Dry season soil conditions and soil nitrogen availability to wet season wetland rice

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1 Soil Science and Plant Nutrition ISSN: (Print) (Online) Journal homepage: Dry season soil conditions and soil nitrogen availability to wet season wetland rice Wilbur Ventura & Iwao Watanabe To cite this article: Wilbur Ventura & Iwao Watanabe (1978) Dry season soil conditions and soil nitrogen availability to wet season wetland rice, Soil Science and Plant Nutrition, 24:4, , DOI: / To link to this article: Published online: 19 Apr Submit your article to this journal Article views: 248 View related articles Citing articles: 18 View citing articles Full Terms & Conditions of access and use can be found at

2 Soil Sci. Plant Nutr., 24 (4), , 1978 DRY SEASON SOIL CONDITIONS AND SOIL NITROGEN AVAILABILITY TO WET SEASON WETLAND RICE Wilbur VENTURA and Iwao WATANABE The International Rice Research Institute, Los Ballos, Loguna, Philippines Received May 4,1978 A pot experiment with Maahas clay soil covered three consecutive crops. After uniform growth of the first crop, the soils were subjected to different moisture conditions during the dry season. Prolonged drying before wet season flooded rice stimulated increased release of mineral nitrogen but moistening of the dry soil for a dryland crop or by occasional rain during the dry season reduced nitrogen use from the soil in the next wet season. One cycle of alternate wet and dry soil preparation for 20 days before transplanting rice improved soil nitrogen availability and plant uptake of fertilizer nitrogen. The initial growth of rice was retarded after flooding the previously moist dryland or dried soil, but not in the continuously flooded soils. Losses of applied nitrogen were sma)) in continuously flooded soils and were greater in the previously moist dryland and dry treatments. Uptake of soil nitrogen, however, was much higher in the air-dried soil treatment and in the dry with alternate wet and dry preparation treatments. Total nitrogen uptake (soil+fertilizer) was also greater in those dry treatments. Uptake of soil nitrogen in the wet-season crop was roughly proportional to the amounts of ammonia measured just before transplanting. The proportion of the uptake of immobilized fertilizer nitrogen to available soil nitrogen was constant among treatments. Release of immobilized fertilizer nitrogen was also greatly enhanced by soil drying. For 1976 wet-season crop, the availability of fertilizer nitrogen immobilized in the 1975 wet season was three times higher than that of native soil nitrogen. Additional Tndex Words: soil nitrogen availability, soil condition, wetland rice. Nitrogen nutrition of wetland rice depends largely on available soil nitrogen (5, 7). Efficient use of soil nitrogen is an important consideration for minimizing dependency on purchased fertilizer for rice production. Amounts of potentially available or mineralizable soil nitrogen in tropical rice soils were surveyed to evaluate soil nitrogen fertility (4). But the amount of ammonia released by the flooding of air-dried soil gives only the potential capacity of available soil nitrogen (14, 15). The actual amount of soil nitrogen absorbed by the wetland rice plant during its growth period is affected by factors such as drying during fallow (15), puddling and land preparation (8), temperature (17, 18), and mid season drying (8, 9). A large area of tropical Asia's rainfed rice land stays dry during the dry season, either for a dryland crop' or as fallow because of insufficient water. Occasio"nal rains 535

3 S3Q" W. VENTURA and I. WATANABE during the dry season may cause fluctuations in soil moisture. The conversion of wetland rice soil to a dryland soil during part of the year may have marked effects on the nitrogen-supply process of the soil in the succeeding wet-season crop. A common practice of preparing rice fields is wetting and ploughing at least two times with soil drying in between, primarily for thorough land preparation and better control of weeds. Such land preparation may affect soil nitrogen availability because alternate drying and wetting cycle is known to enhance nitrogen loss (9, 10). Soil nitrogen becomes available at earlier stage of rice growth in the tropical rice soils than in temperate soils (6), but SHIGA and VENTURA (13) could not find this trend in the Philippines, particularly in dry season when temperature was lower at the earlier stage of rice growth. Both papers did not pay attention to the soil condition before transplanting. In the tropics, soil conditions before the wet season are presumed to have more pronounced effects on the growth and nitrogen uptake of wet season wetland rice than in a temperate region, because of high temperature from an earlier stage of rice growth. Our study experimentally demonstrated the effects of soil drying after one cropping season on the availability of