Production Benefits of Double-Cropping Rice Under Optimized Application of Nitrogen and Potassium Fertilizers Combined with Chinese Milk Vetch and Straw Co-Returning to Fields

DONG Chun-hua, MA Chao-hong, HU Ke-xin, HE Yun-long, LI Wan-min, CHU Fei,YANG Zeng-ping

1. Hunan Institute of Soil and Fertilizer, Changsha 410125, PRC;

2. Institute of Plant Protection and Soil Fertilizer, Hubei Academy of Agricultural Sciences, Wuhan 430064, PRC;

3. Agricultural Technology Extention Center of Anxiang County, Anxiang 415600, PRC

Abstract In order to explore the technology and effects of reducing nitrogen and potassium fertilizer applications in double-cropping rice, a field plot experiment was conducted to study the effects of optimized application of nitrogen and potassium fertilizers combined with returning Chinese milk vetch and straw to fields on yield,fertilizer utilization efficiency, net photosynthetic rate (Pn), stomatal conductance(Gs), intercellular CO2 concentration (Ci), chlorophyll content (SPAD value) and soil physical and chemical properties in late rice harvest period. The results showed that the optimized application of nitrogen and potassium fertilizers combined with the integrated technology of Chinese milk vetch and straw co-returning to the field could enhance the photosynthetic efficiency of double-cropping rice, increase rice yield,and enhance soil biological activity, especially T4 treatment involving the returning of Chinese milk vetch and straw to the field instead of 30% nitrogen fertilizer achieved the highest rice yield, fertilizer use efficiency, net photosynthetic rate and soil biological activity. Compared with the conventional fertilization treatment T2,the total rice yield of T4 treatment increased by 4.1%, among which the early rice and late rice increased by 6.3% and 2.4%, respectively; Pn, Gs and SPAD values of flag leaves at full heading stage significantly increased, and the contents of soil active organic carbon, alkali hydrolyzed nitrogen, available phosphorus and readily available potassium significantly increased.

Key words Double-cropping rice; Optimized application of nitrogen and potassium fertilizers; Chinese milk vetch and straw co-returning to fields; Rice yield; Soil physical and chemical properties

1. Introduction

compaction, farmland acidification, lake water eutrophication and greenhouse effect due to low nitrogen fertilizer utilization efficiency and serious nitrogen loss. Therefore, the optimization of fertilizer nitrogen application technology or the substitution of organic nitrogen to reduce the application of fertilizer nitrogen and reduce agricultural environmental risks had become a major issue that modern agriculture needed to solve.At present, the optimization of fertilizer nitrogen application technology was mainly carried out through the postponing nitrogen fertilization technology,and the organic nitrogen was mainly to replace part of the fertilizer nitrogen with green manure, straw and organic fertilizer. The coordinated return of green manure and straw to the field to reduce the application of fertilizer nitrogen was currently a relatively economical and generally accepted measure.The green fertilizer of rice field in south China mainly refered to Chinese milk vetch, whose nitrogen content of fresh samples was about 0.6%. After being turned over and returned to the field, it can increase soil fertility and improve soil structure, playing an important role in improving rice yield and quality.Research of ZHOU X

et al

.showed that rice yield increased significantly after Chinese milk vetch was turned over, which could increase output value of rice,reduce production cost and improve nutrient utilization efficiency. Straw mainly refered to the straw which contained not only organic nitrogen but also rich potassium; returning rice straw to the field played an important role in the recycling of nutrient resources,reducing environmental pollution, improving soil fertility, increasing rice yield, and improving potash utilization. Studies had shown that returning rice straw to the field and fertilizer application can increase the annual yield and biomass of double-cropping early and late rice, and early rice was superior to late rice.

Both Chinese milk vetch and straw co-returning to fields and optimized application of nitrogen and potassium fertilizers technology can reduce the application amount of nitrogen and potassium fertilizers. The integration of the two technologies can minimize the amount of nitrogen and potassium fertilizers. At the same time, the combination of Chinese milk vetch and straw can also regulate the carbon-nitrogen ratio, so that the biological activity in soil can be activated to the maximum extent. In order to explore the technology and effects of reducing nitrogen and potassium fertilizer application in double-cropping rice, the effects of optimized application of nitrogen and potassium fertilizers combined with returning Chinese milk vetch and straw to fields on double-cropping rice yield, photosynthetic characteristics and soil physical and chemical properties change characteristics were researched, and its benefit was evaluated, which was of great significance to guide the rational reduction of double-cropping rice fertilizer, the improvement of soil fertility and the sustainable development of agriculture.

