Baizhao Ren,Xia Li,Shuting Dong,Peng Liu,Bin Zhao,Jiwang Zhang*

State Key Laboratory of Crop Biology and College of Agronomy,Shandong Agricultural University,Tai'an 271018,Shandong,China

Keywords:Tillage system Summer maize Soil physical properties Root

ABSTRACT The standard cultivation system in the North China Plain is double cropping of winter wheat and summer maize.The main effects of this cultivation system on root development and yield are decreases in soil nutrient content and depth of the plow layer under either long-term no-tillage or rotary tillage before winter wheat sowing and no tillage before summer maize sowing.In this study,we investigated the combined effects of tillage practices before winter wheat and summer maize sowing on soil properties and root growth and distribution in summer maize.Zhengdan 958(ZD958)was used as experimental material,with three tillage treatments:rotary tillage before winter wheat sowing and no tillage before summer maize sowing(RTW+NTM),moldboard plowing before winter wheat sowing and no tillage before summer maize sowing(MPW+NTM),and moldboard plowing before winter wheat sowing and rotary tillage before summer maize sowing(MPW+RTM).Tillage practice showed a significant(P＜0.05)effect on grain yield of summer maize.Grain yields under MPW+RTM and MPW+NTM were 30.6%and 24.0%higher,respectively,than that under RTW+NTM.Soil bulk density and soil penetration resistance decreased among tillage systems in the order RTW+NTM＞MPW+NTM＞MPW+RTM.Soil bulk densities were 3.3%and 515%lower in MPW+NTM and MPW+RTM,respectively,than that in RTW+NTM,and soil penetration resistances were respectively 17.8%and 20.4%lower,across growth stages and soil depths.Root dry matter and root length density were highest under MPW+RTM,with the resulting increased root activity leading to a yield increase of summer maize.Thus the marked effects of moldboard plowing before winter-wheat sowing on root length density,soil penetration resistance,and soil bulk density may contribute to higher yield.

1.Introduction

Since the 1950s,the predominant cultivation model in the North ChinaPlain hasbeen thewheat-maizedouble cropping-system.In practice,rotary tillage is performed before sowing of winter wheat,and summer maize is sown with a no-till seeder without other seedbed preparation,directly after the harvest of winter wheat(RTW+NTM).The long-term effects of no-tillage include changes in the physical properties,fertility,and moisture of soil[1,2],leading to increased soil bulk density and penetration resistance[3].In China,the depth of the plowed soil layer has decreased in most regions under rotary tillage to＜15 cm,less than the 35 cm reported in the USA[4].Poor water infiltration and crop root development are major problems in areas of poorly drained heavy clay soils where no-tillage cultivation is used,and long-term benefits from no-tillage are not easily achieved[5].Tillage is a widely used soil management technique used to break crusts on the soil surface and modify soil physical properties.Soil bulk density[6–8],penetration resistance[9,10],water content and distribution[8,11],and pore size distribution[12]differ among tillage systems and regions.Once soils are sufficiently compacted to restrict root development,appropriate tillage can increase root depth[13].Deep tillage breaks up high-density soil layers and increases water infiltration and movement in soil,root growth and development,and crop yield[14].Different tillage practices have different effects on soil physical properties,but soil-loosening treatments are not required every year[15].

The two most common soil compaction indices are soil bulk density and penetration resistance.Bulk density is defined as the weight of dry soil that occupies a known volume.Soil penetration resistance is a simple way to measure soil strength.Greater bulk density reduces the growth of roots and shoots and inhibits nutrient uptake[16,17].Greater penetration resistance limits root growth and water and nutrient uptake by crops[18].Soil physical properties are critical for early root growth,enabling roots to absorb more nutrients and water[13].Changes in soil physical properties influence root growth and distribution,affecting crop growth and yield.The relationship between soil compaction and yield is not straightforward,but crop yields depend on root growth,which is strongly affected by soil compaction.A compacted soil layer impedes crop root growth in the top layer[19].High degrees of soil compaction and bulk density have been shown to confine corn roots almost entirely to the top 60 cm of soil[20].The effect of tillage on the vertical distribution of roots has been investigated in previous studies[13–15],but the results have been inconsistent.For this reason,examination of the combined effects of tillage before winter wheat and summer maize sowing on the morphology and spatial distribution of maize roots is desirable.

