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Soil Science and Plant Nutrition (2006) 52, 291–299 doi: 10.1111/j.1747-0765.2006.00037.x

© 2006 Japanese Society of Soil Science and Plant Nutrition

Blackwell Publishing, Ltd.ORIGINAL ARTICLENitrogen fixation by soybean with ridge tillageORIGINAL ARTICLE

N2 fixation of nodules and N absorption by soybean roots associated with ridge tillage on poorly drained upland fields converted from rice paddy fields

Tomoki TAKAHASHI, Hisashi HOSOKAWA and Morio MATSUZAKIHokuriku Research Center, National Agriculture Research Center, Niigata, 943-0193 Japan

Abstract

Sowing on elevated ridges is effective in reducing wet injury of soybean plants cultivated in upland fieldsconverted from rice paddy fields. Therefore, we investigated the effect of ridge tillage (RT) on soybean Naccumulation properties. We compared the amounts of plant N associated with N2 fixation of nodules andfrom soil and fertilizer in the RT treatment with amounts in conventional tillage (CT) in two fields in 2002–2003. Both fields were upland fields converted from rice paddy fields (Typic Hydraquents). The maindifference between the fields was the presence of a field underdrain. The amounts of Rb and K accumulatedin the shoots were also determined to estimate soybean root distribution. The grain yields with RTincreased in both fields from 106% to 129% compared with CT. Increased pod number and seed weightwere the major factors responsible for the yield increase. anova indicated that RT significantly increasedthe activities of both N2 fixation of nodules and N absorption by roots until R1 (flowering stage). The ratioof Rb and K accumulated in the shoots indicated that with RT, the root distribution was more abundant inthe superficial layers compared with CT. Thus, RT reduced wet injury during the rainy season that over-lapped the flowering stage. Nitrogen accumulation from N2 fixation until the R7 stage with RT was signifi-cantly higher than that with CT. We concluded that RT was effective in increasing N2 fixation of nodules inpoorly drained upland fields converted from rice paddy fields.

Key words: soybean, tillage, wet injury, nitrogen uptake, upland field converted from rice paddy field.

INTRODUCTION

Wet injury is a major constraint on soybean, Glycinemax (L.) Merr., cultivation in upland fields convertedfrom rice paddy fields (UFCPs) in Japan. Soil structureis poorly developed in many of these fields. In addition,these fields display a low permeable layer under theplow layer as a result of puddling for the preceding ricecultivation. In most cases, wet injury occurs duringthe initial stages of soybean growth, which overlaps therainy season (from June to July) in Japan. According toSugimoto (1994), the earlier the stage at which soybeanexperiences wet injury, the greater the decrease in yield.

Declining soybean yield mainly results from a decreasein pod number by N deficiency. Takahashi et al. (2005)confirmed Sugimoto’s (1994) conclusion based on asurvey of 33 farmers’ UFCPs that had experiencedwet injury. They concluded that a decrease in the podnumber, which resulted from poor drainage, was themajor reason for yield decrease.

Sowing soybean seeds on elevated ridges increasedthe yield in poorly drained UFCPs (Hosokawa 2004,Hosokawa et al. 2005). In contrast to ridge tillage (RT)in the Corn Belt of the USA (Hatfield 1998), the majorobjective of raised ridges in Hosokawa’s case was toalleviate wet injury and not soil erosion. According tothe studies conducted by Hosokawa, RT, whereby theseedbed is raised to 15 cm, increased the yield to 110%or 120% of that in conventional cultivation (i.e. tillagewithout raised seedbeds). Hosokawa concluded that theapparent decrease in the water table by RT increased theoxygen concentration around the rhizosphere, whichresulted in increases in branch and pod numbers per

Correspondence and present address: T. TAKAHASHI, Godo4433, Omaezaki city, Kaigan Sachi Branch, ShizuokaAgriculture Experiment Station, 437-1613, Japan. E-mail:[email protected] 23 June 2005. Accepted for publication 10 January 2006.

292 T. Takahashi et al.


plant. However, the characteristics of both N2 fixationof nodules and N absorption by roots associated withRT have not been fully elucidated. These N accumu-lation characteristics should be clarified because soybeanyield is affected by soil N fertility when wet injury is notsevere (Takahashi et al. 2005). After clarification, theRT method could be applied to various fields.

