1. Introduction
Sunflower (
Helianthus annuus L.) belongs to the
Compositae family and is a native crop in North America. It was introduced into Europe in 1510 by Spanish explorers [
1]. Additionally, sunflower is one of the most important new edible oil crops worldwide and it is also a desirable crop that can be satisfactorily cultivated in intensive dryland rotations [
2]. Its oil has a high fatty acid content that is not synthesized by humans’ (linoleic and oleic acids) and it is used widely in the food industry in place of olive oil [
3,
4]. Furthermore, sunflower oil is used for biodiesel production, thus reducing the negative impacts of non-renewable energy sources in the environment and in society [
5].
In many countries all over the world, the increase in fertilizer price in combination with the increasing cost of fossil fuels has caused producers to consider conservation agriculture, which includes lower impact cultivation practices such as no tillage cultivation, permanent crop residues on the surface, and crop rotation [
6,
7]. Conservation tillage is able to significantly improve soil properties (physical, biological, and chemical) and other biotic factors, reduce soil erosion, improve the water infiltration, and help in the reduction of the production costs [
8,
9,
10].
Nitrogen (N) is the most essential nutrient in agricultural production. It plays an important role in crop photosynthesis and in the total biomass increase of the
Hellianthus annuus. Zubillaga et al. [
11] showed that the sunflower biomass increased from 19 to 40% when an optimum nitrogen amount was applied. In addition, nitrogen fertilizer had positive effects on the dry yield of sunflower according to other research [
12,
13]. On the other hand, nitrogen is considered to be one of the most expensive inputs in agricultural management [
14].
It is well known that legumes have the ability to fix atmospheric N. Haugaard-Nielsen et al. [
15] indicated that nitrogen uptake by legumes was higher in a rotation system compared with a monoculture system. Miller et al. [
16] showed that pea fixes more atmospheric N than lentil and cowpea.
Pisum sativum residues in a rotation system have beneficial effects on agronomic plant characteristics and on total crop production, wherefore the nitrogen requirements of the succeeding crops are reduced and less additional nitrogen fertilizer is applied [
17]. According to Skoufogianni et al. [
18], the incorporation of pea into a rotation system (pea–maize and pea–sunflower) increased the biomass and the nitrogen use efficiency.
The use of alternative cultural practices such as using no tillage (direct seeding) can be enhanced by the farmers. The no tillage practice is one of the three fundamental principles of conservation agriculture. This practice has better protection against soil erosion and offers a greater efficiency in nutrient uptake by plants [
19,
20].
The contour tillage is a more sustainable practice in comparison with that usually expected in flat fields (in straight lines) or for along-the-slope tillage. Adverse effects become more pronounced under intensive rainfall events. Contour cultivation on fields with a high inclination percentage can decrease soil erodibility, thus increasing topsoil resistance [
21].
Information on the effect of cover crop using legumes for no tillage cultivation in combination with different planting systems (parallel and perpendicular to the contour) in sunflower cultivation in sloping land is lacking.
For this reason, we examined the effect of a two-year rotation system, including Triticosecale-Pisum sativum for winter cultivation and Helianthus annuus for summer cultivation, with different tillage practices (conventional and no tillage), being planted parallel or perpendicular to the contour on a dryland sloping field, on sunflower growth and quality under a Greek climate.
The aims of this work were to measure the plant height, leaf area index, specific leaf area, plants’ total nitrogen and protein content, and N uptake of sunflower cultivated under the above agricultural conditions.
3. Discussion
Our study showed that the plant height was higher for the no tillage practice. Our results are in disagreement with other investigations [
22,
23]. According to their study, the plant height was higher in conventional tillage in comparison with the no tillage cultivation. Sessiz et al. [
22] found that the plant height ranged from 128 to 137 cm for conventional tillage and from 122 to 128 cm for the no tillage practice. In our research, the height of the plants varied from 64.9 to 80.1cm using conventional management and from 78.5 to 86.5 cm in the no tillage treatments. Furthermore, Mourad et al. [
23] observed that the plant height was better in conventional tillage compared with no tillage, but in the second year of experiments, the height was lower in comparison with the first year in both cultivation practices. In our investigation, the results showed that in the second year, the plants’ height was higher. It is clear that after the second cultivation year, the incorporation of
Triticosecale-
Pisum sativum residues helped increase the final crop height.
Data from our study site indicate that the no tillage planting parallel to the contour had the highest LAI content. A statistically significant difference was only observed in the second measurement (27 August 2015 and 16 August 2016) between the different tillage practices in both of the growing years. Mujeed-ul-Haq et al. [
13] noticed similar LAI values with our findings when 110 kg N ha
−1 were applied in sunflower cultivation. In other research, the LAI reached a value of 5.13 and 5.37 m
2 m
−2 in conventional (reduced) and traditional tillage 90 days after sowing, respectively [
23]. Aboudrare et al. [
24] stated that the LAI was lower (2.39 m
2 m
−2) than our results using the no tillage cultivation. Only a few studies have addressed the relation between LAI and different tillage practices under sloping conditions for sunflower cultivation.
In our investigation, the plants’ total nitrogen ranged from 4.153% in the NTC-PEC treatment to 4.388% in the NTC-PAC during the first year. In the second year, the nitrogen values were higher than for the first year. Between the two growing years, the increase in nitrogen content ranged from 1 to 2%. Murillo et al. [
25] found out that the total nitrogen content was from 0.76 to 3.48, which were lower values compared with our results.
In addition, the no tillage management had positive effects in the protein values compared with conventional practice. A statistically significant difference was observed in the second year of cultivation. Scheiner et al. [
26] mentioned that the protein content was from 20.2 to 23.6%, which were lower values in comparison with our study.
Regarding N-uptake, the NTC-PAC treatment performed better with higher values compared with the other practices. The N-uptake reached values of 235 and 265 kg ha
−1 in the first and second years, respectively. The same results have been mentioned in other studies [
11,
27]. The findings of Lopez-Bellino et al. [
28] showed that the no tillage treatment positively impacted N-uptake, in agreement with our results. Skoufogianni et al. [
18] found that the sunflower N-uptake reached a value of 221 kg ha
−1 when pea was incorporated into the soil.
Our investigation indicated that pea residues incorporated into the field can provoke a significant increase in all of the above measurements.
5. Conclusions
In this research, we evaluated the impact of no tillage on plant height, LAI, SLA, plants’ nitrogen, and proteins of sunflower in a rotation system intercropping Triticosecale-Pisum sativum under natural rainfall in comparison with conventional agriculture. In addition, we tested the effect of the planting direction (parallel and perpendicular to the contour).
The results showed that plant height was higher in the no tillage planting parallel to the contour cultivation. LAI (3.69 the first year and 4.01 the second year) had the maximum values in the NTC-PAC treatment. No tillage planting parallel to the contour direction (NTC-PAC) treatment, also had a positive impact on the SLA values.
The total nitrogen and proteins of the sunflower plants were better in the second year of the experiments and the highest increase was found for the CTC-PAC treatment (2%). Furthermore, the no tillage practice affected the N-uptake in both of the cultivation years.
To sum up, in a sloping dryland field, sunflower can be a promising crop when cultivated in a rotation system intercropping Triticosecae – Pisum sativum using the no tillage planting parallel to the contour processing in a Greek climate.
The incorporation of leguminous residues into fields can significantly increase the nitrogen content and many abandoned sloping lands could be cultivated.