Tomato (Solanum lycopersicum
L.) is one of the most cultivated horticultural crops worldwide with an area of 4.8 million hectares and with production of 182 million tonnes in 2017 (FAOSTAT, 2019) [1
]. About 20% of this production is used by canning industries for paste, peeled, diced, etc., tomato products. In this context, Italy is the second-largest world producer of processing tomato (~5 million tonnes) (WPTC, 2019) [2
In recent years, one of the key agricultural challenges has been how to limit the negative impact of agricultural practices and increase crop production sustainability [3
]. Nitrogen (N) administration represents an example of such a challenge; indeed, the lack of this element is one of the major yield-limiting factors [5
], while an excess can cause environmental pollution [6
Nowadays, in some Italian areas known for processing tomato production, the marketable yield reaches at least 70 t ha−1 and more than 110 t ha−1 in the most suitable district. A potential increase in marketable yield and the possibility to apply N fertilizers at specific times have induced farmers to reconsider their N management strategies.
Yield is the main parameter adopted in comparisons among different agronomic management types. Ronga et al. [7
] reported that in processing tomato, high yields were obtained with leaf area index (LAI) of about 2.0–3.0 m2
. In addition, a high specific leaf area increases the assimilates available for fruit growth [8
]. Leaf senescence and its chlorophyll content, as well as dry matter production and photoassimilates distribution, affect crop yield and are important parameters that should be evaluated in plant growth and crop yield improvement studies [9
]. Moreover, agronomic management and pedoclimatic conditions (such as tillage, nutrient availability, and weather conditions) might affect the efficiency of source organs and the allocation of dry matter production to the plant organs [13
Nitrogen fertilization affects processing tomato yield and fruit quality parameters requested by canning industries, such as total solids, soluble solids, pH, and acidity [9
]. A fully ripe tomato fruit contains 93–96% water, which dissolves the carbohydrates, organic acids, minerals, vitamins, and pigments that represent the total soluble solids, measured as °Brix. Total soluble solids are a key parameter in processing tomato productions. In fact, tomato paste is produced and sold based on its total soluble solids content; thus, the total soluble solids dictate the factory yield. Higher total soluble solids in incoming fruits means that fewer tons of tomatoes will be needed to produce a given amount of tomato paste.
The sink–source relationship and leaf nitrogen content affect dry matter production; moreover, yield is correlated with both source capacity and sink strength [8
]. A higher allocation of biomass to fruits is a key crop goal to achieve the highest crop yield.
Several works have been carried out on the N fertilization and N uptake of drip-irrigated processing tomatoes, but with contrasting results. In Turkey, Erdal et al. [15
] displayed that 160 kg ha−1
of N was suitable to maximize fruit yield. Similar results were reported in Italy by Tei et al. [16
] supplying 200 kg ha−1
of N. On the other hand, researchers working in Australia [17
], in Brazil [18
], and in Spain [19
] reported that N rates over 300 kg ha−1
were required to achieve the highest fruit yield.
To the authors’ knowledge, only a few research papers have reported the relationships between dry matter partitioning (including root) and yield in processing tomato [20
]; moreover, these studies did not consider the effects of N fertilization on processing tomato sustainability.
The aim of our work was to display the best N rate, able to improve processing tomato production sustainability. Therefore, in the present study, two-year field trials were carried out to investigate the effects of different nitrogen rates (0, 50, 100, 150, 200, and 250 kg of N ha−1) on the agronomic, physiological, economic, and environmental aspects of processing tomato grown under conventional management in the Mediterranean area.
Processing tomato represents an important economically valuable industrial crop requiring huge amount of external inputs like fertilizers, irrigation water, and pesticides that, if not well managed, can negatively affect environmental sustainability [40
]. Hence, researchers are called to give strategic and useful information on agronomic management to improve processing tomato’s production sustainability.
Several studies have reported the effects of cultural practices on the yield and quality of this horticultural crop [21
], however, without information on economic and environmental impacts.
One of the most important issues in tomato crop production regards N administration and its soil availability [43
]. More than any other nutrient, N affects vegetative growth, yield, and quality attributes [37
Water availability, phosphorus and N fertilization, different growth seasons, shading levels, soil salinity, etc., affect dry matter partitioning in tomato crops [8
]. However, a few papers published on dry matter partitioning considered both the above- and the belowground biomass. In addition, the effects of agricultural practices should also be considered in terms of economic and environmental aspects, as well as the impact on crop yield and quality [49
]. Studies are frequently lacking in useful information on environmental impacts [28
]. From this point of view, the present study investigated the effects of different N rates on dry matter accumulation and partitioning in a processing tomato crop while also focusing on MNR and GWP, which are very important parameters of the production process.
Our results highlighted the impact of the environment (year) on both dry matter accumulation and its distribution to the different tomato plant organs. Variability in climatic conditions across the years (mainly related to air temperatures and rainfall distributions) occurred during the growing seasons and strongly affected tomato crop growth, as also reported in similar areas for the same crop [7
] and for quinoa (Chenopodium quinoa
Willd.) cultivated in the Apulia region [50
]. Indeed, the second growing season was drier and hotter than the first one, which influenced the lower values of dry matter accumulation and marketable yield of the tomato crop.
