Optimizing water and phosphorus management to improve hay yield and water‐ and phosphorus‐use efficiency in alfalfa under drip irrigation

Abstract Alfalfa (Medicago sativa L.) is an important forage legume in arid areas, but limited water resources and low fertilizer utilization have restricted its agricultural development. Meanwhile, studies on the effects of integrated water and phosphorus on production performance and water‐use efficiency and phosphorus‐use efficiency of alfalfa, especially on hay yield, phosphorus accumulation, and total phosphorus uptake are rarely reported under drip irrigation. The treatments were a factorial combination of three irrigation rates (5,250, 6,000, and 6,750 m3/ha per year) and four P rates (0, 50, 100, and 150 kg/ha per year) and consisted of 12 treatments for water and P management, arranged in a randomized complete block design with three replicates. Total hay yield and water‐use efficiency and phosphorus‐use efficiency of alfalfa in P2 treatment were significantly greater than those in the P1 and P3 treatments (p < .05), and the total hay yield of alfalfa with phosphorus application increased by 7.43%–29.87% compared with that in the nonphosphorus (P0) treatment under the same irrigation amount. The total phosphorus and available phosphorus concentrations in the 0–20 cm soil layer were greater than those in the 20–40 cm and 40–60 cm soil layers compared with those in the P0 treatment. Correlation analyses showed that total hay yield was significantly positively correlated with total phosphorus uptake and water‐use efficiency (p < .01). The accumulated phosphorus concentration was significantly positively correlated with total phosphorus and available phosphorus concentration (p < .01) and was positively correlated with the phosphorus‐use efficiency (p < .05). The membership function method was used to evaluate all the indicators, and the three treatments that had the greatest influence on the production performance of alfalfa were, in order, W2P2 > W3P2 > W1P2. Therefore, an irrigation rate of 6,000 m3/ha and a phosphorus application rate of 100 kg/ha per year should be considered as the best management for both high yield and water‐use efficiency and phosphorus‐use efficiency of alfalfa.


