4.1. Application of P Fertilizer Affects Winter Survival of Alfalfa
The cold resistance of alfalfa refers to its ability to survive in winter and regenerate in the following Spring [
29], which directly affects its persistence and production capacity. Plant nutrition is one management factor to improve alfalfa winter survival. Fertilization with P and K has long been known to improve the persistence and yield of alfalfa [
30,
31]. Jung and Smith [
9] reported that P fertilization was essential for plant survival. Some reports indicated no impact, and occasionally a negative effect of enhanced P and K nutrition on winter survival. Gross et al. [
32] found that plant density decreased with P fertilization. Berg et al. [
7] also reported that fertilization with P decreased plant population. Sanderson and Jones [
23] found that delayed application of P on fall-planted alfalfa did not affect plant density. In our study, despite some reductions in the alfalfa population following emergence, the winter of 2019–2020 was warmer than usual, and the winter survival rates of alfalfa in the two sites were both more than 80%; the effects of P fertilizer on winter survival were obvious in the Tuzuo experiment site, where the winter survival rate of which reached 93.4% at 45 kg ha
−1 in 2020, whereas it reached 65.3% at 90 kg ha
−1 in 2021. The winter of 2020–2021 was colder than usual, and although some alfalfa plants survived in the first winter, but died in the winter of 2020–2021, the alfalfa under a proper amount of P fertilization had a greater overwintering rate, especially for the Siziwang experiment site, where the maximum winter survival rate in 2020 did not have a significant difference from the control, whereas the maximum winter survival rate in 2021 had a significant difference from the control. The suitable amount of P fertilizer for a greater winter survival rate was 45~90 kg ha
−1 in the two sites. The effect of P fertilizer on winter survival was related to other factors, including temperature, soil test P levels, and the content of other mineral elements, which may be responsible for this overwintering difference. The initial P content of the soil was relatively high, and in a warm winter, the addition of P would not significantly affect the overwintering of alfalfa. If more severe stress had been placed on the soils to supply P to the plant, such as done today with exceptionally high yields and the cold winter of 2020–2021, the initial P soil supply may not be sufficient to carry the crop through the winter [
12,
33]. The other interpretation of the confusion may be that it resulted from inaccuracies associated with estimating plant numbers by counting crowns from aboveground instead of directly digging and counting plants [
3].
4.2. Application of P Fertilizer Affects Alfalfa Yield and Yield Components
P is a crucial element for alfalfa growth and agronomic performance [
34]. When soil P is limited, plants tend to show a positive growth response to P addition [
35]. Berg et al. [
7] found that the addition of P fertilizer can increase alfalfa yield and stand persistence. Malhi et al. [
27] reported that the alfalfa forage yield increased with P application, but its magnitude of response to added P was lower at Botha than at Lacombe, and the residual effect of large single P application on forage yield lasted at least for five years. However, the addition of P to P-deficient plants stimulated the growth of shoots but not roots [
34]. In our study, proper application of P fertilizer increased alfalfa forage yield, and the effects of applying P fertilizer on alfalfa forage yield were significantly different at the two sites. Applying 90 kg P
2O
5 ha
−1, the forage yield increased in the two sites; in Siziwang County, the forage yield of 2020 reached 9101.5 kg ha
−1, which was an increase of 54.0% relative to the control. Because a large amount plants died in the cold winter of 2020–2021 and the consumption of fertility, the forage yield of 2021 decreased relative to that of 2020; the forage yield under 90 kg ha
−1 reached 8386.8 kg ha
−1, which was increased by 39.2% relative to the control. To obtain the ideal forage yield, the optimum amount of P
2O
5 application was 108.1 kg ha
−1. In Tuzuo County, the forage yield of 2020 under 90 kg P
2O
5 ha
−1 reached 13,335.6 kg ha
−1, which was an increase of 89.9% relative to the control, and reached 14,287.2 kg ha
−1 in 2021, which was an increase of 53.9% relative to the control. The optimum P
2O
5 application rate for alfalfa production was 78.3 kg ha
−1 for Tuzuo County. The test P levels of the two soil types were different, and the different contents of other nutrient elements and water conditions in the two sites may also have led to the different demands of P fertilizer for higher forage yield. Insufficient or excessive supply of P fertilizer would reduce the forage yield of alfalfa to some extent, and only appropriate application of P fertilizer can maximize higher yield and the ideal economic benefits [
36].
