Introduction
Alfalfa (Medicago sativa L.) is an important forage crop due to its relatively high yield, palatability, digestibility and excellent nutritional value. Alfalfa is the most efficient forage legume (Zhang et al. 2016) and plays an important role in promoting the development of animal husbandry in China (Geisseler et al. 2010). Additionally, alfalfa is a kind of water-loving plant, and irrigation significantly affects the yield and quality of alfalfa (Ismail and Almarshadi 2013; Altinok et al. 2015; Rahmana et al. 2016). However, recent research has shown that irrigation water has little effect on alfalfa plant height and stem diameter but plays an important role in increasing the branch number, thus indirectly affecting the hay yield (Saeed and El-Nadi 1997). Under water stress, the growth rates of mature alfalfa leaves and stems decreased significantly, and the hay yield decreased (Fiasconaro et al. 2012). The number and length of the stem nodes of alfalfa plants increased and the photosynthetic rate of the leaves became stronger under the full irrigation rate (Zhang et al. 2016). Therefore, an appropriate irrigation quota is conducive to improve the hay yield of alfalfa (Fosu-Mensah and Mensah 2016).
Like water, phosphorus (P) is an indispensable nutrient element in plants and one of the main limiting factors of crop yield (Rehim et al. 2016; Ijaz et al. 2018). After the application of P fertilizer, the grass yield per unit area and plant regeneration speed of alfalfa was affected by P fertilizer (Song et al. 2018). The results show that irrigation rate can not only regulate the fertilizer effect but also affect the absorption of water by herbage (Singh 1999). Additionally, the increase in alfalfa hay yield and nutrients is due to the positive coordination of water and fertilizer application (Berg et al. 2018). Therefore, reasonable coupling of water and fertilizer is of great significance to alfalfa yield.
The oasis area of Xinjiang is one of the main areas of alfalfa cultivation in China. The soil in this area is mainly grey desert soil. Because of the lack of water and poor soil fertility in this area, alfalfa cultivation is difficult. In particular, the success of plantings in the planting year has a great influence on the hay yield and nutrient quality of alfalfa in the later periods of alfalfa growth. Hence, a rational combination of water and fertilizer is the key to a high yield of alfalfa in the same year. In recent years, research on alfalfa has mainly focused on autumn dormancy characteristics, growth period, mowing methods, and grass yield (Gu et al. 2018), while research on alfalfa production performance in the year of establishment is relatively limited. Therefore, the objectives of this study were to establish an optimal fertilization model, to clarify the effect of water and P coupling on the nutrition quality and the P use efficiency (PUE) of alfalfa under drip irrigation and to determine the relationship between PUE and the hay yield of alfalfa.
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Fig. 1: Average temperature and precipitation in growth stages during 2014-2015 (from Shihezi Meteorological Bureau, Xinjiang)
Materials and Methods
Experimental site description
Field experiments were conducted in 2014 at the agricultural College Test Station of Shihezi, Xinjiang, China (44°26'N, 85°95'E) and the experimental field of Shihezi Tianye Group Agricultural Demonstration Park (44°31'N, 85°52'E) in 2015. The physical and chemical properties of the 0–20 cm plough layer soil are shown in Table 1. The physical and chemical properties of the soil are basically the same in the two test sites, so their effects on the subsequent water test results of this paper are negligible. The soil type was grey desert soil.
Experimental design:
The alfalfa crop was grown using different irrigation amounts, i.e., 3750, 4500, and 5250 m3·ha-1, under varying P levels (0, 50, 100, and 150 kg P2O5 ha-1) with three replications. The experiment was designed following a randomized complete block design with factorial arrangement. The crop was irrigated six times each year, on April 26, June 10, June 26, July 12, August 13 and August 28, 2014 and on May 2, June 19, July 1, July 16, August 14, and August 29, 2015. The average precipitation and temperature in each month during the test period are shown in Fig. 1.
In this experiment, WL354HQ alfalfa seeds taken from Beijing Zhengdao Ecology Technology Co., Ltd. were sown on April 19, 2014 and on April 16, 2015, using the drill method with a sowing depth of 1.5～2.0 cm, row spacing of 20 cm, and seed rate of 18.0 kg ha-1 in plots with an area of 5.0 m × 8.0 m. A walkway 1.0 m wide between the plots was left to prevent water and P infiltration between the plots. The drip irrigation belt was shallowly buried in the 8～10 cm surface soil at a distance of 60 cm. In addition to the water and fertilizer factors, other management methods were carried out according to the local high-yield field of alfalfa under the conditions of drip irrigation.
