Yield, Nutrient Quality and Water and Phosphorus Recovery E ciencies of Alfalfa under Different Drip Irrigation and Phosphorus Levels in Northern Xinjiang, China

Junying Liu The College of Animal Science & Technology, Shihezi University Shengyi Li The College of Animal Science & Technology, Shihezi University Yanliang Sun The College of Animal Science & Technology, Shihezi University Xuanshuai Liu The College of Animal Science & Technology, Shihezi University Weihua Lu The College of Animal Science & Technology, Shihezi University Chunhui Ma The College of Animal Science & Technology, Shihezi University Qianbing Zhang (  qbz102@163.com ) The College of Animal Science & Technology, Shihezi University


Background
Alfalfa (Medicago sativa L.) is a perennial leguminous forage with high yield, good nutritive value and wide adaptability 1 ; it is known as the "king of forage". Alfalfa is a important ecological functions, and is widely grown in semiarid areas 2 , has the effect of improving soil fertility and is widely grown in northwest China (the planting area of alfalfa is about 2.62 × 10 5 ha), where precipitation is scarce and evaporation is high. Water resources are the key that restricts alfalfa production and directly or indirectly affect hay yield and quality of alfalfa 3 . Some studies demonstrated that su cient irrigation could signi cantly increase the hay yield of alfalfa 4 , mainly because water supply signi cantly affects the growth and development of alfalfa plants, hay yield, nutrient quality and water-use e ciency (WUE) 5 , while a water shortage could signi cantly reduce the hay yield of alfalfa 6 . In addition to water, phosphorus (P) is an indispensable nutrient element for alfalfa and is one of the main factors for increasing crop yield 7 .
Alfalfa is a crop with a high P demand, and long-term planting reduces the available phosphorus (AP) in the soil 8 . A lack of P in soil will affect the growth and development of alfalfa, so P application can improve the hay yield of alfalfa 9 , and P concentrations of alfalfa increase with increased P fertilizer 10 . P application also plays a signi cant role in regulating water, and proper P application can improve WUE 6 .
Crude protein (CP), neutral detergent bre (NDF) and acid detergent bre (ADF) are three important nutrient quality indicators for forage. An earlier study suggested that the nutritive value of alfalfa was positively correlated with CP and negatively correlated with NDF and ADF 11 . P application can affect nutrient quality by increasing the CP and adjusting the NDF and ADF in plants 12 . Alfalfa hay yield decreased gradually, and CP also decreased with an increase in planting years. P application is a good way to solve the problem, but phosphorus-recovery e ciency (PRE) remains low 13 . Most of the phosphate fertilizer applied to the soil cannot be fully utilized in the current season in actual production.
Usually, 80% of P fertilizer is adsorbed by metal ions in soil or converted to insoluble forms in the soil [14][15] , which enriches soil P and causes environmental pollution while restricting the growth and development of alfalfa. Water and fertilizer are two main factors affecting the growth of alfalfa, and there are synergistic effects 16 . At the same time, P concentration and hay quality parameters (CP and ADF) are closely related to plant growth capacity and plant mass; then, any water effect on v and mass would also promote changes in P concentration and hay quality 17 . However, the mechanism underlying the interaction between water and P and plant growth leading to changes in plant phosphorus concentration, CP and ADF concentration are still unclear. It is of interest to distinguish the effects of water-P interaction on P concentration and hay quality between (i) the indirect effect through plant growth and (ii) direct effect through water de cit and P nutrition.
Since the 21st century, drip irrigation technology has been rapidly and extensively promoted in the oasis district of Xinjiang, which has brought about major changes in the production technology of Xinjiang's agriculture due to the high e ciency of water-saving and increasing production. For the rst time, drip irrigation technology has been successfully applied to alfalfa cultivation at the 148th Regiment in the Xinjiang Production and Construction Corps, China, in 2008, and has been widely promoted. However, researches on the effect of water-P interaction on hay yield, nutrient quality and WUE and PRE of dripirrigated alfalfa between different cuts are scarce, and how much irrigation and P application can make alfalfa hay yield and nutritional quality achieve the best, it is still unclear in the actual production of alfalfa. Therefore, it is of great practical signi cance to determine the optimal water and phosphorus interaction mode for improving alfalfa hay yield and nutritional quality of each cut. The objectives of this study were to clarify the relationship between the production performance, nutrient quality, WUE and PRE rate of drip-irrigated alfalfa between different cuts, and to provide a theoretical basis for the formulation of high-quality and high-yield management measures of drip irrigation alfalfa in the world with the same longitude and latitude.

