Evaluation of Different Shallow Groundwater Tables and Alfalfa Cultivars for Forage Yield and Nutritional Value in Coastal Saline Soil of North China

Freshwater shortage and soil salinization are the major constraints for alfalfa (Medicago sativa L.) growth in coastal salt–alkali soil of North China. In this study, we analyzed the effects of shallow groundwater tables and alfalfa cultivars on forage yield and nutritional value. A field simulation experiment was conducted during the growing season of 2019–2021 with three groundwater depths (80, 100, and 120 cm) and five alfalfa cultivars (Magnum 551, Phabulous, Zhongmu No. 1, Zhongmu No. 3, and WL525HQ) under subsurface pipe systems. Alfalfa forage was harvested six times in total during the growing season. Results revealed significant variation among alfalfa cultivars for forage yield at each shallow groundwater depth. The greatest forage yield was recorded in cultivar Phabulous (32.2 and 35.9 t ha−1 in 2020 and 2021) when planted at 100 cm shallow groundwater depth. Forage yield during the first harvest was 24.6–25.7%, exhibiting the highest ratio of the total annual yield. The effects of shallow groundwater depth, cultivar, and their interaction were significant (p < 0.01) on the turn-green ratio of alfalfa. Cultivar Zhongmu No. 1 had the highest turn-green ratio at the 100 cm groundwater depth, while cultivar WL525HQ showed the lowest turn-green ratio at each groundwater depth. Moreover, crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) content were also significantly affected by shallow groundwater depth, cultivars, and their interaction at different harvests. Cultivars Magnum551, Zhongmu No. 1, Zhongmu No. 3, and Phabulous furnished the highest CP, while cultivar WL525HQ performed the poorest in terms of CP in this study. These results propose that planting the cultivar Phabulous at a groundwater depth of 100 cm could be a suitable agronomic practice for alfalfa forage production in the coastal salt–alkali area of North China.


Introduction
Soil salinization and alkalization have become more serious in numerous coastal regions throughout the world [1]. Saline-alkali soil is not only unfavorable for agricultural productivity, but is also unfavorable for water storage and soil nutrient availability, inevitably leading to drought [2]. Both drought and salt-alkali stress adversely influence crop growth via restricted water uptake and the presence of excessive salt [3,4]. Coastal saline-alkali land is an important cultivable land resource in China [5]. However, few plants can grow in this region due to the high soil salinity and freshwater scarcity [6,7]. Salinity stress affects crop water transpiration and water retention capacity [8]. Agricultural

Experimental Design
Groundwater modeling system experiments were conducted to determine the effects of three shallow groundwater table depths and five alfalfa cultivars on the forage yield and quality. The tested alfalfa cultivars included Magnum 551, Phabulous, Zhongmu No. 1, Zhongmu No. 3, and WL525HQ. The source of tested cultivars is displayed in Table 1. The simulation experiment was arranged in a split-plot design with shallow groundwater table depths (D1, 80 cm; D2, 100 cm; D3, 120 cm) and alfalfa cultivars (C1, Magnum 551; C2, Zhongmu No. 1; C3, Zhongmu No. 3; C4, Phabulous; C5, WL525HQ) as the main plots and subplots, respectively. All treatments were replicated five times. The simulation equipment for the groundwater depth was made of 1.0 cm diameter polypropylene fiber composite plates. The rectangular simulation boxes were closed on the bottom and were connected to the plastic pipe via flexible polyvinyl chloride pipe. A plastic pipe and tank system were used to control the water table depth and to provide a volumetric measure of the amount of water required to maintain a relatively steady underground water level. The entire simulation box assembly was set on reinforced concrete slab, constructed of 1.5-m length, 1.5-m width, and 1.5-m height. The complete setup is shown in Figure 2.

