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Agriculture, Ecosystems and Environment

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Alfalfa planting significantly improved alpine soil water infiltrability in theQinghai-Tibetan PlateauZe Huanga,b,1, Lei Suna,1, Yu Liub, Yi-Fan Liub, Manuel López-Vicentec, Xue-Hong Weia,Gao-Lin Wua,b,⁎

a Animal Science College, Tibet Agriculture & Animal Husbandry University, Nyingchi, 860000, Chinab State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling, Shaanxi, 712100, Chinac Department of Soil and Water, Experimental Station of Aula Dei, EEAD-CSIC, Zaragoza 50059, Spain

A R T I C L E I N F O

Keywords:Alpine soilAlfalfa grasslandSoil water infiltration rateRoot dry weight densityComprehensive index of soil water infiltrationcapacity

A B S T R A C T

The Qinghai-Tibetan Plateau is undergoing serious water and soil conservation problem resulted from grasslanddegradation, that is not conductive to the sustainability of grassland ecologicla function and agriculture pro-ductivity. The soil infiltrability has significance in reducing runoff yield and soil loss. However, characterizationof the soil infiltration capacity of planted grasslands, such as alfalfa (Medicago Sativa), with different growingyears in this high-altitude alpine region is still lacking. In this study, the variation of soil infiltration rate andcharacteristics were analyzed in alfalfa grasslands with three growth years (two-, four- and seven-year-old). Acorn cropland was used as the control field, and the infiltration was monitored using the automatic soil in-filtrability measurement system. To determine the influence factors on infiltration rate (IR), soil characteristicsand root dry weight density (RDWD) at 0–30 cm soil depth were measured. The results indicated that thecomprehensive index of soil water infiltration capacity (FIR) of the seven-year-old alfalfa grassland (0.69) wasthe highest, followed by the two- (0.05) and four-year-old (0.11) grasslands, being these values significantlyhigher than that of the corn field (-0.85, P<0.05). SWC was positively correlated with IR during the fastinfiltration stage (IR-I). The soil bulk density and organic matter were negatively correlated with IR, whileRDWD was positively related to IR. Our results showed that alfalfa planting improved soil infiltrability, and thisupgrade increased with the age growth. This study provided significant information to introduce soil-erosion-control practices for alpine soil and highlights the importance of ecological function of planted grasslands inalpine area of the Qinghai-Tibetan Plateau.

1. Introduction

The increasing demands for food caused by the rising populationlead to the greater pressure on agriculture ecosystem (Schiefer et al.,2016). The overuse of natural resources generally result in the de-gradation of ecosystem functions, especially in the ecological sensitiveareas. The alpine grasslands are sensitive to the global climate change,they are suffering serious degradation of ecological functions, due toovergrazing and human activities (Wang et al., 2018, 2011; Zeng et al.,2013). To improve this problem, it was critical to increase agriculturalproductivity and avoid the negative effects on environment simulta-neously (Schiefer et al., 2016). Artificial grassland planting is a mostfast and effective way to restore the degraded grasslands and to im-prove the forage yield (Chen et al., 2019), especially for the legumes

grasslands. The legumes grassland planting can significantly improvegrassland productivity and provide high-quality forage (Huang et al.,2018). The legumes functional group also can promote nutrient cycleand improve soil quality, such as the higher soil infiltrability in legumesthan the other functional groups (Gould et al., 2016; Huang et al.,2017). Many studied have indicated that soil water infiltration is a keyprocess in the terrestrial water cycle (Cerdà, 1997; Fischer et al., 2014),and rainfall infiltration is an important component for the replenish-ment of the “soil reservoir” (Huang et al., 2017). Soil moisture is themost decisive factor for maintaining crop and natural vegetationgrowing. Thus, high soil water infiltration rates (IR) are critical forvegetation restoration, especially in regions with limited water re-sources. Moreover, the characteristics of the process of soil water in-filtration are critical indicators to evaluate the effects of soil water

https://doi.org/10.1016/j.agee.2019.106606Received 3 May 2019; Received in revised form 14 July 2019; Accepted 16 July 2019

⁎ Corresponding author at: Animal Science College, Tibet Agriculture & Animal Husbandry University, Nyingchi, 860000, China.E-mail address: wugaolin@nwsuaf.edu.cn (G.-L. Wu).

