Pyrolysis Improves the Effect of Straw Amendment on the Productivity of Perennial Ryegrass (Lolium perenne L.)

Abstract

The use of straw as a soil amendment is a well-known and recommended agronomy practice, but it can lead to negative effects on the soil and crop yield. It has been hypothesized that many problems related to the burying of straw can be overcome by pyrolyzing it. The objective of this study was to determine the effect of straw and its biochar on the biomass production of perennial ryegrass. A pot-based experiment was conducted with three factors: (i) the crop species used as feedstock, (ii) raw or pyrolyzed organic material, and (iii) the rate of organic amendments. The soil in the pots was amended with straw and biochar produced from Miscanthus (Miscanthus × giganteus) or winter wheat (Triticum aestivum L.). After soil amendment application, perennial ryegrass (Lolium perenne L.) seeds were sown. During two years of the experiment, the perennial ryegrass above-ground biomass production and root biomass and morphology parameters were determined. Straw and biochar resulted in higher perennial ryegrass above-ground biomass compared with that of the non-fertilized control. However, straw amendment resulted in lower plant yields of above-ground biomass than those of the biochar treatments or the mineral fertilizer control treatment. The feedstock type (Miscanthus or wheat) significantly affected the perennial ryegrass yield. No difference was observed among wheat and Miscanthus biochar, while among straws, Miscanthus resulted in lower perennial ryegrass productivity (the higher rate of straw and biochar as soil amendments resulted in relatively high perennial ryegrass productivity). The organic amendments resulted in relatively high root biomass and length. The root:shoot ratio was lower in the treatments in which biochar was used, whereas feedstock species and amendment rate were not statistically significant for any of the root biomass and morphometric parameters. The results suggest that the use of pyrolyzed straw can be a reliable strategy instead of straw, increasing ryegrass growth and productivity.

1. Introduction

The addition of crop straw to soils is a well-known and recommended practice for conserving soil and improving water retention. Returning straw to soils is a basic component of various conservation tillage systems [1,2]. Straw, as an organic amendment, has a beneficial effect on the physical, chemical and biological properties of soil. It improves soil structure, reduces soil bulk density, activates soil organic phosphorus, and increases soil nitrogen with beneficial effects on crop yields [3,4,5]. Straw incorporation into soil increases soil organic carbon (SOC), and thus mitigates global warming and climate change [6,7,8,9]. According to Zhang et al. [10], straw amendment also increases available nitrogen and phosphorus content and enhances urease, phosphatase and invertase activity levels in the soil. Some studies [2,11,12] have shown that straw removal increases soil bulk density, has a negative effect on root growth, and thus causes crop yields reduction. Xu et al. [2] reported that straw return increased crop yield stability in wheat–maize systems.

Conversely, straw application can lead to negative effects on the soil and crop yields. The decomposition of straw in soil uses plant-available nitrogen, which can reduce crop growth [13,14]. In typical soil conditions on arable land, straw application should be combined with N fertilizers to increase crop yield and improve soil fertility [15]. Therefore, crop straw decomposition rates may vary with fertilization rates, soil depths, soil moisture and temperature conditions under different climates. The net effect of straw decomposition, largely depending on fertilization, is lower in the unfertilized soil [16]. In addition, crop straw contacts with mineral soil may enhance straw decomposition through altering the microbial community [17]. These results highlighted the importance of reasonably managing straw incorporation to increase straw-derived soil organic carbon (SOC) and alleviate existing SOC mineralization [18,19].

A solution to these problems could be represented by the conversion of straw into biochar through pyrolysis. During the past decade, biochar has been recognized as a very promising soil amendment, improving soil fertility and carbon sequestration [14,20,21] and reducing the emission of CO₂, CH₄ and N₂O [22]. Thus, it is considered to be one of the most feasible methods for carbon sequestration and the reduction in greenhouse gas emissions from soil [23,24]. The decomposition of biochar is typically 10–100 times slower than that of uncharred biomass, e.g., crop residues [25]. Biochars are produced from organic heterogeneous feedstock materials that vary in their chemical and physical properties. This variability depends not only on the parameters involved in pyrolysis but also on the materials used to produce biochar [26,27]. Biochar has been shown to affect the physical, chemical, and biological properties of soil [28,29,30]. Many researchers have reported that biochar is characterized by an alkaline pH, high microporosity, specific surface area, and cation exchange capacity, which promotes the retention of organic and inorganic compounds [31,32,33]. Therefore, nutrient availability in the soil may be enhanced by adding biochar because of the increased cation adsorption [34] or because of an increased pH in acidic soils [35,36]. Wu et al. [37] reported that biochar can effectively reduce Zn and Cd concentrations in soil, but As and Cu concentrations increase due to an increase in pH and dissolved organic carbon.

