Optimizing N Application for Forage Sorghum to Maximize Yield, Quality, and N Use Efficiency While Reducing Environmental Costs

Abstract

Investigating the responses of forage crop yield, quality, and nitrogen (N) use efficiency to different N application rates is beneficial for guiding proper N fertilization regimes and for reducing reactive N environmental pollution. A field experiment was conducted to investigate the effects of different N application rates on above-ground dry matter yield, forage quality, crop N uptake, N use efficiency (NUE), and ecosystem economic benefits (EEBs) of forage sorghum cultivated on the Longdong Loess Plateau in 2019 and 2020. Five N application rates were tested, namely 0, 80, 160, 240, and 320 kg·ha⁻¹ (referred to as N₀, N₈₀, N₁₆₀, N₂₄₀, and N₃₂₀, respectively). The maximum above-ground dry matter yield (22.3 t·ha⁻¹ in 2019 and 18.0 t·ha⁻¹ in 2020) was obtained at an N application of 160 kg·ha⁻¹. Forage sorghum crude protein (CP) content increased significantly with increasing N application rates (the CP content at N₃₂₀ was 7.4% and 8.6% in 2019 and 2020, respectively). In contrast, neutral detergent fiber (NDF) and acid detergent fiber (ADF) were only affected by high N application rates (NDF and ADF were significantly higher in N₃₂₀ compared with N₀ and N₉₀). The relative feed value (RFV) was significantly higher in N₀ compared with N₃₂₀. Crop N uptake was significantly higher in N₁₆₀ compared with N₀ (25.7% increase to 249.4 kg·ha⁻¹ in 2019 and 40.5% increase to 247.4 kg·ha⁻¹ in 2020, respectively). NUE decreased linearly as N rates increased, but NO₃⁻–N residue (0–200 cm), reactive N loss (Nr loss), and greenhouse gas (GHG) emissions increased. Private profitability and EEB were the largest at N₁₆₀ (private profitability at N₁₆₀ was 514.2 USD·ha⁻¹, and EEB at N₁₆₀ was 392.7 USD·ha⁻¹). Above-ground yield and optimum forage quality must be maximized, while simultaneously safeguarding farmer income and reducing environmental pollution from N fertilizers. Therefore, the optimum N application rate for forage sorghum cultivation in the dry areas of the Loess Plateau is recommended at 160 kg·ha⁻¹.

1. Introduction

Sorghum is the fifth largest food crop in the world. Global sorghum production in 2021 was 6216.7 × 10⁴ t, of which about 44% was used as forage, while the Americas and Asia are the major producing regions accounting for 83.8% and 10.9% of global forage sorghum production during 2000–2014, respectively (data source: official website of Food and Agriculture Organization of the United Nations). The forage sorghum industry fulfills two primary functions, namely production and ecology. On the one hand, forage sorghum is an important food source for livestock, and its various nutrients and crude fibers contained are irreplaceable by other feeds and grains. On the other hand, the rotation of sorghum with legumes (e.g., soybean) effectively improves soil fertility and nitrogen (N) stocks and aids in soil and water conservation [1]. Therefore, ensuring that enough forage is available is the key to the high-quality development of the livestock industry. Human living standards are continuously improving as the economy develops, and the demand for animal products, such as meat, eggs, and milk is increasing annually. For example, the production of meat, eggs, and milk in China during 2021 was 66.2% higher than 20 years before (data source: official website of National Bureau of Statistics of China). However, the shortage of high-quality forage has become a limiting factor for the sustainable management of China’s livestock industry. For example, domestic soybean production in 2020 was 1.96 × 10⁷ t, while the amount of imported soybeans (1.00 × 10⁸ t) was five times higher than domestic production (data source: official website of National Bureau of Statistics of China). Forage sorghum has increasingly been considered as the ideal feedstock, since it produces a high biomass and is well-adapted to growth conditions with high soil salinity and low precipitation [2]. Forage sorghum roots are able to access water in soils as deep as 270 cm [3]. This can increase yields by up to 540% compared with maize during severe drought conditions [4]. Forage sorghum can therefore be used effectively in arid and semi-arid areas. Forage sorghum also has a high nutritional value and its consumption thus leads to better livestock performance [5] Miron et al. [6] reported that feeding sorghum to cows increased their milk fat content by 9.2% compared with cows raised on corn. As a result, an increased usage of forage sorghum can significantly reduce the feed shortfall.

Cultivation practices such as fertilization and irrigation are essential to improve forage biomass and quality, as well as nutrient use efficiency. N is the primary nutrient that determines plant productivity [7]. A properly applied N amount can improve crop light energy usage, thus affecting photosynthetic product accumulation. The optimal amount of N required for crops depends on the region and variety. Sawargaonkar et al. [3] reported the optimum N application rate for forage sorghum to be 90 kg·ha⁻¹. Marsalis et al. [8] indicated that maximum dry matter yield (24.6 t·ha⁻¹) can be achieved with an N application rate of 218 kg·ha⁻¹. However, excessive N fertilizer application can limit crop yield and reduce N use efficiency (NUE) [9,10]. Most studies have indicated that a proper N application rate effectively increases the crude protein (CP) content of forage sorghum. Nematpour et al. [11] reported that an N application rate of 112.5 kg·ha⁻¹ increased the CP content of sorghum by 48.8% compared with no N application. N application and its effects on neutral detergent fiber (NDF), acid detergent fiber (ADF), and crude ash (ASH) remain controversial. Specifically, the NDF and ADF content of sorghum treated with N can be reduced by up to 6.0% and 5.8%, respectively, compared with no N application [12]; however, Sher et al. [13] indicated the opposite, namely that N application increases sorghum ADF and NDF content. Most studies have concluded that sorghum ADF, NDF, and ASH content are influenced by genes rather than N application [8,14]. Therefore, further research is needed to investigate how N application affects forage sorghum quality.