soil nitrogen in a succeeding wetland rice crop. MATERIALS AND METHODS Three successive rice crops were grown to simulate field cropping conditions in pot culture in the 1975 wet season and the 1976 dry and wet seasons. The Maahas clay soil used had 0.14% N, ph 6.6, CEC of 35 meq/l00 g soil, 1.28% organic C, 443 ppm P, 976 ppm K, and 11.1 % Fe. It was taken from continuously flooded plots at the International Rice Research Institute (IRRI) and the wet soil material was passed through 2 mm sieve and then 2.8 kg (dry equivalent weight) was put to each glazed porcelain pot (1/50 ml). For the 1975 wet-season crop, all pots were flooded. To label a part of soil nitro_ gen half of the pots were treated with UN-labelled ammonium sulphate ( atom % excess) at 180 mg N/pot and the other pots were similarly treated with non]abelled nitrogen. The fertilizer was applied in 20 ml solution and it was immediately mixed by hand with the entire soil in the pot. Each pot also received 40 mg of each of P206 and KIO. Rice was grown as one plant hill per pot. At crop maturity, pots were selected at random for the analysis of total and 15N contents in plant and soil. After harvest of the wet-season crop, different dry season soil conditions Were set: (I) flooded planted, (II) flooded unplanted, (III) dryland planted, (IV) dryland unplanted, (V) kept air-dried, and (VI and VII) kept outdoors. Each treatment had four pots for each of the two sets. The pots kept outdoors received moisture from occasional dry.season rain. All other pots were kept in the greenhouse. For the dryland treatments the pot soil was kept at field moisture capacity. There was an interval of 20 days between harvest of the first and planting of the second crop and Soil preparation started 2 weeks before transplanting. No fertilizer was applied. Two

4 Soil Moisture Conditions and Nitrogen Availability 537 seedlings of rice were uniformly transplanted as one plant hill in every p o t ~ " After harvest of the upland treatment in the 1976 dry-season crop (143 days after transplanting), all outdoor pots were moved into the greenhouse. To simulate the wetting process in rainfed rice fields, half of the pots that had been kept outdoors were immediately water-saturated, puddled, and then left unwatered until cracks appeared on the soil surface, and thereafter the soil was continuously submerged before transplanting the 1976 wet-season crop (Treatment VII). The whole period of soil preparation covered 20 days. All other dry land and dry treatments were kept puddled and submerged for 9 days before transplanting. Before applying fertilizer and transplanting the third crop, soil samples were taken from every pot and analysed for exchangeable ammonia, nitrate, total nitrogen, and 15N. The set of pots that did not receive llin-labelled fertilizer in the first cropping reveived 205 mg N as UN-labelled ammonium SUlphate (5.424 atom % excess) for each pot. The set of pots that received UN-labelled fertilizer in the previous wet-season crop received the same amount of unlabelled ammonium sulphate. Application of nitrogen fertilizer was similar to that in the first crop. Each pot received also 40 mg of each of Pa06 and K 2 0. All pots were kept flooded. Treatments were arranged in a randomized block design. Two 10-day-old rice seedlings were transplanted as one hill at the center of each pot. In addition, one seedling was grown for 20 days at the side of the pot, and analysed for total nitrogen and UN. Rice growth was measured at 20-day intervals. At maturity, the plants were harvested and total nitrogen and llin content of the plants were analysed. A day after harvest, the soil of each pot was mixed and the soil samples were taken for analysis of exchangeable-mineral nitrogen and total nitrogen, and 15N. Immediately after sampling, the soil was extracted with 2 N KCl and the exchangeable ammonium and nitrate were determined by steam distillation of the soil extract with MgO and DEVARDA'S alloy. Total nitrogen content was analysed by the Kjeldahl method. The distillates obtained from the determination of exchangeable and total nitrogen were acidified with dilute H 2 SO. and evaporated to dryness, then treated with methanol to remove excess boric acid. After further drying, llin was generated from these samples using the RITTENBERG method described by PROKSCH (12). Isotope 15N was analysed by emission spectrometry using a JASCO NIA-l 15N analyser. Total nitrogen of the dried and ground plant materials was analysed by the micro.;. Kjeldahl technique and UN by the emission spectrographic method as simplified by YONEYAMA and KUMAZAWA (16) also using the JASCO NIA-1 15 N analyser. RESULTS The 1975 wet-season crop had uniform treatments under flooded condition. No difference was observed in dry matter production (average 15.6 g straw and 16.6 g grain/pot) and plant nitrogen uptake (231 mg N/pot in straw and grains) for the 1975 wet-season crop.

