Growth and Iron Uptake of Lowland and Aerobic Rice Genotypes under Flooded and Aerobic Cultivation

With increasing water shortages in China, rice (Oryza sativa L.) cultivation is gradually shifting away from continuously flooded conditions to partly or even completely aerobic conditions. The effects of this shift on the growth and iron (Fe) nutrition of different aerobic and lowland rice genotypes are poorly understood. A field experiment was conducted to determine the effects of cultivation system (aerobic vs. flooded), genotype (five aerobic rice varieties and one lowland rice variety), and Fe fertilization [no Fe and 30 kg ha^?1 ferrous sulfate (FeSO₄·7H₂O] on rice grain yield and Fe nutrition. Plants were sampled at tillering and physiological maturity. In both aerobic and flooded plots, Fe application significantly increased shoot dry weight, shoot Fe concentration, and shoot Fe content at tillering but not physiological maturity. At physiological maturity, grain yield and Fe and grain harvest indices were significantly lower in aerobic than in flooded plots. Shoot dry weight and shoot Fe content differed among genotypes at tillering and at physiological maturity. The grain harvest index of aerobic rice genotype 89B-271-17(hun) was significantly greater than that of the other five genotypes when no Fe was applied. Because soil Fe fertilization did not improve the Fe nutrition of rice in aerobic plots, the results indicate that the shift from flooded to aerobic cultivation will increase Fe deficiency in rice and will increase the problem of Fe deficiency in humans who depend on rice for nutrition.     

Arsenic speciation in xylem exudates of rice growing in arsenic-contaminated soil

Rice has a much enhanced arsenic (As) accumulation compared with other cereal crops. How As is transported in xylem exudates of rice growing in soil remains unknown. We quantified the chemical species of As in xylem exudates of rice growing in arsenic-contaminated soil (62.5 mg As per kg soil) under flooded and aerobic conditions. In the aerobic treatment, the predominant species of As in the xylem sap was arsenate [As(V)], accounting for 64–88%, and the remainder was arsenite [As(III)]. In the flooded treatment, dimethylarsinic acid (DMA) was detected in the xylem sap besides As(III) and As(V), and the percentages of the three species As was 11–20%, 26–77%, and 12–54%, respectively. As speciation in the soil solution was dominated by As(V) under aerobic conditions, and dominated by As(III) under flooded conditions. DMA was detected at low concentrations in some of the flooded solution samples. Our results show that As speciation in the xylem sap differs in different water management regimes and correlate with As speciation in the soil solution strongly.     

Iron Toxicity in Lowland Rice

Lowland rice is a staple food for more than 50% world population. Iron toxicity is one of the main nutritional disorders, which limits yield of lowland rice in various parts of the world. The toxicity of iron is associated with reduced soil condition of submerged or flooded soils, which increases concentration and uptake of iron (Fe^{2 +}). Higher concentration of Fe^{2 +} in the rhizosphere also has antagonistic effects on the uptake of many essential nutrients and consequently yields reduction. In addition to reduced condition, increase in concentration of Fe^{2 +} in submerged soils of lowland rice is associated with iron content of parent material, oxidation-reduction potential, soil pH, ionic concentration, fertility level, and lowland rice genotypes. Oxidation-reduction potential of highly reduced soil is in the range of –100 to –300 mV. Iron toxicity has been observed in flooded soils with a pH below 5.8 when aerobic and pH below 6.5 when anaerobic. Visual toxicity symptoms on plants, soil and plant tissue test are major diagnostic techniques for identifying iron toxicity. Appropriate management practices like liming acid soils, improving soil fertility, soil drainage at certain growth stage of crop, use of manganese as antagonistic element in the uptake of Fe^{2 +} and planting Fe^{2 +} resistant rice cultivars can reduce problem of iron toxicity.     