Currently, excessive N application in China accounts for 20% of the cultivated area, and national N fertilizer application levels have been increasing. Projections indicate that this will continue until 2050 [15]. Excessive N application results in significant soil NO₃⁻–N accumulation [16] and thus decreased NUE. Increased soil N levels that remain unavailable to crops can lead to environmental problems such as soil acidification, groundwater contamination, and greenhouse gas (GHG) emissions [17,18,19]. Currently, no research has been conducted to the environmental effects of reactive N (Nr) losses and GHG emissions from N fertilization during forage sorghum growth.

The Longdong Loess Plateau is an important area for livestock development in Gansu Province, and forage sorghum is kind of ideal fodder crops for cultivation in this area due to its high yield, quality, and strong stress resistances. Currently, the appropriate N fertilizer rates to maximize crop yield and forage quality while minimizing environmental costs in this area have not been determined. We thus conducted a two-year continuous field experiment to determine how different N application rates affect forage sorghum agronomic parameters (above-ground dry matter yield, forage quality, and N uptake), NUE, environmental impacts (Nr losses, GHG emissions), and ecosystem economic benefits (EEBs). These results are essential for identifying optimal N management strategies and to promote sustainable forage sorghum production with low environmental costs.

2. Materials and Methods

2.1. Site Description

The experiment was conducted during 2019 and 2020 at the Loess Plateau Research Station of Lanzhou University (35°39′ N, 107°51′ E) in Qingyang city, Gansu province, China. The experimental site is located in a rainfed agriculture region. According to meteorological data (2001–2020), the mean precipitation and air temperature in this area is 579.1 mm and 10.1 °C, respectively. Monthly precipitation and air temperature for 2019 and 2020 are provided in Figure 1, and total precipitation during the forage sorghum growing season was 590.1 and 472.1 mm during 2019 and 2020, respectively. The 0–60 cm soil layer characteristics were as follows: bulk density 1.36 g·cm⁻³, organic matter content 15.30 g·kg⁻¹, total soil N 0.88 g·kg⁻¹, NO₃⁻–N 49.53 mg·kg⁻¹, available P (Olsen-P) 21.53 mg·kg⁻¹, and pH 8.2.

2.2. Experimental Design and Management

The experiment was conducted in a randomized complete block design with five N rates of 0, 80, 160, 240, and 320 kg·ha⁻¹ (expressed as N₀, N₈₀, N₁₆₀, N₂₄₀, and N₃₂₀). Each treatment was conducted in triplicate. Urea (N, 46%) was applied in two applications, with 30% of the N fertilizer applied as a basal fertilizer and 70% applied at the jointing stage. Superphosphate (P₂O₅, 16%) and potassium sulfate (K₂O, 51%) were used as basal fertilizer for all plots with a single application of 120 kg·ha⁻¹ P₂O₅ and 150 kg·ha⁻¹ K₂O, respectively.

The forage sorghum cultivar ‘F10’ was used. ‘F10’ is a promising variety with salinity and drought tolerance. The experimental plot area was 24 m² (4 m × 6 m). The planting density was 67,500 plants·ha⁻¹ with a row spacing of 0.5 m and a plant spacing of 0.3 m. In 2019, forage sorghum was sown on 26 May and harvested on 20 October. In 2020, sorghum was sown on 19 April and harvested on 18 September. In both years, ploughing was performed after crop harvest. The predecessor crop to sorghum in 2018 were winter wheat and the two years of experiments were conducted in the same plot. No irrigation or pesticides were used during the experimental period.

2.3. Measurements and Calculations

2.3.1. Crop Sample Collection and Measurement

Plant height, stem diameter, NDVI, and LAI were all measured approximately every 15 days. Plant height and stem diameter were measured on five selected representative plants in each plot using a straightedge and vernier caliper. Plant NDVI and LAI were measured using a Green Seeker handheld optical sensing instrument (DELTRAN, Deland, FL, USA) and LAI-2000 (LI-COR, Lincoln, NE, USA). Five plants were randomly selected from each plot at the jointing, heading, flowering, filling, and harvesting stage, and the five plants were subdivided into stems, leaves, and ears, cut into lengths of 3–5 cm and weighed respectively. They were oven-dried at 105 °C for 30 min, then at 75 °C until a constant weight was obtained, and finally weighed again. The forage sorghum was harvested at the milk stage. Fresh yield was measured for each plot and above-ground dry matter yield was obtained after measuring water content. The moisture content of the above-ground portion of forage sorghum at the harvest stage in 2019 and 2020 was 69.9% and 69.4%, respectively. Dry samples were crushed and passed through a 0.425 mm mesh, and N concentration, neutral NDF, ADF, and ASH were determined using the Kjeldahl, Van Soest’s, and high-temperature scorching methods [20], respectively. One mixed sample was analyzed for each plot.

