Effect of split application of phosphorus on the growth ofAzolla and low land rice

TheAzolla pinnata (Vietnam) inoculated in rice field 10 days after transplanting (DAT) at a rate of 500 kg ha^–1 fresh biomass along with phosphorus fertilizer application produced a mat on the water surface at 30 DAT. The three split application of phosphorus as 4.4, 2.2 and 2.2 kg P ha^–1 applied at 10, 15 and 20 DAT, respectively produced 67% more biomass and 57% more Nitrogen inAzolla than those obtained by applying 8.8 kg P ha^–1 at 10 DAT. Whereas, the two splits of phosphorus as 6.6 and 2.2 kg and 4.4 and 4.4 kg P ha^–1 applied 10 and 15 DAT, respectively produced 20 and 33% more biomass and 14 and 27% more Nitrogen only.The three split application of phosphorus also increased the grain and straw yields, panicle number and weight, nitrogen concentration and its uptake in rice significantly over application of the entire amount once only. An increase of 10% grain yield and 13% straw yields was observed when 8.8 kg P ha^–1 was applied in three splits rather than applied at one time. On the average an increase of 24% grain and 23% straw yields in rice were observed due toAzolla intercropping and 22% and 16%, respectively due to phosphorus application. The intercropping ofAzolla with rice along with phosphorus application also increased the fertility level of soil by increasing the total nitrogen, organic carbon and available phosphorus of soil.     

On-farm evaluation of leaf color chart for need-based nitrogen management in irrigated transplanted rice in northwestern India

Nitrogen use efficiency (NUE) in rice is low due to the inefficient management of fertilizer N by farmers. We evaluated a leaf color chart (LCC) as a simple tool for improving the time and rate of N fertilizer use in farmers’ fields for 4 years (2000–2003) in irrigated rice in northwestern India. Application of N fertilizer whenever leaf greenness was less than shade 4 on the LCC (the critical LCC value) produced rice grain yields on a par with blanket recommendation of applying 120 kg N ha⁻¹ in three equal splits in different years, but it resulted in an average saving of 26% fertilizer N across villages and seasons. In most situations, there was no significant advantage of applying 20 kg N ha⁻¹ as basal N at transplanting on grain yield and NUE of rice compared with no basal N. Use efficiencies of fertilizer N were higher when N was applied using LCC with a critical value of 4 than the recommended practice of applying 120 kg N ha⁻¹ in three equal split doses on all sites and in all years. The LCC with a critical value of 4 for real-time N management can be efficiently used to increase NUE in all types of inbred rice cultivars presently popular with the farmers of the Indian Punjab. The LCC is a cheap and easy-to-use tool that allows real-time N management by farmers on a large area leading to improved fertilizer N use efficiency, and reduced risks associated with fertilizer N application.     

Nitrogen use efficiency in different rice-based rotations in southern China

Experiments in fields and micro-plots were conducted to investigate the optimal cropping system and nitrogen (N) fertilizer application rate and timing. The treatments consisted of Chinese milk vetch–rice (CMV–R) rotation with five N fertilizer application rates (0, 120, 180, 240, 300 kg N ha^?1) during the rice-growing season, and fallow–rice (F–R) and wheat–rice (W–R) rotations with only one N application rate (240 kg N ha^?1) each. Rice yield increased with increasing N fertilizer application rate under CMV–R rotation, and achieved highest yield under CMV–R180. There is a decreasing trend when N application rate exceeded 180 kg N ha^?1. Rice yield was always higher under CMV–R240 compared to W–R240 and F–R240. During the 2012 rice season, the fertilizer N-use efficiency, residual N fertilizer in soil and N fertilizer recovery efficiency of CMV–R180 reached largest under CMV–R rotation with different N treatments. Furthermore, the fertilizer N-use and recovery efficiencies of CMV–R240 and F–R240 were far higher than those of W–R240. In 2013, fertilizer N-use efficiency was the highest (>?50%) at the heading stage, which was nearly twice as much as the efficiencies during the basal and tillering stages. The N fertilizer loss rate during the basal stage was significantly higher than that at the tillering and heading stages, which was up to 60%. CMV–R rotation with 180 kg N ha^?1 achieved the highest rice yield of 9454 kg ha^?1 and high fertilizer N-use efficiency (40.6%) under a relatively lower N application rate. Therefore, Chinese milk vetch–rice cropping system could be a promising approach for decreasing fertilizer inputs to prevent N pollution problems and increasing rice yield, especially for the intensive rice-based cropping systems in southern China.     

