Subsurface Drainage and Biochar Amendment Alter Coastal Soil Nitrogen Cycling: Evidence from ¹⁵N Isotope Tracing—A Case Study in Eastern China

Abstract

Subsurface drainage and biochar application are conventional measures for improving saline–alkali soils. However, their combined effects on the fate of nitrogen (N) fertilizers remain unclear. This study investigated the combined effects of subsurface drainage and biochar amendment on the fate of nitrogen (N) in coastal saline–alkali soils, where these conventional remediation measures’ combined impacts on fertilizer N dynamics remain seldom studied. Using ¹⁵N-labeled urea tracing in an alfalfa–soil system, we examined how different drainage spacings (0, 6, 12, and 18 m) and biochar application rates (5, 10, and 15 t/ha) influenced N distribution patterns. Results demonstrated decreasing in drainage spacing and increasing in biochar application; these treatments enhanced ¹⁵N use efficiency on three harvested crops. Drainage showed more sustained effects than biochar. Notably, the combination of 6 m drainage spacing with 15 t/ha biochar application achieved optimal performance of ¹⁵N use, showing N utilization efficiency of 46.0% that significantly compared with most other treatments (p < 0.05). ¹⁵N mass balance analysis revealed that the plant absorption, the soil residual and the loss of applied N accounted for 21.6–46.0%, 38.6–67.5% and 8.5–18.1%, respectively. These findings provide important insights for optimizing nitrogen management in coastal saline–alkali agriculture, demonstrating that strategic integration of subsurface drainage (6 m spacing) with biochar amendment (15 t/ha) can maximize N use efficiency, although potential N losses warrant consideration in field applications.

1. Introduction

More than 1 × 10⁹ hectares of global land are affected by saline–alkali conditions, and this area is expanding at a rate of approximately 2 × 10⁶ hectares per year [1]. By effectively improving and developing these saline–alkali lands into arable land, the cultivated area can be directly increased, alleviating the problem of insufficient food production space. This is especially beneficial in densely populated regions where arable resources are relatively scarce, as the development of saline–alkali land helps expand food-production space and enhance food self-sufficiency [2]. However, saline–alkali land is often characterized by a high salinity, poor soil permeability, and low fertility [3,4]. A crucial pathway to developing saline–alkali agriculture is by using scientific amendments like biochar and engineering approaches like subsurface drainage to rehabilitate saline–alkali land, enhance crop nutrient absorption, and increase crop yields.

Biochar, derived from the pyrolysis of biomass such as wood, straw, husks, manure, and sludge [5], can be used as an ingredient in mixed products or on its own. It has widespread applications in soil improvement and resource-utilization efficiency enhancement [6]. Specifically, biochar can reportedly improve soil moisture retention and crack resistance [7]. Another study has confirmed that applying high amounts of biochar significantly increases soil organic matter, total nitrogen, total phosphorus, water-soluble organic carbon, ammonium nitrogen, available phosphorus, and microbial biomass nitrogen content, thereby enhancing soil fertility and ensuring stable crop yields in dryland areas [8]. Using corn straw biochar can enhance the effectiveness of urea nitrogen, promote soil microbial activity, and improve crop nutrient-absorption efficiency [9]. Given these characteristics of biochar, its application should be considered as one of the means to enhance crop production efficiency on saline–alkali land. However, its efficacy in saline–alkali soils remains underexplored, particularly when combined with drainage.

Subsurface drainage involves the installation of drainage pipes underground to remove excess moisture from the soil, and it is currently widely used to improve coastal saline–alkali land [10,11,12]. First, subsurface drainage effectively prevents waterlogging caused by excessive water accumulation in the soil. Waterlogging can lead to soil hypoxia, inhibiting the respiratory function of crop roots. Second, subsurface drainage can improve soil structure. Long-term water accumulation can result in soil compaction. Conversely, subsurface drainage can restore soil looseness, increase air exchange within the soil, and enhance its permeability and water conductivity, aiding crop roots for better expansion and absorption of nutrients and moisture [13]. Additionally, the soil environment created by subsurface drainage can promote the decomposition of organic matter, enhance soil fertility, and increase nutrient supply. Most importantly, subsurface drainage can remove soluble salts from the soil [14], alleviating the stress caused by salinity.