Site Description
The eld experiment was conducted during 2016 and 2017 at Tianye group agricultural demonstration park (44°26′ N, 85°95′ E), Shihezi City, Xinjiang, China. The experimental site was located in an arid temperate continental climate zone with large diurnal temperature variations. Mean annual temperature was 11.2 (2016) and 14.0°C (2017), and annual precipitation was 395 (2016) and 203 mm (2017). The previous crop was cotton (Gossypium spp.). The physical and chemical properties of the 0-20 cm plough layer soil are shown in Table 1.
There were 8 irrigation times in each growing season, the speci c irrigation time was 8-10 days before harvest and 3-5 days after harvest, and the P fertilizer was evenly divided into four times and applied to the soil with irrigation under drip irrigation, beginning at the branching stage of spring growth following

Soil sample collection
Soil samples of 0-20 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 ne soil was sifted through a 100 mesh sieve for reserve 18 .

Sampling and Measurements
The hay yield of each cut of alfalfa was measured by cutting three 1 m × 1 m quadrats in each plot at the The alfalfa plants in the sample plot (cut height 5 cm) were cut with scissors and weighed, and the yield of fresh alfalfa forage was recorded. A sample of 300 g per plot fresh alfalfa was taken back to the laboratory. The samples were rst oven-dried at 105°C for 30 min and then at 65°C to a constant mass.
Crude protein (CP) was determined by the semimicro Kjeldahl method. The neutral detergent bre (NDF) and acid detergent bre (ADF) were determined according to procedures of Van Soest 19 . The relative feeding value (RFV) was calculated by NDF and ADF using the following equation 20 : RFV = (88.9 -0.779 × ADF) × (120 / NDF) / 1. 29 (1) In the process of measuring alfalfa hay yield, three fresh alfalfa samples were dried and crushed. Forage P concentration was determined using the molybdenum-antimony spectrophotometric method 21 .
The P concentration of the alfalfa was multiplied by the respective yields to calculate the shoot P uptake based on the hay yield. The P uptake based on the hay yield was added together to determine the total P uptake and was converted to kg P uptake ha -1 . Phosphorus recovery efficiency (PRE) was calculated as the following equation 21 : where U p and U 0 are the P taken up by alfalfa from soils with (U p ) and without (U 0 ) added P and F p is the amount of P applied, and the result expressed as a percentage.
The WUE of alfalfa was calculated using the following equation 23 : where HY is the alfalfa hay yield, and ET (evapotranspiration) is the crop water consumptio 16 . Then: where P is precipitation, U is the groundwater recharge, I is the amount of irrigation, F is the deep drainage, R is the runoff, and ΔW is the change in soil moisture from the beginning to the end of the trial 24 , soil moisture content was determined by drying method. According to the conditions during the experiments (no slope, deep water table), the contributions of groundwater recharge, runoff and deep drainage were negligible.
Total phosphorus (TP) was determined by the sulfuric acid-perchloric acid decoction molybdenum antimony colorimetric method, and AP was determined by the NaHCO 3 extraction molybdenum antimony colorimetric method 25 .

Economic bene ts
Economic bene t analysis refers to the assessment and evaluation of the size or level of economic bene ts, and the analysis and Research on the causes of its formation 26 .

EB=YB-TC (5)
YB=Y-P (6) TC=LRC+SC+WC+PC+WEC+LC+HC (7) where EB is the Economic bene t, YB is the yield bene t, TC is the total cost, Y is the yield of alfalfa, P is the price of alfalfa, LRC is the land rent cost, SC is the seed cost, PC is the phosphorus cost, WE is the water and electricity cost, LC is the labor cost, and HC is the harvesting cost.

