Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657, JapanJapan International Research Center for Agricultural Sciences, 1-1, Ohwashi, Tsukuba, Ibaraki, 305-8686, Japan
r t i c l e i n f o
rticle history:eceived 30 March 2012eceived in revised form 6 September 2012ccepted 9 September 2012
Development of alternative soil fertility management is opted to mitigate the side effects associated withthe excessive use of synthetic fertilizer. Soil fertility management practice through strengthening internalnutrient cycling (agroecological management: AEM) was examined in lowland rice (Oryza sativa) farmingin Tochigi Prefecture, Japan. The principal study fields were established AEM (EAEM), where AEM wasestablished in 1999 and transitional AEM (TAEM), where AEM has been practiced since 2009 and thefield is still in transient stage. A field with conventional nutrient management (CNM) was studied forreference purposes.
In EAEM field, potential amount of soil N supply was estimated at 41.9 g N m−2 in spring 2011. Compo-nents in AEM maintained labile N pool were basic mineralizable N (21.0 g N m−2) and annually changingmineralizable N (20.9 g N m−2). Annually changing mineralizable N include soil drying effect (3.6 g N m−2),internal inputs (13.2 g N m−2) such as rice straw, rice bran, spring and winter weed as well as externalinputs (4.1 g N m−2) such as biological N2 fixation, precipitation and guano. Estimated NH4–N releasedfrom soil during the cropping season was 28.5 g N m−2 in 2011. As the N uptake of rice was 11.5 g N m−2
in 2011, EAEM supplied sufficient N to satisfy N demand by the rice plants. Relatively large amount of
NH4–N released in EAEM implies that the recycling of on-site N sources would supply substantial amountof N to rice plants. In TAEM (two and three year practice), limited amount of NH4–N (17.8 g N m−2) avail-able during the cropping season compared with EAEM emphasized the importance of long-term practiceto ensure effects of AEM through increase of basic soil mineralizable N. The research result suggests thatAEM might be a sustainable and appropriate soil fertility management option for lowland rice farming ifmeasures such as adjusting application rate of rice bran are employed to control soil N supply.
. Introduction
Nitrogen (N) is one of the essential nutrients in agriculturalroduction. Over the past several decades, the intensive use ofynthetic N fertilizers in combination with high-yielding varieties,rrigation and agrochemicals for plant protection has resulted in aignificant increase in productivity of major cereal crops such asice (Oryza sativa L.), wheat (Triticum spp.) and maize (Zea mays.) (Evans, 1993). Among the major cereals, rice is the most widelylanted staple crop which supports more than half of the world’sopulation (FAO, 2004). With appropriate supply to rice plants,
can contribute to grain filling by improving the photosynthetic
apacity and enhancing carbohydrate accumulation in culms andeaf sheaths (Mae, 1997). N is also a critical element for promot-ng growth and tillering during the vegetative stage. It enhances
spikelet production during the early panicle formation stage in riceplants.
However, as the synthetic fertilizer became a dominant sourceof N supply in agricultural production, a number of issues haveemerged. They are excessive N influx into the terrestrial and coastalecosystems, degradation of water quality, heavy dependence onfossil fuels and vulnerability to fluctuation in fuel prices. In orderto mediate these negative impacts and to ensure more sustain-able agricultural production, there is a need to supply N from othersources.
One approach is to increase soil N mineralization throughstrengthening internal N cycling (Drinkwater and Snapp, 2007).Soil fertility management practice relying on internal nutrientcycling (herein after referred to as agroecological management:AEM) avails against fossil fuel dependence and environmental
degradation. Examples of AEM include incorporating crop residue,intercropping leguminous crops and growing catch vegetation.
A large number of research has demonstrated effectiveness ofAEM for upland cereals and vegetables (LaRue and Patterson, 1981;
yland et al., 1996; Thorup-Kristensen et al., 2003). However,ome studies have reported reduced yields. Nutrient deficiency haseen identified as a key constraint under alternative nutrient man-gement practices (Mäder et al., 2002). For instance, wheat grownnder AEM was reported to suffer from insufficient N supply whichesulted in lower yields compared with conventional nutrient man-gement (Baresel et al., 2008).
However, a recent review demonstrated that relative yieldifference between conventional and alternative managementsary substantially among the crop types (De Ponti et al., 2012).s for rice, although AEM showed relatively high performance,esearch is still limited on N availability and N supply capac-ty under AEM (De Ponti et al., 2012). Since irrigated lowlandice accounts for 50% of the total rice area and contributes to5% of the global total rice harvest (IRRI, 2011), it is importanto elucidate potential and shortcomings of AEM for lowland ricearming.