In agricultural ecosystems, nitrogen serves as a primary growth-limiting nutrient for crops, leading to the intense competition for soil nitrogen between crops and microorganisms [15]. Therefore, nitrogen is a research hotspot in various types of soils and crops [16]. The sources of nitrogen absorbed by plants are twofold: soil and fertilizers. Soils with strong productivity have better synergy between seasonal nitrogen fertilizer input and absorption, meaning that the nitrogen fertilizer applied in the season can be maximally absorbed by the crops [17]. The synergy of biochar and subsurface drainage may alter soil characteristics, creating comfortable conditions for root development and subsequently enhancing crops’ ability to absorb fertilizer nitrogen; however, this process has rarely been studied, and the efficiency of crop fertilizer nitrogen absorption and its distribution after improvement with biochar and subsurface drainage on coastal saline–alkali soils has not been quantitatively evaluated, which has limited the practical assessment of soil improvement efficacy.

In previous studies, ¹⁵N isotope labeling and tracing techniques have often been used to distinguish between nitrogen fertilizer and soil nitrogen [18], especially in research on nitrogen fertilizer utilization efficiency and its behavior. Utilizing ¹⁵N isotope labeling and tracing techniques is beneficial for better understanding the behavior of fertilizer nitrogen. In the present study, we used ¹⁵N-labeled urea as a source of fertilizer, with salt-tolerant alfalfa suitable for planting in coastal saline–alkali land as the plant material. Different spacings of subsurface drainage and varying amounts of biochar were designed to observe the movement and distribution of fertilizer nitrogen (¹⁵N) and to statistically analyze the seasonal crop utilization efficiency of ¹⁵N under different treatments. This work focused on (1) quantifying the differences in alfalfa ¹⁵N utilization efficiency under different spacings of subsurface drainage and the combined application of biochar, (2) clarifying the movement and balance of ¹⁵N under different treatments, and (3) identifying combinations that maximize ¹⁵N use efficiency while minimizing N losses.

2. Materials and Methods

2.1. Experimental Location

The field trial was conducted over an 8-month period (March–October 2024). The trial was performed in the planting area of Cixi Hangzhou Bay Ningbo Haihan Agricultural Development Co., Ltd., located at a geographical coordinate of 30°10′ N latitude and 121°13′ E longitude (Figure 1). Located within the subtropical zone, the region displays four distinct seasons. According to climate data from the past decade, the area receives 1422 mm of rainfall and accumulates 1850 h of sunlight annually. The salinity level of the alkaline soil in the study area ranges from a minimum of 10 g/kg to a maximum of 280 g/kg. Before the experiment, the soil underwent appropriate improvement measures, including leaching irrigation, plant cultivation, and application of organic fertilizers. The precipitation during the experimental period is shown in Figure 2a, and temperature data are depicted in Figure 2b.

Before experiment commencement, the physical and chemical indicators of the soil at a depth of 0–60 cm were measured, and results are shown in

Table 1. Physical and chemical properties of the soil.

Soil Depth /cm	pH	Moisture /%	Bulk Density /g/cm3	Organic Matter /g/kg	Total Nitrogen /g/kg	Total Salt Content /g/kg	Available Nitrogen /mg/kg	Available Phosphorus /mg/kg	Available Potassium /mg/kg
0–20	8.17	23.2	1.43	9.32	0.62	1.61	85.6	9.86	187.5
20–40	8.46	23.6	1.46	8.82	0.49	1.84	74.1	9.94	174.3
40–60	9.12	24.1	1.54	8.41	0.44	2.27	62.3	8.36	166.5

Note: Data represent the mean value of three replications.

and

Table 2. Soil mechanical composition.

Soil Depth/cm	Mechanical Composition (%)
	>0.05 mm	0.05–0.01 mm	0.01–0.005 mm	0.005–0.001 mm	<0.001 mm
0–20	37.2	46.5	9.5	1.2	5.6
20–40	33.4	51.1	9.5	2.8	3.2
40–60	29.6	51.7	10.2	3.1	5.4

Note: Data represent the mean value of three replications.