Nitrogen accumulation properties of soybean are morecomplex than those of other crops because half or moreof the accumulated N in soybean is derived from N2

fixation of nodules. Several methods have been devel-oped to estimate the ratio of N derived from N2 fixation(Herridge and Peoples 1990), including the N accumu-lation difference between nodulating soybeans andnon-nodulating isoline, 15N dilution method, acetylenereduction method and the relative ureide method. Amongthese methods, the relative ureide method offers numerousadvantages for the study of wet injury because: (1) itis reliable for field experiments, (2) it is applicable forestimation through a wide range of growth stages, (3) itdoes not require special apparatus for estimation(Herridge et al. 1990; Takahashi et al. 1992).

The objective of the present study was to evaluatethe effect of RT on both N2 fixation of nodules and Nabsorption by roots using the relative ureide method.

MATERIALS AND METHODS

Fields and soilsTo evaluate the effects of RT on N2 fixation of nodulesand N absorption by roots of soybean, we used twoUFCPs with different soil moisture conditions, sites Aand B, located in Joetsu City, Niigata prefecture, Japan.Both sites had been used as paddy fields prior to 2001and were converted to upland fields in the spring of 2002.There were no climatic differences between the two fieldsbecause the fields were separated by 100 m. The investi-gations were conducted from 2002 to 2003. Table 1shows the soil classification, pH, soil texture and soil N

properties, and the height of the water table in bothfields. Figure 1 shows the time-course of the volumetricwater content of the soil at both sites. Severe wet injurywas observed at site B with conventional tillage (CT),where an underdrain was not installed and the soilexhibited higher clay content. The color of the leaves hadturned yellow and leaves from the lower nodes had fallenduring the rainy season at site B. These were typicalsymptoms of wet injury, as described by Sugimoto (1994).Site A had an underdrain and was drier than site B. Wetinjury at site A during the rainy season was not assevere as that at site B.

Cultivation methodsIn 2002 and 2003, the soybean cultivar “Enrei” wassown on 2 June and 3 June and harvested on 1 Octoberand 2 October, respectively. Tillage, making of ridges,sowing and basal application of fertilizers were conductedin one process using a machine developed by Hosokawa(2004). The spacing of each row was 75 cm long and twoseeds were sown at 18 cm intervals. Height of the seed

Table 1 Soil classification, texture, pH, soil N, amount of mineralized N and height of water table in the two fields

Site A Site B

Classification of cultivated soils in Japan† Mottled Gley Lowland soils Strong Gley Lowland soilsSoil Taxonomy‡ Typic Hydraquents Typic HydraquentsClay content (%) 27 45Silt content (%) 30 41Sand content (%) 44 19pH (H2O) 5.5 5.0Soil N (%) 0.15 0.28Mineralized N (mg N kg−1) 50.3 69.3Height of water table (m)§ 0.7 0.4

†Cultivated Soil Classification Committee (1995); ‡Soil Survey Staff (1994). §Measurement was conducted in October 2003.

Figure 1 Difference in the volumetric water content betweentwo cultivation treatments (ridge tillage [RT] and conventionaltillage [CT]) at 5 cm depth from the soil surface at sites A andB in 2003.

Nitrogen fixation by soybean with ridge tillage 293


locations was 15 cm above the inter-row field surface.In CT the process was identical to that for RT, except forthe lack of ridges. Hence, the only difference between thetreatments was the vertical location of the seeds. Basalapplication of fertilizers was conducted by side-dressingat a distance of 5 cm from the seeds. The respectiveamounts of nitrogen, phosphorus and potassium usedfor basal application were 1.6, 6.0 (as P2O5) and 8.0 (asK2O) g m−2. The nitrogen fertilizer applied consisted ofammonium sulfate. We did not use organic fertilizers orapply any topdressing.

Analytical methods for soilsSoil samples to measure the amount of mineralized Nwere taken in March 2002 from the plow layer (approxi-mately 13 cm depth). The amount of mineralized N wasdetermined by incubation at 30°C under field moistureconditions (Methods of Soil Environment Analysis Com-mittee 1997) for 4 weeks. The amount of mineralizedN was calculated from the sum of nitrate, nitrite andammonium N. Soils used for the N measurementdisplayed fresh moisture conditions without drying.