Quality indices for animal feed, including dry matter intake (DMI), dry matter digestibility (DMD), and relative feed value (RFV) were calculated using the following formulas [21,22]:

$$\mathrm{DMI} \left(\%\right)=\frac{120}{\mathrm{NDF}}$$

(1)

$$\mathrm{DMD} \left(\%\right)=88.9-0.779\times \mathrm{ADF}$$

(2)

$$\mathrm{RFV} \left(\%\right)=\mathrm{DMD}\times \frac{\mathrm{DMI}}{1.29}$$

(3)

2.3.2. Evaluation of NUE

The N recovery rate (RE) and partial-factor productivity of applied N (PFP) were used to evaluate NUE, and these indicators were calculated as follows [23]:

$$\mathrm{RE} \left(\%\right)=\left(\frac{{\mathrm{U}}_{\mathrm{N}}-{\mathrm{U}}_0}{\mathrm{N}}\right)\times 100\%$$

(4)

$$\mathrm{PFP} \left(\mathrm{kg}·{\mathrm{kg}}^{-1}\right)=\frac{{\mathrm{Y}}_{\mathrm{N}}}{\mathrm{N}}$$

(5)

where U_N is N uptake (kg·ha⁻¹) with N fertilizer, U₀ is N uptake (kg·ha⁻¹) without N fertilizer, Y_N is above-ground dry matter yield (kg·ha⁻¹) with N fertilizer, and N is N fertilizer input (kg N·ha⁻¹).

2.3.3. Soil Sample Collection and Measurement

Soil samples were collected between a depth of 0–200 cm at 20 cm intervals on pre-sowing dates, as well as three growth stages (jointing, flowering, and harvesting). Soil samples were collected, and sealed in plastic bags, from two randomly selected locations per plot and stored in a refrigerator until required for analysis. Soil NO₃⁻–N content was determined using an automatic discontinuous chemical analyzer (Smart-Chem 450, French). The NO₃⁻–N residue (NR) in each soil layer was calculated as follows [24]:

$$\mathrm{NR}\left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)={\mathrm{NC}}_{\mathrm{i}}\times {\mathrm{BD}}_{\mathrm{i}}\times \mathrm{SD}\times 0.1$$

(6)

where NC_i is the soil NO₃⁻–N content of the ith soil layer (mg·kg⁻¹), BD_i is the soil bulk density of the ith soil layer (g·cm⁻³), SD is soil depth (cm), and 0.1 is a conversion factor.

2.3.4. Reactive N Loss and Footprint Calculations

Nr loss and NF were calculated using the following equations [25]:

$$\mathrm{Nr} \mathrm{loss} \left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{m}}{ \mathrm{Rate}}_{\mathrm{i}}\times {\mathrm{F}}_{\mathrm{i}}+{\mathrm{N}}_2{\mathrm{O}}_{\mathrm{direct}}–\mathrm{N}+{\mathrm{NO}}_3–\mathrm{N}+{\mathrm{NH}}_3–\mathrm{N}$$

(7)

$$\mathrm{NF} \left(\mathrm{kg} \mathrm{N}·{\mathrm{t}}^{-1}\right)=\mathrm{Nr} \mathrm{loss}/{\mathrm{Y}}_{\mathrm{N}}$$

(8)

where i represents agricultural input (N fertilizer and other inputs), Rate_i is the agricultural materials input application rate, and F_i represents the Nr emission factor during the production and transportation of agricultural products. Rate_i and F_i values for this study are listed in

Table 1. Application rate of agricultural inputs in the forage sorghum production system.

Agricultural Materials	N Fertilizer (kg N·ha−1)	P Fertilizer (kg P2O5·ha−1)	K Fertilizer (kg K2O·ha−1)
Rate	0/80/160/240/320	120	150

and

Table A1. Nr losses and GHG emission for production and transportation of various agricultural inputs in the forage sorghum production system.

Item	Unit	Nr Losses (10−3 kg N·Unit−1)	GHG Gases Emission (kg CO2 eq·Unit−1)	Reference
N production and transportation	kg N	7.15	8.30	Zhang et al. (2013) Cui et al. (2013) Yue et al. (2013)
P production and transportation	Kg P2O5	0.184	0.79
K production and transportation	Kg K2O	0.146	0.55

[26,27,28]. Y_N represents the total above-ground dry matter yield with the application of N fertilizer (t·ha⁻¹). N₂O_direct–N, NO₃–N, and NH₃–N are Nr losses due to N₂O emissions, NO₃⁻ leaching, and NH₃ volatilization during N fertilizer application. Nr losses, represented by N₂O_direct–N, NO₃–N, and NH₃–N from N fertilizer, were calculated according to fitted models based on previously published reports (Figure A1) [29,30,31,32,33,34,35,36,37,38]. The models were:

$${\mathrm{N}}_2{\mathrm{O}}_{\mathrm{driect}}–\mathrm{N} \left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)=0.0195\times \mathrm{N}+0.3095$$

(9)

$${\mathrm{NO}}_3–\mathrm{N} \left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)=0.0497\times \mathrm{N}+1.8391$$

(10)

$${\mathrm{NH}}_3–\mathrm{N} \left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)=5.0215\times {\mathrm{e}}^{\left(0.0091\times \mathrm{N}\right)}$$

(11)

where N is the N application rate (kg N·ha⁻¹).

