In the work we present here, we assessed how conventional and organic farming systems affected the water relations of soils and crops in the 2017 and 2018 growing seasons. Our study was conducted in the DOK system comparison trial, established as a collaboration of the Swiss Research Institute of Organic Agriculture and Agroscope in 1978 near Basel, Switzerland.We used the DOK trial to test if soil moisture and soil surface evaporation are affected by conventional and organic farming, and if organically and conventionally grown plants differ in their root water uptake depth, stomatal conductance and leaf area. Based on these assessments, we made a rough evaluation of the total water use of the studied organic and conventional farming systems. The goal of this study was to assess how organic and conventional farming systems affect the water relations of soils and crops. In particular, we tested if organic and conventional farming systems lead to marked differences in soil moisture, soil evaporation, as well as root water uptake depth and the stomatal conductance of crops. Our results suggest no differences with regard to soil water evaporation but higher soil moisture in the rooting zone of organically compared to conventionally managed systems. The differences were statistically significant for wheat but not for soybean. Soil water retention curves suggest that soil physical properties do not explain the observed differences in soil moisture between the two systems at the trial site. Also, no differences in root water uptake depth between plants grown in organically or conventional farming systems was detected. We found, however, that organically grown wheat exhibited a generally lower stomatal conductance compared to conventionally grown wheat. Soybean showed similar tendencies but treatment effects on gs and soil moisture were smaller. In addition,rolling bench we found that winter wheat had lower dry matter yield and leaf area in organic farming systems compared to conventional farming systems.

In summary, our study suggests that lower stomatal conductance and smaller leaf area under organic compared to conventional farming can reduce water use and resulted in higher soil moisture in organically compared to conventionally managed wheat at the DOK trial site. Our study revealed trends of higher volumetric soil moisture in the organic compared to the conventional treatment . Specifically, soil moisture under wheat was higher in the organic compared to the conventional treatment at 10–30 cm in 2018 and a similar tendency was observable for soybean in 2017. Previous studies found higher water holding capacities under organic farming . These studies suggest that organic carbon and higher aggregate stability are responsible for these patterns. Previous analyses of soil properties at the DOK trial have reported higher contents of org C and higher aggregate stability in organic compared to conventional systems . The observed trend in soil moisture between the two treatments in our study could thus be the result of a larger storage capacity in organic compared to conventional soils driven by the previously reported differences in physicochemical properties. Importantly, however, differences in soil moisture between the two treatments were most expressed when soils became dry . This suggests that water holding capacity is not the driver of the observed patterns as differences in water holding capacity should become most evident in wet soils. Interestingly, Kundel et al. found similar soil moisture patterns at the same trial for winter wheat in 2017, but their effects were strongest under ample soil water conditions. Alternatively, the observed trends in volumetric soil moisture between the two treatments could be the result of differences in water retention capacities resulting in a less efficient water extraction from the organic compared to the conventional soils. This would mean that the higher volumetric soil moisture observed in the organic compared to the conventional trials, in particular in drying soils, are merely the result of residual water that is not available to the plants. The water retention capacity of a soil is determined by soil physical properties such as soil texture but also org C content.Given that org C content was higher in organic compared to conventional soils,we determined soil water retention capacities in the two treatments with pF curves.We,however,did not find any difference in the pF-curves between the organic and the conventional treatments .

However, technical limitations only enabled measurements of pF values at volumetric moisture contents greater than 13.7 vol%. But looking at the soil moisture data of wheat in 2018, we already observe treatment differences in moisture contents greater than 40% at − 10 to − 30 cm soil depth. Also, the amount of measured soil moisture contents below 13.7 vol% is relatively small and only observed at − 10 cm. In soybean, moisture values below 13.7 vol% were measured mainly at − 20 cm without resulting in significant treatment differences. As a consequence, we conclude that the observed trends in soil moisture patterns between the two treatments are unlikely a result of differences in soil matrix potentials. Comparing the relationship between δ18O and δ2 H values of soil water with those of precipitation can reveal information on soil hydrological processes such as infiltration, residence time or evaporation . Precipitation stable isotope values of the Basel GNIP station as well as precipitation samples collected at the DOK trial site, i.e. the local meteoric water line , plotted close to the GMWL . In contrast, when soil water δ18O and δ2 H values were plotted in a dual isotope space we observed that the slope of soil water lines was significantly less steep than that of the LMWL . Given that evaporation fractionates oxygen and hydrogen isotopes in water differently, the shallow slopes of the soil water lines suggest evaporative water loss from the soil to the atmosphere . When dividing the soil water line into single soil water lines for individual soil depths, we observed less steep slopes in shallower compared to deeper soil layers . This suggests, that evaporative water loss is more pronounced in shallow compared to deep soil layers . Most importantly, the slopes of these depth-specific evaporation lines did not differ between the organic and conventional treatments , which suggests that evaporative water loss does not differ between the organic and the conventional treatments. In turn, this implies that differences in evaporative water loss cannot explain the observed trend in soil moisture between the two treatments. Amooh and Bonsu suggested a negative correlation between soil organic matter and evaporative water loss. Despite higher soil organic matter contents and higher weed coverage in organic compared to conventional systems , the different farming systems did not affect evaporative water loss in our case. However, soil coverage or tillage did not differ between the treatments investigated in this study. Such measures are often part of sustainable farming approaches and have been shown to considerably affect soil water evaporation.

