Effects of Humic Acid-Irrigation Coupling Techniques on Temperature, Radiation and Rice Growth in Songnen Plain, China

: In the past decades, the application of organ fertilizer in agricultural soils has attracted wide attention. However, few studies have carefully explored the effects of humic acid on soil and canopy temperature, radiation, the physiological process of plant leaves, especially coupling with the different irrigation methods. To provide a better growing environment for crops and explore the best regulation mode of humic acid and irrigation coupling techniques in the farmland soil environment in the Songnen Plain Heilongjiang Province, through field experiment, we selected rice as the test crop and applied humic acid in the soil with different irrigation methods. The temperature conditions, radiation, agronomic and fluorescence characteristics were monitored by different stages. The effects of different humic acid and irrigation coupling techniques on the temperature and radiation changes during different growth stages were discussed, and the subtle differences of agronomic and fluorescence characteristics in different growth stages of rice plants were compared. The results showed that the humic acid application with different irrigation methods was not beneficial to the maintenance of soil temperature, the differences among the different treatments, were no found significant at 5% probability statistically. However, the differences of radiation interception was obvious, and the best value was CT5 treatment, there were also similarities to plant height. The fluorescence indexes and leaf chlorophyll relative content (SPAD) had the differences with the change of humic acid application rate and irrigation methods. Over all, under the humic acid application rate of 1500 kg·ha-1 with the control irrigation method, could bring the best humic acid and irrigation effects.

. Survey of the study area.

Humic acid The humic acid fertilizer was produced by Yunnan Kunming Grey Environmental Protection
Engineering Co., Ltd, China (Fig.2). The organic matter≥61.4%, the moisture≤2.51%, the pH value was 5.7, the worm egg mortality rate≥95%, and the amount of fecal colibacillosis≤3%. The fertilizer contained numerous elements necessary for plants, such as carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc, boron, copper, chlorine, molybdenum and silicon. The contents of harmful elements including arsenic, mercury, plumbum, cadmium and chromium were much lower than the test standard.

Experimental design and observation methods
In this experiment, three irrigation practices, namely: control irrigation (C), wet irrigation (W) and flood irrigation (F) were designed (Table 1). Note: θs is soil saturated water content, before "～" is the lower limit of irrigation and after "～" is the upper limit of irrigation; C represents control irrigation; W represents wet irrigation; F represents flood irrigation; the numbers represents water layer depth; the percentage represents percent content of soil saturated water content.
Control irrigation (C) of rice had no water layer in the rest of the growing stages, except for the shallow water layer at the re-green stage of rice which was maintained at 0-30 mm and the natural dryness in the yellow stage [22]. The irrigation time and irrigation quota were determined by the root soil moisture content as the control index. The upper limit of irrigation was the soil saturated moisture content, the lower limit of soil moisture at each growth stage was the percentage of saturated moisture content, and the TPIME-PICO64/32 soil moisture analyzer was used to determine the soil moisture content at 7:00 a.m. and 18:00 p.m., respectively [22,23]. When the soil moisture content was close to or lower than the lower limit of irrigation, artificial irrigation occurred until the upper irrigation limit was reached. The soil moisture content was maintained between the upper irrigation limit and the lower irrigation limit of the corresponding fertility stage. Under the wet irrigation (W) and flood irrigation (F) conditions, it was necessary to read the depth of the water layer through bricks and vertical ruler embedded in the field before and after 8:00 am every day to determine if irrigation was needed. If the irrigation was needed, then recorded the water meter before and after each irrigation. The difference of before and after was the amount of irrigation [23].
Phosphorus was applied once as a basal application. Potassium fertilizer was applied at twice: once as a basal fertilizer and at the 8.5 leaf age (panicle primordium differentiation stage), at a 1:1 ratio [22].
The cultivar "Suijing No.18" was used for the experiment. In first year, on May 21, the rice started to transplant to the field and on May 16 in second year. The crops were harvested on September 20 in first year and September 28 in second year. This study was performed with a randomized complete block design with three replications. The length and width of each plot were 10.0 m and10.0 m, respectively (area = 100 m 2 ). The rice was also planted around the cell as a protection row. A 40 cm deep plastic plate was embedded between the plots to prevent underground water-fertilizer exchange in each plot. The plant protection and pesticide application measures and field management conditions in each plot were consistent. A soil temperature sensor (HZTJ1-1) was buried in each experimental plot to monitor the temperature of each soil layer (5 cm, 10 cm, 15 cm, 20 cm and 25 cm depth ) and the canopy temperature was measured by the crop canopy temperature measurement system from 08:00 to18:00 o'clock. The transmission of photosynthetically active radiation was measured during 11:00 to 13:00 by using a SunScan Canopy Analysis System (Delta T DevicesLtd., Cambridge, UK), data during the crop growing season were recorded in every day [23].
Plant measurements were taken during the periods of tillering to ripening on days with no wind and good light. The fluorescence parameters were measured by the portable fluorescence measurement system (Li-6400XT, America), the detection light intensity was 1 500 μmol m -2 s -1 , saturated pulsed light intensity was 7 200 μmolm -2 s -1 . The functional leaves were dark adaptation, which lasted for 30 min, then the maximum photosynthetic efficiency of PSII (Fv/Fm) was measured. Photochemical quenching (QP) and non-photochemical quenching (NPQ) were measured with natural light. Simultaneously, the leaf chlorophyll relative content (SPAD) was monitored using SPAD 502 (Konica Minolta, Inc., Tokyo, Japan). For the plant agronomic characteristics, the distance from the stem base to stem tip was measured with a straight ruler to quantify plant height [23].
Statistical analysis Experimental data obtained for different parameters were analyzed statistically using the analysis of variance technique as applicable to randomized complete block design. Duncan's Multiple Range Test was employed to assess differences between the treatment means at a 5% probability level. All statistical analyses were performed using SPSS 22.0 for Windows [24].

