ABSTRACT

The use of biochar in agriculture remains controversial due to the amounts applied and the small size of the particles which can cause respiratory problems when inhaled. With the aim of evaluating the effects of cattle manure biochar (CMB) on the chemical attributes of the soil and on plant growth and nutrition, a greenhouse experiment was conducted, with Urochloa brizantha ‘BRS Paiaguás’ grown in pots over five crop cycles, in weathered soil (Oxisol). CMB pellets, both enriched and not enriched with potassium phosphate (PK), were produced using cassava starch as the binder, in a ratio of 2:2:1 (biochar: PK fertiliser: binder) and 4:1 (biochar: binder), respectively. The CMB was produced at a pyrolysis temperature of 450 °C. The experimental design was completely randomised in a 2 x 2 x 2 + 1 factorial scheme, with five replications. The treatments were non-pelletised and pelletised CMB, with and without liming, and with and without PK, and an additional treatment with no fertiliser. The CMB improved the chemical properties of the soil, correcting acidity, increasing nutrient availability and improving the production of Urochloa brizantha ‘BRS Paiaguás’. The use of CMB enriched with phosphorus and potassium behaved as a slow-release organomineral fertiliser in Urochloa brizantha ‘BRS Paiaguás’.

Key words
Organomineral; Slow-release fertiliser; Use of waste; Pyrolysis

INTRODUCTION

Mineral fertilisers are widely used to achieve high productivity, which in the long term promotes the depletion of natural resources, and imbalances in the environment (PATHY; RAY; PARAMASIVAN, 2020PATHY, A.; RAY, J.; PARAMASIVAN, B. Biochar amendments and its impact on soil biota for sustainable agriculture. Biochar, v. 2, p. 287-305, 2020.). In this respect, biochar offers a sustainable alternative for reducing pressure on natural resources, and for safely disposing of organic waste in the environment.

The term biochar, inspired by the so-called ‘Terra Preta de Índio’ in the Amazon region, refers to a solid material, rich in carbon, obtained through the thermochemical transformation of biomass with little or no oxidising agent, a process known as pyrolysis (ALMEIDA et al., 2022ALMEIDA, S. G. C. et al. Biochar production from sugarcane biomass using slow pyrolysis: characterization of the solid fraction. Chemical Engineering and Processing - Process Intensification, v. 179, e109054, 2022.; BRTNICKY et al., 2021BRTNICKY, M. et al. A critical review of the possible adverse effects of biochar in the soil environment. Science of The Total Environment, v. 796, e148756, 2021.). During pyrolysis, the lignocellulosic material aromatises and the functional groups that contain oxygen are reduced, affording the carbon greater stability (HE et al., 2021HE, Y. et al. Geographical location and water depth are important driving factors for the differences of suspended particulate organic matter (SPOM) in lake environment across nationwide scale: evidences from n-alkane fingerprints. Science of the Total Environment, v. 752, e142948, 2021.). Unlike charcoal, biochar is applied to the soil, with the aim of conditioning the soil environment to increase the carbon stock, the cation and water retention capacity, and the availability of nutrients (AGBEDE; OYEWUMI, 2022AGBEDE, T. M.; OYEWUMI, A. Benefits of biochar, poultry manure and biochar-poultry manure for improvement of soil properties and sweet potato productivity in degraded tropical agricultural soils. Resources, Environment and Sustainability, v. 7, e100051, 2022.; BISTAet al., 2019BISTA, P. et al. Biochar effects on soil properties and wheat biomass vary with fertility management. Agronomy, v. 9, p. 623-634, 2019.).

The use of biochar is justified by the possibility of recycling large amounts of organic waste, reducing the risk of pollution associated with disposing of these materials in the environment (BISTA et al., 2019BISTA, P. et al. Biochar effects on soil properties and wheat biomass vary with fertility management. Agronomy, v. 9, p. 623-634, 2019.). In addition, unlike incorporating organic waste into the soil, biochar increases carbon storage in the soil as the material has a high concentration of stable carbon, which reduces the emission of polluting gases into the atmosphere (GWENZI et al., 2016GWENZI, W. et al. Comparative short-term effects of sewage sludge and its biochar on soil properties, maize growth and uptake of nutrients on a tropical clay soil in Zimbabwe. Journal of Integrative Agriculture, v. 15, p. 1395-1406, 2016.).