In this case study at a farmer’s fields in Japan, wenvestigated availability of soil N supply and contribution ofarious N supply components under AEM in lowland ricearming.
. Materials and methods
.1. Research design
The research was conducted at farmer’s lowland paddy fields inagawano, Nogi-town, Simotsuga-county, Tochigi prefecture, Japanlatitude 36◦14′N, longitude 139◦46′E) during the summer monthsrom June to October in 2010 and 2011. The soil type of this studyrea is Andosol.
Under AEM, rice straw was incorporated into soil in autumn,nd so were the weeds that grew during off season in springnd rice bran at transplanting. Table 1 presents N inputs at thetudy fields. Autumn ploughing was conducted to enhance ricetraw decomposition. Since the change in soil fertility is reportedo take several years of AEM practice (Kobayashi et al., 2007;amaki et al., 2002), we compared the established AEM (EAEM)eld where AEM has been practiced since 1999 and a transi-ional AEM (TAEM) field where AEM started in 2009. In 2011,nother AEM field was included at the proximity of the EAEM
eld. The farmer reduced the amount of rice bran by 30% in011 (1.1 g N m−2), and, hence, the field is referred to as EAEM-. A field with conventional nutrient management (CNM) wasncluded for reference purposes. The CNM field received synthetic
able 1 input in the study fields in 2010 and 2011.
Field Nitrogen input (g N m−2)
Rice strawa Rice branb Guanoc
2010EAEM 3.4 1.6 0.1
TAEM 3.4 2.1 0.2
CNM 0.0 0.0 0.0
2011EAEM 5.0 1.6 0.1
EAEM-L 5.0 0.5 0.1
TAEM 3.6 2.1 0.2
CNM 5.9 0.0 0.0
nput data was mainly obtained from the interview with the farm owner with additionala For 2010, nominal N content of rice straw (0.57% referred from Tanaka, 1978) was mu
or 2011, N content of rice straw (2010 harvest) was multiplied by the harvest amount frran application rate.b N content (2.4%) of rice bran was multiplied by the applied amount.c N content (0.47%) of guano was multiplied by the applied amount.d Slow release fertilizer (N content: 14.0%) was applied at a rate of 40 g m−2 for 2010 ane Nominal N content (2.0%) was multiplied by the application amount and the efficienc
arch 137 (2012) 251–260
slow-release fertilizer (Super SR Coat M, Sumitomo Chemical Co.,Ltd. N:P:K = 14:14:14) and supplementary cow manure in 2010.In 2011, however, only the synthetic slow-release fertilizer wasapplied at the same N application rate as in 2010 due to thedifficulty in obtaining cow manure. The total N input in CNMaccorded with the conventional practice in this area. The fieldswere located within the distance of ca. 750 m with little differencein the elevation. AEM fields were puddled twice or three timesbefore transplanting to control weeds without using herbicides.CNM field had received herbicide application until 2009, but wassubjected to the same weed control as AEM fields in 2010 and 2011.No weed infestation causing rice yield reduction was observed inany of the study fields. To avoid phosphorous deficiency, guano(Surya Guano, Asunaro Corporation Ltd.) was applied to the AEMfields.
All the other cultural practices were identical among the studyfields. The rice cultivar was Koshihikari (O. sativa L.), the mostwidely grown variety in Japan (MAFF, 2009). On June 15th in2010 and on June 13th in 2011, two to three seedlings perhill of 5.5 leaf age were mechanically transplanted at a den-sity of 12 hills m−2. No agrochemical agent was used to controlpests and diseases in any field. Ground water was used forirrigation, and the water depth was kept at more than 5 cm aftertransplanting. Mid-summer drainage was not practiced. N con-tents in irrigation water were low (NH4–N: 0.26 mg L−1, NO3–N:0.17 mg L−1). After heading, intermittent irrigation was practiceduntil ca. 10 days before harvest, and the field was drained there-after.
2.2. Measurements
2.2.1. Soil analysisDiagonal transect method was used to collect soil samples,
which were taken three times from the study fields. The firstsampling was in autumn, 2010 (October, 17th), at EAEM, TAEMand CNM fields. The second sampling was in spring, 2011(May 18th) at EAEM, TAEM and CNM fields. The third samp-ling was in autumn, 2011 (October, 12th) at EAEM, EAEM-L,TAEM and CNM fields. Core samples were taken from the ploughlayer (0–15 cm) at three sites in one study field; three cores
of soil samples were composited at each field. For autumnin 2010, samples were also collected from the subsoil layer(15–20 cm). The soil samples were sieved (2 mm mesh) and usedfor incubation.
