1. Introduction
Durum wheat (
Triticum turgidum L. subsp.
durum (Desf.) Husn.) is an economically important crop cultivated worldwide. Europe-28 is by far the largest world durum wheat producer. In 2017, it was grown on 2.7 million hectares only in the European Union (EU), providing an output of about 9 million tons. The cultivation area of durum wheat in Europe is mostly concentrated in the Mediterranean region: Italy, Spain and France together account for 80% of total EU production [
1]. Italy is the top EU producer country and a traditional durum wheat growing region as it dedicates half of the total EU durum wheat area to this crop, thus accounting for 45% of the entire EU production, with a yield of about 3.2 t ha
−1.
Grain quality has become one of the most important goals for the breeders and growers [
2,
3], because it is essential in obtaining premium prices and meeting markets needs for high-quality end-products of durum wheat such as pasta, couscous and burghul [
4].
There is no simple and complete definition for the quality of durum wheat [
4,
5]. Grain protein content, color and gluten strength are considered the most important features needed for use in pasta and bread production. Grain protein content is known to be influenced by climatic parameters, genetic factors, nitrogen fertilizer rate, time of nitrogen application, residual soil nitrogen and available moisture during grain filling [
6,
7,
8,
9]. The yellow color is due to the carotenoid pigment content in the whole kernel, and it is commercially identified as the yellow index in semolina. Besides their role as an important aesthetic parameter, the carotenoids have important nutritional and health characteristics [
10]. While yellow index was found to be affected by weather conditions, cultivar and N rate and timing [
10,
11,
12], less is known about the effect of N source and S fertilization on this quality index.
Gluten strength contributes to the ability of dough to rise and maintain its shape as it is baked. Gluten strength is commonly estimated using the sodium dodecyl sulfate (SDS) sedimentation test that, depending on the protein quality, provides a good indicator of pasta cooking quality [
13,
14,
15]. Ercoli et al. (2011) [
15] found that SDS value increases with the increase of inorganic N (from 120 to 180 kg ha
−1) and S (from 0 to 60 kg ha
−1) rate.
The use of nitrogen is normally considered a key factor in cereal crops and numerous studies on the best N fertilization rates and timing have been conducted. In fact, if on the one hand it has been proved that nitrogen positively affects grain yield and quality, on the other hand N fertilizer management is pivotal to avoiding N losses caused by leaching, runoff, denitrification or volatilization [
11,
16,
17].
After taking all this into account, the use of organic N fertilizers may be a further option, together with other cropping management and practices, to reduce nitrate pollution and improve the environmental sustainability of conventional farming systems [
12]. Thus, another feature can be added to the definition of the quality of wheat products [
18].
Moreover, fertility management was the identified key factor in limiting both yield and grain protein content in the organic wheat management [
12,
19,
20]. The results on common wheat (
Triticum aestivum L.) emphasize the importance of a sufficient supply of soils with organic fertilizers as well as the need to improve the availability of organic nitrogen [
19,
21]. This latter option might be accomplished by trying to regulate the degradation and mineralization of organic matter (OM) in the soils, which is the traditional role assigned to heterotrophic microbes [
22]. The number of these microorganisms, and in particular those which oxidize sulfur (S), was found to be: (i) greater in some rizospheres (e.g., canola and wheat) than in bulk soil controls [
23]; and (ii) stimulated by S fertilization and soil OM [
24]. A recent study conducted in the Canadian prairie showed that common wheat biomass production in organic systems was positively related (among other factors) to the plant tissue S concentration [
21]. Thus, S application to the soil might have a synergistic effect with organic N fertilization of durum wheat, determining higher yields and better quality. This hypothesis could be particularly verified in those agroecosystems that extend along the Mediterranean coast, in which soil temperature and water availability during the winter season do not drastically reduce mineralization capacity by the soil biota.
Sulfur is an essential element for all organisms since it is present in many molecules (amino acids, oligopeptides, vitamins and many secondary metabolites) and it is involved in several biochemical processes. Plants absorb S as sulfate ion (SO
42−) from soil solution and use it in key steps of their metabolism [
25]. Furthermore, findings provide evidence for the uptake and metabolization of elemental S also at the leaf level [
26,
27]. However, the fact that symptoms of deficiency appear earlier in young leaves than mature ones suggests that S is relatively immobile in mature leaves and that the re-distribution from vegetative tissues to wheat kernels is noticeably less than that of N and P [
28]. These are significant findings when considering foliar S application from flag leaf emergence to anthesis, aiming to reach greater efficiency in the S fertilization [
29,
30].