5 538 W. VENTURA and I. WATANABE Without fertilizer application, the second crop (1976 dry season) had stunted plant growth. Rice plants grown in soil kept at field moisture capacity were bigger and had greater nitrogen uptake but matured 10 days later than rice grown in the flooded soil. Nitrogen uptake in straw and grain was 64.8 mg N/pot in the flooded rice and 75.8 mg N/pot in field moisture capacity pots. The soil kept outdoors received a total of 394 mm of rainfall for the dry season (143 days): 196 mm during the last 10 days of December, 44 mm in January, 9 mm in February, 31 mm in March, 55 mm in April, and 59 mm for the first 2 weeks of May. Table 1 shows the available nitrogen content for each treatment of soil after 9 days of flooding before transplanting the 1976 wet-season (third) crop. Flooding and puddling of the soil that was kept air-dried for the entire dry season released large amounts of nitrogen in ammonium form, indicating that ammonification was enhanced. One cycle of wetting and drying also favoured releases of exchangeable ammonium nitrogen, but the ammonification process was accompanied by nitrification probably caused by the subsequent drying of the flooded soil. Keeping the soil at field moisture capacity and unplanted stored a large amount of nitrate nitrogen but not ammonium nitrogen as compared with the planted treatment because of possible depression of nitrification under the plants. The release of residual labelled nitrogen from the 1975 wet-season fertilization was roughly proportional to the release of soil nitrogen (Table 1). Although sufficient mineral nitrogen was present as a result of heavy mineralization and fertilizer application, the growth of plants was inhibited at 20 days after transplanting in dryland soils that were previously moist and then dried (Table 2). Plants in continuously flooded soils showed greater growth and nitrogen uptake, which came mostly from the applied fertilizer. Table 1. Available nitrogen in soil just before fertilizer application and transplanting for 1976 wet-season crop. Soil condition in previous dry season Available N in soil (mg/pot)!' NH,-N NO.-N Total Residual Total Residual Flooded, planted 45.3 be 0.8e S.Ob O.le Flooded, fallow O.Ob O.Oc Field moisture capacity, planted b O.le Field moisture capacity, fallow 5.04 O.Od Sb Air-dried 215.4& 11.0& 4.8b 0.2e Dry, with occasional rain b Dry, with occasional rain (AWD)Z) 70.3b 2.9b 98.0& ) In a column, means followed by a common letter are not significantly different at the S% level by DMRT. I) AWD: alternate wet and dry soil preparation.