长期稻草还田对紫色水稻土肥力和生产力的影响

通过8年淹水条件下一季中稻的田间定位试验,研究了长期稻草还田以及稻草与不同化肥配合施用对紫色水稻土生产力和土壤肥力的影响。结果表明,稻草与N、P、K化肥配合施用能维持或提高紫色水稻土的生产力和土壤肥力;水稻获得了持续高产,土壤有机质和全氮含量提高,土壤磷的有效性增加,速效钾与试验前基本平衡。纯化肥处理尤其是N、NP、NK处理水稻产量、土壤氮和钾含量逐年降低,不能维持系统生产力和土壤肥力。稻草还田对翌年水稻具有显著的增产作用,8年平均稻草还田处理比对照增产39.5%,稻草的增产作用还随着稻草还田时间的延长而逐年升高。稻草还田携入的钾与化学钾肥具有相同的营养功效,稻草可替代部分化学钾肥。     

Chemical stabilization of soil organic nitrogen by phenolic lignin residues in anaerobic agroecosystems

This review summarizes independent reports of yield decreases in several agricultural systems that are associated with repeated cropping under wet or submerged soil conditions. Crop and soil data from most of these agroecosystems have led researchers to attribute yield decreases to a reduction in crop uptake of N mineralized from soil organic matter (SOM). These trends are most evident in several long-term field experiments on continuous lowland rice systems in the Philippines, but similar trends are evident in a continuous rice rotation in Arkansas, USA and with no-till cropping systems in North American regions with cool, wet climatic conditions in Spring. Soil analyses from some of these systems have found an accumulation of phenolic lignin compounds in SOM. Phenolic compounds covalently bind nitrogenous compounds into recalcitrant forms in laboratory conditions and occurrence of this chemical immobilization under field conditions would be consistent with field observations of reduced soil N supply. However, technological shortcomings have precluded its demonstration for naturally formed SOM. Through recent advances in nuclear magnetic resonance spectroscopy, agronomically significant quantities of lignin-bound N were found in a triple-cropped rice soil in the Philippines. A major research challenge is to demonstrate in the anaerobic agroecosystems that these lignin residues bind sufficient quantities of soil N to cause the observed yield decreases. A key objective will be to elucidate the cycling dynamics of lignin-bound N relative to the seasonal pattern of crop N demand. Anaerobic decomposition of crop residues may be the key feature of anaerobic cropping systems that promotes the accumulation of phenolic lignin residues and hence the covalent binding of soil N. Potential mitigation options include improved timing of applied N fertilizer, which has already been shown to reverse yield decreases in tropical rice, and aerobic decomposition of crop residues, which can be accomplished through field drainage or timing of tillage operations. Future research will evaluate whether aerobic decomposition promotes the formation of phenol-depleted SOM and greater in-season N mineralization, even when the soil is otherwise maintained under flooded conditions during the growing season.     

Adaptation of green manure legumes to adverse conditions in rice lowlands

Poor adoption of sustainable pre-rice green manure technology by lowland farmers is frequently associated with unreliable legume performance under adverse environmental conditions such as marginal soils, short photoperiod, and unfavorable hydrology. A series of field and microplot experiments were conducted at the International Rice Research Institute (IRRI) in 1991 and 1992 to screen and evaluate 12 promising flood-tolerant legumes for adaptation (N accumulation and biological N₂ fixation) to a range of environmental stresses, frequently encountered in rice lowlands. Legumes belonging to the genera Sesbania and Aeschynomene were grown for 8 weeks at 10×10 cm spacing: (1) in a fertile control soil and in four marginally productive irrigated lowland rice soils (sandy Entisol, P-deficient Inceptisol, acid Ultisol, and saline Mollisol); (2) during short- (11.7h) and long-day (12.3 h) seasons in a favorable irrigated lowland soil; and (3) in an aerobic soil (drought-prone rain-fed lowland) and a deep-flood-prone lowland soil (1 week seedling submergence). A large variability in N accumulation was obsersed among legume species and across different environments, ranging from less than 1 to over 70 mg N plant^-1. The nitrogen derived from the atmosphere (Ndfa) accounted on average for 82% of total N accumulation. Sesbania virgata was least affected by unfavorable soil conditions but its Ndfa was the lowest among the tested species (less than 60%). Stem nodule formation did not convey a significant advantage to legumes grown under adverse soil conditions. However, flooding reduced N₂ fixation less in stem-nodulating than in solely root-nodulating species. Most species drastically reduced N accumulation under short-day conditions. Aeschynomene afraspera and S. speciosa were least affected by photoperiod. The considerable genetic variability in the germplasm screened allows the selection of potentially appropriate legumes to most conditions studied, thus increasing N accumulation in green manures.     

