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Article

Comparative Efficiency of Nitrogen Fertilization Levels in Two Sorghum Hybrids for Bioenergy Production

by
Antonio M. Cabrera-Ariza
1,2,*,
Miguel Aguilera-Peralta
2 and
Rómulo Santelices-Moya
2
1
Centro de Investigación de Estudios Avanzados del Maule, Universidad Católica del Maule, Avenida San Miguel 3605, Talca 3460000, Chile
2
Centro de Desarrollo del Secano Interior, Facultad de Ciencias Agrarias y Forestales, Universidad Católica del Maule, Talca 3460000, Chile
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(9), 2026; https://doi.org/10.3390/agronomy14092026
Submission received: 31 July 2024 / Revised: 2 September 2024 / Accepted: 3 September 2024 / Published: 5 September 2024
(This article belongs to the Section Agricultural Biosystem and Biological Engineering)
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Abstract

:
To achieve sustainable and profitable production of sorghum for energy purposes, it is crucial to ensure the efficient use of the nutrients necessary for its growth and development. This research investigates the influence of diverse management practices on biomass production, nutrient use efficiency, and nitrogen balance in two sorghum hybrids cultivated for bioenergy applications. A comprehensive field study was conducted over two growing seasons, evaluating the effects of fertilization methods and crop rotation strategies. Results indicate that high nitrogen (HN) fertilization increased dry biomass production (up to 20.7 Mg ha−1) and nutrient removal (up to 343.5 kg K ha−1) in both sorghum hybrids. The H128 hybrid showed higher nutrient use efficiency, especially for phosphorus, while the nitrogen balance was positive under HN but varied under low nitrogen (LN), with the H133 hybrid experiencing a net nitrogen loss at LN. These findings contribute valuable insights into sustainable sorghum cultivation for bioenergy production, highlighting the importance of tailored management practices in achieving optimal crop performance.