6 Soil Moisture Conditions and Nitrogen Availability 539 Table 2. Dry matter weight and nitrogen uptake by the rice plant at 20 days after transplanting, 1976 wet season. Dry matter N uptake (mg/pot)l) Soil condition in previous wt. dry season (mg/pot) Total Fertilizer Residual Flooded, planted 43S& 16.2& 13.0& 0.2b Flooded, fallow 459& 17.2& 12.1& 0.3& Field moisture capacity, planted 10S bo 4.3b 3.4bO 0.04 Field moisture capacity, fallow 151 b 4.9 b 3.9b 0.04 Air-dried 135bo 4.8b 2.3 bo 0.2b Dry, with occasional rain 94 bo 3.2b 2.0e 0.04 Dry, with occasional rain (AWD)2) b le4 1) In a column, means followed by a common letter are not significantly different at the S% level by DMRT. I) Alternate wet and dry soil preparation. Table 3. Tiller numbers and yield of wetland rice in the 1976 wet season as affected by different soil conditions in the previous (1976) dry season. Soil condition in previous dry season Tiller numbers l ) Panicle Straw Grain no./pot wt. wt. 20DAT 40DAT 60DAT 8SDAT (g/pot) (g/pot) Flooded, planted 6& 28& 27& 19b 19b 23.6b 24.0b Flooded, fallow 6& 28& 27& 20& 20& 27.1& 26.6&b Field moisture capacity, planted Field moisture capacity, fallow 2b 7d 104 lot b IS.7 Air-dried 2b ISb 21b 20& 19&b 22.Sb 2S.6& Dry, with rain 2b 6' IS.2o Dry, with rain (AWD)') 2b 9c IS.2o 23.Sb 1) In a column, means followed by a common letter are not significantly different at the S% level by DMRT. I) Alternate wet and dry soil preparation. Table 3, however, shows that growth of plants improved with age in the previous dryland and dry treatments. The grain yield in the air-dried treatment was comparable to that in continuously flooded soil. Also, the growth of rice improved at the later stages in alternate wet and dry preparation treatments. The soil that was watered occasionally by dry-season rain behaved similarly to the previously moist soil and both had inferior plant growth and yield. Keeping the soil in fallow for the previous crop did not significantly improve grain yield. Utilization of fertilizer nitrogen was the best in continuously flooded soils (Table

7 540 W. VENTURA and I. WATANABE Table 4. Balance sheet of labelled nitrogenl) in wetland rice for the 1976 wet season. Soil condition in previous dry season Recovery') Unaccounted for Crop (%) Soil (%) Total (%) (%) Flooded, planted SS.l' 40.8ab 9S.9' 4.10 Flooded, fallow S8.S a 3S.9 bo 94.4 S.6o Field moisture capacity, planted 38.2d 4S. S" 83.7be 16.3ab Field moisture capacity, fallow 46.0bO 3S.4bo 81. 4becl 18.6ab Air-dried 43.0cd d 22.8' Dry, with rain 40. 3d 41. lab 81.4bo 18.6ab Dry, with rain (AWD)I) 49.3b 37.0bo 86.3b 13.7b 1) In a column, means followed by a common letter are not significantly different at S% level by DMRT. I) Based from the amount of labelled N added (3S0 mg per pot). ') Alternate wet and dry preparation. Table S. Soil nitrogen uptake 1 ) 1976 wet season as influenced by soil conditions during previous dry season. Total N Soil condition in previous Soil N uptake dry season uptake (mg/pot) (mg/pot) Soil N Residual in total labelled N uptake in soil N (%) uptake (X) Flooded, planted 320.8c d 39.9' 6.3ab Flooded, fallow 380.3b 17S.4o a Field moisture capacity, planted S 4S.1 d S.9b Field moisture capacity, fallow r 36.8' 6.0b Air-dried 'b Dry, with rain JOS.Oc S3.7 S.6b Dry, with rain (AWD)2) 406. lab 233.Sb S7.S b 6.3ab 1) In a column, means followed by a common letter are not significantly ditterent at the S% level by DMRT. I) Alternate wet and dry soil preparation. 4), and was lower in previously moist dryland and dried treatments. One cycle of alternate wet and dry preparation of 20 days duration, however, improved plant uptake of applied nitrogen. The total recovery of labelled nitrogen (plant and soil) showed losses of less than 6% in the continuously flooded soils. Loss was the greatest in the air-dried treatment. Although prolonged air drying before the third crop caused more losses of fertilizer, it was compensated by greater use of soil nitrogen, so that total nitrogen uptake Was even greater than in continuously flooded soil (Table S). Alternate wet and dry prep. aration before transplanting increased both fertilizer nitrogen and soil nitrogen use.