Variation in soil temperature under different humic acid-irrigation coupling techniques
In this experiment, the temperature data for the 5~25 cm of the soil plow layer were analyzed. The average temperatures of the soil layer during the entire growth period of the plant are shown in (Table 2). Across all treatments, the soil temperature showed the same trend, which was generally divided into two parts. The first part, from 5 cm to 15 cm, was the soil surface, the soil temperature gradually decreased, the decreased in temperature was approximate 3-4℃. The second part, from 15 cm to 25 cm, the soil temperature rose slowly and continued to increase, the increased in temperature was below 1℃. Under the different humic acid and irrigation coupling techniques conditions, the temperature differences of 15 cm to 25 cm were smaller than those 5 cm to 15 cm. The surface soil temperature (5cm and 10cm) under the five humic acid application modes, general trend of change showed the order T2>T3>T1>T4>T5, while under the three irrigation conditions the general trend showed the order C>W>F.
Based on the above data, among all the different humic acid and irrigation coupling techniques, although there were differences, we had no found significant at 5% probability statistically (P>0.05), it could be concluded that the humic acid application with different irrigation was not beneficial to the maintenance of soil temperature.

Variation in canopy temperature under different humic acid-irrigation coupling techniques
The various in the canopy temperature under the different humic acid and irrigation coupling techniques conditions are shown in Fig.3. The analysis of the difference between canopy temperature and atmosphere temperature showed that the periods of major changes in difference of canopy temperature and atmosphere temperature were all related to atmosphere temperature. When comparing the difference of canopy temperature and atmosphere temperature at the beginning and end of the monitoring period, the average changes in each treatment were small. Under the flood irrigation conditions, no significant changes were found in the difference of canopy and atmosphere temperature with different humic acid application (P>0.05). However, in the observations of the control irrigation and wet irrigation, under T2 treatment conditions, the difference of canopy and atmosphere temperature was below 0℃, which indicated that these treatments occurred stress effects. In the other treatments, the difference status was more than 0℃, which indicated that these treatments, water and nutrient were supplied sufficiently, the growth of rice was sustainable and would not change with the actual changes in the growth environment. Fig.3 The canopy temperature and atmosphere temperature in different treatments Note: C represents control irrigation; W represents wet irrigation; F represents flood irrigation; T1,T2,T3,T4,T5 represents five fertilization treatments. respectively. C-T represents the canopy and atmosphere temperature.

Variation in radiation interception under different humic acid-irrigation coupling techniques The variation in radiation interception under the different humic acid and irrigation coupling techniques conditions is
shown in Fig.4. Overall, the interception in terms of radiation was mainly completed during three plant growth stages which none-interception stage (0-20days), fast-interception stage (20-70days) and slow-interception stage (70-100days). The change in radiation interception was nonlinear over time, and radiation interception differed under the influence of the different humic acid and irrigation coupling techniques. However, the change trend of radiation interception under the different treatments was synchronous. Under the different humic acid conditions, the greatest radiation interception was obtained in T5 treatment, and the minimum value was recorded in T2 treatment. In addition, the radiation interception under control and flood irrigation was more than wet irrigation.
As a whole, in the two years, the experiment showed that when humic acid was applied in the soil 1500 kg·ha -1 under control irrigation conditions, the values of radiation interception were the maximum, which were 983.3 MJ·m -2 and 1034.8 MJ·m -2 , respectively.

Fig.4 Dynamic change of accumulation interception amount in different treatments
Note: C represents control irrigation; W represents wet irrigation; F represents flood irrigation; T1,T2,T3,T4,T5 represents five fertilization treatments. respectively.

Variation in plant height under different humic acid-irrigation coupling techniques
The performance in terms of plant height were similarities to radiation interception, which gradually increased as shown in Fig.5. The main growth in plant height occurred during the jointing stage. Across all humic acid and irrigation coupling techniques, the plant height was significantly different at the tillering, jointing and the heading stages. Moreover, there were significant differences among gradients under the different humic acid and irrigation coupling techniques in terms of plant height. According to the results from the tillering stage observation, the plant height in WT2 treatment was the lowest value in the two growth seasons, while that in CT5 treatment had the largest value. In the first growth season, as compared with the other treatments, the plant height in CT5 treatment increased by 7.51% (P<0.05), 6.09% (P<0.05) and 5.45% on average at the tillering, jointing and heading stage; while in the second growth season, the plant height increased by 9.57% (P<0.05), 8.16% (P<0.05) and 7.48% (P<0.05), respectively. The increased in plant height of the two growth seasons was significant.