Synthetic fertilizerd Cow manuree Total
0.0 0.0 5.20.0 0.0 5.85.6 3.0 8.6
0.0 0.0 6.70.0 0.0 5.60.0 0.0 5.98.6 0.0 14.5
information as follows.ltiplied by the amount of rice straw harvested in 2010 from the respective field.om the respective field. For EAEM-L, the data from EAEM was used except the rice
d 61 g m−2 for 2011.y index for cow manure (30%).
s Rese
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ap1s1bp
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A. Tanaka et al. / Field Crop
In order to examine soil N supply and availability, anaerobicncubation was conducted to measure ammonium N (NH4–N) using
et or air-dried soil samples. As mineralization is a microbiologi-ally mediated process, anaerobic incubation has been regarded ashe most appropriate method (Yoshino and Dei, 1977). In anaerobicubmerged incubation, 20 g of soil sample was put into a glass tube15 mm in diameter with 12 cm height) and deionized water wasdded to leave no head space below a rubber stopper which sealedhe tube firmly. The incubation tubes were kept in an incubator at0 ◦C for 4 or 10 weeks. During the incubation, the gas producedas released once or twice a week through the glass tube attached
o the rubber stopper of incubation tube. After the incubation, theoil and upper water was decanted to the 200 mL polyethylene tubeith KCl solution with a final concentration of ca. 2 mol L−1 KCl. TheH4–N concentration was determined after 1 h extraction by shak-
ng at room temperature with indophenol blue method (Nakatani,981) using AutoAnalyzer II (Technicon Co. Ltd.). Soil drying effectas calculated by subtracting the amount of NH4–N in wet soil
ncubation from the amount of NH4–N mineralized from air-driedoil incubation.
Air-dried and sieved (2 mm mesh) soil samples were usedor the analysis of soil chemical properties. The soil pH (H2O)1:2.5 dry soil:water ratio) was measured with a pH meter (B-12; Horiba Ltd.). Electrical conductivity (EC) (1:5 dry soil:wateratio) was measured with an EC meter (B-173; Horiba Ltd.).ir-dried and finely ground soils were measured for total
and N contents using NC analyzer (Sumigraph NC-220F;umika Chemical Analysis Service, Ltd.). After H2O2 treatment,and content was measured with the sieve (>0.2 mm mesh).ilt and clay content was measured with Laser Diffractionarticle Size Analyzer (LS I3 320; Beckman Coulter Inc.). Avail-ble P was determined by Bray 2 method (Bray and Kurtz,945) using UV–vis recording spectrophotometer (UV-2400PC;himadzu Scientific Instruments, Inc.). The cation-exchange capac-ty (CEC) was determined by the procedure of Nakatsukat al. (1987) with AutoAnalyzer II (Technicon Co. Ltd.) andxchangeable cations were measured by ICP Plasma Atomic Emis-ion Spectrometer (ICPE-9000; Shimadzu Scientific Instruments,nc.).
Soil bulk density was measured for the soils taken by 100 mL core sampler (5 cm diameter, 5 cm height) at twolots in EAEM, TAEM and CNM at the depth of 3–8 cm and0–15 cm from the plough layer and ca. 20 cm in the sub-oil layer respectively. Collected samples were oven-dried at05 ◦C for more than two days. Bulk density was determinedy mass of soil divided by the volume of the core sam-ler.
To estimate the amount of N supplied from the soil duringhe cropping season, bulk density was multiplied by the amountf mineralized NH4–N of wet soil from each layer after the 10eeks incubation. The total available soil N supply was summed
y the amounts from the two layers. For the subsoil layer, themount of NH4–N mineralized from the soil collected in autumn010 was used for all estimation due to stability of mineralized
in un-disturbed subsoil. The potential amount of soil N sup-ly in AEM fields was calculated by adding basic mineralizable, N contents in the following inputs: rice straw, rice bran, weed,iological N Fixation (BNF), guano and precipitation. As the N con-ents in BNF and precipitation were not measured in this study,he nominal value from the previous study (Hasegawa, 1992) wassed. Due to low N contents in irrigation water (Section 2.1) andigh recovery rate of N in applied rice straw (97%) in soil for
owland rice (Yoneyama and Yoshida, 1977), neither N amountsdded in irrigation water, or lost by denitrification and leachingas included in calculating the potential amount of soil N sup-ly.
arch 137 (2012) 251–260 253
2.2.2. Rice plant analysisChanges in the number of tillers and plant height were peri-
odically recorded for 21 hills (triplicates of seven hills) in eachfield. Plant height was measured from the base of the stemto tip of the upper most reaching leaf. Targeting the above 21hills, SPAD values were recorded for the second fully expandedleaf avoiding the midvein (Mimoto et al., 1995) using a chloro-phyll meter (SPAD-502Plus, Konica-Minolta Holdings, Inc.). Theleaf area index (LAI) was estimated by a light interceptometer(LAI-2000, Li-cor Inc.) in three randomly chosen plots for eachfield.