Soil sample collection:
Soil samples of 0–20 cm, 20–40 cm and 40–60 cm were taken from soil drills in each plot by the "S" sampling method in October of each year. Five soil samples from the same soil layer were mixed to make composite soil samples. After removing impurities such as alfalfa roots and stones, the soils were brought back to the laboratory and dried to constant weight in an oven at 65℃. The soil samples were ground, and the fine soil was sifted through a 100 mesh sieve for reserve.
Measurement index and method
Hay yield determination: In the initial flowering stage (5–10% plants in bloom), alfalfa plants with uniform growth were selected using the “S” sampling method in the experimental plot. Using a 1 m ×1 m sample, scissors were used to cut the alfalfa plants in the sampling plot (stubble height 5 cm) to weigh the plants, and the fresh grass yield of plants was recorded. This process was repeated three times, and the hay yield (kg ha-1) of alfalfa was calculated (Formula 1). The concrete formula is as follows:
Y = FY × (1- M (%)) (1)
Y represents the alfalfa hay yield, FY represents the alfalfa fresh grass yield and M represents the alfalfa moisture contents
Determination total phosphorus and available phosphorus in soil
The total phosphorus (TP) content of soil was determined using the sulfuric acid perchlorate digestion method, and the available phosphorus (AP) content was determined using NaHCO3 leaching molybdenum antimony KangFa determination (Lu 2000).
Calculation of water use efficiency (WUE) and agronomic efficiency of phosphorus fertilizer (AEPF) of alfalfa
The WUE of alfalfa was defined as the ratio of alfalfa hay yield (Y, kg ha-1) to transpiration ET (mm), that is,
WUE=Y/ET (2)
The ET was calculated by the following formula (Li et al. 2018):
ET=P+I+Δω (3)
P, I and Δ omega represent mowing twice during rainfall, irrigation quantity and 0–2 m of difference in the soil water content, respectively.
The AEPF of alfalfa was calculated using the following equation:
AEPF = (Yf－Y0) / Fi (4)
AEPF represents the agronomic efficiency of P fertilizer, Yf represents the alfalfa hay yield in the fertilized area, Y0 represents the alfalfa hay yield in the non-fertilized area, and Fi represents the amount of P fertilizer used.
Data processing and analysis
Microsoft Excel 2010 was used for data processing, and DPS 7.05 (Data Processing System, China) was used for data processing and analysis. The data were analysed by two-way ANOVA to determine the significance of individual factors and their interactions. The LSD test was used to compare the differences among means, and Origin 8.0 software (OriginLab OriginPro, U.S.A.) was used for drawing.
Results
Hay yield of alfalfa
The hay yield of alfalfa first increased and then decreased with increasing P application and reached the highest value under the 100 kg·ha-1 treatment (Table 2). Under the same P application conditions, the hay yield of the 4500 m3 ha-1 treatment with increasing irrigation amount was significantly greater than those of the 3750 m3 ha-1 and 5250 m3 ha-1 treatments (P < 0.05). There were no significant differences between the treatments except 3750 m3 ha-1 and 5250 m3 ha-1, which were cut for the first time in 2014 (P < 0.05). The hay yield of alfalfa in the second cut was higher than that in the first cut under the same water P treatment (except the 5250 m3 ha-1 condition in 2014).
WUE and AEPF of alfalfa
The WUE and AEPF of alfalfa increased first and then decreased with increasing P application (Table 3), and the WUE of alfalfa under P application was significantly higher than that without P application (P < 0.05). Except for 0 kg ha-1 and 100 kg ha-1 in 2014, there were no significant differences in the water use efficiencies of alfalfa under 50 kg ha-1 in 2015, and there was a significant difference in the water use efficiency under different irrigation amounts and the same P application (P < 0.05). Appropriate irrigation rates and P levels of at 4500 m3 ha-1 and 100 kg ha-1, respectively, significantly improved the WUE and AEPF of alfalfa. The WUEs of the first and second cut alfalfa were 1.66 kg mm-1 ha-1 and 1.94 kg mm-1 ha-1, respectively, in 2014, and it was 1.59 kg mm-1 ha-1 and 2.36 kg mm-1 ha-1, respectively, in 2015.