Conversely, high sorption of biochar has some drawbacks. Some plant nutrients, e.g., Ca, P, N, may be immobilized, resulting in nutrient deficiency and further inhibiting plant growth [38]. Biochar was recognized as a soil amendment that increased the sorption and reduced the toxicity of organic pollutants, e.g., herbicides or polycyclic aromatic hydrocarbons. However, biochar also reduces their biodegradation; thus, immobilized pollutants may become bioavailable with time [39]. This effect may also result in lower herbicide efficacy, which leads to greater need in the application time, herbicide doses, and control operations [38].

In a previous study, Głąb et al. [40] reported that biochar improved the physical properties of sandy soil, e.g., soil porosity and water retention characteristics. By adding biochar, the macroporosity and mesoporosity of silty loam soil were significantly increased, thus improving aeration and water availability for plant roots [30]. A similar result was confirmed in research on the use of biochar as an additive to compost [41]. This characteristic of biochar is ascribed to its highly porous structure and large surface area [27]. Biochar has a beneficial effect on soil hydraulic properties. However, biochar application may lead to the risk of water repellency, particularly for sandy soil [39]. However, other studies have reported no impact. Jeffery et al. [42] found no significant effects of biochar application on sandy soil water retention. Hardie et al. [43] proposed the following three mechanisms by which biochar application might increase soil porosity: (i) pore contribution from the high-porosity biochar material, (ii) modification of the pore system by creating packing or accommodation pores, and (iii) improved aggregate stability. However, all of these mechanisms might lead to different outcomes in different soil–climate–management combinations [44].

The chemical properties of biochar, such as minerals, volatile organic compounds and free radical content, can potentially influence microbial activity. Therefore, biochar affects soil enzyme activity that catalyzes various biochemical processes, including soil organic matter turnover and elemental cycles [45,46]. Lehmann et al. [28] reported that the application of biochar affects the activity of soil fauna and microorganisms. However, these effects depend on biochar characteristics, doses, and soil properties [47].

Biochar amendment can improve the yield of cereals, root crops and fiber crops in tropical, subtropical and temperate regions [48,49,50,51]. However, crop productivity responses to biochar may be affected by different factors, such as biochar feedstocks, application rate, crop species, and soil properties [51,52,53,54]. There are some findings in which excessive biochar application was shown to inhibit plant biomass production [23,55]. Research on the effects of biochar on perennial grasses has often presented inconsistent results. Positive effects on switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman) have been reported by Bonin et al. [54]. A similar effect was obtained for red clover (Trifolium pratense L.) [56]. However, Adams et al. [57] reported no effect for sericea lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don). The results of Saha et al. [58] indicate that the use of only biochar was not sufficient to improve plant growth. Therefore, to achieve environmentally safe and high-quality production of creat (Andrographis paniculata (Burm. f.) Wall.), it is recommended to use a combination of biochar and mineral fertilizers [57]. Gondek et al. [36] also reported a significant increase in the yield of perennial ryegrass (Lolium perenne L.) after amending the soil with biochar enriched with mineral fertilizers.

The beneficial effect of biochar application on plant yields is strictly connected with root growth and traits. According to a meta-analysis by Xiang et al. [59], biochar application increases root biomass, root volume, surface area, root length and the number of root tips. However, root diameter is usually not affected by biochar application. This suggests that biochar application benefits root morphological development to alleviate plant nutrient and water deficiency rather than to maximize root biomass accumulation. A similar effect was confirmed for biochar produced from Miscanthus and applied to spring barley (Hordeum vulgare L.). Lateral and fine roots of barley contributed the most to total root lengths [60]. According to Xiang et al. [59], the responses of root traits to biochar application were generally greater in annual plants than in perennial plants, probably as an effect of lower mean root diameter, higher root length and dry matter of perennial plants, e.g., grasses, than annual crops [61,62].

