1. Introduction
Organic amendment is a well-known and common practice for providing
nutrients to crops and conserving soil and water. The use of straw as an
organic amendment is widely recommended in various conservation tillage
systems (Xu et al., 2019). Straw is usually used as a mulch after crop
harvesting because of its beneficial effects on the soil physical,
chemical and biological properties. It reduces the soil bulk density and
improves the soil structure and soil pore system (Głąb and Kulig, 2008).
The incorporation of straw into the soil increases the soil organic
carbon (SOC) storage and thus mitigates climate change (Wang et al.,
2019). According to Zhang et al. (2016), straw amendment increases the
available nitrogen and phosphorus content in the upper soil layers and
enhances the urease, phosphatase and invertase activity levels in the
lower soil layers. The incorporation of straw into soil increases crop
biomass production (Zhao et al., 2016). Xu et al. (2019) reported that
straw return increased the crop yield stability in wheat-maize systems.
According to Getahun et al. (2018), the incorporation of straw residues
into soil had beneficial effects on root development and thus affected
aboveground biomass production.
In grasslands, straw amendment is used to inhibit invasion by non-native
plants. Increased availability of N is known to promote the growth of
these plants. Reducing soil N levels, such as through the use of straw
for carbon amendment, inhibit the establishment of invasive species
(Desserud and Naeth, 2013). Additionally,
Alpert and Maron (2000) observed that carbon amendments may negatively
affect non-native species and have very little effect on the native
grasses.
On the other hand, the application of straw 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). Since this phenomenon
appears under typical soil conditions in arable land, straw application
should be combined with N fertilizer to increase crop yield and improve
soil fertility (Wu et al., 2018).
The solution to the problem of decreasing plant-available N is to
convert straw into biochar through pyrolysis. During the past decade,
biochar has been recognized as a very promising soil amendment that
results in soil improvement and carbon sequestration that may mitigate
climate change (Peake et al., 2014; He et al., 2017). The application of
biochar increases crop production through different mechanisms, i.e.,
providing a liming effect, increasing the soil water and nutrient
retention and reducing N leaching
(Karhu
et al., 2011; Hunt et al., 2010;
Jeffery et al., 2011; Oram et al.,
2014). The positive effect of biochar amendment on root growth was
confirmed by Rafique et al. (2020) in research with maize. This effect
can be increased by P fertilizer. However, the effects of biochar
application on crop production are sometimes contradictory. The results
depend on the feedstock used for the biochar production, the parameters
of the pyrolysis process and soil and the climate conditions
(Mierzwa-Hersztek et al., 2018).
Moreover, there is little information on the long-term impacts of straw
and biochar on perennial plants in soil under natural environmental
conditions. Most studies examining the biochar effects on plants have
been performed on annual crops in agricultural systems. Relatively
little is known about how biochar affects plant competition in
grasslands. The grasslands in Europe are the main source of forage for
ruminant production systems (Doben et al.,
2019; Hopkins and Wilkins, 2006). The
botanical composition of grassland is one of the most critical
parameters for forage quality and productivity. Grasslands are regarded
as diverse and species-rich ecosystems and are usually dominated by
perennial ryegrass (Lolium perenne L.) in West and Central Europe
according to Habel et al. (2013).
Mineral or organic fertilizer affects the grassland botanical
composition. The tall grasses in grassland communities become more
dominant under increased levels of N fertilizer. The results also showed
a decrease in the number of species with increasing N fertilizer for
forbs and especially legumes (Čámská and Skálová, 2012; Dindova et al.,
2019). The most important nutrient responsible for biomass production is
N. Even so, the use of N fertilizer to increase dry matter yield
inhibits the diversity and results in changes in the botanical
composition of meadow sward (Aydin and Uzun, 2005; Kacorzyk and Głąb,
2017). The relationship between species richness and nutrient supply has
been widely investigated (Wassen et al. 2005, Oelmann et al. 2011). Lee
and Lee (2000) stated that fertilizer with N stimulates grass growth but
depresses the growth of legumes. The effects of biochar on the growth of
plant species can be explained by changes in the soil physical-chemical
properties, such as increased soil pH, increased soil nutrient
availability, or altered soil water retention (Jeffery et al., 2011; van
de Voorde et al., 2014; Jones et al., 2012). These soil features affect
the outcomes of competition between plant species and thus plant
community composition. Legumes in particular have been found to respond
strongly to the addition of biochar to soils (Jeffery et al., 2011).
Some reports have suggested that biochar can increase the competitive
ability of legumes in grassland communities (Oram et al., 2014). This
effect has been ascribed to N immobilization by the microbial community
and the stimulation of biological N fixation (Rondon et al., 2007).
Biochar amendment was shown to significantly increase L. perenneyields and shift the plant community from grasses to herbs
(Schimmelpfennig et al., 2014). The stimulation of these changes was
explained by higher SOC contents and increased water retention. It has
been recognized that biochar amendment changes plant community
composition through two mechanisms: (i) affecting the seed germination
process and plant establishment and (ii) affecting the growth and
development of the plant species and the functional groups of plants
(van de Voorde et al., 2014). According to Beltman et al. (2007), the
species richness was negatively correlated with aboveground biomass. The
interactions between the aboveground and belowground plant biomass
present high potential for integrating knowledge of the agriculture
practices, soil environment and plant response (Bardgett, 2018).
We hypothesized that straw amendment improves aboveground and
belowground productivity of the red clover/grass mixture, but biochar as
a soil amendment results in a greater effect. The objectives of this
study were to (i) determine the effect of straw and biochar amendment on
the yields of a red clover/grass mixture and on the root morphology and
(ii) determine the effects of the carbon amendments on the botanical
composition of the grassland plant communities.