不同节水稻作模式对土壤理化特性和土壤酶活性影响研究

针对南方丘陵区季节性干旱现状与目前稻田单一化的连作耕作制度对农田环境造成的不利影响因素,通过4a田间定位试验研究了节水稻作模式下土壤理化性状和土壤酶活性变化规律影响.结果表明,与该区常规稻作模式相比.在试验设计范围内,节水稻作模式下稻田的土壤理化性状明显改善,随着耕种年限增加,土壤容重下降,而孔隙度增加.土壤通透性增强,有效阻止土壤次生潜育化和土壤酸化.提高了土壤pH值;就土壤酶活性而言,节水稻作模式处理酶活性常规稻作模式相比.除转化酶外,其它酶活性均有不同程度的增加,尤其对磷酸酶与脲酶影响最大.     

不同氮素用量对高肥力稻田水稻-土壤-水体氮素变化及环境影响分析

研究了分次施氮条件下不同氮紊用量对高肥力稻田水稻-土壤-水体含氮量的变化，结果表明：不同施氮水平与植物吸氮置、土壤含氮量以及田面水、渗漏水全氮含量之间具有很强的相关性。但总的来说，氮素利用率不高，有70％～80％左右的化肥氮排入到环境中，对土壤．水体和大气造成污染。在移栽期时，氮紊损失严重。土壤古氮量在水稻生长的前3个时期变化不大，但最终土壤氮素效应明显，低于150kg／hm^2的施氮量不利于土壤肥力的保持。同时用差值法估算出化肥氮对土壤氮的贡献量占化肥氮排入环境量的比例为30％～40％。田面水全氮浓度在每次施肥后第一天达到高峰．一周后全氮浓度显著下降，从环境角度，施肥后一周内是防止田面水氮素流失的主要时期。通过差值法估算的渗漏水中氮含量占化肥氮排入环境氮的比例很小，说明化肥氮当季渗漏淋失的较小。但从总量上却不可忽视，特别是施氮量大于225kg／hm^2时，会对环境造成很大的污染。     

CO2/heat fluxes in rice fields: Comparative assessment of flooded and non-flooded fields in the Philippines

The seasonal fluxes of heat, moisture and CO₂ were investigated under two different rice environments: flooded and aerobic soil conditions, using the eddy covariance technique during 2008 dry season. The fluxes were correlated with the microclimate prevalent in each location. This study was intended to monitor the environmental impact, in terms of C budget and heat exchange, of shifting from lowland rice production to aerobic rice cultivation as an alternative to maintain crop productivity under water scarcity.The aerobic rice fields had higher sensible heat flux (H) and lower latent heat flux (LE) compared to flooded fields. On seasonal average, aerobic rice fields had 48% more sensible heat flux while flooded rice fields had 20% more latent heat flux. Consequently, the aerobic rice fields had significantly higher Bowen ratio (0.25) than flooded fields (0.14), indicating that a larger proportion of the available net radiation was used for sensible heat transfer or for warming the surrounding air.The total C budget integrated over the cropping period showed that the net ecosystem exchange (NEE) in flooded rice fields was about three times higher than in aerobic fields while gross primary production (GPP) and ecosystem respiration (R_e) were 1.5 and 1.2 times higher, respectively. The high GPP of flooded rice ecosystem was evident because the photosynthetic capacity of lowland rice is naturally large. The R_e of flooded rice fields was also relatively high because it was enhanced by the high photosynthetic activities of lowland rice as manifested by larger above-ground plant biomass. The NEE, GPP, and R_e values for flooded rice fields were −258, 778, and 521 g C m⁻², respectively. For aerobic rice fields, values were −85, 515, and 430 g C m⁻² for NEE, GPP, and R_e, respectively. The ratio of R_e/GPP in flooded fields was 0.67 while it was 0.83 for aerobic rice fields.This short-term data showed significant differences in C budget and heat exchange between flooded and aerobic rice ecosystems. Further investigation is needed to clarify seasonal and inter-annual variations in microclimate, carbon and water budget of different rice production systems.     