High crop yields depend on optimum root growth and suitable soil conditions[19].For this reason,it is important to evaluate soil tillage systems for sustainable agricultural production.Effects of tillage on soil properties and root growth have been studied worldwide[6–15].However,few studies have examined the combined effects of tillage before winter wheat and summer maize sowing on soil properties and maize root growth and distribution in double cropping systems.The objectives of the present study were to compare the combined effects of different tillage practices before winter wheat and summer maize sowing on root growth,soil penetration resistance,and soil bulk density after tilling,and toevaluatetherelationshipsofrootgrowth,soil properties,and grain yield response to tillage practices during the summer maize growing season in a winter wheat–summer maize double-cropping system in the North China Plain.

2.Materials and methods

2.1.Research site and soil conditions

This study was conducted in Mazhuang research field(35°59′N,117°00′E,86 m a.s.l.),Shandong province,North China Plain,China,from 2012 to 2013.The experimental region has a temperate continental monsoon climate.The mean total precipitation was 405 mm.The region has loam soils with a pH of 7.9.Mean organic matter,total N,rapidly available phosphorus (P2O5)and exchangeable potassium (K2O)contents in the upper 30 cm ofsoil were 15.71 g kg-1,1.12 g kg-1,46.65 mg kg-1,and 98.15 mg kg-1.Undisturbed soil cores(with diameter and height both 50 mm)were taken from different soil layers for measuring bulk density.In the top 30-cm layer,the soil bulk density was 1.54 Mg m-3.

2.2.Experimental design

The summer maize hybrid Zhengdan 958(ZD958),the most popular maize cultivar in China,was used as the experimental material.Each plot was 9 m×40 m=360 m2.Fifteen rows of maize were spaced 0.6 m apart in each plot.Experimental treatments consisted of rotary tillage before sowing winter wheat and no-tillage before sowing summer maize(RTW+NTM),moldboard plowing before sowing winter wheat and no-tillage before sowing summer maize(MPW+NTM),and moldboard plowing before sowing winter wheat and rotary tillage before sowing summer maize(MPW+RTM).RTW or RTM plots were tilled twice to a depth of 10 cm with a rotary cultivator.The MPW plots were plowed to a depth of 25 cm and then tilled twice to a depth of 10 cm with the rotary cultivator.The NTM plots were sown with no-till seeder without any other seedbed preparation.The details of the tillage treatments are described in Table 1.Each treatment was replicated three times in a completely randomized block design.

2.3.Sample collection

2.3.1.Soil bulk density

Bulk density was determined at the sowing date of maize(DS),sixth leaf stage(V6),twelfth leaf stage(V12),tasseling stage(VT),milk ripe stage(R3),and physiological maturity stage(R6)according to Ritchie and Hanway[21].For each sampling season,soil cores(diameter and height both 50 mm)were taken at 0–10 cm,10–20 cm,20–30 cm,30–40 cm,40–50 cm,and 50–60 cm depths.Three bulk density samples were randomly selected for measurements.

2.3.2.Soil penetration resistance(PR)

Penetrationresistancewasmeasuredat10-cmintervalsfromthe soil surface to 40 cm soil depth at DS,V6,V12,VT,R3,and R6 stagesusingamechanicalconepenetrometerwithaconehaving a 30°angle,length 45 cm,and diameter 21.5 mm(TJSD-750-II,Zhejiang Top Instrument Co.LTD,Zhejiang,China).Ten samples were randomly selected for measurement in each field location.

