**1. Introduction**

Agrivoltaic (AV) systems are currently being implemented in a number of countries as an approach for the dual use of arable land for renewable energy and agricultural production [1,2]. It has been shown that both land productivity and farm income can be increased by the additional energy generated through AV [1–5]. Recently, first concepts for the integration of AV into prospective farming systems—e.g., in combination with farming robots—have been proposed [6]. However, considering the land use conflict between food and energy production, a sustained adequate agricultural yield needs to be guaranteed if AV systems are to be used. This necessitates further field studies on the performance of agricultural crops under AV. The implementation of AV is currently being investigated in field trials by several researchers [2,5,7–9]. So far, a number of crops have been assessed for their suitability for cultivation underneath AV, including lettuce [8], corn [10], potatoes, winter wheat [9], and fruit vegetables (such as cherry tomatoes and chili peppers) [7]. Additionally, grass-clover has been investigated as a perennial forage crop [9]. These studies have shown that sufficient crop yields can be achieved in the partial shade of the photovoltaic (PV) modules of AV systems, but agricultural yield reductions of up to 20% can occur [8,9,11]. By contrast, in hot and dry weather conditions, reduced solar radiation

and microclimatic alterations under AV (e.g., lower soil [8,9] and air temperatures [9] and potential advantages in water use efficiency [12]) can be beneficial for crop production and lead to increased yields [7,9].

The present study was conducted on-farm within a field trial with four different crops (celeriac, grass-clover, potato and winter wheat) cultivated underneath an AV system. The crops were cultivated as part of a crop rotation under organic management. This setup was chosen because, to date, no AV studies have been conducted under organic field management conditions. Furthermore, organic farming generally strives to reduce external inputs "by reuse, recycling and efficient management of materials and energy in order to maintain and improve environmental quality and conserve resources", as a matter of principle—as described by the International Federation of Organic Agriculture Movements (IFOAM) [13]. Thus, organic farming also addresses energetic self-sufficiency and the replacement of fossil energy resources. As such, AV would appear an appropriate approach in this context. Further details on the field trial were reported by Weselek et al. [9]. Harvestable crop yields decreased by 18.7% (wheat), 18.2% (potatoes) and 5.3% (grassclover) in 2017, but increased by 2.7% (wheat) and 11% (potatoes) in 2018. Grass-clover yields in 2018 were reduced by 7.8% [9]. The results were linked to quite distinct climatic differences between the years; 2018 brought lower precipitation, higher temperatures and greater solar irradiance. In the same time frame, 246 MWh of energy were generated by the AV facility in the first cropping year, which corresponds to about 83% of the electrical yield a conventional ground-mounted PV installation covering the same area would have achieved [14]. Hence, even with a reduction of harvestable crop yield of 18.7% for winter wheat in 2017, overall land productivity was increased by about 56% in comparison to single crop and PV production [14]. The results further emphasized findings from previous studies [3] on the benefits of AV regarding land use and land productivity.

As a recent study showed, long term land productivity and market certainty are often seen as the main arguments favoring the implementation of AV from farmers' perspective [15]. This emphasizes the need for agricultural field trials. However, experimental data on the impact of AV on crop production are scarce; few data are available for field vegetables and, in particular, root vegetables. In 2017, vegetables were cultivated on a total area of 2.2 million hectares in Europe [16]. As comparatively high market revenues can be achieved with vegetables, the impacts of AV on cultivation and harvestable crop yields will be of major interest. Celeriac (*Apium graveolens* L. var. *rapaceum*), also known as turnip-rooted celery or knob celery, is a celery variety cultivated primarily in Central and Eastern Europe [17,18]. In contrast to common celery (*Apium graveolens* var. *graveolens*) and leaf celery (*Apium graveolens* var. *secalinum*), this biennial crop forms large bulbs in the first cropping year—which consist of hypocotyl, tap root and stem in equal proportions [17]. Celeriac bulbs have white flesh and can be used both cooked and raw. In 2018, organic celeriac was cultivated on a total of about 219 hectares in Germany, producing 6853 tons of harvested celeriac bulbs [19].

The aim of our study was to investigate how celeriac (a common field vegetable) would be affected if it were cultivated underneath the solar panels of an AV system (Figure 1). In addition to examining parameters such as crop development and yields, the study examined, for the first time, how altered microclimatic conditions in the partial shade of the AV facility affected the chemical composition—and consequently, the quality—of celeriac.

**Figure 1.** Celeriac plants growing underneath the agrivoltaic (AV) facility in 2017. The reference site is located behind the facility. (source: Bauerle/University of Hohenheim). **Figure 1.** Celeriac plants growing underneath the agrivoltaic (AV) facility in 2017. The reference site is located behind the facility. (source: Bauerle/University of Hohenheim).