Agronomic evaluation of rice cultivation systems for water and grain productivity

Field experiments were conducted during summer (March–July) and kharif (June–September), 2008 at the wetland farm, Tamil Nadu Agricultural University, Coimbatore, India, to study the performance of different rice cultivation methods on productivity and water usage using the hybrid CORH-3 as a test crop. Treatments consisted of different rice cultivation methods, namely, transplanted rice (conventional), direct sown rice (wet seeded), alternate wetting and drying method (AWD), system of rice intensification (SRI) and aerobic rice cultivation. Results revealed that maximum number of tillers m^?2, higher shoot and root length at maturity were recorded under SRI followed by transplanted rice, while aerobic rice produced lower growth parameters in both the seasons. Chlorophyll content at flowering was higher under SRI in two seasons studied (42.74 and 39.48 SPAD value, respectively) and transplanted rice compared to aerobic rice and AWD. In both summer and kharif seasons, SRI produced higher grain yield (6014 and 6682 kg ha^?1), followed by transplanted rice (5732 and 6262 kg ha^?1), while the lowest grain yield (3582 and 3933 kg ha^?1) was recorded under aerobic rice cultivation. Under SRI, 5 and 6.7% increase in grain yield and 12.6 and 14.8% water saving were noticed compared to transplanted rice, respectively, during summer and kharif seasons. In respect to water productivity, the SRI method of rice cultivation registered the highest water productivity (0.43and 0.47 kg m^?3), followed by AWD and aerobic rice cultivation. The conventional rice cultivation and direct sown rice produced lower grain yield per unit quantity of water used.     

不同覆盖旱作水稻对后茬大麦生长和土壤氮素的影响

通过田间试验研究水稻不同覆盖旱作栽培方式对后茬大麦生长及土壤氮素动态变化的影响。结果表明，盖草处理后茬大麦表层土壤硝态氮含量最高，而土壤铵态氮含量由高到低依次为：水作〉盖草〉覆膜〉裸露，且硝态氮、铵态氮从表层（0～20cm）到底层（60～80cm）依次降低。具有明显的层次性。各处理都表现出0～40cm土层氮素表观盈余，水作、盖草和裸露处理后茬分别比覆膜处理后茬高40．5％，39．5％和36．1％。大麦籽粒产量以盖草处理后茬摄高，分别比水作后茬和裸露后茬增加837kg／hm^2和251kg／hm^2。后茬大麦各生育阶段对氮素的累积吸收量均以前茬盖草早作处理最高。覆膜、盖草和裸露旱作水稻后茬大麦的氮肥利用率分别比水作稻后茬大麦高16．1％，17．5％和13．8％。土壤氮素转化和大麦产量均表明，半腐解秸秆覆盖旱作水稻-大麦轮作是一种较合理的种植模式。     

