## 3 Results and Discussion
### Plants Isotopic Values (C-N-S), Comparison Between Regions
#### 3.1.2 \(\delta^{15}\)N of Plants  
The stable isotope ratio of nitrogen (\(\delta^{15}\)N) in all the studied plants ranges from -\(\delta\) 4.0% to 55.1% (except for citrus, 12.1%; see Table 1 and Figures 3 and 4). The observed difference between the samples, approximately \(\delta\)10%, cannot be fully explained by the differences in rainfall (Figure 1). It can be attributed to the agricultural regime, climate, altitude, distance from the coast, and different crops. Furthermore, it seems that in some plant species, the high \(\delta^{15}\)N (>4%) could be attributed to the proximity to the sea (Ilia region and Sparta area) and/or in arid climates (Nemea area, Sparta area and Heraklion region) since the \({}^{15}\)N isotope grows in drought [48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59].  
In addition, a difference in nitrogen isotope values is observed between northern, central, and southern Greece, most likely indicating that climate significantly impacts the final \(\delta^{15}\)N values of plants. The nitrogen isotopic values in northern Greece (Kozani and Naousa regions) and central Greece (Fthioidta region) are less than \(\delta\)2%, while in Peloponnese (apart from one sample from Messinia of western Peloponnese) and in Crete the nitrogen isotopic values are more than \(\delta\)2% (Figure 3). [PERSON] and [PERSON] [48] demonstrated that the slope of the variation in \(\delta^{15}\)N with precipitation is equal to 0.38/100 mm for both Greek plants and herbivores (the calculated slope refers to a range of annual precipitations from 300 to 1200 mm).  
The \(\delta^{15}\)N variation with rain intensity (precipitation) for the plants measured in this work equals 0.48/100 mm (see Figure 5; the data from the Ilia region are not included). This value differs from the previous work [48], possibly due to significant area variations. Even so, it demonstrates a small correlation between nitrogen and climate. The altitude  
Figure 2: Carbon values (\(\delta^{13}\)C) versus temperature. The inserted figure illustrates a temperature map of Greece.  
The studied plants from the same locality, such as the Messinia, Ilia, and Fthiotida regions and Kiato area, show a difference between each other in nitrogen isotope value of more than 81-2% (Figure 6). This difference is likely due to the use of fertilizers. The same plants from the same area (nearby fields) show different nitrogen isotope values, indicating the use of manuring/fertilization. Different types of fertilizers differentiate the nitrogen isotope [14; 57]. Among the studied samples, only one plant showed a noteworthy higher \(\delta^{15}\)N value (812.1%), which was probably due to the use of animal manure. It is well known that nitrogen in earth is obtained through the use of organic fertilizer inputs. This is explained by the N volatilization during the N mineralization process. The latter enhances heavy nitrogen isotope (\(\delta^{15}\)N) enrichment in the N sources available for plants, resulting in higher plant \(\delta^{15}\)N value [62]. In fact, animal manure presents nitrogen values higher than +88% [63], since animals eat plants (with \(\delta^{15}\)N values that reflect nitrogen isotopes of soil N (84-8%), except for N-fixing plants, which demonstrate \(\delta^{15}\)N values similar to those of atmospheric N (80%)), and thus, their wastes show \(\delta^{15}\)N enrichment due to the fractionation processes (volatilization of ammonia and denitrification or bacterial reworking of N). In contrast, synthetic fertilizers show lower nitrogen values compared to those of organic fertilizers [14; 62]. The grains (barley and wheat) exhibit values of 2 \(\pm\) 1.3%, demonstrating the use of conventional fertilizers. [PERSON] et al. [60] observed a significant difference between the nitrogen isotopic values for conventionally grown (2.1 \(\pm\) 0.3%) and organically grown barley (>3.4 \(\pm\) 0.5%) grains. The main reason for this difference is the type of fertilizer used.  
Figure 5: \(\delta^{15}\)N of Greek plants versus precipitation (mean values) [48].