At physiological maturity, yield and yield components weredetermined by rough grain weight from the three areas of 1.0 m2
(12 hills) in which most plants exhibited the average number ofpanicles. The plant samples were oven-dried at 80 ◦C for morethan 72 h. All panicles were counted and separated by hand-threshing. Grain yield and grain weight were expressed at 15% grainmoisture content. Rough grain yield (unhusked) was the actualmeasured value including unfilled grain. Completely filled grainyield (unhusked) was calculated by multiplying the number ofspikelets per unit area, by the ripening ratio and individual grainweight. Ripening ratio was defined as the filled grains (specificgravity ≥1.06 g cm−3; Matsushima, 1966) as a percentage of totalnumber of spikelets. Imperfectly filled grains were manually sepa-rated into sterile and partially filled spikelets (Satake and Yoshida,1978). Each spikelet was pressed between the thumb and fore-finger to check sterility. Spikelet sterility ratio was calculated bydividing sterilized spikelet numbers by total grain spikelet num-bers. Imperfectly filled grain ratio was determined by dividing thenumber of imperfectly filled spikelets by total grain spikelet num-bers.
Rice plants harvested at physiological maturity were usedfor plant N analysis. The plants were divided into culms andsheaths, leaves, and panicles. Subsequently these parts weregrinded and N concentration was determined by NC ana-lyzer (Elementar, Vario MAX CN) in 2010 and NC analyzer(Sumigraph NC-220F; Sumika Chemical Analysis Service, Ltd.)in 2011. The amount of N accumulation by rice plants wascalculated by multiplying the dry-matter weight by the N con-tent.
For dry weight of weed, plant samples were taken from50 cm × 50 cm quadrats at three spots in each field. Weed plantsamples were collected three times: in October 2010, May 2011and October 2011. Collected plant samples were oven dried at 90 ◦Cand grinded. N concentration and the amount of N accumulation inweed were determined in the same manner as for rice plants.
Statistical analysis (ANOVA) was conducted using JMP 9.0 (SASInstitute, USA). All results were considered significant at P < 0.05.
2.3. Air temperature
Fig. 1 presents daily mean (a) and daily maximum (b) air tem-peratures recorded at the Oyama meteorological station by theAutomated Meteorological Data Acquisition System (AMeDAS) ofthe Japan Meteorological Agency (JMA) in 2010 and 2011 alongwith the 30-year mean temperatures for the period of 1981–2010.The meteorological station was located 12.5 km away from theresearch field, and the elevation differed by less than 20 m. Theyear 2010 had a record hot summer, especially in July and August,when rice plants are sensitive to temperature anomalies. In August2010, both daily mean and maximum temperatures were about2.0 ◦C higher than 2011 (Table 2). In 2011, temperature was
erratic: maximum temperature was higher than in 2010 exceed-ing 35 ◦C in the period from July 9th to 19th and August 8thto 15th (Fig. 1b). Also, a number of typhoons hit the region in2011.
254 A. Tanaka et al. / Field Crops Research 137 (2012) 251–260
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Fig. 1. Daily mean (a) and daily maximum (b) air temperatures in 2010 and 2011 and their
Meteorological Data Acquisition System (AMeDAS) of Japan Meteorological Agency (JMA)and EAEM were August 13th and August 14th respectively in 2010. For 2011, heading dat
Table 2Monthly means for daily mean and maximum air temperatures in 2010 and 2011.
30-year mean (1981–2010) recorded at Oyama monitoring station of the Automated. *Arrows indicate heading date in CNM, August 11th. Heading (50%) dates for TAEMe for CNM was August 11th. Heading dates for EAEM, and TAEM were August 12th.
