The control of soil polarization in Populus simonii and Quercus liaotungensis forests by forage litter on the Loess Plateau, P.R.China

Nhu Trung Luc•Zengwen Liu•Yuanhao Bing•Xiaoxi Zhang•Thi Huong Nguyen

The control of soil polarization in Populus simonii and Quercus liaotungensis forests by forage litter on the Loess Plateau, P.R.China

Nhu Trung Luc1,2•Zengwen Liu3,4•Yuanhao Bing5•Xiaoxi Zhang1•Thi Huong Nguyen2,3,6

Soil polarization in pure forest stands affects the stability and sustainable developmentof the ecosystem. The most effective approaches to the prevention of soil polarization may be the use of forage litter as fertilizer or the direct establishment of tree-grass communities.We investigated monospecific plantations of Populus simonii and Quercus liaotungensis.Such plantations have been established throughout the Loess Plateau of China.A 120-day decomposition incubation experiment with forest humus soilmixed with 7 common leguminous forage litterspecies was performed to study the uses of forage litter in controlling soilpolarization in pure forests.The addition of forage litter of Astragalus adsurgens,Lespedeza bicolor, and Vicia villosa to the soilofpure P.simonii forestclearly improved the soilquality,these forage species are suitable for planting with P.simonii,whereas Melilotus officinalis, Medicago sativa,and Onobrychis viciifolia litter produced obvious deterioration,thus these forages should not be planted with P.simonii.Coronilla varia litter showed no significant influence.The addition of forage litter of M. sativa to the soil of pure Q.liaotungensis forest clearly improved the soilquality,and C.varia and V.villosa also yielded improvements butwere slightly less effective than M.sativa litter,these forages are suitable to be planted with Q.liaotungensis.However,L.bicolor showed obvious deterioration,followed by O.viciifolia and A.adsurgens, and M.officinalis had no significant influence.These species should not be planted with Q.liaotungensis.

Soilpolarization·Forage litter·Populus simonii·Quercus liaotungensis

Introduction

Pure forest plantations are important for controlling soil erosion and improving the ecological environment on the Loess Plateau ofChina.However,afterlong-term growth or continuousplanting forseveralgenerations,pure forestoften shows numerous problems,e.g.,growth recession,poor regeneration,and soildeterioration(Joshietal.1997;Liu etal. 1998;Liu and Qiang 2002;Zhang and Cai2004;Zhang etal. 2006;Li et al.2008;Mi et al.2013).The reasons for the changes in pure forest over time include selective nutrientrecycling,impacts of litter on soil,and sparse understory vegetation,which cause the soil properties to deviate from their original equilibrium to produce positive or negative effects on the development of the stand.This process was defined as‘‘soilpolarization’’by Liu etal.(2007).Previous studies have indicated that in the central hilly area of the Loess Plateau(e.g.,Huangling,in Shaanxiprovince),a pure Populus simonii forestthathad grown for a long time was already showing several negative polarization problems, e.g.,decreasesin the contentofsoilorganic matter,available Nand P,CEC,urease,sucrase and protease activities,and the abundance of microorganisms.Moreover,a pure Quercus liaotungensis forest was showing serious negative polarization asa substantiallossofavailable P and Kfrom the soil and a decrease in protease and sucrase activities,whereas positive polarization occurred in the form of increased abundance of microorganisms and activity of urease(Liu etal.2007).To oppose further deterioration of the ecosystems in this area(Du etal.2007;McVicaretal.2007),this study was conducted to controlsoilerosion and improve the ecologicalenvironmenton the Loess Plateau through the use of pure forest plantations in conjunction with important forestvegetation.

The analysis of the causes of soil polarization in pure forestshasindicated thattheuseofforagelitterasfertilizeror the direct establishmentof tree-grass communities may be one ofthe mosteffective approaches to preventing negative polarization and promoting positive polarization,due to not only the resulting rapid growth and the wide range of potential litter sources but also the distinct improvement occurring in soilproperties(Zhang etal.2008;He etal.2013; Luo etal.2013).Forexample,Zhao etal.(2012)showed that the decomposition of M.sativa can significantly improve enzyme activities as well as the amount of soil organic matter,total N,available P and K,and microorganisms. Similar results were obtained from our previous studies (Bing etal.2014a,b).Based on this information,this study examined pure forests of P.simonii and Q.liaotungensis, which grow throughout the Loess Plateau.Humus was collected from the forests,and a 120-day decomposition incubation experimentwith forest soils mixed with 7 common leguminousforage litterswas conducted to study the effects offorage litterson soil.The resultsofthe study are expected to furnish scientific guidance for the prevention of soilpolarization and the improvementoflocalpure forests.