1 These authors contributed equally to this work and are co-first authors.

Agriculture, Ecosystems and Environment 285 (2019) 106606

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conservation and the ability of anti-soil erosion practices (Kroulik et al.,2007). High IR favor a decrease of the soil erosion risk (Tongway andLudwig, 1990). Overall, soil infiltrability is significant to evaluate hy-drological processes and to improve the soil and water conservationfunction in grassland ecosystem. Thus, more researches should be paidto understand the soil infiltration processes and capacity of artificialgrassland, especially in alpine area, where occured serious degradationof grassland ecological function and soil erosion.

Previous studies have indicated that the existence of plants favorsrainfall infiltration and reduces runoff yield, although these effects varywith different plants (Thompson et al., 2010; Zhang et al., 2011). Plantsinfluence soil hydraulic properties by means of changing the soil phy-sical and chemical characteristics (Wang et al., 2007). The plant rootsystem is the most important biotic factor influencing soil character-istics (Peres et al., 2013; Gould et al., 2016). Zuazo and Pleguezuelo(2008) suggested that roots could reinforce the soil, increase the soilsurface roughness and soil permeability by penetrating the soil mass,which increases the soil infiltrability and antierodibility (Cerdà et al.,2017). Thus, the characteristics of root distribution have significanteffects on the soil water infiltration capacity (Barley, 1953; Wu et al.,2017a,b). Besides, higher infiltrability benefits plant growing and soilwater recharge (Cerdà, 1997; Zuazo and Pleguezuelo, 2008). Accordingto published articles, soil water infiltration capacity is mainly influ-enced by soil texture, soil structure, bulk density, porosity and soilorganic matter (Zhang et al., 2007; Wu et al., 2016; Huang et al., 2017).Hu et al. (2019) recently indicated that the increase of the root networkdensity would lead to the increase of soil macroporosity and macroporevolume. Because most root’s diameters are higher than soil pores, soilappears as a compacted material and the soil bulk density near the rootsis higher than far from the roots (Zuazo and Pleguezuelo, 2008). Devittand Smith (2002) indicated that the effect of the shrub (Larrea tri-dentata) root channels on the soil infiltration capacity would be morepronounced when the plant root system become larger. Thus, we

hypothesize that the infiltration capacity of grasslands will change withincreasing the plantation age owing to the changes in the root traits.

Gould et al. (2016) have reported that the existence of legumes canreinforce soil strength and against disturbance, due to its larger rootbiomass and root diameter than other functional groups. Thus, it wasessential to study the effects of legumes on the degradation grasslandecosystems. Alfalfa (Medicago sativa) is so-called the ‘king’ of thegrazing grasses, which are the conventional plants in the pastoral areasof the Qinghai-Tibetan Plateau (Chen et al., 2017). Alfalfa is marked byhigh yield values and endurance capacity, and it also has a significanteffective capacity to prevent soil erosion and to improve soil quality(Wang et al., 2008a). Alfalfa is a perennial plant which can use manyyears by once cropping. And the planting age significantly influencedthe variation of soil characteristics (Zhao et al., 2017). Soil character-istics through carefully selected can provide the information for fieldmanagement practices (Andrews et al., 2002). However, there is still agap about how the alfalfa grasslands impact on soil water infiltrabilitywith increasing time.

Soil water infiltrability has many ecological significances, such asdetermining the amount of runoff, preventing soil erosion and influ-encing vegetation growth (Kroulik et al., 2007; Thompson et al., 2010).The previous studies mainly concentrated in the different underlyingsurface types (Hu et al., 2018) and in the degraded alpine meadows(Zeng et al., 2013). However, the available studies about soil hydraulicproperties and soil and water conservation function of planted grass-lands in alpine area of the Qinghai-Tibetan Plateau were few. There isstill a lack of field experimental data about the changes in the soil in-filtration capacity of planted grasslands with different growth year inthis high-altitude alpine region.

Hence, this study aimed to investigate the variation causes of soilinfiltration rate in alfalfa grasslands with distinct ages (two-, four- andseven-year-old) in alpine area of the Qinghai-Tibetan Plateau, and thento analyze the main soil influencing factors on soil water infiltrability.

