Residual N and P in soils at the end of rice season
In all the treatments, soil total N increased by the end of the 2-year experiments than that of the initial state. Planting japonica rice increased soil total N in the 0–20 cm layer (Fig. 1a). The improvement rates for total N under different treatment methods were as follows: NTJ (2.63%), CTJ (2.37%), NTG (2.11%), and CTG (2.05%). At the end of 2 years, the total N content was the highest under the NTJ treatment (1.95 g kg-1), and lowest under the CTG treatment (1.93 g kg-1). Based on the increase in the soil total N content during the fallow season and the rice season, the means of total N content increased in fallow seasons F1 and F2 by 5.25 mg kg-1 and 7.00 mg kg-1 in the 2-year experimental period. The average increase in soil total N content of R1 and R2 was 10.80 mg kg-1 and 20.5 mg kg-1, respectively.
Compared with the total P content in the initial soil sample (983 mg kg-1), the total P content in the soil increased annually and japonica rice contributed to the increase (Fig. 1b). The improvement rates for total P under different treatment methods were NTJ treatment (1.73%), CTJ treatment (1.53%), NTG treatment (1.12%), and CTG treatment (0.92%). After 2-years experiments, the total P content in the paddy soil was the highest (1000 mg kg-1) in the NTJ treatment and the lowest (992 mg kg-1) in the CTG treatment. The average values of the total P content increase in F1 and F2 were 1.75 mg kg-1 and 1.80 mg kg-1, respectively. The average values of the soil total P content increases in R1 and R2 were 3.50 mg kg-1 and 6.00 mg kg-1, respectively. This indicated that the increase in soil total P during the rice season was higher than that during the fallow season.
Both no-till and the japonica rice variety contributed to the increase of residual N and P in the soils at the end of rice season, but the difference was not significant (p > 0.05) (Fig. 2a, 2b). The increase in soil N and P stocks during the rice season was significantly higher than that during the fallow season (p < 0.05). Compared with the initial N stocks in the rice field (4613.1 kg N ha-1), the increase rate for the N pool in the rice field soil under the different treatments was: NTJ (3.12%) > CTJ (2.69%) > NTG (2.67%) > CTG (2.45%). At the end of the two rice seasons (R1E and R2E), the increases in soil N stocks in the rice fields were 42.34 kg N ha-1 and 62.56 kg N ha-1, respectively, and at the end of the two fallow seasons (F1E, F2E) was 11.85 kg N ha-1 and 9.35 kg N ha-1, respectively.
Compared with the P stocks in initial rice fields soil (2400.6 kg P ha-1), at the end of 2-year experiment, the increase rate in P stocks in the rice fields soil based on different treatments were: NTJ (3.12%) > CTJ (2.69%) > NTG (2.67%) > CTG (2.45%). At the end of the two rice seasons (R1E and R2E), the increase in soil P stocks in the rice fields was 5.44 kg P ha-1 and 10.42 kg P ha-1, respectively, and at the end of the two fallow seasons (F1E, F2E) was 2.56 kg P ha-1 and 1.81 kg P ha-1, respectively.
Nutrient uptake by rice tissues
The no-till treatment with glutinous rice did not have a significant effect on the total N content of the rice straw (p > 0.05), but it significantly increased the total P content in the rice straw and total N and total P content in rice grains (p < 0.05). In addition, the content of total N and total P in rice grains was much higher than that in rice straw (p < 0.05) (table 2). During the 2-year experimental period, the change in total N content in rice straw and grain remained consistent under the different treatment conditions. The order of total N content in rice straw treated during different periods was NTG (R1E, 8.20 g kg-1; R2E, 8.31 g kg-1)> CTG(R1E, 8.16 g kg-1; R2E, 8.27 g kg-1)> NTJ(R1E, 8.11 g kg-1; R2E, 8.21 g kg-1) > CTJ (R1E, 8.20 g kg-1; R2E, 8.31 g kg-1)(Table 2). The total N content of straw treated with NTG remained the highest (8.20 g kg-1–8.31 g kg-1). Compared with the CTJ treatment, the total N content of straw increased by 1.61%–2.09%. No significant difference in the total N content of rice straw differed among the different tillage method and rice variety treatments (p > 0.05). However, the total N content of the grains under NTG treatment increased by 6.79%–9.77% than the CTJ treatment, which was significantly different (p < 0.05).
At RE, the order of total P content in rice straw under different treatment conditions was NTG > NTJ > CTG > CTJ, whereas the order of total P content in rice grains was NTG > CTG > CTJ > NTJ. The total P content of rice straw (1.22 g kg-1–1.25 g kg-1) and grains (2.35 g kg-1–2.38 g kg-1) under the NTG treatment remained the highest. The content of total P in rice straw (1.03 g kg-1–1.06 g kg-1) under the CTJ treatment and that of rice grains (2.11 g kg-1–2.15 g kg-1) under the NTJ treatment were the lowest. Compared with CTJ, NTG significantly increased the total P content in rice straw by 17.92%-18.45%, and the total P content of rice grain treated with NTG was 10.70%-11.37% higher than that treated with NTJ, both of them showed a significant difference (p < 0.05).
