| Glyphosate was developed by Monsanto Company in 1974.As a non-selective,broad spectrum,systemic and strongly conductive herbicide,glyphosate has been widely applied to weed control in agriculture and forestry and aquatic environment,especially used in research about transgenic crops in recent years.Glyphosate became the most widely-used herbicide in forestry slash disposal took the place of controlled burning,because controlled burning was forbidden in many regions which resulted in ecological problems such as soil erosion and soil degradation in southern China.As an organic pollutant,glyphosate might pollute soil environment and forest products because of toxicity and high residue although it has greatly increased efficiency of weed control and promoted forestry productivity.Chinese fir(Cunninghamia lanceolata(Lamb.)Hook)is one of the south advantage with timber species,with fast-growth,good in material,extensive use and soon,which play an important role in forestry production.The deficiency of available phosphorus has been one of the main factors which resulted in soil fertility seriously declined in Chinese fir forest land and posed threat to the sustainable development of Chinese fir plantation over time.As a consequence,there is an inevitable coexistence of glyphosate residue and deficient phosphorus stress while a large dose of glyphosate was used in southern Chinese fir forest land.A large number of studies show that there might be a competition between glyphosate and phosphorus for adsorption sites in soil,which could affect chemical behavior and availability of phosphorus in soil and phosphorus utilization of Chinese fir.In view of this,in order to compare effect of glyphosate on phosphate adsorption and desorption process in different acidic soils,and reveal the influence of glyphosate on phosphorus availability in different soil,and understand the morphological and physiological response of Chinese fir to glyphosate,and explore the functional mechanisms of glyphosate to phosphorus utilization of Chinese fir,we design adsorption isothermal experiment and soil culture experiment and pot experiment.This study will provide a scientific theoretical basis for reasonable use of glyphosate in forestry production.The findings are as follows:(1)Phosphate quantity absorded by yellow soil was greater than red soil,but glyphosate could reduce the quantity in two kinds of soils.Langmuir equation was used to describe the phosphate adsorption behavior in red soil and yellow soil affected by glyphosate.According to the relevant characteristic parameters,in the absence of glyphosate treatment(control),the maximum phosphate adsorption capacity in red soil was significantly lower than that in yellow soil.With the increase of glyphosate,Xm rised in red soil,first risen and then fallen in yellow soil.Phosphate adsorption was a spontaneous endothermic reaction both in red soil and yellow soil,and the higher the concentration glyphosate was,the smaller the adsorption process of spontaneous tendency was.The Km×Mb in control group of red soil and yellow soil were both largest which reflect adsorption intensity,4 mmol/L glyphosate treatment group were both smallest.It showed that the glyphosate weaken the adsorption intensity of two kinds of soil to phosphate.View from the Langmuir adsorption isotherm,adsorption quantities of the two soils to phosphate under the control were both highest,and the adsorption quantities were elevated when equilibrium concentration of phosphate increased.However,when the equilibrium concentration of phosphate continually increased,adsorption quantity of red soil was relatively easy to reach saturation,so the adsorption ability to phosphate of yellow soil is stronger than red soil.Adsorption quantity of the two soils to phosphate declined with the increase of glyphosate,but the change trend of adsorption isotherm is different due to the soil types and concentrations of glyphosate.Phosphate desorption quantity and ratio in yellow soil were less than that in red soil in the same initial phosphate concentration,but adding glyphosate significantly affected phosphate desorption behavior in red soil.Compared to control,glyphosate obviously inhibited desorption of high concentration phosphate(>0.6 mmol/L),while glyphosate didn't affect desorption of low concentration phosphate(<0.6 mmol/L),even had a promoting role in red soil.Phosphate desorption quantity in the presence of glyphosate had little change,but the desorption ratio risen with the increase of concentration glyphosate in yellow soil.(2)Addition of glyphosate increased the available phosphate in acidic soils in certain concentration range.Compared with treatment of check(CK),on average,glyphosate treatment of 2 mg/kg,10 mg/kg and 20 mg/kg improved the available phosphate content by 10.29%,36.14%and 66.41%respectively in red soil.Compared with glyphosate treatment of 2 mg/kg,glyphosate treatment of 10 mg/kg and 20 mg/kg improved available phosphate content by 8.58%and 8.58%in yellow soil,so the activation of glyphosate in the test concentration range to available phosphate was better in red soil than yellow soil.The effect of glyphosate was related to the change of inorganic phosphate component.In red soil,activation of glyphosate to Al-P and Fe-P was strongest,then Ca-P,while O-P content increased.The inorganic phosphate components delined in different degrees with the addition of glyphosate in yellow soil.Correlation analysis showed that available phosphate content and pH reached to significant positive correlation(P<0.05),and there was no significant correlation between inorganic phosphate components(P>0.05),while Al-P and Fe-P content,O-P and Ca-P content tended to be waxing and waning in red soil.There was significant negative correlation(P<0.05)between available phosphate and Al-P,Fe-P,while there is no significant correlation between O-P and Ca-P,so Al-P and Fe-P were major contributor to available phosphate in yellow soil.However there was no significant correlation between pH and available phosphate and inorganic phosphate component(P>0.05)in yellow soil.(3)Glyphosate affected growth of Chinese fir seedling.Under low concentration of glyphosate,biomass increment of seedlings was significantly higher than CK(P<0.01),roots and branches were developed.Under high concentration glyphosate,it was significantly lower than CK(P<0.05),at the same time fir leaves appeared yellowing.When glyphosate concentration was 50 mg/kg,seedlings all died.Physiological indexes of seedling under different treatment showed that:With the increase of glyphosate,relative conductivity,MDA content,SOD activity,POD activity of leaves overall risen,proline content,soluble protein declined,chlorophyll a and chlorophyll b and total chlorophyll were at first risen and then declined,while shikimic acid content had no significant change.(4)The macro and micronutrient accumulation in roots and leaves of Chinese fir seedlings were affected by glyphosate.With the increase of glyphosate,Cu,Ca and Mg increased in roots,while there was no significant change about accumulation of Mn,Fe and Zn.In addition,the accumulation of Cu in Chinese fir leaves increased and Mn first rised and then decreased,Zn increased after risen.There was no obvious change in leaves about accumulation of Fe,Mg and Ca under different concentrations of glyphosate.Correlation analysis showed that there was a significant positive correlation between accumulation of Cu,Mg,Ca in roots and the concentration of glyphosate(P<0.01),while no significant correlation between Fe,Zn,Mn,P,and the concentration of glyphosate(P>0.05).There was also a significant positive correlation between accumulation of Cu in Chinese fir seedling leaves and the concentration of glyphosate(P<0.01),and no significant correlation between other macro and micronutrient and the concentrations of glyphosate(P>0.05).(5)In conclusion,effect of glyphosate on phosphate availability in acidic soil and growth of Chinese fir seedling embodied in:Glyphosate did not reduce the phosphate availability.Instead,glyphosate occupied adsorption sites for the phosphate in soil by competition,and decreased adsorption effective phosphate in soil,and promoted the release of adsorbable state phosphate in acidic soil;Glyphosate decreased inorganic phosphate in soil,so glyphosate could promoted the dissolution of indissolvable state phosphate in acidic soil;It was important to note that glyphosate could also affect the normal physiological functions in leaves and nutrients uptake and conduction in roots of Chinese fir seedling,and affect the growth of Chinese fir seedling,eventually reduced the biomass and survival rate.In the process of weeding in forestry,we should control the dosage of glyphosate within a reasonable range,and stayed away from Chinese fir rhizosphere soil to reduce adverse effect of glyphosate. |
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