秸秆覆盖旱作对稻田甲烷排放和水稻产量的影响

为较全面评价秸秆覆盖旱作水稻栽培模式的生态意义,采用田间试验研究了常规淹水（F）、秸秆覆盖旱作（NF-M）和无覆盖旱作（NF-ZM）3种栽培模式稻田甲烷排放、水稻产量及土壤养分的变化规律。结果表明：3种水稻栽培模式的甲烷排放均集中在水稻生育期的前20d;在水稻生育期内,秸秆覆盖旱作稻田甲烷的排放总量为11.12g·m^-2,显著高于常规淹水稻田的7.78g·m^-2和无覆盖旱作稻田的4.23g·m^-2。秸秆覆盖旱作稻田的水稻产量为8.60t·hm^-2,与常规淹水处理没有显著差异,但二者均显著高于无秸秆覆盖旱作处理的6.78t·hm^-2;与常规淹水处理相比,秸秆覆盖旱作还可以提高水稻单株生物量10g以上。秸秆覆盖旱作还可以显著提高稻田表层土壤有机质含量,维持和改善表层土壤养分状况,对实现农业可持续性有重要意义。因此,在水资源缺乏地区,秸秆覆盖旱作是一种值得考虑的替代传统淹水栽培的水稻栽培模式,同时秸秆覆盖旱作还田也是一种值得推广的稻田秸秆管理技术。     

Improvement in nitrogen availability, nitrogen uptake and growth of aerobic rice following soil acidification

A yield decline and increase in soil pH under continuous cropping of aerobic rice have been reported in previous studies. However, the underlying mechanisms governing the poor growth and low yield of aerobic rice following an increase in soil pH are unknown. The objective of the present study was to determine the effect of soil acidification on the soil nutrient availability, plant nutrition and growth of aerobic rice grown in continuously cropped aerobic soil. Two pot experiments were conducted using soil from a field where aerobic rice had been grown for 13 consecutive seasons. Soil was acidified by adding 50–300 mL of 0.05 mol L^–1 sulfuric acid to 3.0 kg of air-dried soil to achieve a range of soil pH levels. Rice was grown aerobically with N rates of 0–1.2 g per pot using urea or ammonium sulfate. Soil chemical properties were measured as were leaf nutrient concentrations, plant growth parameters, and the above-ground N uptake. A 5.5-fold and 1.5-fold increase in soil ammonium and nitrate were observed, respectively, after adding sulfuric acid. Plant growth and N uptake improved significantly with soil acidification, regardless of N rates or N sources, and were associated with an improvement in plant N nutrition. The application of N had greater positive effects on plant growth and N uptake than soil acidification. The growth response to soil acidification reduced as the rate of N application increased. These results suggest that the yield decline of continuous aerobic rice is probably associated with a reduction in soil N availability and plant N uptake as a result of a gradual increase in soil pH.     

Do abiotic factors cause a gradual yield decline under continuous aerobic rice cultivation? A pot experiment with affected field soils

Abstract

Aerobic rice is a water-saving technology in which rice grows in non-puddled and non-saturated (aerobic) soil without ponded water. A gradual decline in rice yield was found in field plots at the farm of the International Rice Research Institute, Los Baños, Philippines, where rice has been cultivated continuously for 10 cropping seasons under aerobic rice conditions. We investigated whether abiotic soil factors lead to the observed yield decline. An aerobic rice pot experiment was conducted using field soils from flooded rice plots and from the 10-season-long aerobic rice cultivated plots (referred to as 1st-season and 11th-season aerobic rice, respectively). Subtreatments consisted of soil sterilization by oven heating (at 95°C or higher for 24?h) and a control treatment. The above-ground biomass of 1st-season aerobic rice was significantly greater than that of 11th-season aerobic rice in both the oven-heating and control treatments. Oven heating increased soil N availability and above-ground biomass accumulation over the control in both 1st-season and 11th-season aerobic rice, but the above-ground biomass in the oven-heated 11th-season aerobic rice was still significantly lower than that of the oven-heated and even the untreated (control) 1st-season aerobic rice. These results suggest that abiotic factors contribute to the gradual yield decline observed in the field plots.     