Little is known about the effect of biochar amendments on perennial ryegrass productivity, particularly below ground biomass and root system morphology. We hypothesize that: (i) biochar of wheat or Miscanthus straw increases perennial ryegrass productivity, (ii) biochar application gives a better result than non-pyrolyzed straw, and (iii) this effect can be modified by feedstock material and application rate. The objective of this study was to determine the effect of different straw and biochar rates and biochar feedstock types on the biomass production of perennial ryegrass with detailed characteristics of root system.

2. Materials and Methods

2.1. Experimental Design and Treatments

A pot-based 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 above sea level (a.s.l.)) in the period from 2017 to 2018. The climate of the experimental site, situated in the south of Poland, is temperate. The average annual temperature during the study period was 7.7 °C. The pots were placed in a rainfall shelter with no walls and with a transparent glass roof to exclude precipitation but to ensure natural light and ventilation. During the experiment, soil water content was maintained at the maximum water holding capacity with daily watering using a drip irrigation system (Rain Bird Inc., Tucson, AZ, USA) equipped with a soil moisture sensor ECH2O EC5 (Decagon Devices, Pullman, WA, USA). The experiment was conducted with a completely randomized design with three replications considering three factors: (i) crop species used as feedstock, (ii) raw or pyrolyzed organic material, and (iii) rate of organic amendments. The pots (diameter of 0.22 m, volume of 0.009 m⁻³) were filled with soil with a loamy sand texture (Figure S1 in Supplementary Materials). The physical and chemical characteristics of the soil are presented in

Table 1. Basic soil physical and chemical properties (means± standard deviation).

pH (H2O)		5.67 ± 0.05
Soil organic carbon	g kg−1	6.43 ± 0.08
Total N	g kg−1	0.54 ± 0.01
P	mg kg−1	188 ± 11
K	mg kg−1	305 ± 29
Ca	mg kg−1	207 ± 17.9
Mg	mg kg−1	236 ± 12.5
Electrical conductivity	µS cm−1	32.2 ± 4.35
Solid particle density	g cm−3	2.65 ± 0.06
Bulk density	g cm−3	1.78 ± 0.04
Sand	g kg−1	850
Silt	g kg−1	90
Clay	g kg−1	60

.

Organic amendments were added to the soil in the pots in 2017. Biochar was produced from the biomass of the following two species: Miscanthus (Miscanthus × giganteus) or winter wheat (Triticum aestivum L.) (Figure S2). The Miscanthus straw and winter wheat straw were air dried at 70 °C, ground to fine particles (<4 mm), and mixed to ensure homogeneity. The plant material was pyrolyzed in an electrical laboratory furnace, equipped with a temperature controller, at a temperature of 300 °C for 15 min, with limited air access to reduce C losses [63,64]. The furnace heating occurred at a rate of 10 °C min⁻¹. The time and temperature were set according to the research of Lu et al. [65], Mendez et al. [66], Gondek et al. [67] and Domene et al. [68]. The basic chemical characteristics of the feedstocks are presented in

Table 2. Chemical properties of organic amendments used in pot experiment (means ± standard deviation).

Parameter	Unit	Triticum aestivum L		Miscanthus × giganteus
		Straw	Biochar	Straw	Biochar
pH (H2O)		5.84 ± 0.15	6.52 ± 0.60	6.18 ± 0.43	6.28 ± 0.42
EC	µS cm−1	4.48 ± 0.21	378 ± 21	3.23 ± 0.45	345 ± 18
DM	g kg−1	952 ± 0.2	966 ± 2	947 ± 0.3	977 ± 1
Ash	g·kg−1	59 ± 2	134 ± 5	54 ± 1	87 ± 3
Total C	g·kg−1	441 ± 2	628 ± 2	456 ± 2	651 ± 6
Total N	g·kg−1	7.16 ± 0.32	12.4 ± 0.36	3.97 ± 0.29	7.31 ± 0.09
K	g·kg−1	4.95 ± 0.66	11.9 ± 0.29	1.33 ± 0.06	2.81 ± 0.17
P	g·kg−1	1.04 ± 0.05	1.17 ± 0.04	0.73 ± 0.04	0.94 ± 0.06

EC: electrical conductivity; DM: dry matter.