Materials and methods

Study sites

Soil from P.simonii and Q.liaotungensis pure forest plantations was collected from several sites located in the Huangling region of Shaanxi province,China(Table 1). This area is located in a typical hilly region of the Loess Plateau.The climate of the region is warm temperate and semi-humid,with an average annualtemperature of 9.4°C. The average annualrainfallis 630.9 mm,the average frostfree period is approximately 150 days,and the relative humidity is approximately 64%.The soilis a typicalgrey cinnamon soil.

Collecting soil samples in pure forest and forage litter

We collected soils from P.simonii and Q.liaotungensis pure forestplantations thathad reached the mature growth stage.In these plantations,the vegetative growth density was greater than 0.9,and the understory vegetation was less than 15%.Standard 20 m×20 m plots were established in typical locations in the two forests,and the site factors and tree growth indices were measured in these plots.The plots(Table 1)were divided into five quadrats, each 1 m×1 m.The humus layer of the soilwas collected at a depth of 0-10 cm after removing the litter from the ground.The samples from each stand were then mixed to form a composite sample and transported to the laboratory after leaves,roots,and gravel had been removed from the soilsamples with 5 mm mesh sieves.In the laboratory,all litter samples were gently washed and oven-dried at65°C for 24 h to reach constant weight.The dry litter samples were then ground with a laboratory mill(φ=1 mm).

Litter(leaves and stems)from A.adsurgens,L.bicolor, V.villosa,C.varia,M.officinalis,M.sativa,and O.viciifolia was collected,rinsed in water,air-dried,and sifted using 5 mm mesh sieves.Some oflitter samples were used for determination of their initial nutrient contents.

Decomposition incubation of mixtures of soil with forage litter

Prepared fresh soil samples and litter were mixed at a dry weight ratio of 100:2(with the original forest soil with no litter as a control).A totalof 2.5 kg mixed soilwas placed in impermeable plastic pots.Each treatmentwas repeated 3 times.First,a measured amount of distilled water was added to each pot to adjust the soil moisture to 50%of saturated field capacity,covered with plastic film with 4 holes for ventilation to reduce water loss,and incubated at room temperature.The weight of the pots was recorded every 3 days.The stability of the moisture content of the soilwas maintained with a water sprayer.The weightof the pots was kept constant.Incubation under these conditions of cultivation was maintained for 120 days.Following cultivation,soil samples were placed on clean plates,and the remaining leaf litter was carefully removed.The platedilution method was then used to count numbers of microorganisms per unit of fresh soil.

Table 1 The characteristics of the studied pure forests

Measurements of soil and litter properties

Fresh soil was collected to measure the content of microorganisms.The remaining soil samples were air-dried, then ground to measure the following biological and chemical properties.

Chemical properties

The chemicalproperties of the soilwere measured with the following methods(Lu 2000).Soil pH was measured by glass electrode method(the ratio of soil to water was 1:2.5).Organic matter was determined with the potassium bichromate titrimetric method.Available nitrogen was measured with the micro-diffusion technique.Available phosphorus was measured by NaHCO3extraction with the molybdenum blue colorimetric method.Available potassium was measured with the ammonium acetate extraction flame photometric method.Cation exchange capacity was measured with the sodium acetate-ammonium acetate flame photometric method.

Biological properties

The quantity of microorganisms was measured with the dilution plating method(Nanjing Institute of Soil Science, Chinese Academy of Science,1985)(bacteria-beef extract peptone agar culture medium,fungi-potato dextrose agar culture medium,actinomyces-GAO 1st synthetic culture medium).Urease activity was measured with the phenol sodium–sodium hypochlorite colorimetric method.Sucrase activity was measured with the Na2S2O3titration method. Catalase activity was measured with the KMnO4titration method.Dehydrogenase activity was measured with the triphenyltetrazolium chloride colorimetric method.Alkaline phosphatase activity was measured with the disodium phenyl phosphate colorimetry method.Protease activity was measured with the ninhydrin colorimetric method. Polyphenol oxidase activity was measured with the iodometric titration method(Guan 1986).