| INTRODUC TI ON
Alfalfa (Medicago sativa L.) is a perennial leguminous forage that has the largest area of forage cultivation in China because of its high yield, rich protein concentration, strong biological nitrogen fixation ability, and wide adaptability (Huang et al., 2018). However, alfalfa consumes large amounts of water; water has been the major factor limiting its development, especially in dry areas with little precipitation (Gu et al., 2018), and irrigation is required to maximize alfalfa yields . Therefore, water-saving irrigation for alfalfa cultivation has become the fundamental way to sustainably develop the alfalfa industry in Xinjiang. Alfalfa has many branches and abundant stems and leaves, so it needs high levels of nutrients for growth and development. Phosphorus is a basic plant nutrient constitutes nucleic acids, phospholipids, adenosine triphosphates, and many coenzymes. It is involved in the synthesis of substances and various physiological and biochemical processes in plants (Syers, Johnston, & Curtin, 2008). However, the gray desert soil in Xinjiang is a typical phosphorus-deficient soil with a very low available phosphorus concentration, which limits the development of agriculture in Xinjiang. Initially, the application of phosphate fertilizer was to alleviate this situation; however, the solubility of phosphorus is low, its mobility is poor, and it is easily fixed by metal ions after the application of phosphate fertilizer to the soil, making the phosphorus-use efficiency in a growing season only 5%-25% .
All the rest of the phosphorus is in the form of recalcitrant phosphorus residues in soil, which enriches the phosphorus in the soil. This situation restricts the growth and development of plants while causing phosphorus pollution in the soil and wasting limited phosphorus resources (Mai, Xue, Feng, Yang, & Tian, 2018). However, without the addition of phosphorus fertilizer over a long period, the soil available phosphorus concentration will gradually decrease (Yao et al., 2012), and the crop yield will decrease accordingly. In the past few decades, crop yields have not increased proportionally with increasing fertilizer inputs (Wang et al., 2017). Therefore, it is necessary to improve fertilizer-use efficiency through appropriate fertilization to ensure high crop yield and maintain soil fertility (Song et al., 2018).
Some studies have demonstrated that phosphorus and water have synergistic effects on plant growth (Thompson, Doerge, & Godin, 2000).
Within a certain range of water and phosphorus levels, irrigation can effectively improve the absorption, transformation, and utilization of fertilizers by crops. Appropriate fertilization can reduce the negative effects of soil water deficiency on crop growth and development to a certain extent (Yang, Guo, Wang, Yang, & Yang, 2012) and can also increase the phosphorus concentration in plants (Gu et al., 2018) with increased phosphorus uptake. Therefore, the appropriate management of water and fertilizer can not only increase crop yield and reduce irrigation and phosphorus application but can also reduce total phosphorus and increase available phosphorus in soil (Schärer et al., 2010). Meanwhile, the combination of water and fertilizer can effectively improve the water-use efficiency and phosphorus-use efficiency of alfalfa (Lenssen et al., 2010), which is beneficial for reducing the loss of agricultural water in the field and the excessive use of phosphate fertilizers. In many studies on water and fertilizer management in alfalfa, single factors such as water and phosphorus have often been used to evaluate the influence of different irrigation amounts or phosphorus application rates on the production performance of alfalfa (Mallarino & Rueber, 2013). However, the relationships between water-use efficiency and phosphorus-use efficiency and hay yield, total phosphorus uptake and hay yield, and soil total phosphorus and available phosphorus under both irrigation and phosphorus application are rarely reported. Therefore, this study carried out a 3-year study on the effects of different irrigation amounts applied via subsurface drip irrigation and phosphorus application rates on hay yield and soil phosphorus concentrations. The objectives of the present study were to (a) examine the effects of different levels of irrigation and phosphorus on yield and water-use efficiency and phosphorus-use efficiency in alfalfa and (b) examine the impacts of the phosphorus application on phosphorus concentration accumulation in the plants and the soil. The results of present investigation are of significant importance for providing practical guidance for the development of a reasonable and efficient water and fertilizer irrigation system for high quality and high yield in alfalfa under drip irrigation.

| Site description
The field experiment was conducted during 2016-2018 at Tianye Group Agricultural Demonstration Park (44°26′N, 85°95′E), Shihezi City, Xinjiang, China. The experimental site was located in a temperate continental climate zone. The area is dry and rainless, and the diurnal temperature varies greatly. The mean annual temperature is 11.2-13.98°C, the annual precipitation was 203.1-394.9 mm, and the annual pan evaporation is approximately 1,000-1,500 mm. The soil type was a gray desert soil (Chinese soil classification) or Aridisol (United States Department of Agriculture (USDA) classification). The previous crop was cotton (Gossypium spp.). The physical and chemical properties of the 0-60 cm plow layer soil are shown in Table 1.