.

The following treatments were applied: wheat straw (MSW), wheat biochar (MBW), Miscanthus straw (MSM) and Miscanthus biochar (MBM). A treatment with mineral fertilization but without organic amendments (M) was used. A control treatment (CTR), without mineral fertilization and without any amendments, was also used. Mixtures of straw, biochar and soil were prepared with two amendment rates: 1.0% and 2.0% (with straw and biochar mass as a percentage of the whole sample mass) which corresponds to 30 and 60 t ha⁻¹, respectively. Mineral fertilization was applied in 2017 and 2018 at rates of 0.10 g N kg⁻¹, 0.04 g P kg⁻¹ and 0.12 g K kg⁻¹ (calculated as the mass of the element per dry matter of soil), which corresponds to 300 kg N ha⁻¹, 120 kg P ha⁻¹ and 360 kg K ha⁻¹, respectively. After amendment application, perennial ryegrass (Lolium perenne L.) was sown in the pots at a rate of 3.5 g per pot (90 kg ha⁻¹).

2.2. Plant Yields

The plants were harvested three times per year, in May, June and August of 2017 and 2018. The dry matter (DM) of the yield was determined by harvesting the above-ground plant biomass and drying at 70 °C to a constant weight. Based on the RDMD and the dry matter of the above-ground yields, the root:shoot ratio (RSR) was determined using Equation (1).

$$RSR=\frac{RDMD\times V_p}{SDM}$$

(1)

where V_p is the total volume of the pot, RDMD is the root dry matter density, and SDM is the mean annual shoot dry matter.

2.3. Root Measurements

Roots were sampled in 2018 using the soil core method. Three samples per pot were collected using a 50 mm diameter core. The roots were washed using a hydro-pneumatic elutriation system [69] to remove mineral particles from the samples. Before scanning, any organic contamination was removed manually, and digital images were obtained with an Epson Perfection 4870 photo scanner (Seico Epson Corp., Suwa, Japan) and saved at a resolution of 1200 dpi. The images were analyzed using Aphelion 3.2 image analysis software (ADCIS S.A. and Amerinex Applied Imaging, Herouville, Saint-Clair, France), and the root morphometric characteristics were then calculated. The procedure for image analysis described by Bauhus and Messier [70] was used, which comprises four main steps: filtering, segmentation, preparing root skeletons and morphometric measurements. The obtained root length was divided into eight diameter classes (<0.02, 0.02–0.05, 0.05–0.1, 0.1–0.2, 0.2–0.5, 0.5–1.0, 1.0–2.0, and >2.0 mm), and calculations were then performed. The root length density (RLD) was calculated by dividing the total root length (L) by the volume of the soil sample (V) using Equation (2).

$$RLD=\frac{L}{V}$$

(2)

The mean root diameter (MRD) was calculated as the weighted mean of the root length (l_i) with particular diameter classes (d_i) as weights using Equation (3).

$$MRD=\frac{{{\displaystyle \sum }}_{i=1}^n{l}_i{d}_i}{{{\displaystyle \sum }}_{i=1}^n{d}_i}$$

(3)

The root surface area (RSA) and root volume density (RVD) were calculated as the product of the root segment lengths (l_i) and their diameters (d_i) using Equations (4) and (5), respectively.

$$RSA={{\displaystyle \sum }}_{i=1}^n\pi d_i{l}_i$$

(4)

$$RVD={{\displaystyle \sum }}_{i=1}^n\pi {\left(\frac{d_i}{2}\right)}^2{l}_i$$

(5)

After scanning, the roots were dried at a temperature of 70 °C to determine the root dry matter density (RDMD). Then, the specific root length (SRL) was calculated as a ratio of RLD and RDMD using Equation (6).