We used the method referred to as “ratio of similarelements” (Takahashi 1996) to evaluate the root systemof soybean. This method is based on the fact that theabsorption behavior of rubidium is similar to potassiumfor plants (Takahashi 1996) and on the assumption thatK is located in more superficial layers than Rb becauseof the application of fertilizers. The tendency of plantroots to be distributed in deeper layers was evaluated byusing the Rb/K ratio in plants. We verified the assumptionby analyzing the Rb/K ratio of exchangeable cations inthe soil profiles. Soil samples from 0–10, 10–20 and 20–30 cm depths were taken in October 2002 for estimationof the Rb and K distribution toward the vertical soilprofiles. Exchangeable Rb and K of soils were extractedtwo times with a 1 mol L−1 pH 7 ammonium acetatesolution for 1 h with shaking. The soil to solution ratiowas 1/50 g mL−1.

In 2003, the field moisture content of each plot wasmeasured by inserting time domain reflectometry(TDR) probes at a 5 cm depth from the surface.

Sampling procedures and analytical methods for soybeansSoybean plants in each plot were sampled (0.75 m2) atthe V4 (only in 2003), V5, R1, R3, R5 and R7 stages tomeasure dry weight and amounts of accumulated N, Kand Rb in shoots. The terminology of the growth stagesof soybean followed that proposed by Fehr et al. (1971).At each sampling time, four plants were selected andthe xylem sap was collected from a stem cut close to theground using cotton for 2 h. The sampled xylem sap wasfrozen immediately until an analysis of the composition

of the xylem sap solution was carried out. Samplingfrom the V4 to R7 stages was duplicated. Sampled plantswere dried at 80°C immediately, the dry weight wasmeasured and the plants were ground for N, Rb and Kmeasurements. Total plant N content at each stage wasdetermined using the combustion method (Rapid-N III,Elementar Inc., Hanau, Germany). Rb and K in theshoots of the soybean plants were extracted using50 mL of 0.1 mol L−1 HCl for 0.5 h with shaking; thesample to solution ratio was 1/50 g mL−1 (Takahashiet al. 1991).

Estimation of the N2 fixation ratio was carried outaccording to the method of Takahashi et al. (1992). Inbrief, nitrate, amide and ureide concentrations in thexylem sap solution were determined by colorimetry usingthe methods of Cataldo (1974), Herridge (1984) andYoung and Conway (1942), respectively. The N of thenitrate and amide forms was considered to correspondto the N absorbed from soil or fertilizer by the roots.Ureide form N was considered to be derived from N2

fixation of nodules. The amount of N derived from N2

fixation of the nodules, soil or fertilizer was estimatedas follows:

NSi = NSi−1 + (TNi − TNi−1) FFi (1)

NFi = NFi−1 + (TNi − TNi−1)(1 − FFi) (2)

In these equations, NS, NF and TN, are the amountof N derived from soil or fertilizer, N derived from N2

fixation and total N, respectively. FF is the ratio of theamount of N derived from N2 fixation per total N in thexylem sap solution and i indicates the sampling step. Itmust be noted that the above estimation is likely to beambiguous if TN in the plant is lost by, for example,falling leaves. Hereafter, we use the terms plant N todesignate the total amount of accumulated N in a plant,fixed N as the amount of N derived by N2 fixation ofnodules and absorbed N as the amount of N derivedfrom soil and fertilizer by root absorption.

At the harvest stage, 1.5 m2 of soybean was sampledand used to measure grain yield, dry weight, pod number,seed number and seed weight. Sampling at harvest wasconducted in four duplications in 2002 and three dupli-cations in 2003. Grain yield was expressed as 15% ofthe seed moisture content.

RESULTS AND DISCUSSION

Growth and yield of soybeanThe average temperature, solar radiation and precipita-tion for 2002 and 2003 are shown in Fig. 2. The weatherconditions were different in 2002 and 2003. The rainyseason lasted from 11 June and 12 June and ended on23 July and 1 August in 2002 and 2003, respectively.



