Introduction

Plants require mineral nutrients to complete their growth and development, and, of those, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are classified as macronutrients due to relatively large amounts (Maschner, 1995). N functions as an essential component of amino acids, nucleic acids, chlorophyll, vitamins, and thus is greatly responsible for quantitative growth of plants (Maathuis, 2009). More than 90% of soil P is strongly fixed, and this results in extremely low availability of plants. P plays a role to constitute nucleic acids, cellular energy (e.g. ATP) and cell membrane (Maathuis, 2009). K, varying between 0.1 and 1.0 mM in the soil solution, is relatively easily absorbed, and directly involve cellular metabolic reactions via activating a multitude of enzymes and precisely manipulate water homeostasis like stomatal opening/closure (Britto and Kronzucker, 2008).

In context of an availability of mineral nutrients, nutrient use efficiency (NUE) is widely adopted as an important index to evaluate the uptake, utilization and recovery of mineral nutrients by crop plants (Dobermann, 2007). Therefore, many studies have addressed to emphasize an importance of nitrogen use efficiency (NUE) as a crop functionality (Bhardwaj et al., 2021; Miao et al., 2021; Lee et al., 2022; Ma et al., 2022; Lee et al., 2023).

Sorghum-sudangrass hybrid is a subtropical crop preferring the relatively higher temperature for greater production and nutritional quality (Choi et al., 2022), and, the cultivation scale is increasingly extended to compensate livestock feed requirement (Pedersen, 1996; Reddy et al., 2005). Additionally, due to an advantage of summer season-producing system ensuring greater biomass (Choi et al., 2017), sorghum-sudangrass hybrid is required to be more tolerant/resistant against abiotic stresses such as drought and high temperature compared to other forage crops (Yoon et al., 2007; Uzun et al., 2009).

Despite many reports on fertilization efficiency and biomass production of forage crops, there is limited information on sorghum-sudangrass hybrid. Therefore, it is necessary to know whether difference application rates of N, P₂O₅ or K₂O affect nutrient use efficiency and biomass production, and the objective of the current study is to suggest the optimal NPK fertilization rate with an evaluation of nutrients (N, P₂O₅, K₂O) use efficiency based on nutrient uptake and biomass production of sorghum-sudangrass hybrid.

Materials and Methods

Experimental setup and treatment

This work was performed at the experimental field in Chungbuk Agricultural Research and Extension Service (CARES) from May to August, 2022. Experimental plot was arranged with a randomized block design (60 m² per treatment, 3 replications). Seeds of sorghum-sudangrass hybrid (cv. SX17, heading type) were strip-sown with a furrow of 50 cm (width, 40 kg ha^-1) at 10^th May, 2022. Compared to the optimum range of soil chemistry for forage crop production (NAAS, 2022), an experimental soil showed lower organic matter (OM) and exchangeable potassium (K), whereas pH, available phosphate (P₂O₅) and exchangeable calcium (Ca) were higher (Table 1). Compost was applied as a basal fertilizer at a rate of 10 tons ha^-1. The standard fertilization was 175 kg N, 150 kg P₂O₅ and 150 kg K₂O ha^-1, and N was split with three times as a rate of 40 (basal): 30 (20 days after sowing, DAS): 30% (45 DAS), and total requirement of P₂O₅ and K₂O were applied as a basal fertilization on 4^th May, 2022. To develop variable N, P₂O₅ and K₂O levels, the dose was divided into four different levels; 0, 50, 100 and 200 % to the standard fertilization rate (Table 2). An experimental soil was analyzed according to NAAS method (NAAS, 2010). The pH and EC were measured with pH/EC meter (sampled soil : ddH₂O = 1:5). Total soil carbon (dried soil) was measured with a C/N analyzer (VarioMax CN Analyzer, Elementar GmbH, Germany), and transformed to organic matter with multiplying by 1.724. An available phosphate extracted with Lancaster method was measured with UV-Spectrometer (720nm, UV 1900i, Shimadzu, Japan). The cations (5 g of dried soil) were analyzed with ICP-OES equipment (INTEGRA XMP, GBC, Australia) after an extraction (50 mL of 1 N CH₂COONH₄, pH 7.0).