However during application, biochar is lost through drift because of the small size of the low-density particles, which may result in the poisoning of those carrying out the application due to inhaling any toxic compounds in the biochar dust (LI; BAIR; PARIKH, 2018LI, C.; BAIR, D. A.; PARIKH, S. J. Estimating potential dust emissions from biochar amended soils under simulated tillage. Science of the Total Environment, v. 625, p. 1093-1101, 2018.).

In an attempt to get around the limitations of using high doses and reduce the effects of the dust, one alternative is to produce biochar pellets enriched with nutrients, similar to organomineral fertilisers. In this respect, the first hypothesis is that, even in small quantities, biochar enriched with nutrients acts as a soil conditioner. The second hypothesis is that pelletisation eliminates or reduces the emission of dust, and affords gradual release of the nutrients. The aim of this study, therefore, was to evaluate the effects of applying cattle manure biochar on the chemical properties of the soil and on the growth of Urochloa brizantha ‘BRS Paiaguás’.

MATERIAL AND METHODS

The experiment was conducted in two phases. The first phase involved developing a protocol for the production of biochar pellets, while the second phase included the cultivation of U. brizantha ‘BRS Paiaguás’.

The biochar used in the experiment came from cattle manure (CMB) collected in an area reserved for feeding dairy cows at the Institute of Agricultural Sciences (ICA) of the Federal University of Minas Gerais (UFMG), Brazil. From this raw material, spheres measuring approximately 4 cm in diameter were prepared manually and then dried at 103 ± 2 °C for 48 h to constant weight to ensure they were completely dehydrated.

To produce the biochar, the dried spheres were placed in a hermetically sealed steel container in an industrial muffle furnace and subjected to slow pyrolysis (ALMEIDA et al., 2022ALMEIDA, S. G. C. et al. Biochar production from sugarcane biomass using slow pyrolysis: characterization of the solid fraction. Chemical Engineering and Processing - Process Intensification, v. 179, e109054, 2022.). The pyrolysis reactor was adjusted to a heating rate of 5 °C min^-1, until reaching 450 °C, remaining at that temperature for 30 min, which is the residence time for the biomass to produce the CMB. The biochar was cooled to room temperature inside the muffle furnace and then crushed and sieved through a 0.25 mm mesh.

The biochar was characterised (Table 1) as per the method described by Rajkovich et al. (2012)RAJKOVICH, S. et al. Corn growth and nitrogen nutrition after additions of biochars with varying properties to a temperate soil. Biology and Fertility of Soils, v. 48, p. 271-284, 2012.: the total C and total N content were determined by dry combustion at 950 °C using an elemental analyser, and the other nutrients by inductively coupled plasma mass spectrometry (ICP-MS) following digestion in a microwave oven at 175 ± 5 °C for 4.5 to 10 min.

During the first phase, CMB was used together with potassium phosphate mineral fertiliser, KH₂PO₄ (PK). Cassava starch was used as the binding component to promote consistency of the granules. From earlier tests, the proportions determined for the production of pelletised organomineral fertiliser was 40% fertiliser, 40% biochar and 20% binder. For the treatments with pelletised biochar with no added fertiliser, the proportion was 80% biochar to 20% binder.

The fertiliser was ground and mixed with biochar and cassava starch. Distilled water was then added to the mixture, which was homogenised in a mortar to a paste-like consistency. The mixture was transferred to a polyethylene mould containing holes 5.0 mm in diameter and 5.0 mm in height. The mould filled with the mixture was placed in an oven to dry at 65 °C for 30 min (SANTOS et al., 2019SANTOS, S. R. et al. Biochar association with phosphate fertiliser and its infl uence on phosphorus use efficiency by maize. Science and Agrotechnology, v. 43, e025718, 2019.; TORRES et al., 2020TORRES, W. G. A. et al. Phosphorus availability in soil amended with biochar from rice rusk and cattle manure and cultivated with common bean. Science and Agrotechnology, v. 44, e014620, 2020.).

For the second phase, was conducted in a greenhouse, with Urochloa brisantha ‘BRS Paiaguás’ in pots of 1.5 dm³. The soil used in the experiment was collected in the 0 to 20 cm layer of an Oxisol from an area of native Cerrado.