Environmental factors affecting nitrogen efficiency in flooded tropical rice

How environmental factors modify the effect of time of nitrogen application on grain yield and yield components of rice (Oryza sativa L.) were studied under field experiments during the 1975 dry and wet seasons on a fertile Maahas clay soil (an isothermic clayey mixed Aquic Tropudalf). Two rice varieties, one early maturing (IR28) and one medium maturing (IR26), which are highly responsive to nitrogen fertilizer, were tested. These varieties were planted monthly from January through December. Three nitrogen levels were applied in a single or in split doses to examine the maximum nitrogen efficiency under different climatic conditions.Low solar radiation values during the reproductive growth periods of both varieties reduced the spikelet number per unit area. IR26 rice, with an excessive number of spikelets per panicle, was more sensitive to solar radiation. It produced a high percentage of unfilled spikelets at low solar radiation.Nitrogen levels higher than 120 kg N ha^–1 were required to obtain maximum grain yields from both varieties under high solar radiation values during the reproductive growth periods. At low solar radiation levels, 60 kg N ha^–1 was adequate to get an optimum grain yield with high fertilizer efficiency.Increased grain yields greater than 1.0 t ha^–1 were obtained when the nitrogen fertilizer was applied at appropriate times. Proper application minimized the adverse effects of low solar energy.     

Effects of different levels of chemical Nitrogen (urea) onAzolla and Blue-green algae intercropping with rice

Application of higher levels (60 and 90 kg N ha^–1) of nitrogen fertilizer (Urea) inhibited the growth ofAzolla pinnata (Bangkok) and blue-green algae (BGA) though the reduction was more in BGA thanAzolla. Inoculation of 500 kg ha^–1 of freshAzolla 10 days after transplanting (DAT) in the rice fields receiving 30, 60 and 90 kg N ha^–1 as urea produced an average of 16.5, 15.0 and 13.0 t ha^–1 fresh biomass ofAzolla at 30 DAT, which contained 31, 31 and 27 kg N ha^–1, respectively. The dry mixture of BGA (60%Aulosira, 35%Gloeotrichia and 5% other BGA on fresh weight basis) inoculated in rice field 3 DAT at a rate of 10 kg ha^–1 showed a mat formation at 80 DAT with an average fresh biomass of 8.0, 5.8 and 4.2 t ha^–1 containing 22, 17 and 12 kg N ha^–1, respectively with those N fertilizer doses.Application ofAzolla showed positive responses to rice crop by increasing the panicle number and weight, grain and straw yields and nitrogen uptake in rice significantly at all the levels of chemical nitrogen. But, the BGA inoculation had a significant effect on the grain and straw yields only during the dry season in the treatment where 30 kg N was applied. During the wet season and in the other treatments performed during the dry season no significant increase in yields, yield components and N uptake were observed with BGA.The intercropping ofAzolla and rice in combination with 30, 60 and 90 kg N ha^–1 as urea showed the yields, yield attributes and nitrogen uptake in rice at par with those obtained by applying 60, 90 and 120 kg N ha^–1 as urea, respectively but, the BGA did not. The analysis of soil from rice field after harvest showed thatAzolla and BGA intercropping with rice in combination with chemical fertilizer significantly increased the organic carbon, available phosphorus and total nitrogen of soil.     

Effect of green manure on the yield,N uptake and floodwater properties of a flooded rice,wheat rotation receiving15N urea on a highly permeable soil