Fig. 1. Location of the studied site in the Tibetan Plateau.

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To achieve this goal, we used parallel plots and a maize field as controlcrop. Soil physical and chemical parameters, and the characteristics ofthe root system were measured; and soil infiltration rate were assessedby using an automatic soil water infiltrability measurement system.This research aimed to fill the gap of soil water infiltration studies inalfalfa grasslands located in high-altitude alpine regions, which wasconductive to the sustainable development of grassland ecologicalfunctions in alpine area of the Qinghai-Tibetan Plateau.

2. Materials and methods

2.1. Study site

The field experiment was conducted in Milin county of the Qinghai-Tibetan Plateau that is located in the southwest part of the Qinghai-Tibetan Plateau (93°25′ E, 29°50′ N), at an altitude of 2900m a.s.l.(Fig. 1).The region is a plateau with a temperate sub-humid monsoonclimate, and the mean annual temperature is of 8.2℃. The mean annualrainfall is 641mm, and 85% occurred from June to September. Thepresent soil type is mainly leached cinnamon soil with sandy texture.The sand, clay and silt contents of the surface soil are 59.3%, 23.5% and17.2%, respectively. The value of soil pH is 5.5-6.5. The study site is flatand plough layer is thin (the soil thickness is 0.4-0.8m). This site wascultivated with hulless barley (Hordium vulgare) before planting alfalfa(Medicago sativa).

2.2. Experimental design

Alfalfa is the most common legume species used in vegetation re-storation. Three alfalfa grasslands of two-, four- and seven-year-oldwere selected to conduct this study, these grasslands were planted in2010, 2013 and 2015, respectively. And a maize (Zea mays) field as thecontrol crop. Alfalfa was sown in drill, and the sowing rate, sowingdepth and row spacing was 45 kg hm−2, 2 cm and 30 cm, respectively.Seeding for maize was carried out with a row space of 50 cm andplanting space 25 cm, each hole with two seeds. Three 50×50mparallel plots were established at each treatment and in the controlcrop, with a distance between each plot of at least 100m. The fieldmeasurements of IR and of the soil characteristics were made in mid-July 2017 before harvest, and three repetitions were made at each plot.Thus, a total of 36 series of measures were obtained to provide a sounddata base.

2.3. Soil sampling and analysis

Soil water infiltration was measured by using the automatic soilinfiltrability measurement system that is a point source method forestimating the complete infiltration process. The experimental appa-ratus includes a peristaltic pump, a camera and a computer with themathematical model allows estimating IR from the wetted soil surface.The consist of this device showed in Fig. 2, more details about the ex-periment can find in Mao et al. (2016); Wu et al. (2016) and Huanget al. (2017). The total measurement time of each infiltration processwas 90min, and this process was divided into three stages according tothe variations of IR (Wu et al., 2016). The three stages were the fastinfiltration (IR-I; 0–15min), the stable change (IR-II; 15–45min) andthe remaining stable (IR-III, 45–75min) stages. In addition, the averageIRs during the last 15min was the steady IR (SIR, 75–90min), and theaverage IRs during the whole experiment, 0–90min, was the average IR(AIR) (Fig. 3).

Soil and roots characterization included the following parameters:soil water content (SWC), bulk density (BD), soil organic matter (SOM),root biomass (RB), and the root dry weight density (RDWD). All theseparameters were measured for the top 0–30 cm soil depth at 10-cmintervals. SWC was measured by using the soil-drilling (4-cm diameter)method. BD was determined by using a cutting ring (5-cm diameter and

5-cm height). SOM was measured by using the oxidation with po-tassium chromate method (Walkley and Black, 1934). RB was de-termined by using a 9-cm dimeter root augur, and roots were separatedfrom the soil particles by using a 2-mm sieve. Then, the roots were driedat 75 ℃ for 48 h to constant weight. RDWD was calculated as the ratiobetween the dry root biomass and the soil volume.