The physical and chemical properties of the soil were characterised as per Teixeira et al. (2017)TEIXEIRA, P. C. et al. Manual de métodos de análise de solo. 3. ed. Brasília: Embrapa, 2017. 574 p.: sand = 780 g kg^-1, silt = 100 g kg^-1, clay = 120 g kg^-1, pH in water = 5.6, available phosphorus (Mehlich 1) = 1.3 mg kg^-1, remaining phosphorus = 37.3 mg L^-1, potassium (Mehlich 1) = 11.4 mg kg^-1, calcium = 1.07 cmol_c dm^-3, magnesium = 0.44 cmol_c dm^-3, aluminum = 0.05 cmolc dm^-3, hydrogen = 0.98 cmol_c dm^-3, base saturation (V) = 58.49%, cation exchange capacity at pH 7.0 (T) = 3.32 cmol_c dm^-3, and soil organic carbon (SOC) = 0.49 dag kg^-1.

The treatments were arranged in a 2 x 2 x 2 + 1 factorial scheme, with five replications (n = 45), in a completely randomised design, comprising nonpelletised and pelletised biochar, with and without a soil acidity corrector, and with and without the addition of mineral fertiliser (PK), and an additional treatment of natural soil (Figure 1).

For the treatments with added acidity corrector, 1.5 g limestone (calcium and magnesium carbonate in a ratio of 4:1, Ca:Mg) was applied per pot to raise the base saturation to 60%. The incubation period of the corrective was 30 days, during which time the soil moisture was maintained close to field capacity. Mineral fertiliser was applied at a rate of 200 mg dm^-3 P and 250 mg dm^-3 K (Table 2).

Ten seeds of U. brisantha ‘BRS Paiaguás’ were sown per pot. Thinning was carried out 13 days after sowing, leaving five plants per pot, which were grown for 129 days. During this period, the aerial part of the plants was cut five times depending on their growth, defining the cutting height as per the recommendations for pasture management, i.e. 20 cm above ground level whenever the height reaches 40 cm.

As the N concentration of the biochar was low, a top dressing of nitrogen was applied, using 45 mg dm^-3 N in the form of urea after each cutting (Table 1).

After each cutting, shoot dry matter was determined in a forced air circulation oven at 65-70 °C for 72 hours; the macroand micronutrient content of the leaf blade was also determined. The accumulated amount of each nutrient in the aerial part of the plants were estimated by multiplying the levels of each nutrient by the production of dry matter.

Root dry matter and the chemical attributes of the soil were determined after the fifth cut at the end of the cultivation period (TEIXEIRA et al., 2017TEIXEIRA, P. C. et al. Manual de métodos de análise de solo. 3. ed. Brasília: Embrapa, 2017. 574 p.).

The Shapiro-Wilk test and Levene’s test were used to verify the normality and homogeneity of the data, respectively. The treatments were compared using the F-test at 5%, and the additional treatment was compared to the other treatments using Dunett’s test at 5%. The R v. 3.6.3 statistical software (R Development Core Team) was used.

RESULTS AND DISCUSSION

Chemical attributes of the soil

The application of cattle manure biochar (CMB) to the soil, whether in pelletised or non-pelletised form and irrespective of liming or fertilisation, reduced soil acidity. Likewise, the application of agricultural limestone, both with or without CMB or PK fertiliser, raised the soil pH to close to seven. In the treatments that included biochar, soil pH values were significantly higher compared to the control treatment (Table 3). While in the liming treatments, the soil pH increased close to neutral, regardless of whether the biochar was pelletised or not (Table 3).

Correction of the soil acidity by the CMB can be attributed to its alkaline pH, possibly due to the presence of basic cations, such as Ca⁺², Mg⁺² and K⁺, in the ash in the form of alkali metal oxides and hydroxides from the pyrolysis process (TORRES et al., 2020TORRES, W. G. A. et al. Phosphorus availability in soil amended with biochar from rice rusk and cattle manure and cultivated with common bean. Science and Agrotechnology, v. 44, e014620, 2020.). The presence of basic compounds in the CMB ash, such as oxides, e.g. CaO, MgO and K₂O, and hydroxides, such as Ca(OH)₂, Mg(OH)₂ and KOH, is clearly related to the alkaline pH of the biochar (Table 1). In this respect, the higher the ash content, the higher the pH value of the biochar (SARFARAZ et al., 2020SARFARAZ, Q.et al. Characterization and carbon mineralization of biochars produced from different animal manures and plant residues. Scientific Reports, v. 10, e955, 2020.). The percentage ash in the CMB was 36.2%, a relatively high value, which explains the pH of 9.8 (Table 1).