Litter N,P and K contents:after digested by H2SO4and H2O2,contents of N,P and K were measured using continuous flow analyzer,ultraviolet spectrophotometer and flame photometer,respectively(Table 2).

Data processing

The data were analyzed using Microsoft Excel 2010. Differences between mean effects of the forage litter types on the pure forest soil properties were evaluated using an LSD multiple comparisons test.The addition of forage litter had complex effects on the 16 indexes of soil biochemistry.The same type of litter affected the quantity and direction of change of various soil properties.To assess the improvements in soil polarization resulting from the use of forage litter and to facilitate comparative analyses,a comprehensive principal component analysis method was used to analyze the rate of improvement of 15 biochemical properties(pH excluded)relative to the corresponding control values.This analysis was performed with SPSS 19.0,and the comprehensive principal component value Fiwas calculated(Fig.1).Fi＞0 implies that the addition of forage litter can improve soil properties(by decreasing the rate of negative polarization or increasing the rate of positive polarization);in contrast, Fi＜0 implies that the forage litter causes the soil properties to deteriorate.

Results and analysis

Effects of forage litter in the controlof soil polarization in Populus simonii forest

The biochemical properties of the soil from the P.simonii forestshowed various changes as a resultof the addition of forage litter(Appendix Table 3).A.adsurgens litter significantly increased the contentof available nutrients(N,P, and K)in the soil,the activity of urease and catalase,andthe growth of fungi and actinomycetes.L.bicolor litter significantly increased the content of organic matter, available N,available K,and the activity of urease,phosphatase,and catalase in the soil,and itpromoted the growth of microorganisms.V.villosa litter significantly improved the contentof available nutrients and the activity of urease, catalase,and dehydrogenase in the soil and promoted the growth of fungi.C.varia litter significantly increased the contentof available nutrients and the urease activity in the soil,and itpromoted the growth of actinomycetes,bacteria, and fungi.M.officinalis litter significantly improved the content of available nutrients and catalase activity and promoted the growth of microorganisms but significantly reduced the activity of urease and sucrase.M.sativa litter significantly increased the content of available nutrients and catalase activity and promoted the growth of bacteria and fungi.O.viciifolia litter significantly increased the organic matter,the content of available P and K,and the activity of catalase,phosphatase,and polyphenol oxidase in the soil,and itpromoted the growth of bacteria and fungi but significantly reduced CEC and the activity of urease and sucrase.

Table 2 Initial nutrient contents of litter form 7 forage species(g kg-1)

Fig.1 Combined effects of forage litter on the soil of pure broad-leaved forest(a P.simonii;b Q.liaotungensis)

The results of this study may be compared with those of previous studies showing that negative soil polarization problems had occurred in pure P.simonii plantations, producing decreases in the content of organic matter in the soil,reductions of available N and P,a reduction of CEC,decreased activity of urease,sucrase,and protease, and reductions in microbial biomass.This comparison showed that L.bicolor and O.viciifolia litter could significantly alleviate the negative polarization of organic matter content in P.simonii forest.Litter of all species except O.viciifolia significantly alleviated the negative polarization of available N content.The litter of M.officinalis showed the most marked effect.Litter of all species except L.bicolor significantly alleviated the negative polarization of available P content and increase available P content by 1.56%.No species of litter significantly eased the negative polarization of CEC content, and O.viciifolia litter significantly exacerbated this trend. Litter of A.adsurgens,L.bicolor,V.villosa,and C.varia significantly alleviated the negative polarization of urease activity,whereas M.officinalis and O.viciifolia significantly exacerbated the negative polarization.No species of litter significantly alleviated the negative polarization of sucrase activity,and M.officinalis and O. viciifolia litters significantly exacerbated this trend.No species of litter significantly alleviated the negative polarization of protease activity.

Based on the effects of forage litter on the abovementioned negative polarization trends as well as on other biochemical properties of the soil,a principal component analysis(Fig.1a)showed that the addition of forage litter to the soil of the pure P.simonii plantation resulted in measurable improvement in the case of A.adsurgens,L. bicolor,and V.villosa,whereas litter of M.officinalis,M. sativa,and O.viciifolia resulted in marked deterioration;C. varia litter had no obvious effect.