| Experimental design
The experiment was conducted in a split plot based on a randomized complete block design with three replications and consisted of three levels of irrigation rates and four levels of phosphorus fertilization. The three levels of irrigation rates were 5,250 m 3 /ha (W 1 ), 6,000 m 3 /ha (W 2 , actual irrigation amounts on high yield of alfalfa in local field), and 6,750 m 3 /ha (W 3 ). The four phosphorus fertilizer rates in the experiment were (as P 2 O 5 equivalent) 0 kg/ha (P 0 ), 50 kg/ha (P 1 ), 100 kg/ha (P 2 ), and 150 kg/ha (P 3 ). Three replications were performed for the 12 water-phosphorus management treatments, and each plot size was 5 by 8 m equal 40 m 2 . The distance between each plot is 1.5 m to avoid the lateral movement of water from irrigation level to another.
The WL354HQ alfalfa seeds (Beijing Zhengdao Ecological Technology Co., Ltd.) were sown on April 19, 2015. The crop was sown with a seed drill at a seed rate of 18 kg/ha with a row spacing of 20 cm, and the sowing depth was 2.0 cm. The full quota of irrigation water per plot was delivered across 8 inlaid drip irrigation belts with working pressure of 0.1 MPa and diameter of 12.5 mm. The dripper spacing of drip irrigation belt was 20 cm, and the irrigation water discharge of drip irrigation belt was 3.2 L/h. The drip irrigation belts were buried under the surface 8-10 cm deep with a spacing of 60 cm. The drip irrigation belt was laid with 3 rows of alfalfa for one pipe (Figure 1). The specific amount of irrigation was controlled by a water meter at the intake of the plot. The main pipe diameter in the drip irrigation system was 75 mm. There were 8 irrigation events during the growth year. The amounts of irrigation water application (IWA) to each plot during the irrigation regime were determined by using the following equation (El-Mageed, El-Sherif, Ali, & El-Wahed, 2017): where IWA is the irrigation water application (m 3 ), A is the area (m 2 ), ETc is the reference evapotranspiration (mm/day), Li is the irrigation intervals (day), and Ea is the application efficiency (%).
The phosphorus fertilizer was applied together with the irrigation water under drip irrigation, beginning at the branching stage of spring growth following winter dormancy and subsequently 3-5 days after the first, second, and third cuts. The phosphate fertilizer used was monoammonium phosphate (P 2 O 5 52%) with good water solubility. To keep the test only affected by the phosphate fertilizer, and based on the monoammonium phosphate (NH 4 H 2 PO 4 ) containing nitrogen fertilizer, the effect of nitrogen fertilizer on the production of alfalfa was offset by adding urea (CN 2 H 4 O) to maintain the consistency of the test, as shown in Table 2

| Forage yield
The yield of alfalfa was measured by taking a sample of 1 m × 1 m at the early flowering stage (10% blooming) and cutting four times a year. The alfalfa plants in the sample plot (cut to 5 cm) were cut with scissors and weighed, and the yield of fresh alfalfa forage was recorded three times for every treatment. Three samples of 300 g fresh alfalfa were taken back to the laboratory. The samples were first oven-dried at 105°C for 30 min and then at 65°C to a constant mass. The forage yield (t/ha) was calculated using the following equation: where FY is the fresh yield (t/ha) and MC is the moisture concentration.
In the process of measuring alfalfa hay yield, three fresh alfalfa samples were dried and crushed. The phosphorus concentration was determined using the molybdenum-antimony antispectrophotometric method (Fan, Du, et al., 2016). The total phosphorus uptake of alfalfa was the sum of the phosphorus concentration in the four cuttings of alfalfa plants. The total phosphorus uptake of alfalfa was calculated using the following equation: where THY was total hay yield (kg/ha) and APC is the accumulated phosphorus concentration.

| Phosphorus-use efficiency
The phosphorus-use efficiency of alfalfa was calculated using the following equation: where PUE was phosphorus-use efficiency (%), Yi was total phosphorus uptake in the phosphorus application treatment, Yc is the total phosphorus uptake in the Nonphosphorus (P 0 ) treatment, and TPA is the total phosphorus application in the phosphorus application treatment.

| Water-use efficiency
The hay yield water-use efficiency of alfalfa was calculated using the following equation (El-Mageed et al., 2017): where WUE is the hay yield water-use efficiency (kg/m), THY is the total hay yield (kg/ha), and WA is the water applied (m 3 /ha).

| Soil phosphorus concentration
The "S"-shaped sampling method was adopted to take soil samples at 0-20 cm, 20-40 cm, and 40-60 cm with the soil drill in each plot. Samples of different soil depths were taken at one site after taking the fourth cut, with five replications. Impurities such as roots and stones were removed, and the soil samples were brought back to the laboratory to dry in a cool and ventilated place. The soil was sieved through a 2 mm sieve and placed in a plastic self-sealing bag for the determination of total phosphorus and available phosphorus in soil. Total phosphorus was determined by the sulfuric acid-perchloric acid decoction molybdenum antimony colorimetric method, and available phosphorus was determined by the NaHCO 3 extraction molybdenum antimony colorimetric method (Mehlich, 1984).