2.3.3.Root sampling

Root sampling was performed using the core method[22]at V6,V12,VT,R3,and R6 stages.For each plant sample,a plant was dried at 70±5°C and then weighed,and a block of soil surrounding the plant(60 cm long×24 cm wide×10 cm deep or 60 cm long×24 cm wide×20 cm deep)was removed.Each10-cm layer of soil down to 20 cm depth and each 20-cm layer from 20 cm to 60 cm depth was placed into a nylon netting bag.The roots were washed and collected after soil was passed through a 0.5-mm sieve using a hose and nozzle.Roots were then arranged and floated in shallow water on a glass tray(24 cm×32 cm)before scanning with an Scanjet(HP Scanjet 8200;Hewlett-Packard,Palo Alto,USA)and determining the number of root tips and root lengths with an image analyzer(Delta-T Area Meter Type AMB2;Delta-T Devices,Cambridge,UK).Root length density was calculated by dividing root length by soil volume.Root dry matter was determined after drying at 70±5°C to constant weight.

Table 2–Effects of tillage treatment on bulk density(Mg m-3)in different soil layers(2013).

2.4.Grain yield

Thirty ears harvested by hand from the three middle rows in each plot were used to determine yield(moisture content was approximately 14%)and ear traits including length,width,weight,row number,kernels per row,bald tip length,cob weight,and cob width.

Grain yield(kg ha-1)=harvested ears(ears ha-1)×kernel number per ear×1000-kernel weight(g 1000 kernels-1)/106×(1-moisture content%)/(1–14%).

2.5.Statistical analysis

Comparisons among treatments were based on Duncan's test at the 0.05 probability level(P＜0.05).Analysis of variance(ANOVA)was performed for grain yield,root dry matter weight,root length density,and soil bulk density using SPSS 17.0(SPSS Institute Inc.).

3.Results

3.1.Soil bulk density

Table 2 shows the effects of tillage system on bulk density through the 0–60 cm soil profile at each growth stage.Soil bulk density among tillage systems followed the order RTW+NTM＞MPW+NTM＞MPW+RTM.At the DS stage,the lowest bulk density in the 0–10 cm layer was observed in the MPW+RTM system,whereas the lowest bulk densities in the 10–20 and 20–30 cmlayerswerefoundintheMPW+NTMandMPW+RTM systems.AttheR6stage,thehighestbulkdensityinalllayerswas alsofoundintheRTW+NTMsystem.However,greatdifferences were observed among tillage systems in the 0–40 cm layer.Soil bulk density was greater at the end of the experimental season than at the beginning,and the greatest increase(15.0%)was observed in 0–10 cm layer in the MPW+RTM system.The average bulk densities in RTW+NTM,MPW+NTM,and MPW+RTM were 1.54,1.50,and 1.48 Mg m-3,respectively.

Soil bulk density increased with soil depth in all tillage systems.The largest increase from surface to bottom layer for all seasons was found in the MPW+RTM system,with a mean of 11.9%.The highest bulk density observed among alllayers was 1.67 Mg m-3in the 20–30 cm layer in the RTW+NTM system.The lowest bulk density was observed in the 0–10 cm layer in MPW+RTM.In the 0–10 cm layer,soil bulk density differed significantly among tillage treatments in the orderRTW+NTM ＞MPW+NTM ＞ MPW+RTM.In10–20,20–30,and 30–40 cm layers,bulk density of RTW+NTM was highest among tillage treatments.Soil bulk densities of MPW+NTM and MPW+RTM were 3.3%and 5.2%lower,respectively,that that of RTW+NTM across growth stages and soil depths(Table 2).The higher soil bulk density in RTW+NTM(Table 2)indicated that rotary tillage before winter wheat sowing and no-tillage before summer maize sowing produced an increase in soil compaction during the summer maize growth period.

Table 3–Effects of different tillage treatments on soil penetration resistance(MPa)in different soil layers(2013).

3.2.Soil penetration resistance

Soil penetration resistance across the 0–60 cm soil profile at different maize growth stages decreased in the order RTW+NTM＞MPW+NTM＞MPW+RTM(Table3).Soil penetration resistance initially increased and subsequently decreased with increasing soil depth in all treatments across the growing season.The greatest penetration resistance in all layers was observed in RTW+NTM.Soil penetration resistances of MPW+NTM and MPW+RTM were respectively 17.8%and 20.4%lower than those of RTW+NTM.