亚洲热带地区的土壤资源及土地利用

Tropical Asia is a region comprising South and Southeast Asia and under strong influence of the Asian monsoon climate.It is characterized by and extremely high population density and by high land use intensity.Paddy rice cultivation is the most important form of agriculture in the greater aprt of the region.Soil resources of tropical Asia have a specific feature in comparison with tropical Africa and America.Ultisols dominate in uplands,and lowland soils like Inceptisols and Histosols are relatively abundant.The latter point is made clearer if we take the landforms of the region with a vast extent of lowlands into consideration.Geologically,tropical Asia with the Himalayan orogeny and active volcanism exhibits a conspicuous contrast to tropical Africa and America with the dominance of the shield structure.This along with the monsoon climate should have determined the basic features of landforms and soil,and acoordingly all the agricultural and social characteristics of tropical Asia today,Although paddy rice cultivation in the lowland is highly sustainable,upland cultivation in extensive Ultisol areas tends to be handicapped by low fertility and high erodibility of the soil,resulting in low sustainability.Land shortage is compelling people to exploit slopelands in hills and mountains,on the one hand,and thus far unutilized coastal lowlands,on the other,Both of these new reclamations are facing to serious land degradation problems todary.Torpical Asia will continue to be the most densely populated region of the wolrd with ever-increasing population.In order to meet the increasing food demand lowlang rice cultivation should beintensified by the infrastructure development to ease the stresses on slopelands and vulnerable coastal lowlands.At the same time,upland crop production in Ultisol areas should be stabilized and enhanced,providing integrated nutrient management and measures for soil conservation.     

Soil Chemical and Biological Activities under Vegetable Intensive Colocasia-based Cropping System in Indian Sub-Himalayas

ABSTRACT

Lack of crop diversification with suitable vegetable-based cropping system is a major constraint in limiting the productivity and sustainability of north-western Indian sub-Himalayas. To find out a sustainable vegetable-based cropping system in this region, a three year colocasia-based vegetable intensive experiment was conducted at Hawalbagh, Almora, India in a sandy clay loam soil under sub-temperate climatic conditions. Seven colocasia-based vegetable cropping systems along with rice–wheat system were compared under recommended package and practices. The system productivity in terms of colocasia equivalent yield was highest under colocasia–onion–frenchbean (52.38 Mg ha^?1) system. Sustainable yield index was highest with colocasia–gardenpea–frenchbean system (0.86). After 3 years, total soil organic carbon (0–5%), available N (2–22%), P (–7% to 14%) and K (3–15%) concentrations were increased in all cropping systems except rice–wheat system, where negative balance of available P (7%) was observed over that of initial soil. Significantly higher soil microbial activity, soil carbohydrate, dehydrogenase, protease, acid and alkaline phosphatases activity were observed under colocasia–onion system. The results suggest that colocasia–onion–frenchbean system with higher productivity improves soil fertility and enhances enzymatic activities.     

不同耕作方式下覆草旱作稻田土壤肥力的特征

通过始建于2003年中国南方季节性干旱区(江西省余江县)的双季稻田定位试验,于2005～2007年研究了水稻覆草旱作和免耕覆草旱作对稻田土壤理化性质和生物学性质的影响。结果表明,覆草旱作、免耕覆草旱作的耕层土壤容重和总孔隙度与常规水作的差异不显著。与常规水作相比,免耕覆草旱作显著提高土壤有机质、全氮、碱解氮和土壤基础呼吸;与常规水作相比,覆草旱作和免耕覆草旱作均显著提高土壤微生物生物量碳含量、脲酶和蔗糖酶活性。由此可知,覆草旱作和免耕覆草旱作可以作为该区积极推行的具有培肥地力作用的节水型稻作栽培模式。     

Land-use management for sustainable rice production and carbon sequestration in reclaimed coastal tideland soils of South Korea: a review