Statistical Analysis
The effects of water and P on the hay yield, CP, RFV, WUE, PRE and P concentration of alfalfa were examined using two-way (W, P, W×P) ANOVA for each of the 8 harvest dates separately. The means were compared using Duncan tests at P < 0.05. The statistical analyses were determined with 7.05 (Data Processing System, China).
The subordinate function evaluation method was used to comprehensively evaluate the optimal treatment using the following formulas: where X is the measured value of each index of the sample; UX(+) is the positive correlation low function value of each index; and UX(-) is the negative correlation low function value of each index 18 .

Water × Phosphorus Interaction
The water and P interaction of each index was analysed in 2016 and 2017. The irrigation amount and P application had signi cant effects on each index for same cut moments ( Table 2). The irrigation amount and P application had a signi cant effect on alfalfa hay yield (P < 0.01), but the interaction W×P was not signi cant (P > 0.05) for alfalfa hay yield at any cut moment. The irrigation amount had a signi cant effect on RFV and on CP and WUE (P < 0.01) at all cut moment. The P application signi cantly affected WUE (P < 0.05), P concentration and PRE (P < 0.01) at all cut moments in both seasons. In general, the interaction W×P was not signi cant for the productivity indicators evaluated, for the majority of the cut moments (P > 0.05).

Phosphorus Concentration of Alfalfa
The P concentration of alfalfa was signi cantly affected by the irrigation level and P application rate in both years (Table 3). Under the same irrigation amount, P concentration increased gradually (2016) or increased rst and then decreased (2017) with increased P fertilizer; the maximum P concentration was attained under the P 2 and P 3 treatments. P concentration ranged from 0.19 to 0.28% for the P 1 , P 2 and P 3 treatments, across irrigation levels, cut moments and seasons. The P 1 , P 2 and P 3 treatments increased P concentration by 0.05%-0.14% compared with the P 0 treatment (Table 3). Under the same P application treatment, there was no obvious regularity of P concentration in alfalfa under different irrigation treatments, for which the maximum P concentration was attained under W 2 treatments in 2017.

Hay Yield
The hay yield of each cut of alfalfa increased rst and then decreased with increased P fertilizer under the same irrigation amount (Table 4). In general, the maximum hay yield was attained under the P 2 treatment and was signi cantly greater than that in the P 0 treatment (P < 0.05), in the majority of the cut moments and across water levels and seasons. Under the W 1 treatment, hay yield of alfalfa was signi cantly higher than that at the P 0 treatment (P < 0.05) except for the second and third cut (P > 0.05) during the rst growing season. The P 2 treatment resulted in the highest hay yield of alfalfa at all water levels in the second growing season (Table 4). Under the P 0 and P 1 treatments, the hay yield in the W 2 and W 3 treatments was signi cantly higher than that in the W 1 treatment in the rst cut (P < 0.05), while under the P 2 and P 3 treatments, there was no signi cant difference among different irrigation treatments with respect to the hay yield of alfalfa (P > 0.05). Except for W 3

CP Concentration and RFV
The CP concentration increased rst and then decreased with increased P fertilizer; the maximum CP concentration was attained in the P 2 treatment under the same irrigation amount ( Table 5). The CP concentration increased rst and then decreased with increasing irrigation amount; the maximum CP concentration was attained in the W 2 treatment under the P 2 treatment. P 0 , P 1 and P 3 had similar patterns in the rst and second cut. The maximum CP concentration was attained in the W 2 P 2 treatments under water-P interaction treatments.
The RFV gradually increased with increasing P fertilizer under the same irrigation amount; the maximum hay yield was attained under the P 3 treatment and only in the second cut (2017), and the same treatment.

WUE
Under the same irrigation amount, WUE followed the same trend across P levels as alfalfa hay yield; thus, it increased rst and then decreased with increased P fertilizer. The maximum WUE was attained under the P 2 treatment (Table 6). Except for the third cut difference in WUE, the other cuts in all P 2 treatments were signi cantly higher than the P 0 treatment (P < 0.05). The maximum WUE was attained under the P 1 treatment, except for the fourth cut of the P 2 treatment in 2016. The other P application treatments increased with increasing P fertilizer within the same water level. The P 1 , P 2 and P 3 treatments increased by 0.20%-4.75% (2016) and 1.31%-6.22% (2017) compared with the P 0 treatment. The WUE in 2017 was higher than that in 2016 for the rst three cuts, while the WUE in 2017 was lower than that in 2016 for the fourth cut.

