**3. Results**

#### *3.1. Plant Biomass and N Uptake*

Plant biomass was influenced by treatments and the growing year. Higher biomass of fruits and shoots were obtained in systems where clover was incorporated into the soil with and without plastic mulch compared to no-till cover crop-based systems in 2016 (Table 2). Only the treatment where clover was incorporated under plastic mulch resulted in higher root biomass that year.


**Table 2.** Plant dry biomass as a ffected by tillage and cover crop management.

CT-CC: conventionally tilled + cover crop; CT-CC-PM: conventionally tilled + cover crop + plastic mulch; CT-NC: conventionally tilled without cover crop; CT-NC-PM: conventionally tilled without cover crop + plastic mulch; NT-CC-SW: no-till + cover crop + supplemental weeding; NT-CC: no-till + cover crop; NT-NC: no-till without cover crop (weedy control). SE = standard error. Values followed by different letters are significantly different at *p* < 0.05.

In 2017, dry biomass of fruits, shoots, and roots revealed the outperformance of plastic mulch systems over the other systems mainly where clover was incorporated as green manure. The dead mulch had the lowest performance among residue managemen<sup>t</sup> techniques for all the biomass components. The supplemental weeding over the dead mulch increased fruits', roots', and shoots' dry matter. Generally, plants of all treatments had better performance in that season compared to 2016.

Nitrogen uptake in both seasons followed almost the same trend of the plant biomass which was the main contributor to it (Table 3). Total N uptake in 2016 was higher in conventionally tilled plots over no-till with no significant di fferences between clover incorporated and clover incorporated in soil covered with plastic mulch. In 2017, N uptake was the lowest in no-till plants and the highest in plastic mulch system with green manure due to the large N uptakes in shoots, roots, and fruits. Di fferences in nitrogen concentration among treatments in the di fferent plant parts were not statistically significant (data not presented).


**Table 3.** N uptake by tomato plants as affected by tillage and cover crop management.

Values followed by different letters are significantly different at *p* < 0.05.

#### *3.2. Yield Components and Fruit Quality*

Treatments and growing season both had effects on yield components. Irrespective of residues managemen<sup>t</sup> and the presence of plastic mulch, higher total fruit number was obtained in conventionally tilled systems with respect to no-till in 2016 due to the higher number of red fruits (Table 4). However, the production of marketable and unmarketable fruits depended on the treatment adopted. The CT-NC produced the highest marketable fruits and had the lowest proportion of unmarketable fruits number among CT systems. Marketable fruits in that year were lower than 2017 due to the presence of disease and physiological disorder incidences. In 2017, systems where plastic mulch was preceded with clover as green manure produced the highest number of fruits due to the higher production of red marketable (similar to CT-NC-PM) and unmarketable tomatoes alike and resulted in the highest fresh yield for each type, compared to the other systems. The proportion of discarded fruits of the whole fruit production in 2017, however, was not affected by tillage and cover crop presence. All conventionally tilled systems especially where green manure was present produced more green fruits than no-till in that year.


**Table 4.** Number of tomato fruits obtained in each system as affected by tillage and cover crop management.

Values followed by different letters are significantly different at *p* < 0.05.

Therefore, total yield (Table 5) in both years was drastically reduced under no-till-dead mulch conditions, at least 85% in 2016 and 66% in 2017 compared with incorporated clover, with higher productivity where a supplemental weeding was performed in 2017. However, the effect of the different treatments on yield depended on the season. In 2016, production under plastic mulch conditions was similar to tilled systems without cover crop and kept bare during the season (CT-NC). The highest production was achieved where clover was turned as green manure without a plastic mulch (CT-CC) and this was due to the high number and singular weight of tomato fruits. In 2017, the total productivity reached its highest value (60–70 t ha−1) in plastic mulch systems. Squarrose clover incorporated and covered with plastic mulch was obviously the best performing among the different residues managemen<sup>t</sup> systems. Despite these results, the system where clover was incorporated without plastic

mulch seemed to be more stable than the other systems; in 2017, all systems except CT-CC and the weedy control showed an increase in their production.


**Table 5.** Tomato yield obtained in each system as a ffected by tillage and cover crop management.

> Values followed by different letters are significantly different at *p* < 0.05.

