| Grassland ecosystem is one of the most widely distributed vegetation type and is also the most important and most widely carbon distribution ecosystem,plays an important role in global carbon cycle and climate regulation.Songnen grassland,located in China’s Northeast Plain,is a typical meadow steppe.But natural and human factors causes the salinization and the degree of salinity increases year by year.Regarding to the restoration of salinization of Songnen grassland,many scientist shave done a lot of work,which mainly focused on aboveground species cover,species density,biomass or physical and chemical properties of soil etc.It is a surprise that there was no study that has been carried out from the whole plant perspective,especially from belowground net primary productivity(BNPP)and root turnover.In this work,we studied the heavily saline-alkali grassland in Changling county,Jilin province.Six experimental treatments was employed and randomly arranged as follows:free grazing(FG),no ploughing(NP),only ploughing(OP),ploughing + fertilizer(PF),ploughing + mulching(PM),ploughing +mulching + fertilizer(PFM).Each treatment was repeated four times.BNPP was determined using root ingrowth Donuts and root turnover was determined using improved root window’s method.From the whole plant perspective,we explored the changes of aboveground and belowground in the restoration process,especially for the response of root turnover to different restoration measures.The results will fill in the gap of the response of roots in belowground ecological restoration theory and enrich the knowledge of belowground grassland ecology.According to the results of 2011,2012 and 2013,the results was summarized as follows:1.The annual vegetation of Chloris virgata and Suaeda glauca are dominant species in the first two restoration years,and the relative density of Chloris virgata is 32-99%,and Suaeda glauca is 3-60%,no perennial vegetation was found.The vegetation cover,species density and aboveground net primary productivity(ANPP)of PM and PFM treatments are significantly higher than other treatments.Chloris vigata is the single dominant species in those treatments(relative density is more than 80%and aboveground cover can reach 50%).those results implied that PM and PFM are favor of growth and restoration of aboveground vegetation;2.The maximum BNPP values of 2012-2013 of different restoration treatments are 28.4 g m-2 and 15.1 g m-2,and the minimum values are 0.8 g m-2 and 0.7 g m-2.Due to only existence of annual vegetation and no perennial vegetation,the restoration is not stable.Inter-annual effect on BNPP is significant(P<0.05),BNPP dramatically decreased in the second year(2013)and BNPP of 2012 year is 1.6-1.9 times of 2013 year.But there was no significant difference for ANPP(P>0.05),indicating that more biomass is allocated to the aboveground during the restoration process.The main mechanism is that the soil organic matter was less,salinity and pH were high,which was difficult for plants roots growth.In order to resist this harsh environments,plants have to choose to increase the aboveground growth,and producing more photosynthetic products for their own growth and reproduction.This is the main growth strategy for annual vegetation.There was significant difference for BNPP among different treatments(P<0.001),BNPP of PM and PFM treatments are higher than other treatments,which results in the favor of underground growth;3.There was about 40%root death after occurring two weeks,then death rate gradually reduces,and a little difference between root survival curve of different restoration measures.The roots of spring and summer birth easily died and the remainder is only up to about 20%at end of autumn,and 40%roots born in autumn still existed until winter.The spring and summer are main period of seed germination and fast-growing,these roots will die after absorbing nutrients,while root respiration rate slows as autumn temperature drops,and therefore the death rate is lower;4.Root turnover only has one peak and the peak is in the summer(July),whatever 0-10 cm or 10-20 cm soil in 2013.Root turnover is faster in summer,slower in spring and autumn.In the growing season July,rainfall and temperatures reach the peak of year and plants grow fast,therefore root turnover increases and reaches a peak of year.Root turnover of 2013 of different restoration treatments were between 0.46 ± 0.03 and 0.55 ± 0.05.Root turnover of OP,PF and PFM treatments is lower,which indicate more biomass accumulation and better growth conditions;5.Different restoration measures have no significant effect on root turnover of 0-20 cm soil.But for 0-10 cm soil,average root lifespan of PM and PFM treatments are higher than FQ OP and PF treatments,while average root lifespan of PM and PFM treatments are higher than OP and FG treatments in 10-20 cm soil.Root turnover of 10-20 cm soil(0.52 ± 0.02)is higher than 0-10 cm soil(0.47 ± 0.02).This shows that different restoration measures have no effect on root turnover in a short time,but soil layers affect average root lifespan(P<0.001).PM and PFM treatments can improve plant growth environment,be favor of colonization and growth of surface roots and extend root lifespan which help the restoration of saline-alkali grassland;From the results of three years,we get the following conclusions:from the whole plant perspective,ploughing can improve soil structure,increase soil porosity,reduce salt of surface soil and mulching can intercept plants seeds,increase the number of seeds in seed bank and organic matter of surface soil,reduce evaporation of surface soil,increase species cover,so the combination of this two restoration measures are favor of restoration of saline-alkali grassland;but the assumption of little fertilizer(25 kg N ha-1)will accelerate restoration does not hold;Various combinations of restoration measures can only improve physical and chemical properties of surface soil(0-2 cm),reduce soil pH and salinity,and are not obvious for deep soil,so rooting and germination of perennial vegetation are very difficult.This is the reason of no perennial vegetation in the restoration process.In summary,ploughing and mulching reduce the salinity and pH of surface soil(0-2 cm),increase the soil organic matter,obtain the higher ANPP,BNPP,vegetation coverage and species density,so these are suitable restoration measures for annual vegetation in heavily degraded saline-alkali grassland.Therefore,restoration and reconstruction of Songnen saline-alkali grassland not only follow the law of nature,but also consider the technical feasibility and economic investment.We hope our work can provide some experimental basis for predicting the trends of belowground biomass in complex environmental factors and guide people to improve and mitigate salinization,extend grassland sustainable development. |
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