| Statistical analysis
All the plant data collected were statistically analyzed by SPSS 20.0

| Total hay yield of alfalfa
The total hay yield first increased and then decreased with increasing phosphorus application and reached a maximum in the P 2 treatment at the same irrigation level. The hay yield in the P 2 treatment was significantly higher than that in the P 1 and P 3 treatments (p < .05) ( Table 3). The total hay yield in W 2 and W 3 treatments were significantly higher than that in the W 1 treatment (p < .05), except that the hay yield in the W 3 treatment was significantly higher than that in the W 1 and W 2 treatments under the P 0 , P 1 , and P 3 treatments in 2018 (p < .05). The total hay yield in the phosphorus application treatments increased by 7.43%-29.87% compared with nonphosphorus (P 0 ) treatment under the same irrigation amounts, This indicated that the total hay yield decreased gradually with time.

| Accumulated phosphorus concentration and total phosphorus uptake of alfalfa
The accumulated phosphorus concentration and total phosphorus uptake of alfalfa increased gradually in 2016 and increased first and then decreased in 2017 and 2018 with the increase in phosphorus application at the same irrigation level (Table 4). The accumulated phosphorus concentration and total phosphorus uptake in the P 1 , P 2 , and P 3 treatments were significantly higher than those in the P 0 treatment. The accumulated phosphorus concentration in the phosphorus treatments increased by 19.99%-36.87% compared with that in the P 0 treatment (p < .05).
The total phosphorus uptake in phosphorus treatments increased by 29.23%-60.16% compared with that in the P 0 treatment (p < .05). The total phosphorus uptake in the W 2 and W 3 treatments increased by 2.06%-13.81% and 0.73%-13.14%, respectively, compared with that in the W 1 treatment (p < .05). This indicated that the addition of phosphate fertilizer effectively promoted the absorption of phosphorus in plants.

| Water-use efficiency and phosphorus-use efficiency of alfalfa
The water-use efficiency of alfalfa increased first and then decreased with increasing phosphorus application and reached a maximum in the P 2 treatment at the same irrigation level. The water-use efficiency of the P 2 treatment was significantly higher than that in the P 1 , P 3 , and P 0 treatments (p < .05), and the P 2 treatment increased by 12.55%-29.79% compared with that in the P 0 treatment (Table 5). It was indicated that phosphorus application significantly improved the water-use efficiency. The water-use efficiency in the W 1 treatment was significantly higher than that in the W 2 and W 3 treatments (p < .05). Thus, the water-use efficiency decreased with the increasing irrigation amount. The phosphorus-use efficiency of alfalfa decreased with an increasing phosphorus application at the same irrigation level.

| Soil total phosphorus
The total phosphorus concentration of the soil in the phosphorus treatment increased gradually with the increase in the phosphorus application rate and reached a maximum in the P 3 treatment, except in the W 1 treatment in the 40-60 cm soil layer in 2016 ( Figure 3).
The total phosphorus concentration in P 3 treatment was significantly higher than that in the P 0 treatment at 0-60 cm at the same irrigation level (p < .05). The total phosphorus in the phosphorus treatment increased by 21.9%-117.8% in the 0-20 cm soil layer, by 9.9%-72.9% in the 20-40 cm soil layer, and by 13.60%-82.6% in the 40-60 cm soil layer compared to those in the P 0 treatment. The total phosphorus in the no-phosphorus treatment in the 0-20 cm soil layer showed a decreasing trend, but total phosphorus showed an increasing trend in 0-60 cm soil layer under the phosphorus treatment. The total phosphorus concentration decreased gradually with the increasing soil depth, reaching a maximum in the 0-20 cm soil layer and a minimum in the 40-60 cm soil layer.

| Soil available phosphorus
The available phosphorus increased gradually or increased first and then decreased with the increase in the phosphorus application rate and reached a maximum in the P 2 /P 3 treatments, and the available phosphorus in the phosphorus treatment was significantly higher than that in the P 0 treatment at the same depth (p < .05) (Figure 4).