Effects of forage litter in the control of soil polarization in Quercus liaotungensis forest

The biochemical properties of the soil of pure Q.liaotungensis plantation showed various changes afterthe addition of forage litter(Appendix Table 4).The litter of A. adsurgens greatly improved the amounts of available K and available P and the activity of urease,sucrase,phosphatase,protease,and polyphenol oxidase in the soil(increased by 99,70,16,128,96%,respectively).The litter of L.bicolor significantly improved the content of organic matter and available K and the activity of CEC,urease, sucrase,phosphatase,and protease in the soiland promoted the growth of actinomycetes.The litter of V.villosa significantly improved the content of available nutrients and all enzyme activities but that of urease in the soil and promoted the growth of bacteria and actinomycetes but significantly reduced urease activity.The litter of C.varia significantly improved the availability of nutrients in the soil,the activities of all enzymes but urease,catalase,and dehydrogenase,and the growth of bacteria and fungi,but significantly reduced urease activity.The litter of M.officinalis significantly improved the concentrations of available nutrients and the activities of allenzymes except for CEC and polyphenoloxidase,and promoted the growth of bacteria and actinomycetes.The litter of M.sativa significantly improved the availability of nutrients in the soil, the activity of sucrase,and the growth of fungi but significantly decreased urease activity.The litter of O.viciifolia significantly improved the contentof available K and CEC activity and the activities of all enzymes except for protease and polyphenoloxidase and promoted the growth of bacteria and actinomycetes butsignificantly reduced the activity of protease.

The results of this study may be compared with those of previous studies showing thatnegative soilpolarization problems had occurred in pure Q.liaotungensis plantations.In these plantations,the long-term growth of pure forest had resulted in serious negative polarization problems,e.g.,a substantial loss of available P and K in the soil and a decrease in protease and sucrase activity as well as a positive polarization in the amount of microorganisms and the activity of urease.This comparison showed that all litter of all forage species except L.bicolor and O.viciifolia markedly reduced the negative polarization of available P and slowed the negative polarization of available K;all species of litter sharply reduced the negative polarization of sucrase activity,with M.sativa showing the most marked effect.Litter of A. adsurgens,L.bicolor,V.villosa,C.varia,and M.officinalis substantially decreased the negative polarization of protease activity,whereas O.viciifolia litter strongly exacerbated this effect.Additionally,the treatments with forage litter affected several positive polarization trends observed in the original forest soil.Litter of A.adsurgens, L.bicolor,M.officinalis,and O.viciifolia significantly promoted the positive polarization of urease activity.L. bicolor litter was the most effective;however,the remaining two types of forage litter had marked negative effects on this trend in urease activity.The litter of V. villosa,M.officinalis,C.varia,and O.viciifolia significantly increased the amount of bacteria,C.varia and Medicago sativa significantly promoted the growth of fungi,and L.bicolor,V.villosa,M.officinalis,and O. viciifolia contributed markedly to the growth of actinomycetes.However,note that fungi were the most important constituents of the microorganism community.In this context,of the 7 species of litter,only C.varia and M.sativa contributed substantially to the positive polarization of soil microorganism content.

Based on the effects of forage litter on the abovementioned negative polarization trends as well as on other biochemical properties of the soil,a principal component analysis(Fig.1b)showed that the soil of the pure Q. liaotungensis plantation was measurably improved as a result of the addition of M.sativa litter,followed by C. varia and V.villosa litter.However,the addition of L.bicolor litter caused obvious deterioration,and the same was the case(but to a lesser extent)for O.viciifolia and A. adsurgens litter.The effect of M.officinalis litter was not obvious.

Discussion

The differing biological properties of the two forest tree species examined in this study resulted in differences in the amounts of soil nutrients taken from and returned to the soil.Some soil nutrients cannot be supplemented,and nutrientloss results(Miand Liu 2014).Moreover,due to the environment of the monoculture-dominated community, the decomposition of forest litter and the leaching and secretions associated with rain and root activity produce combined effects that result in destructive changes to the biochemical properties of the soil(Mi and Liu 2014;Lu¨ etal.2006).These processes are the fundamentalcause of the negative polarization of the pure forest soil.Under these circumstances,it is effective to rapidly improve the properties ofthe pure forestsoiland to preventthe negative polarization of the soil through measures involving the rapid mitigation of soil nutrient loss through the decomposition of the litter of other plants(Liu etal.2012).These measures also improve other biochemicalproperties of the soil.