.

2.2. Experimental Design

The experimental design comprised four spacings of subsurface drainage (0 m (CK), 6 m (S1), 12 m (S2), and 18 m (S3)) combined with three application rates of biochar (5 t/ha (C1), 10 t/ha (C2), and 15 t/ha (C3)), resulting in a total of 4 × 3 = 12 treatments, each replicated three times (Figure 3). The subsurface drainage pipes were made of PVC corrugated pipes wrapped in non-woven fabric, buried at a depth of 1 m, with a gradient of 1‰. The raw material for biochar was peanut shells, carbonized at temperatures ranging from 350 °C to 500 °C. The biochar produced exhibited the following characteristics: pH 9.7, total nitrogen 5.6 g/kg, organic carbon 446.3 g/kg, and specific surface area 9.12 m²/g. Aside from the differences in subsurface drainage spacing and biochar application rates, all other field-management measures for each treatment were retained.

Before sowing, 1000 kg/ha of organic fertilizer was mixed into the top 0–10 cm of soil. The organic fertilizer, formulated through the fermentation of effective microorganisms (EM)-activated solution combined with rice straw, soybean meal, and poultry manure, had a nutrient composition of 4.5% nitrogen, 2.5% phosphorus pentoxide, and 1.5% potassium oxide. For this study, Medicago sativa L. cv. ‘Zhongmu No. 3’ was used as the experimental plant material., and alfalfa seeds were sown on March 12 at a 25 kg/ha density. Irrigation was performed three times throughout the entire experiment, with amounts of 880, 960, and 830 m³/ha on 27 March, 2 July, and 17 August, respectively. Alfalfa was harvested three times on 4 June, 29 July, and 2 October.

A fragmented planting design was implemented, comprising multiple 60 m × 40 m (2400 m²) cultivation units. A 60 cm deep isolation membrane separated the fragments to prevent lateral moisture seepage. Each spacing of the drainage pipes occupied one fragment, and a 6 m × 6 m sub-area was set up in the middle of adjacent drainage pipes within the same fragment for the application of biochar. Each fragment contained three sub-areas, applying three different amounts of biochar. In the center of each sub-area, a 1 m×1 m micro-area was established, where 75 kg/ha of ¹⁵N double-labeled urea (abundance 19.6%) was applied. Each treatment was replicated three times, resulting in a total of 36 micro-areas.

2.3. Measurement Projects and Methods

At each harvesting of alfalfa, a random sample of 500 g was collected and subjected to high-temperature killing at 105 °C for 30 min. After the killing process, the temperature was lowered to 75 °C for drying until a constant weight was achieved. The dried samples were mechanically homogenized and passed through a 100-mesh sieve for measuring the total nitrogen and ¹⁵N atom percentage in the plants. After the last harvest of alfalfa, soil samples were collected layer by layer within the micro-area using a five-point sampling method with a soil auger at depths of 0–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm. After collection, soil samples from the same layer were mixed uniformly, dried, and ground to be used for measuring soil ¹⁵N levels.

The total nitrogen in plant and soil samples was determined using the Kjeldahl method [19] after digestion with H₂SO₄-H₂O₂. The ¹⁵N atom percentage in the dry samples of soil and plants was measured using a Finnigan-MAT-251 isotope-ratio mass spectrometer (Finnigan, Bremen, Germany).

2.4. Data Calculation

The calculation method for crop fertilizer nitrogen-utilization efficiency (¹⁵N utilization efficiency) was as follows [20]:

$$N\mathrm{dff}={\mathrm{C}}_{\mathrm{s}}\times {\displaystyle \frac{{\mathrm{E}}_{\mathrm{s}}}{{\mathrm{E}}_{\mathrm{f}}}}$$

(1)

$$NUE=\left({\displaystyle \frac{N\mathrm{dff}}{M_{\mathrm{f}}}}\right)\times 100\%$$

(2)

where Ndff is the amount of nitrogen in alfalfa plants derived from ¹⁵N labeled urea (kg/ha), C_s is the total nitrogen content in the plant samples (kg/ha), E_s is the ¹⁵N atom percentage in the plant samples (%), E_f is the ¹⁵N atom percentage of the labeled nitrogen fertilizer (%), and M_f is the ¹⁵N application amount (kg/ha).