| Pearson's correlation analysis
The hay yield of alfalfa was significantly positively correlated with total phosphorus uptake and water-use efficiency (p < .01). The accumulated phosphorus concentration was significantly positively correlated with the total phosphorus and available phosphorus concentration (p < .01) and was positively correlated with the phosphorus-use efficiency (p < .05). Total phosphorus was significantly positively correlated with available phosphorus (p < .01). There were no significant correlations among the other indicators (p > .05).  Table 6 above. The total phosphorus and available phosphorus were matched with other indicators using only 0-20 cm data; while the total phosphorus and available phosphorus were fitted with all data from 0 to 60 cm, and P 0 was removed from the phosphorus-use efficiency fitting data. The determinant coefficient (R 2 ) values for the phosphorus accumulation concentration and phosphorus-use efficiency were small, so they were not included in the figure below.

| Linear nonlinear equations between significantly related paired indicators
The R 2 of available phosphorus and phosphorus accumulation was the largest, followed by that of total phosphorus and available phosphorus. This indicated that the accumulation of phosphorus came from the available phosphorus concentration in the soil, while the available phosphorus concentration in the soil was limited by the supply of total phosphorus. Maintaining the total phosphorus concentration in the soil was beneficial for increasing the level of available phosphorus, thus ensuring the supply of phosphorus nutrients in alfalfa.

| Membership function analysis
Since each treatment performed differently for the different indicators, it was not sufficient to evaluate the optimal irrigation amounts and phosphorus application with any single indicator. The total hay yield, accumulated phosphorus concentration, total phosphorus uptake, water-use efficiency, and phosphorus-use efficiency were positive indicators, while total phosphorus concentration was a negative indicator. The membership function values of the averages of 7 indicators were sorted by their comprehensive value. The larger the average value, the higher the comprehensive value, and vice versa.
The comprehensive assessment of various indicators of alfalfa by membership function analysis showed that W 2 P 2 > W 3 P 2 > W 1 P 2 were the top three treatments for alfalfa production performance (Table 7).

| Effects of water and phosphorus management on hay yield, phosphorus accumulation, and phosphorus uptake of alfalfa
The irrigation amount and phosphorus application rate had significant effects on the total hay yield of alfalfa. The research demonstrated that phosphorus application could help to increase the chlorophyll concentration in alfalfa leaves, thereby improving the photosynthetic rate, and then increase the phosphorus concentration in alfalfa plants, resulting in higher yields in phosphorus-deficient soil (Song et al., 2018). In this study, phosphorus application significantly increased the hay yield, and the accumulated phosphorus concentration and the phosphorus uptake of alfalfa increased gradually with increasing phosphorus application (Tables 3 and 4).  (Table 4), while the hay yield increased first and then decreased (Table 3). Therefore, phosphorus application can of alfalfa in the W 2 and W 3 treatments was significantly higher than that in the W 1 treatment under the same phosphorus application (Table 3). This result indicates that the increase in irrigation amounts was more conducive to the improvement of hay yield than the increase in phosphorus application. Studies have shown that irrigation can increase the growth rate and leaf area of alfalfa (Avramova et al., 2015), enhance the photosynthetic leaves of alfalfa and increase the accumulation of photosynthetic products, thereby improving the hay yield (Li, Wan, Wang, & Li, 2018).
Low soil moisture levels lead to crop water deficits and inhibit crop growth (Jia et al., 2009). Therefore, the effect of water stress on photosynthesis is reflected in the change of plant biomass. In this study, phosphorus application improved the growth of alfalfa and then increases the hay yield under drought stress (Table 3).
Studies have shown that phosphorus regulates root growth under soil water deficit conditions mainly by changing the water status of roots, improving the root water potential, and increasing the absorption of soil water by the root system and the amount of transpiration and evaporation, and then promoting the growth of aboveground and underground plant parts (Liu et al., 2015). In addition, phosphorus application can significantly enhance the stability of tissue and cell membranes, increase stomatal conductance and reduce sensitivity to drought, thereby enhancing drought resistance and improving alfalfa growth (Mickky, Abbas, & El-Shhaby, 2018).

