| Green manure（GM）has widely received a considerable attention by research community in recent decades as a source of soil nutrients while polyacrylamide（PAM）is a synthetic polymer that has shown potentials of improving physical conditions of soil and water retention.The objectives of the present study were:Assessment of physicochemical properties of purple soil under GM and PAM in Huidong County,as well as to investigate the combined effects of GM and PAM on chemical quality of tobacco as GM succeeding crop.Our main goals were to improve the fertility of purple soils in Huidong County,resolving soil water stress issues and preserving degraded soils in Huidong County,as well as to improve the quality of tobacco crop.The study was established in Huidong County/Xiaoba,Liangshang Yi Autonomous Prefecture,Sichuan,China.The experiment was laid out in fully factorial design of 5×3 in which five application rates of PAM 0 kg/ha,30 kg/ha,90 kg/ha,150 kg/ha,and 210 kg/ha and three varieties of GM:Alfalfa（Medicago Sativa L.,45 kg/ha）,Hairy Vetch（Vicia Villosa Roth,45kg/ha）,and White Clover（Trifolium repens L.,30 kg/ha）were applied in 15 field plots（4m×2m each）replicated thrice.Results showed that the soil properties and tobacco chemical quality indicators differed significantly among GM varieties treated plots with different PAM application rates,and the important findings are summarized below.（1）Regarding the dynamic of water content in legumes’plots that received different doses of PAM,the results showed that only time effect exhibited a significant difference,and the peak was observed at 7th week in 9 weeks of monitoring soil water content.（2）Among the GM varieties studied,T.repens showed a strong interaction with PAM for soil properties enhancement.For instance,SOM increased significantly in plots applied T.repens,with PAM increase.The lowest mean value was 0.26±0.031%and found in plots without PAM while the highest mean value was 0.86±0.031%and found in plots with the highest PAM dose,yet the means difference was statistically significant（p<0.05）.（3）T.repens was the most soil acidifying GM among the legumes varieties studied,while V.villosa and M.sativa showed high buffering capacity,irrespective of PAM dose applied.The lowest mean value found in T.repens’plots（4.94±0.089）was observed with 30kg/ha and significantly different from the highest mean value（6.24±0.14）that found with the highest PAM dose.It turns out that,PAM dose increase reversed T.repens effect.M.sativa was the legume with higher soil p H buffering capacity and the p H values were in range of 7.20±0.17–7.46±0.046 which did not differ from the p H value of the soil before the experimental set up（7.5）.（4）V.Villosa significantly enhanced available soil nitrogen with PAM dose increase compared to T.repens and M.Sativa.The lowest mean value of available N found with V.villosa was 43.66±1.52 mg/kg when 30 kg/ha of PAM was applied,while the highest mean value was found to be 72.01±0.89 mg/kg when 210 kg/ha of PAM was applied.（5）M.sativa was the most effective legume to reincorporate base cations（Ca and Mg）insoil,while T.repens was the least.For instance,the lowest mean value of Ca in plots applied M.sativa was 1721.83±26.65 mg/kg and found when no PAM applied,while the highest mean value was 3274.10±315.96 mg/kg,and found when 150 kg/ha of PAM was applied.The mean values of Ca observed in T.repens plots were in range of 32.22±4.03-283.10±3.31 mg/kg.Likewise;the response mechanism of Mg to the treatments was almost similar to that of Ca.（6）Our results demonstrated that PAM and GMs combined are a good alternative of preserving sodic soils.The application of PAM with GM legumes dramatically decreased the concentration of Na+in soil,specifically,more prominent results were found in plots treated with T.repens.The highest mean value of Na recorded with T.repens was2612.33±88.34 mg/kg when no PAM applied,while the lowest mean value was2081.67±1.86 mg/kg when 150 kg/ha of PAM applied.（7）Tobacco chemical quality indicators were determined to examine their contents in tobacco leaves.The results showed that middle leaves are more likely to accumulate Cl and K than upper leaves.However,30 and 150 kg/ha PAM were generally likely to increase Cl in either upper or middle leaves for all GM varieties,while the results for PAM effect on K were inconclusive.The highest mean value of Cl observed in middle leaf（0.91±0.031%）was in plots of V.villosa with 30 kg/ha of PAM,while the lowest mean value of Cl observed in middle leaf（0.37±0.025%）was also found in plots of V.villosa with the highest PAM dose,yet these values were significant different（p<0.05）.On the other hand,the highest mean value of K content in tobacco middle leaf（3.98±0.22%）was obtained in plots of M.sativa that did not receive any dose of PAM,while the lowest mean value was 2.20±0.35%and recorded in V.villlosa plots without PAM.Considering the potassium and chlorine content which determine the rate of combustibility of tobacco leaf,the results of the study showed that the potassium content was higher than chlorine content,suggesting the high burning speed of tobacco leaf.In this essense,the results showed that V.villosa enhanced K/Cl ratio more than any other legume.The highest mean value of K/Cl ratio recorded in upper leaf of tobacco grown with V.villosa（13.48±1.98）was when no PAM applied,while the lowest mean value（4.28±0.34）was recorded with 150 kg/ha of PAM.Middle leaf recorded mean values of K/Cl ratio ranging from 3.50±0.28 with 150 kg/ha of PAM to 7.10±1.47 with 210 kg/ha of PAM and these mean values were significant different.Basically,V.villosa was generally the most effective among the winter legumes studied and maintained adequate economic returns,because it enhanced most of soil properties and chemical components of tobacco leaf with no PAM or lower doses of PAM.To sum up this,the combined impacts of GM and PAM could be a solid option to compensate for the limiting characteristics of purple soils,resulting in an overall improvement of tobacco chemical quality. |
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