The importance of S in plant nutrition is highlighted by the fact that a limited availability of this element causes both direct (biomass reduction) and indirect production loss [
28,
31,
32,
33]. The indirect effects on plants productivity are attributable to the role that S plays in the synthesis of several metabolites such as Sulfur-containing Defense Compounds (SDC) involved in the physiological response to biotic and abiotic stresses [
34,
35,
36]. Such considerations have led to studying grain yield and quality responses to S fertilizer and thus developing improved N and S fertilization strategies [
11,
16,
37,
38].
Although many studies have been conducted on the influence of N and S fertilization on common wheat characteristics such as growth, yield, quality and technological properties [
28,
29,
30,
38,
39,
40], still very little is known about the effect of S and N nutrition on grain yield and quality of durum wheat.
Studies conducted in the Mediterranean basin show different results leading to different conclusions. Garrido-Lestache et al. (2005) [
11] in a three-year field experiment in southern Spain found that soil or leaf application of S had no effect on quality indices, with the exception of ash content. Conversely, Lerner et al. (2006) [
41] in Argentina described a positive effect of sulfur fertilization on wheat quality traits. A similar result was observed in Southern Italy [
42] and by Ercoli et al. (2011) [
15] in Central Italy, even if they did not find a significant effect of S fertilization on grain protein content. Moreover, to the best of our knowledge, no studies have been conducted on durum wheat aiming to test simultaneously the interaction of S fertilization type with both mineral and organic N source.
Thus, the aims of the present study were: (1) to evaluate the effect of different N-S fertilization rates and types on grain yield and quality of three durum wheat cultivars representative of the Mediterranean region; and (2) to verify the hypothesis that soil S fertilization has a synergetic effect with organic N fertilization, on improving grain yield and quality of durum wheat.
4. Discussion
Results of this study demonstrate that fertilization type and rate have a strong influence on durum wheat yield, yield components and quality characteristics of grain. However, as also found by other authors [
6,
11,
17], crop growth, yield and quality traits are mainly a function of environmental conditions. In fact, significant second and third order interactions were found which confirm the year on year variations for durum wheat production. This variability may be due to changes in the rainfall amount and distribution throughout the growing seasons [
48].
Overall, organic fertilization was a determining factor in the observed reduction of grain yield and quality. As other studies confirm, mineral N fertilization gives better results as compared to organic fertilization, either in terms of yield and protein content [
49,
50]. These studies reported that winter wheat receiving organic fertilization had yields up to 19% lower than that fertilized with mineral N, on average. In our study, considering only the N effects and the same N rate, mineral nitrogen fertilization gained significantly higher grain yield than NO, ranging from +21% to +23%.
Similarly, protein content and SDS test values were higher for mineral fertilization as compared to the organic one, and they increased with the increase of nitrogen rate. These results are consistent with those by other authors and corroborate the issue of N availability in the case of organic fertilization, which is limited during the crop reproductive phases and always lower when compared to mineral nitrogen [
12,
50]. Systems that are based on organic fertilization usually have very different seasonal N cycle and availability than those that use mineral fertilizers. Thus, reliance on organic N sources requires an understanding of organic N mineralization–immobilization and turnover patterns in relation to crop N demands and N loss pathways. Besides the dependence of mineralization on pedoclimatic conditions [
51], it usually takes many years to mineralize past organic fertilizer and support crops with appropriate N availability [
52]. Moreover, even if a balance is reached, winter crops generally suffer significant yield reduction due to slow mineralization during their growth cycle. Even though commercial organic fertilizer, such as that used in this study, contains this reduction due to its low C/N ratio and higher nutrient availability, the yield gap was found to be significant even after six years [
53]. However, a long-term study would better clarify if a multi-year application of an organic fertilizer may reduce this yield gap, thanks to the positive effects that organic matter has on physical, chemical and biological properties of the soil.
In this study, yield gain for NM fertilization treatments was at 14.5% between 120 and 200 kg ha
−1 while it was just at 3% from 160 to 200 kg ha
−1. On the contrary, no significant increase was detected with the increase of NO rate (from 160 to 200 kg ha
−1). Ercoli et al. (2011) [
15] found an increase of 20% (from 120 to 180 kg ha
−1 of NM) in similar climatic conditions. Other authors in Spain reported no grain yield response to NM rates of up to 100 kg ha
−1 [
11,
54,
55]. Such a large difference is probably because wheat yield is influenced by N rate only when the amount of rainfall exceeds 450 mm during the growing season, as reported by Lopez-Bellido et al. (1996) [
48] in a long-term experiment. In fact, in the wetter year (2011), we obtained significantly different yields at each N rates (both NM and NO), whereas in the drier year (2012), NM160 and NM200 yields were similar. In 2012, only the NM120 yield was significantly lower, supporting the finding that the crop fertilized with 120 kg ha
-1 of N, often has sub-optimal yield performance [
15].