The principle of ¹⁵N balance was that the input of ¹⁵N equals the sum of the loss of ¹⁵N and the recovery of ¹⁵N. The amount of ¹⁵N recovery was the total amount of ¹⁵N absorbed by the plants plus the total amount of ¹⁵N in the soil from 0 to 80 cm [19].

2.5. Data Analysis

Significant differences (p < 0.05) among treatments were identified through Duncan’s multiple-range test with SPSS 17.0 software.

3. Results

3.1. Impacts of Different Spacings of Drainage Pipes and Biochar Application on ¹⁵N Absorption by Alfalfa

The amount of ¹⁵N absorbed by the three cuts of alfalfa was highest in the second cut, followed by the third cut, whereas the first cut was at a relatively low level (Figure 4). The differences between treatments were evident in the first cut. Compared with the no-drainage-pipe treatment (CK), the drainage-pipe treatments significantly increased the ¹⁵N absorption in the first cut of alfalfa. The amounts of ¹⁵N absorbed under treatments S1, S2, and S3 in the first cut were 6.4, 5.7, and 4.7 kg/ha, respectively, which were 43.7%, 37.1%, and 24.0% higher than those under CK (3.6 kg/ha). Under the same drainage-pipe spacing, increasing the amount of biochar resulted in a 9.5–24.3% increase in ¹⁵N absorption in the first cut of alfalfa.

The differences in ¹⁵N absorption in the second cut of alfalfa among treatments were more pronounced than in the first cut. Specifically, as the spacing between drainage pipes gradually decreased, the overall ¹⁵N absorption displayed a marked increase (p < 0.05); when the amount of biochar applied increased, the ¹⁵N absorption of alfalfa also increased, but this increase reached a significant (p < 0.05) level only when biochar application increased from C1 to C3. From the perspective of promoting ¹⁵N absorption by alfalfa, the drainage-pipe spacing had a greater impact on the second cut than the application of biochar. Under the drainage pipe treatments, the average ¹⁵N absorption for S1, S2, and S3 was 14.1, 12.8, and 10.6 kg/ha, respectively, higher than CK (8.2 kg/ha).

With decreased drainage-pipe spacing from S3 to S1, the ¹⁵N absorption of the third cut of alfalfa significantly (p < 0.05) increased, but when it decreased from S2 to S1, this difference was significant (p < 0.05) only at the C3 biochar application level. Compared with drainage pipes, the effect of biochar application on ¹⁵N absorption by alfalfa was relatively small, which was similar to the results observed in the second cut. In most cases, increasing the amount from C1 to C2 or from C2 to C3 did not result in a significant increase in ¹⁵N absorption by alfalfa, with a significant increase (p < 0.05) occurring only with increased biochar application from C1 to C3. The drainage pipes also promoted ¹⁵N absorption in the third cut of alfalfa, with absorption amounts of 11.9, 10.8, and 8.9 kg/ha under S1, S2, and S3, respectively, all found to be higher than those under CK (6.8 kg/ha).

A synergistic effect of drainage pipe and biochar application was observed among all 12 treatments. This synergy maximally promoted ¹⁵N absorption in all three cuts of alfalfa. Under the combination of S1 and C3, the absorption amounts of alfalfa for ¹⁵N reached their highest values of 6.8 (kg/ha) (first cut), 15.0 (kg/ha) (second cut), and 12.8 (kg/ha) (third cut). Compared with the CKC1 treatment, the ¹⁵N absorption amounts for the first, second, and third cuts of alfalfa under the S1C3 treatment increased by 116.0%, 111.9%, and 113.1%, respectively.

3.2. Impacts of Different Spacings of Drainage Pipes and Biochar Application on Soil ¹⁵N Distribution in Profile

With increased spacing of drainage pipes from S1 to S3, the total residual amount of ¹⁵N in the 0–80 cm profile significantly increased (Figure 5). Under S1 conditions, the average total residual amount in the profile was 30.2 kg/ha, whereas under S3, it increased to 40.9 kg/ha (C1), with an increase of 26.2%. However, no significant differences (p > 0.05) were observed in the total residual amount (37.3–39.5 kg/ha) among the three biochar application rates.

