche differentiation of plants in the uptake of soil N through N form;d)Tempoal niche differentiation of plants in the uptake of soil N.The primary results were as follows,in typical forest ecosystems in northeast China:(1)There was considerable amino acids N pool in soils.The content of soil DON in different forest types ranged from 64.5 to 90.9 ?g N/g,the coefficient of variation was 39%,and 3.4%-13.7%was composed by free amino acids N.The content of soil free amino acids ranged from 2.58 to 6.66 ?g N/g,and the coefficient of variation was 50%;the content of NH4+ranged from 3.21 to 11.29 ?g N/g,and the coefficient of variation was 55%;the content of NO3-ranged from 2.05 to 7.07 ?g N/g,and the coefficient of variation was 50%.The pool of soil free amino acids N was in the same magnitude as inorganic N.(2)The components of dominant soil amino acids were different among forest types.Soil dominant amino acids accounted for 47.8%-60.9%of total amino acids,and they included arginine(0.45-1.24 ?g N/g),histidine(0.34-0.94 ?g N/g),serine(0.32-0.64 ?g N/g),leucine(0.26-0.53 ?g N/g),aspartic acid(0.13-0.59 ?g N/g).glycine(0.11-0.35 ?g N/g),glutamic acid(0.14-0.40 ?g N/g)and proline(0.16 ?g N/g).Arginine.histidine and aspartic acid were the dominant soil amino acids that commonly existed in six forest types,and proline was only dominant in the soils of Fraxinus mandshurica plantation.Although the components of soil dominant amino acids were significantly different among forest types,basic amino acids(arginine and histidine)existed continuously in six forest types.This suggests that the adsorption of soil solid phase could potentially play an important role in the cycling of amino acids N in soils.(3)Although forest community influenced the pool size and component of soil dissolved N,the influence was weaker than the regulation of season.First,there were marked differences in the contents of soil dissolved N between forest types,but the differences varied with the change of season;second,the coefficients of variation among forest types ranged from 39%to 69%,which tends to be smaller than that among months(61%-94%);third,soil dissolved N showed a consistent variation tendency with season.The average contents of NH4+,NO3-and amino acids N were 14.79,10.67 and 9.16 ?g N/g at the beginning of growing season,respectively,and were 6.34,2.97 and 3.49 ?g N/g at the height of growing season,respectively,decreasing by 57%,72%and 62%,respectively.Plant phenology and soil potential drivers(e.g.moisture,microbial biomass-N and DON)together drove the seasonal dynamics of soil inorganic N and amino acids N.(4)Plant species can take up soil organic N.In mixed broad-leaved forest,48%,55%,80%and 99%of glycine-derived N was taken up directly from glycine by Betula platyphylla,Ulmus davidiana,Prinsepia sinensis and Sphagnum palustre,respectively.Although excess 13C was not detected in the roots of Acer ginnala,it could take up glycine-derived N.In Larix gmelinii forest,97%,88%,60%,71%and 58%of glycine-derived N was taken up directly from glycine by Larix gmelinii,Betula platyphylla,Rhododendron simsii,Rhododendron tomentosum and Sphagnum palustre,respectively.After the injection of glycine solution into soils,the mineralization of glycine by soil microbes and extracellular enzymes,and the subsequent respiration of plant roots following uptake,could potentially lead to the loss of 13C of glycine tracer.(5)The uptake of different soil N forms by plant roots resulted from opportunistic response but not preference.First,with the change of soil environment and neighbouring plant species,plants showed different uptake patterns of NH4+,NO3-and glycine N.For example,in mixed broad-leaved forest(August 2016),Betula platyphylla showed a higher uptake rate of glycine N,and no difference was observed between the uptake rate of NH4+ and NO3-;in Larix gmelinii forest,Betula platyphylla showed a higher uptake rate of NO3-,and no difference was observed between the uptake rate of NH4+ and glycine N.Second,with the change of month(July and August 2018).plants of the two forest types also changed the uptake pattern of different soil N forms.For example,the uptake rates of three N forms by Rhododendron tomentosum were NH4+>glycine N>NO3-in July,while it appeared as glycine N>NH4+>NO3-in August.Therefore,the uptake of different soil N forms by plant roots was highly plastic,and this could be beneficial for plant species to acclimate to different habitat environments and alleviate competition between species.(6)N niche differentiation phenomenon existed for co-existing plant species.First,N niche differentiation appeared in the uptake of different soil N forms.The isotope tracer experiments were conducted three times for the study,and the uptake patterns of different forms of soil N by co-existing plant species were discovered to be different.In Larix gmelinii forest(August 2016),for example,the uptake pattern of N by Larix gmelinii appeared as NO3->glycine N>NH4+;Betula platyphylla also showed a higher uptake rate of NO3-,and no difference was observed between NH4+ and glycine N;the uptake pattern of N by Rhododendron simsii and Rhododendron tomentosum appeared as NH4+>NO3->glycine N;Sphagnum palustre also showed a higher uptake rate of NH4+,but no difference was observed between NO3-and glycine N.Second,N niche differentiation appeared in the temporal uptake of soil N.From July to August in 2018,the pattern of the change of N uptake rate among co-existing plant species appeared to be different.In mixed broad-leaved forest,for example,the uptake rate of NH4+ by plants varied with month:Betula platyphylla increased,Acer ginnala,Prinsepia sinensis and Sphagnum palustre decreased,while Ulmus davidiana showed no significant difference.With the complementary use of soil limited N resources by co-existing plant species,it could be beneficial for species' co-existence,and for maintaining community's stability and ecosystem functioning.As mentioned above,amino acids N and inorganic N compose soil available N resources in forest ecosystems in northeast China,which can be potentially important for the growth and development of plants.Compared with forest community,season can show a stronger role in regulating the pool size and composition of soil inorganic and organic N.The adsorption of soil solid phase,however,can also play an important role in the cycling of soil amino acids N.Plants can utilize various N resources in soils,and meanwhile,their phenology can be strongly influenced by season.Therefore,the uptake of soil N by plant roots mainly results from opportunistic response but not preference,and is highly plastic.Based on this,co-existing plant species can establish the mechanism of N niche differentiation to complete the complementary use of soil limited N resources.For example,N niche differentiation can appear in the uptake of different forms of soil N and in the temporal uptake of soil N.This can alleviate competition for N by co-existing plant species in forest community,and can be beneficial for the stability of community and ultimately for maintaining the functioning of ecosystem.This research investigated the availability of soil N in typical forest ecosystems in northeast China,and explored some key processes in the cycling of plant-soil N.We hope that this research can add some knowledge for the new system of soil N cycling developed after 1990s,and can also play a role in the forest management of the region in the future.