2. Materials and Methods

2.1. Test location

The test was carried out in Lanxi Town, Heshan District, Yiyang City, Hunan Province (28°34′33″N,112°25′43″E). With the subtropical monsoon humid climate, it was featured by warm winter and hot summer, four distinct seasons, abundant precipitation and uniform seasonal distribution. Rich in heat resources, the average annual temperature was 13～20℃, and the average annual precipitation was 800～1 600 mm. The experimental soil was a red-yellow mud plot with medium fertility, with a clay soil texture. The basic physical and chemical properties of the 0～20 cm soil layer were: pH of 5.43, organic matter of 22.1 g/kg, total nitrogen of 2.18 g/kg, total phosphorus of 0.86 g/kg, total potassium of 14.8 g/kg,alkaline hydrolyzed nitrogen of 181.0 mg/kg, available phosphorus of 26.0 mg/kg, readily available potassium of 99.8 mg/kg, slowly-available potassium of 190.6 mg/kg.

2.2. Test materials

Tested rice varieties: early rice was Xiangzaoxian 45, and late rice was Xiangwanxian 12.

Test fertilizers: urea (containing 46.0% N),superphosphate (containing PO12.0%), potassium chloride (containing KO 60.0%).

2.3. Experiment design

Set up 5 treatments, namely T1 (CK): no fertilization, no Chinese milk vetch or straw returning to the field; T2: conventional fertilizer, no Chinese milk vetch or straw returning to the field. T3, T4, T5:reduce fertilizer nitrogen by 20%, 30%, and 40%,respectively, and use the returning of Chinese milk vetch and straw to the field instead of reduced fertilizer nitrogen, overturn Chinese milk vetch and return it to the field about 10 d before rice transplanting. The plot area was 40 m, with random block arrangement and 3 repetitions.

The total nutrients of nitrogen, phosphorus and potassium were basically consistent in all treatments.Fertilization rate for early rice: N, POand KO were 10, 4 and 6 kg/667mrespectively; fertilization rate for late rice: N, POand KO were 12, 3 and 8 kg/667m, respectively. Nitrogen and potassium fertilizers treated with T2 were used as base fertilizer and tiller fertilizer at a ratio of 3 ∶2. The nitrogen and potassium fertilizers treated with T3, T4 and T5 were used as base fertilizer, tiller fertilizer, and panicle fertilizer for 3 times. The nitrogen fertilizer application ratio for early rice was 6 ∶2 ∶2, and the potassium fertilizer application ratio was 5 ∶3 ∶2. The nitrogen and potassium fertilizer application ratio for late rice was 5 ∶3 ∶2. The straw returning to the field means that the early rice straws were crushed and all returned to the field, the late rice straws in a high stubble were left, and the harvested parts were crushed and all returned to the field. Both green manure and phosphate fertilizer were used as base fertilizer at one time. The specific settings of each processing were shown in Table 1 below.

Table 1 Fertilization design for different treatments of double-cropping rice

2.4. Measurement items and methods

2.4.1. Measurement index

Before the test, 0～20 cm ploughed soil was taken using the plum blossom five-point sampling method to measure the pH value, organic matter,total nitrogen, total phosphorus, total potassium,alkali hydrolyzed nitrogen, available phosphorus,readily available potassium and slowly available potassium content; the content of total nitrogen, total phosphorus and total potassium in rice and straw were measured during the harvest of late rice to determine the soil pH and organic matter, total nitrogen, total phosphorus, total potassium, activated organic carbon,alkali hydrolyzed nitrogen, available phosphorus,readily available potassium and slowly available potassium content; the leaf chlorophyll content(SPAD value), leaf photosynthetic rate (Pn), stomatal conductance (Gs) and intercellular COconcentration(Ci) were measured at the early and late rice panicle differentiation stage; when harvesting, each plot was individually harvested, received and dried to measure the weight of rice and straw.