3. Results
3.1. Soil analysis
The pH was similar among EAEM, TAEM and CNM indicatingaround 6.0. EC was generally low between 0.085 and 0.125 dS m−1.The lowest bulk density was found in 3–8 cm of the plough layer inEAEM (0.50 g cm−3). In each
field, bulk density was higher in the subsoil layer rangingbetween 0.78 g cm−3 and 0.82 g cm−3. The soil texture and chemicalproperties are presented in Table 3. Silt is dominant among the soilcomponents in all the study fields accounting for 66.6–70.2%. The
A. Tanaka et al. / Field Crops Research 137 (2012) 251–260 255
Table 3Physical and chemical properties of the plough layer in the study fields.
Field Soil texture (%) Exchangeable cations (cmolC kg−1) CEC (cmolC kg−1) Total-N (g kg−1) Total-C (g kg−1) Available Pa (mg P2O5 kg−1)
a Bray 2 method phosphorous (P). C, carbon; Ca, calcium; CEC, cation exchange capacity; K, potassium; Mg, magnesium; N, nitrogen; Na, sodium; P2O5, phosphorouspentoxide; SOC, soil organic carbon.
Table 4Amount of NH4–N after 4- or 10-week anaerobic incubation using wet soil sampled from the study fields in autumn 2010 and spring and autumn 2011.
Spring and autumn, 2011 (mg N 100 g−1)EAEM 12.7 22.7a 5.7b 12.5abEAEM-L – – 4.9bc 13.0abTAEM 6.9 15.6 b 4.5c 11.7bCNM – 13.6 c 6.7a 14.0a
N re not
Csah((
wms2iTw
l2tthN
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N
N contents accumulated in weed were highest in EAEM followedby TAEM and CNM. In the case of spring 2011, N contents in EAEM,
Table 6Estimated amount of NH4–N released from the soil sampled in autumn 2010 andspring 2011 soil during the cropping season among the study fields.
Field Plough layera Subsoil layera Total
Autumn 2010 (g N m−2)
ote: including the NH4–N at the start of incubation. Means sharing a same letter a
EC as well as the total N and C contents were similar among thetudy fields in the range of 37.3–48.8 cmolC kg−1, 4.0–5.5 g N kg−1
nd 43.8–68.6 g C kg−1, respectively, with TAEM having slightlyigher CEC, total N and total C. Available P was higher in EAEM-L54 mg P2O5 kg−1) and EAEM (48 mg P2O5 kg−1) followed by TAEM36 mg P2O5 kg−1) and CNM (20 mg P2O5 kg−1).
Amount of NH4–N mineralized from fresh soil under incubationas significantly different among the study fields (Table 4). NH4–Nineralized in EAEM under both 4- and 10-week incubation was
ignificantly higher than other fields in autumn 2010 and spring011 but for autumn 2011. EAEM demonstrated higher mineral-
zation ratio at early growth stage (assumed as 4-week incubation).he amount mineralized for 4 weeks accounted for 56% of the 10-eek incubation value in EAEM, whereas that in TAEM was 44%.
After air dry treatment, NH4–N mineralized from the ploughayer was indifferent among the study fields in spring or autumn011 (Table 5). Mineralized NH4–N was lower in autumn 2010 thanhe other timings. The soil sampled in autumn 2010 was kept inhe refrigerator for almost a year before air dry treatment. It mayave affected microorganisms in the soil, in reducing mineralizedH4–N.
From the soil sampled in autumn 2010, the estimated amount
f NH4–N released during the cropping season was high inAEM (21.0 g N m−2) followed by CNM (17.5 g N m−2) and TAEM14.1 g N m−2) (Table 6). From the soil sampled in spring 2011,
able 5mount of NH4–N after 4-week anaerobic incubation of air dried plough layer soilampled from the study fields in autumn 2010, and spring and autumn 2011 (mg N00 g−1).
ote: including the NH4–N at the start of incubation.
statistically different from each other by Tukey–Kramer HSD test ( = 0.05).
the estimated amount of NH4–N released was also high inEAEM (24.6 g N m−2). In spring 2011, estimated amount of avail-able soil N supply with rice bran application was higher inEAEM (28.5 g N m−2) than in TAEM (17.8 g N m−2). The mineralizedNH4–N from rice bran was higher in EAEM (4.4 g N m−2) than inTAEM (2.9 g N m−2) although rice bran application rate was higherin TAEM (Table 1). Potential amount of soil N supply in EAEMand in TAEM in spring 2011 was estimated at 41.9 g N m−2 and33.2 g N m−2, respectively (Table 7).
a Referred from Table 6.b Referred from Table 11.c Referred from Table 1.d Due to low N contents in irrigation water (Section 2.1) and high recovery rate of N in applied rice straw (97%) in soil for lowland rice (Yoneyama and Yoshida, 1977),
n was i
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3
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t
TR
Cta
either N amounts added in irrigation water, or lost by denitrification and leachinge Nominal N contents (Hasegawa, 1992).f Estimated amount of soil dry effect for 4 cm soil depth.