#### **2. Material & Methods 2. Material & Methods**

### *2.1. Site Description & Field Experiment 2.1. Site Description & Field Experiment*

Celeriac was cultivated as part of an on-farm field experiment using a four-year crop rotation (along with winter wheat (*Tricticum aestivum*), potato (*Solanum tuberosum*) and grass-clover) [9]. The field trial was performed on a commercial farm managed according to biodynamic principles (Demeter) as described in [9]. Details on the design of the AV facility were described by Trommsdorff et al. [14]. In both 2017 and 2018, celeriac was grown on a strip 24 m long and 19 m wide under an AV system, with a total size of 0.3 ha. Additional celeriac was grown on an adjacent reference area (REF) without solar panels (Figure 2). To avoid any shading of the REF site, it was located at a distance of 20 m from the AV facility. On both sites, four trial plots of 1 m² were defined. To reduce border effects—in particular under the AV facility—the plots were located at least 4 m from the sites′ borders. Celeriac plantlets (*Apium graveolens* L. var. *rapaceum*, Goliath variety) were sown in seed trays and planted out around development stage 13 (according to BBCH (Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie) scale for root and stem vegetables [20]) at a density of 45,000 plants per hectare. In both years, planting took place on 5 May. The celeriac cropping area was fertilized with 15 t composted cow manure per hectare between mid-February and mid-March. Biodynamic preparations (20 l per hectare each of horn manure and horn silica) were sprayed according to Demeter guidelines twice a year. Weed control was mainly conducted by currycombing before planting (twice) and hoeing after planting (up to four times). Additional hand weeding was performed if weed pressure became high within the rows. In 2017, the preceding crop was perennial grass-clover; in 2018, it was potato. For further information on field man-Celeriac was cultivated as part of an on-farm field experiment using a four-year crop rotation (along with winter wheat (*Tricticum aestivum*), potato (*Solanum tuberosum*) and grass-clover) [9]. The field trial was performed on a commercial farm managed according to biodynamic principles (Demeter) as described in [9]. Details on the design of the AV facility were described by Trommsdorff et al. [14]. In both 2017 and 2018, celeriac was grown on a strip 24 m long and 19 m wide under an AV system, with a total size of 0.3 ha. Additional celeriac was grown on an adjacent reference area (REF) without solar panels (Figure 2). To avoid any shading of the REF site, it was located at a distance of 20 m from the AV facility. On both sites, four trial plots of 1 m<sup>2</sup> were defined. To reduce border effects—in particular under the AV facility—the plots were located at least 4 m from the sites0 borders. Celeriac plantlets (*Apium graveolens* L. var. *rapaceum*, Goliath variety) were sown in seed trays and planted out around development stage 13 (according to BBCH (Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie) scale for root and stem vegetables [20]) at a density of 45,000 plants per hectare. In both years, planting took place on 5 May. The celeriac cropping area was fertilized with 15 t composted cow manure per hectare between mid-February and mid-March. Biodynamic preparations (20 l per hectare each of horn manure and horn silica) were sprayed according to Demeter guidelines twice a year. Weed control was mainly conducted by currycombing before planting (twice) and hoeing after planting (up to four times). Additional hand weeding was performed if weed pressure became high within the rows. In 2017, the preceding crop was perennial grass-clover; in 2018, it was potato. For further information on field management, see [9].

### agement, see [9]. *2.2. Microclimate*

Microclimate was monitored via eight microclimate stations (i.e., four per treatment) on the celeriac cropping area, each assigned to one of the trial plots. Each microclimate station was equipped with different sensors and recorded various microclimatic parameters. Air temperature and humidity were measured at a height of 2 m using a VP-4 sensor. Soil temperature and moisture were measured at a depth of approximately 25 cm using a 5TM sensor. Due to tillage operations, soil sensors were only installed during the celeriac cropping period from 8 June to 10 October in 2017, and from 9 May to 22 October 2018.