ABSTRACT

The properties of secondary salt-affected soils developed from improper irrigation and drainage management and their effects on rice growth and yield are well documented. However, relevant information on coastal reclaimed tideland (RTL) soils, which are classified as primary salt-affected soils developed through salt-accumulated sediments is lacking. In this paper, we reviewed the physical and chemical properties of RTL soils in comparison with non-RTL soils and analyzed the relationship between rice production and soil salinity in RTL to suggest agricultural management practices for sustainable rice production and soil carbon sequestration in RTL. Similar to the secondary salt-affected soils, RTL soils were characterized by high alkalinity, salinity, and sodicity, and rice yield was negatively correlated with salinity. However, it was also found that lower fertility (e.g., organic matter and phosphorus) of RTL soils than non-RTL soils might also hamper rice growth and thus carbon input via plant residues in RTL soils. Correlation between years after reclamation and soil properties of RTL showed that cultivation of rice with annual fertilization and organic matter inputs increased soil fertility but salinity and sodicity did not show a significant tendency of change, suggesting that natural desalinization in RTL soils is hard to be achieved with conventional rice cultivation. Therefore, it is suggested that fertilization management as well as salinity management via drainage, gypsum application, tillage, and proper irrigation may be necessary to improve rice production and carbon sequestration in RTL soils.     

Mild drying of sandy soil can physically limit the uptake of phosphorus by rainfed lowland rice in northeast Thailand

ABSTRACT

Poor response of rice to phosphorus (P) fertilization and low phytoavailability of soil P have been reported in sandy rainfed fields in northeast Thailand. In order to evaluate the effects of mild soil drying on the uptake of P by rainfed lowland rice, we carried out nutrient omission trials for nitrogen (N) and P at Ubon Ratchathani Rice Research Center under rainfed and flooded conditions. The surface soil was classified as sandy loam. To avoid severe soil drying and drought stress in the rainfed field, soil water potential at a depth of 20 cm was maintained at the field capacity (> ?20 kPa) by flush irrigation. The effects of flooding and drying on the soil properties were also evaluated in the laboratory using soils with diverse textures in and around the center. In the field experiments, the above-ground biomass of rice plants (RD6) did not respond significantly to P fertilization in the rainfed field, although it responded positively to N fertilization. Root length in the surface 10 cm under the rainfed condition was significantly smaller than that under the flooded condition due partly to the increased soil hardness upon drying, but this could not quantitatively explain the large discrepancy of P uptake observed between the rainfed and flooded conditions. Under the rainfed condition, the P uptake did not increase significantly, even when the concentration of soil Bray P was tripled by transferring the surface soil from the flooded to the rainfed field. From the laboratory experiments, it was further suggested that soil P was supplied mainly by diffusion and that the effective diffusion coefficient for P can become less than one-tenth of the value in the flooded field when the sandy soil with clay at around 10% dried to ?100 kPa. Our results suggest that the uptake of P by the rainfed lowland rice grown in sandy soil can be limited physically by mild soil drying that reduces the supply of P to roots by diffusion rather than the chemical extractability of soil P.     

Effect of Water and Rice Straw Management Practices on Soil Organic Carbon Stocks in a Double-Cropped Paddy Field

ABSTRACT

Water and rice straw (RS) management practices can potentially affect the accumulation of soil organic carbon (SOC) in agricultural soils. Field experiments were conducted in two consecutive rice-growing seasons (wet and dry) to evaluate SOC stocks under different water (continuous flooding [CF], alternate wetting and drying [AWD]) and RS management practices (RS incorporation [RS-I], RS burning [RS-B], without RS incorporation and burning [WRS]) in a double-cropped paddy field. RS-I under AWD had higher volumetric water content than the same RS management under CF at tillering in both growing seasons. Total SOC was significantly higher under AWD at tillering in both wet and dry seasons and after harvesting in the dry season compared with CF. The same trend was also observed for C:N ratio at tillering and after harvesting in the dry season. RS-B plots had lower SOC stocks than RS-I and WRS plots across most of the measuring periods regardless of the growing seasons. SOC stocks were 33.09 and 39.31 Mg/ha at RS-B and RS-I plots, respectively, in the wet season, whereas the respective values were 21.45 and 24.55 Mg/ha in the dry season. Incorporation of RS enhanced SOC stocks under AWD irrigation, especially in the dry season before planting. Soil incorporation of RS in combination with AWD could be a viable option to increase SOC stocks in the double-cropped rice production region as it is strongly linked with soil fertility and productivity. However, the environmental consequences of RS incorporation in irrigated lowland rice production system should be taken into consideration before its recommendation for paddy field on a large scale.