## 3 Results and Discussion
### The Carbon and Water Balances and the Water Use Efficiencies  
Over the entire extended watermelon cultivation period of six months, from June to November, photosynthetic CO\({}_{2}\) uptake exceeded CO\({}_{2}\) losses from respiration, resulting in a net C input of 49.0 g C m\({}^{-2}\). By subtracting the C removed through harvesting (278.9 \(\pm\) 70.1 g C m\({}^{-2}\)), the NBE yields a value of +229.9 g C m\({}^{-2}\) over the experimental period, indicating that this biodegradable multiched watermelon crop, managed with organic fertilization and residual green manure, is a net carbon source. In terms of carbon balance in the field, all terms of Equation (5) are reported in Figure 9. The largest amount of imported organic C is due to the green manure of non-marketable watermelon fruits left on the field, followed by the green manure of watermelon plants and broccoli, and the C input from NEE. The biodegradable mulching film contributes 12% to the carbon input, while organic fertilization has the lowest contribution. The substantial amount of C removed by harvest reduced the C stored in the soil to 19.3 g C m\({}^{-2}\) over the investigated period. This finding is in accordance with results by [59; 60] who reported increased carbon sustainability through conservative practices applied to horticultural water-consuming crops under Mediterranean conditions. However, the directly measured ASOC from soil analysis of samples in the 0-0.30 m layer (352.2 \(\pm\) 52.2 g C m\({}^{-2}\)) was much higher than the estimated C temporarily stored in the soil over the six-month experimental period (19.3 g C m\({}^{-2}\)). Conversely, this latter value is much lower than the uncertainties of the SOC measurement, being the 7% of the standard deviation of the carbon stock measured in the soil samples at the beginning of the trial (289.8 g C m\({}^{-2}\)). This result indicates the following: (i) it is extremely difficult to unambiguously establish the net C stored in the soil, as many input/output terms must be evaluated for a complete carbon balance [40; 61; 62]; (ii) C losses can be determined on a crop growth seasonal scale, but several years of continuous cultivations and green manure incorporation are necessary to detect a significant difference in soil carbon storage [40]; (iii) measurements of SOC are subject to great uncertainties, as indicated by large value spreads around the mean [63], likely linked to the spatial variability [64; 65] and short-term time processes at the atmosphere-pedosphere interface [66], often making single point measurements ambiguous. The water balance on an extended seasonal scale showed that the cumulative inputs were: precipitation, 139 mm; irrigation, 583 mm; and 5 mm of water supplied by the green manure of fresh materials (broccoli, watermelon plants and non-harvested fruits). Conversely, the outputs of evaporation and transpiration were 178 and 186 mm, respectively. Figure 8: Cumulative values of C absorbed by photosynthesis (GPP) and released by respiration (Reco) in the whole experimental period (1 June–30 November 2023). These amounts, added to the water stored in the soil (272 mm), resulted in a significant amount of water lost through deep percolation (91 mm). This quantity represents 16% of the water supplied by irrigation. Similar values of deep percolation water losses were found by [67] in watermelon cultivation under a Mediterranean climate, whereas [68] found higher values of deep percolation in semi-arid continental conditions. From the above-reported values of total gross primary production, net ecosystem exchange, and water terms, along with the marketable yield, it is possible to calculate all defined water use efficiencies. In the present case, we found \(\mathrm{WUE_{ph}}=-1.38\) g C kg H\({}_{2}\)O\({}^{-1}\), \(\mathrm{WUE_{ph}}=-0.70\) g C kgH\({}_{2}\)O\({}^{-1}\), \(\mathrm{WUE_{env}}=-0.14\) g C kg H\({}_{2}\)O\({}^{-1}\).