| Effects of water and phosphorus management on water-use efficiency and phosphorus-use efficiency of alfalfa
The water-use efficiency and phosphorus-use efficiency are important criteria for determining whether the irrigation and phosphorus application amounts are reasonable. The water-use efficiency is a physiological index used to describe the growth of alfalfa, especially the relationship between harvest yield and crop water consumption (Lamm, Harmoney, Aboukheira, & Johnson, 2012). In this study, the water-use efficiency decreased with increasing irrigation amounts, and increased first and then decreased with increasing phosphorus application (Table 5). This result indicated that fertilization had obvious water regulating effect, and proper fertilization could improve water-use efficiency (Thompson et al., 2000). Fertilization can increase the soil water holding capacity (Wang, Liu, & Dang, 2011), and successfully matching fertilizer availability with crop absorption improves water-use efficiency and increases yield (Agami, Alamri, El-Mageed, Abousekken, & Hashem, 2018). Other studies have indicated that fertilizers increase the formation of soil aggregates (>0.25 mm) and the levels of nutrients in soil (Liu et al., 2013). Most importantly, the application of phosphate fertilizer promotes plant root development, improves water absorption capacity in roots, and thus improves water-use efficiency (Fang, Xu, Turner, & Li, 2010). Phosphorus application is also beneficial to the distribution F I G U R E 4 Soil available phosphorus concentration under different treatments (%).Different capital letters indicate significant differences in the different irrigation levels under the same P application conditions (p < .05). Different small letters indicate significant differences in the different P levels under the same irrigation conditions (p < .05) of photosynthetic products in the aboveground parts of the plant, which is very important for improving yield and water-use efficiency (Hu et al., 2015).
Research has shown that phosphorus application can promote water-use efficiency, but additional water does not promote phosphorus-use efficiency. Phosphorus-use efficiency is related to the degree to which plants mobilize phosphorus from insoluble sources or absorb soluble phosphorus from the soil solutions (Shenoy & Kalagudi, 2005). Elevated mineral nutrient availability occurs when the soil undergoes drying and rewetting cycles under lower irrigation levels. More intense drying (such as that occurring at high temperatures or over long durations) leads to increased mineralization when the soil is re-watered compared with that in continuously wet soil (Bünemann et al., 2013). Phosphorus had high solubility in W 3 , and water can act as a solvent to dissolve less-soluble phosphorus; the dissolution reduces total phosphorus and increases available phosphorus (Williams, King, Duncan, Pease, & Penn, 2018). Under suitable irrigation conditions, alfalfa phosphorus uptake and phosphorus-use efficiency could be improved (Tables 4 and 5). In this study, the phosphorus-use efficiency ranged from 8.76% to 26.96%,