1. Introduction

Sorghum bicolor (L.) Moench is highly productive C4 crop that is mainly utilized for animal feed, fodder, forage, and human food. Sorghum demonstrates several advantageous characteristics, including high water and N-use efficiencies [1,2,3] and drought tolerance [4], and exhibits limited flood tolerance [5,6]. Sorghum is also an important source of fiber and feedstock for biofuel production [7,8,9]. In particular, its use for renewable energy generation has become an attractive alternative in the context of the search for more sustainable and less polluting energy sources. However, to achieve sustainable and profitable production of sorghum for energy purposes, it is crucial to ensure the efficient use of the nutrients necessary for its growth and development [10]. Tissue mineral nutrient concentrations and total removal of nutrients in the harvested plant biomass can affect the long-term suitability and sustainability of cropping systems, as well as the conversion efficiencies of bioenergy feedstocks by combustion or fermentation [7,11]. The nutritional needs of the species are very similar to those of corn; the removal value recorded in fiber sorghum (which differs only slightly from that from biomass) is respectively around 10 kg of N, 3 kg of P2O5, and 12 kg of K2O per ton of dry matter produced [12]. To enhance nutrient use efficiency in sorghum cultivation for energy purposes, various strategies and agronomic techniques have been proposed. These may include selecting sorghum varieties that are more efficient in nutrient utilization, applying fertilizers precisely and timely, proper water management, and implementing conservation tillage systems. Additionally, the use of innovative technologies such as precision agriculture and biofertilization can also contribute to improving nutrient use efficiency in sorghum cultivation [13]. Notably, the interaction of N fertilization and supplementary irrigation (SI) has been shown to significantly increase the aboveground biomass (AB) of grain sorghum, with AB varying significantly with different levels of SI (p ≤ 0.001) and N fertilization (p ≤ 0.01) [14]. Furthermore, factors such as year, N rate, and their interaction can affect the distribution of biomass components, including stem, total dry matter (DM), and leaf DM [10].
Soil conservation and nutrient retention are crucial for agricultural sustainability. To assess the effectiveness of nutrient cycling, it is essential to evaluate the balance between nutrient inputs and outputs. Nutrients can be replenished through organic matter decomposition or external sources like fertilizers. Factors like plant uptake, harvesting, and losses due to leaching, runoff, erosion, and denitrification can influence soil nutrient levels. However, quantifying these processes presents significant challenges [15].
Most nitrogen in the soil is contained in the organic matter (e.g., humified), which on average contains 5% nitrogen. The organic matter of the soil is progressively decomposed and oxidized by microorganisms and atmospheric agents, releasing the nitrogen it contains. According to Masoni and Pampana [16], it has been assumed that soil humus is degraded at an average annual rate of 1%. Thus, in the soil where the test was carried out, having an average organic matter content of 1.8%, about 54 kg of nitrogen ha−1 year−1 released from the 50 cm-deep soil layer has been estimated. Atmospheric deposition brings different amounts of nitrogen to the soil depending on the amount of rainfall and its concentration of nitrogen. In coastal Tuscany, this can be considered to be on average 2 mg of nitrogen per liter [16]. Based on this value, the nitrogen supply from rainfall during the reference period of the test has an average value of 15.7 kg ha−1 year−1.
Additionally, Masoni and Pampana [16] noted that the nitrogen lost by denitrification does not reach consistent levels in normal Tuscany conditions, with a maximum loss of 5 kg ha−1 of nitrogen per year. Nitrogen leaching is influenced by factors such as the magnitude and distribution of rainfall, soil texture and its organic matter content, evapotranspiration, the dose of nitrogen fertilizer distributed, the type of fertilizer used, and the time of distribution. Nitrogen leaching is an unavoidable phenomenon that occurs even in the absence of nitrogen fertilization, as nitrates are naturally produced by the mineralization of organic matter and the oxidation of the ammonium ion in all soil. For the soil where the experiments were carried out, leaching of 25 kg ha−1 year−1 of nitrogen was estimated.
Regarding sorghum, a study in India found that in a sorghum–castor rotation, there was a loss of 38 kg ha−1 of potassium (K) and a gain of 10 kg ha−1 of nitrogen (N) and 22 kg ha−1 of phosphorus (P) [17]. Rego and Seeling [18] documented elevated levels of soil total nitrogen and enhanced nitrogen uptake, ranging from 35 to 47 kg ha−1, in subsequent sorghum crops within various cropping systems based on pigeonpea. Notably, these positive effects persisted even when harvested residues were removed from the system.
In this context, the main objective of the study is to understand how different management practices impact biomass production, nutrient use efficiency, and nitrogen balance in the cultivation of two sorghum hybrids for energy purposes.

2. Materials and Methods

2.1. Study Area

This research was carried out at the Enrico Avanzi Agricultural and Environmental Research Center (CIRRA) at Pisa University (Italy). The experimental site is located in San Piero a Grado, at 43°40′ N latitude and 10°21′ E longitude, 5 m above sea level and 2 km from the coast. The soil type was Xerofluvent (20.1% clay, 40.5% silt, and 39.4% sand), characteristic of the lower Arno River alluvial plain. This area is known for its shallow water table (1.8 m deep in the driest conditions) and adequate nutrient levels (1.8% organic matter, 1.3 g kg−1 total nitrogen, 8.8 mg kg−1 available phosphorus, and 128.3 mg kg−1 exchangeable potassium). Soil samples were collected in replicates at a depth of 0–30 cm, providing a standard deviation for measured variables. Bulk density and initial mineral nitrogen content were also measured to aid in the nitrogen balance.

2.2. Climate and Weather Conditions

The experiment was conducted over two consecutive growing seasons from April 2006 to mid-September 2007. Figure 1 shows the seasonal weather conditions during the trial period. In 2006, July was the warmest month, with average temperatures exceeding 21 °C and reaching highs of over 30 °C. During the winter months (January to March), minimum temperatures were slightly below historical averages. Conversely, summer temperatures in July surpassed 34 °C, while September and October experienced temperatures of 26 °C and 23 °C, respectively. The year 2007 saw a mild January and February, with August emerging as the hottest month, a departure from typical patterns. Rainfall distribution remained consistent across both years, featuring heavy autumn rains and summer droughts in July and August. However, annual precipitation fell short of long-term averages (940 mm), totaling 705 mm in 2006 and 632 mm in 2007.

