1. Introduction
Cereal production, especially wheat, is increasing worldwide due to the burgeoning population, which is projected to increase to 9.1 billion in 2050 [
1]. Wheat is a staple crop for achieving the food security of the world’s population [
2].
Food consumption in the Maghreb (North-West Africa) countries, including Libya, Tunisia, Algeria, Morocco, Mauritania, etc., is essentially based on cereals. Wheat, therefore, occupies an important place in grain production, with a significant proportion in the agricultural system of these countries. Algerians consumed 11,100 million metric tons (MMT) of wheat during 2021/22, and the predicted consumption for 2022/23 is about 11,150 MMT [
3]. The improvement of wheat production becomes necessary to meet daily needs and to achieve food self-sufficiency in these regions.
However, many constraints harm wheat production, such as limited and irregular rainfall, since a majority of the cereals-producing areas in Algeria are in arid and semiarid zones [
4]. Furthermore, durum wheat produces less efficiently under low-yielding conditions. Indeed, in the presence of environmental changes, the number of grains per unit of area (number of grains·m
−2) of wheat becomes the component responsible for yield sensitivity [
5]. More so, cultivation techniques, including tillage, fertilization systems, and plant protection strategies, are poorly mastered by farmers [
6,
7].
One of the main reasons for limited yield and grain quality is the lack of knowledge in nitrogen (N) fertilization practice.
The differences in yield were also related to differences in water and nitrogen use efficiencies. Raun et al. [
8] reported that usually, 33% of applied N fertilizer is retained or present in the soil while the rest is lost by leaching and volatilization, which is directly related to the degree of fertilizer use efficiency. In another study [
9], the authors demonstrated that nitrogen use efficiency (NUE) could achieve 87% for wheat if nitrogen (N) application takes into account soil characteristics and climatic conditions. Indeed, NUE is closely related to the amounts and distribution of rainfall. Furthermore, the synchronization of nitrogen inputs and the optimal timing of its application play a key role in increasing NUE and minimizing nitrogen loss [
10]. In addition, increasing the GY and enhancing the NUE can be managed by adding supplement units of N under low amounts and irregular distribution of rainfall with respect to the environment [
11,
12,
13].
There are various nitrogen use efficiency indicators [
14,
15,
16], such as agronomic efficiency (AE), which depends on soil characteristics and environmental factors and decreases with increasing nitrogen application [
14,
17]. For a total delivered rate of 100 kg (N), the average AE in Africa was estimated at 13.6 kg dry matter (DM)·kgN
−1. However, the world average of AE was estimated at 19.6 kgDM·kgN
−1 [
15]. The apparent recovery efficiency (ARE) is also used as a nitrogen use efficiency indicator. Its target value is between 50% and 90%. The lower values under this interval cause soil and water pollution by N. Higher values deplete the soil stock of nitrogen [
18]. In Africa, the average ARE value is close to 63% for cereals, with a world average of 55%, for wheat. Different work represents that the range of ARE varies between 22% and 68% to genotype and N rate with a value of 57% [
15,
17]. The NHI is close to the physiological efficiency (NUEp). The good or bad efficiency of nitrogen distribution in the plant at maturity is expressed by a nitrogen harvest index (NHI). It can vary from 47 to 80% [
19] and can be improved by decreasing the nitrogen concentration of the stems [
20]. It seems that increasing stem size increases NHI in low nitrogen amounts, and the phenomenon is reversed in nitrogen-unlimited conditions [
21].
In Mediterranean conditions, the N application rate depends mainly on rainfall in the vegetative period [
22,
23]. Ren et al. [
24] demonstrated that rainfall in the early stages favored high yields with 210 UN in wet years, but the application of 150 UN resulted in the highest grain yields in dry years. The variation in N application rate was extensively studied in Mediterranean conditions [
25,
26,
27,
28]. In the Algerian regions, there has been anarchy in the application of nitrogen over the years [
29] due to the lack of knowledge in the management of nitrogen fertilization practices [
7,
30]. Furthermore, the optimal application of nitrogen with fewer environmental problems in these areas is unknown. Indeed, in a similar region, Ayadi et al. [
31] demonstrated that 150 UN increased GY but reduced NUE. In this context, to achieve economic benefits and environmental conservation from N leaching, we hypothesize that decreasing N rates under 150 UN by a regular step would conserve grain yield without reducing NUE. Based on this assumption, the aim of this study is (i) to examine how the addition of gradually increasing amounts of N affects the GY, the GPC, and the NUE and verify if these parameters enhance GY under Algerian semiarid conditions, (ii) to investigate the relationship between rainfall amounts and NUE, (iii) to determine the principal components presenting a direct effect on GY and the main indicators of NUE, and (iv) to suggest a sustainable approach using an environment-friendly N amount with a minimum of N losses and economic profitability.