Field studies were conducted for two years on a rapidly percolating loamy sand (Typic Ustochrept) to evaluate the effect of green manure (GM) on the yield,¹⁵N recovery from urea applied to flooded rice, the potential for ammonia loss and uptake of residual fertilizer N by succeeding crops. The GM crop ofSesbania aculeata was grownin situ and incorporated one day before transplanting rice. Urea was broadcast in 0.05 m deep floodwater, and incorporated with a harrow. Green manure significantly increased the yield and N uptake by rice and substituted for a minimum of 60 kg fertilizer N ha^–1. The recovery of fertilizer N as indicated by¹⁵N recovery was higher in the GM + urea treatments. The grain yield and N uptake by succeeding wheat in the rotation was slightly higher with GM. The recovery of residual fertilizer N as indicated by the¹⁵N recovery in the second, third and fourth crops of wheat, rice and wheat was only 3, 1 and 1 per cent of the urea fertilizer applied to the preceding rice crop. Floodwater chemistry parameters showed that the combined use of the GM and 40 kg N ha^–1 as urea applied at transplanting resulted in a comparatively higher potential for NH₃ loss immediately after fertilizer application. The actual ammonia loss as suggested by the¹⁵N recoveries in the rice crop, however, did not appear to be appreciably larger in the GM treatment. It appeared the ammonia loss was restricted by low ammoniacal-N concentration maintained in the floodwater after 2 to 3 days of fertilizer application.     

Leaching losses of urea-N applied to permeable soils under lowland rice

Application of 120 kg urea-N ha^–1 to lowland rice grown in a highly percolating soil in 10 equal split doses at weekly intervals rather than in 3 equal split doses at 7, 21 and 42 days after transplanting did not significantly increase rice grain yield and N uptake. Results suggest that leaching losses of N were not substantial. In lysimeters planted with rice, leaching losses of N as urea, NH ₄ ⁺ , and NO ₃ ^- beyond 30 cm depth of a sandy loam soil for 60 days were about 6% of the total urea-N and 3% of the total ammonium sulphate-N applied in three equal split doses. Application of urea even in a single dose at transplanting did not result in more N leaching losses (13%) compared to those observed from potassium nitrate (38%) applied in three split doses. Nitrogen contained in potassium nitrate was readily leached during the first week of its application. More N was lost from the first dose of N applied at transplanting than from the second or third dose. Data pertaining to yield, N uptake and per cent N recovery by rice revealed that the performance of different fertilizer treatments was inversely related to susceptibility of N to leaching.     

Nitrogen-15 balances and nitrogen fertilizer use efficiency in upland rice

Field experiments were conducted in the 1984 and 1985 wet seasons to determine the effect of N fertilizer application method on¹⁵N balances and yield for upland rice (Oryza sativa L.) on an Udic Arguistoll in the Philippines. The test cultivars were IR43 and UPLRi-5 in 1984 and IR43 in 1985. Unrecovered¹⁵N in¹⁵N balances for 70 kg applied urea-N ha^–1, which represented N fertilizer losses as gases and movement below 0.5 m soil depth, ranged from 11–58% of the applied N. It was lowest (11–13%) for urea split applied at 30 days after seeding (DS) and at panicle initiation (PI), and highest (27–58%) for treatments receiving basal urea in the seed furrows. In all treatments with basal-applied urea, most N losses occurred before 50 DS.Heavy rainfall in 1985 before rice emergence resulted in large losses of native soil N and fertilizer N by leaching and possibly by denitrification. During the week of seeding, when rainfall was 492 mm, 91 kg nitrate-N ha^–1 disappeared from the 0.3-m soil layer in unfertilized plots. Although rainfall following the basal N application was less in 1984 than in 1985, the losses from basal applied urea-N were comparable in the two years. Daily rainfall of 20–25 mm on 3 of the 6 days following basal N application in 1984 may have created a moist soil environment favorable for ammonia volatilization.In both years, highest grain yield was obtained for urea split-applied at 30 DS and at PI. Delayed rather than basal application of N reduced losses of fertilizer N and minimized uptake of fertilizer N by weeds.     

The value of poultry manure for wetland rice grown in rotation with wheat

Poultry manure applied alone or in combination with urea at different N levels was evaluated as a N source for wetland rice grown in a Fatehpur loamy sand soil. Residual effects were studied on wheat which followed rice every year during the three cropping cycles. In the first year, poultry manure did not perform better than urea but by the third year, when applied in quantities sufficient to supply 120 and 180 kg N ha^–1, it produced significantly more rice grain yield than the same rates of N as urea. Poultry manure sustained the grain yield of rice during the three years while the yield decreased with urea. Apparent N recovery by rice decreased from 45 to 28% during 1987 to 1989 in the case of urea, but it remained almost the same (35, 33 and 37%) for poultry manure. Thus, urea N values of poultry manure calculated from yield or N uptake data following two different approaches averaged 80, 112 and 127% in 1987, 1988 and 1989, respectively. Poultry manure and urea applied in 1:1 ratio on N basis produced yields in between the yields from the two sources applied alone. After three cycles of rice-wheat rotation, the organic matter in the soil increased with the amount of manure applied to a plot. Olsen available P increased in soils amended with poultry manure. A residual effect of poultry manure applied to rice to supply 120 or 180 kg N ha^–1 was observed in the wheat which followed rice and it was equivalent to 40 kg N ha^–1 plus some P applied directly to wheat.     