2.3. Experimental Design

Sorghum experiments were carried out with two different levels of nitrogen fertilization: high nitrogen (HN) and low nitrogen (LN). Two sorghum hybrids were utilized: H128, an early-maturing hybrid, and H133, an early–medium hybrid, both of which are fiber sorghums. The trial was set up on plots of 2000 m2, with a total area of 8000 m2. The experimental design was a 2 × 2 Latin square, with the treatments being the hybrid (H128 vs. H133) and the nitrogen level (HN vs. LN). Each plot was replicated four times, resulting in a total of 16 plots (4 replicates × 2 nitrogen levels × 2 hybrids). The same hybrid was consistently grown in the same plot across both years to maintain uniformity. In the first season, both hybrids were planted on 10 April 2006, and harvested on 15 September 2006. In the second season (2007), planting took place on 8 April, and the harvest on 14 September.

2.3.1. Common Practices

For both nitrogen management levels, several common agricultural practices were employed. Weed control was achieved using Pendimethalin® herbicide (BASF, Ludwigshafen, Germany) (Pendimethalin 400 g L−1) at a rate of 0.5 L per hectare, followed by deep soil tillage. Seeding was conducted with precision pneumatic seeders (DamaxR PNL Mt. 4. DAMAX S.A., Córdoba, Argentina), planting seeds at a depth of 20 mm and a density of 20 plants per square meter (0.25 m row spacing, 0.2 m within the row, and 13 kg seed ha−1). The final harvest was performed with a forage harvester (Claas Jaguar 870, CLAAS KGaA mbH, Harsewinkel, Germany) in mid-September, 10–20 days before the flowering stage, when the plants reached maximum dry matter and cellulose content. The BBCH growth stage was recorded at harvest to ensure accurate assessment. After harvesting, soil restoration was carried out through deep soil tillage.
The plant nutrient needs were calculated based on agronomic guidelines derived from prior sorghum plantations conducted in the same study area. These guidelines provided a baseline for determining the specific nutrient requirements under local environmental and soil conditions. Using this site-specific data, we adjusted the nitrogen application rates and other management practices to optimize nutrient use efficiency and biomass production.

2.3.2. High Nitrogen Management

The high nitrogen management involved higher fertilization doses: 70 kg ha−1 of urea [32.2 kg of nitrogen (N)] and 80 kg ha−1 of triple superphosphate [36.8 kg of phosphorus (P)] in pre-sowing, and 90 kg ha−1 of urea after sowing (panicle initiation stage, approximately 32 days after emergence).

2.3.3. Low Nitrogen Management

The low nitrogen management involved lower fertilization doses: 40 kg ha−1 of urea and 50 kg ha−1 of triple superphosphate in pre-sowing, and 60 kg ha−1 of urea after sowing. All other operations were the same as in the high nitrogen management.

2.4. Biomass

Above-ground biomass was estimated by manually harvesting and sampling four replicate plots of 10 square meters each from each experimental field. Approximately 6 kg from each biomass sample were weighed, dried in an oven at 105 °C until constant weight, and re-weighed to determine dry matter content. Samples were collected monthly from June onward to monitor plant growth. To minimize border effects, plants located within the main cultivated area, excluding the outer rows, were selected. Statistical analysis was performed using the Student’s t-test at p ≤ 0.01 to compare biomass yield of different treatments. Data related to each sorghum sampling were subjected to a two-way block ANOVA using the CoStat program version 6.205, with factors being the level of nitrogen management and the hybrid employed. The data were tested for normality using the Shapiro–Wilk test before conducting the analysis of variance (ANOVA). If necessary, data transformations were applied to meet the assumptions of ANOVA, including normality and homogeneity of variances. The statistical significance of differences between averages was analyzed with the Student’s t-test for p ≤ 0.01, performed only on parameters significant for the analysis of variance.

2.5. Nutrient Use Efficiency (NUE)

In the laboratory, the nitrogen, phosphorus, and potassium content of the plant material was analyzed for each season at harvest time during 2006 and 2007. Nitrogen was measured using the Kjeldahl–Tecator method [19] with a Kjeltec 2100 distiller (FOSS Analytical A/S, Hillerød, Denmark), phosphorus was determined using the Ames method [20], and potassium was analyzed following the Kalra method [21]. The same plant samples used for biomass production assessment were utilized for elemental analysis. A Student’s t-test was employed to compare mean values across different treatments. Nitrogen use efficiency (NUE) was calculated according to Vitousek [22], defined as
NUE = Dry biomass (g)/Nutrient content (g)
where ‘Nutrient content’ is the total nutrient quantity in aboveground woody tissue.