2.4. Soil water infiltration capacity index

The principal components analysis (PCA) was used to determine thecomprehensive index of IR. The field measures were standardized byusing the min-max method before doing the statistical analyses. Eachprincipal component (PC) of eigenvalue was retained when it was morethan 1. In this study, there were two PCs with the eigenvalue more than1, and the two PCs explained 97.88% of total variation (Table 1). Themost critical parameters to explain IR were determined from the PCA,and the scores of PC-1 and PC-2 were calculated as follow (Andrewset al., 2002; Armenise et al., 2013):

FPC-1=0.111·XIR-I + 0.283·XIR-II + 0.275·XIR-III + 0.261·XSIR +0.243·XAIR (1)

FPC-2=0.616·XIR-I – 0.0.071·XIR-II – 0.237·XIR-III – 0.289·XSIR +0.378·XAIR (2)

The comprehensive index of IR (FSIR) was calculated as follow:

FSIR = (67.900·FPC-1+ 29.978·FPC-2) / 97.878 (3)

2.5. Statistical analysis

The one-way analysis of variation (ANOVA) was used to analyze thesignificance of difference of soil characteristics and IR between thedifferent alfalfa grasslands. The difference was significant at 0.05 level.The comprehensive index of IR was determined by PCA. The statisticaldescriptive analyses of all data were conducted by using SPSS software(version 19.0). All figures were created with SigmaPlot software (ver-sion 14.0).

3. Results

3.1. Soil characteristics of different alfalfa grasslands

The SWC at the top 0–30 cm soil depth of the two-year-old alfalfagrassland was lower than that of the other two grasslands and of thecorn field, and the difference was significant at 0–10 cm soil depth(Fig. 4a, P < 0.05). The SWC at the 10–30 cm soil depth of the alfalfagrasslands was lower than SWC in the corn field at the same soil depth.SWC at the 10–30 cm soil depth of the seven-year-old alfalfa grasslandwas approximately 30%, being this value 15% higher than SWC in thetwo- and four-year-old alfalfa grasslands, respectively.

The values of BD at the 0–10 cm layer in the corn field and the four-year-old alfalfa grassland were significantly higher than the values ofBD in the two- and seven-year-old alfalfa grasslands (Fig. 4b,P < 0.05). The BD at the 10–30 cm soil depth in the corn field was thehighest, followed by the seven-year-old grassland, whereas BD in thissoil layer was the lowest in the two-year-old grassland. The SOM con-tent at the 0–30 cm soil depth in the corn field was significantly higherthan those values obtained in the alfalfa grasslands (Fig. 4c, P < 0.05).Moreover, the SOM content at the 0–10 cm soil depth of the two-year-old alfalfa grassland (3.27 ± 0.05 g kg–1) was the lowest. And the SOMcontent of seven-year-old alfalfa grassland were 5.88 ± 0.24 g kg–1

and 5.10 ± 0.16 g kg–1 at 10–20 cm and 20–30 cm soil depth, respec-tively, which with a mean value that was ca. two times lower than thatobserved in the corn field. The RDWD at the 0–30 cm soil depth of thealfalfa grasslands increased with increasing the growth year (Fig. 4d).The RDWD of the seven-year-old alfalfa grassland was the highest

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(40.76 ± 10.5 kg m–3) at the 0–10 cm soil depth, which was sig-nificantly higher than that of the corn field (13.73 ± 8.55 kg m–3,P < 0.05).

3.2. Soil water infiltration rates of the different alfalfa grasslands

On balance, the IRs in the alfalfa grasslands were higher than themeasured rates in the corn field (Table 2). IR-I of the seven-year-oldgrassland was significantly higher than IR-I in the other grasslands andin the corn (P < 0.05). IR-II and IR-III decreased with increasing thegrowth year of the grasslands. The highest SIR appeared in the two-year-old alfalfa grassland, and this value was ca. 15% and 35% higherthan the values of SIR in the four- and seven-year-old grasslands, re-spectively. The highest AIR appeared in the seven-year-old grassland,followed by the two- and four-year-old grasslands, although the dif-ferences between the AIR values of the distinct grasslands were notsignificant.

The highest FIR (comprehensive index of soil water infiltration ca-pacity) value appeared in the seven-year-old alfalfa grassland(FIR= 0.69), which meant that the oldest grassland had the best in-filtration capacity (Fig. 5). Compare with seven-year-old alfalfa grass-land, the four- (FIR=0.11) and the two-year-old (FIR= 0.05) grass-lands showed lower FIR. All grasslands had significantly higher FIRvalues than the value obtained in the corn field (FIR = -0.85;P < 0.05).