2.4.2. Test methods

The contents of total nitrogen, total phosphorus and total potassium of the plant were measured with concentrated HSO-HOdigestion, flow injection analyzer and flame photometer. The chemical properties of soil were determined by conventional analysis method, namely: pH value was measured by potentiometer method with water: soil ratio of 2.5 ∶1(v/v). Soil organic matter content was determined by potassium dichromate volumetric method. Soil total nitrogen, total phosphorus and total potassium content were determined by semi-micro kelvin method, sodium hydroxide melting-molybdenum antimony colorimetric method and sodium hydroxide melting-flame spectrophotometry, respectively.Content of alkali hydrolyzed nitrogen, available phosphorus and readily available potassium in the soil was determined by alkaline hydrolysis diffusion method, sodium bicarbonate extraction-molybdenum antimony colorimetric method and ammonium acetate extraction-flame photometric method, respectively.

Flag leaves were selected to determine chlorophyll content (SPAD value), leaf photosynthetic rate (Pn), stomatal conductance (Gs) and intercellular COconcentration (Ci) at the full heading stage of rice. The chlorophyll content was measured with SPAD 502 chlorophyll content analyzer, and the leaf photosynthetic rate, stomatal conductance and intercellular COconcentration were measured with Li-6400 (Portable Photosynthesis System).

The weight of straw was weighed by steelyard and the weight of rice was weighed by electronic scale.

2.5. Data processing

The mapping and statistical analysis of the data were performed with Excel 2010 and SPSS 13.0 software respectively.

3. Results and Analysis

3.1. Effects of different treatments on rice yield

It can be seen from Table 2 that the yield of rice and straw in the fertilization treatment was significantly higher than that in the T1 (CK), and the total amount of rice and aboveground biomass in the T4 was the highest. Compared with T1, the yield of early rice treated with T2, T3, T4, and T5 increased by 50.9%, 56.2%, 60.4% and 53.1%, and the total aboveground biomass increased by 58.6%, 57.2%,60.6% and 52.0%, respectively; the rice yield of late rice increased by 62.2%, 64.1%, 66.0% and 63.5%,respectively, and the above-ground biomass increased by 54.4%, 63.1%, 64.2% and 62.1%, respectively;the paddy yield of double-cropping rice increased by 56.9%, 60.5%, 63.4% and 58.7%, and the total aboveground biomass increased by 56.4%, 60.3%,62.5% and 57.3%, respectively. Compared with T2,the total output of two crops treated with T4 increasedby 4.1%, with early rice and late rice increasing by 6.3% and 2.4%, respectively.

Table 2 Rice, straw yield and total aboveground biomass of early rice, late rice and annual double-cropping rice with different treatments kg/hm2

3.2. Effects of different treatments on chlorophyll content of early and late rice leaves at full heading stage

It can be seen from Fig. 1 that SPAD value of leaves treated with fertilization was higher than that of the control treatment without fertilization, and SPAD value of leaves treated with T4 was the highest.Compared with the control treatment (T1) without fertilization, the SPAD values of early rice leaves of T2, T3, T4, and T5 increased by 3.5%, 3.7%, 4.4%and 1.2% respectively, where T2, T3 and T4 were all significantly higher than T1; compared with T1,the SPAD value of late rice leaves being treated with T2, T3, T4 and T5 increased by 6.5%, 9.9%, 10.1%and 7.2% respectively, among which T3 and T4 were significantly higher than T1.

Fig. 1 Effects of different treatments on the chlorophyll content of early and late rice leaves at full heading stage

3.3. Effects of different treatments on net photosynthetic rate of flag leaf of early and late rice at full heading stage

The net photosynthetic rate (Pn) reflected the amount of organic matter (or glucose) accumulated or increased per unit time of rice leaves. It can be seen from Fig. 2 that the flag leaf Pn of the early and late rice fertilization treatments was higher than that of the no fertilization treatment, and the difference between those with early rice fertilization treatment and without fertilization treatment was significant; the flag leaf Pn of the T4 was the highest, followed by the T5.Compared with the control (T1), the flag leaf Pn of early rice of T2, T3, T4, and T5 treatments increased by 12.0%, 14.6%, 18.5% and 16.1%, respectively, and T2, T3, T4 and T5 were all significantly higher than those of T1; compared with T1, the flag leaf Pn of late rice of T2, T3, T4 and T5 treatments increased by 5.6%,6.4%, 7.2% and 6.7%, respectively, whereas there was no significant difference.

