Studies On The Effects Of Grain Yield, Rice Quality And Nitrogen Use Efficiency In Irrigated Rice System Based On Real-time And Site-specific Nitrogen Management

The poor fertilizer-nitrogen use efficiency (FNUE) is a critical issue in irrigated ricesystem in China. The International Rice Research Institute has coordinated a project onReaching Toward Optimum Productivity (RTOP) since 1997. The goal of RTOP projectis to improve nutrient management strategies of irrigated rice system through on-farmresearch on site-specific nutrient management and real-time N management (RTNM). Toexplore the possibility to improve the nitrogen use efficiency in irrigated rice in China,the site-specific nitrogen management (SSNM) is developed based on the previousfindings from RTOP. To evaluate the effect on agronomic performance in terms of grainyield, grain quality and NFUE of RTNM and SSNM, the on-farm experiments wereconducted in 2004 and 2005 in Xuhsan village, where located at Xinpu town, Xiaonancounty, Hubei province. The results show as follows:We used two varieties for experimental materials, such as two-line hybrid riceLiangyoupei9 (LYP9) and three-line hybrid rice Shanyou63(S63) both in 2004 and 2005.We set eight treatments in 2004, including N1(fertilizer-N omission), N2～N7 (RTNM,fertilizer-N application based on whether weekly SPAD readings dropped below than theexpected SPAD thresholds of 35, 36.5, 38, 39.5, 41 and 42.5 respectively), and N8(SSNM, fixed time but adjustable fertilizer-N rate based on SPAD reading). We set sixtreatments in 2005, including N1 (N omission), N2～N5 (RTNM with SPAD threshold of34, 36, 38, 40 for S63, and 36, 39, 42, 45 for LYP9), and N6 (SSNM). The results showas follows:(1) The N rate increasing follows the increase of SPAD threshold under RTNM. Itshows significant linear correlation between the SPAD threshold and actual fertilizer-Nrate under RTNM for both varieties. To maintain the same expected SPAD reading withinthe whole growth season, S63 needed one more fertilizer-N topdressing than that of LYP9,and the total application rate of nitrogen increased by 30 kg N.hm^-2-45 kg N.hm^-2compared to LYP9. And the nitrogen rate of S63 increased by10 kg N.hm^-2-20 kg N.hm^-2 expected SPAD threshold. However, the differences of the biomass were not significantamong various expected SPAD thresholds treatments under RTNM when the SPADthreshold was higher than a proper value. LYP9 had higher dry matter accumulationcapacity than that of S63 in the same rate of N-fertilizer, due to LYP9 accumulated by10% more dry mater during heading to maturity than that of S63, through both of themhad the similar accumulation rate in term of dry matter production before heading. Thismight be the main biology base of LYP9 with higher yield.(3) There was a binomial relationship between the SPAD threshold for RTNM andthe grain yield. Based on the regression equation with LYP9 and S63, the optimum SPADthreshold of Shanyou63 was lower than LYP9. It indicated that lower SPAD thresholdwould be recommended for the slimily varieties or combination as the S63 under nitrogenmanagement. SSNM; however, it should choose higher SPAD threshold for the analogousvarieties or combination like Liangyoupei9.The harvest grain yield of S63 in N omission plot was higher than that of LYP9,but this trend was reversed when fertilizer-N rate reached 120 kgN.hm^-2and/or above, theincrease percentage of grain yield of LYP9 comparing to S63 was by 7%. The investigateresults showed that the number of average glume per centiare across all treatments wasby 1.32×10⁴ higher than that of S63, which might explain that LYP9 had a barger "Sink"to get the higher yield.