8 Soil Moisture Conditions and Nitrogen Availability 541 Table 6. The effect 'oc 1976 dry-season conditlons on the availability of newly immobilized and native soil nitrogen at the time oc the harvest oc 1976 wet season. % tin absorbed to % UN absorbed to Availability Treatment total soil nitrogen immobilized nitrogen ratio A Bl) BIA Flooded, planted Flooded, Callow Field moisture capacity, planted Field moisture capacity, Callow Air-dried Dry, with rain Dry, with rain (A WD)') ) Calculated Crom Table 7. I) Alternate wet and dry soil preparation. Table 7. Balance sheet of labelled nitrogen 1 ) of the residual experiment Cor three crop seasons, Labelled Labelled Labelled Unaccounted N in soil N left in TotalN for (X) Previous season Present soil before N uptake soil condition condition by plants soil after recovery crops (mg/pot) crop (%) To To (mg/pot) (mg/pot) 180mg 90.7 mg First crop (wet season)!) Flooded field flooded S S.4 Second crop (dry season) Flooded flooded, planted 7.1' FMC. planted 4.6 b Third crop (wet season) Flooded, planted flooded 83.0& 8.1d 65. lab 88.0& 12o0 b 6 Ob Flooded, fallow flooded 84.3& 12.4c 66. 7&b 87.8& 12.2b 6.1b Field moisture capacity. planted Field moisture capacity, fallow flooded 82.2' & 88.2& 11.8b S.9b flooded 80.1' S ab 82.5b 17.5' 8.8& Air-dried flooded 86.4' 16.6' 64. lab b S. 7b Dry, with rain flooded 77.0& b 82.1b 17.9a 9.0a D ~ with, rain AWD)') flooded 77. Sa 14.6b 61.7ab 86.3& 13.7a b 6.9ab 1) In a column for each cropping, means followed by a common letter are not significantly different at the 5% level by DMRT. I) Initial labelled N based from amount added (180 mg!pot). ') Alternate wet and dry. soil preparation.

9 542 W. VENTURA and I. WATANABE Nitrogen uptake from soil during the third crop was roughly proportional to the amount of ammonium nitrogen already present in soil before transplanting (Tables 1 and 5). Greater dependence on soil nitrogen by plants grown after soil drying was already apparent in nitrogen uptake at 20 days after transplanting (Table 2). A fraction of soil nitrogen uptake came from the residual labelled nitrogen which was labelled during 1975 wet-season crop (Table 5) and the ratio of the residual nitrogen to the available soil nitrogen was almost constant among treatments (6%). As presented by BROADBENT and NAKASHIMA (2), an availability ratio was calculated (Table 6). It was demonstrated that the nitrogen immobilized two crops before (l year before) was three times more available than the native available soil nitrogen. The fate of applied nitrogen was determined for three cropping seasons. Of the 180 mg nitrogen per pot applied, the first crop took up 44% and 50% remained in the soil as immobilized nitrogen (Table 7). Remineralization of immobilized nitrogen was slow, releasing only 8 and 5% of it for the second crop of wetland and dryland rice, respectively. Soil conditions during the previous dry season affected availability of immobilized nitrogen for the next wetland rice crop. The greater uptake of residual nitrogen from air-dried and alternate wet-dry treatments may show greater mineralization activity by soil drying. At the end of the third crop, about 35% of the applied nitrogen still remained in immobilized form in the soil and from 12 to 18% was lost during three crops in this experiment. DISCUSSION BIRCH (1) showed the relationship of length of soil drying to the mineral nitrogen level after wetting. In our experiments the soil kept dry for 140 days greatly increased the availability of soil nitrogen. But drying for such a long period is the extreme case. An occasional rain produces nitrate and also decreases the drying effect on soil nitrogen mineralization as shown in this experiment. Because moisture is sup. plied by capillary movement to the dry surface soil in the field, the dry condition in the field would not be as severe as in our pot experiments. Consequently, the airdried treatment we used may provide knowledge of the extreme case for the dry season in the tropics. To explain our results, the hypothesis is that drying in soil caused opposing effects on the rice plant after the flooding. The soil drying increased the amount of available soil nitrogen (positive effect). On the other hand, soil organic matter became easily decomposable by drying effect which promoted soil reduction after flooding (IS) (presumably negative effect). The growth retardation, owing to the delayed settling of seedlings, in the previous dry or moist treatments indicates that harmful factors accumulated after flooding. Plant response might be determined by the balance of the two opposing factors. High availability of soil nitrogen released by air-drying likely overcomes the growth retarda_ tion after settling of seedlings.

