2.2. Experimental design and treatments
A field experiment was conducted at the experimental station of the Department of Agricultural and Environmental Chemistry, University of Agriculture in Krakow (50°04′ N, 19°51′ E, 280 m a.s.l.) in the period from 2014‑2018. The climate of the experimental site, which is situated in southern Poland, is considered temperate. The average annual temperature during the study period was 9.7 °C, and the average annual precipitation was 639 mm (Figure 1). The soil in the experimental field was a Eutric Cambisol (loamy sand) (IUSS Working Group WRB, 2015). Table 2 lists the soil characteristics based on the samples collected in 2014 before the start of the trial.
The straw and biochar were applied on March 2014. The soil amendments were mixed with the soil using a rotary harrow at a depth of 10 cm. After the amendment application, the seeds of the red clover/grass mixture were sown at a seeding rate of 60 kg ha-1; the mixture included perennial ryegrass (L. perenne L.) cv. Victorian (with a 20% rate in mixture) and cv. Solen (20%), timothy (Phleum pratense L.) cv. Erecta (15%), meadow fescue (Festuca pratensis Huds.) cv. Ardeinia (10%), tall fescue (F. arundinacea Schreb.) cv. Alix (10%), Italian ryegrass (L. multiflorum L.) cv. Gaza (10%), Westerwolds ryegrass (L. westerwoldicum Breakw.) cv. Mowester (10%), and red clover (Trifolium pratense L.) cv. Dajana (5%).
The field experiment consisted of 24 plots areas of 1 m2. The experiment was established as a randomized block design with three replications. The following treatments were applied: wheat straw at a rate of 5 t ha-1 (WS), miscanthus straw at a rate of 5 t ha-1 (MS), wheat biochar at a rate of 5 t ha-1 (WBH), wheat biochar at a rate of 2.25 t ha-1 (WBL), miscanthus biochar at a rate of 5 t ha-1 (MBH), and miscanthus biochar at a rate of 2.25 t ha-1 (MBL). A treatment with mineral fertilizer but without organic amendments (MCTR) was used, and a control treatment (CTR) without mineral fertilizer and without any amendments was also tested. The mineral fertilizer was applied every year at rates of 100 kg N ha-1, 40 kg P ha-1 and 120 kg K ha-1.