AEM and CNM were 6.6 g N m−2, 5.2 g N m−2 and 4.6 g N m−2,espectively.
.3. Plant growth characteristics
Plants elongated linearly in all the three fields in both yearsFig. 2, 2010(a), 2011(a)). In 2010, plant height was tallest in CNMollowed by TAEM and EAEM. At heading elongation stopped ando changes in plant height occurred thereafter. Lodging started at0 DAT in TAEM and the lodging area expanded while lodging inAEM was limited at harvest (data not shown). In 2011, CNM alsoecorded the tallest plant height (Fig. 2, 2011(a)). At 72 DAT, bentower nodes were observed in EAEM. With the heavy rain at 81 DAT,ice plants in EAEM started to lodge. At 100 DAT, the typhoon #15it the area with the heavy rain (23.5 mm h−1) and strong wind22.5 m s−1). Rice plants in all the fields lodged to certain degreeue to the typhoon.
Tiller development was most vigorous in CNM in both yearsFig. 2, 2010(b), 2011(b)). In 2010, tiller development in EAEM wasnitially slowest among the study fields, but grew faster than thosen TAEM during the late vegetative stage. In 2011, similar tillerevelopment was observed in EAEM and TAEM during the vege-ative stage. At harvest, the average panicle number (m−2) differedignificantly among the fields (Table 9).
The SPAD value increased in all three fields at the early vegeta-ive stage in 2010 (Fig. 2, 2010(c)). In 2010, SPAD value declinedontinuously in CNM and TAEM from the late vegetative stagehereas, in EAEM, it continued to increase through to the panicle
nitiation stage (c, 41 DAT) and remained high during the repro-uctive and ripening stages. In 2011, SPAD value in EAEM initially
ollowed the similar pattern as in 2010 (Fig. 2, 2011(c)), but it con-inued to rise until heading recording higher value than in 2010.
LAI was highest in CNM among the study fields throughouthe crop life span in 2010 and 2011 (Fig. 2, 2010(d)). In 2010, at
able 8ough grain yields and completely filled grain yields among the study fields in 2010 and
Field Rough grain yield (g m−2)
2010 2011
EAEM 664a 707a
EAEM-L – 720a
TAEM 572b 570c
CNM 655a 663b
ompletely filled grain yield and rough grain yield are unhusked yields. Means sharing aest ( = 0.05). Statistical analysis is not conducted for the completely filled grain yield sinnd grain weight.
ncluded in calculating the potential amount of soil N supply.
heading, LAI in CNM was about 1.5 times that of EAEM and TAEM.Initially at 26 DAT, LAI in EAEM was smaller than that in TAEM, butit increased faster during the late vegetative stage. After heading,LAI in CNM continued to increase. In 2011, LAI was also highest inCNM (Fig. 2, 2011(d)). LAI in EAEM was larger in 2011 than in 2010.Due to lodging, LAI for EAEM could not be recorded at 86 DAT in2011.
3.4. Yields, yield components and nitrogen uptake
In 2010, rough grain yields were comparable in EAEM and CNMwith that in TAEM being significantly lower, whereas completelyfilled grain yield was highest in EAEM among the study fields(Table 8). For 2011 also, the rough grain yield was highest in AEMfields, while TAEM had the lowest rough grain yield. Completelyfilled grain yield was highest in EAEM followed by EAEM-L, CNM,and TAEM.
Among the yield components of the study fields (Table 9), therewas no significant difference in one thousand completely-filledgrain weight and the number of spikelets per panicle in eitheryear. The number of panicles per area was high in CNM followedby EAEM and TAEM in 2010. In 2011, the number of panicleswas highest in EAEM-L whereas TAEM recorded lowest paniclenumber as in 2010. In a comparison between 2010 and 2011, thenumber of panicles was greater in 2011 (240 panicles m−2) thanin 2010 (205 panicles m−2) on average across the treatment. In2010, the ripening ratio was significantly higher in EAEM thanin CNM due to the significantly lower spikelet sterility ratio inEAEM, which resulted in higher yield of completely filled grainsin EAEM than in CNM. In 2011, the ripening ratio was higher in
TAEM than CNM, but the spikelet sterility ratio was not signifi-cantly different among the study fields ranging between 6.6% and7.9%. The imperfectly filled grain ratio was high in CNM and low inTAEM.
2011.
