Effect of Reducing Chemical Fertilizer on Rice Yield，Output Value,Content of Soil Carbon and Nitrogen after Utilizing the Milk Vetch

Xing ZHOU, Zaiming LI， Jian XIE, Yulin LIAO, Zengping YANG, Yanhong LU, Jun NIE*, Weidong CAO

1. Soil and Fertilizer Institute of Hunan Province, Hunan Academy of Agricultural Sciences, Changsha 410125, China;

2. College of Resources and Environment, Hunan Agricultural University，Changsha 410128, China;

3. Agricultural Development Bureau of Yunxi Area, Yueyang 414009, China;

4. Scientific Observing and Experimental Station of Arable Land Conservation (Hunan), Ministry of Agriculture, Changsha 410125,China;

5. Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China

Hunan is a large green manure producing province[1]. In the 1970s, the sowing area with green manure reached 2 million hm2in Hunan Province. In the 1980s, the sowing are with green manure drops substantially under influence of largescale application of chemical fertilizer,adjustment of agricultural industry structure, transferring of rural labor,and shortage of policy support[2-5]. Astragalus sincus(Chinese milk vetch)is the most important green manure in southern paddy fields in China.What’s more, the application of Astragalus sincus would increase soil fertility, re-duce fertilization of chemical fertilizer,and improve rice yield and quality,which is of great significance for sustainable development of agricultural production[1,6-8]. In the research, a located field experiment was carried out to study the effects of different amounts of chemical fertilizer usage on rice yield,economic benefits of rice,soil carbon (C)and total nitrogen(TN)under ploughing back of Chinese milk vetch for 5 consecutive years, in order to better explore the role of Chinese milk vetch in production of doublecropping rice.

Materials and Methods

Materials

The test materials included earlyseason rice,Liangyou No.25 and lateseason rice,Jinyou No.163 in 2012.

Introduction of test site

Wanyuanqiao Village, Sanxianhu Village, Nan County, Hunan Province is situated in a transitional zone from mid-subtropical area to northern subtropical area, at 29°13′N and 112°28′E, with an elevation of 30 m. It has a monsoon humid climate, involving annual mean temperature at 16.6 ℃,annual mean precipitation at 1 237.7 mm, and annual sunshine hour of 1 775.7 h. The test soils were purple soils with pH value of 7.7, containing organic matter at 48.4 g/kg, total N at 3.28 g/kg, total P at 1.28 g/kg, total K at 22.2 g/kg, alkali-hydrolyzale nitrogen at 261 mg/kg, available P at 15.6 mg/kg,and available K at 98 mg/kg.

Test design

Chinese milk vetch was ploughed back for 5 consecutive years during 2008 -2012. Early-season rice was sown in the end of March 2012 and transplanted in middle or late April.Later-season rice was sown in middle June and transplanted in middle and late July. The test included 6 treatments(Table 1).

In the test, N, P and K fertilizers referred to urea, superphosphate and potassium chloride. Specifically, base fertilizers were applied 1 d before transplanting; additional fertilizers were applied in the peak of tillering period;P and K fertilizers were applied as base fertilizer before transplanting.After the application of base fertilizer,the fertilizers should be ploughed back to soils with depth of 5 cm,with a rake.Subsequently,Chinese milk vetch was ploughed back 5 d before transplanting and decomposed a little with shallow water. The test region was 20 m2,and the test set three repetitions as per randomized block design. At harvest in 2012,the rice was dried individually,followed by measurement of rice yield in different test regions. Furthermore,soil samples were collected with depth of 0-15 cm from farmlands in a shape of S.

Table 1 Fertilization schemes of different treatments

Table 2 Grain yield of early rice under different fertilization kg/hm2

Measurement items and methods

The content of carbon was measured as per concentrated sulfuric acid-potassium dichromate heating method and total N was measured as per concentrated sulfuric acid digestion-kjeldah method[9].

Data processing

The test data were processed with Excel 2003 and variance analysis was conducted with SPSS 19.0.

Results and Analysis

Yield of rice in different treatments

Yield of early-season rice As shown in Table 2, the differences early-season rice yield showed none statistical significance between CF and CK. The reduction of yield in CF is possibly caused by consecutive rainy days in spring in 2012 with low temperature, which indicated that simple application of chemical fertilizer would not guarantee yield of early-season rice under disastrous climate. Compared with CK, the increases of yields in treatments A1and A2were larger,increasing by 22.1% and 16.9%, respectively (P＜0.05),which incorporated that based on use of Chinese milk vetch,the applications of 100%chemical fertilizer,and 80%N and K fertilizers would maintain stable yield or even increase yield of early-season rice.

As shown in Table 2, compared with CF, the yield of early-season rice in treatments A1and A2increased by 12.5% and 7.7% , respectively; the yield of A3maintained similar to that of CF;yield of A4decreased to some extent. These indicated that with fixed or increased yield of paddy rice, only 60%-80% N and K fertilizers can be applied for ploughing back of Chinese milk vetch at 22 500 kg/hm2, suggesting that the application of Chinese milk vetch would dramatically reduce the production cost of rice and increase rice production effect.