PRE
In this study, P 0 was a treatment without P application, so PRE was not calculated for this treatment.
Under the same irrigation level, the PRE gradually increased with P fertilizer; the PRE in the P 1 and P 2 treatments was signi cantly higher than that in the P 3 treatment (P < 0.05) (

Comprehensive evaluation
We used the following ve indicators of alfalfa for a comprehensive evaluation: Phosphorus concentration, hay yield, CP, RFV, WUE (Table 8). According to the comprehensive ranking of alfalfa production indicators under different inoculation treatments, the top three inoculation treatments were as follows: W 2 P 2 > W 2 P 1 > W 3 P 2 .
The economic bene ts of the best group of treatments are calculated according to the membership function. Economic bene t analysis of alfalfa production in the irrigation of 6.0 ML ha -1 and 100 kg P ha -1 treatment is shown in Table 9. The net bene t of alfalfa production was 2 505-2 616 $ ha -1 .

Discussion
The hay yield of alfalfa is greatly affected by water and fertilizer conditions [27][28] . Research showed that the application of phosphate fertilizer can signi cantly increase the photosynthetic rate 21 , accelerate the growth of alfalfa plants, and then increase the hay yield of alfalfa within the range of suitable P application, which promotes the accumulation of P in alfalfa plants 29 . In addition, under the condition of drip irrigation, the appropriate rate of phosphate fertilizer signi cantly promoted the total hay yield of alfalfa 30 . In this study, low P application increased the P concentration of alfalfa plants (Table 3) and then increased the hay yield of each cut of alfalfa. This was in accordance with work reported by Maiorana et al. (2001) 31 in Europe and by Berg et al. (2018) 32 in America south of West Lafayette for the lowest dose of phosphoric fertilizer assured the best qualitative responses of alfalfa. Therefore, the use of low P (0 kg ha -1 ) application in alfalfa production can achieve relatively high hay yield.
Research showed that crop mass accumulation is related to P application, and it has a certain promoting effect on the accumulation of dry matter in alfalfa 33 , but excessive P application could result in a decrease in the dry matter of alfalfa, because there is a certain threshold of absorbed P for dry matter production 34 . In addition, excessive P uptake by plants could have a competitive effect with the uptake of other elements by plants, resulting in unbalanced nutrition, which would reduce plant yield 21 . When phosphorus application exceeds the maximum phosphorus uptake by alfalfa, the hay yield of alfalfa plants decrease, which harms plant growth and development 29 .
Moreover, using the relationship between nitrogen (N) and P concentrations, crop nutrition can be examined in more detail 33 . It was showed that P can promote N assimilation and provides a material basis for protein synthesis 35 . Furthermore, the crop yield has a positive shift as N supply increases in conditions of high soil P availability, while has a negative shift in conditions of low soil P availability. In this study, in the range of 0-100 kg P ha -1 , alfalfa hay yield of each cut increased gradually with the increase of P application rate (Table 4). This is mainly because (i) P application can promote the increase of N in soil 33 , and (ii) the allometric relationships of absorbed P and N with shoot mass lead to the relationship between %P and %N, and this ratio between P and N concentrations increases as %N decreases as a consequence of the dilution of N by increasing crop mass 33 . The functional relationship proposed by Lemaire and Belanger (2020) 35 relating crop N uptake and crop mass accumulation during the time course of crop growth illustrates this very well. Therefore, P concentration is closely linked to absorbed N in plant. However, irrigation promotes the growth of plants and enhances the water absorption function of root system, thus alfalfa can obtain more nutrients and water 3 . Therefore, N concentration is affected by water to some extent, and in turn CP is also affected by water stress 33 . It has been showed that there is a certain threshold for the P absorption by alfalfa plants. Below this threshold, P can promote alfalfa growth and development 23 . When the P application exceeded the maximum absorption of P by alfalfa, P had a negative impact on plant growth and development and alfalfa hay yield decreased. As alfalfa hay yield is closely related to CP concentration, excessive P may limit the availability of N, and then reduce the CP concentration. Of course, the speci c impact mechanism needs further research. Consequently, the CP increased rst and then decreased with increasing irrigation amount (Table 5).