2.3.5. GHG Emissions and CF Calculations

GHG emissions and C footprints (CF) were calculated using the following equations [25]:

$$\mathrm{GHG} \left({\mathrm{kg} \mathrm{CO}}_2 \mathrm{eq}·{\mathrm{ha}}^{-1}\right)={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{m}}{ \mathrm{Rate}}_{\mathrm{i}}\times {\mathrm{G}}_{\mathrm{i}}+265\times {\mathrm{N}}_2{\mathrm{O}}_{\mathrm{total}}–\mathrm{N}\times 44/28$$

(12)

$${\mathrm{N}}_2{\mathrm{O}}_{\mathrm{total}}–\mathrm{N} \left(\mathrm{kg} \mathrm{N}·{\mathrm{ha}}^{-1}\right)={\mathrm{N}}_2{\mathrm{O}}_{\mathrm{driect}}–\mathrm{N}+1\%\times {\mathrm{NH}}_3–\mathrm{N}+2.5\%\times {\mathrm{NO}}_3–\mathrm{N}$$

(13)

$$\mathrm{CF} \left({\mathrm{kg} \mathrm{CO}}_2 \mathrm{eq}·{\mathrm{t}}^{-1}\right)=\frac{\mathrm{GHG}}{{\mathrm{Y}}_{\mathrm{N}}}$$

(14)

where i represents agricultural input (N fertilizer and other inputs), Rate_i is the application rate of agricultural materials input, and G_i represents the GHG emission factor during the production and transportation of agricultural products. Rate_i and G_i values used in this study are listed in Table 1 and Table A1 [26,27,28]. The global warming potential of N₂O is 265 times that of CO₂ on a mass basis [39]. The factor used to convert N₂O–N to CO₂ was 44/28. N₂O_total–N represents the total N₂O–N loss from direct and indirect pathways. The indirect N₂O–N emissions were the sum of 1% NH₃–N and 2.5% NO₃–N [40]. Y_N represents total above-ground dry matter yield with the application of N fertilizer (t·ha⁻¹).

2.3.6. N Fertilizer-Derived Ecosystem Economic Benefits

Accounting for ecosystem and human health costs, the estimated N-derived yield benefits (B_Y), private profitability, and EEB were calculated using the following equations [25]:

$${\mathrm{B}}_{\mathrm{Y}} \left(\$·{\mathrm{ha}}^{-1}\right)=\left({\mathrm{Y}}_{\mathrm{N}}–{\mathrm{Y}}_0\right)\times {\mathrm{S}}_{\mathrm{Price}}$$

(15)

$$\mathrm{Private} \mathrm{profitability} \left(\$·{\mathrm{ha}}^{-1}\right)={\mathrm{B}}_{\mathrm{Y}}–{\mathrm{N}}_{\cos \mathrm{t}}–{\mathrm{L}}_{\cos \mathrm{t}}$$

(16)

$$\mathrm{EEB} \left(\$·{\mathrm{ha}}^{-1}\right)={\mathrm{B}}_{\mathrm{Y}}–{\mathrm{N}}_{\cos \mathrm{t}}–{\mathrm{L}}_{\cos \mathrm{t}}–{\mathrm{E}}_{\cos \mathrm{t}}–{\mathrm{H}}_{\cos \mathrm{t}}$$

(17)

where Y_N represents total above-ground dry matter yield with the application of N fertilizer, and Y₀ represents the total above-ground dry matter yield without N fertilizer. S_Price was 0.25 USD·kg⁻¹ and represents the forage sorghum price for silage [25]. The N_cost and L_cost are the N fertilizer and labor costs associated with N fertilizer application. They were calculated by multiplying the amount of N or labor applied by the corresponding price (N fertilizer = 0.64 USD·kg⁻¹; single-person labor cost = 1.47 USD·h⁻¹; data source: http://zdscxx.moa.gov.cn/month/nycsc3/zlsc317#nycsc3/ (accessed on 20 August 2022). E_cost represents ecosystem damage costs caused by Nr losses, while H_cost represents the human health costs caused by various Nr losses resulting from N fertilizer application. These were calculated as follows [41]:

$$\begin{array}{c}{\mathrm{E}}_{\mathrm{cost}}={\mathrm{C}}_{\mathrm{GHG}}+{\mathrm{C}}_{\mathrm{eu}}+{\mathrm{C}}_{\mathrm{acid}}=({\mathrm{CO}}_2\times 0.0204)+\left(1.12\times {\mathrm{NO}}_3–\mathrm{N}+0.24\times {\mathrm{NH}}_3–\mathrm{N}+0.0018\times \mathrm{N}\right)+\\ \left(1.87\times {\mathrm{NH}}_3–\mathrm{N}+0.021\times \mathrm{N}\right)\end{array}$$

(18)

$${\mathrm{H}}_{\cos \mathrm{t}}=(0.30\times {\mathrm{N}}_2{\mathrm{O}}_{\mathrm{total}}–\mathrm{N}+0.20\times {\mathrm{NO}}_3–\mathrm{N}+3.30\times {\mathrm{NH}}_3–\mathrm{N}$$

(19)

where C_GHG, C_eu, and C_acid represent GHG emission, water eutrophication, and soil acidification damage costs [42]. CO₂ is the total GHG emissions from N fertilizer production, transportation, and application. The CO₂ market price was 0.0204 USD·kg⁻¹ [43]. The eutrophication impact restoration cost for NO₃–N and NH₃–N was 1.12 and 0.24 USD·kg⁻¹, respectively [44]. The soil acidification damage restoration cost for NH₃–N was 1.87 USD·kg⁻¹ [44,45]. The costs of eutrophication and soil acidification damage per kg of N fertilizer were 0.0018 and 0.021 USD·kg⁻¹ [45], respectively. The human health cost per unit of N₂O_total–N, NO₃–N, and NH₃–N were 0.30, 0.20, and 3.30 USD·kg⁻¹ [46], respectively.