Yield of later-season rice As shown in Table 3, average yield of later-season rice in CK was 6 266.7 kg/hm2, which indicated that basic yield of later-season rice was higher in the paddy fields with purple soils,possibly caused by high temperature in summer, and more released nutrients from soils. Furthermore, the yields of CF and treatments A1-A4averaged 7 085.0 kg/hm2, which increased by 13.1% compared with CK.Compared with CF, the yields of laterseason rice in treatments A1and A2grew a little, and in treatments A3 and A4decreased a little, showing none statistical significance(P＜0.05).These demonstrated that in paddy fields with purple soils, ploughing back of Chinese milk vetch at 22 500 kg/hm2before transplanting of early-season rice would release nutrients for later-season rice,so that only 60%-80%N or K fertilizers can be applied for later-season rice.

Total yield of double-season rice It can be concluded from Table 4 that annual total yield of early and laterseason rice averaged 12 650.2 kg/hm2of treatments CF and A1-A4, with significant increase of yield(P＜0.05).

As shown in Table 4, compared with CF, total yields of early and laterseason rice in treatments A1and A2grew by 7.2% and 3.9% respectively,with little yield increase (P ＜0.05).Yields of treatments A3and A4decreased a little, showing none statistical significance. Specifically, yields of treatments A1and A3grew by 1 779.1 and 1 359.6 kg/hm2compared with treatment A4, with increases at 15.2%and 11.6%. According to total yields,with ploughing back of Chinese milk vetch at 22 500 kg/hm2, rice yield can be guaranteed with N and K fertilizers applied of 40%-80%, and it is proved the yield can be the best with N and K fertilizers at 80%.

Table 3 Grain yield of late rice under different fertilization treatments in 2012 kg/hm2

Table 4 Total grain yield of double-rice under different fertilization treatments in 2012 kg/hm2

Table 5 Economic benefits of rice under different treatments yuan/hm2

Economic benefits of rice in different treatments

The economic benefits of rice in different treatments were analyzed with consideration of chemical fertilizer, Chinese milk vetch seeds, labor force cost, as well as incomes from Chinese milk vetch. What’s more, the prices of seeds of Chinese milk vetch,chemical fertilizer and rice took references in 2012.For example,the prices of seeds, N, P2O5, K2O were 18, 4.67,5.2 and 5.47 yuan/kg, respectively.The cost of labor was 120 yuan/d as per the standard in 2012. The test results indicated that compared with CK(Table 5), average income of earlyseason rice reduced by 3.76% in treatments of CF, A1-A4, because of influence from disastrous climate. As shown in Table 5, compared with CF,the income from treatment A1reached the highest and the increase was as high as 9.18% , followed by A2at 5.92%.As for treatment A3,the income of early-season rice kept the same as that of CF.In treatment A4,the income reduced by 6.42%. In conclusion, with ploughing back of Chinese milk vetch at 22 500 kg/hm2, the application of N and K fertilizers at 60%-80% would significantly increase the income from early-season rice.

Compared with CK, average income of later-season rice in treatments of CF, A1-A4performed better than that of early-season rice (Table 5). For instance, the increased incomes reached 706.39 yuan/hm2and the increase reached 4.34%. Specifically, economic benefits of different treatments with later-season rice proved similar to that of early-season rice, but the increase of income was poorer. Compared with CF, the increase income of treatment A1with later-season rice was the best, with the increased income at 606.58 yuan/hm2and the growth reached 3.58%;the income of treatment A2increased a little;the incomes of treatments A3and A4declined a little. These indicated that ploughing back of Chinese milk vetch has effects in increasing benefits from later-season rice.

It can be concluded from Table 5 that average incomes from treatments CF,A1-A4maintained the same as that of Ck. Compared with CF, the increased incomes of rice in treatment A1, early-season rice or later-season rice,performed the highest,by 5.92%,followed by treatment A2by 4.08%.The increased income of treatment A3kept the same as that of CF and of treatment A4decreased by 3.83%.

The content of organic carbon of soils in different treatments

The content of organic carbon of soils grown with early-season rice Before the test, the content of organic carbon was 27.9 g/kg.As shown in Fig.1, the content of organic carbon in CK increased to certain extent compared with the test before, which is possibly related to rice stubbles,runoff, irrigated water and climate.Compared with CK, the content of organic carbon in soils improved by 6.1%and 4.6%in treatments of A4and CF, showing significant differences(P＜0.05),followed by treatment A with none statistical significance. In addition,the contents of treatments A3and A2declined by 12.4% and 22.6%, respectively, with significant differences(P＜0.05), possibly because of mineralization of organic carbon by 80% or 60% N and K fertilizers with ploughing back of Chinese milk vetch. It showed that the content of organic carbon in treatments of A1and A4with early-season rice showed none statistical significance with CF. The contents of A2and A3reduced by 26.0% and 16.2%compared with Cf, showing significant differences (P＜0.05), which indicated that the application of 80%and 60%N and K fertilizers 5 years later is adverse for accumulation of organic carbon in soils with early-season rice with ploughing back of Chinese milk vetch.These incorporated that with ploughing back of Chinese milk vetch at 22 500 kg/hm2, the content of organic carbon changed from decreasing to increasing as N and K fertilizers decreased from 100%to 40%and the accumulation of organic carbon on soils with 80% N and K fertilizers proved the least.