The mineral composition of the raw material (RAJKOVICHet al., 2012RAJKOVICH, S. et al. Corn growth and nitrogen nutrition after additions of biochars with varying properties to a temperate soil. Biology and Fertility of Soils, v. 48, p. 271-284, 2012.) and the pyrolysis temperature are responsible for the content and composition of the ash in biochars (SOUZA et al., 2021SOUZA, C. S. et al. Induced changes of pyrolysis temperature on the physicochemical traits of sewage sludge and on the potential ecological risks. Scientific Reports, v. 11, e974, 2021.) and, consequently, the ability of biochars to neutralise soil acidity, as found in this study. In addition to the increase in pH, at higher pyrolysis temperatures there is a greater reduction in volatile materials, greater concentration of the inorganic fraction, greater dissolution of water-soluble salts and, consequently, higher values for electrical conductivity (SANTOS et al., 2019SANTOS, S. R. et al. Biochar association with phosphate fertiliser and its infl uence on phosphorus use efficiency by maize. Science and Agrotechnology, v. 43, e025718, 2019.).

In the case of tropical soils, which are highly weathered and acidic due to the replacement of exchangeable bases by H⁺ or AI⁺³ ions, applying biochar can be considered an alternative for mitigating the effects of soil acidity and ensuring crop productivity (APORI et al., 2021APORI, S. O. et al. Effect of co-applied corncob biochar with farmyard manure and NPK fertiliser on tropical soil. Resources, Environment and Sustainability, v. 5, e100034, 2021.; YAO et al., 2019YAO, L. et al. Responses of Phaseolus calcaltus to lime and biochar application in an acid soil. PeerJ. v. 7, e6346, 2019.). However, the doses of biochar must be adjusted so that the pH of the soil does not exceed 6.6 to avoid any harmful effects to the plants, such as reducing the availability of cationic micronutrients due to precipitation reactions (BRTNICKY et al., 2021BRTNICKY, M. et al. A critical review of the possible adverse effects of biochar in the soil environment. Science of The Total Environment, v. 796, e148756, 2021.).

The available P content of each treatment that included the application of biochar, both in pelletised or non-pelletised form, was higher than in the control treatment (Table 3). The P content of the soil fertilised with pelletised biochar with PK were 420.3 and 401.3 mg kg^-1, in the treatments with and without liming, respectively (Table 3). On the other hand, applying non-pelletised biochar with PK fertiliser resulted in a P content of 81.4 and 74.6 mg kg^-1, respectively, with and without liming (Table 3). It was found that pelletising the biochar, especially together with the use of phosphate fertiliser, helped to increase the available P content of the soil compared to the corresponding treatment with non-pelletised biochar, with 207.9 and 41.3 mg kg^-1, respectively (Table 3), a variation of 403.39%.

The increase in available P in the soil with the application of CMB, whether pelletised or not, is primarily due to its presence in the biochar, since the nutrient is only volatilised at pyrolysis temperatures of approximately 700 °C. Biochars, in addition to being sources of P, reduce the phosphate fixation reactions in soils due to the negative surface electrical charges of anionic functional groups, while the soluble silica present in biochars competes for the phosphorus fixation sites (TORRES et al., 2020TORRES, W. G. A. et al. Phosphorus availability in soil amended with biochar from rice rusk and cattle manure and cultivated with common bean. Science and Agrotechnology, v. 44, e014620, 2020.). The increase in available P in treatments corrected with biochar may also be related to the increase in soil pH and alkaline metals, which reduce P adsorption reactions to the Fe and Al oxides found in large quantities in the most-weathered tropical soils, such as Oxisols (APORI; BYALEBEKA, 2021APORI, S. O.; BYALEBEKA, J. Contribution of corncob biochar to the chemical properties of a ferralsol in Uganda. Arabian Journal of Geosciences, v. 14, e1290, 2021.).