3.3. Influence of the soil characteristics on the infiltration rates

As expected, IR-I, IR-II, IR-III, SIR and AIR were negatively corre-lated to SWC, and SWC at the top 0–20 cm soil depth showed a sig-nificant and negative correlation with IR-III and SIR (Table 3). IR-I wassignificantly and positive correlated with RDWD at the 0–10 cm and10–20 cm soil depth (P < 0.05). IR-III and SIR was significantly andnegative correlated with BD at the 10–20 cm and 20–30 cm soil depth.AIR was significantly and positive correlated with RDWD at the0–10 cm soil depth, while AIR was significantly and negative correlatedwith BD at the 0–10 cm soil depth, and with SOM at the 10–20 cm and20–30 cm soil depth.

4. Discussion

The influence of the crust is definitive in the agriculture fields andalfalfa induce change on this property, due to the roots and the growth

Fig. 2. A scheme of the experimental device for the measurement of soil infiltration rate.

Fig. 3. Soil water infiltration process of corn cropland, two-, four- and seven-year-old alfalfa grasslands.

Table 1Principal component analysis (PCA) of the soil water infiltration rates in thealfalfa grasslands with different growth year, and in the corn field.

Principal component PC-1 PC-2

Eigenvalue 3.395 1.499Variance 67.900 29.978Cumulative 67.900 97.878Loading valueAverage infiltration rate in stage I (IR-I) 0.377 0.924Average infiltration rate in stage II (IR-II) 0.960 −0.106Average infiltration rate in stage III (IR-III) 0.932 −0.355Steady infiltration rate (SIR) 0.885 −0.433Average infiltration rate (AIR) 0.823 0.567

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and cover of the plants (Di Prima et al., 2018). The soil erosion degreewas mainly influenced by soil structure characteristics and rainfall in-tensity (Wu et al., 2017a,b). The increasing coverage of surface soilwould effectively reduce runoff through reducing raindrop impact onthe soil surface, decreasing the water runoff rate and increasing in-filtration (Zuazo and Pleguezuelo, 2008; Kalhoro et al., 2018; Keesstraet al., 2019).Studies have indicated that the planted grasslands not onlyhave the significance of improving productivity, but also have manyenvironmental benefits in the Qinghai-Tibetan Plateau (Feng et al.,2010; Wu et al., 2010).

This study suggested that the planting of alfalfa grasslands havesignificantly improved the soil infiltrability compared with a control

crop. And this positive change became more pronounced with in-creasing the plantation age (Alagna et al., 2017). Zhao et al. (2017) alsohad reported that grassland presented the higher capacity of improvingsoil structure than cropland. The differences of IR-II, IR-III, SIR and AIRbetween the grasslands with different growth year were not significant,but the highest IR-I appeared in the seven-year-old grassland, whichresult in the highest comprehensive index (FIR) in the seven-year-oldalfalfa grassland. This result demonstrated that the IR-I played the de-cision role to the soil infiltrability, and soil water infiltration capacitywas enhanced with the increasing growth year of the alfalfa grasslands.The soil and water conservation capacity of alfalfa grasslands wouldfurther increase with the increasing growth year. Cerdà et al. (2017)

Fig. 4. Variation of soil water content, bulk density, organic matters and root dry weight density in corn cropland, two-, four- and seven-year-old alfalfa grasslands.Different lowercase in different year-old alfalfa grasslands and corn cropland indicates the difference was significant at 0.05 level.

Table 2Soil infiltration rates (mean ± SE) of corn cropland, two-, four- and seven-year-old alfalfa grasslands.