2.4. Statistical Analysis

Agronomic traits, forage quality, dry matter yield, crop N content, crop N uptake, NUE, NO₃⁻–N accumulation, Nr loss, CF, and NF were compared using Analysis of Variance (ANOVA) and the Duncan method (p = 0.05) in SPSS (26.0, SPSS Inc., Chicago, IL, USA). The data in the graphs are expressed as mean ± standard error. GraphPad Prism (9.1.1, GraphPad Software, Inc., San Diego, CA, USA) was used to generate the graphs.

3. Results

3.1. Dynamics of Plant Height, Stem Diameter, LAI, NDVI, and Dry Matter Accumulation

During the forage sorghum growth period, plant height, stem diameter, LAI, and NDVI generally increased at first, whereafter they remained either stable or decreased (Figure 2). In 2019, forage sorghum plant height, stem diameter, and NDVI were not significantly different between treatments (p > 0.05). In 2020, plant height, stem diameter, and NDVI at N₁₆₀ were 3.3 m, 2.3 cm, and 0.8, respectively, which were 7.7%, 4.7%, and 9.3% significantly higher compared with N₀ (p < 0.05), respectively. During the two-year harvest stages, LAI peaked in N₁₆₀ with values of 2.7 in 2019 and 2.4 in 2020, respectively, which were 5.1% and 12.3% higher compared with N₀, respectively (p < 0.05).

Above-ground total dry matter accumulation (TDMA) either gradually increased, or first increased and then decreased, with increasing N application rates under the different growth stages (Figure 3). At the heading, flowering, and filling stages in 2019, above-ground TDMA values in N₁₆₀ were 8.6, 11.8, and 18.6 t·ha⁻¹, respectively, which were 113.6%, 28.8%, and 14.0% significantly higher compared with N₀ (p < 0.05). At the harvest stage in 2019, above-ground TDMA was the highest in N₁₆₀ (22.3 t·ha⁻¹), but it did not differ significantly from the other N application treatments (p > 0.05). At the heading and flowering stages in 2020, above-ground TDMA values in N₃₂₀ increased significantly by 65.3% and 79.8%, respectively, compared with N₀ (p < 0.05). Moreover, N₁₆₀ had the highest above-ground TDMA (18.0 t·ha⁻¹) at the harvest stage in 2020, which was 23.8% significantly higher compared with N₀ (14.5 t·ha⁻¹) (p < 0.05). During the harvest stage in 2019 and 2020, stem and ear dry matter accumulation (DMA) was highest in N₁₆₀ (15.6 and 13.2 t·ha⁻¹ for stems; 3.3 and 1.79 t·ha⁻¹ for ears). However, leaf DMA did not differ significantly among all N application treatments during the two years (p > 0.05).

3.2. NDF, ADF, ASH, CP, and RFV

At the respective harvest stages during the two years, NDF was the highest in N₃₂₀ (58.0% in 2019 and 59.7% in 2020) and significantly higher compared with N₀ and N₈₀ (

Table 2. The NDF, ADF, ASH, CP, DMI, DDM, and RFV of forage sorghum at the harvest stage under different N application rates in 2019 and 2020.

Years	Treatments	NDF (%)	ADF (%)	ASH (%)	CP (%)	DMI (%)	DDM (%)	RFV (%)
2019	N0	51.5 ± 2.2 b	29.6 ± 2.5 b	4.8 ± 0.3 a	6.0 ± 0.3 c	2.3 ± 0.1 a	65.9 ± 2.0 a	119.6 ± 8.3 a
	N80	51.8 ± 1.0 b	29.3 ± 1.4 b	4.8 ± 0.4 a	6.9 ± 0.3 b	2.3 ± 0.1 a	66.1 ± 1.1 a	119.0 ± 4.3 a
	N160	55.5 ± 3.1 ab	28.1 ± 2.1 b	4.7 ± 0.2 a	7.0 ± 0.1 ab	2.2 ± 0.1 ab	67.0 ± 1.6 a	113.0 ± 8.0 ab
	N240	55.2 ± 1.5 ab	30.6 ± 1.5 ab	4.9 ± 0.9 a	7.3 ± 0.1 ab	2.2 ± 0.1 ab	65.2 ± 1.1 ab	110.2 ± 5.0 ab
	N320	58.0 ± 3.1 a	33.3 ± 2.2 a	4.8 ± 0.3 a	7.4 ± 0.2 a	2.1 ± 0.1 b	63.0 ± 1.7 b	101.8 ± 8.4 b
2020	N0	53.9 ± 0.44 b	28.5 ± 0.1 b	5.7 ± 0.2 a	7.8 ± 0.2 b	2.2 ± 0.1 a	66.7 ± 0.1 a	115.0 ± 1.0 a
	N80	56.3 ± 1.6 b	30.1 ± 1.5 b	6.0 ± 0.4 a	8.2 ± 0.2 ab	2.1 ± 0.1 ab	65.4 ± 1.1 ab	108.2 ± 4.8 ab
	N160	54.0 ± 0.8 b	28.4 ± 0.1 b	6.1 ± 0.4 a	8.6 ± 0.2 a	2.2 ± 0.1 ab	66.8 ± 0.1 a	115.0 ± 1.7 a
	N240	52.4 ± 2.7 b	28.1 ± 2.0 b	5.5 ± 0.2 a	8.6 ± 0.1 a	2.3 ± 0.1 a	67.0 ± 1.6 a	119.9 ± 8.7 a
	N320	59.7 ± 2.5 a	32.3 ± 0.6 a	5.1 ± 0.2 a	8.6 ± 0.1 a	2.0 ± 0.1 b	63.7 ± 1.4 b	99.3 ± 0.2 b

In the same year, different lowercase letters within a column indicate significant differences at p < 0.05 among the different N treatments.