4. Discussion
In both growing seasons, the effect of the years was highly significant for GY, its components (Nspike/m
2, Ngrains·m
−2, and TGW), and the GPC. Under rainfed conditions, the increase in N fertilizer rate increased GY significantly, as reported in several studies [
32,
40]. The maximum grain yield was noted in the wet year (WY2018) with the maximum nitrogen rate (150 UN).
The significant difference in GY (4 t·ha
−1) between the two trial years (DY2017 and WY 2018) can be explained in light of the good distribution of rainfall over the wheat vegetative development on the GY. It occurs during the vegetative growth period, where the crop receives a total of 230 and 15 mm rainfall in the wet and dry years, respectively. The favorable soil moisture regime might have contributed to better N uptake and N translocation to the sinks [
41], thereby increasing the yield attributing characters, which finally contributed to higher grain yield in the wet year. Moreover, the effect of N inputs was well expressed by the assimilation of N during the pre-anthesis like that was observed in the wet year (2018) and reported in similar results [
42,
43,
44].
The GY was correlated to the number of grains·m
−2, which was the main component in the development of yield in the wet year (2018). Indeed, the path coefficient analysis during the wet year (2018) demonstrated that the number of grains·m
−2 has a direct effect on GY that match with other works on wheat [
45,
46] and can be explained by the reducing of N amount at tillering and increasing at recovery growing stages [
47], contrary to the dry year (2017). In this year, the N inputs favored the TGW over the number of grains which is indicated by a positive and indirect effect between the number of grains·m
−2 and TGW (
Table 5). This positive correlation between the number of grains·m
−2 and TGW supports the results achieved by Gweyi-Onyango et al. [
48] on rice crops. However, the study of Giambalvo et al. [
42] on durum wheat showed a significant decreasing effect in the number of grains·m
−2 and TGW.
The TGW is a key component of the final yield. The year effect was very significant, but the N input had a less significant effect on grain weight in the dry year (2017), and the Bousselam variety did not express its potential (TGW < 35 g) (
Table 3). This could be explained by the climate scene, where the cumulative rainfall of the three months (March, April, and May) was less than 15mm. The average temperature could not increase above 20 °C until June (
Figure 1). In addition, these weather variations were not in favor of the dynamics of accumulation of dry matter in grain (source-sink relations) at the beginning of grain filling and even before this stage, where biomass production is carried out [
31,
49].
Derbal et al. [
22] also stated this reasoning; they concluded that rainfall conditions in semiarid areas provide the best opportunity to produce high-quality durum wheat. Indeed, in the wet year (2018), results have shown that the Bousselam variety expressed its potential on TGW (>50 g), and the effect of N application was not significant.
The presence of an inverse relationship between grain yield and GPC has been deduced in several studies [
50,
51], while some studies pointed out a simultaneous increase in GY and GPC with adequate N fertilization during the vegetative stage followed by one late topdressing in irrigated wheat grains [
52,
53]. In the current study, the grain protein content increased with increasing N application over the two years (
Table 3). The results are in agreement with previous studies, which showed that the protein content increased linearly with the increase in N application [
54,
55]. Furthermore, at a low nitrogen concentration in the plant, there is an insufficient translocation of assimilates from the organs to the grain, while in a situation of excess nitrogen in the soil, the plants often stop nitrogen uptake due to the nitrogen concentration of amino acids in the plant organs [
56].
In the dry year (2017), the Bousselem variety recorded a high GPC (17%) at 120 UN only, while in the wet season, despite the high yield [6.9 t/ha], the GPC was only 12% even with N dose of 150 UN. The higher GPC in the dry year could be explained by the presence of nitrogen assimilates absorbed through the amount of rainfall (41.91 mm) recorded in June, which corresponds to the grain-filling stage. Abdellaoui et al. [
57] reported that a variation in GPC was affected by post-anthesis nutrients and weather conditions and confirmed by Giunta et al. [
58], who found that N uptake in post-anthesis allows getting both high-grain protein and yields. Brown and Petrie [
59] also reported that the late intake of N caused an increase in protein content.