We used the cryogenic extraction technique to obtain water from soil samples for stable isotope analysis following the procedure described in Newberry et al. . Several previous studies have revealed that the cryogenic extraction method can introduce isotope artefacts to theextracted soil water and that factors such as soil texture can influence the magnitude of these artefacts . As such, soil water isotope values obtained with the cryogenic extraction methods need to be interpreted with the consideration of these methodological artefacts. For the data that we present here, it is unlikely that potential artefacts influence the main findings of our study. This is, because it was the main objective of our study to assess differences in soil hydrology between the organic and the conventional treatment. Potential artefacts associated with the cryogenic extraction of soil water are thus identical for samples from both treatments and although these artefacts might introduce errors in absolute δ18O and δ2 H values, they do not influence the comparison of the two treatments. Given that previous studies have shown higher bulk density and root penetration resistance in conventional compared to organic farming systems , we expected root water uptake depths from deeper soil layers of plants in the organic treatment compared to the conventional treatment. Surprisingly, our results did not confirm our expectations. We detected no significant differences in RWU depth between the two treatments in both years and in both species . This is in line with recent findings of Sun et al. showing no effect of organic and conventional cropping systems on water uptake patterns of pea and barley. However,grow table hydroponic soybean plants in the organic treatment showed slightly deeper RWU depths than plants in the conventional treatment, especially when soil moisture was progressively decreasing between DOY 173 and 214 in 2017. This suggests that soybean plants in the organic treatments have access to deeper soil water and thus a larger soil water pool than soybean plants grown in the conventional treatment. Easier root growth in organic soils which have a lower bulk density or a larger water uptake horizon by greater arbuscular mycorrhizal colonization under the organic regime is a possible explanation for the observed patterns in soybean.

However, it is possible that RWU patterns similar to the ones observed on soybean were not detected on winter wheat since wheat plants in the conventional treatment were treated with the growth regulator chlormequat chloride . CCC has been shown to increase the rooting depth of wheat . It could therefore be possible that effects of organic farming on RWU depth could have been countervailed by the application of CCC in the conventional treatment. Also, the differences in root morphology between wheat and soybean having an adventitious and a tap root system, respectively, may be a possible reason for the contrasting patterns observed between the two crops. We found that stomatal conductance of wheat was significantly lower under the organic compared to the conventional treatment and that there was a similar tendency for soybean . This suggests lower per leaf area transpiration rates of wheat growing in the organic compared to the conventional treatment. Given that water retention did not differ between the treatments and soil moisture under wheat was generally higher in the organic treatment at 10–30 cm soil depth, the observed differences in gs of wheat are most likely not a response to moisture availability. gs as well as photosynthetic assimilation were shown to be positively related to N availability . However, we observed mixed results for plant N concentrations and slightly higher assimilation rates in organically grown wheat in 2018 . Hence, N availability can also not explain gs patterns observed in this study. It is possible that plant intrinsic factors or pathogen-induced differences in hydraulic conductivity or root water uptake resulting from the different pest managements of the organic and conventional treatments were responsible for these patterns. Conventionally grown wheat received two fungicide sprayings per season, while organically grown wheat was not treated with any pesticides during 2017 and 2018 . It is known that pathogens infecting xylem tissue can impair plant water transport and status, resulting in lower gs . However, this explanation is rather speculative, also because conventionally grown crops have been shown to be more vulnerable to xylem cavitation compared to organically grown crops . In soybean, farming systems did not affect gs significantly. Sun et al. already found less pronounced effects of organic and conventional farming systems on hydraulic traits of pea compared to barley. It could therefore be that the ability of N-fixation and thus better N nutrition countervailed farming system effects on legumes also in our experiment. Nevertheless, reducing gs and increasing the transpiration efficiency of crops is of great interest in order to maintain yields under increasingly occurring drought events . Thus, our results showing lower gs of wheat under organic crop management point towards the potential of organic farming as a more sustainable and resistant farming system in a changing climate. We analyzed the bulk δ18O values of winter wheat leaves and soybean beans as an independent measure of treatment effects on gs. δ18O values in plants are driven by the δ18O values of the plants’ source water and the evaporative 18O enrichment of leaf water .