; p < 0.05). ADF was also the highest in N₃₂₀, which was 12.6% and 13.5% significantly higher in 2019 and 2020 compared with N₀ (p < 0.05). N application did not significantly affect ASH in either of the years (p > 0.05). At the harvest stage in 2019 and 2020, CP values did not differ significantly between N₁₆₀, N₂₄₀, and N₃₂₀ (p > 0.05), but they were significantly higher compared with N₀ (p < 0.05). During the two-year experiment, there were no significant differences in DMI, DDM, and RFV at N₀, N₈₀, N₁₆₀, and N₂₄₀ (p > 0.05). In 2019 and 2020, DMI, DDM, and RFV values were the lowest in N₃₂₀, and were all significantly lower compared with the other treatments (N₀, N₈₀, N₁₆₀, and N₂₄₀; p > 0.05).

3.3. Above-Ground Crop N Uptake

Stem and leaf N content decreased as the growth period progressed (Figure 4). At the harvest stage in 2019 and 2020, stem N content increased overall with increasing N rates, and the stem N content in N₃₂₀ increased significantly by 44.9% in 2019 and 40.8% in 2020 compared with N₀ (p < 0.05). Moreover, leaf N content values were the lowest in N₀ at the harvest stage in 2019 and 2020 (26.2 and 28.8 mg·g⁻¹) and were significantly lower compared with N₃₂₀ (28.1and 31.6 mg·g⁻¹). At the harvest stage for both years, total N uptake tended to first increase and then decrease with increasing N application rates. Total N uptake values peaked at N₁₆₀ (249.4 kg·ha⁻¹ in 2019 and 247.4 kg·ha⁻¹ in 2020) and were 25.7% and 36.8% significantly higher compared with N₀, respectively (p < 0.05). Stem N uptake values in N₀ were the lowest at the harvest stages (97.4 kg·ha⁻¹ in 2019 and 76.5 kg·ha⁻¹ in 2020) and were significantly lower compared with N₁₆₀, N₂₄₀, and N₃₂₀ for both years.

3.4. Soil PFP, RE, and NO₃⁻–N Residue

In both years, PFP and RE tended to decrease with increasing N rates (Figure 5). PFP and RE responses towards N rates were fitted to linear equations for both years. Maximum PFP (240.2 kg·kg⁻¹ in 2019 and 180.6 kg·kg⁻¹ in 2020) and RE (45.3% in 2019 and 41.1% in 2020) values were obtained at an N rate of 80 kg·ha⁻¹. Moreover, minimum PFP (38.7 kg·kg⁻¹ in 2019 and 34.6 kg·kg⁻¹ in 2020) and RE (10.0% in 2019 and 14.2% in 2020) values were obtained at an N rate of 320 kg·ha⁻¹.

At the jointing stage for both years, NO₃⁻–N remained at a high level in the top 0–50 cm soil layer but remained at a low level for the 50–200 cm depths (Figure 6). At the flowering and harvesting stages for both years, NO₃⁻–N tended to leach into deeper soil layers (>50 cm). NO₃⁻–N residue in the 0–200 cm soil layer was not affected by N rates during the jointing stages in 2019 and 2020 (p > 0.05). NO₃⁻–N residue at the flowering stages for both years first increased and then decreased with increasing N rates, and NO₃⁻–N residue was the highest in N₂₄₀ (470.0 kg·ha⁻¹ in 2019 and 519.9 kg·ha⁻¹ in 2020). At the harvest stages for both years, NO₃⁻–N residue increased with increasing N rates and peaked at N₃₂₀ (412.2 kg·ha⁻¹ in 2019 and 307.9 kg·ha⁻¹ in 2020); these values in 2019 and 2020 were 151.3% and 183.2% significantly higher compared with N₀, respectively (p < 0.05).

3.5. Nr Losses, GHG Emissions, NF, CF, and EEB

The two-year average Nr losses generally increased with increasing N application rates (Figure 7). The Nr losses increased from 7.2 kg N ha⁻¹ in N₀ to 119.0 kg N ha⁻¹ in N₃₂₀, in which about 56.0–70.0% of the Nr losses were derived from NH₃ volatilization, followed by NO₃⁻ leaching (21.3–31.1%). Only minor Nr losses were caused by direct N₂O emission (4.3–10.0%). Total N₂O emission, as well as N production and transportation, were the main contributors to GHG emissions. Among all the treatments, total N₂O emission induced GHG emissions accounted for 48.8–53.8% of the total GHG emissions, while N fertilizer production and transportation induced GHG emissions accounted for 38.5–43.5%.

The average NF values for the two years were 0.4, 1.0, 1.8, 3.5, and 6.5 kg N t⁻¹ for N₀, N₈₀, N₁₆₀, N₂₄₀, and N₃₂₀, respectively. The NF values for N₈₀, N₁₆₀, N₂₄₀, and N₃₂₀ were 144.5%, 340.8%, 761.5%, and 1492.7% higher compared with N₀ (p < 0.05). The average two-year CF values for N₀, N₈₀, N₁₆₀, N₂₄₀, and N₃₂₀ were 19.6, 92.2, 156.7, 247.6, and 335.5 kg CO₂ eq t⁻¹, respectively. Total N₂O emission, as well as N production and transportation, accounted for 48.8–53.8% and 0–43.5% of CF, followed by P (1.5–27.4%) and K (1.3–23.8%) production and transportation.