Soil TP contents
The TP contents in the soil with P application were significantly higher than that without P application under the same irrigation conditions in the same soil layer (Table 4). Except for the 0–20 cm soil layer, the TP content from the 100 kg ha-1 treatment was significantly greater than those of the 0 kg ha-1 and 50 kg ha-1 treatments under 4500 m3 ha-1 conditions (P < 0.05), and that of the 150 kg ha-1 treatment reached the maximum value. Except in the 20–40 cm soil layer, the TP content from the 4500 m3 ha-1 treatment was significantly higher than those of the 3750 m3 ha-1 and 5250 m3 ha-1 treatments under 50 kg ha-1 (P < 0.05), and the total P contents of the other treatments increased with increasing irrigation. The content of total P in the 0–20 cm soil layer was the highest, and that at 60 cm was the lowest under the same irrigation and P application conditions.
Soil AP content
The AP contents in the soil with P application were significantly higher than that without P application under the same irrigation conditions in the same soil layer (Table 5). The content of available P in the soil first increased and then decreased with increasing P application. The contents of available P in the soil reached maximum values for 3750 m3 ha-1 and 4500 m3 ha-1 under the 100 kg ha-1 treatment, and those of the 100 kg ha-1 treatment were significantly higher than those of the 0 kg ha-1 treatment (P < 0.05). The content of soil available P reached a maximum value of 5250 m3 ha-1 under the 150 kg ha-1 treatment, and those of the 100 kg ha-1 and 150 kg ha-1 treatments were significantly higher than those of the 0 kg ha-1 treatment (P < 0.05). The content of soil available P varied with increasing irrigation amount under the same conditions. In the same soil layer, for 0 kg ha-1, the content of available P in the 4500 m3 ha-1 treatment was the highest; in other conditions, and that of the 5250 m3 ha-1 treatment was significantly higher than that of the 3750 m3 ha-1 treatment (P < 0.05). The content of soil available P decreased with increasing soil depth under the same irrigation and P application conditions.
 1061-1067.files/02 doi 15.1386 IJAB-19F-197 (7) 1061-106713379.png)
Relationship between TP and AP contents in soil
To clarify the relationship between the soil TP and AP contents, the soil TP and AP contents were fitted and the results show that (Fig. 2) quadratic equations fit the soil TP and AP contents better than linear
 1061-1067.files/02 doi 15.1386 IJAB-19F-197 (7) 1061-106713640.png)
Fig. 2: Relationships between total phosphorus and available phosphorus in soil
 1061-1067.files/02 doi 15.1386 IJAB-19F-197 (7) 1061-106713723.png)
Fig. 3: Relationships between soil TP and AP and hay yield of alfalfa
equations, but the determination coefficient (R2) difference was not large. In addition to the linear fitting of 2015, two kinds of fitting equations had R2 above 0.7; with the highest value reaching 0.77, showing that there was a close relationship between the soil TP and AP contents and that the soil AP content was strongly influenced by the soil TP content. With increasing soil TP, the soil available P increased gradually. 1061-1067.files/02 doi 15.1386 IJAB-19F-197 (7) 1061-106714225.png)
Relationship between soil TP and AP and hay yield of alfalfa
To understand the relationship between TP and AP content in soil and hay yield of alfalfa in the year of planting, the TP and AP contents in soil were fitted with the alfalfa hay yield. The two-year test results (Fig. 3) showed that there were good linear relationships between the AP and TP contents and alfalfa hay production, with fitting coefficients (R2) less than 0.7 for the TP content and those greater than 0.7 for the AP content, indicating that soil AP content contributes more to alfalfa hay yield than soil TP content.