2.6. Nitrogen Balance

The apparent nitrogen balance was also calculated using the method proposed by Masoni and Pampana [16]. The formula used to calculate nitrogen balance is as follows:
ΔN = Contributions (fertilizers + atmospheric inputs + mineralization + crop residues) − Losses (nitrogen removed with the biomass collected + leaching + denitrification)
The proportion of nitrogen removed from the soil by leaching was estimated using historical data of agronomic tests carried out in the same location in San Piero A Grado [16]. Nitrogen volatilization was not considered, as it does not reach significant levels in Italian soils [16].
To determine the nitrogen, phosphorous, and potassium content, the same methods described before have been used. Plant samples, comprising 15 plants per plot, underwent these analyses at the final harvest. Notably, the examination of nitrogen return to the field through the roots left after cutting was conducted on the same 15 plants. Both the root systems and the biomass of these 15 plants were sampled and analyzed for weight and nitrogen content, with evaluations performed for each hybrid and level of fertilization.

3. Results

3.1. Biomass Production

The high nitrogen (HN) treatment yielded higher dry biomass compared to the low nitrogen (LN) treatment for both hybrids and growing seasons under study (Figure 2). In 2006, hybrid H128 showed greater responsiveness to the HN management, whereas hybrid H133 had superior yield under HN management in 2007. Regarding dry matter, significant differences were identified between the two cultivation fertilization levels in 2006, with HN treatment being the most productive. Similarly, in 2007, significant differences were observed between the two levels of crop fertilization at harvest time, averaging 20.7 Mg ha−1 for HN and 14.4 Mg ha−1 for LN. No significant differences were found between the hybrids in either experimental year, nor was there a significant hybrid × management interaction. Results of ANOVA for aboveground biomass showed significant effects of management (p < 0.01) but no significant hybrid or interaction effects (Table 1).

3.2. Nutrient Use Efficiency

For the assessment of nutrient uptake by biomass at harvest, the elemental content (N, P, and K) was analyzed in the fiber sorghum plant for each hybrid and fertilization level. Table 2 presents the obtained results.
The concentration of the major macro-nutrients in sorghum plants varies depending on the hybrid and the level of crop fertilization. It is noteworthy that there is a higher concentration of P and K in the H133 hybrid compared to H128 for both levels of crop fertilization. Statistically, there are significant differences in the concentration of N, P, and K in sorghum plants between the different hybrids (H128 and H133). There are also significant differences in the management for phosphorous, and in the interaction (hybrid × management) for the nitrogen concentration.
Nutrient removal (Table 3) in this case is determined by the harvest of the aboveground part of the sorghum plant. Obtaining values that range from 114–205, 14.4–30.2, and 175.9–343.5 kg ha−1 for N, P, and K, respectively. Nutrient removal is higher in HN compared to LN, corresponding to the higher yields in HN compared to LN. Nutrient removal is also greater in the H133 hybrid for both levels of crop fertilization, except for nitrogen in HN, confirming the higher nutrient concentration in the H133 hybrid compared to the H128 hybrid.
The nutrient use efficiency for the two levels of crop fertilization and the two hybrids is shown in Table 4.
From the analysis of the table, it can be deduced that the H128 hybrid is more efficient in nutrient use than the H133 hybrid, especially concerning phosphorus (P).