$$SRL=\frac{RLD}{RDMD}$$

(6)

Root tissue density (RTD) was calculated as the ratio of RDMD to RVD using Equation (7).

$$RTD=\frac{RDMD}{RVD}$$

(7)

2.4. Statistics

An analysis of variance (ANOVA) for a randomized design was performed using the statistical software package Statistica v. 13.3 (StatSoft Inc., Tulsa, OK, USA) to evaluate the significance of different organic amendments, their rate, and feedstock type on perennial ryegrass biomass production and root morphometric parameters (Tables S1–S3 in Supplementary Materials). To avoid psuedoreplication, we used mean values for the sample replication per pot in ANOVA analyses. The distribution of the data was checked for normality using a Shapiro–Wilk test. The homogeneity of variance was checked using Levene’s test. The multiple means comparison was made using a Bonferroni test with an adjusted level of significance. Pearson’s correlation analysis was used to analyze the correlation between root morphological parameters and above-ground biomass production (Table S5). For data where significant values of correlation coefficients (r) were observed, the results of linear regression models are presented in graphical form, together with regression equations and coefficients of determination (R²).

3. Results

3.1. Biochar Effects on Perennial Ryegrass Yields

The highest above-ground biomass of ryegrass was obtained for the first cut in 2018 (0.965 kg DM m⁻²); in comparison, the second and the third cuts yielded 0.120 and 0.115 kg DM m⁻², respectively (

Table 3. Above-ground biomass production (kg DM m⁻²) of perennial ryegrass. MBM, Miscanthus biochar with mineral fertilization; MBW, wheat biochar with mineral fertilization; MSM, Miscanthus straw with mineral fertilization; MSW wheat straw with mineral fertilization; M, mineral fertilization; CTR, without mineral fertilization and without any amendments.

Treatment	2017			2018			Annual Mean
	1st Cut	2nd Cut	3rd Cut	1st Cut	2nd Cut	3rd Cut	1st Cut	2nd Cut	3rd Cut
1% MBM	0.396	0.319	0.200	1.075 b	0.194	0.120	0.735	0.256	0.160
2% MBM	0.367	0.410	0.210	1.197 ab	0.218	0.129	0.782	0.314	0.169
1% MBW	0.379	0.394	0.228	1.048 b	0.148	0.154	0.713	0.271	0.191
2% MBW	0.362	0.337	0.277	1.224 a	0.230	0.129	0.793	0.283	0.203
1% MSM	0.131	0.238	0.124	0.838 de	0.065	0.103	0.484	0.151	0.113
2% MSM	0.066	0.189	0.087	0.811 ef	0.038	0.110	0.439	0.113	0.099
1% MSW	0.149	0.166	0.257	0.921 c	0.091	0.128	0.535	0.128	0.193
2% MSW	0.055	0.058	0.283	0.864 d	0.099	0.129	0.459	0.078	0.206
M	0.341	0.310	0.198	0.877 d	0.070	0.083	0.609	0.190	0.140
CTR	0.050	0.112	0.069	0.794 f	0.048	0.068	0.422	0.080	0.068
Means for feedstock species × material interaction
MBM	0.381	0.364 a	0.205 b	1.136 a	0.206 a	0.124	0.759 a	0.285	0.165
MBW	0.370	0.365 a	0.252 a	1.136 a	0.189 a	0.142	0.753 a	0.277	0.197
MSM	0.098	0.213 b	0.105 c	0.825 c	0.051 c	0.107	0.461 c	0.132	0.106
MSW	0.102	0.112 c	0.270 a	0.893 b	0.095 b	0.128	0.497 b	0.103	0.199
Means for amendments rate
1%	0.263 a	0.279	0.202	0.970 b	0.124 b	0.127	0.617	0.202	0.164
2%	0.213 b	0.249	0.214	1.024 a	0.146 a	0.124	0.618	0.197	0.169
Means for feedstock species
Miscanthus	0.240	0.289 a	0.155 b	0.980 b	0.129	0.116 b	0.610	0.209 a	0.135 b
Wheat	0.236	0.239 b	0.261 a	1.014 a	0.142	0.135 a	0.625	0.190 b	0.198 a
Means for amendment material
Biochar	0.376 a	0.365 a	0.229 a	1.136 a	0.197 a	0.133	0.756 a	0.281 a	0.181 a
Straw	0.100 b	0.162 b	0.188 b	0.859 b	0.073 b	0.118	0.479 b	0.118 b	0.153 b

For each column, mean values with different letters are significantly different (p < 0.05); Bonferroni post hoc test; superscripts used only for significant differences according to ANOVA (see Table S1 in Supplementary Materials).