The effect of drainpipe spacing on the 0–20 cm soil layer was similarly significant. The ¹⁵N residue in the 0–20 cm soil layer under the S1 treatment was only 9.6–11.2 kg/ha, whereas under the S3 spacing, the ¹⁵N residue reached 16.9–18.2 kg/ha. The ¹⁵N residue under the CK treatment was even twice as high as that under S1, indicating that installing drainpipes especially with smaller spacing reduced the ¹⁵N residue in the plow layer. The amount of biochar also affected the ¹⁵N residue in the plow layer, but the impact was much smaller than that of the drainpipe spacing. Specifically, increased biochar amount slightly decreased the ¹⁵N residue in the 0–20 cm soil layer.

For the two lower sampling layers (40–60 and 60–80 cm depth), which were beyond the main root zone of alfalfa, the effect of drainpipe spacing on the ¹⁵N residue was not significant in the 40–60 cm layer, where the S3 (4.3–5.4 kg/ha) and CK (4.6–5.1 kg/ha) treatments were slightly lower than the other two treatments. The amount of biochar also had no significant impact on the ¹⁵N residue in the 40–60 cm soil layer. In the 60–80 cm soil layer, the ¹⁵N residue increased with decreased drainpipe spacing. Under the S3 treatment, the ¹⁵N residue was 2.9–4.1 kg/ha, whereas under the S1 treatment, it increased to 4.1–6.3 kg/ha, indicating that smaller drainpipe spacing promoted the migration of ¹⁵N to deeper soil layers.

3.3. Impacts of Different Drainpipe Spacings and Biochar Application on Alfalfa ¹⁵N Utilization Efficiency

Under the S1 spacing, the ¹⁵N utilization efficiency of alfalfa plants ranged from 40.5% to 46.0%, demonstrating statistically significant (p < 0.05) elevation compared with CK (21.6% to 28.4%), with an increase of 87.7% to 113.1% (Figure 6). Similarly, under the S2 spacing, the ¹⁵N utilization efficiency of alfalfa plants (36.5% to 41.2%) was also significantly (p < 0.05) higher than that of CK, with an increase of 69.1% to 90.7%. Although the ¹⁵N utilization efficiency of alfalfa plants under the S3 spacing (31.2% to 35.0%) was also greater than that of CK, the increase was relatively smaller, ranging from 44.4% to 61.9%. Therefore, drainpipe drainage can significantly improve alfalfa’s ¹⁵N utilization efficiency, but the enhancement effect diminished as the drainpipe spacing increased.

Under the same drainpipe spacing, the ¹⁵N utilization efficiency of alfalfa plants exhibited a positive correlation with biochar application rates. For example, under the S1 treatment, the ¹⁵N utilization efficiency of C2 and C3 treatments increased by 6.6% and 13.6%, respectively, compared with the C1 treatment. Under the S2 treatment, the ¹⁵N utilization efficiency of C2 and C3 treatments increased by 8.0% and 12.8%, respectively, compared with C1. In the S3 treatment, the C3 treatment increased by 12.1% compared with C1. Notably, the impact of biochar application on alfalfa’s ¹⁵N utilization efficiency was less significant than that of drainpipe spacing.

From the perspective of improving ¹⁵N utilization efficiency, the S1C3 treatment showed the best effect on alfalfa nitrogen-utilization efficiency. Under this treatment, the ¹⁵N utilization efficiency was 46.0%, which did not differ significantly from that under S3C2 treatment (p > 0.05), while surpassing other treatments.

The crop ¹⁵N use rate showed a negative correlation with the residual ¹⁵N content in the 0–20 and 20–40 cm soil layers (Figure 7), indicating a balance between the crop ¹⁵N uptake and the residual ¹⁵N in shallow soil layers. However, no significant correlation was observed between the crop ¹⁵N use rate and the residual ¹⁵N content in 20–40 and 40–60 cm soil layers. Additionally, a marked positive correlation was found between crop ¹⁵N use rate and ¹⁵N uptake by the three harvests of alfalfa.