10 Soil Moisture Conditions and Nitrogen Availability 543 The improvement of soil nitrogen availability by alternate submergence and drying preparation indicates the effectiveness of even a short and gradual drying of the wet and puddled soil. The better utilization of fertilizer nitrogen as compared to the other dry treatments suggests that gradual wetting of soil. for a reasonable period of time before fertilization, does not increase losses of fertilizer nitrogen. This procedure alleviated harmful effects after flooding without greatly decreasing soil nitrogen.availability. A consideration of the period of submergence before fertilization and transplanting appears important. Nine days of soil flooding before transplanting was shorter than the usual practice. It takes at least 3 weeks to bring down the redox potential of a submerged soil to a stable level of activity (11). Therefore. if the period of flooding before transplanting was longer. any harmful factors accumulated at an earlier stage would be decreased. Converting wetland rice soils into dryland farming during the dry season made nitrification active during the period of moist soil conditions and the large amount of nitrate formed was stored in soil in the absence of an absorbing rice plant. It would not be expected, however, for the nitrate to stay long enough after flooding the soil to be of use to the rice seedlings. In the continuously flooded treatment, fertilizer nitrogen efficiency was high, but use of soil nitrogen was low. In the air-dried treatment, the opposite was observed. Because the total nitrogen uptake by a plant is limited. the efficiency of fertilizer nitrogen competes with the use of soil nitrogen. It is. therefore. one-sided to pay attention only to the efficiency of fertilizer nitrogen. Loss of fertilizer nitrogen was the highest in the air-dried treatment (Table 4), but the loss of immobilized UN (Table 7) was the lowest in this treatment. This implies that the behaviour of fertilizer nitrogen is different from that of the available (mineralizable) soil nitrogen. The finding also gives argument against the A-value concept (3) that presumed similarity in the behaviour in plant uptake of fertilizer nitrogen and available soil nitrogen. The amount and rate of immobilized UN were high (from 34 to 50%, Tables 4 and 7) as compared with other reports. especially with that of YOSHIDA and PADRE (20) in which the rate of immobilization was about 20%. Consequently, total recovery of applied UN in this experiment was also high and reached 95%. In this experiment, the fertilizer was applied either in the continuously flooded soils or in soils that were flooded and puddled for 9 days and not during the day of soil submergence, which may account for the greater nitrogen recovery. including the amount of immobilized nitrogen. Our results show that the effect of soil conditions on recovery of fertilizer nitrogen is drastic. Furthermore, the differing C: N ratio might have contributed to the difference in rates of nitrogen immobilization. Keeping the soil unplanted in the previous season decreased the rate of immobilization (Table 4), perhaps because there were no crop residues that could widen the C : N ratio of the soil. The fraction of immobilized nitrogen in the nitrogen uptake from soil nitrogen