| Effects of water and phosphorus management on soil total phosphorus and available phosphorus
The phosphorus application and irrigation amounts had significant influence on the changes in total phosphorus and available phosphorus concentration in the different soil layers. The results showed that the soil total phosphorus and available phosphorus were the highest in the 0-20 cm soil layer and decreased gradually with increasing soil depth, and total phosphorus and available phosphorus in the 0-20 cm soil layer were significantly greater than that in the 20-60 cm (Figures 3 and 4). This is mainly because the total phosphorus and available phosphorus in the soil accumulated at 0-20 cm, while the soil layer at 20-60 cm showed a slightly increasing deficit.
The application of phosphate fertilizer to the soil accelerates the recycling of organic matter in the soil, which reduce the adsorption of phosphorus in the soil, promotes the dissociation of phosphorus in the soil, and improves the phosphorus fertility of the soil (Baker, Johnson, Confesor, & Crumrine, 2017). Meanwhile, phosphate fertilizer enters the soil with water and tends to accumulate on the surface; the root system in the soil brings deeper soil nutrients to the surface to provide nutrients for the aboveground plant parts (Fan, Mcconkey, Wang, & Janzen, 2016), thereby increasing the total phosphorus and available phosphorus concentration in the 0-20 cm soil layer.
Water can dissolve phosphorus as a solvent under suitable soil moisture concentration, which can reduce total phosphorus and increase available phosphorus (Williams et al., 2018). However, this process of change is a relatively slow process, so the change is small.
The abovementioned phosphorus accumulation and total phosphorus uptake in plants first increased and then decreased in 2017 and 2018 and reached a maximum in the P 2 treatment, which was significantly greater than that in the P 3 treatment (Table 4). However, the total phosphorus in the P 3 treatment was greater than that in the P 2 treatment in the 0-20 cm soil layer (Figure 3). The phosphorus application in the P 3 treatment was higher than that in the P 2 treatment and was not better absorbed by plants, leading to further accumulation of total p and available p in the soil. Therefore, too much phosphorus application leads to low alfalfa yield, soil environmental pollution, and economic waste. In this study, total phosphorus and available phosphorus in the phosphorus application treatments decreased gradually over the years compared with that in the P 0 treatment in the 0-20 cm soil layer (Figures 3 and 4). It could be concluded that alfalfa cultivation on the gray desert soil would reduce the original available phosphorus concentration, deplete the soil nutrients, reduce soil fertility, and reduce the yield at the same time.
In summary, when the phosphorus amounts in soil are lower than that required by plants, the consumption of phosphorus in soil is greater than that accumulated, and the phosphorus nutrient level in soil decreases gradually; when the amount of phosphorus in soil is higher than that required by plants, the phosphorus pool in the soil will increase continuously. Therefore, to maintain the balance of soil phosphorus accumulation and consumption and to save phosphorus resources in agricultural production, the minimum required amount of phosphorus should be the amount of fertilizer used to maintain the balance of soil phosphorus input and consumption. In this way, the economic risks of fertilization and the environmental pollution of the soil are avoided while the profit per unit area from fertilization is guaranteed.

| Membership function analysis for evaluation of the optimum combination of phosphorus application and irrigation
In this study, the accumulated phosphorus concentration was significantly positively correlated with total phosphorus and available phosphorus concentration (p < .01). Total phosphorus was significantly positively correlated with available phosphorus (p < .01) ( indicators. In this study, it was found that the optimal water-phosphorus management mode was the W 2 P 2 treatment, that is, irrigation at 6,000 m 3 /ha and P application at 100 kg/ha (Table 7). This was an appropriate water-phosphorus combination model for high quality and high yield alfalfa production under drip irrigation. This treatment effectively improved the hay yield of alfalfa, promoted the absorption of available phosphorus by alfalfa plants, improved the water-use efficiency and phosphorus-use efficiency, increased the available phosphorus concentration in soil, and reduced the total phosphorus concentration.

| CON CLUS ION
Moderate management of water and phosphorus was the key measure for high efficiency alfalfa production under drip irrigation.
Phosphorus application significantly improved the water-use efficiency and played a role in water transfer. During the whole growing season of alfalfa, the phosphorus-use efficiency was higher under the low fertilizer levels, but the effect of phosphorus application on the water-use efficiency was not obvious. Phosphorus application significantly increased the available phosphorus concentration in the 0-20 cm soil layer, and available phosphorus gradually decreased with increasing soil depth. The optimal water-phosphorus combination was irrigation at 6,000 m 3 /ha and phosphorus application at 100 kg/ha per year, which resulted in high quality and high yield of alfalfa under drip irrigation.

ACK N OWLED G M ENTS
The research was supported by the National Natural Science

CO N FLI C T O F I NTE R E S T
The authors declare that they have no conflict of interest.

E TH I C A L A PPROVA L
The study did not involve any human or animal testing.

I N FO R M E D CO N S E NT
Written informed consent was obtained from all study participants. Vertical stratification of soil phosphorus as a concern for dissolved phosphorus runoff in the Lake Erie basin.