3.3. Nitrogen Balance

The nitrogen balance in fiber sorghum was conducted for the second year of experimentation (2007) as the yields in the first year were not representative. The year 2007 was considered a standard growth season, rather than conducting a nutrient balance for both years of experimentation.
Inputs were provided through fertilizer application, with 160 kg ha−1 of urea in the HI and 100 kg ha−1 of urea in the LN.
Additionally, nutrient return to the field was analyzed in fiber sorghum through the roots that remained in the field after harvest. Three root systems were collected for each hybrid and crop management level, and their weights and nutrient contents were analyzed. The nutrient return ranged from 115.6–203.41, 61.45–111.91, and 41.2–59.3 kg ha−1 for nitrogen (N), phosphorus (P), and potassium (K), respectively, depending on the hybrid and crop management level.
The value assigned to atmospheric inputs was 13.4 kg N ha−1, as calculated previously, with mineralization at 54 kg N ha−1 and leaching at 25 kg N ha−1. The nitrogen balance is presented in Table 5:
As can be appreciated from the table, at the HN management level, positive net balances were observed for both hybrids, with H133 showing a significantly higher balance than H128. In contrast, at the LN management level, H128 exhibited a notable positive net balance, while H133 showed a slightly negative balance. This negative balance in H133 may not necessarily indicate a significant nitrogen loss but rather an efficient use of the nutrient, suggesting that the fertilization rate was close to optimal for this hybrid. Conversely, the positive nitrogen balances observed, particularly under HN management, could signal a potential risk of nitrogen loss if not carefully managed, underscoring the importance of fine-tuning fertilization strategies.

4. Discussion

4.1. Biomass

The 2006 season was significantly impacted by insect predation on seeds, potentially leading to waterlogging in certain areas. Despite a sowing rate of 20 seeds per square meter, average emergence rates one month after sowing were lower than expected. Under low nitrogen conditions, hybrids H128 and H133 exhibited emergence rates of 46% and 43%, respectively. High nitrogen conditions resulted in lower emergence rates for both hybrids, averaging 34%. In contrast, the 2007 season saw a significant improvement in emergence, with an average of 13 plants per square meter for both hybrids, indicating a 65% emergence rate.
The analysis of biomass accumulation trends during the 2006 and 2007 growth seasons revealed that in the first year of the experiment, fiber sorghum exhibited constant growth until August for both levels of crop management. From August to September, biomass accumulation remained constant in the HN treatment but increased in the LN treatment.
Statistically significant differences were observed in August and at harvest (September) for both years, based on the level of crop fertilization. This pattern, consistent for both hybrids, may be attributed to varying crop responses to climatic conditions. In 2006, there was higher rainfall (55.4 mm) from June to July, whereas in the same period in 2007, only 13.8 mm of rainfall occurred, resulting in lower biomass accumulation.
As previously mentioned, a statistically significant difference was found at harvest between the two crop management levels in both years, with HI being more productive. This difference may be attributed to higher potassium levels, as nitrogen is not considered a key element for biomass production, as indicated by other authors [23,24,25]. For instance, Ceotto et al. [26] found that sorghum productivity under nitrogen 0 treatment was comparable to that with partial and full fertilization, even after 5 years. Moreover, there were no statistically significant differences between the two hybrid varieties during the two years of experimentation.
Our findings confirm that crop management fertilization and the cultivation year significantly influence sorghum biomass production. Similar sorghum yield values have been reported in other studies, with variations primarily attributed to irrigation doses. For example, Curt et al. [27] conducted a trial in Spain achieving yields ranging from 18 to 48 Mg ha−1, depending on different irrigation doses. Hallam et al. [28] proposed various fertilization levels in Iowa (USA), resulting in yields between 15.3 and 20.7 Mg ha−1. Habyarimana et al. [29] conducted an experiment in Italy on sorghum, applying varying irrigation amounts to different hybrids, resulting in crop yields ranging from 20 to 51 Mg ha−1. In northern Italy, Barbanti et al. [30] tested different fertilizer doses in fiber and sweet sorghum, with yields ranging from 17.7 to 24.2 Mg ha−1. Giovanardi et al. [31], in the Friulian plain (Italy), using irrigated sorghum hybrids, obtained yields ranging between 19 and 40 Mg ha−1. Zhao et al. [32] conducted a trial in Beijing (China) with five different sorghum hybrids and varying irrigation doses, achieving yields ranging from 13.2 to 35.2 Mg ha−1.

