| Direct seeding rice has advantages in less labor, low labor intensity and low cost, and meets the needs of the development of simplified cultivation. However, the direct seeding rice shortens the growth period and has shallow root penetration, a high risk to lodging and a difficulty to achieve high grain yield and poor stability in the yield. Therefore, research on improving the yield of direct seeding rice, especially in mechanical direct seeding rice, has great significance in the development of simplified cultivation in rice production. Nitrogen (N) is one of the most important decision factors of crop yield. Though much work has been done on the effect of N application on rice yield formation, little is known about the response of rice growth, yield and quality formation to N rates and its physiological mechanism in mechanical direct seeding rice. In this study, two japonica rice varieties, Huaidao No.5 and Lianjing No.7, were field-grown and were sown in drill by simulating mechanical direct seeding. Four N levels, ON (no nitrogen), 120N (120 kg N/ha),240N (240 kg N/ha) and 360N (360 kg N/ha), and five nitrogen levels, ON, 90N (90 kg N/ha),180N (180 kg N/ha),270N (270 kg N/ha) and 360N (360 kg N/ha), were designed in 2013 and 2014, respectively. The grain yield, grain quality and use efficiencies of N, phosphorus (P) and potassium (K) under the different N levels and the physiological mechanism were investigated. The main results are as follows.1. Under the condition of the total amount of N application during the whole growing season from 0 to 360 kg ha-1, the grain yield was increased and then declined with the increase in N rates, and showed consistent performance between the two study years. The order of the grain yield was 240N> 360N> 120N> ON in 2013 and 270N> 360N> 180N> 90N> ON in 2014. The yield increase after N application was mainly due to the increase in total spikelets per unit area, which was resulted from the increase in both spikelet number per panicle and panicle number per unit area. With the increment of N rates, the panicle number per unit area was increased, the spikelet number per panicle was increased first and then decreased, and the peak appeared in the 240N or 270N level. The seed-setting rate and 1000-grain weight were decreased slightly with the increase in N rates. In addition, higher dry matter production, especially higher accumulation of dry matter after heading, and high harvest index also contributed to the increase in grain yield when N rates was increased from 0 to 240 kg ha-1 or from 0 to 270 kg ha-1.2. With the increase in N rates, the uptake of N, P, K was increased, but the agronomic use efficiency, recovery use efficiency and partial factor productivity of N fertilizer were decreased. The internal use efficiency of N or P was decreased, whereas the absorption amount of N or P per unit grain yield was increased, with the increase in N rates. The internal use efficiency of K was increased first and then decreased with the increase in N application. The remobilization of N, P, and K from stems during the grain filling period was increased with the increase in N rates.3. Compared with those under any other N rates, leaf area index, SPAD value, photosynthetic rate of the flag leaves, root oxidation activity and activities of the key enzymes involved in sucrose-to-starch conversion in grains were higher under the N rate at 240 or 270 kg ha-1, which should be an important physiological basis for a higher grain yield under such a N rate.4. With the increase in N rates, the milling quality and nutrient quality were improved, but the apparent quality was decreased, the gel consistency was shorten, and the taste of rice became poor. Application of N fertilizer increased the starchy peak viscosity and breakdown viscosity and reduced setback viscosity.In summary, under the condition of this experiment, the suitable amount of nitrogen rate in drilling and direct seeding rice is 240 to 270 kg ha-1. High grain yield and higher nutrient use efficiency could be achieved under such N levels. Higher root activity, photosynthetic rate of levels and activities of the key enzymes involved in sucrose-to-starch conversion in grains contributed to the higher grain yield and higher nutrient use efficiency under this amount of N fertilizer. Application of N fertilizer could not improve the eating quality of drilling and direct seeding rice. Further research is needed to develop the crop management that could realize the coordinative development of high grain yield and good quality in drilling and direct seeding rice. |
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