Infiltration rate Corn Alf. 2 year Alf. 4 year Alf. 7 year

IR-I (mm h–1) 74.40 ± 4.04b 77.67 ± 4.81b 93.80 ± 6.81b 157.07 ± 12.88aIR-II (mm h–1) 22.80 ± 1.62a 31.03 ± 3.70a 30.85 ± 0.78a 28.80 ± 4.85aIR-III (mm h–1) 14.25 ± 1.01a 21.57 ± 3.53a 19.72 ± 0.03a 17.23 ± 2.97aSIR (mm h–1) 11.58 ± 0.72a 18.53 ± 3.37a 15.90 ± 0.29a 13.78 ± 2.44aAIR (mm h–1) 26.19 ± 1.64b 33.57 ± 2.35ab 35.05 ± 1.34ab 42.85 ± 4.90a

Note: IR-I, IR-II and IR-III: average infiltration rates in stage I, II and III, respectively. SIR: steady infiltration rate; AIR: average infiltration rate. Different lowercase inthe same line indicates the difference was significant at 0.05 level.

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have also suggested that the capacity of reducing runoff and soil losseswas higher in older planted land.

These observed effects of plants on soil water infiltration capacitywas mainly due to the improvement of the soil characteristics, becausethe soil is the interface for infiltration (Cerdà, 1997; Sun et al., 2018).As many studies have suggested that the cultivation of alfalfa induced ahigher soil surface roughness and control the runoff generation andthen increase infiltration (Cerdà et al., 2017; Zhao et al., 2018). In thisstudy, IR-I was positively correlated with SWC, while IR-II and IR-IIIwere negatively correlated with SWC. Devitt and Smith (2002) alsosuggested that the rainfall infiltration rate was greater when the soilsurface was moist. However, when the infiltration process prolonged,higher SWC can hinder water infiltration (Wang et al., 2008b).

In addition, higher SOM content were not beneficial for soil waterinfiltration. This result did not agree with the generally acceptedstatement, which suggests that the higher SOM content would reduceBD, improving the soil water infiltration capacity (Huang et al., 2017;Eviner and Chapin, 2003). This apparent discrepancy may be explainedby the soil texture in this study, sandy type, which is characterized withthe lowest water holding capacity, although a previous study showedthat IR increased with increasing the content of sand (Sirimarco et al.,

2018). Some studies have also indicated that IR decreased with in-creasing the macro-organic matter content at the top 0–10 cm soil depthin a sandy loam soil (Barley, 1953). Thus, lower soil surface moistureand SOM favored higher soil water infiltration in the sandy soil. But thewater holding capacity is lower in sandy soil that the water cannoteffectively used by plants. In this study, the BD, SWC and SOM at thetop 0–30 cm soil depth of the alfalfa grasslands decreased compared tothe corn field. However, soil BD, SWC and SOM increased with in-creasing growth year of the alfalfa grasslands. The increasing SOMcontents can improve soil water holding capacity and increase effectivewater content in sandy soil (Libohova et al., 2018). The increase of BDis generally related to a reduction in soil porosity and macroporosity(Bonetti et al., 2019). This process led to the decrease of IR-II and IR-III.Namely, IR-I increased, while IR-II and IR-III decreased with increasingthe plantation age of the alfalfa grasslands.

The variation of IR was related to the changes of BD, SOM contentand SWC, as there were suggested in other studies (Neris et al., 2012;Sun et al., 2018). The changes in the soil characteristics were related tothe plant root traits (Angers and Caron, 1998; Gould et al., 2016). Inthis study, IR-II was significantly and positive correlated with RDWD.This result was accordance with the findings of Devitt and Smith(2002), who suggested that plant root plays an essential role in influ-encing soil water infiltrability. The reason that may contribute to thisobservation is related to the alternation of wet and dry periods, whenthe coarse roots of alfalfa would shrink and expand, forming stablechannels that favor soil water infiltration (Archer et al., 2002).

