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.