). In 2017, the first and second cuts had similar results, with 0.229 and 0.253 kg DM m⁻², respectively. The treatments used significantly affected perennial ryegrass yields in 2017 and 2018 during the two-year experiment.

The soil amendments, i.e., straw and biochar, significantly increased annual productivity in comparison to that of the control (CTR) treatment (Figure 1a). However, when mineral fertilization was applied (M), the annual yields (0.940 kg DM m⁻², on average) were higher than those of the treatments with straw (0.570 kg DM m⁻²), but lower than those of the treatments with biochar (1.218 kg DM m⁻²).

The feedstock (Miscanthus or wheat) also significantly affected plant yield. However, this effect occurred in interaction with amendment, biochar or straw. Miscanthus biochar did not significantly influence perennial ryegrass production in comparison with wheat feedstock. However, when unprocessed straw was used, the selection of Miscanthus or wheat made a difference. In 2018, the use of Miscanthus straw (MSM) as a soil amendment resulted in lower perennial ryegrass productivity (0.983 kg DM m⁻²) than that of the MSW treatment with wheat straw (1.116 kg DM m⁻²). This effect was more pronounced in 2018 than in 2017. The rate of amendment application was also a significant factor. The higher rate (2%) of tested soil amendments resulted in higher perennial ryegrass productivity in 2018. This effect was significant only during the first and second cut (Table 3). However, in 2017, the amendment rate affected grass yields only in the first cut. Surprisingly, a higher yield was observed for the 1% rate (0.263 kg DM m⁻²) than for the 2% rate (0.213 kg DM m⁻²).

3.2. Root Morphometric Parameters

The soil amendments, straw and biochar, significantly affected root biomass (Figure 1b, Table S4). The increase in RDMD as a result of mineral fertilization (M) (1.71 mg cm⁻³) is larger than the increase in RDMD as a result of CTR (1.25 mg cm⁻³). The organic amendments used here resulted in higher root biomass than the control treatments, M and CTR. Biochar used as a soil amendment resulted in significantly higher RDMD values (2.34 mg cm⁻³) than those of straw amendment (1.85 mg cm⁻³). The mean value of RLD for the perennial ryegrass was 79.1 cm cm⁻³. The organic amendments resulted in higher RLD (81.9 cm cm⁻³) in comparison with those of CTR (53.5 cm cm⁻³) (Figure 2a).

The RLD values for pots with biochar amended soil (85.3 cm cm⁻³) were significantly higher than that for treatments with straw (71.2 cm cm⁻³). The highest value of RLD, above 90 cm cm⁻³, was observed for the 1% MBM and 2% MBW treatments. The RLD for particular root diameters is presented in Figure 3.

The most frequent fraction of the RLD for all tested treatments was that with diameters of 0.02–0.05 mm, representing approximately 51% of all root lengths. Roots with diameters above 0.2 mm occurred in only 1% of all roots. The materials used as organic soil amendments significantly affected RLD in root diameter fractions of 0.02–0.05 mm only (Figure 4). Biochar application resulted in a higher RLD value (44.9 cm cm⁻³) in this root size class, while straw addition resulted in an RLD value of 38.4 cm cm⁻³.

The RLD distribution in the root diameter fractions was reflected by the MRD results (Figure 2d). The highest MRD was observed for the 2% MBW treatment (0.062 mm), whereas other treatments resulted in MRD in the range of 0.05–0.06 mm. Only the control treatments, CTR and M, were characterized by relatively thin roots, with MRDs of 0.049 and 0.046 mm, respectively.