During the forage sorghum life cycle, as N rates increased, the N-derived benefits initially peaked at N₁₆₀ (up to 624.4 USD·ha⁻¹) and then decreased; however, the ecological and human health costs continuously increased (

Table 3. Mean costs and benefits of forage sorghum production under different N treatments across 2019–2020.

Treatments	N-Derived Benefits (USD·ha−1)	N Costs (USD·ha−1)	Labor Costs (USD·ha−1)	Ecological Costs (USD·ha−1)	Health Costs (USD·ha−1)	Private Profitability (USD·ha−1)	Ecosystem Economic Benefits (USD·ha−1)
N0	0.0	0.0	3.9	14.1	6.3	−3.9	−24.3
N80	266.7	51.2	7.8	50.3	15.4	207.7	142.1
N160	624.4	102.4	7.8	90.7	30.8	514.2	392.7
N240	217.6	153.6	7.8	139.8	59.4	56.2	−143.0
N320	156.8	204.8	7.8	207.0	115.2	−55.8	−377.9

). Private profitability increased with increasing N application rates to 514.2 USD·ha⁻¹ (N₁₆₀), and then decreased to 56.2 USD·ha⁻¹ (N₂₄₀) and −55.8 USD·ha⁻¹ (N₃₂₀). Finally, EEB was similar in trend to private profitability.

4. Discussion

4.1. Plant Growth, Dry Matter Yield, and Nutrient Quality

Appropriate N fertilizer application significantly increased forage sorghum height and stem diameter [3,47]. Specifically, plant height and stem diameter were significantly higher (7.7% and 9.3%) at N₁₆₀ compared with N₀ at the harvest stage in 2020, which agreed well with the results of Afzal et al. [48], who reported that forage sorghum height and stem diameter were 25.2% and 71.4% higher at an N application rate of 57.5 kg N·ha⁻¹ compared with the zero N treatment. Several studies have reported that N fertilizer influences LAI [49]. Our study showed that forage sorghum LAI first increased and then decreased with increasing N application rates at the harvest stage during both years. Moreover, the largest LAI values appeared in N₁₆₀ at the harvest stage, which were 5.1% and 12.3% significantly higher compared with N₀, respectively. This may be due to insufficient plant nutrient supply in N₀ (no N applied), which results in fewer and smaller plant leaves. Additionally, excessive N application results in denser leaves, thereby creating inadequate lighting for the lower and middle plant parts, thus leading to premature leaf wilting and decreased LAI.

Sorghum yield can greatly be increased by utilizing moderate N application rates [50,51], but excessive or insufficient N application rates can reduce yield [14]. When N application levels are too high, excessive leaf growth and poor population ventilation is promoted, and the lower and middle leaves do not receive enough light, which negatively affects photosynthetic product formation and leads to yield reduction. In this study, the forage sorghum N requirement was sufficient at a 160 kg·ha⁻¹ N application rate. In contrast, when the N application rate exceeded 160 kg·ha⁻¹, the above-ground dry matter yield at the harvest stages decreased. Lower NDF and ADF levels improve food digestibility in ruminants, and thus increases nutrient intake. Therefore, lower NDF and ADF levels exhibit better forage quality. A high N application rate (N₃₂₀) significantly increased NDF and ADF at the harvest stages. Tang et al. [51] similarly concluded that sorghum NDF and ADF increased by 4.5% and 1.4%, respectively, at a 240 kg·ha⁻¹ N application rate compared with no N application. ASH content at the harvest stage was not affected by N input. Qu et al. [52] and Zhang et al. [53] generated similar results and demonstrated limited effects of N application on sorghum ASH, but a more significant impact from genes. Furthermore, Monti et al. [54] reported that ASH was influenced more by P and K fertilizer. For CP content, the results of this study are similar to previous studies, which concluded that sorghum CP content at the harvest stage significantly increased with increased N application rates [14,51]. Highly desirable quality elements, such as DMI, DDM, and RFV, benefit forage quality and thereby enhance the capacity of livestock to utilize forage nutrients [21,55]. In our study, high N application rates (N₃₂₀) significantly reduced forage sorghum RFV, whereas low and medium N application rates (N₀, N₈₀, N₁₆₀, and N₂₄₀) did not produce significant differences.

4.2. Crop N Uptake, NUE, and NO₃⁻–N Residue

An appropriate N application rate can improve crop production capacity and promote plant N uptake. During both growing seasons in this study, forage sorghum N uptake at the harvest stages was significantly higher in N treatments compared with N₀. This result is consistent with other studies [56,57]. N fertilization may increase crop N uptake by stimulating root growth. Furthermore, the crop N uptake in different N rates at the harvest stage was higher than the amount of N fertilizer we applied. On the one hand, a considerable part of the N absorbed by crops comes from the mineralized N, and the application of N fertilizer increases the soil N source, which can improve soil physicochemical properties and increase the number and activity of soil microorganisms, thus promoting the mineralization of soil organic N [58]. On the other hand, global N deposition has continued to increase in the last hundred years, and the rate of increase is rising [59]. He et al. found that total airborne N inputs to a maize-wheat rotation system on the North China ranged from 99 to 117 kg N ha⁻¹ yr⁻¹ [60]. Therefore, crop N uptake can be higher than the applied N fertilizer. N application increased stem N uptake at the harvest stages, which was similar to previous studies. For example, Cosentino et al. [61] showed that the forage sorghum stem N uptake increased by 69.6%, 58.5%, and 67.0% at 60, 120, and 180 kg·ha⁻¹ N application rates, respectively, compared with the zero N treatment. Abunyewa et al. [62] also found similar results.