Sampling and analysis

The shoots (leaves + stems) of sorghum-sudangrass hybrid were taken at harvest stage (85 days after sowing, DAS) from each treatment (50 × 50 cm, 3 replications), immediately divided into leaves and stems after washing with deionized-distilled water, and dried at 72°C for 48 hrs. After measuring dry weight (biomass), the powdered leaves and stems (500 mg, DW) were mixed with 5 mL of 368 mmol L^-1 of salicylic acid in 84.7% sulfuric acid (H₂SO₄, v/v) for 24 h and wet-digested at 300°C for 6 hrs, followed by the addition of several drops of hydrogen peroxide (H₂O₂), transferred to a 100 mL volumetric flask and diluted to 100 mL with deionized water for the mineral analysis. Nitrogen was determined using a C/N analyzer (VarioMax CN Analyzer, Elementar, Germany) with manufacturer’s manual. Phosphorus was measured using the molybdate-blue colorimetry method (UV-2450, Shimadzu, Japan). Potassium (K) was measured using an ICP-OES equipment (INTEGRA XMP, GBC, Australia) according to manufacturer’s manual.

Nutrient use efficiencies (NUEs) were calculated with samples taken at the harvest stage using the following formulas (Dobermann, 2007).

Where Y is yield (kg ha^-1) of crop with fertilization and Y₀ is without fertilization. U is uptake (kg ha^-1) by aboveground biomass with fertilization and U₀ is without fertilization. F means fertilization rate (kg ha^-1).

Statistical analysis

Data were analyzed using one-way ANOVA, and, if p < 0.05, were subjected to Duncan multiple range test (DMRT) to detect significant differences among the means (RStudio VER. 4.2.2).

Results and Discussion

Growth and aboveground biomass yield

Improving nutrient use efficiency of crop plants is essential for effective fertilization management. This study focused on evaluating the use efficiency of N, P, or K in a sorghum-sudangrass hybrid (cv. SX17). Agronomic growth parameters of the hybrid plant grown under different levels of N, P₂O₅ or K₂O fertilization were measured at harvesting stage (Table 1). Indeed, standard fertilization rates of N, P₂O₅ or K₂O significantly promoted plant height. Leaf length and width varied significantly with N rates, showing the greatest at 175 kg N, and K₂O fertilization (75 and 150 kg ha^-1) also noticeably increased leaf length. By contrast, stem diameter was not significantly affected by varying levels of N, P₂O₅ or K₂O fertilization. Interestingly, the yield of sorghum-sudangrass hybrid, measured as dry matter (kg ha^-1), was not greatly affected by different levels of N, P₂O₅ or K₂O fertilization, however there was a consistent trend corresponding to fertilization rates, except for at the highest supply level (Table 3 and Fig. 1). In fact, studies have documented that growth performance of sorghum varieties is reported to have a diverse range by genotype, production phase, harvest age, grain quality and nutritional content (Shegro et al., 2012; Sajimin et al., 2018). Furthermore, biomass yield of sorghum was obtained with 14.5 -21.1 Mg ha^-1 affected by various NPK fertilization rates (Weldegebriel et al., 2018), and greater contributor to yield improvement was N fertilization and followed by P₂O₅ and K₂O (Li et al., 2010). From our results, the recommended rates of N (175 kg ha^-1), P₂O₅ (150 kg ha^-1) or K₂O (150 kg ha^-1) were found to be highly effective for biomass production in the sorghum-sudangrass hybrid. Water and N interactions were highlighted as major factors influencing growth and yield, as suggested Gonzalez-Dugo et al. (2010) and Sadras et al. (2016). Overall, higher fertilization rates of N, P₂O₅ or K₂O directly enhanced agronomic traits including biomass production. Nevertheless, an increase N, P₂O₅ or K₂O fertilization rates was not effective to growth promotion.

Fig. 1.

Yield response of sorghum-sudangrass hybrid under different levels of N, P₂O₅ or K₂O fertilization. Sorghum-sudangrass hybrid plants were harvested at 80 days after sowing from each treatment (50 × 50 cm, n = 3).