RSA was calculated as a derivative of the RLD and root diameter. The highest RSA was observed in the 2% MBW treatment (1.81 cm² cm⁻³) (Figure 2f). The lowest RSA value (0.815 cm² cm⁻³) characterized the CTR treatment. RSA was significantly higher for all the treatments with biochar amendment (1.54 cm² cm⁻³) than with straw (1.31 cm² cm⁻³). A similar relationship was observed for RVD (Figure 2c). The lowest RVD values were in the control treatments, CTR and M. Biochar application, in contrast with straw treatments, resulted in higher RVD. There were no significant differences in SRL between treatments (Figure 2e). The mean value of SRL was 383 m g⁻¹, and the value varied from 326 to 428 m g⁻¹. The RTD values ranged from 0.367 to 0.576 mg cm^−3, with a mean value of 0.427 mg cm⁻³ (Figure 2b). The highest root tissue density was observed in roots from the control treatments, CTR (0.576 mg cm⁻³) and M (0.515 mg cm⁻³). In all the other treatments, RTD was below 0.5 mg cm⁻³. Low values (i.e., below 0.4 mg cm⁻³) of RTD were obtained in the 1% MBW, 2% MSM and 1% MSW treatments.

Based on the above-ground biomass and RDM, the RSR was determined. The RSR was significantly lower for all treatments where biochar was used as a soil amendment (0.91) (Figure 1c). However, straw application resulted in a significantly higher RSR (1.18). Low RSR (i.e., below 1.0) was observed in the 1% and 2% MBM, 1% MBW and M treatments. Feedstock species and amendment rate were not statistically significant for any root morphometric parameters. It was found that some root morphometric parameters, i.e., RDMD, RSA, RVD, and RLD, were positively correlated with the mean annual above-ground biomass productivity of perennial ryegrass (Table S5, Figure 5a–d). Higher values of these root morphometric features resulted in higher above-ground productivity. The correlation coefficients ranged from 0.544 (RSA) to 0.446 (RVD). For other root characteristics, i.e., MRD, SRL and RTD, correlation coefficients were below 0.3 (p > 0.05).

4. Discussion

Biochar or straw application resulted in a higher biomass of perennial ryegrass than that of the unfertilized control. However, straw amendment resulted in lower yields than those with mineral fertilizers. According to Wang et al. [13], the negative effect of straw application on plant yield is ascribed to the process of straw decomposition in the soil, which uses plant-available nitrogen. Conversely, Wang et al. [9] reported that buried straw can increase the rice yield but had no influence on wheat yield compared with those of the non-straw control treatment. According to Wang et al. [9], the final effect of straw amendment on crop productivity depends on the straw rate, N content in the straw and amount of N applied as mineral fertilization. In our research, 0.10 g N kg⁻¹ of soil was applied, which corresponded to approximately 300 kg N ha⁻¹. The straw rate was calculated as 30 and 60 t ha⁻¹ (1% and 2%, respectively). Mean annual yield of perennial ryegrass with mineral fertilization was of 0.940 kg DM m⁻² (corresponding to 9.4 t DM ha⁻¹). Hopkins et al. [71] reported a similar DM yield for perennial ryegrass (approximately 9 t DM ha⁻¹) at an N fertilization rate of 250 kg N ha⁻¹ in similar climate and soil condition, whereas without N fertilization, DM yield was reduced to below 5 t DM ha⁻¹. In the current research, perennial ryegrass yielded 5.7 t DM ha⁻¹ at non-fertilized control. Despite high N fertilization, straw application resulted in reduced grass yields. However, this effect was noticed in the first year (4.50 t DM ha⁻¹) in relation to the control treatment with mineral fertilizers (8.49 t DM ha⁻¹). Huang et al. [72], in their meta-analysis, ascribed this effect to the phenomenon that crop residues with high C:N ratios result in microbial N immobilization and a temporary decrease in crop-available N. Frequent application of crop straw without sufficient N fertilization results not only in N immobilization, but also in the accumulation of heavy metals, and may adversely affect plant growth and soil quality [14]. Moreover, straw incorporation into the soil stimulates microbial activity by nutrient supply, thus accelerating SOC mineralization [15]. The accelerated SOC decomposition may outweigh the overall positive effects of straw addition on soil C sequestration, resulting in small net gains or even net losses in soil C storage [15,73].