The content of organic carbon of soils in different treatments As shown in Fig.2, the content of organic carbon in soils with later-season rice improved in varying degrees in different treatments. Specifically, compared with Ck, the content increased by 5.6% in the treatment of A4, with significant differences (P ＜0.05); treatments CF, A, and A3showed none statistical significance with that of CK;the content of treatment A2declined by 17.8% , with significant differences.With ploughing back of Chinese milk vetch, the content of soils with laterseason rice in treatments A1and A3 maintained similar to that of CF; the accumulated organic carbon was the least in treatment A2, decreasing by 17.7% compared with CF; the content of treatment A4increased by 5.8% ,with significant differences.

The content of total N in different treatments in maturing stage

The content of total N in different treatments in maturing stage As shown in Fig.3,compared with CK,the content of total N in most treatments showed significant differences with that of CK, except of treatment CF.Specifically, the content increased by 15.8% and 6.6% and of treatments A3and A2decreased by 4.7%and 19.5%.Compared with CF,the content of total N showed significant differences in dif-ferent treatments, and the content in treatment A1grew by 4.7%,which indicated that with the quantity of chemical fertilizer the same, ploughing back of Chinese milk vetch at 22 500 kg/hm2would considerably improve the content of total N in soil layers of earlyseason rice in maturing stage;the contents in treatments A2and A3dropped by 21.0% and 6.5% and in treatment A4enhanced by 13.7%. These indicated that the change of total N was similarly as 60%-100% N and K fertilizers were applied, with the change of organic carbon in paddy fields with earlyseason rice, provided ploughing back of Chinese milk vetch, of which the content of organic carbon was the least in the soils with 80%N and K fertilizers.

The content of total N in soil layers in different treatments with laterseason rice in maturing stage As shown in Fig.4,only the content of total N in treatment A2showed significant differences with treatment CK (P ＜0.05). Specifically, the content in treatment A2declined by 13.9%,which indicated that with ploughing back of Chinese milk vetch, the application of 80% N and K fertilizers in soils with later-season rice is adverse for the accumulation of total N in soils.In maturing stage of later-season rice,the content of total N in treatment A1 increased by 5.5% compared with CF,and in treatment A2declined by 12.5%;the contents in rest treatments showed none statistical significance with that of CF. These suggested that the change trend of total N in soils with later-season rice in maturing stage is basically similar to that of early-season rice.

Conclusions and Discussions

In crop production, after application of green manure, rice yield maintained the same or even increased a little compared with the treatments with chemical fertilizers or N fertilizers reduced by 15%-40%[10-12]. The application of Chinese milk vetch and chemical fertilizer would meet rice demands on available nutrients, as well as other nutrients demands to maintain rice yield. In the test, in the treatment with local method, ploughing back of Chinese milk vetch would maintain or facilitate rice yield. With ploughing back of Chinese milk vetch,80%N and K fertilizers would increase rice yield and 60% N and K fertilizers would maintain stable rice yield. In terms of economic benefits and ecological benefits, compared with the treatment with just chemical fertilizer,ploughing back of Chinese milk vetch at 100% would improve incomes from rice production,and 80%N and K fertilizers would maintain incomes from rice production.

With application of organic fertilizer and inorganic fertilizer, the fertilization of organic matter to soils would considerably improve the content of organic carbon[8,13-15].In the test,single fertilization of chemical fertilizers would effectively increase the content of organic carbon, because chemical fertilizer would facilitate root growth and increase biomass of underground part.These are similar to the research conducted by A. Mandal et al.[16]and Yin et al.[17]. However, with root growth promoted, C-to-N proportion would drop resulting from application of single chemical fertilizer. What’s worse,the decrease of C-to-N proportion would lead to decomposition of increased organic carbon, and ctivated decomposition of original organic carbon,which is adverse for accumulation of organic carbon in soils[13,18]. Therefore, the effects of chemical fertilizer on organic carbon in soils are much complicated, which is related to crop,soil property,and soil mineralization.

In the test, in the treatment with application of Chinese milk vetch at 100%, the contents of organic carbon and total N showed insignificant increase in terms of organic carbon and total N, compared with the treatment with 100% chemical fertilizer, which is possibly related to soil property, crop variety, soil mineralization and test circle. With effects of Chinese milk vetch, the contents of organic matter and total N changed in the form of decreasing-lowest-increasing as N and K fertilizers declined from 100% to 40% and rice yield was decreasing.Specifically, in the treatment with 80%N and K fertilizers,the contents of organic matter and total N kept the lowest,and were significantly lower compared with the treatment without fertilization of Chinese milk vetch or chemical fertilizers. These incorporated that with Chinese milk vetch applied, the contents of organic carbon and total N is of correlation with N and K fertilizers and rice yield, which requires further exploration.

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