3.3.Root dry matter

Average root dry matter(RDM)of maize across the whole soil profile and growth period was affected by tillage,decreasing in the order MPW+RTM＞MPW+NTM＞RTW+NTM in the 10–60 cm layer(Fig.1-b,c,d),and in the order MPW+NTM ＞MPW+RTM ＞ RTW+NTM in the 0–10 cm layer(Fig.1-a).The RDM initially increased and then decreased as a singlepeak curve across the full growth period under all tillage treatments.The RDMs under the MPW+RTM and MPW+NTM treatments were significantly higher than that under RTW+NTM.At the V6 stage,the respective RDMs of MPW+NTM and MPW+RTM were 49.3%and 24.5%higher than that under RTW+NTM,.At the V12 stage,the RDMs of MPW+NTM and MPW+RTM were 27.5%and 34.9%higher than that of RTW+NTM,respectively.At the VT stage,the RDMs of MPW+NTM and MPW+RTM were 58.1%and 68.4%higher than that of RTW+NTM,respectively.At the R3 stage,the respective RDMs of MPW+NTM and MPW+RTM were 77.0%and 57.8%higher than that of RTW+NTM.

RDM in each soil layer also differed greatly among tillage treatments.At 0–10 cm,RDMs of MPW+NTM and MPW+RTM were respectively 44.2%and 29.8%higher than that of RTW+NTM(Fig.1-a).At 10–20 cm,RDM of MPW+NTM and MPW+RTM were 69.7%and 121.8%higher than that of RTW+NTM(Fig.1-b).At 20–40 cm,RDM of MPW+NTM and MPW+RTM were 798.1%and 275.5%higher than that of RTW+NTM(Fig.1-c).At 40–60 cm,RDMs of MPW+NTM and MPW+RTM were 68.6%and 97.5%higher than that of RTW+NTM(Fig.1-d).

Fig.1–Effects of different tillage treatments on root dry matter weight(g cm-3)in different soil layers(2013).RTW+NTM,rotary tillage before sowing winter wheat and no-tillage before sowing summer maize;MPW+NTM,moldboard plowing before sowing winter wheat and no-tillage before sowing summer maize;MPW+RTM,moldboard plowing before sowing winter wheat and rotary tillage before sowing summer maize.Means and standard errors based on three replicates are shown.

Fig.2–Effects of tillage treatment on root length density(RLD,km m-3)in different soil layers(2013).RTW+NTM,rotary tillage before sowing winter wheat and no-tillage before sowing summer maize;MPW+NTM,moldboard plowing before sowing winter wheat and no-tillage before sowing summer maize;MPW+RTM,moldboard plowing before sowing winter wheat and rotary tillage before sowing summer maize.Means and standard errors based on three replicates are shown.

3.4.Root length density

In general,root length density(RLD)differed greatly among tillage treatments,decreasing in the order MPW+RTM＞MPW+NTM＞RTW+NTM.During the full growth period,RLD initially increased and then decreased as a single-peak curve under all tillage treatments.At VT,the RLDs in MPW+NTM and MPW+RTM were respectively 27.3%and 54.8%higher than that in RTW+NTM.Soil depth also had a great effect on RLD,with 80%of roots accumulating in the top 0–10 cm layer,and RLD declining markedly with depth in the layers below.There were differences among MPW+RTM,MPW+NTM,and RTW+NTM treatments in the disturbed 0–40 cm zone(Fig.2-a,b,c),but the effects of tillage were minimal below the undisturbed 40–60 cm zone(Fig.2-d).In the 0–10 cm layer,the RLD of MPW+RTM was highest and that of RTW+NTM lowest among all tillage treatments(Fig.2-a).In the 10–60 cm layers,there were no significantdifferences between MPW+RTM and MPW+NTM treatments,but the RLDs of MPW+RTM and MPW+NTM were significantly greater than that of RTW+NTM(Fig.2-b,c,d).