Discussion
Irrigation has an important effect on the hay yield of alfalfa of different stubble heights. Research shows that the hay yield of alfalfa increases primarily linearly with increasing irrigation (Grimes et al. 1992). The WUE of alfalfa first increases and then decreases with increasing irrigation rate (Song et al. 2019). The annual WUE of alfalfa is the highest under suitable water supply conditions, while the annual WUE of alfalfa is the lowest under sufficient water supply conditions (Sun et al. 2018). This study showed that an increased amount of irrigation was beneficial to the formation of alfalfa plantings and hay yield, but when the irrigation reached a certain amount, the yield increase effect was not obvious (Table 2). At the early stage of alfalfa growth, under drought conditions, increasing irrigation can promote the water absorption and plant growth of alfalfa (Zhang et al. 2017), mainly because excessive irrigation increases alfalfa lodging, which is not conducive to alfalfa photosynthesis or dry matter accumulation, and ultimately reduces the hay yield of alfalfa.
Fertilization is one of the key measures to improve the forage yield of alfalfa. Studies show that fertilization can not only improve the nutritional quality but also increase the growth rate and plant height of alfalfa, thus improving the hay yield of alfalfa (Fan et al. 2016). The results of this study showed that the application of P significantly increased the hay yield of alfalfa in the year of planting, but when the application of P was too high, the hay yield did not increase significantly (Table 2). This result may occur because the P fertilizer applied promoted the accumulation of dry matter to a certain extent but a decrease in dry matter was caused by excessive P application, mainly because alfalfa plants have a certain "threshold" range of P uptake and utilization; when a certain threshold is reached, it can effectively promote the growth of alfalfa (Colomb et al. 2007; Bai et al. 2013), while absorbing excess or greatly exceeding the maximum of absorption, and the P content of alfalfa plants decreases (Berg et al. 2018). Additionally, P can promote the growth of the plant root system, improve the root water absorption ability, increase hay yield and improve WUE (Fang et al. 2010). In our study, when irrigation rate and P were applied at 4500 m3 ha-1 and 100 kg ha-1, respectively, the WUE and AEPF of alfalfa improved (Table 3), and the hay yield showed the same pattern (Table 2). As a consequence, reasonable coupling of water and P increased the absorption of water and P by alfalfa and then increased the hay yield.
The application of P had an important effect on the TP and AP content in different soil layers. Research shows that P is one of the most important elements for plant growth. The P for plant growth mainly comes from soil, and inorganic P in soil mainly depends on the P absorbed by plants. Additionally, the availability of inorganic P of different forms in soil to plants is different (Gu et al. 2018). The results showed that the TP increased with increasing P application and irrigation (Table 4), while the AP increased first and then decreased (Table 5). This result may be due to the spatial variability and vertical migration of soil P and the diffusion of P promoted by water. In addition, this result may be related to the low content of AP in soil and the short movement distance of this type of P, generally approximately 3–5 cm (Batool et al. 2015).
In our study, under the same irrigation amount and the same P application, the contents of TP and AP in the 0–20 cm soil layers were generally the highest (Table 4 and 5), possibly because P has an obvious "surface agglomeration" phenomenon (Lombi et al. 2004; Fraser et al. 2019) that makes the content of TP and AP in the 0–20 cm soil layer significantly greater than that in the 40–60 cm soil layer. Because the content of soil AP was greatly affected by the content of soil TP (Zhao et al. 2018), there was a linear relationship between these parameters (Fig. 2). With increasing TP, the AP gradually increases.
In conclusion, the contents of TP and AP in the water-phosphorus coupled treatment were significantly higher than those in the other non-phosphorus treatments and decreased gradually with increasing soil depth, indicating that water-phosphorus coupling had an impact on the soil P content and then affected the absorption of P by the alfalfa roots. Research shows that alfalfa roots are mainly concentrated in the 20–60 cm soil layer, of which 20–40 cm soil layer roots account for 35.8% of the total root system (Jiang et al. 2017). Therefore, alfalfa topdressing should be applied as deep as possible to meet the soil nutrient needs of deep alfalfa roots (20–60 cm).
Conclusion
Appropriate irrigation rates and phosphorus levels of 4500 m3 ha-1 and 100 kg ha-1, respectively, significantly improved the hay yield, WUE and AEPF of alfalfa in the planting year. The contribution rates of AP to hay yield were the largest and could be used as indicators of growth traits.
Acknowledgements
The research was supported by the National Natural Science Foundation of China (31660693), the Project Funded by the China Postdoctoral Science Foundation (2018T111120, 2017M613252), the Youth Innovation Talent Cultivation Program of Shihezi University (CXRC201605) and the China Agriculture Research System (CARS-34).
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