The different NUE indicators were significantly higher in the wet year (2018), with 468 mm of rainfall than in the dry year (2017), with only 195 mm. Several works go towards this positive and significant improvement of NUE with moisture. Gauer et al., 1962 [
54] have demonstrated that moisture conditions make possible the improvement of NUE and thus increased GY, especially in rainfall areas where supplement irrigation was fundamental to ameliorate the crop yield and NUE of some crops [
14]. Souissi et al. [
10] reported that irrigation has a positive effect on NUE. But during the pre-anthesis phase, adequate soil moisture can increase NUEs, and the water deficit during this phase can limit N movement and may ensure a reduction in N uptake and NUE [
41]. In addition, the two major physiological processes underlying grain N supply for wheat are late post-anthesis uptake and translocation of N stored in vegetative parts before flowering [
20,
60], which influence the improvement of nitrogen use efficiency. More so, it is pertinent to point out that in both years of the current study, the indicators AE and ARE were higher, with 120 kg of N application rate.
The positive correlations between GY and NUE (AE and ARE) (
Figure 3) are consistent with those demonstrated by Fageria et al. [
61]. They reported that a significant positive association between GY and NUE can improve grain yield. Nevertheless, through the path analysis, it appears that the positive correlation between GY and AE (r = 0.67 ***) is instead due to the positive and indirect effect of ARE (
p = 0.86*0.78), which means that it is important to take into account the mineralization of N in the soil. Indeed, González-Montaner et al., 1997 [
45] showed that the increase of the AE is dependent on mineral N in soil and environmental factors and confirmed the importance of improving ARE in breeding programs to improve NUE.
López-Bellido et al. [
62] recommended the application of N fertilizer primarily as a top dressing in durum wheat between tillering and stem elongation stages to improve crop NUE and reduce losses through the leachate process and runoff losses. In this study, N was applied at tillering and stem elongation stages, contributing to the increase of NUE.
For NHI, the values were steady for the two years separately, with a slight difference between them (
Table 4). It was not influenced significantly by the fertilizer N rates and was higher than that obtained under the same conditions by Benchelali et al. [
23]. Fageria [
61] reported that the higher NHI positively correlates with GY in the wet year. Furthermore, the study of the two parameters NHI and NUEp can explain the mechanism of nitrogen use of the different applied rates in the two study years. It could be assumed that the presence of water favored the remobilization of pre-anthesis assimilated nitrogen towards the grains, thus increasing the values of NHI and allowing protein synthesis [
19,
20].
The second objective of this investigation was to determine an eco-friendly, economical and agronomical N application rate for highly refined research on the N use efficiency of local varieties.
According to
Figure 5, where agronomic (NUE, GY, GPC), economic (MRR), and environmental (apparent loss of N) parameters were combined, the 120 UN rate showed better results. In a wet year (2018), the application of N at 120 UN recorded both a higher grain yield (6.27 t·ha
−1) and grain quality 11.4% GPC, gave the highest marginal return (15.64) and was found to be more eco-friendly with high apparent recovery efficiency (ARE = 81.08%). Moreover, in the dry year (2017), the application of 120 UN recorded the highest GY (1.2 t·ha
−1), similar to the treatment T7 = 150 UN, and reached the highest GPC (17.5%) and ARE = 29.78%.
The field trials should be extended to other local and modern varieties for their quality potential (GPC), especially to test N applications with different splitting schedules of N. This is to practice 4R N management (right source, right rate, right time, and right place) [
63].
Results were uneven through the years, but they put us at the same N application rate (120 kg). In the semiarid regions, reducing the amount of nitrogen application for economic benefit (MNR) and environmental conservation (apparent N loss) is interesting.
The results obtained in this work should help to improve the accuracy of the nitrogen fertilizer advice and additional N management, both in terms of the level to be applied and weather variation. This work also shows that the current evolution of fertilizer application practices ensures economic sustainability and the quality of cereal production while respecting the environment.