Precipitation during the rainy season was relatively lowin 2003. Abundant precipitation was recorded fromlate August to early September in 2003. Figure 1 showschanges in the water content of soils in 2003. Watercontent of the soil at site B was always higher than thatat site A. In contrast to site B, the difference in the watercontent of the soil between the two croppings appearedonly during the rainy season at site A. Such poor drainageat site B was caused by the absence of an underdrain andhigher soil clay content. The days after sowing (DAS) foreach growth stage were 31 (V5), 50 (R1), 66 (R3), 79(R5), 98 (R7) and 121 (harvest) in 2002, and 22 (V4),32 (V5), 51 (R1), 70 (R3), 85 (R5), 108 (R7) and 121(harvest) in 2003. The flowering stage (R1) occurredlate or at the end of the rainy season in both years.

Yield and related properties of soybean are shown inTable 2. anova showed that although RT cultivationincreased the soybean yield in both years (Table 3), theaverage yield in each year was quite different, reflectingthe difference in the weather conditions. The growth ofthe soybean plants was relatively less active at site Bthan at site A. The level of significance was less than5% for the dry weight, pod number and seed weightbetween the two sites, but 7.4% for the grain yield. Thislower significance of the grain yield might result from alarger deviation of the values of grain yield. Increased

yield with RT was observed at both sites A and B.Although site B experienced more severe moisture con-ditions (Fig. 2), the yield at site B with RT was higherthan that at site A without RT. Hosokawa et al. (2005)revealed that the height of the water table from thesurface in the case of RT was 10–15 cm lower than thatfor CT. This apparent decrease in the water table wasconsistent with the height of the raised ridges. Furthermore,O2 concentration within the plow layer did not decreasebelow 13% for RT, although these values decreased to3% for CT. The results indicated that RT amelioratedthe soil air conditions during the rainy season, whichresulted in an increase in yield with RT. The samemechanisms underlying the reduction of wet injury mighthave operated in our experiments because we used thesame machine and fields as Hosokawa et al. (2005).

In previous studies on the effects of RT on soybeanyield, a significant yield increase has not been confirmed(Eckert 1987; Fausey 1990; Vyn et al. 1990). The differ-ence between our results and those of Hosokawa et al.(2005) and the results from other investigators may beascribed to differences in the field moisture conditions.For example, Hatfield et al. (1998) showed that theupper limit of the water table of the field where theyconducted RT experiments ranged from 0.7 m to 6 m,with an average value of 3 m. The corresponding values

Figure 2 Average temperature, solar radiation and precipitation for each 5-day period during the cropping season of soybean at sitesA and B. Arrows indicate the rainy season in a normal year.

Table 2 Yield and related properties of soybean at harvest

Grain yield (g m−2)

Dry matter (g m−2)

Pod number (m−2)

Seed number (m−2)

Seed weight (g)

2002 Site A CT 353 539 755 1,260 0.280RT 378 577 786 1,270 0.296

Site B CT 348 515 734 1,210 0.286RT 375 586 800 1,310 0.287

2003 Site A CT 317 506 668 1,090 0.289RT 398 601 742 1,250 0.320

Site B CT 292 435 519 941 0.311RT 331 481 537 943 0.351

RT, ridge tillage; CT, conventional tillage.



of sites A and B in our study were lower, 0.7 m and0.4 m, respectively, in October (Table 1). It is reasonableto assume that the water table during the summer seasonwas higher than that in October because the fields thatsurrounded both sites were flooded for paddy rice cultiv-ation and RT treatment might be more effective undersuch conditions.

N2 fixation of nodules and N absorption by rootsThe ratio of N derived from N2 fixation in the xylem sapsolution, FF in Eq. 1, is shown in Fig. 3. Although thepeak occurred relatively earlier in 2003, the time-courseof the ratio was almost the same throughout all plots;that is, the ratio was low at the V5 stage, increased andtouched a peak from the R1 to R5 stages, and decreasedafter the R5 stage. There was no clear difference betweenRT and CT. Although it appears that the ratio of fixed Nwas too high at the V4 stage or too low at the V5 stage in2003, the reasons for such phenomena have not beendetermined.