Effects of Biochar and Nitrogen Application on Rice Biomass Saccharification,Bioethanol Yield and Cell Wall Polymers Features

Rice is a major food crop that produces abundant biomass wastes for biofuels. To improve rice biomass and yield, nitrogen (N) fertilizer is excessively used, which is not eco-friendly. Alternatively, biochar (B) application is favored to improve rice biomass and yield under low chemical fertilizers. To minimize the reliance on N fertilizer, we applied four B levels (0, 10, 20, and 30 t B ha⁻¹) combined with two N rates (low-135 and high-180 kg ha⁻¹) to improve biomass yield. Results showed that compared to control, the combined B at 20–30 t ha⁻¹ with low N application significantly improved plant dry matter and arabinose (Ara%), while decreasing cellulose crystallinity (Crl), degree of polymerization (DP), and the ratio of xylose/arabinose (Xyl/Ara), resulting in high hexoses (% cellulose) and bioethanol yield (% dry matter). We concluded that B coupled with N can alter cell wall polymer features in paddy rice resulting in high biomass saccharification and bioethanol production.     

Effect of green manuring with Sesbania aculeata and nitrogen fertilization on the performance of direct-seeded flood-prone lowland rice

Green manuring of rice with dhaincha (Sesbania aculeata) is widely practised under irrigated puddle-transplanted conditions. In flood-prone lowlands, the rice is established through direct seeding early in the season and flooding occurs after 1–2 months of crop growth following regular rains. The low yields are due to poor crop stands and difficulty in nitrogen management under higher depths of water. The effect of green manuring with dhaincha intercropped with direct-seeded rice vis-à-vis the conventional practice of incorporating pure dhaincha before transplanting was investigated under flood-prone lowland conditions (up to 50–80 cm water depth) at Cuttack, India. Treatment variables studied in different years (1992, 1994 and 1995) were: rice varieties of different plant heights, crop establishment through direct seeding and transplanting, varying length of periods before dhaincha incorporation, and urea N fertilizer levels. Dhaincha accumulated 80–86 kg N ha^-1 in pure stand and 58–79 kg N ha^-1 when intercropped with direct-seeded rice in alternate rows at 50 days of growth. The growth of rice improved after dhaincha was uprooted manually and buried in situ between the rice rows when water depth was 10–20 cm in the field. The panicle number was lower but the panicle weight was higher with dhaincha green manuring than with recommended level of 40 kg N ha^-1 applied as urea. The grain yield was significantly higher with direct seeding than with transplanting due to high water levels (>60 cm) immediately after transplanting. Dhaincha manuring was at par with 40 kg N ha^-1 as urea in increasing the yield of direct-seeded and transplanted crops. The highest yield of direct-seeded crop was obtained when 20 kg N ha^-1 was applied at sowing and dhaincha was incorporated at 50 days of growth. The results indicate that green manuring of direct-seeded rice with intercropped dhaincha is beneficial for substituting urea fertilizer up to 40 kg N ha^-1 and augmenting crop productivity under flood-prone lowland conditions.     

Studies on the substitution of inorganic fertilizers with organic manure and their effect on soil fertility in rice—wheat rotation

Field studies on the substitution of N and P fertilizers with farm yard manure (FYM) and their effect on the fertility status of a loamy sand soil in rice—wheat rotation are reported. The treatments consisted of application of 12 t FYM ha^–1 in combination with graded levels of N and P. Application of fertilizer N, FYM and their different combinations increased the rice yield significantly. There was no significant response to P application. The magnitude of response to the application of 12 t FYM and its combined use with each of 40 kg and 80 kg N ha^–1 was 0.7, 2.2 and 3.9 t ha^–1 respectively. Application of 120 kg N ha^–1 alone increased the yield by 3.9 t ha^–1, and was comparable to rice yield obtained with 80 kg N and 12 t FYM ha^–1. This indicated that 12 t FYM ha^–1 could be substituted for 40 kg N as inorganic fertilizer in rice. In addition FYM gave residual effects equivalent to 30 kg N and 13.1 kg P ha^–1 in the succeeding wheat. The effect of single or combined use of inorganic fertilizers and FYM was significantly reflected in the build up of available N, P, K and organic carbon contents of the soil. The relationship for predicting rice yield and nutrients uptake were also computed and are discussed.     