3.4. Balance of ¹⁵N Under Different Drainpipe Space and Biochar Application

Under different drainpipe spacings and biochar application conditions, the balance of ¹⁵N (including crop absorption, soil residue, and loss) showed variations (

Table 3. ¹⁵N balance under different drainpipe spacings and biochar application conditions.

Drainage Spacing	Biochar Application Rate	Total Applied 15N (kg/ha)	Crop Absorbed 15N (kg/ha)	Soil Residual 15N (kg/ha)	15N Loss (kg/ha)
S1	C1	75	30.40 ± 1.56 bc	31.03 ± 0.81 g	13.57 ± 0.96 a
	C2	75	32.40 ± 0.98 ab	30.67 ± 0.34 g	11.93 ± 1.31 bc
	C3	75	34.53 ± 1.72 a	28.97 ± 1.18 h	11.50 ± 0.54 bcd
S2	C1	75	27.40 ± 0.54 de	35.37 ± 0.05 e	12.23 ± 0.49 ab
	C2	75	29.60 ± 1.14 cd	33.93 ± 0.65 ef	11.47 ± 0.49 bcd
	C3	75	30.90 ± 1.14 bc	32.97 ± 0.65 f	11.13 ± 0.49 bcd
S3	C1	75	23.40 ± 1.80 f	41.10 ± 1.14 cd	10.50 ± 0.65 cd
	C2	75	22.97 ± 1.75 f	41.93 ± 1.28 c	10.10 ± 0.62 de
	C3	75	26.23 ± 0.94 e	39.73 ± 0.45 d	9.03 ± 0.49 ef
CK	C1	75	16.20 ± 0.33 g	50.63 ± 0.29 a	8.17 ± 0.61 f
	C2	75	18.33 ± 0.94 g	49.90 ± 0.62 a	6.77 ± 0.33 g
	C3	75	21.27 ± 0.74 f	47.37 ± 0.29 b	6.37 ± 0.45 g

Note: CK, S1, S2, and S3 represent drainpipe spacings of 0 m (no drainpipe), 6 m, 12 m, and 18 m, respectively; C1, C2, and C3 represent biochar application rates of 5, 10, and 15 t/ha, respectively. Significant differences (p < 0.05) were marked by different superscript letters (a–g), based on Duncan’s multiple range test. The values in the table represent the mean ± standard deviation.

). Compared with CK, the drainage-pipe treatment increased the absorption of ¹⁵N by alfalfa plants while reducing the ¹⁵N residue in the soil, but it also increased the ¹⁵N loss. This pattern became more pronounced as the drainpipe spacing decreased. Under the same drainpipe spacing, increasing the biochar application rate generally enhanced alfalfa’s ¹⁵N absorption while slightly reducing the ¹⁵N residue in the soil and the ¹⁵N loss.

Under different drainpipe spacings and biochar application rates, 21.6% to 46.0% of the exogenous ¹⁵N applied was absorbed by the crop, 38.6% to 67.5% remained in the soil, and 8.5% to 18.1% was lost through leaching, gaseous losses, or other unknown losses (Figure 8). A good treatment was considered one that can increase the crop’s ¹⁵N absorption ratio while decreasing the ¹⁵N soil residue and loss ratios. However, according to the results of this study, no treatment exhibited all these beneficial effects simultaneously.

4. Discussion

Alfalfa is one of the suitable crops for planting in coastal saline–alkali soils. It is characterized by its salt tolerance and ease of management. After harvesting, alfalfa can serve as forage for livestock. Therefore, alfalfa yield is one of the key concerns of the local coastal agricultural industry. Similarly to other crops, alfalfa yield is closely related to nitrogen supply. This study proposed using alfalfa’s nitrogen absorption as an indicator to assess the soil’s production capacity and evaluate the effects of measures like subsurface drainage and biochar application on the improvement in coastal saline–alkali soils. Furthermore, the fate of externally applied ¹⁵N in the alfalfa–soil system under these two measures was investigated.

