Therefore, based on those experiments, and taking into account the highest recommended doses of the selected products according to their labels, ground losses for each selected formulation were estimated as indicated in Table 2 and applied using a manual sprayer to each soil column. After application, a rainfall event of 120 mm was immediately simulated during a period of 2 h to imitate an average rainfall event from the Tarragona region for the treatment months. 2 L of rain water were supplied to each soil column. The leachate water from each column was collected in consecutive 200 mL samples, and the water leaching time was recorded. A second experimental process was repeated one week later, only in the sandy soil columns, increasing the time between fungicide application and rain event simulation to 24 h, to study the interaction of the pesticides with the rhizosphere. In this case, leachate water samples were collected every 500 mL, as a lower concentration of the fungicides was expected. As in the previous study, the time between water sample collection was recorded. The mass balance of the leached PPPs was calculated for each soil column condition, taking into account the amount of fungicide losses and the concentration contained in each active ingredient. The experimental results were statistically analysed using RStudio .
The Shapiro–Wilk test was used to check the normal distribution of the variables and their homoscedasticity. ANOVA test was performed to compare differences between the different hypotheses. HSD Tuckey test was conducted to find the group means that are different from each other. The same procedure was applied to analyse the statistical differences between the soil leaching flowrates and the hydroponic study results. Although 6 fungicides were applied on the soil surface,indoor garden only 3 were detected in the leached water from the sandy soil columns, and only one was leached through the loam soil columns. Oxathiapiprolin, laminarin, and acibenzolar-S-methyl were not detected in the leaching water from any of the studied soil columns and conditions. The absence of these compounds in the leachates could be due to the low calculated ground losses taking into account the recommended application dose,but also to the high soil sorption coefficient in the case of oxathiapiprolin and the high biodegradation rates described for laminarin and acibenzolar-S-methyl in aquatic and soil environments . Fig. 2 indicates that copper shows a linear increasing trend for the three column types in both soils, whereas dimethomorph and zoxamide behave differently in bare soil columns than in vegetation covered soil columns. Both accumulated leached synthetic fungicide products continued to maintain the linear trend in cover cropped soil while bare soil columns showed a sigmoid curve for dimethomorph and an exponential curve for zoxamide. This may indicate that at the beginning, the bare soil columns were capable of retaining greater amounts of these fungicides, but later on, the retention capacity of cover cropped columns were greater.
The mass balance assessment shows that fungicides leached through the cover cropped sandy soil columns by a range of 7–64% . Specifically, dimethomorph was leached from 37 to 64%, whereas these leaching values were 16–21% for zoxamide and 7–15% for copper. These results are in agreement with pesticide leaching values found by Singh et al. in packed soil columns for metolachlor and terbuthylazine . The greatest leaching of dimethomorph in comparison to zoxamide is in accordance with the reported sorption coefficient for these compounds.Although our results show a high dependence on soil texture, no significant differences were observed between covered soil columns and bare soil columns in sandy soil. These results are in disagreement with Dousset et al. who found that lower amounts of pesticides leached through cover cropped soil columns than bare soil columns . These differences can be explained due to the soil composition. While our sandy soil has a sandy-loam texture, the soil used by Dousset et al. had a sandy texture, and as it has been seen in Table 3, the texture of the soil affects the water leaching rates, thus the leaching of the fungicide products. Futhermore, differences in cover species types or water loading rates employed in each study can explain fungicide mass leaching differences. However, mass balance studies performed on loam soil columns show that fungicide leaching was much lower than that from sandy soil columns, probably due to the longer soil interaction time as well as the higher organic matter and clay content of the loam soil, except for Cu which showed greater mass leaching in the loam soil columns. It is important to notice that loam soil had a very high concentration of copper , whereas it was low in sandy soil , so copper leaching was strongly affected by copper soil composition. The greater retention of fungicides by loam soil is in agreement with different studies that have found that organic matter and clay play a relevant role in enhancing the soil retention of pesticides and copper . Furthermore, our results show that the use of covers on loam soils significantly reduced the amount of copper leached through the columns probably due to the longer water soil contact time in these columns .
Since no differences on fungicide leaching between cropped and uncropped columns were found when a rain event was simulated immediately after fungicide application in sandy soil columns ,we tried to observe if the same behaviour occurred with a 24 h elapsed between application and rainfall, as this is conventionally done as a cultural practice in a real vineyard. The sandy soil columns were chosen because they resulted in the highest fungicide leaching rates . Fig. 3 shows that the leaching of fungicides in bare soil columns is greater after 24 h of application than when the rain event was produced immediately after the application . This is mainly due to the accumulation of fungicides in the soil, as the second study was performed only one week after the first one. Even so, the fungicide mass leaching in the covered soil columns were lower than in the first study. In fact, our findings show that applying fungicides 24 before rain event resulted in a significant reduction in the amount of leached fungicides between covered soil columns versus bare soil columns . Despite this, no statistical differences were observed for dimethomorph, most likely due to the compound’s very high overall leached amount . Furthermore, results show that although there are no statistical differences between covered cropped strategies for the leaching of zoxamide, monoculture covered columns resulted in a greater reduction of copper leaching than those observed in polyculture covered columns. This is in agreement with the fact that raygrass is a metal-accumulating plant , but also due to the fact that MC columns had greater root biomass than PC columns . Overall, our study demonstrates that the presence of cover crops significantly reduce the quantity of fungicides leached through the soil.