The no-till and japonica rice treatment was beneficial to improving the N content of rice grains. In the 2-year experiment, N accumulation in rice grains showed an upward trend (Fig. 3a), and P accumulation in rice grains showed a similar trend (Fig. 3b). The N and P accumulation in the no-till rice fields were 2.23 kg N ha-1 and 0.14 kg P ha-1 higher than those in the conventional tillage treatment, respectively. Japonica rice increased the N accumulation of the rice grains by 2.65 kg N ha-1 and P accumulation by 0.37 kg P ha-1, as compared with that of glutinous rice. Both of no-till and glutinous rice variety contributed to the increase in P accumulation in rice straw. At the end of the second rice season (October 2017), the N and P accumulation in rice straw showed an upward trend (Fig. 4a, 4b). The N and P accumulation in rice straw under the NTG treatment was the highest (70.30 kg N ha-1, 10.58 kg P ha-1), and was 9.38% and 26.40% higher than that of CTJ treatment, respectively, all of which were significantly different (p < 0.05). In the 2-year experiment, the no-till treatment increased the N and P accumulation in rice straw by 1.02 kg N ha-1 and 0.37 kg P ha-1, respectively, as compared with the conventional tillage treatment. The accumulation of N and P in rice straw in the glutinous rice treatment was 2.08 kg N ha-1 and 0.53 kg P ha-1 higher than those of japonica rice.
There was a slight improvement in rice grain yield in the no-till treatment (p > 0.05); however, rice variety had a significant effect on the improvement of rice grain yield (p < 0.05). The grain yield of japonica rice was higher than that of glutinous rice. At the end of the two rice seasons, the grain yield of rice treated with the NTJ treatment was highest (R1E: 7090 kg ha-1; R2E: 7332 kg ha-1) (Table 3), and yield was increased by 11.23% (R1E) and 11.62% (R2E), as compared with that of the corresponding NTG treatment, respectively. The yield of grain in the CTJ treatment was the second and was 11.17% (R1E) and 11.24% (R2E) higher than that of the CTG treatment for the same period. However, the NTJ treatment only increased grain yield by 1.04%–1.34% compared with that of the CTJ treatment, whereas the NTG treatment increased yield by only 1.00% compared with that of the CTG treatment. The no-till treatment improved the yield of rice straw (0.03%–1.69%), but the difference was not significant (p > 0.05). However, rice variety and the yield of rice straw were significantly correlated (p < 0.05). The yield of rice straw in the NTG treatment was always the highest (8292 kg ha-1–8360 kg ha-1) (Table 3) and was 2.51% (R1E) and 1.54% (R2E) greater than the corresponding NTJ treatment. The yield of rice straw in the CTG treatment was the second highest, and a significant difference occurred between the CTG and CTJ treatments (p <0.05). There was a greater yield of rice straw (2.83% [R1E] and 2.99% [R2E]) under the CTG than CTJ treatment. The yield under the NTG treatment only increased by 0.03%–0.89% compared with that of the CTG treatment, whereas the yield in the NTJ treatment increased by only 1.20%–1.69% compared with that of the CTJ treatment.
Soil nutrient balance analysis
By calculating the N/P balance in the rice fields during two consecutive seasons, it was found that no-till and japonica rice were conducive to improving the in-situ reduction of N and P in the rice fields (Tables 4 and 5). The environmental loss of N and P during the planting season was lower than that during the rice season. Compared with the rice and planting seasons in the first year, the N and P environmental losses decreased during the rice and planting seasons in the second year.
The environmental losses of N and P in the rice fields mainly occurred during the rice season. At different time periods, the environmental losses of N and P in rice fields treated with CTG were the largest. After 2-year experiments, N losses in the NTJ, NTG, and CTJ treatments were reduced by 15.03 kg N ha-1, 11.55 kg N ha-1, and 6.72 kg N ha-1, respectively, as compared with that of the CTG treatment. The corresponding P losses were reduced by 8.02 kg P ha-1, 7.84 kg P ha-1, and 3.96 kg P ha-1.
Economic benefits under different tillage methods and rice varieties
From the perspective of economic benefits, the production cost of the no-till treatment was lower than that of conventional tillage method (Table 6). The no-till method saved 1500 yuan ha-1 for machine tillage and the same cost for fertiliser and rice seeds was assumed (the difference between the prices of the two rice seeds was negligible). The average price of japonica rice seeds (Xiushui-134) was approximately 2640 yuan t-1, and the average price of glutinous rice seeds (Zhenuo-65) was approximately 2900 yuan t-1 [23]. The NTJ treatment had the highest output-investment ratio (6.40), and the real income per hectare was 18896.5 yuan. The NTJ treatment increased revenue by 2,033.9 yuan (12.06%) compared with that of the CTG treatment, 1,764.0 yuan (10.30%) compared with that of the CTJ treatment, and 317.3 yuan (1.71%) compared with that of the NTG treatment.