Response of three sorghum varieties to N supply and plant density in a tropical environment

Field trials were conducted at Samaru, Nigeria to investigate the growth, yield and grain quality response of three grain sorghum (Sorghum bicolor L. Moench) varieties (L. 187, SK 5912 and FFBL) to N fertilization under varying plant densities (33300, 50000 or 66600 plants ha^–1). Year × N interactions were significant for yield components and so were variety × N and variety × plant density interactions. Grain yield increased 41, 42, and 126% with application of 60 kg N ha^–1, the optimum N rate, a response which was associated with variations in grain weight per panicle, panicle weight and grain number. Varieties SK 5912 and FFBL produced more straw in response to added N than did var. L. 187 while yield components in var. SK 5912 and L.187 responded better than those in var. FFBL. Yield components declined in var. SK 5912 and L. 187 as plant density was increased to either 50000 or 66600 plants ha^–1. Grain crude protein (CP) content and protein yield were increased 8 and 52% respectively by 60 kg N ha^–1 but CP content declined as plant density was increased. Grain tannin content was virtually unaffected by increasing N supply. Optimum plant density for grain sorghum production in this environment is in the range of 50000 plants ha^–1.     

Urea-rubber matrices as slow-release fertilizers

The efficacy of a prototype slow-release fertilizer, urea-rubber matrix (URM) was assessed under flooded rice conditions. URM cuboids of size 0.5 × 1.0 × 0.4 cm were applied in comparison with prilled urea at levels of 50, 130 and 200 kg N ha^?1. URM was placed at the soil/solution interface in intimate association with rice seedlings whereas prilled urea was broadcast into the floodwater to simulate the normal application method of South East Asian farmers. URM cuboid sizes of 0.25 × 0.5 × 0.4 cm, 0.5 × 0.5 × 0.4 cm and 1.0 × 1.0 × 0.4 cm were similarly evaluated at a single rate of 130 kg N ha^?1; a broadcast URM treatment was also included. Different methods of prilled urea application including deep placement and split application were also studied at a single rate of 130 kg N ha^?1. It was found that the build-up of floodwater N (urea + NH ₄ ⁺ ) from URM during the 13 days following application was almost negligible irrespective of level or method of application. This was thought to result in low losses of N through ammonia volatilization as shown by higher rice grain yields in comparison with prilled urea at all levels of application. Deep-placed urea gave a comparable grain yield to that of broadcast URM. There was no significant difference in grain yield between URM applied by placement and by broadcast, suggesting that URM can be effectively applied either in intimate association with rice seedlings or by broadcasting to the rice field before, or after, planting. An attempt to predict the release of urea from URM was made using a diffusion-based simulation model. It was found that the model underestimated the actual release of urea from URM within the rhizosphere, probably due to the extensive penetration by rice roots of the URM cuboids.     

Long term effects of inorganic fertilizer inputs on crop productivity in a rice-wheat cropping system

Three levels of N (40,80,120 kg N ha^-1) and P (0,17.5,35 kg P ha^-1), and 2 levels of K (0,33 kg K ha^-1) were tested for 19 years in rice and wheat crops of a rice-wheat cropping system in a fixed layout of 3×2×2 factorial partially confounded design along with one control and 3 replications. From this trial, data of 7 treatments, i.e. 0-0-0, 40-35-33, 80-35-33, 120-35-33, 120-0-0, 120-35-0 and 120-0-33 kg ha^-1 N-P-K respectively were compared for yield trends, changes in response functions, soil organic -C and available N,P,K status. Soil organic - C decreased in unfertilized plots by 62% (over initial value of 0.45%) but increased by 44, 40 and 36% in plots receiving 120-35-33, 120-35-0 and 80-35-33 kg ha^-1 N-P-K respectively. Available N was also greatest in these same three treatments. Available soil P increased by about 5 fold in 15 years in treatments supplied with fertilizer P, but no significant change was detected in treatments without P addition. Yields of rice and wheat exhibited linear declining trend in all treatments. The highest rate of decline (89 kg ha^-1 year^-1 in rice and 175 kg ha^-1 year^-1 in wheat), however, was found when 120 kg ha^-1 N was applied alone. The least rate of decline of 20 kg ha^-1 year^-1 in rice and 58 kg ha^-1 year^-1 in wheat was observed when 40-35-33 kg ha^-1 N-P-K respectively was applied to both the crops. At currently recommended levels of NPK (120-35-33 kg ha^-1), the rate of decline in yields was 25 kg ha^-1 year^-1 for rice and 62 kg ha^-1 year^-1 for wheat. Possible causes of these yield declines are discussed.     