The high levels of available P in treatments with pelletised CMB with P and K fertiliser show that the P in the pellet was released more slowly in order to avoid fixation reactions in the soil (KIM; HENSLEY; LABBÉ, 2014KIM, P.; HENSLEY, D.; LABBÉ, N. Nutrient release from switchgrass-derived biochar pellets embedded with fertilisers. Geoderma, v. 232-234, p. 341-351, 2014.). Some authors consider the formation of cation bridges between the biochar matrix and the phosphorus. CMB has high levels of cations, such as Ca and Mg (Table 1), which could favour this type of connection and contribute to a slower release of phosphorus into the soil (LUO et al., 2021LUO, W. et al. A potential Mg-enriched biochar fertiliser: excellent slow-release performance and release mechanism of nutrients. Science of the Total Environment, v. 768, e144454, 2021.). Furthermore, in the case of weathered and acidic soils, the acidic pH of the medium can interfere with the surface charges of organic groups in the biochar pellets by protonation, which increases the affinity of the pellets for P, with a weaker binding energy in relation to P retention by inner-sphere complexation (FRAZÃO et al., 2019FRAZÃO, J. J. et al. Agronomic effectiveness of a granular poultry litter-derived organomineral phosphate fertiliser in tropical soils: soil phosphorus fractionation and plant responses. Geoderma, v. 337, p. 582-593, 2019.).

The use of CMB pellets enriched with mineral fertiliser contributed to higher levels of exchangeable K in the soil, both with or without the application of soil acidity corrector, corresponding to 376.4 and 410.7 mg dm^-3, respectively (Table 3). Similar to the phosphorus, the highest levels of K were obtained with the application of CMB pellets, particularly with PK fertilisation, of 203.9 and 21.4 mg dm^-3, with and without pelletisation, respectively. (Table 3).

The greater specific surface area of the nonpelletised CMB may have contributed to a greater adsorption of potassium on the negative electrical charges of the micropores, releasing the nutrient more slowly into the soil. Furthermore, K is itself capable of catalysing the production of surface oxygenated compounds in the biochar matrix, which are converted into carboxylic and phenolic groups that increase the density of negative charges to retain cations in the structure (DOMINGUES et al., 2020DOMINGUES, R. R. et al. Enhancing cation exchange capacity of weathered soils using biochar: feedstock, pyrolysis conditions and addition rate. Agronomy, v. 10, p. 824-841, 2020.). Some authors have found potassium fertilisers to be more efficient when applied together with biochars due to fewer losses from leaching (ORAM et al., 2014ORAM, N. J. et al. Soil amendment with biochar increases the competitive ability of legumes via increased potassium availability. Agriculture, Ecosystems & Environment, v. 191, p. 92-98, 2014.).

The calcium levels in treatments that included the application of biochar, regardless of pelletisation or liming, were higher in relation to the control treatment. Furthermore, irrespective of biochar pelletisation or mineral fertilisation, the levels of calcium and magnesium were higher in the liming treatments (Table 3). The joint application of CMB and limestone therefore helped significantly to increase the levels of calcium and magnesium in the soil.

Increases in the levels of Ca and Mg in the treatments with CMB and agricultural limestone are related to the presence of these nutrients in the biochar (Table 1) and in the acidity corrector. The elements Ca and Mg also contribute to the slower release of the organomineral fertiliser, since they stabilise the mineral nutrients in the biochar matrix by electrostatic surface attraction, precipitation, or ion exchange, and increase the pore structure, forming a greater number of adsorption sites (AN et al., 2020AN, X. et al. High-efficiency reclaiming phosphate from an aqueous solution by bentonite modified Biochars: a slow release fertiliser with a precise rate regulation. ACS Sustainable Chemistry & Engineering, v. 8, p. 6090-6099, 2020.).

Calcium and magnesium play an important role in the factors of soil acidity. As discussed above, in this study the exchangeable acidity was neutralised by the use of CMB and limestone, considering that both Ca and Mg from the acidity corrector and from the CMB transported the toxic Al⁺³ into the soil solution that, at a pH greater than 5.5, is precipitated in the form of Al(OH)₃ and is no longer available to the plants (SHETTY; PRAKASH, 2020SHETTY, R.; PRAKASH, N. B. Effect of different biochars on acid soil and growth parameters of rice plants under aluminium toxicity. Scientific Reports, v. 10, e12249, 2020.). On the other hand, the highest values for potential acidity (H + Al) were found in treatments with no limestone but with the P and K mineral fertiliser (Table 4).