Yang et al. [74] reported a beneficial effect on winter wheat root system vigor, total root length, root surface area, and root tip number when 6 t ha⁻¹ of maize straw and 120 kg N ha⁻¹ was applied. Straw mulching also resulted in an increase of root length density of wheat when maize straw was used at 8 t ha⁻¹ [75]. The current research did not confirm this effect. Straw application reduced root biomass and deteriorated morphological parameters. This effect can be result of the reduction in ryegrass biomass production in the straw treatment. Xu et al. [76] reported that high root length was observed when straw was applied together with relatively high N fertilization. Thus, the effect of straw application on root system growth and morphology depends on the proportion between the amount of straw and N fertilization.

Increased crop yield is a commonly reported benefit of adding biochar to soils [49,50,77]. Current research confirmed this effect. Biochar was widely recognized as a soil amendment that can improve the physical, chemical and biological properties of soil. It increases the sorption of nutrients and water retention in soils [29,34]. Biochar increases pH and thus reduces toxic metal availability for plants [37]. However, the results of published studies on the effects of biochar on crop yields range from increased, neutral and decreased compared to unamended soils [78,79]. This variability is usually ascribed to biochar rate, soil properties and climate conditions. Mclennon et al. [79] observed that biochar application up to a rate of 17.8 t ha⁻¹ did not affect the yield of grass mixture. The greatest positive effect is observed in extremely high biochar rates of 100 t ha⁻¹ [80]. Relatively large positive effects have also been reported for sandy soil. This suggests that yield improvement may also be an effect of the higher water holding capacity of the soil [78]. Larger differences between the treatments and control were observed in the first year than in the second year. According to Zhang et al. [81], rice grain yield was improved in the first and second years following biochar application. Similarly, Madari et al. [82] observed that soybean yield increased with biochar rates until the third year after its application. They did not find an interaction effect of biochar and mineral fertilizers.

After biochar application, higher root lengths and biomass were observed. This effect was also reported in a review by Agegnehu et al. [83]. According to Olmo [50], biochar addition increased the root length of wheat and decreased the root diameter and root tissue mass density, which is ascribed to better resource acquisition. Prendergast-Miller [60] observed that roots are attracted to the biochar. Biochar controls plant root nutrient acquisition as a nutrient source and by altering soil nutrient content. According to the meta-analysis by Xiang et al. [59], the effect of biochar application on root biomass and morphology varied remarkably among plant functional groups. Root biomass, root length and specific root length in annual plants are higher than those in perennial plants. They also stated that there is no significant effect of biochar feedstock type or C:N ratio on the responses of root traits to biochar application.

The root:shoot ratio was significantly higher in the control and treatments in which straw was used compared to treatments with biochar, which may be due to the higher nutrient and water availability after biochar application. This phenomenon can be explained by optimal partitioning theory. Plants allocate their biomass to the organ that has the most limited resource [84]. The expression of such an optimization is the change in allocation between the biomass of shoots and roots in response to nutrient availability. When water supply or nutrient availability increases, plants allocate less mass to their roots because less effort is required to acquire these resources [85,86].

5. Conclusions

Straw as a soil amendment is a well-known and recommended agronomy practice, but it can lead to negative effects on the soil environment and crop yield. Pyrolyzed straw can solve these problems. Our investigation shows that biochar is a better soil amendment than straw in perennial ryegrass production. Straw amendment results in lower grass yields than those with biochar or mineral fertilizers. However, this effect depends on feedstock type and rate of amendment. Miscanthus straw increase perennial ryegrass yield in comparison with wheat straw. However, when Miscanthus is used for biochar production, this does not significantly influence perennial ryegrass production. The higher rate of the soil amendments resulted in relatively high perennial ryegrass productivity in the second year of cultivation. All of the organic amendments used in this experiment resulted in relatively high root biomass and length. However, the root:shoot ratio is lower when biochar was used as a soil amendment. It can be concluded that pyrolyzed straw is a better soil amendment than raw straw for perennial ryegrass cultivation.