Experimental Design
Groundwater modeling system experiments were conducted to determine the effects of three shallow groundwater table depths and five alfalfa cultivars on the forage yield and quality. The tested alfalfa cultivars included Magnum 551, Phabulous, Zhongmu No. 1, Zhongmu No. 3, and WL525HQ. The source of tested cultivars is displayed in Table 1. The simulation experiment was arranged in a split-plot design with shallow groundwater table depths (D1, 80 cm; D2, 100 cm; D3, 120 cm) and alfalfa cultivars (C1, Magnum 551; C2, Zhongmu No. 1; C3, Zhongmu No. 3; C4, Phabulous; C5, WL525HQ) as the main plots and subplots, respectively. All treatments were replicated five times. The simulation equipment for the groundwater depth was made of 1.0 cm diameter polypropylene fiber composite plates. The rectangular simulation boxes were closed on the bottom and were connected to the plastic pipe via flexible polyvinyl chloride pipe. A plastic pipe and tank system were used to control the water table depth and to provide a volumetric measure of the amount of water required to maintain a relatively steady underground water level. The entire simulation box assembly was set on reinforced concrete slab, constructed of 1.5-m length, 1.5-m width, and 1.5-m height. The complete setup is shown in Figure 2. The local air-dried saline soil was put into simulation boxes. The simulation boxes were packed to a bulk density of 1.3 g cm −3 with salty-alkaline soil collected from the 1.5 m depth of the soil profile near the experimental site. Water with low salt concentration was applied based on the change in the water volume of plastic tank. The simulation equipment used in this study determined the alfalfa water use from shallow groundwater depths. The local air-dried saline soil was put into simulation boxes. The simulation boxes were packed to a bulk density of 1.3 g cm −3 with salty-alkaline soil collected from the 1.5 m depth of the soil profile near the experimental site. Water with low salt concentration was applied based on the change in the water volume of plastic tank. The simulation equipment used in this study determined the alfalfa water use from shallow groundwater depths.
For sowing, 2 g of seeds were planted on the furrows on 20 August 2019. The row spacing of alfalfa was kept 25 cm in the simulation boxes. Before sowing seeds, the base fertilizer was applied as manure (3.6 t ha −1 ), diammonium phosphate (800 kg ha −1 ; (N, 21.2% and P2O5, 53.8%), and potassium sulphate (180 kg ha −1 ; (K2O,54.1%). Weeds were pulled out manually all through the growing period. Alfalfa forage was harvested six times during the growing season at the height of 5-cm above the soil surface when 20% of the stems had flower buds. Stubble height for the last harvested alfalfa was 10 cm, which ensured alfalfa plant through winter safely. Six harvests were made on the following dates: 13 May, 10 June, 8 July, 4 August, 6 September, and 7 October 2020. In 2021, seven harvests were made on the following dates: 13 May, 6 June, 27 June, 20 July, 12 August, 11 September, and 17 October.

Sampling and Measurements
The number of turn-green plants in the simulation box was counted on 28 April 2020. Observed sprouting was considered to be turning green. All aboveground plants from simulation box in each experimental unit (3 depths × 5 cultivars × 5 replicates) were harvested and weighed separately to estimate the fresh forage yield. Afterwards, subsamples were weighed and then placed in forced air oven for 48 h at 65 °C and weighed again to determine the dry forage yield and yield proportion by harvest. Then, oven-dried samples were ground into a fine powder, passed through a 0.15 mm sieve, and analyzed for forage quality attributes, i.e., crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF), according to AOAC International Methodology [31]. The nitrogen content was determined by the Kjeldahl method [32] and CP content was calculated by the formula of N × 6.25 [33]. The NDF and ADF content were measured by the Van Soest method [34].

Statistical Analysis
Response variables include forage yield, CP, NDF, ADF, and turn-green ratio. The data for all treatments was presented on the basis of five replicated measurements. The For sowing, 2 g of seeds were planted on the furrows on 20 August 2019. The row spacing of alfalfa was kept 25 cm in the simulation boxes. Before sowing seeds, the base fertilizer was applied as manure (3.6 t ha −1 ), diammonium phosphate (800 kg ha −1 ; (N, 21.2% and P 2 O 5 , 53.8%), and potassium sulphate (180 kg ha −1 ; (K 2 O,54.1%). Weeds were pulled out manually all through the growing period. Alfalfa forage was harvested six times during the growing season at the height of 5-cm above the soil surface when 20% of the stems had flower buds. Stubble height for the last harvested alfalfa was 10 cm, which ensured alfalfa plant through winter safely. Six harvests were made on the following dates: 13 May, 10 June, 8 July, 4 August, 6 September, and 7 October 2020. In 2021, seven harvests were made on the following dates: 13 May, 6 June, 27 June, 20 July, 12 August, 11 September, and 17 October.

Sampling and Measurements
The number of turn-green plants in the simulation box was counted on 28 April 2020. Observed sprouting was considered to be turning green. All aboveground plants from simulation box in each experimental unit (3 depths × 5 cultivars × 5 replicates) were harvested and weighed separately to estimate the fresh forage yield. Afterwards, subsamples were weighed and then placed in forced air oven for 48 h at 65 • C and weighed again to determine the dry forage yield and yield proportion by harvest. Then, oven-dried samples were ground into a fine powder, passed through a 0.15 mm sieve, and analyzed for forage quality attributes, i.e., crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF), according to AOAC International Methodology [31]. The nitrogen content was determined by the Kjeldahl method [32] and CP content was calculated by the formula of N × 6.25 [33]. The NDF and ADF content were measured by the Van Soest method [34].