11 S44 W. VENTURA and I. WATANABE was constant among the treatments (Table 5). If a large amount of atmospheric nitrogen was fixed and incorporated into the available-soil-nitrogen pool under the presence of rice plant as suggested by YOSHIDA and ANCAJAS (19), residual un uptake ratio to nitrogen uptake that originated from sources other than fertilizer might be lowered after the growth of rice plant as compared with that after fallow. This dilution effect was not found (Table 5). Based from the error of analysis the contribution of atmospheric nitrogen, if any, might be less than 25% of plant nitrogen uptake. Experiments demonstrate the need of an appropriate manipulation scheme to use the soil nitrogen that is released after a prolonged dry period. An improvement in the method of land preparation as well as judicious use of nitrogen fertilizer based on the amount and pattern of soil nitrogen release is indicated. REFERENCES 1) BIRCH, H.F., Nitrification in soils after different periods of dryness, Plant Soil, 12, (1960) 2) BROADBENT, F.E. and NAKASHIMA, T., Reversion of fertilizer nitrogen in soils, Soil Sci. Soc. Arne,.. Proc., 31, 648-6S2 (1967) 3) FRIED, M. and DEAN, L.A., A concept concerning the measurement of available soil nutrients. Soil Sci., 73, (19S2) 4) KAWAGUCHI, K. and KYUMA, K., Paddy soils in tropical Asia. Part I. Description on fertility characteristics, South Asian Studies, n, 3-24 (1974) 5) KOYAMA, T., Practice of determining potential nitrogen supplying capacity of paddy soils and rice yield, J. Sci. Soil Manure. Japan. 46, (197S) (in Japanese) 6) KOYAMA, T., Soil plant nutrition studies on tropical rice. Ill. The effect of soil fertility status of nitrogen and its liberation upon the nitrogen utilization of rice plants in Dangkhen paddy Soil Soil Sci. Plant Nutr., 17, (1971) 7) MITSUI, S., Inorganic Nutrition, Fertilization and Soil Amelioration for Lowland Rice, Yokendo Co., Tokyo, 19S4. p ) OYAMA, N., Nitrogen supplying patterns of paddy soil for rice in temperate area in Japan-cft'ect of application of organic matters and soil managements on the pattern, J. Sci. Soil M a n u r ~ Japan, 46, (197S) (in Japanese) 9) PATRICK, W.H., Jr., QUIRK, W.A., PETERSON, F.J., and FAULKNER, M.D.. Effect of continuous submergence versus alternate flooding and drying on growth, yield and nitrogen uptake of rice J. Agron., 59, (1967) 10) PATRICK, W.H., Jr. and WYATT, R., Soil nitrogen loss as a result of alternate submergence and drying, Soil Sci. Soc. Amer. Proc., 28, 647-6S3 (1964) 11) PONNAMPERUMA, F.N., Physicochemical properties of submerged soils in relation to fertility, IRIU Res. Paper Ser. S, p.32 12) PROKSCH, G., Application of mass and emission spectrometry for UN/UN ratio determination in biological material, In Isotopes and Radiation in Soil-Plant Relationships Induding Forestry International Atomic Energy Agency, Vienna, pp s 13) SmoA, H. and VENTURA, W., Nitrogen supplying ability of paddy soils under field conditions in the Philippines, Soil Sci. Plant Nutr., 21, (1976) 14) SHIOIRI, M., AOMINE, S., UNO, Y., and HARADA, T., Effect of drying of paddy soils J. Sci. SOil Manure, Japan, 15, (1941) (in Japanese) 15) SHlOIRI, M., The effect of soil drying during fallow period of lowland rice, Report Agrie. ExPII Station. Ministry oj Agric., 64, 1-24 (1948) (in Japanese)

12 Soil Moisture Conditions and Nitrogen Availability S4S 16) YONEYAMA, T. and KUMAZAWA, K., A simple determination of UN abundance in plant powder sample, J. Sci. Soil Manure, Japan, 45, (1974) (in Japanese) 17) YOSHINO, T. and DEI, Y., Patterns of nitrogen release in paddy soils predicted by an incubation method, Japan Agric. Res. Quart., 8, (1974) 18) YOSHINO, T. and DEI, Y., Prediction of nitrogen release in paddy soils by means of the concept of effective temperature, J. Central Agric. Exptl. Station,lS,l-62 (1977) (in Japanese, English summary) 19) YOSHIDA, T. and ANCAJAS, R.R., Nitrogen fixing activity in upland and flooded rice fields, Soil Sci. Soc. Amer. Proc., 37, (1973) 20) YOSHIDA, T. and PADRE, B.C., Jr., Effect of organic matter application and water regimes on the transformation of fertilizer nitrogen in a Philippine soil, Soil Sci. Plant Nutr., 11, (1975)