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.