4.1. Straw and biochar effects on aboveground biomass
The reported research showed that straw application can lead to negative effects on the soil environment and crop yields. The decomposition of straw with a high C:N ratio in soil results in microbial N immobilization and a temporary decrease in the crop-available N and thus can reduce crop growth (Wang et al. 2018; Huang et al., 2013). However, this study did not confirm this thesis. The analysis of the data from the experimental trial with the red clover/grass mixture amended with straw and biochar produced from wheat and miscanthus revealed a positive effect on the root system and the aboveground biomass. As expected, the first cut had a significantly higher biomass than the second and third cuts. This effect is usually ascribed to the weather conditions, soil moisture and the dynamics of humic substance mineralization (Verlinder et al. 2009; Kacorzyk and Głąb, 2017). The beneficial effects of straw amendment on crop productivity were consistent with Zhao et al. (2016) and Xu et al. (2019), who reported that straw return increased the crop yield of annual crops. According to Getahun et al. (2018), straw residues incorporated into soil have beneficial effects on root development and thus affect aboveground biomass production. This aligns with current studies on root morphology. The positive effects of straw application were observed for most root morphometric parameters. Interestingly, the same effect was observed with straw even after the pyrolysis process was applied.
Some reports have shown that biochar is a source of nutrients for plants and thus improves crop yields (Lehman and Joseph, 2015; Rafique et al., 2020; Mierzwa-Hersztek et al., 2017). In this study, an increase in the yield was obtained after the application of organic materials before and after thermal conversion. According to Alburquerque et al. (2013), crop yields are influenced less by the dose of biochar than by the type of feedstock used in the production of biochar. They studied 5 types of biochar (olive stone, almond shell, wheat straw, pine woodchips, and olive-tree pruning) introduced in doses from 15‑225 t ha-1. The authors stated that the effects of biochar addition on plant dry biomass were greatly dependent upon the biochar application rate and the biochar type. This effect was associated with the biochar nutrient content. In the present study, the results in the red clover and grass mixture did not confirm the effects of the type of feedstock and its rate. The straw and biochar rates were likely too low to show this effect. However, the rates used here were more likely to be representative of the actual field use. According to Reed et al. (2017), the greatest positive yields arise from biochar applications of above 30 t ha-1, which is a much greater rate than that applied in this study. Although such high rates of biochar are theoretically possible, from a practical point of view, the application of high quantities of biochar for large field areas on commercial farms is not reasonable because of the economic costs involved in production, processing and transport.
Carbon amendments, such as the application of straw, are used in grasslands to inhibit invasion of non-native plants by reducing the soil nitrogen (Desserud and Naeth, 2013; Alpert and Maron, 2000). This study confirmed this effect when miscanthus and wheat straw were used. However, biochar did not show such an effect. It was expected that the biochar application would also alter the competitive ability of plant species in grassland communities. In particular, leguminous species have been shown to benefit from biochar amendment (Rondon et al., 2007). Some mechanisms were recognized to explain the higher competitive ability of legumes with biochar as the soil amendment: (i) reductions in available N, (ii) increases in soil pH, (iii) increases in the content of soil P, K, Mg and other nutrients, and (iv) stimulation of biological nitrogen fixation (Jeffery et al., 2011; Oram et al., 2014; Rondon et al., 2007). The most important nutrient responsible for biomass production was nitrogen. However, the N fertilizer used to increase the DM yield inhibited the diversity and resulted in changes in the botanical composition of meadow sward (Aydin and Uzun, 2005). Oram et al. (2014) reported that biochar had a beneficial effect on red clover under N limiting conditions due to an increase in the K availability. According to Mevlut et al. (2007), large concentrations of P are associated with low floristic diversity but play a favourable role in the occurrence of legumes. Concentrations of K appear to increase diversity (Aydin and Uzun, 2005). In this study, the highest red clover content was observed in the control plot, which was not fertilized. In all other treatments with organic amendments, the red clover nearly disappeared. This effect may be ascribed to the N availability in soil. According to previous authors’ report (Mierzwa-Hersztek et al., 2017) there is no effect of straw and biochar at rates 5 and 2.25 t ha-1 on the N content in soil four years after organic amendment application. Changes in the botanical composition are strictly related to herbage production (Kacorzyk and Głąb, 2017). In this study, this effect can be ascribed to the higher participation of D. glomerata L. and the lower participation of forb species in the control treatment.