PFP and RE are common indicators that express crop NUE in different ways. In this study, PFP and RE decreased significantly with increasing N application. Ju [63] and Abunyewa et al. [62] showed that excessive N application resulted in reduced plant yield increase. This is possibly due to the fact that a high N fertilizer application depresses plant root growth, and the subsequent reduction in root length and absorption area affects root system nutrient uptake rates [64,65].

Soil mineral N content gradually increases with increasing N fertilizer application [66], and NO₃⁻–N is the main N form present in the soil, which is used by crops. In both growing seasons, NO₃⁻–N residue in the 0–200 m soil layer increased significantly with increasing N application. Scordia et al. [38] similarly concluded that N application rates of 120 and 240 kg·ha⁻¹ significantly increased soil NO₃⁻–N residue. Wang et al. [67] also showed that soil NO₃⁻–N residue was highly correlated with N application rate. In this study, NO₃⁻–N residue in N₃₂₀ at the harvest stage in 2020 increased by 74.1% compared with N₁₆₀. This indicates that a two-year successive application of excessive N fertilizer leads to significant NO₃⁻–N residue in deeper soil layers. Ju et al. [68] showed that 20.9–48.4% of N fertilizer remains in the soil after the crop is harvested. Since farmers tend to apply large amounts of N fertilizer every year, a large amount of NO₃⁻–N remains in the soil profile after crop harvest, and if heavy rains are encountered, NO₃⁻–N can move deeper. This NO₃⁻–N is unavailable for crop use, ultimately entering the groundwater and atmosphere through leaching, nitrification, and denitrification, thereby causing potential environmental risks [67,69]. The use of cover crops and controlled-release urea can be promising measures to increase crop NUE and reduce the N losses to the environment [70]. In our study, NO₃⁻–N residue in 2020 was lower than in 2019, this may be caused by higher rainfall in 2019 than in 2020. High intensity precipitation can cause NO₃⁻–N residue in the soil to leach deeper into the soil, resulting in ineffective NO₃⁻–N residue below the root zone, which cannot be detected [71]. In addition, NO₃⁻–N residue in the 0–200 cm soil layer at harvest stage was higher than the amount of N fertilizer applied in 2019 and 2020. This is understandable since the in-season N fertilizer is not the sole source of NO₃⁻–N residue in soil, the NO₃⁻–N residue before sowing, mineralization of soil organic N, and increasing global N deposition in agroecosystems also made a big contribution

4.3. Nr losses, Private Profitability, and EEB

Increased N fertilizer application has played an irreplaceable role in improving crop yields, as well as the economic benefits for farmers. However, the overuse of N fertilizer has resulted in increased Nr losses, which has negatively impacted the global N cycle and has caused numerous environmental problems. To achieve sustainable land use, farmland Nr losses must be understood. NO₃⁻ leaching, N₂O emission, and NH₃ volatilization are the main Nr loss pathways from farmland [72]. In this study, NO₃⁻ leaching, N₂O emission, and NH₃ volatilization increased with increasing N application. Zhang et al. [25] and Yao et al. [73] reached similar conclusions: Nr losses increased with increasing N application. Therefore, a suitable N fertilizer application rate must be selected to prevent major environmental problems.

EEB and private profitability reached a maximum value at N₁₆₀. Therefore, when accounting for dry matter yield, a 160 kg·ha⁻¹ N application rate can maximize forage sorghum above-ground dry matter yield while maintaining a high private profitability and EEB. This study also demonstrated that using common urea (CU) results in higher ecological and health costs. Zhang et al. [25] showed that the ecological and health costs of controlled-release urea and urea blends (BU) were 14.7% and 20.1%, respectively, lower than CU. Moreover, BU can be applied at once, which reduces the labor cost by half. Therefore, widespread BU use can be considered in future agricultural production.

Both in-season N fertilizer application and soil basic fertility condition before sowing have notable influences on yield performance and Nr losses, thus influencing private profitability and EEB. This two-year N application study was performed on a typical cropland with history of farmer conventional fertilization and management. The basic fertility of the cropland used in this study has good representatives of soil fertility condition in the area. So the yield response to fertilizer N application and Nr losses, private profitability, and EEB can possibly vary significantly when the same study is performed on sites with different soil basic fertilities. This two-year successive study was conducted on the same typical cropland to investigate the cumulative effect of two seasons of N application on yield performance, quality, and N-use efficiency in forage sorghum production. This is essentially important in optimizing N application for forage sorghum to maximize yield, quality, and N use efficiency while reducing environmental costs.

5. Conclusions

This study indicated that forage sorghum above-ground dry matter yield and N uptake initially increased and then decreased with increasing N application (both reached a maximum value at the 160 kg·ha⁻¹ N rate, with a two-year average of 20.1 t·ha⁻¹ and 248.4 kg·ha⁻¹, respectively). Similarly, forage sorghum CP content at the 160 kg·ha⁻¹ N rate was significantly higher compared with the zero-N treatment, but NDF, ADF, and RFV were not significantly different. Nr losses, GHG emission, NF, and CF continuously increased with increasing N application. Private profitability and EEB were maximized at the 160 kg·ha⁻¹ N rate. Simply increasing the N fertilizer amount will not increase yield, but will indeed raise the economic costs for farmers and negatively impact the environment. Therefore, we propose that 160 kg·ha⁻¹ is the ideal N application rate for forage sorghum in the dry region of the Loess Plateau. This will ensure optimum yield and forage quality, as well as improve farmer income and reduce environmental pollution.