In this study, the hay yield (Table 4) and CP concentration (Table 5) of each cut of alfalfa reached the maximum under W 2 P 2 treatment. The allometric growth function showed that the CP concentration of forages can be directly related to the quantity of forage harvested 35 . Therefore, the water-P interaction may rst cause the change of alfalfa yield, and then change the quality (CP concentration) of alfalfa, making the hay yield and CP concentration of alfalfa reach the maximum under W 2 P 2 treatment, which indicated that reasonable irrigation and P application could improve the CP concentration of alfalfa.
Research showed that ADF and NDF decline allometrically with crop mass accumulation and hence forage production 36 . So, any decline in crop mass, due to either water stress or P de ciency should have an effect on forage quality 35 . At the same time, there was no obvious trend of RFV in response to the irrigation amount and P application (Table 5). This was in accordance with work reported by Lemaire and Belanger (2020) 35 for NDF. This is mainly due to both forage nutritive value decline and plant phenology progression was only correlated with time 35 .
Alfalfa WUE and PRE are important criteria for determining whether the irrigation amount and P fertilizer are reasonable. WUE is a physiological index used to describe the growth of alfalfa, especially the relationship between harvest yield and crop water consumption. Research showed that there is a positive link between WUE and hay yield 37 . Previous studies have shown that there are synergistic effects between water and fertilizer and that fertilization has an obvious water-regulating effect [38][39] In this study, although the WUE was highest in the W 1 P 2 treatments (Table 6), the alfalfa hay yield of W 1 P 2 treatments were relatively low (Table 4), which was mainly due to N nutrition of alfalfa was severely depress by water stress 37 , and lower plant N nutrition will lead to lower plant P nutrition 33 , which will reduce alfalfa hay yield. At the early stage of alfalfa growth, under drought conditions, increasing irrigation can promote the water absorption and plant growth of alfalfa 40 , 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. Appropriate fertilization can improve WUE and transform "ineffective" water to water that is "effective" for plant growth 41 . The interaction of water and P, to a certain extent, can reduce the irrigation amount and fertilization application in grassland management of alfalfa, achieving the goal of saving water and reducing fertilization costs while maintaining yield.
In the case of water de cit, crop root development is hindered, nutrient absorption capacity is reduced, and fertilizer e ciency is limited. When water is excessive, soil nutrient leaching occurs, soil permeability is reduced, root respiration and nutrient uptake by crops are hindered 42 . In this study, PRE increased rst and then decreased with increased irrigation (Table 7). Therefore, the irrigation amount will indirectly affect the distribution of crop roots, which in turn affects the absorption of water and nutrients by roots.
On the contrary, the study in Europe showed that water levels did not affect hay yield and PRE of alfalfa 31 , which is different from the results of this study, probably because of the rainfall in the experimental area of this study was less than 200 mm, alfalfa was more sensitive to irrigation water, while there are good winter rainfalls (over 600 mm), which levelled the effects of irrigation 31 . It can be seen that the coupling of water and P under drip irrigation can improve the PRE of alfalfa plants.
In addition, land is one of the main limiting factors to get the most economic bene ts from planting alfalfa in Xinjiang, China. Because the lease fee of land was 857 $ ha -1 , while the cost of irrigation water (including water fee and electricity fee) was only 257 $ ha -1 (Table 9). Therefore, the most economical way for alfalfa growers is to maximize the hay yield of alfalfa. In this study, the net economic bene ts of alfalfa production reached 2 505-2 616 $ ha -1 (Table 9). Studies in northern China showed that the economic bene t of planting alfalfa was signi cantly better than that of planting grass 43 , largely due to the market value of alfalfa was higher than that of grass, while irrigation has little impact on the economic bene t of alfalfa growers.

Conclusions
Water-P interaction signi cantly improved hay yield, nutrient quality, WUE and PRE of each cut of alfalfa. Suitable increase in the irrigation amount can improve the PRE of alfalfa, so the effective interaction of water and P can promote P through the effect of water and can regulate water through the effect of P.
Therefore, moderate irrigation (6.0 ML ha -1 ) and P fertilizer (100 kg P 2 O 5 ha -1 ) combined with application, the alfalfa has higher WUE and PRE, and can signi cantly promoted the further improvement of alfalfa hay yield and nutrient quality of each cut.