4.2. Nutrient Use Efficiency (NUE)

The average values of different nutrients (9.93, 1.34, and 13.25 g kg−1 for N, P, and K, respectively) are similar, except for nitrogen, to others found in the literature. For example, Gutierrez-Miceli et al. [33], calculated concentrations of 1.7, 2.1, and 10.9 g kg−1 in sorghum plants for N, P, and K, respectively. In turn, Giovanardi et al. [31], obtained a nitrogen concentration in sorghum plants of 7.2 g kg−1, more in line with the results obtained here and similar to those obtained by Torbert et al. [34], with values ranging from 7.7 to 9.4 g kg−1.
The nutrient return ranged from 114.6–203.41, 61.45–111.91, and 41.2–59.3 kg ha−1 for nitrogen (N), phosphorus (P), and potassium (K), respectively, depending on the hybrid and crop management level. In contrast, the study conducted by Putthacharoen et al. [35], reported values around 147, 27, and 304 kg ha−1 for N, P, and K, respectively. The removal values can be compared with other experiments. For instance, in the research conducted by Giovanardi et al. [31], the amount of removed nitrogen is 180 kg ha−1. In Thailand, for example, the amount of nutrients removed with sorghum harvesting is 79, 25, and 51 kg ha−1 of N, P, and K respectively [35]. The differences in removal between different experiments can be explained by different environmental conditions and the use of different sorghum hybrids.
Regarding nitrogen use efficiency in sorghum, there are diverse studies and varied results obtained. Nitrogen use efficiency values range from 77 to 133, are consistent with those reported by Torbert et al., [34], who obtained values ranging from 108–130 kg kg−1.
Sigua et al. [14], for example, obtained that sorghum with 85 and 170 kg N ha−1 and supplemental irrigation treatment had the greatest NUE of 60.5 and 57.1%, respectively. In other study, Maw et al. [10], examined nitrogen use efficiency (NUE) in sorghum, calculating physiological NUE-DM (dry matter) and NUE-LEY (ethanol yield). Results showed variability based on nitrogen rate and year. In 2010, NUE-DM remained consistent across nitrogen rates, while in 2011, rates between 56N and 224N exhibited reduced NUE-DM. Environmental conditions favored higher NUE-DM at high N rates in 2010, contrasting with less favorable conditions in 2011. These findings provide valuable insights into sorghum’s response to different nitrogen fertilizer rates, showcasing a range of NUE values.
Regarding the NUE, other authors, using herbaceous species for energy purposes, have obtained values of 135, 526, and 78 respectively for N, P, and K for Miscanthus [36]. Instead, Beale and Long, [37], obtained results of 66–111, 333–556, and 86–161, respectively for N, P, and K for maize crops. Nitrogen use efficiency values range from 77 to 133, consistent with those reported by Torbert et al., [34], who obtained values ranging from 108–130 gdry biomass gnutrient−1.