Fig. 2 Effects of different treatments on the netphotosynthetic rate of flag leaves of early and late rice at full heading stage

3.4. Effects of different treatments on stomatal conductance of flag leaf of early and late rice at full heading stage

Stomatal conductance (Gs) reflected the amount of channel opening for gas exchange between plant leaves and the outside world. It can be seen from Fig. 3 that the Gs of flag leaf in early and late rice fertilization treatment were all higher than that of no fertilization treatment, and the Gs of flag leaf in the T4 was the highest. Among them, compared with those treated with, Gs of early rice treated with T2,T3, T4 and T5 were increased by 9.3%, 9.3%, 18.6%and 14.0%, respectively. Compared with those treated with T1, Gs of late rice treated with T2, T3, T4 and T5 were increased by 16.7%, 35.7%, 38.1% and 23.8%,respectively. However, there was no significant difference between the treatment of early rice and late rice.

3.5. Effects of different treatments on intercellular CO2 concentration of flag leaves of early and late rice at full heading stage

Fig. 3 Effects of different treatments on the stomatal conductance of the flag leaf of early and late rice at full heading stage

Intercellular COconcentration (Ci) referred to the concentration of carbon dioxide in the intercellular spaces of the mesophyll in the leaves. It can be seen from Fig. 4 that the leaf Ci of early and late rice under fertilization treatment was lower than that under the T1 without fertilization, and that of the leaf Ci under T4 was the lowest. Compared with the T1, the leaf Ci of early rice in the T2, T3, T4 and T5 decreased by 5.1%, 6.0%, 9.3% and 7.6%, respectively. Compared with the T1, the leaf Ci of late rice in the T2, T3, T4 and T5 decreased by 4.3%, 5.8%, 6.2% and 5.1%,whereas there was no significant difference between the early rice and late rice treatment.

Fig. 4 Effects of different treatments on the intercellular CO2 concentration of flag leaves of early and late rice at full heading stage

3.6. Utilization efficiency of nitrogen, phosphorus and potassium fertilizer in different fertilization treatments

The utilization efficiency of nitrogen, phosphorus and potassium fertilizers for early rice, late rice and double-cropping rice treated by fertilization were higher than conventional fertilization treatment, with T4 being the highest, followed by T3; the efficiency of nitrogen and potassium fertilizer in early rice was higher than that in late rice, while the efficiency of phosphorus fertilizer in early rice was lower than that in late rice. Compared with T2, the nitrogen use efficiency of early rice in the T3, T4, and T5 increased by 18.6%, 19.1%, and 7.2%, respectively,the phosphate utilization efficiency increased by 8.2%, 15.1%, and 8.2%, and the potassium utilization efficiency increased by 19.8%, 23.5% and 11.9%;the nitrogen use efficiency of late rice in the T3, T4 and T5 increased by 13.6%, 18.3% and 10.4%, the use efficiency of phosphate fertilizer increased by 12.7%, 16.0% and 10.0%, and the use efficiency of potassium fertilizer increased by 21.2%, 29.9% and 11.5% respectively; the nitrogen use efficiency of double-cropping rice in the T3, T4 and T5 increased by 13.7%, 15.9% and 8.9%, respectively, the use efficiency of phosphate fertilizer increased by 10.1%,14.8% and 9.4%, and the use efficiency of potassium fertilizer increased by 20.2%, 26.7% and 11.3%,respectively. (Table 3).

3.7. Soil physical and chemical properties of different treatments

The content of soil organic carbon, active organic carbon, total nitrogen, total phosphorus, total potassium, alkali hydrolyzed nitrogen, available phosphorus, readily available potassium in fertilizer treatment at late rice harvest were higher than the control (only total potassium of T1 was slightly higher than that of T2). Moreover, the treatment of returning of Chinese milk vetch and straw to the field instead of part of fertilizer nitrogen (T3, T4, and T5) was higher than conventional fertilization treatment. Among the T3, T4, and T5, the T4 showed the lowest organic carbon content, total phosphorus and total potassium content, and the highest active carbon content, alkali hydrolyzed nitrogen, available phosphorus, and readily available potassium content (Table 4). Compared with conventional fertilization treatment, organic carbon content of T3, T4 and T5 increased by 2.5%,1.0% and 1.1% respectively, total nitrogen content increased by 1.5%, 5.3% and 1.9% respectively, total phosphorus content increased by 5.1%, 3.4% and 5.1% respectively, total potassium content increased by 4.3%, 0.7% and 1.3%, respectively, active organic carbon content increased by 2.6%, 3.8% and 3.2%,respectively, alkali hydrolyzed nitrogen content increased by 1.6%, 8.8% and 4.8%, respectively,available phosphorus increased 23.4%, 32.0% and16.8%, and readily available potassium content increased 16.1%, 24.1% and 19.0%, respectively(Table 4).