Alfalfa forage yield can be described as the product of three components: plants area
−1, shoots plant
−1, and mass shoot
−1 [
17]. Adequate soil P at planting is essential to establishing productive stands of legumes [
23]. Supplemental P should be placed according to the native soil fertility and soil P-fixing capacity [
37]. Previous reports on the effect of P fertilizer on alfalfa density have had different results. Markus and Battle [
30] and Berg et al. [
38] found that decreased alfalfa populations were observed in response to enhanced P fertilization. Sanderson and Jones [
23] found that the delayed application of P to fall-planted alfalfa did not affect plant density. In a decade-long alfalfa fertility study, stand densities in plots fertilized with both P and K were greater than stand densities of plots fertilized with P alone [
39]. In our study, applied P fertilizer improved plant population density in the two sites; however, the amount of P fertilizer required for high plant population in the two sites was different. The differences in the results are related to the initial P levels of soils in various regions. In the two years, plant losses totaled 27 plants m
−2 in Siziwang County and 41 plants m
−2 in Tuzuo County. As the climate and environmental conditions of Tuzuo County during the alfalfa planting period were better than those of Siziwang County, Tuzuo County had more initial plants m
−2. Plants die not only in winter but also during the growing season [
38]. As the plant grows, intraspecific competition gradually intensifies later, including competition for water, light, and nutrients. Moreover, the winter-injured plants may survive through the first harvest but subsequently die later in summer [
38]. These reasons may lead to Tuzuo County losing more plants m
−2.
Improved plant nutrition is helpful to increase plant persistence. P fertilizer application can increase the number of leaves and stems, promote root development, increase the forage yield, and improve soil fertility [
40]. Application of P fertilizer increased the initial plant population, but plants were also lost during the growing period; the magnitude reduction of the plants m
−2 was equal to that of the no fertilization treatment. The decline in the alfalfa population may have resulted from enhanced interplant competition for light, water, and nutrients that eliminated smaller, less vigorous plants from the genetically heterogeneous population of plants that comprise an alfalfa stand. This is because extensive shading and low nutrition content cause preferential death of the weakest plants in the population [
41]. Robust, P-responsive plants have greater mass and have more rapid regrowth after harvest, eventually crowding out smaller, less vigorous, slower-growing alfalfa plants [
38].
The shoots number of alfalfa is the number of branches derived from the taproot, which are needed for forage yield [
42]. Shoots plant
−1 is the yield component that is thought to increase as stand density declines to maintain a high yield [
43]. The application of P fertilizer can increase the shoots number and yield of alfalfa [
44]. In our study, the shoots plant
−1 of alfalfa in Siziwang County increased with the P fertilization and shoots plant
−1 of the second harvest was larger than that of the first harvest. However, the shoots plant
−1 in Tuzuo County also increased with the P fertilization in 2020, while the shoots plant
−1 in 2021 did not have an obvious difference between P fertilization treatments. There was a slight increase in shoots plant
−1 at the same harvest in the two years. The greater forage yield needed more shoots plant
−1, except for the second harvest of 2020 and the third harvest of 2021 in Tuzuo County. Berg et al. [
38] found that slight increases in shoots plant
−1 occurred as plant populations decreased, but these changes were independent of P fertilization. The shoots plant
−1 of alfalfa is determined by genetics, which would be affected by nutrition and other environmental conditions to a certain extent, but it will not continue to increase with the improvement of nutritional conditions [
17].
Mass shoot
−1 is the most important component of forage yield [
18]. Increased mass shoot
–1 has consistently been associated with the improved agronomic performance of alfalfa, and improved soil fertility is a strategy to increase mass shoot
−1 [
25]. When applying an appropriate amount of P fertilizer, the shoot mass increased in a consistently positive relationship with forage yield. Two components determine mass shoot
−1: mass shoot height
−1 and height shoot
−1. The latter has a more direct impact on forage yield [
12]. In our study, shoot mass and plant height both increased when the P application rate increased, and were also linearly correlated with forage yield. Rapid shoot initiation rates after harvest permit shoot regrowth to resume quickly after harvest, resulting in high plant height and shoot mass [
45]. Plants receiving P initiated shoot growth quicker, leading to higher plant height and greater shoot mass [
25].