Statistical Analysis
Response variables include forage yield, CP, NDF, ADF, and turn-green ratio. The data for all treatments was presented on the basis of five replicated measurements. The analysis of variance (ANOVA) was performed using SAS 8.0 statistical software (SAS Institute, Cary, NC, USA). Mean comparisons between the treatments were tested by the Fisher's least significant difference (LSD) test using 95% confidence interval [35]. Data were expressed as mean ± standard deviation (SD). All figures were created using Sigmaplot 12.5 (Systat Software, Inc., San Jose, CA, USA).

Analysis of Variance
The effect of shallow groundwater table depth on CP, NDF, and ADF was significant (Table 2). Similarly, the effect of the alfalfa cultivar on total dry forage yield, CP, NDF, ADF, and turn-green ratio was also significant (p < 0.01). The groundwater table depth significantly affected the dry forage yield of the first, second, and sixth harvests, and the percent of dry forage yield of each harvest in the total yield (p < 0.01). Likewise, the effect of the cultivar on the dry forage yield of the first, third, and fifth harvests, and the percent of dry forage yield in all the harvests was significant (p < 0.05). The interaction between groundwater depth and cultivar was significant for all the studied parameters (p < 0.01).  *, ** and ***: Significant correlation at the 5, 1, and 0.1% levels of probability, respectively. CP: crude protein, NDF: Neutral detergent fiber, ADF: acid detergent fiber.

Forage Yield Changes with Groundwater Depth and Cultivar
Forage yield at a groundwater depth of 100 cm was higher compared with the groundwater depths of 80 cm and 120 cm in 2020 and 2021 ( Figure 3). The sequence of the forage yield at different groundwater depths in 2020 was as follows: 100 cm > 120 cm > 80 cm. Alfalfa cultivar Magnum551 had the highest forage yield (30.9 t ha −1 ), which was approximately 60.9% higher than the value for cultivar Zhongmu No. 3 at groundwater depth of 80 cm. The forage yield of cultivar Phabulous was 81.9% higher than that of Zhongmu No. 3 at a groundwater depth of 100 cm. The highest and lowest forage yields were recorded for the cultivars Phabulous (32.2 t ha −1 ) and Zhongmu No. 3 (17.1 t ha −1 ) at a groundwater depth of 100 cm, respectively. In 2021, the average forage yield of five cultivars at a groundwater depth of 100 cm was 3.3% and 10.1% higher than that at groundwater depths of 80 and 120 cm, respectively. The average forage yield for Juneng551 and Phabulous was higher than that for the other three cultivars at all groundwater depths. were recorded for the cultivars Phabulous (32.2 t ha −1 ) and Zhongmu No. 3 (17.1 t ha −1 ) at a groundwater depth of 100 cm, respectively. In 2021, the average forage yield of five cultivars at a groundwater depth of 100 cm was 3.3% and 10.1% higher than that at groundwater depths of 80 and 120 cm, respectively. The average forage yield for Juneng551 and Phabulous was higher than that for the other three cultivars at all groundwater depths. The highest forage yields of 25.7%, 25.8%, and 24.6% at groundwater depths of 80, 100, and 120 cm, respectively, were achieved from the first harvest in 2020 ( Figure 4). The proportion of the sixth harvest in total forage yield was the lowest, being 16.1%, 15.0%, and 13.9% at the groundwater depths of 80, 100, and 120 cm, respectively. The sequence of forage yield proportion among the six harvests was as follows: first harvest > fifth harvest > third and fourth harvests > second harvest > sixth harvest. In 2021, the proportion of first, second, third, and fourth harvests in total forage yield had reached 71.0%, 73.9%, and 68.8% at the groundwater depths of 80, 100, and 120 cm, respectively. The highest forage yields of 25.7%, 25.8%, and 24.6% at groundwater depths of 80, 100, and 120 cm, respectively, were achieved from the first harvest in 2020 ( Figure 4). The proportion of the sixth harvest in total forage yield was the lowest, being 16.1%, 15.0%, and 13.9% at the groundwater depths of 80, 100, and 120 cm, respectively. The sequence of forage yield proportion among the six harvests was as follows: first harvest > fifth harvest > third and fourth harvests > second harvest > sixth harvest. In 2021, the proportion of first, second, third, and fourth harvests in total forage yield had reached 71.0%, 73.9%, and 68.8% at the groundwater depths of 80, 100, and 120 cm, respectively.