The results of a previous preliminary study and of this study showed thatcertain losses oforganic matter from thesoil occur in the pure P.simonii plantation and that the impact on the organic matter content depended,in some cases,on the species of forage litter.L.bicolor and O. viciifolia litter significantly alleviated the loss of soil organic matter.This result is consistent with the findings of Liu et al.(2006)on the use of leguminous plants to improve tobacco planting soil.However,the effects of other types of litter on soil organic matter loss are not obvious. These differing results may be due to the varying biological characteristics of forage litter(Li et al.2011a,b).The addition of litter did not change the organic matter content significantly.The reason for this outcome may be that the decomposition of the forage stimulates the process of organic decomposition to a certain extent.This process consumes a large amount of the available N and P,which are essential nutrients for microbial activity.Accordingly, the complementary effects of certain species of litter relative to available N and P are weakened.

The results of this study showed that in the Huangling region,almost all of the studied types of forage litter could significantly increase the content of available N in broad-leaved forest soil and mitigate the observed negative polarization trend of alkali-hydrolyzale N.This result is similar to the findings of Li et al.(2011a,b).In certain cases,however,a mitigation effect of this type was not observed if leguminous plantlitter rich in available N was added.These results are not consistent with the abovementioned findings.The addition of litter shows similar effects on available P(Zhao et al.2012).An important reason for the above results may be that the litter itself and the decomposition of soil organic matter used the available N and P because microbial decomposition requires appropriate C/N and C/P ratios(Li et al.2010; Zhao et al.2011).In the two types of woodland,the addition of all types of litter significantly alleviated the negative polarization of available K or enhanced its positive polarization trend,as previously shown by other studies(Zhang et al.2008).The reason for this outcome is that K is a non-structural element(Osono and Takeda 2004)that is more easily released than N or P(Zhao et al. 2011).The release of a large amount of water-soluble K will increase the content of available K in the soil.Additionally,the use of all of the studied types of litter had almost no significant effect on the CEC of the soil.This result indicates that the litter had little effect on the conservation of fertilizer for the soil.

A certain degree of increase in enzyme activities occurred as a resultof the biologicaleffects of the litter.This result is similar to the findings of Li et al.(Liu et al.2010; Li et al.2011a,b;Wang et al.2012).Increase in the amount of decomposition substrates or the stimulation of certain antioxidant protection systems by substances released during decomposition may be involved.Certain types of litter inhibited the activity of particular enzymes, weakening the positive polarization and even intensifying the negative trend.This result is similar to the findings of Sun et al.(2011).The above phenomenon may be due to effects on the enzyme of changes in the content of certain nutrients(Sun et al.2011).Changes in microbial activity are also influenced by multiple factors even if the same type of litter is added.Different effects on microbial activity are observed in different types of soil because this activity is influenced by the substrate quality of the litter (Qi et al.2004),the content of particular chemical compounds,and the nutrients in the soil.The microbialspecies, the quantity of microorganisms,and even the microbial flora(Kourtev etal.2002)would change,and these effects require further research and explanation.

Conclusions

The addition of forage litter to the soil of pure P.simonii plantation resulted in measurable improvement in the case of A.adsurgens,L.bicolor,and V.villosa;The addition of forage litter to the soilof pure Q.liaotungensis plantation resulted in obvious improvement in the case of M.sativa, followed by C.varia and V.villosa.

AcknowledgmentsThis study was financially supported by a grant from National Natural Science Foundation of China(31070630).The authors sincerely thank the anonymous reviewers and editors for criticisms,helpful comments,and discussions that improved earlier versions of this manuscript.

Appendix

See Tables 3 and 4.

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21 March 2014/Accepted:14 July 2014/Published online:2 June 2015

©Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

✉Zengwen Liu zengwenliu2003@aliyun.com

Nhu Trung Luc trungfrc@gmail.com;2597782062@qq.com

Yuanhao Bing yy198766@163.com

Xiaoxi Zhang zhangxiaoxi712100@gmail.com

Thi Huong Nguyen huongnguyenmsc@gmail.com

1Institute of Soil and Water Conservation,Northwest A&F University,Yangling 712100,Shaanxi,China

2Department of Agriculture and Rural Development of Lao Cai,Lao Cai City 330100,Vietnam

3College of Natural Resources and Environment,Northwest A&F University,Yangling 712100,Shaanxi,China

4Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China,Ministry of Agriculture, Yangling 712100,Shaanxi,China

5College of Forestry,Northwest A&F University, Yangling 712100,Shaanxi,China

6Department of Natural Resources and Environment of Lao Cai,Lao Cai City 330100,Vietnam