A field trial was conducted at Wroclaw University of Environmental and Life Sciences in Poland between 2013–2017. The field trial was located in a suburban area at a distance of 10 km north west of the centre of Wroclaw. The field plots were located at an altitude of 147 m above sea level, in the catchment area of the river Dobra, a right-bank tributary of the river Widawa. The geographical position is defined by the following coordinates: 51°10'N, 17°07'E.
The field experiments were conducted on a light soil defined as very light alluvial soil, on loose sand and sandy gravel. Each year, soil samples (from each plot) were analysed. Samples were taken at a depth of 0–30 cm at the start of the vegetation period. Analysis were performed according to the following methods:
the soil reaction (pH/KCl) by the potentiometric method
the available forms of potassium and phosphorus by Egner-Rhiem method
magnesium by Schachtschabel method
The soil is visibly alkaline, the content of phosphorus ranges from medium to high, and potassium from high to very high, and the content of Mg is medium [see supplementary online material Table S1].
The research was conducted in one of the warmest of Lower Silesia's regions. In the first half of the 20th century the average annual air temperature was 8.6°C, the average temperature of the vegetation period (IV-IX) was 14.7°C. The groundwater level was 85 cm below the ground level. The mean annual temperature during the study was higher than the long-term trend. The monthly and annual average precipitation sums varied. The lowest precipitation was reported in 2015. Specific temperature and precipitation breakdown are described in Tables S2 and S3 [see supplementary online material].
Material and Management Practices
This study was conducted in an experiment with a Salix viminalis var. gigantea (1047 willow clone). This variety is intended for biomass producers. It was planted obtained from a nearby plantation (planted in 2010) immediately before planting. Willows were planted in a twin row configuration. Cuttings 20 cm in length were planted in 100 cm wide interrows with 60 cm spacing (16,700 cuttings per hectare). White clover of the Romena cultivar was sown in the amount of 10 kg ha-1 (hand-sown during plantation set-up). The willow plants were not harvested after the first vegetation period and the clover was not mown during the studied period. For the first two years in tested undersowing cultivation clover covered about 95% of the area. Clover dying was observed because of little access to light and drought in the third year of the experiment. To determine the yield, willow plants were harvested after fifth vegetation periods. The chemical composition was determined on four and five-year-old plants.
The pre-forecrop was winter oil seed rape (OSR) and the forecrop was fallow land. The majority of weed species in the year before the plantation was identified, and these included: Elymus repens (L.) Gould., Viola arvensis Murr. and Thlaspi arvense L. In July and September 2012, two sprays of Roundup 360 SL herbicide at a dose of 5 l ha-1 were applied and autumn ploughing was performed. In spring, the field was evened out with a cultivator. After these treatments, weeds were no longer controlled until the end of the experiment.
Mineral fertilizers were applied annually in the spring in April. Fertilized plots annually received 35.2 kg P ha-1 and 83 kg K ha-1 as urea, treble superphosphate and muriate of potash. Urea (50 kg N ha-1) was applied on nitrogen fertilized plots.
The number and dry mass of weeds was assessed each year in June. A 50 cm x 50 cm metal frame was placed on the ground between the rows at four places within each plot in the treatment. The numbers and species of weeds were recorded inside each frame [see supplementary online material Fig. S1]. The weed biomass was assessed by weighing their fresh mass. The dry matter of the weeds was established by the drying method. The fresh mass of weeds was dried at 60°C for 24 hours and then for five hours at 105°C. After harvest in December 2017, absolute dry mass yield and water content in plants were determined based on 4 randomly chosen shoots from every plot. The shoot fragments were cut at a distance of 20 cm on both sides of the gravity centre point and subsequently dried at 60°C for 1 week until constant weight was reached, and then dried for 5 hours at 105°C (modified norm PN-R-04013:1988). The phloem and wood were separated by hand. After debarking the shoots, the phloem and wood were dried, milled and weighed in the same way as the shoot fragments and then chemically analysed. The weight of fresh and dry stem pieces was determined with an accuracy of 0.01 g. Total shoot dry weight per plot was calculated by multiplying the total fresh weight of all sampled shoots for each net plot by the dry matter content. Plant shoot dry weight was calculated as total shoot dry weight per net plot divided by the number of living plants in each net plot.
At the beginning and end of the vegetation period, the diameter and height of the main shoot were measured 20 cm above the ground and plant losses were determined. The plant was designated as dead if no living tissue was detected at 5 cm above the ground.
Plant material was collected at the end of the growing season (December 2016 and 2017). Stems were collected from ten randomly chosen plants. Plant material was dried and minced, and chemical analysis was performed:
Crude ash – determined by burning of 2 g plant material at 600°C in an electric furnace. After loss on ignition the crucibles were placed in a desiccator to cool down, and then weighed to the nearest 0.001g
The Kjeldahl method using the Kjeltec apparatus was applied for total nitrogen determination
Potassium and calcium concentrations were determined by flame photometry using Flapho 4, [Carl Zeiss Jena]
Phosphorus concentration was determined by a colometric method with vanadium and molybdenum reagents using Spekol 10 [Carl Zeiss Jena]
Magnesium concentration was determined by a colometric method with yellow titanium using Spekol 10 [Carl Zeiss Jena]
Five years (2013–2017) of one-factor field experiment was established with the use of a random blocks method. The study was based on rigorous 1-factor field trials. The study compared weed assessments, morphological features, yield and chemical composition of: a) basket willow in monoculture without nitrogen fertilization, b) basket willow in undersowing cultivation with white clover and c) basket willow monoculture fertilized with nitrogen at a dose of 50 kg ha-1. The area of plots was 12m2 with 4 replications of each treatment.
All examined parameters were statistically evaluated, using variance analysis, at a 0.05 level of confidence. Plant mortality variations between treatments were compared using data from square root transformations. The conversion was performed in order to meet the assumptions of the standard normal distribution. The results were statistically analysed using STATISTICA 13.0 PL. Homogeneous groups were determined by Tukey’s multiple range test using consecutive letters starting from “a” – the most beneficial value – to “e” – least beneficial in terms of economic value.