Completely filled grain yield (g m−2)
2010 2011
551a 532a– 513ab358b 388b448ab 476ab
same letter are not statistically different from each other by Tukey–Kramer HSDce it was calculated from the number of grains per area multiplied by ripening ratio
A. Tanaka et al. / Field Crops Research 137 (2012) 251–260 257
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In 2010, the above-ground dry weight was significantly high inNM, whereas, in 2011, it was comparable between EAEM-L, CNMnd EAEM (Table 10). TAEM, on the other hand, had lowest dryeight in both 2010 and 2011. In both years, N concentration and
e study field EAEM, TAEM and CNM. Error bars indicate S.D. Note: heading refers to
amount in leaves, culms and sheathes and panicles at physiologicalmaturity were comparable in CNM and EAEM (Table 11). Rice plantN accumulation was comparable between EAEM and CNM, but wassignificantly less in TAEM.
258 A. Tanaka et al. / Field Crops Research 137 (2012) 251–260
Table 9Yield components among the study fields in 2010 and 2011.
Field Number of panicles (m−2) Number of spikelets per panicle Ripening ratio (%) Spikelet sterility ratio (%)
Means sharing a same letter are not statistically different from each other by Tukey–Kramer HSD test ( = 0.05).
Fig. 3. Simplified diagram of nitrogen flow in EAEM from the harvest in 2010 until the pre-transplanting in 2011. Note: each figure is referred from Table 7.
Table 10Plant mass dry weight in the study fields at physiological maturity in 2010 and 2011.
Field Total (g m−2) Leaf (g m−2) Culm and sheath (g m−2) Panicle (g m−2)
Means sharing a same letter are not statistically different from each other by Tukey–Kramer HSD test ( = 0.05).
s Rese
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the processes of C and N metabolisms in soil as well as plant assim-
A. Tanaka et al. / Field Crop
. Discussion
.1. Soil nitrogen supply under AEM
Nitrogen deficiency has been reported as the major limitationf AEM on crop yield (Baresel et al., 2008; Tonitto et al., 2006). Bothhe total N amount and temporal supply pattern must match therop N demand to ameliorate the limitation.
Available N in EAEM was estimated at 28.5 g N m−2 during the011 cropping season. It was more than double the amount of Nbsorbed by the rice plants in EAEM (11.5 g N m−2). The N accumu-ation in EAEM is identical to that in CNM (Table 11), which suggestshat EAEM can supply comparable amount of N with CNM. In TAEM,n the other hand, soil N supply was estimated at 14.9 g N m−2. Theow level of soil N supply limited N accumulation in TAEM ricelants (8.0 g N m−2). The difference in soil N supply between EAEMnd TAEM indicates the importance of continuing AEM practices inrder to provide sufficient N.
Congruence of soil N supply pattern with crop N require-ent has been reported as an even greater challenge than total
supply under AEM. For example, in the winter wheat and grass-lover sequence, release of N is large in autumn, when N demandy wheat plants is still limited, and becomes less at heading orowering (Baresel et al., 2008). Excess supply in the early develop-ent and deficiency later in the ripening stage (Pang and Letey,
998) reduced yields and grain quality. By contrast, synchronyf soil N supply and rice plant requirement was observed in thistudy. Rice plant development in EAEM was initially slow andecame vigorous during the late vegetative stage (Fig. 2). At head-
ng and grain-filling stages, EAEM retained green leaf area ashown in higher SPAD values than other fields. This ‘stay green’henotype (Thomas and Smart, 1993) indicates high level of Nineralization maintained after heading. It agrees with the past
tudies (Noji et al., 1993; Kobayashi et al., 2007; Miura, 2007)hat rice plants, grown in field with plant residue being incor-orated, uptake more N in the period from panicle initiation tohe ripening stage. The lower sterility under heat stress of EAEMn the record hot summer of 2010 may be ascribed to high Nupply at heading (Tanaka et al., 2011). The present study alsohowed the risk of lodging due to excess soil N supply in theegetative stage as observed in EAEM, 2011. However, as demon-trated in EAEM-L, management practice can control N release. Itmphasizes that estimating soil N supply at the beginning of crop-ing period and controlling N release is key to stabilize yields inEM.