Regarding fruit quality, firmness is a mechanical property relevant for both processing and fresh tomatoes. It defines the susceptibility of the fruits to mechanical damage during harvest and transportation as well as to environmental ones like drought and temperature changes. Therefore, plants with higher firmness are less prone to qualitative and quantitative losses and have a longer shelf life [25]. Firmer fruits are preferred for processing purposes to maintain the form and integrity of fruits during transformation. Fruit firmness was the same in all treatments in 2015 and tended to be lower in no-till systems in 2017. The TSS and pH values did not show statistically significant di fferences among the systems in both years (Table 6). Both factors are important for the final yield, energy saving, and conservation of tomato. Regarding the nutraceutical quality measured only in 2017, vitamin C content increased by at least 32% in plants grown over the dead mulch having 31 mg 100g−<sup>1</sup> FW. Vitamin C and polyphenols are reported to be the major antioxidant hydrosoluble components in tomato and an increase in their content would be an added value for fresh and processing markets where losses during transformation may occur. In our case, total phenols and the antioxidant activity were not influenced by di fferent tillage and cover crop residues management.


**Table 6.** Marketable fruit basic and nutraceutical characteristics from each of the systems in comparison.

Values followed by different letters are significantly different at *p* < 0.05.

#### *3.3. Weed Biomass and Soil Characteristics*

Weed biomass at harvest of 2016 was the highest in no-till systems similarly to the weedy control (Table 7), whereas in 2017 the dead mulch succeeded to decrease weed biomass although not at the level of conventionally tilled systems. No e ffect of supplemental mowing over the dead mulch was seen at harvest time.


**Table 7.** Weed biomass measured in each system at harvest.

\* Weed biomass measured on the remaining bare soil of the 1 m<sup>2</sup> area assessed. Values followed by different letters are significantly different at *p* < 0.05.

Soil moisture in 2016 did not show statistical differences among treatments throughout the season, although a trend for higher moisture content under the plastic mulch compared to bare and dead mulch soil at the top 10 cm of the soil was confirmed statistically only in mid-season. In early season 2017, almost all conventionally tilled plots had higher moisture content than no-till systems to a depth of 20 cm, both with and without the dead mulch.

Almost 45 days after cover crop incorporation in 2016 (7 July), soil nitrates content was the highest where clover was incorporated and covered with plastic mulch (CT-CC-PM). Lower NO3− were found in soil of plastic mulch without cover crop (CT-NC-PM) and the system where clover was incorporated (CT-CC), while no significant mineralization was seen on dead mulch (NT-CC) (Figure 2). Almost 65 days after clover incorporation/soil preparation, soil nitrates increased in all tilled systems, having a higher nitrates concentration compared to dead mulch. N mineralization in plastic mulch with tilled clover reached a peak after 90 days of clover incorporation (20 August). Nitrogen mineralization continued till 4 months after clover incorporation (20 September), where soil the nitrates content was the highest in plastic mulch systems without significant effect of the green manure. In 2017, after almost 10 days of cover crop incorporation (22 May), nitrogen release started. Nitrates concentration was the highest in plastic mulch with clover (CT-CC-PM) similar to the first season, followed by green manure without plastic mulch and being almost double the lowest concentration found in dead mulch soil. Significant mineralization of green manure clover on bare soil (CT-CC) was detected 24 days after cover crop incorporation. Later in the season, major differences in soil nitrates among managemen<sup>t</sup> systems, except a peak in CT-CC-PM after 75 days of CC incorporation, were not detected until early September with all tilled systems higher than no-till. Contrary to 2016, a very low mineralization occurred in the system of plastic mulch without the green manure clover.

Soil mechanical strength is an important soil parameter that defines the level of soil compaction. As soil bulk density increases and total porosity decreases, soil resistance to root penetration increases, restricting root growth as well as water and air movement throughout the profile [26]. In our case, penetrometer readings measuring the soil strength at the end of the growing season (September) showed differences among both no-till dead mulch systems (NT-CC and NT-CC-SW) and all tilled systems in the first 5 cm of the soil profile, whereas differences in soil resistance were seen till almost 20 cm depth in 2017 (Figure 3). In both seasons, no system surpassed the 2000 kPa, the growth-limiting compaction threshold in the topsoil [27].

**Figure 2.** The soil nitrates concentration as affected by cover crop residues managemen<sup>t</sup> in 2016 (**upper charts**) and 2017 (**lower charts**) trials. Letters of statistical significance correspond to treatments comparison within the same date of assessment. Values followed by different letters are significantly different at *p* < 0.05.

**Figure 3.** Penetrometer soil resistance (KPa) in the different cover crop residues managemen<sup>t</sup> systems in 2016 (**left**) and 2017 (**right**). \*\*\* Represents statistically significant differences (*p* < 0.001).