Our study found that increasing the amount of biochar promoted alfalfa’s absorption of ¹⁵N owing to the following reasons. (1) The abundant functional groups on the surface of biochar give it strong adsorption capacity, improving soil aggregate structure, enhancing aggregate stability, increasing soil porosity [21], and promoting root growth and development of alfalfa, thereby increasing its nutrient absorption. (2) The application of biochar increased the retention of water and ¹⁵N nutrients in the soil, thereby increasing the amount of ¹⁵N available for alfalfa absorption [22]. Xie [23] demonstrated that biochar may promote plant growth and improve nitrogen-use efficiency by increasing nutrient availability and enhancing microbial activity. (3) Coastal saline–alkali soils contain Na⁺, and increased biochar application reduces Na+ stress on plants [24]. Subsurface pipe drainage physically removed portion of Na⁺ from the plow layer through leaching effects, and its combination (with biochar) may synergistically enhance the mitigation of Na⁺ stress.

Notably, the promoting effect of biochar on ¹⁵N absorption weakened from the first to the third harvest. The reason may be that with the movement of soil moisture and root growth, more biochar migrated into areas outside the root zone, reducing its beneficial effects in the root zone. Conversely, drainpipe drainage continued to significantly promote alfalfa’s absorption of ¹⁵N during the second and third harvests. Some studies suggest that the installation of drainpipes promotes the decomposition and transformation of soil organic matter, increasing nutrient mineralization and thus the mineral nitrogen content in the plow layer [25], which may increase alfalfa’s absorption of ¹⁵N. However, other researchers argue that increased drainage leads to the loss of easily soluble mineral nutrients in the plow layer [26], which can reduce alfalfa’s absorption of ¹⁵N. This study leaned toward the idea that drainpipe drainage, especially with smaller spacings, helped reduce salinity in the root zone, alleviating salt stress on the crops and promoting the absorption of water, nutrients, and dry matter accumulation. An earlier study demonstrated that the 33.96–46.02% of soil salt storage changes in the 0–150 cm were drained away by subsurface pipes [27]. The process of accumulating dry matter required extracting large amounts of nitrogen, including the applied ¹⁵N, from the soil.

Our result further showed that drainpipe drainage and biochar application affected soil ¹⁵N residues. However, drainpipe drainage had a more significant effect on the ¹⁵N in the entire soil profile, whereas the effect of biochar on ¹⁵N was limited to the plow layer (Figure 5). The reason may be the direct effect of drainpipe drainage on soil ¹⁵N residues; by contrast, the effect of biochar on ¹⁵N residues was indirect. Drainpipe drainage influenced water movement throughout the entire soil profile above the pipes, with water acting as the carrier for ¹⁵N, directly affecting where ¹⁵N was ultimately retained in the soil layers. Meanwhile, biochar’s effect on ¹⁵N retention in deeper soil layers may be due to its promotion of alfalfa’s absorption of ¹⁵N (Figure 4), which led to a decrease in the amount of ¹⁵N remaining in the surface layer.

The subsurface drainage significantly improved the alfalfa plants’ efficiency in utilizing ¹⁵N. The reduced drainage-pipe spacing resulted in greater improvements in ¹⁵N use efficiency because of the alternating wet and dry conditions that formed with smaller pipe spacing. They were more conducive to the mineralization of ¹⁵N [28], thereby promoting the crop’s absorption of ¹⁵N. Additionally, subsurface drainage improved the root environment for crop absorption, such as porosity, oxygen, moisture content, and salinity [29], which may also be an important reason for the increase in ¹⁵N utilization efficiency. Our study demonstrated that the application of biochar can improve alfalfa’s ¹⁵N utilization efficiency, consistent with the findings of Yan [30] and Shi [31]. Yan showed through column-leaching simulations that biochar helps slow the release of nitrogen from ammonium sulfate in chemical fertilizers, thereby reducing nitrogen loss and increasing crop efficiency in utilizing external nitrogen. Shi [31] showed that biochar significantly improves soil bacterial communities, increasing the abundance of nitrogen conversion functional genes. In turn, fertilizer nitrogen transformed into the soil mineral nitrogen pool. From an agronomic perspective, the increased nitrogen use efficiency of crops is of paramount importance. In this study, crop nitrogen use efficiency was closely associated with soil ¹⁵N residual content and ¹⁵N losses. Taking biochar application as an example, the biochar application rate was found to have a highly significant (p < 0.01) effect on soil ¹⁵N residue. Higher biochar application rates led to greater utilization amount of current-season fertilizer nitrogen by crops, which simultaneously reduced nitrogen residue in soil and decreased the risk of ¹⁵N losses.