Taking into account the effects of N fertilization, as expected, the N-250 rate resulted in the highest dry matter accumulation and LAI in both the 2004 and 2005 growing seasons. Moreover, comparing N-250 to treatment N-0, an increase in dry matter accumulation and maximum LAI (on average, +130% and +280%, respectively) over the years was observed. Our results are in accordance with results found in Italian tomato cultivation [51
] and even in experimental trials performed in California [21
]. Finally, similar results were also highlighted for durum wheat and broccoli cultivated in Southern Italy by Tedone et al. [52
] and Conversa et al. [53
], respectively. Both authors reported that high nitrogen applications are required to achieve the highest values in terms of vegetation nutritional status and dry matter in both durum wheat and broccoli.
The present study highlighted the same behavior in terms of dry matter accumulation for treatments N-100, N-150, and N-200, regardless of growing seasons and, therefore, of the total dry matter allocation, suggesting no plant physiological changes in the source–sink relationship affected by N rates in the range between 100 and 200 kg ha−1
. According to these results, no translocation efficiency improvement could be obtained by applying more than 100 kg of N ha−1
. Similar results were also reported in comparisons between different farming systems (organic vs. conventional) and nitrogen fertilizers (organic vs. mineral) by Ronga et al. [7
Focusing on the dry matter distribution to the sink organ, the most striking differences as an effect of N rates were recorded in the second growing season (2005). N supply from 50 to 200 kg ha−1
resulted in the highest dry matter allocated to fruits, while the N-0 and N-250 treatments favored more accumulation to leaf, stem, and root than fruit. This behavior seemed to be related to the need to improve the source strength in the unfertilized control (N-0), while the highest N rate gave excessive and imbalanced crop growth of vegetative luxuriance. Ronga et al. [10
] and Higashide and Heuvelink [41
] pointed out the importance of a balanced leaf area as a factor in obtaining satisfactory tomato production.
The highest rates of dry matter accumulation to the fruits were clearly observed starting from the third sampling (T3, corresponding to the middle July in both years), while an opposite trend was found for root, stem, and leaf. Interestingly, the highest dry matter allocation to the root was measured in 2005 under scant natural rainfall, suggesting an improved increase in root growth by dripline in the drought environment conditions. Moreover, Poorter et al. [54
] and Hermans et al. [55
] also reported a higher biomass allocation to root than other organs under limited soil resources such as water and N.
After fruit set, the increased rates in dry matter allocation to fruits, coinciding with a decrease to the other organs, measured in our research agree with the results of Scholberg et al. [21
]. Indeed, the same authors reported that high-yielding crops, such as tomato, displayed values of ~65% of the dry matter allocated to fruit [also called the harvest index (HI)]. Tollenaar [56
] also reported in maize crop a high yield according to the greatest dry matter production, although the HI did not vary, as also showed in our study, as not affected by N supply.
Fertilization affected the unmarketable, marketable, and total yields and fruit quality (in terms of brix t per
ha) similarly in both years, resulting in increasing effects up to 200 Kg ha−1
(without significant differences from 250 Kg ha−1
). Similar research, performed in the same location on peeling tomato (cv. Galeon), reported increasing yield up to N-200 [46
]. The authors also reported no significant effects in tomato production moving from N-200 to N-250 supply. The yield performance under high N rates was also in accordance with other investigations carried out in a similar area [10
] and under different environmental conditions [57
Inappropriate N administration can reduce crop production and NUE, resulting in increased fertilizer losses and in reduced tomato crop sustainability [46
]. NUE is a complex physiological parameter depending on plant genetic background and affected by environmental factors. It has two components: N uptake efficiency (NUpE) and N utilization efficiency (NUtE) [59
In the present study, the N-100 and N-150 rates showed the highest values of NUE and NupE in both years. Despite the highest marketable tomato yield being achieved by supplying 200–250 Kg of N ha−1
, these N fertilization levels displayed intermediate and low values of NUE and NUpE, respectively. Considering the importance of N management as a key factor in achieving satisfactory yields, the results of our research identified treatment N-200 as the optimal rate to obtain high tomato production and avoiding N losses like volatilization and leaching, which both contribute to the impact due to GWP [59
In the field experiments, tomato crops were well watered in both years. Our results showed the highest values of crop water productivity and fruit water productivity in the N-200 and N-250 fertilized plants, confirming the important relationship between soil water content and N fertilization that was also reported in other studies [62
]. In addition, similar results were reported by Mastalerczuk et al. [64
] on festulolium, confirming that lack of nitrogen results in lower values of water use efficiency.
Crop MNR and environmental impacts due to N administration are two aspects frequently ignored by farmers. In this experiment, no significant increases in MNR were detected above 200 kg of N ha−1
in both the years of research. On the other hand, N rates less than N-200 did not result in a satisfactory marketable yield; however, no significant differences in MNR were noticed for the N-150 rate. In the Mediterranean environment, low N supplies also resulted in unacceptable values of MNR for wheat; moreover, the production trend resulting from N fertilization was affected by interannual climatic variability [49
Farmers adopt huge amounts of external inputs for agricultural productions, contributing to GWP via greenhouse gases, like carbon dioxide, methane, and nitrous oxide. Hence, the implementation of sustainable agronomic management to mitigate the production of GHGs is a crucial point [65
]. In our study, treatment N-200 resulted in the lowest GWP impact in both the growing seasons, especially due to the highest marketable yields. The GWP of processing tomato production per
1 ha obviously increased according to N fertilization. Interestingly, our results on GWP, both per
marketable yield and per
cropped area, are in agreement with data reported by Ronga et al. [40
] and Ntinas et al. [65
] who investigated the GWP of processing tomato cultivation in similar areas.