Photosynthetic active radiation (PAR) was estimated by photosynthetically active photon flux density (PPFD) using a QSO-S sensor. All parameters were recorded with data loggers (EM50G). Data loggers (and the sensors mentioned above) were obtained from METER Group AG (Munich, Germany). In addition to the data collected in the field trial, meteorological data for comparison were obtained from Agricultural Meteorology Baden-Wuerttemberg, published by the Agricultural Technology Centre Augustenberg (LTZ) [21]. The weather station nearest to the field trial was located at Billafingen (47.83◦ latitude 9.13◦ longitude), 2 kilometers away. Mean monthly temperature and accumulated precipitation are shown in Figure 3 (data taken from Billafingen weather station [21]). Note that values recorded in the field trial cannot be directly compared with those recorded at the weather station, as they are located at different spots and their instruments have not been calibrated. Furthermore, in 2018, no values were recorded at our field trial from 11 to 13 December due to a power outage. *Agronomy* **2021**, *11*, x FOR PEER REVIEW 4 of 18

**Figure 2.** Setup of the field experiment in 2017 and 2018 with location of celeriac within the crop rotation. The experimental site was split into a reference (REF) and agrivoltaic (AV) site. The diagram is a schematic illustration and not to scale. (image source: BayWa r.e., modified). **Figure 2.** Setup of the field experiment in 2017 and 2018 with location of celeriac within the crop rotation. The experimental site was split into a reference (REF) and agrivoltaic (AV) site. The diagram is a schematic illustration and not to scale. (image source: BayWa r.e., modified).

> *2.2. Microclimate*  Microclimate was monitored via eight microclimate stations (i.e., four per treatment) on the celeriac cropping area, each assigned to one of the trial plots. Each microclimate station was equipped with different sensors and recorded various microclimatic parame-Climatic conditions varied greatly between the two years. In 2017, annual accumulated precipitation was 1351 mm, annual solar was radiation 1180 kWh/m<sup>2</sup> , and mean annual temperature was 8.6 ◦C. In 2018, accumulated precipitation was 916 mm, annual solar radiation was 1204 kWh/m<sup>2</sup> , and mean annual temperature was 9.7 ◦C.

### ters. Air temperature and humidity were measured at a height of 2 m using a VP-4 sensor. *2.3. Crop Monitoring & Harvest*

December due to a power outage.

Soil temperature and moisture were measured at a depth of approximately 25 cm using a 5TM sensor. Due to tillage operations, soil sensors were only installed during the celeriac cropping period from 8 June to 10 October in 2017, and from 9 May to 22 October 2018. Photosynthetic active radiation (PAR) was estimated by photosynthetically active photon flux density (PPFD) using a QSO-S sensor. All parameters were recorded with data loggers (EM50G). Data loggers (and the sensors mentioned above) were obtained from ME-TER Group AG (Munich, Germany). In addition to the data collected in the field trial, meteorological data for comparison were obtained from Agricultural Meteorology Baden-Wuerttemberg, published by the Agricultural Technology Centre Augustenberg (LTZ) [21]. The weather station nearest to the field trial was located at Billafingen (47.83° latitude 9.13° longitude), 2 kilometers away. Mean monthly temperature and accumulated precipitation are shown in Figure 3 (data taken from Billafingen weather station [21]). Note that Crop development was monitored over two growing seasons, beginning in May (both years) immediately after the celeriac was planted and lasting until shortly before final harvest. The last monitoring dates were 26 September in 2017 and 18 October in 2018. In each of the defined plots, 12 individual plants were selected and tagged. Of these, 10 plants were monitored and two were kept as backup in case of plant losses. Crop development was monitored every two weeks. Crop height was measured using a folding rule. Leaf area index (LAI) was measured using a plant canopy analyzer (LAI-2200C, LI-COR Biosciences, Lincoln, Dearborn, MI, USA). On each monitoring date, twelve single measurements were taken per plot: six measurements between plants within the rows, and six measurements between rows. The final harvest was performed on the farm's actual harvest dates. The 12 selected plants in each plot were harvested manually. Each celeriac plant was separated into aboveground and belowground biomass. Remaining roots were roughly removed from

calibrated. Furthermore, in 2018, no values were recorded at our field trial from 11 to 13

the bulbs. The aboveground biomass from each plot was weighed and subsequently dried for 48 h at 60◦C to determine dry matter yield. Diameter and weight of each celeriac bulb was measured. For the analysis of chemical composition, bulbs were peeled, washed with distilled water and ground (Thermomix, Vorwerk, Wuppertal, Germany). The resulting fibrous pulp was freeze-dried at 0.34 mbar and −32◦C until completely dry and then stored at −20◦C for further analysis. *Agronomy* **2021**, *11*, x FOR PEER REVIEW 6 of 19

**Figure 3.** Monthly mean temperature (red curve) and monthly accumulated precipitation (cyan bars) in 2017 and 2018. Data from Agricultural Meteorology Baden-Wuerttemberg, Billafingen weather station. **Figure 3.** Monthly mean temperature (red curve) and monthly accumulated precipitation (cyan bars) in 2017 and 2018. Data from Agricultural Meteorology Baden-Wuerttemberg, Billafingen weather station.