Our experiments revealed that the ¹⁵N residual proportion at 40–60 and 60–80 cm depths in the S1 treatment (accounting for the total ¹⁵N in the 0–80 cm soil layer) was significantly higher than those in the other two subsurface drainage treatments. The reason may be that under this spacing, water infiltration was smoother, but it can also increase the risk of ¹⁵N leaching. The higher total ¹⁵N loss in the S1 spacing (Figure 7) supported this hypothesis. Overall, the ¹⁵N residual in the 60–80 cm soil layer was relatively low, so we inferred that the total loss of ¹⁵N was primarily from gaseous losses, with less leaching to soil layers below 80 cm. We further speculated that the slight reduction in ¹⁵N loss after biochar application may be due to its control over volatilization losses.

In this study, no treatment simultaneously promoted alfalfa’s ¹⁵N absorption while reducing ¹⁵N residue or decreasing ¹⁵N losses. Therefore, although the 6 m spacing and 15 t/ha biochar application rate were satisfactory choices in terms of improving ¹⁵N utilization efficiency, in practical applications, attention should be paid to the potential loss risks of external nitrogen under this treatment. Moreover, gaseous and leaching losses were difficult to monitor in this work, leading to the estimation of losses by subtracting the soil residual ¹⁵N and crop absorbed ¹⁵N from the total ¹⁵N. Future studies should refine the observation of ¹⁵N losses.

A limitation in our study was the relatively small scale of the ¹⁵N-labeled experimental plots. The larger trial areas would yield findings more representative of actual production conditions. Practically, the coastal saline-alkali soil reclamation must prioritize cost considerations, particularly biochar expenses, due to the vast scale of coastal farmlands requiring treatment and the correspondingly massive biochar quantities needed. For countries like China that actively promote coastal saline soil cultivation as long-term arable land, inputting higher initial costs (even with short-term economic losses) was acceptable. Conversely, nations utilizing such lands only for emergency or intermittent cultivation should conduct thorough cost–benefit analyses before implementation.

5. Conclusions

Narrowing the spacing of drainage pipes and increasing the application of biochar promoted the ¹⁵N absorption in three cuts of alfalfa. The effect of drainage pipes was more persistent than that of biochar. Compared with the control (CK) treatment (3.6, 8.2, and 6.8 kg/ha), the ¹⁵N absorption by alfalfa under drainage pipe treatments (4.7–6.4, 10.6–14.1, and 8.9–11.9 kg/ha for the first, second, and third cuts, respectively) was significantly higher. Drainage pipes had a noticeable effect on the total ¹⁵N residue in the 0–80 cm soil profile, and narrower spacing of drainage pipes promoted the migration of ¹⁵N from shallow to deeper soil layers. However, biochar application had only a slight effect on the ¹⁵N residue in the 0–20 cm soil layer and no significant effect on the total ¹⁵N residue. Among all treatments, S1C3 showed the best improvement in alfalfa nitrogen-use efficiency. Under this treatment, the alfalfa ¹⁵N utilization efficiency was 46.0%, statistically comparable with that of S3C2 while demonstrating significant (p < 0.05) superiority over remaining treatments. Under the influence of different drainage-pipe spacings and biochar applications, 21.6–46.0% of the exogenous ¹⁵N was absorbed by the crops, 38.6–67.5% remained in the soil, and 8.5–18.1% was lost through leaching, gaseous emissions, or other unknown losses that were estimated rather than directly measured. Our comprehensive results indicated that drainage-pipe installation combined with biochar application had different effects on the fate of ¹⁵N. A 6 m spacing and 15 t/ha biochar application promoted the absorption of exogenous nitrogen by alfalfa and improved nitrogen-use efficiency. However, the potential risk of ¹⁵N loss should be considered in practical applications of these combined measures.