Effects of mulch,N fertilizer,and plant density on wheat yield,wheat nitrogen uptake,and residual soil nitrate in a dryland area of China

Understanding mulching influences on nitrogen (N) activities in soil is important for developing N management strategies in dryland. A 3 year field experiment was conducted in the Loess Plateau of China to investigate the effects of mulching, N fertilizer application rate and plant density on winter wheat yield, N uptake by wheat and residual soil nitrate in a winter wheat-fallow system. The split plot design included four mulching methods (CK, no mulch; SM, straw mulch; FM, plastic film mulch; CM, combined mulch with plastic film and straw) as main plot treatments. Three N fertilizer rates (N0, 0 kg N ha⁻¹; N120, 120 kg N ha⁻¹; N240, 240 kg N ha⁻¹) were sub-plot treatments and two wheat sowing densities (LD, low density, seeding rate = 180 kg ha⁻¹; HD, high density, seeding rate = 225 kg ha⁻¹) were sub-subplot treatments. The results showed that wheat yield, N uptake, and N use efficiency (NUE) were higher for FM and CM compared to CK. However, soil nitrate-N contents in the 0–200 cm soil profile were also higher for FM and CM compared to CK after the 3 year experiment. Wheat grain yields were higher for SM compared to CK only when high levels of nitrogen or high planting density were applied. Mulching did not have a significant effect on wheat yield, nitrogen uptake and NUE when soil water content at planting was much high. Wheat yield, N uptake, and residual nitrate in 0–200 cm were significantly higher for N240 compared to N120 and N0. Wheat yield and N uptake were also significantly higher for HD compared to LD. When 0 or 120 kg N ha⁻¹ was applied, HD had more residual nitrate than LD while the reverse was true when 240 kg N ha⁻¹ was applied. After 3 years, residual nitrate-N in 0–200 cm soil averaged 170 kg ha⁻¹, which was equivalent to ~40% of the total N uptake by wheat in the three growing seasons.     

Fertilizer requirement of rice-wheat and maize-wheat rotations on coarse-textured soils amended with farmyard manure

Field experiments with rice-wheat rotation were conducted during five consecutive years on a coarse-textured low organic matter soil. By amending the soil with 12t FYM ha^–1, the yield of wetland rice in the absence of fertilizers was increased by 32 per cent. Application of 80 kg N ha^–1 as urea could increase the grain yield of rice equivalent to 120 kg N ha^–1 on the unamended soil. Although the soil under test was low in Olsen's P, rice did not respond to the application of phosphorus on both amended and unamended soils. For producing equivalent grain yield, fertilizer requirement of maize grown on soils amended with 6 and 12 t FYM ha^–1 could be reduced, respectively to 50 and 25 per cent of the dose recommended for unamended soil (120 kg N + 26.2 kg P + 25 kg K ha^–1). Grain yield of wheat grown after rice on soils amended with FYM was significantly higher than that obtained on unamended soil. In contrast, grain yield of wheat which followed maize did not differ significantly on amended or unamended soils.     