The RT treatment increased the amount of plant N insoybean plants during almost all stages aside from thevery early stage (Table 4). We could not evaluate thenet N accumulation at harvest because soybean leavesfell after the R7 stage. Thus, the amount of plant N atharvest was the apparent amount of plant N. Thisamount could not be factored into N2 fixation and theamount derived from root absorption.

anova results of plant N at the R1 stage (end of rainyseason) and R7 stage (the last stage at which we wereable to evaluate the net N accumulation) are shown inTable 5. At the end of the rainy season (R1), the amountof plant N was significantly lower at site B than site A(Table 4). Even under such conditions, RT treatmentincreased significantly both the amount of fixed N andabsorbed N (Table 5). Hence, the absence of significantdifferences between FF for RT and CT (Fig. 3) does not

imply that RT did not increase the amount of fixedN, but indicates that the activities of both fixed Nand absorbed N increased. There are two possibilitiesof mechanisms for promoting early growth of soybean,namely reduction of wet injury and warming of theseedbed. According to Radke (1982) because RT warmsthe seedbed, establishment and plant growth are promotedin early spring. Although we cannot deny that the effectsreported by Radke (1982) could have been effective inour study, we consider that the reduction in wet injurywas more likely to have increased N accumulationfor the following reason. By specifically examining theresults for site A, the beneficial effect of RT on theincrease in the amount of plant N was unclear at the V5stage in 2002 and the V4 stage in 2003, but becamemore evident at the V5 and R1 stages, respectively. Thispattern suggested that RT did not always increase theamount of plant N before, or at the onset of, the rainyseason. In other words, the effect of RT started to appearfrom the rainy season onward. Therefore, this effectcannot be explained by warming of the seedbed. Accord-ing to Torigoe et al. (1981), lower branches of soybeanplant that differentiate at an earlier growth stage have

Table 3 Sources of variation, degrees of freedom and mean squares for soybean properties at harvest in an analysis across years,sites and methods of cultivation

Source df

Mean squares

Grain yield Dry weight Pod number Seed weight

Year (Y) 1 5,687** 16,114** 159,198**** 63.075****Site (S) 1 3,278* 14,197** 42,101*** 7.253**Y×S 1 3,060* 13,144** 51,482*** 13.872***Cultivation (C) 1 11,618*** 25,980*** 16,032* 27.195****Y×C 1 1,992 468 11 12.826***S×C 1 493 11 38 0.373Y×S×C 1 886 2,918 3,504 2.751Error 20 936 2,025 4,813 1.333

*P < 0.1; **P < 0.05; ***P < 0.01; ****P < 0.001.

Figure 3 Comparison of the time-course of the ratio of Nderived from N2 fixation of nodules per plant N (ratio of fixedN) with the differences in years, cultivation treatments andsites. Letters above the x-axis denote the growth stage ofsoybeans. RT, ridge tillage; CT, conventional tillage.



the potential to bear many more nodes and pods, andthe differentiation of these branches is sensitive to plantconditions. Furthermore, they concluded that II, III andIV branches, which corresponded to lower branchesin their nomenclature, tended to bear many nodes andpods. The period when these branches differentiatedcorresponded to the V5 to V7 stages, which coincidedwith the rainy season in our study. Torigoe et al.’s(1981) study implied that plant N accumulation, whichcould be improved by RT in the rainy season, resultedin an increase in pod number (Table 2) through anincrease in the number of branches at earlier stages.

At the R7 stage, RT significantly increased the amountof N derived from N2 fixation, while the amount ofabsorbed N did not increase significantly (Table 5),presumably because of the lower amount of N absorbed

with RT at site A in 2003. The amount of N absorbedwith RT was higher at site A in 2002 and at site B in2003. In other words, RT increased the amount of Nderived from N2 fixation unlike that of N derived fromabsorption by roots through sites and years. It is wellknown that O2 concentration in soil is an importantfactor facilitating N2 fixation of root nodules (Ae et al.1983). It is reasonable to conclude that the effect of RTwas mainly associated with an increase in N2 fixation.High N2 fixation can alleviate N deficiency during therainy season and induce a high value for the seed weight(Table 2) at the maturity stage. Takahashi et al. (1992)considered the possibility that a high leaf area index(LAI) at the R1 or R2 stage resulted in efficient seedgrowth through an increase in the amount of photosyn-thates. Takahashi et al.’s (1992) study pointed out that