4.3. Nitrogen Balance

The nitrogen balance analysis conducted for fiber sorghum in the second year of experimentation (2007) has provided valuable insights into the nitrogen dynamics under different fertilization levels (HN and LN) and for two sorghum hybrids (H128 and H133). As indicated, the net nitrogen balances varied significantly, reflecting the impact of management practices on nutrient contributions and removal.
At the HN fertilization level, both the H128 and H133 hybrids exhibited positive net nitrogen balances, with H133 showing a notably higher balance than H128. In contrast, at the LN fertilization level, H128 displayed a substantial positive net balance, while H133 demonstrated a negative balance, indicating a net loss of nitrogen. These findings underscore the importance of crop management practices in influencing nitrogen utilization efficiency and subsequent environmental impact.
The discussion of our results aligns with the broader context of nitrogen fertilization in sweet sorghum cultivation, as discussed in the literature. The values obtained are similar to those found in the literature. For instance, Rego et al. [16] obtained an overall balance of +2 kg ha−1 of N in a sorghum–pea rotation in the tropics. The optimal nitrogen fertilization rate is crucial for sweet sorghum growth and soil fertility stability. The presence of adequate organic matter and organic nitrogen contents in the soils suggests that, under optimal soil water availability, sweet sorghum nitrogen requirements can be met without external nitrogen inputs. However, we acknowledge that soil organic carbon (SOC) plays a crucial role in nutrient cycling, soil fertility, and biomass production. Although SOC was not the primary focus of our study, we recognize its importance in influencing overall soil health and nutrient availability. Future research should address SOC dynamics more comprehensively to understand its direct impacts on sorghum cultivation. Nevertheless, other experiments [38] highlights that without nitrogen fertilization, the soil becomes strongly nitrogen-impoverished, posing a risk of negligible residual fertility and potential harm to subsequent crops in rotation. In this same study nitrogen input/output was found to be balanced under 120 kg N ha−1 treatment, maintaining the soil nitrogen reserve, while under 180 kg N ha−1 treatment, the system was enriched due to the unused nitrogen input from fertilizer, rain, and irrigation water. This observation aligns with the findings of Lovelli et al. [39], suggesting that the crop’s preference for nutrient uptake shifts when higher nitrogen rates are applied, favoring the more readily available form from fertilizers. This enrichment, though beneficial for subsequent crops, raises concerns about the sustainability of agricultural practices, particularly regarding potential environmental impacts on air and water quality [38].
The long-term experiment on sorghum by Katayama et al. [40] provides insight into soil nitrogen recovery dynamics. They observed increased nitrogen recovery from fertilized plots over time, indicating progressive improvement in soil nitrogen fertility through consecutive nitrogen fertilization. The importance of promoting nitrogen fertilization management strategies, such as using slow-acting fertilizers or split applications, is highlighted, as discussed by the authors.
In agreement with reports by Holou et al. [41], our results emphasize the necessity of nitrogen application through fertilization in very poor soils for sweet sorghum cultivation, even at low rates. This is crucial for achieving adequate biomass yields and maintaining the soil at its initial nitrogen reserve.
In conclusion, our nitrogen balance analysis in fiber sorghum, along with insights from related studies, underscores the complexity of nitrogen management in sorghum cultivation. Balancing nitrogen input and output is essential for sustainable agricultural practices, and careful consideration must be given to environmental impacts. The findings contribute to the broader understanding of nitrogen dynamics in sorghum cultivation and provide valuable information for optimizing nitrogen fertilization practices in sweet sorghum cultivation.

5. Conclusions

The investigation into the impact of diverse management practices on sorghum biomass production, nutrient use efficiency (NUE), and nitrogen balance yielded valuable insights. The study, conducted over two growing seasons, emphasized the intricate interplay between management strategies, hybrid characteristics, and environmental factors influencing sorghum performance for energy purposes. High nitrogen (HN) fertilization consistently demonstrated superior biomass production compared to low nitrogen (LN). The findings underscore the significance of meticulous management practices in enhancing sorghum yields, crucial for sustainable energy production. Hybrid H133 displayed notable superiority in biomass yield during the second experimental year (2007), revealing hybrid-specific responses to management practices. However, the first year witnessed low plant survival rates, affecting overall biomass yield and emphasizing the importance of hybrid selection.
Hybrid H128 exhibited enhanced NUE, particularly in phosphorus utilization. This highlights the importance of selecting hybrids with inherent nutrient utilization efficiencies for optimizing sorghum cultivation for energy purposes.
Positive net nitrogen balances were evident at HN, with H133 displaying significantly higher balances. In contrast, at LN, H128 highlighted a noteworthy positive net balance, while H133 experienced a net loss of nitrogen. This indicates the sensitivity of nitrogen dynamics to varying fertilization management.
The study underscores the need for tailored management practices in sorghum cultivation, considering hybrid-specific responses. Efficient nutrient use and positive nitrogen balances are pivotal for ensuring the sustainability and profitability of sorghum production for energy purposes. Future research should delve deeper into understanding the complex.