Concerning varieties, all the tested genotypes responded similarly to the year-on-year variations of climatic conditions, but were differently sensitive to N fertilization in each year. Particularly, all the three cultivars were markedly sensitive to water shortage which especially reduced the number of kernels per spike and the mean kernel weight. This behavior was also verified by Ercoli et al. (2011) [
15] in medium and late-maturing varieties (Claudio and Creso, respectively), while in early or medium-early varieties they did not find any yield difference between wet and dry season. However, other authors also found short-cycle cultivars decreasing grain yield with the decrease of the rainfall amount in the Mediterranean environment [
11,
56]. The different behavior of cultivars in response to climatic fluctuations is of crucial importance for Mediterranean environments, because of the high year-to-year variability in rainfall and temperature pattern existing in the climate.
In our study, the yield performance of the cultivar Dylan in the drier year was unexpected. This medium-late variety was expected to yield poorly in the most limiting environmental conditions of the second year, while it performed similarly to early-maturing cultivars with NM treatments and even better with NO fertilization. This finding is consistent with the results from the Italian durum wheat network, which show that, in the last seven years (from 2011 to 2017) and in an environment comparable to that of this study, Dylan yielded similarly to Iride and/or Saragolla in five different seasons (2012, 2013, 2014, 2015, and 2017). Although Iride had the same yielding performance of Dylan, it did not have the same quality of grains, showing a significantly lower protein content, yellow index, test weight and grain vitrousness. Saragolla was the lowest yielding variety but that with the highest SDS value.
Concerning the effect of sulfur, it is known that elemental sulfur (ES) has to be oxidized to SO
42− before it is available for plants and that the response to sulfur fertilization can be very variable in wheat. Sulfur uptake and metabolization depends on soil N and S balance, water supply, timing and rates of N and S application [
28,
57,
58]. With regard to grain yield, we found that, for a given N rate, the S application to the soil had a synergistic effect with organic rather than mineral N fertilization. Yield gains obtained from S fertilization ranged between 280 and 310 kg ha
−1 (+7%) within a same NO rate and were caused by a higher number of both spikes per unit area and kernels per spike. Consistently with our findings, several studies reported a similar grain yield increase and suggested that S deficiency leads to a reduction of the number of spikelets or to an increase of floret mortality [
28,
39,
59,
60]. The synergistic effect of S with NO fertilization may be attributed to the higher rates of OM degradation achieved by the improved activity of the heterotrophic S-oxidizing microorganisms and the resulting release of other nutrients [
61,
62]. In fact, there is evidence which shows that organic amendments to the soil promote ES oxidation rates and that some specific rhizosphere (e.g., wheat and canola) may stimulate the proliferation of heterotrophic ES oxidizing microorganisms and arbuscular mycorrhyzal fungi [
21,
23,
24,
63,
64,
65,
66]. Moreover, in a recent study, regression analysis showed that initial soil pH was the most important factor affecting ES oxidation, followed by OM content [
67]. Specifically, in soils with pH above 6.65 and higher S and OM content, the ES oxidation rate was found to be significantly higher than that of the other soils. In our study, pH after fertilization treatments may have played an important role in the ES oxidation dynamic, considering that ammonium nitrate (used to fertilize the NM plots) has soil acidifying potential while organic fertilization often resulted in an increased soil pH [
68].
Even though some authors found that S deficiency may have a significant effect on the synthesis and accumulation of proteins [
29,
69], generally, the S nutrition of wheat has a marked effect on the composition of the seed storage protein, rather than the concentration of total proteins in seeds [
28,
70]. Although we did not find any effect of S application to the soil on grain protein concentration and SDS test [
15,
71,
72], it must be observed that foliar S application significantly increased both these traits. This effect could be due to a better assimilation of N and S, as previously reported by Tea et al. (2007) [
30]. In that same study, the authors demonstrated that S applied by foliar spray was mainly assimilated in the grain and here it may favor N accumulation. Furthermore, many studies, as reviewed by Zhao et al. (1999) [
28], demonstrated that the higher S accumulation in grain determines an increase in disulfide groups (polymeric glutenins), which are related to a higher gluten strength that means a higher SDS sedimentation value. Our results are consistent with those by Ercoli et al. (2011) [
15], who showed that SDS sedimentation values and alveograph W were the quality indices highlighting the highest correlation with S concentration. The same authors argued that a high S concentration in grain is a key factor in obtaining a high quality pasta.