Effects of Shallow Groundwater Depth and Cultivar on Turning Green of Alfalfa
There was a significant difference in the rate of turning green among different alfalfa cultivars at the shallow groundwater table depths of 100 and 120 cm (p < 0.05) ( Figure 5). According to the results, the average greening rate of all alfalfa cultivars with a groundwater depth of 100 cm was about 40.6% and 47.0% higher than that at the groundwater depths of 80 cm and 120 cm, respectively. Cultivar WL525HQ had the lowest greening rate (8.9%), which was approximately 3.

Effects of Shallow Groundwater Depth and Cultivar on Turning Green of Alfalfa
There was a significant difference in the rate of turning green among different alfalfa cultivars at the shallow groundwater table depths of 100 and 120 cm (p < 0.05) ( Figure 5). According to the results, the average greening rate of all alfalfa cultivars with a groundwater depth of 100 cm was about 40.6% and 47.0% higher than that at the groundwater depths of 80 cm and 120 cm, respectively. Cultivar WL525HQ had the lowest greening rate (8.9%), which was approximately 3.

Effects of Shallow Groundwater Depth and Cultivar on Turning Green of Alfalfa
There was a significant difference in the rate of turning green among different alfalfa cultivars at the shallow groundwater table depths of 100 and 120 cm (p < 0.05) ( Figure 5). According to the results, the average greening rate of all alfalfa cultivars with a groundwater depth of 100 cm was about 40.6% and 47.0% higher than that at the groundwater depths of 80 cm and 120 cm, respectively. Cultivar WL525HQ had the lowest greening rate (8.9%), which was approximately 3.

Crude Protein
In the third harvest, Zhongmu No. 3 had the significantly highest CP content (2.03 g kg −1 ) at a groundwater depth of 80 cm (Table 3). At a groundwater depth of 100 cm, Zhongmu No. 3 (2.03 g kg −1 ) had the highest CP content, followed by cultivar Phabulous (2.00 g kg −1 ). During the fourth harvest, cultivar Phabulous had the highest CP content (2.07 g kg −1 ) at a groundwater depth of 80 cm. The CP content of cultivar Magnum551 was highest at a groundwater depth of 120 cm. During the fifth harvest, CP content of cultivar Magnum551 was significantly the highest at a groundwater depth of 100 cm. There was a significant difference in CP content among different harvest periods (p < 0.05) ( Figure 6). The highest CP content of 2.28, 2.34, and 2.25 g kg −1 at groundwater depths of 80, 100, and 120 cm, respectively, was achieved from the sixth harvest. The lowest CP content of 1.85, 1.82, and 1.70 g kg −1 at groundwater depths of 80, 100, and 120 cm, respectively, was achieved from the fifth harvest.

Neutral Detergent Fiber
At the first, second, and fifth harvests, no significant differences were observed in NDF among cultivars at all groundwater depths (Table 4). In the third harvest, the NDF content of cultivar WL525HQ was highest at a groundwater depth of 100 cm, while the NDF content of cultivar Magnum551 was significantly the lowest ( Figure 6). During the fourth harvest, the NDF content of cultivar WL525HQ was highest at a groundwater depth of 100 cm, while the NDF content of cultivar Phabulous was lowest. Cultivar WL525HQ at a groundwater depth of 120 cm had the highest NDF content, which was 15.8% and 16.5% higher than that of Magnum551 and Phabulous, respectively. During the sixth harvest, the NDF content of cultivar WL525HQ was the significantly highest, while that of cultivar Phabulous was the lowest.   Values are the means of five replicates. For each harvest, different small letters in a column denote significant differences among alfalfa cultivars of the same groundwater depth at p < 0.05.