Table 3 The utilization efficiency of nitrogen, phosphorus and potassium fertilizers in different fertilization treatments %

Table 4 The physical and chemical properties of the soil during the harvest of late rice with different treatments

4. Discussion and Conclusion

4.1. Discussion

The application of organic fertilizer was one of the important ways to improve soil fertility. The rational application of nitrogen and potassium was very important in the increase and stability of rice yield by the combined application of organic and inorganic fertilizers. In this study, early rice,late rice and double-cropping rice of T4 involving the returning of Chinese milk vetch and straw to the field instead of 30% fertilizer nitrogen, showed the highest rice yield and total above-ground biomass,and it was followed by T3 involving the returning of Chinese milk vetch and straw to the field instead of 20% fertilizer nitrogen, which was basically consistent with the results of GUO X Y

et al

.and WANG H

et al

.. The yield of early rice and late rice was in the order of T4>T3>T5, which may be related to serious competition for "nitrogen" by microorganisms and relatively insufficient nutrient supply due to relatively low carbon to nitrogen ratio and slow rate of decomposition in the returning of Chinese milk vetch and straw to the field of T5 involving the returning of Chinese milk vetch and straw to the field instead of 40% fertilizer nitrogen. The yield of T4 treatment was relatively high, which may be similar to maximum soil biological activity and accelerated soil nutrient cyclingdue to decomposing process of nitrogen and other nutrients in the process of decomposition of Chinese milk vetch that was similar to that of slow release fertilizer, high synchronism of nutrient requirements of rice and suitable carbon to nitrogen ratio.Rice yield was closely related to photosynthesis,and 70% of the nutrients in the rice filling stage come from the photosynthesis of the flag leaf. Therefore,the flag leaf photosynthesis had a decisive effect on the rice yield to a certain extent. The treatment of the returning of Chinese milk vetch and straw to the field instead of some fertilizer nitrogen (T3, T4 and T5) give a higher flag leaf net photosynthetic rate(Pn) than conventional fertilization treatment and no fertilization treatment. This may be related to the nitrogen provided by milk vetch in the late growth stage of rice, which was basically consistent with the results of SU S

et al

.. The stomatal conductance(Gs) of early rice showed the same trend as Pn, and there may be a certain positive correlation, which was similar to the results of YANG F

et al

..The treatments of returning of Chinese milk vetch and straw to the field instead of some fertilizer nitrogen (T3, T4 and T5), their flag leaf intercellular COconcentration (Ci) was lower than that of the non-fertilization treatment and conventional fertilization treatment, because Ci depends on the COconcentration, stomatal conductance, mesophyll conductance and photosynthetic activity of mesophyll cells in the surrounding air of the leaf; the weaker the net photosynthetic rate of rice, the lower the amount of COused for photosynthesis in the cells,resulting in a higher intercellular COconcentration.Double-cropping rice under different treatments of Chinese milk vetch and straw co-returning to fields(T3, T4 and T5) had higher soil organic matter, active organic matter, total nutrients and available nutrients in the harvest period than conventional fertilization treatment, which was in line with the research results of WANG L

et al

.. Among the co-returning treatments of Chinese milk vetch and straw to the field, T4 involving the return of Chinese milk vetch and straw to the field instead of 30% fertilizer nitrogen give the lowest organic carbon content, the contents of active organic carbon content, alkali hydrolyzed nitrogen, available phosphorus and readily available potassium are the highest, indicating that the proper proportion of Chinese milk vetch and straw returning to the field can improve soil biological activity,promote soil mineralization, and improve fertilizer utilization efficiency, which is consistent with the research results of LIAN Z C.

4.2. Conclusion

The optimized application of nitrogen and potassium fertilizers combined with the integrated technology of Chinese milk vetch and straw coreturning to the field could enhance the photosynthetic efficiency of double-cropping rice, increase rice yield,and enhance soil biological activity, especially, T4 involving the return of Chinese milk vetch and straw to the field instead of 30% nitrogen fertilizer achieved the highest rice yield, fertilizer use efficiency, net photosynthetic rate and soil biological activity.Compared with the conventional fertilization T2, the total rice yield of T4 increased by 4.1%, among which the early rice and late rice increased by 6.3% and 2.4%, respectively; Pn, Gs and SPAD values of flag leaves at full heading stage significantly increased,and the contents of soil active organic carbon, alkali hydrolyzed nitrogen, available phosphorus and readily available potassium significantly increased.