.2. Components of nitrogen supply capacity under AEM
In contrast to CNM, where synthetic N fertilizer accounts for0% of the N supply in Japan (Hasegawa, 1992), AEM relies domi-antly on soil N supply via plant- and microbes-mediated processesDrinkwater et al., 2008). Under EAEM, the size of labile soil N pool,.e. potential amount of soil N supply, was estimated at 41.9 g N m−2,
hich is the sum of basic mineralizable N (21.0 g N m−2) and annu-lly changing mineralizable N (20.9 g N m−2) (Table 7). The formers the amount of N released from soil without any new inputs or soilrying effects, and is responsible for the low basic mineralizable N
n TAEM (Table 7). TAEM also had lower capacity of rice bran min-ralization (Table 6). The extended period of AEM in the EAEM fieldhould have accumulated N from rice straw and changed the micro-ial activities and community in the soil. Inomata and Hirasawa2010) have indeed found a 1.5 times higher microbial activity in
he established AEM fields at the same site.
The latter component of N release from soil is annually chang-ng mineralizable N, which includes soil drying effect (3.6 g N m−2),nternal inputs (13.2 g N m−2) from rice straw, rice bran, spring
arch 137 (2012) 251–260 259
and winter weed, as well as external inputs (4.1 g N m−2) fromother sources such as BNF, precipitation and guano. Drying soilbefore flooding increases N mineralization (Shioiri et al., 1941;Toriyama, 1994), through accelerating the soil organic matterdecomposition (Birch, 1958; Kundu and Ladha, 1995) and increas-ing labile soil N pool. NH4–N released from dried soil tends to havehigher fertilization efficiency as NH4–N is absorbed at the paceof root development instead of rapid uptake (Kai, 1978). Magni-tude of soil drying effect is known to be closely related to soilmoisture content before flooding (Toriyama, 1994). Anomalouslylow precipitation prior to flooding in 2011 (Table 3) might haveenhanced the soil drying effect, which implies the possibility offluctuation in mineralization under AEM due to environmentalfactors.
Rice straw contained 5.0 g N m−2 in EAEM in 2010 (Table 11).In addition to its substantial amount of N, high recovery rate of Nin applied rice straw was similarly reported as 25% in plant and72% in soil by 15N study (Yoneyama and Yoshida, 1977). Rice strawincorporation also enhances heterotrophic N2 fixation under sub-merged soil through immobilizing available N, providing carbon (C)substrate for the increased microbial activities (Roper and Ladha,1995). Thus, the actual N input due to BNF might be more than theamount referred from the previous study (Hasegawa, 1992) whererice straw incorporation was not fully assumed.
Rice bran is another crop residue to recycle N. Despite its smallN contents (Table 1), its application can substantially affect cropgrowth due to its higher decomposability. Lodging was avoidedin EAEM-L as compared with EAEM with the reduction of ricebran application (1 g N m−2) in 2011. Fallow weed plays a dualrole in avoiding N loss and fixing atmospheric N2. Nitrate (NO3–N)remained in soil or produced after harvest are easily lost to deni-trification and leaching, but can be captured by off-season weedand made available for the next season crop (Buresh and DeDatta, 1991). Rerkasem and Rerasem (1984) reported that weedincorporated plots could increase yields compared with the plotswhere weed was removed. In EAEM, 6.6 g N m−2 in weed wasreturned into soil by spring ploughing. Leguminous plant, Astra-galus sinicus, was also observed which could have contributedto BNF.
Thus, this study has demonstrated that large labile N pool can bemaintained through strengthening internal N cycling with manage-ment practices to recycle and capture N within the system (Fig. 3).Lowland rice fields are further made conducive for AEM due tothe anaerobic soil condition which slows down the decompositionof organic substrates (Witt et al., 2000; Sahrawat, 2004). In addi-tion, sustainability of AEM is also found in the economic aspect, asavoidance of fossil-fuel based N fertilizer saves on production costs.
5. Concluding remarks
N availability and its mineralization pattern demonstratedthe potential of AEM as an alternative soil fertility managementpractice for lowland rice farming. The higher dependence of AEMon mineralizable N than CNM would warrant better quantificationof actual amount of N supply from mineralization of soil N andsupplementary organic materials under fluctuating climate and soilconditions. Such quantification was beyond the technical feasibilitywhen nutrient management based on synthetic fertilizers was firstintroduced with the Green Revolution. Since then, however, largeimprovements in scientific understandings have been realized into
ilation. It is now within the reach of technical feasibility to designa system for estimating the nutrient availability under AEM and,thereby, assisting the farmers in making the decision on their soilfertility management practices.
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60 A. Tanaka et al. / Field Crop
cknowledgements
The authors are grateful to the Tateno farm for their kindooperation in conducting this research on their field. Dr. Satoshiakamura and Ms. Yukiko Nishimura at Japan Internationalesearch Center for Agricultural Sciences are also thanked for theirssistance in conducting soil and plant analysis.
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