Acid Detergent Fiber
During the first and second harvests at all groundwater depths, no significant differences were observed in the ADF content among alfalfa cultivars (Table 5). During the third harvest, at a groundwater depth of 100 cm, the ADF content of cultivar WL525HQ was significantly higher, while that of cultivar Phabulous was the lowest. During the fourth harvest, at a groundwater depth of 80 cm, WL525HQ had the significantly highest ADF, while the ADF content of cultivar Zhongmu No. 3 was the lowest (Figure 6). Cultivar WL525HQ was higher in ADF than rest of the cultivars at a groundwater depth of 120 cm. During the fifth harvest, cultivars WL525HQ and Zhongmu No. 3 furnished the highest (3.33 g kg −1 ) and lowest (2.96 g kg −1 ) ADF content, respectively, at a groundwater depth of 100 cm. The highest contents of ADF were noted in cultivar WL525HQ during the sixth harvest at all groundwater depths. When alfalfa is planted in the saline soils of North China, the goal of a consistent supply of high-yielding and quality forage is limited by freshwater shortage and soil salinity. Groundwater is an important water source for alfalfa growth in shallow groundwater table areas. Shallow groundwater had a great influence on the distribution of water in different soil layers and alfalfa production, and the interaction between groundwater depth and alfalfa cultivars was significant ( Table 1). The mean forage yield at 100 cm groundwater depth was noticeably higher than at groundwater depths of 80 and 120 cm (Figure 2), probably resulting from the greater use of shallow groundwater by alfalfa, which translated to lower leaching fractions [12,29]. Another reason could be that the roots of alfalfa are mostly distributed in the 0-100 cm deep soil layer [35]. The forage yield during the first harvest was the highest at the 100 cm groundwater depth, accounting for 25.8% and 21.6% of the annual total forage yield in 2020 and 2021, respectively (Figure 4). The high ratio of forage yield in this study region during the first harvest was attributed to the longer growth period than the subsequent harvests, which also reflects better water use for alfalfa grown in a dry period [36,37]. Furthermore, because of the low temperature and low shading, a large number of fresh leaves were formed, and they increased the leaf/stem ratio [38]. On the contrary, the low dry forage yield of alfalfa for the second harvest in 2020 was most probably due to the long-term dryness and high temperature before harvesting during the growing season, compared with 2021 ( Figure 2). To maintain a higher alfalfa yield, our results suggest that Magnum551 and Phabulous could prove the best cultivars to be planted in this saline region of North China at a shallow groundwater depth of 100 cm (Figure 3).

Effects of Shallow Groundwater Depth, Cultivars, and Harvest Times on Alfalfa Nutritive Value
Many studies have reported that forage quality is increased by increasing the CP and RFV, while quality is decreased with increased NDF and ADF content [25,39]. In the present study, a high content of CP was observed in the first harvest. During subsequent harvests, a significant decrease in CP content from the second to fourth harvests was observed, but the highest CP content was noticed in the fifth and sixth harvests (Table 3). This rise in forage CP content is attributed to a weather-induced increase in the ratio of leaf to stem [33]. The nitrogen content of alfalfa stems is low, while the leaves are rich in nitrogen [26]. Thus, increasing the leaf area led to higher CP levels of forage.
In this study, the percentage of crude fiber (NDF and ADF) for cultivar WL525HQ was recorded as the highest, as reported by another researcher for the same cultivar [40]. For instance, reduction of 0.8% and 1.3% in NDF and ADF content was found in the cultivars WL525HQ and Zhongmu No. 2, respectively [40]. In our case, NDF and ADF first increased, and then decreased with increasing harvest frequency, which could also be attributed to an increase or decrease in the leaf/stem ratio [25]. The NDF content increased from 18.6 to 42.6% between the first and fifth harvests of alfalfa, respectively; this has also been reported by Robinson [41].
Alfalfa cultivars and harvest times appear to be the major factors influencing forage quality, indicating that the nutritional value of alfalfa increased slightly with the increasing leaf/stem ratio [40]. The high temperature and precipitation mainly increased plant height and growth rate [42]. The forage quality of dry forage also suggested that cultivar WL525HQ was lower in terms of quality performance than the other four tested cultivars. Forage quality differences may be attributed to the differences in weather conditions (i.e., temperature and precipitation) and soil type. Less precipitation is not favorable for alfalfa nutritional point of view in arid regions; meanwhile, shallow groundwater can provide optimum level of moisture for alfalfa growth when the precipitation is not sufficient [43]. Hence, water stress resulted in reduced forage quality in their study.

Conclusions
The present study evaluated the forage yield, returning green rate, and forage nutritional value of five alfalfa cultivars grown at three shallow groundwater depths in a coastal saline area. The forage yield at a groundwater depth of 100 cm was higher than that at groundwater depths of 80 and 120 cm. Cultivar Phabulous furnished the highest forage yield at a groundwater depth of 100 cm. The proportion of forage yield from the first to fourth harvests in the total annual yield was over 65%. The turn-green ratio of cultivars Zhongmu No. 1 and Zhongmu No. 3 was the highest at a groundwater depth of 100 cm. The forage nutritional value (CP, ADF, and NDF) was significantly affected by the shallow groundwater table depth, cultivar, and their interaction. Based on the outcomes of this study, it is concluded that planting cultivars Magnum551, Zhongmu No. 1, and Phabulous at a groundwater depth of 100 cm appears to be a desirable practice for optimum forage yield and quality and a returning green ratio when alfalfa is being cultivated in saline soil. Further studies involving the variation of groundwater depth and salt content are required to confirm the shallow groundwater extraction and utilization capacity of salt-stressed alfalfa under field conditions.

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article.