In this study, RDWD was higher in the three alfalfa grasslands thanin the maize cropland. And soil infiltration capacity of the alfalfagrasslands was improved, largely owing to the higher RDWD. The rootsof plants act as preferential flow pathways for water percolation, andthe higher RDWD of alfalfa could form a more opened root channelsystem (Devitt and Smith, 2002). Besides, the growth of the roots al-tered the distribution and size of the pores, while the roots replaced thesoil and occupied the pore spaces with the root growing (Barley, 1953;Eviner and Chapin, 2003). Then, the soil became compacted with thegrowing of roots, resulting in the larger BD around the root system(Eviner and Chapin, 2003). The RDWD increased with increasing thegrowth year of the alfalfa grasslands, which may result in the increaseof BD with increasing the plantation age. This led to the lower IR atinterspace locations than near the roots (Dunkerley, 2000). Further-more, plant roots can enhance soil cohesion by increasing the aggregatestability (Gould et al., 2016). Thus, the existence of root systems can fixthe sandy soil and reduce soil erosion (Bernatek-Jakiel et al., 2017). Inaddition, the increasing root biomass and length would increase theroot water absorption from more extensive and deeper soil layers toobtain more biomass, which may result in the decrease of SWC (Evinerand Chapin, 2003). These results indicated that the plant roots haddirect and indirect effects on soil infiltration capacity. Overall, theplanted alfalfa grasslands could significantly improve the soil infiltra-tion capacity and reduce the soil and water loess. Therefore, alfalfacould be used to fight against land degradation, enhance forage pro-ductivity in the Qinghai-Tibetan Plateau (Keesstra et al., 2018), whichcould achieve the goal of simultaneously improving agriculture pro-ductivity and protecting the sensitive environment.

5. Conclusions

Our results demonstrate that plantation of alfalfa grasslands im-proved the soil water infiltrability in the apline sandy soil throughchanging the soil characteristic, and the soil water infiltrability en-hancement increased with the plantation age. Some soil properties(bulk density and soil organic matter content) were negatively corre-lated with the soil infiltration rates, but the root dry weight density waspositively correlated with the soil infiltration rates. Higher soil watercontent was beneficial for the infiltration rates during the initial andfast (0–15min) stage, but it was adverse during the stable (15–45min),

Fig. 5. A comprehensive index of soil water infiltration capacity (FIR) for corncropland, two-, four- and seven-year-old alfalfa grasslands. Different lowercasein different year-old alfalfa grasslands and corn cropland indicates the differ-ence was significant at 0.05 level.

Table 3Correlation analysis between the soil characteristics of the 0–30 cm soil depthand the soil water infiltration rates (IR).

Soil parameter Soildepth(cm)

IR-I IR-II IR-III SIR AIR

0-10 0.434 −0.553 −0.637* −0.637* −0.008SWC 10-20 0.245 −0.571 −0.589* −0.582* −0.143

20-30 0.175 −0.431 −0.414 −0.406 −0.1080-10 −0.512 −0.375 −0.315 −0.302 −0.587*

BD 10-20 −0.058 −0.602* −0.678* −0.688* −0.42520-30 0.318 −0.517 −0.617* −0.619* −0.0970-10 0.235 −0.375 −0.519 −0.573 −0.096

SOM 10-20 −0.575 −0.490 −0.405 −0.365 −0.706*20-30 −0.521 −0.520 −0.448 −0.404 −0.688*0-10 0.698* 0.150 −0.005 −0.088 0.578*

RDWD 10-20 0.690* 0.076 −0.116 −0.174 0.52120-30 0.514 −0.055 −0.109 −0.134 0.365

Note: IR-I, IR-II and IR-III: average infiltration rates in stage I, II and III, re-spectively. SIR: steady infiltration rate; AIR: average infiltration rate. SWC: soilwater content; BD: bulk density; SOM: soil organic matter; RDWD: root dryweight density.

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remaining stable (45–75min), and steady (75–90min) stages. The soilcharacteristics (soil water content, bulk density and soil organic matter)of the alfalfa grasslands were significantly difference compared with thecontrol crop of maize and changed with increasing the growth year.Moreover, the soil water content, bulk density and soil organic matterof alfalfa grassland increased with the plantation age. Our study evi-dence suggests that the observed improvement of soil water infiltr-ability was beneficial for reducing the soil erosion risk and keepingecological function in alpine area of the Qinghai-Tibetan Plateau.

Acknowledgements

We thank the editors and anonymous reviewers for their con-structive comments and suggestions on this manuscript. This researchwas funded by the National Natural Science Foundation of China(NSFC41722107, 31760692), the Western Light Program of ChineseAcademy of Sciences (XAB2015A04, XAB2018B09), CollaborationProject of Universities and Local Governments of Tibet AutonomousRegion (603318014), and the Youth Talent Plan Foundation of theNorthwest A & F University (2452018025).

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