This can be explained by the positive effect of the plant rhizosphere in biodegradation as well as the plant uptake of synthetic and ecological organic fungicides . Therefore, since covers do not act as a physical resistance against product leaching, it seems that the attenuation of potential groundwater contamination occurs when the fungicide-rhizosphere interaction time is as long as possible prior to a rainfall event to allow phytoremediation to take place. Different hydroponic studies were performed to assess the direct impact of vegetation on the attenuation of fungicides without taking into consideration soil interaction. The concentration decay of fungicides in hydroponic containers followed a first-order kinetic with concentration decay rates ranging from 0.06 to 0.63 d− 1 and half-lives from 1 to 21 days . These kinetic rates are in the range of those found in other hydroponic studies that explored the impact of rooted crops on the attenuation of pesticides . Fungicides can be classified as highly degradable , moderately degradable and poorly degradable . Acibenzolar-s-methyl was the fungicide with the highest kinetic decay rate in any of the studied hydroponic conditions, indicating that this is a highly degradable compound as it was suggested in previous soil leaching studies where it was not detected . Similarly, oxathiapiprolin and laminarin showed kinetic rates of between 0.2 and 0.5 d− 1 for rooted hydroponic containers, indicating moderate biodegradability. In this regard, for laminarin, the presence of roots enhanced the concentration decay from 0.06 d− 1 in the control containers to 0.22 and 0.34 d− 1 for the rooted MC and PC containers, respectively. This positive effect of roots may be explained by the increase in biodegradation due to the root exudates or the plant uptake of this highly polar compound . Finally, dimethomorph and zoxamide were only poorly removed. These findings are consistent with results from leaching studies where these two fungicides and copper were the only ones that leached through soil columns. Overall, hydroponic farming studies show that dimethomorph, zoxamide, and laminarin are highly affected by the presence of rooted crops , indicating that covering has a highly positive impact on enhancing concentration decay, either by biodegradation, sorption, and/or plant uptake.
These results are in agreement with previous hydroponic studies carried out by Lv et al. , who found that the concentration decay of fungicides such as tebuconazole and imazalil is enhanced by the use of highly rooted plants like Phragmites australis. Furthermore, our studies show that using a mixture of cover spices enhances the reduction of dimethomorph under hydroponic conditions. This last finding suggests that plant biodiversity may aid in enhancing soil bioremediation . The removal of fungicides in the hydroponic studies after 10 incubation days ranged from 36 to 98%, so that at least 36% of all tested fungicides degrade after that time. Similarly, the findings for kinetic rates in removal results show that vegetation has a significant positive effect on all tested fungicides, with the exception of oxathiapiprolin. The reduction in concentration of zoxamide, dimethomorph and acibenzolar-s-methyl due to the presence of vegetation increased by 27–51%, 108–148% and 42–50%, respectively. For laminarin, elimination is gathered with the root action, practically reaching total elimination after 10 days , but with a low degradation in the control containers . Overall, hydroponic results indicate that cover crops roots are capable of enhancing the removal of almost all studied fungicides, whereas increasing plant biodiversity can play a relevant role in improving the attenuation of dimethomorph.Furthermore, fungicide application practices are very relevant for enhancing the crop covering effectiveness. Therefore, our results suggest that fungicide spraying should be performed at least 24 h before rain forecasting. This is in line with the European strategy to reduce the use of pesticides and soil pollution. According to Chapagain et al. , covers minimize soil disturbance and erosion, improve soil structure and water-stable aggregates, and support pollinators and beneficial insects, among other things. However, Sharma et al. noticed that over crops can be problematic in some points, including the method of killing, host for pathogens, regeneration, and not immediate benefits of using them.
There is also a concern about water competition concerns between the main crop and the cover crop. But according to Delpuech and Metay , even in the Mediterranean region, where most vineyards are located, the implementation of a cover crop strategy is feasible. The growth of the human population places an ever-increasing demand on freshwater resources and food supply. The nexus of water and food is now well recognized. One promising strategy to sustain food production in the face of competing water demands is to increase the reuse of treated human wastewater. Municipal wastewater reuse for food production has been successfully adopted in some regions of the world. For example, Israel uses ~84% treated wastewater in agriculture production . However, Southern California, a region that suffers from a similar degree of water shortage, currently uses less than ~3% of municipal wastewater in agriculture, while discharging ~1.5 million acre-feet effluent per year into the Pacific Ocean . Secondary municipal wastewater effluent for ocean discharge is often sufficient to support both the nutrient and water needs for food production.