The uptake of N and K in the sorghum-sudangrass hybrid plant was substantially affected by fertilization rates of N or K₂O, whereas P₂O₅ uptake did not differ (Table 4). Compared to the control (0-0-0 kg ha^-1), nitrogen uptake increased by 25.5% at 87.5N kg ha^-1, 25.9% at 175N kg ha^-1 and 60.3% at 350N kg ha^-1. Potassium (K₂O) uptake was 16.9% greater at 75K kg ha^-1, 52.1% greater at 150K kg ha^-1 and 52.9% greater at 300K kg ha^-1 compared to the control. Compared to the standard N fertilization (175 kg ha^-1), a 50% reduced-N application did not affect N uptake, whereas a 200% increase in N supply resulted in a noticeable rise. In contrast, a 50% reduction in K₂O fertilization led to a significant 76.9% reduction in K uptake, while doubling the K₂O fertilization rate resulted in no significant change (100.5%). Based on the results of nutrient uptake and biomass production, nutrient use efficiencies were evaluated (Table 4). Nutrient uptake efficiency (NUpE) showed significant difference (p < 0.05) with N treatments, indicating a decreasing trend with increasing N fertilization. Different levels of P₂O₅ or K₂O fertilization did not affect NUpE. Nutrient utilization efficiency (NUtE) also tended to decrease gradually with increased fertilization rates of N (p < 0.01) and K₂O (p < 0.05). Consequently, nutrient use efficiency (NUE), expressed as the product of NUpE and NUtE, was significant in N treatments (p < 0.01), with values of 40.7 kg kg^-1 (50% of N), 24.6 kg kg^-1 (100% of N) and 3.8 kg kg^-1 (200% of N). P- or K-use efficiency did not differ significantly among different fertilizer- P₂O₅ or - K₂O application rates although the highest NUE was observed at the standard rate, indicating 34.0 kg kg^-1 for P₂O₅ and 8.7 kg kg^-1 for K₂O. An internal utilization efficiency (IE) was reduced by an increasing N or K₂O fertilization rates, whereas remained consistently in P₂O₅. Additionally, a partial factor efficiency (PFE) was also significantly declined in N treatment, whilst an increasing P₂O₅ fertilization resulted in higher PFE. In terms of PFE, an increase in P₂O₅ fertilization could promote biomass production (DW) of sorghum-sudangrass hybrid. In line with previous investigations (Gheith et al., 2022; Sharma et al., 2022; Sigua et al., 2018), higher N fertilization substantially decreased the N use efficiency compared to lower N rate. Additionally, several studies recommend coupling N supply with crop requirements during the cropping season as an ideal practice to improve NUE by reducing N loss (Asibi et al., 2019; Qiu et al., 2015; Srivastava et al., 2018). Zhu et al. (2012) estimated an optimal P₂O₅ fertilization range of 127 to 150 kg ha^-1, calculated from a regression model for P use efficiency, which aligns with our results. There were conflicting results regarding K use efficiency, with some studies reporting no difference (Sharma et al., 2022) and decrease (Brhane et al., 2017) under increasing K₂O fertilization rates. Therefore, our study showed that increased N fertilization rates evidently reduced N availability (uptake and utilization), whereas different P and K rates had an indirectly influence on their use efficiency.

Conclusion

This study highlights the importance of nutrient use efficiency (NUE) in the sorghum-sudangrass hybrid (cv. SX17) under varying levels of nitrogen (N), phosphorus (P), and potassium (K) fertilizations. Overall, the current study demonstrated that higher fertilization rates of N, P₂O₅, or K₂O promoted agronomic traits and biomass production, although there was a threshold beyond which additional fertilization does not yield further growth benefits. Based on the results of yield and nutrient use efficiency, we suggest that the optimal level of NPK fertilization is 175 kg ha^-1 for N, 150 kg ha^-1 for P₂O₅ and 150 kg ha^-1 for K₂O. Despite our suggestion, improving efficient fertilization practices is essential for maximizing nutrient use without causing environmental or economic drawbacks.