The increase in potential acidity from the application of mineral fertiliser can be explained by the fertiliser favouring the mineralisation reactions of the soil organic matter, including the CMB, by the addition of P and K, thereby promoting the release of organic acids and H⁺ protons in the soil (ADEKIYA et al., 2020ADEKIYA, A. O. et al. Biochar, poultry manure and NPK fertiliser: sole and combine application effects on soil properties and ginger (Zingiber officinale Roscoe) performance in a tropical Alfisol. Open Agriculture, v. 5, p. 30-39, 2020.). Estimates of the cation exchange capacity (CEC) of the soil can be infl uenced by its potential acidity.

The highest values for CEC were obtained in treatments that included the application of pelletised CMB compared to the treatments with non-pelletised CMB, of 3.94 and 3.34 cmol_c dm^-3, respectively (Table 4). As a result, better responses were obtained in the treatment with liming and pelletised biochar enriched with P and K, which presented a CEC of 4.77 cmol_c dm^-3, an increase of 43.67% compared to the control treatment (Table 4). Due to the addition of basic cations to the cation exchange complex, the liming and CMB treatments, regardless of pelletisation, gave higher values for base saturation (V) (Table 4).

In this study, the CEC was estimated as the sum of Ca, Mg, K, Al and H+Al. The addition of exchangeable bases by the acidity corrector, biochar and PK fertiliser therefore contributed to increase the CEC. Due to the addition of exchangeable bases, the highest values for base saturation (V) were found in the treatments with liming and CMB enriched with PK fertiliser (Table 4). Increases in the CEC in soils amended with biochars are due to the oxidation of surface C groups forming functional groups in the structure of the biochars, which enable basic cations to be adsorbed (DOMINGUES et al., 2020DOMINGUES, R. R. et al. Enhancing cation exchange capacity of weathered soils using biochar: feedstock, pyrolysis conditions and addition rate. Agronomy, v. 10, p. 824-841, 2020.).

It was found that, regardless of biochar pelletisation or mineral fertilisation, the highest values for soil organic carbon (SOC), were found in treatments that included the application of CMB with no soil acidity corrector, compared to the control treatment (Table 4). The increase in SOC in treatments with the application of CMB, irrespective of pelletisation, can be explained by the carbon-enriched soil from the CMB, which contributes to an increase in the pool of labile organic carbon in the soil when this is mineralised (APORI et al., 2021APORI, S. O. et al. Effect of co-applied corncob biochar with farmyard manure and NPK fertiliser on tropical soil. Resources, Environment and Sustainability, v. 5, e100034, 2021.). In addition to being a source of the element, the organic carbon in biochars has a high degree of aromaticity, due to a reduction in the H/C ratio of the material with pyrolysis and a resistance to biological degradation (SANTOS et al., 2019SANTOS, S. R. et al. Biochar association with phosphate fertiliser and its infl uence on phosphorus use efficiency by maize. Science and Agrotechnology, v. 43, e025718, 2019.). The level and aromaticity of SOC in the CMB, 167.9 g kg^-1 (Table 1), certainly helped to retain nutrients in its matrix and increase the CEC of the soil, as discussed above. The lower levels of SOC in the liming treatments can be attributed to losses in the form of carbon dioxide due to heterotrophic microorganisms in the soil promoting the conditions for mineralisation (MOSHARROF et al., 2021MOSHARROF, M. et al. Combined application of biochar and lime increases maize yield and accelerates carbon loss from an acidic soil. Agronomy, v. 11, p. 1313-1323, 2021.).

Production and mineral nutrition in Urochloa brizantha ‘BRS Paiaguás’

During the first crop cycle, it was found that growth in the control treatment was significantly lower compared to the other treatments, and did not reach the cutting height of 40 cm. For the remaining cuts, the production of shoot dry matter (SDM) was lower in the control treatment, compared to the other treatments (Table 5).

This result can be explained by the low initial availability of nutrients in the soil, while the greater growth when applying CMB can be attributed to the increase in soil pH and nutrient availability (BISTAet al., 2019BISTA, P. et al. Biochar effects on soil properties and wheat biomass vary with fertility management. Agronomy, v. 9, p. 623-634, 2019.), since biochars act as conditioners of the chemical properties of the soil due to a high capacity for retaining nutrients on the surface of the particles and inside the pores (MUSTAFÁ et al., 2022MUSTAFÁ, A. T. et al. Food and agricultural wastes-derived biochars in combination with mineral fertiliser as sustainable soil amendments to enhance soil microbiological activity, nutrient cycling and crop production. Frontiers in Plant Science, v. 13, p. e1028101, 2022.).