Author Contributions

Conceptualization, A.M.C.-A.; methodology, A.M.C.-A.; software, A.M.C.-A.; validation, A.M.C.-A., R.S.-M. and M.A.-P.; formal analysis, A.M.C.-A., R.S.-M. and M.A.-P.; investigation, A.M.C.-A.; resources, A.M.C.-A.; data curation, A.M.C.-A.; writing—original draft preparation, A.M.C.-A.; writing—review and editing, R.S.-M.; visualization, M.A.-P.; supervision, A.M.C.-A.; project administration, A.M.C.-A.; funding acquisition, A.M.C.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 14 02026 g001
Figure 1. Seasonal weather conditions at the field experimental station in growing season 2006 and 2007. T min, minimum temperature; T max, maximum temperature [19].
Figure 1. Seasonal weather conditions at the field experimental station in growing season 2006 and 2007. T min, minimum temperature; T max, maximum temperature [19].
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Agronomy 14 02026 g002
Figure 2. Dry biomass of H128 and H133 sorghum hybrids under two different levels of nitrogen fertilization in 2006 and 2007. HN, high nitrogen; LN, low nitrogen [19].
Figure 2. Dry biomass of H128 and H133 sorghum hybrids under two different levels of nitrogen fertilization in 2006 and 2007. HN, high nitrogen; LN, low nitrogen [19].
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Table 1. ANOVA for aboveground biomass.
Table 1. ANOVA for aboveground biomass.
SourceSum SqMean SqF ValuePr > (F)
Nitrogen46.1346.3711.080.0448 *
Hybrid0.110.110.0250.8838
Nitrogen × Hybrid0.190.190.0440.8465
Residuals12.554.18
* indicates statistical significance at the 0.01 level.
Table 2. Average 2006 and 2007 N, P, and K above- and belowground concentration for two sorghum bicolor hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN).
Table 2. Average 2006 and 2007 N, P, and K above- and belowground concentration for two sorghum bicolor hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN).
N FertilizationHybridAbovegroundBelowground
N
(g kg−1)		P
(g kg−1)		K
(g kg−1)		N
(g kg−1)		P
(g kg−1)		K
(g kg−1)
HNH12810.30 ± 1.230.82 ± 0.2311.17 ± 1.657.40 ± 1.341.06 ± 0.4310.38 ± 1.47
H1338.90 ± 1.151.36 ± 0.3115.97 ± 2.128.43 ± 1.461.12 ± 0.1511.46 ± 1.36
LNH1287.50 ± 1.500.94 ± 0.3511.48 ± 1.844.27 ± 1.151.39 ± 0.4912.42 ± 1.66
H13313.00 ± 1.862.23 ± 0.6514.36 ± 1.996.63 ± 1.001.51 ± 0.2412.60 ± 1.48
Table 3. Annual nutrient (N, P, and K) removal for two sorghum hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN).
Table 3. Annual nutrient (N, P, and K) removal for two sorghum hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN).
H128H133
HNLNHNLN
N (kg ha−1)205.4114.9191.4176.0
P (kg ha−1)16.414.429.330.2
K (kg ha−1)222.7175.9343.5194.4
Table 4. Nutrient use efficiency for two sorghum hybrids (H128 and H133) and two level of crop management (HI and LI) (year 2007).
Table 4. Nutrient use efficiency for two sorghum hybrids (H128 and H133) and two level of crop management (HI and LI) (year 2007).
Nutrient Use Efficiency gdry biomass gnutrient−1
NUE (N)NUE (P)NUE (K)
H128HI97.11215.989.5
LI133.31063.987.1
H133HI112.4734.162.6
LI76.9448.369.7
Table 5. Nitrogen balance for two sorghum hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN) (year 2007).
Table 5. Nitrogen balance for two sorghum hybrids (H128 and H133) and two level of nitrogen fertilization (HN and LN) (year 2007).
Contributions
(kg ha−1)		Removal
(kg ha−1)		Balance
(kg ha−1)
HNH128236.68230.46.28
H133270.81216.454.41
LNH128183.9139.944.0
H133194.6201.1−6.5
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Cabrera-Ariza, A.M.; Aguilera-Peralta, M.; Santelices-Moya, R. Comparative Efficiency of Nitrogen Fertilization Levels in Two Sorghum Hybrids for Bioenergy Production. Agronomy 2024, 14, 2026. https://doi.org/10.3390/agronomy14092026

AMA Style

Cabrera-Ariza AM, Aguilera-Peralta M, Santelices-Moya R. Comparative Efficiency of Nitrogen Fertilization Levels in Two Sorghum Hybrids for Bioenergy Production. Agronomy. 2024; 14(9):2026. https://doi.org/10.3390/agronomy14092026

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Cabrera-Ariza, Antonio M., Miguel Aguilera-Peralta, and Rómulo Santelices-Moya. 2024. "Comparative Efficiency of Nitrogen Fertilization Levels in Two Sorghum Hybrids for Bioenergy Production" Agronomy 14, no. 9: 2026. https://doi.org/10.3390/agronomy14092026

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