Table 4 Estimated amount of N derived from N2 fixation of nodules (fixed N) and of N absorbed by roots (absorbed N)

Site A Site B

Total Fixed N Absorbed N Total Fixed N Absorbed N

CT RT CT RT CT RT CT RT CT RT CT RT

2002V5 1.3 1.3 0.25 0.26 1.1 1.0 0.41 0.83 0.11 0.19 0.30 0.64R1 4.6 5.7 3.2 4.2 1.4 1.5 4.5 5.0 3.3 3.6 1.1 1.4R3 7.3 7.6 5.6 6.0 1.8 1.6 9.6 11 8.0 9.3 1.6 2.1R5 16 14 14 13 2.6 1.9 19 21 16 17 2.7 3.2R7 18 23 14 18 3.1 5.2 26 26 20 20 5.6 6.02003V4 0.29 0.29 0.18 0.11 0.11 0.18 0.21 0.25 0.07 0.13 0.14 0.12V5 1.2 1.7 0.31 0.42 0.9 1.3 0.65 0.76 0.15 0.18 0.50 0.59R1 4.8 5.9 3.4 4.1 1.3 1.8 2.8 2.9 2.0 2.0 0.84 0.90R3 11 11 8.5 8.5 2.6 2.8 8.2 9.7 6.0 7.5 2.2 2.2R5 14 19 11 15 3.4 4.5 12 15 8.8 11 3.1 3.8R7 19 21 12 15 6.0 5.6 18 26 12 16 5.7 9.8

Values are g N m−−−−2.

Table 5 Sources of variation, degrees of freedom and mean squares for soybean N fixation and absorption of R1 and R7 stages inan analysis across years, sites and methods of cultivation

Source df

Mean Squares

R1 R7

Total Fixed N Absorbed N Total Fixed N Absorbed N

Year (Y) 1 0.061* 2.04*** 0.061** 25.2 74.61**** 13.09*Site (S) 1 2.052**** 3.94*** 0.824**** 54.1** 14.46**** 12.69*Y×S 1 0.001 2.45*** 0.273**** 12.8 15.31** 0.111Cultivation (C) 1 0.273*** 0.980** 0.17*** 75.9** 31.22*** 9.78Y×C 1 0.015 0.102 0.019 4.87 2.63 0.334S×C 1 0 0.456 0.015 0.098 1.23 2Y×S×C 1 0.166 0.009 0.101** 30.2 5.7 9.63Error 8 0.114 0.165 0.009 8.27 1.98 3.08

*P < 0.1; **P < 0.05; ***P < 0.01; ****P < 0.001.



N2 fixation decreased after the R5 stage and senescencewas attributed to competition for photosynthatesbetween seed growth and N2 fixation of nodules.Recently, Takahashi et al. (2005) have supported thishypothesis by indicating that the amount of soil N min-eralized until the R1 stage was significantly correlatedwith seed weight, if wet injury did not decrease the podnumber severely. These results are fully consistent withour findings. Therefore, it can be considered that reduc-ing wet injury in the rainy season increased LAI,which led to a high N2 fixation activity of the nodulesand seed weight at the maturity stage.

We must also consider the possibility that the contri-bution of absorbed N was underestimated. The fact thatthe amount of plant N increased with RT (Table 4),while the ratio of N2 fixation of nodules did not changewith CT (Fig. 3), indicated that the amount of N absorbedalso increased with RT. RT increased the amount of Nabsorbed except at site A in 2003. The contribution ofN absorption by roots increased gradually after the R5stage because the ratio of N2 fixation was at a maximumat the R1–R5 stages, after which it decreased (Fig. 3),whereas the amount of total plant N of soybean increaseduniformly (Table 4). Although we could not estimatethe amount of N absorbed after the R7 stage, if it couldbe estimated the effect of RT on the increased amountof N absorbed could be clarified. In contrast, RT didnot always increase the amount of N absorbed at site Ain 2003. We will discuss the relationship between rootextension and the amount of N absorbed in this plot inthe next section.