Table 4–Yield and yield components of summer maize under different tillage treatments.

3.5.Yield

Tillage practice had significant(P＜0.05)effects on grain yield of summer maize(Table 4).Grain yields of MPW+RTM and MPW+NTM were 30.6%and 24.0%higher than that of RTW+NTM,and grain yield of MPW+RTM was 5.2%greater than that of MPW+NTM across years.For yield components,1000-kernel weight and number of ears per hectare differed significantly(P＜0.05)among tillage treatments.The 1000-kernel weights of MPW+NTM and MPW+RTM were respectively 5.3%and 7.2%higher than that of RTW+NTM.The numbers of ears per hectare under the MPW+NTM and MPW+RTM treatments were 4.3%and 9.2%higher than that of RTW+NTM,respectively.The number of kernels per ear did not differ significantly between MPW+RTM and MPW+NTM treatments,but values under these treatments were respectively 12.9%and 11.6%higher than that of RTW+NTM across years(Table 4).

4.Discussion

Tillage practices are key components of soil management systems that affect root growth and development[13–15].As shown in many previous studies,soil bulk density and soil moisture,which play an important role in soil–crop systems,are significantly affected by tillage practices[23–25].Soil penetration resistance and bulk density in the upper profile under no-tillage was higher than that under moldboard plowing[25],chisel plowing[16],and minimum tillage[26],leading to a decrease in root dry matter.The finding that soil penetration resistance and bulk density were higher under RTW+NTM than under MPW+NTM and MPW+RTM indicates that moldboard plowing before winter wheat sowing effectively broke up soil compaction in the deeper soil layers.The resulting root growth of summer maize resulted in a grain yield increase[21].Thus,deep tillage was an effective means of reducing soil compaction in a no-tillage systems,improving the early shoot growth of crops.

In previous studies,increased soil bulk density and penetration resistanceinhibited crop rootdevelopment[5,11].Plant root depth and root density were significantly increased by deep tillage[14].Root surface area and length were significantly lower under a no-tillage treatment than those under tillage[8].Our study showed that the RTW+NTM treatment had higher soil bulk density and penetration resistance,which would reduce root extension and restrict the uptake of nutrients and water,inhibit normal root growth,and accelerate root senescence[5,11],resulting in a marked reduction in shoot and root dry weight.However,the MPW+NTM and MPW+RTM treatments supported higher root dry matter and root length density,indicating that moldboard plowingwasconducivetorootgrowth,increasingthe efficiency of soil nutrient and water absorption by roots and thereby enhancing shoot growth and development.Our study also showed that tillage produced loose soil,which increased root penetration in the plowed layer,and widely distributed roots are beneficial for high yield.The finding that the grain yield of RTW+NTM was lower than that of MPW+NTM and MPW+RTM confirms that the grain yield of summer maize was affected by moldboard plowing before winter-wheat sowing.

In addition,soil penetration resistance and bulk density under the MPW+RTM and MPW+NTM treatments were lower than those under MPW+NTM,leading to better root development and higher yield of summer maize.Grain yield of MPW+RTM was only 5.2%higher than that of MPW+NTM across years.However,the increase in mechanical tilling operations in the MPW+RTM treatment would result in an increase in cost of RMB 900 yuan per hectare over MPW+NTM[27].Thus,the lower frequency of mechanical tilling under MPW+NTM than under MPW+RTM made it more economical and efficient.In summary,moldboard plowing before winter wheat sowing and no-tillage before summer maize sowing led to high yield and efficiency and may be a suitable tillage practice for the double-cropping system used in the Huang-Huai-Hai Plain.

Acknowledgments

This study was funded by China Agriculture Research System(CARS-02-18),National Basic Research Program of China(2015CB150404),Shandong Province Key Agricultural Project forApplication Technology Innovation,and Funds of Shandong “Double Tops”Program(SYL2017YSTD02).