(4) Under Real-time Nitrogen Management, the general trend of nitrogen dry matterproduction efficiency, nitrogen grain production efficiency, nitrogen harvest index,nitrogen agronomy efficiency, nitrogen physiological efficiency, nitrogen partial factorproductivity decreased following the increase of SPAD threshold both in LYP9 and S63.There were highest nitrogen agronomy efficiency,nitrogen recovery efficiency,nitrogenphysiological efficiency and nitrogen partial factor productivity under 35-39,34-36 of theSPAD threshold in LYP9 and S63 respectively. The performance of nitrogen useefficiency in SSNM showed a similar traits as that of the optimal SPAD thresholdtreatment under RTNM both in two varieties.Under the same condition of nitrogen management in terms of application timeand rate between two varieties, nitrogen recovery efficiency of S63 was by 70.70% higherthan that of LYP9 when 90kg N/hm² applied, but nitrogen physiological efficiency of S63was by 16.57% lower than that of LYP9. But when applied more than 120 kg N/hm² ,LYP9 had higher nitrogen agronomy efficiency, nitrogen recovery efficiency and nitrogenphysiological efficiency than that of S63.(5) Under Real-time Nitrogen management, the effect of different SPAD thresholdon grain quality was significant, due to the rate of nitrogen fertilizer increased following (5) Under Real-time Nitrogen management, the effect of different SPAD thresholdon grain quality was significant, due to the rate of nitrogen fertilizer increased followingthe SPAD threshold value increasing. The milled rice rate was significant variation as therate of nitrogen fertilizer application increased. But the highest value of milled rice ratehad different SPAD threshold value with different varieties. The highest value of milledrice rate appeared at SPAD threshold of 39 in LYP9, but for S63, it at the range of 38-40with SPAD threshold. There were lower Chalky ratio and Chalkiness at optimum SPADthreshold value. The head rice rate of LYP9 was significantly higher than S63, the riceshape (length/width) of LYP9 was larger than that of S63 too. But Chalky ratio andChalkiness of LYP9 was lower than S63, there was a significant difference respectively.General evaluate the effects of rice qualities under RTNM, it imight improve millingquality and apparent features at the SPAD threshold ot 38-41 (rate of nitrogen application:120kg N/hm²-165kg N/hm²) for LYP9 and 36-39 (rate of nitrogen application: 120kgN/hm²-165kgN/hm²) for S63 respectively. Under SSNM, the range 38-39 of the SPADthreshold value was optimal for LYP9 and other varieties with similar traits, for S63 andwith similar traits varieties, the optimal SPAD threshold was 36.(6) The changes in temperature (T) and relative humidity (RH) within the canopywere recorded by HOBO data loggers. The following results were obtained: T and RHwithin canopy were affected by the air-T and air-RH of the top above the canopy, canopysize, growing season and growth stage; T and RH within canopy, which showed a trendsimilar as that above the canopy, were stable, respectively lower and higher at night(19:00-07:00), and unstable, respectively higher and lower at daytime (07:00-19:00).Thedaily extreme T and RH within canopy occurred at about 13:30; Differences in T and RHamong different treatments were mainly caused by day T and day RH, and were greater infull heading stage. The daily maximum T (Tmax) and the daily minimum RH (RHmin)varied most among the treatments. The day T and day RH within canopy decreased andincreased respectively as nitrogen level increased.(7) It showed that, leaf N concentration in all the varieties increased during thewhole growth stage following the increasing of the SPAD threshold under RTNM. Theresponse of fertilizer-N rate was different under the same SPAD threshold usingvarieties with different traits of leaf color, specific leaf weight etc.. To obtain the higheryield and improve the nitrogen use efficiency, the SPAD threshold should be adjustedconsidering the varieties traits. Higher threshold value was recommended for the varietieswith dark leaf color and thick leaf. The lower SPAD threshold was recommended for thevarieties with light color and thin leaf.In general, when fertilizer nitrogen management based on SSNM, the SPAD characteristics as LYP9. The optimal SPAD threshold might be determined based onRTNM with various SPAD threshold treatments for the new released varieties.Fertilizer-N Management based on SSNM using optimal SPAD threshold, the grain yieldmay be increased and nitrogen agronomy use efficiency may be improved comparing tothat under traditional farmers' practice.