Impact of organic residue management on nitrogen use efficiency in an annual rice cropping sequence of lowland Central Thailand

Field experiments were conducted in Central Thailand under a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1998 to determine the impact of residue management on fertilizer nitrogen (N) use. Treatments consisted of a combination of broadcast urea (70 kg N ha^–1) with rice straw (C/N 67) and rice hull ash (C/N 76), which were incorporated into the puddled soil 1 week before transplanting at a rate of 5 Mg ha^–1. Nitrogen-15 balance data showed that the dry season rice recovered 10 to 20% of fertilizer N at maturity. Of the applied N, 27 to 36% remained in the soil. Loss of N (unaccounted for) from the soil–plant system ranged from 47 to 54% of applied N. The availability of the residue fertilizer N to a subsequent rice crop was only less than 3% of the initial applied N. During both season fallows NO₃-N remained the dominant form of mineral-N (NO₃+NH₄) in the aerobic soil. In the dry season grain yield response to N application was significant (P=0.05). Organic material sources did not significantly change grain yield and N accumulation in rice. In terms of grain yields and N uptake at maturity, there was no significant residual effect of fertilizer N on the subsequent rice crop. The combined use of organic residues with urea did not improve N use efficiency, reduced N losses nor produced higher yields compared to urea alone. These results suggested that mechanisms such as N loss through gaseous N emissions may account for the low fertilizer N use efficiency from this rice cropping system. Splitting fertilizer N application should be considered on the fertilizer N use from the organic residue amendment.     

Effect of Straw Application on Rice Yields and Nutrient Availability on an Alkaline and a pH-neutral Soil in a Sahelian Irrigation Scheme

Like elsewhere in the Sahel, actual rice yields (3–5 t ha⁻¹) are far below yield potential (±8 t ha⁻¹) in an irrigation scheme in central southern Mauritania. Earlier studies showed that yields are especially low on alkaline soils due to N and P deficiency. We investigated the potential of rice straw application as a mean to improve yields and fertilizer efficiency on an alkaline soil (pH 8.2) and a pH-neutral soil (pH 6.2). Application of 5 t straw ha⁻¹ increased yields by 1.1 t ha⁻¹ on average, independent of soil type and fertilizer dose. Contrary to our study, similar studies in Asia showed little short-term effects of straw on yield and N uptake. Straw application improved N availability, but not P availability. The improved N availability was attributed to N mineralized from the straw, from increased mineralization of soil organic matter (SOM) with a low C:N ratio (< 7.2) and from increased mineral fertilizer N (urea) recovery efficiency. We deduced that improved N fertilizer recovery upon straw application was due to reduced nitrification–denitrification losses. On the alkaline soil, volatilization was important, but that process seemed unaffected by straw application. We hypothesize that the positive effects of straw application at our study site are due to low soil C content (< 43 g kg⁻¹) and low C:N ratio compared to most lowland rice soils in Asia.     

Synchronizing N Supply from Soil and Fertilizer and N Demand of Winter Wheat by an Improved Nmin Method

Excessive nitrogen (N) fertilizer application and poor timing of N fertilizer application to winter wheat are common problems on the North China Plain. To study the possibilities of optimizing the timing and rate of N application, a field experiment was conducted from 1999 to 2001 in a suburb of Beijing. A control (no nitrogen) and two N fertilization strategies (conventional N application and optimized N fertilization) were designed to compare their effects on wheat growth, N nutrient status, grain yield and N balance. The conventional N fertilization strategy was given a fixed N rate of 300 kg N ha⁻¹, which was split, half in autumn and half in spring as a top-dressing. The timing and rate of N and application of the optimized N fertilization strategy were determined by the target value of soil mineral nitrogen demand for three growth periods of wheat, which is related to the target yield, and soil mineral N (N_min) in the effective rooting depth at the beginning of these three periods. Based on the optimized N fertilization strategy, a total of 55 and 65 kg N ha⁻¹ had to be applied to winter wheat in the re-greening and shooting stages of the first and second experimental years, respectively. Compared with the high N rate before sowing in the conventional N fertilization treatment, the optimized N fertilization treatment did not require any N fertilizer before sowing of wheat. Despite a much lower N fertilization rate, no significant difference in N nutrient status, growth during the wheat growing period or grain yield was observed between optimized N and conventional N fertilization treatments. As a consequence of optimizing the rate and timing of the N fertilizer application to match wheat demand, a much lower residual N_min and calculated apparent N loss was found as compared to the conventional N treatment. N recovery for the optimized N fertilization treatment (67% in 1999/2000 and 66% in 2000/2001) was much higher than that of the conventional N fertilization treatment (19% in 1999/2000 and 18% in 2000/2001). In conclusion, the optimized N fertilization strategy can synchronize N demand of wheat and the N supply from soil and fertilizer, and therefore drastically reduce N application rates without any yield losses.