Effect of RT on the development of the root systemThe profile of the ratio of exchangeable Rb to K in soilchanged with soil depth (Fig. 4), which supports the useof the Rb/K ratio in soybean shoots as an indicator ofroot distribution (Takahashi 1996). Lower Rb/K ratiosin plants indicate that the root system of soybean isdistributed in the superficial layers.

The Rb/K rate of soybean was significantly lower withRT at the R1 stage (Tables 6,7), which indicates thatmost of the developing root system was located withinthe plow layer with RT. RT raised the seedbed by usingsoil from the plow layer. Thus, the thickness from thesoil surface to the subsoil was higher in RT than CTand the depth of the surface of the subsoil was the sameas that with CT. The pattern of the Rb/K ratio mayindicate that the root system with RT was located in amore superficial area than that with CT, but it wouldnot necessarily indicate that the root system with RTwas thicker than that with CT. However, this resultdoes imply that most of the developing root system didnot reach the subsoil: this root system was suitable for

reducing wet injury in the rainy season. At the R7 stage,because the value of the Rb/K ratio increased, the rootsystem was distributed in deeper layers than at the R1stage. The significant difference in the Rb/K ratio betweensites A and B at the R7 stage (Table 7) may reflect thedifference in the gradient of the Rb/K ratio between thetwo fields (Fig. 2), but not the soil moisture conditions.No significant differences in the Rb/K ratio were observedbetween RT and CT at the R7 stage, indicating that theroot distribution was oriented toward the vertical direc-tion with RT and that there was no difference with CTat the R7 stage. The fact that the root system with RTwas distributed in the superficial zone in the rainy seasonimplies that after the rainy season the soybean rootswith RT extended more vigorously than with CT. Thisconclusion and the fact that RT increased the uptake of

Figure 4 Ratio of exchangeable Rb and K in vertical soil profileat sites A and B. The ratio is expressed on a weight basis.

Table 6 Ratio of Rb and K in soybean at R1 and R7 stages

Site A Site B

CT RT CT RT

2002R1 14 8.8 11 12R7 13 13 19 252003R1 15 13 16 13R7 20 17 24 29

Values are mole ratio ×100,000.



N derived from soil at the R5 stage or later indicatedthat the soybean roots with RT displayed high activityat the maturity stage of soybean growth. The lowerRb/K ratio with RT at site A in 2003 than with CT isconsistent with the fact that soybean plants in this plotabsorbed a lower amount of N than plants with CTduring the R5–R7 stages (Table 4). While the reason forthis remains to be determined, it appears that soybeanroots with RT did not necessarily extend vigorouslythroughout the plot. As Takahashi et al. (2003) empha-sized, N uptake after the R5 stage is an importantstrategy to increase soybean yield. Further studies shouldbe carried out to elucidate the relationships among RT,root systems and N accumulation during the soybeanmaturity stage.

Conclusions1 RT significantly increased pod number, seed weight

and grain yield in two UFCPs during a 2-year period.2 RT significantly increased the amount of plant N.

Clear differences in the ratio of the amount of Nderived from N2 fixation of nodules per total plantN were not observed between CT and RT.

3 RT increased both N2 fixation of nodules and Nabsorption by roots at the end of the rainy season(R1). Roots were distributed within more superficiallayers than with CT at this stage. Reduction of wetinjury at this stage is considered to increase thenumber of soybean pods.

4 Throughout the growth stages until R7, RT signifi-cantly increased the amount of N derived from N2

fixation. The increase in the amount of N absorbedwith RT was observed at many stages of the experi-ments, but the effect was neither stable nor significant.

ACKNOWLEDGMENTS

We deeply appreciate the technical assistance providedby Mr J. Asaoka and Mr K. Yazaki. We would also like

to thank Mrs A. Okuda for her assistance with thechemical analysis.

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Table 7 Sources of variation, degrees of freedom and meansquares for Rb/K at R1 and R7 growth stages in an analysisacross years, sites and methods of cultivation

Source df R1 R7

Year (Y) 1 36.6**** 81.9***Site (S) 1 0.123 274****Y×S 1 0.122 0.004

Cultivation (C) 1 19.8**** 15.6Y×C 1 0.203 0.666S×C 1 8.12 8.45Y×S×C 1 15.6 0.193Error 8 0.125 5.72

***P < 0.01; ****P < 0.001.



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