At the first cut, the highest values for SDM were obtained in the treatments with non-pelletised biochar, production being 18.28% higher compared to the pelletised biochar. SDM production was 1.86 and 2.20 g pot^-1 in the biochar treatments with and without pelletisation, respectively (Table 5). On the other hand, the treatments with pelletised biochar showed increases at the second and third cuts, demonstrating at the third cut, a significantly greater response, with a mean SDM production of 3.18 g pot^-1, which equals a difference of 19.55%, in relation to the treatments with non-pelletised biochar (Table 5).

Throughout the experimental period, each of the treatments that included the application of CMB, whether in pelletised or non-pelletised form, and regardless of liming or fertilisation with P and K, showed an accumulated production of shoot dry matter (TSDM) greater than the control treatment for the sum of the five cuts (Table 5). Regardless of CMB pelletisation or the application of mineral fertiliser, the highest values for TSDM were obtained with the liming treatments, with mean values of 11.06 and 11.86 g pot^-1 for the non-pelletised CMB and pelletised CMB, respectively (Table 5). Similarly, the highest values for TSDM were obtained in treatments that included the application of P and K, regardless of CMB pelletisation or liming, of 12.10, 13.26, 11.44 and 14.46 g pot^-1, for non-pelletised CMB with no liming, non-pelletised CMB with liming, pelletised CMB with no liming, and pelletised CMB with liming, respectively (Table 5).

There was no effect from pelletisation throughout the five crop cycles, with the TSDM being 10.95 g pot^-1 in the treatments with pelletised CMB, and 10.36 g pot^-1 with the non-pelletised CMB (Table 5). Although there were no significant differences in the sum of the five cuts regardless of liming or fertilisation with NPK, the highest production of TSDM was obtained with pelletised CMB enriched with P and K mineral fertiliser, together with correction of the soil acidity by liming, 14.46 g pot^-1 (Table 5).

The lower production of SDM at the first cut in treatments with the application of CMB pellets showed that the release of nutrients by the pelletised biochar was slower compared to the non-pelletised biochar. As a result, the TSDM of these treatments was greater, especially with the addition of PK fertiliser and liming, since, unlike commercial fertilisers, biochar-based pelletised fertilisers are characterised by a slow and controlled release of nutrients into the soil, which, in the long term, allows better nutrient use by the plants. This change in nutrient release dynamics by pelletised biochar enriched with PK is desirable, as it can avoid the loss of K from mineral fertiliser through leaching, and reduce P fixation, especially in highly weathered, acidic soils that have a low CEC, and are rich in kaolinite and Fe and Al oxides (Santos et al., 2019SANTOS, S. R. et al. Biochar association with phosphate fertiliser and its infl uence on phosphorus use efficiency by maize. Science and Agrotechnology, v. 43, e025718, 2019.).

In the case of P absorption by the plants, the controlled release of nutrients and the lower solubility of the organomineral fertiliser are important characteristics in soils with a high phosphate adsorption capacity, such as Oxisol, as they prevent any increase in the non-labile P pool over time, a pool that is not in equilibrium with the P in the soil solution due to the formation of surface binuclear complexes that reduce the availability of the nutrient (FRAZÃO et al., 2019FRAZÃO, J. J. et al. Agronomic effectiveness of a granular poultry litter-derived organomineral phosphate fertiliser in tropical soils: soil phosphorus fractionation and plant responses. Geoderma, v. 337, p. 582-593, 2019.).

This study showed that the joint application of CMB regardless of pelletisation, the use of agricultural lime, or mineral fertiliser, contributed to an increase in the production of BRS Paiguás grass. Several studies have pointed to the importance of applying biochar together with soil acidity correctors and mineral fertilisers (MOSHARROF et al., 2021MOSHARROF, M. et al. Combined application of biochar and lime increases maize yield and accelerates carbon loss from an acidic soil. Agronomy, v. 11, p. 1313-1323, 2021.; YAO et al., 2019YAO, L. et al. Responses of Phaseolus calcaltus to lime and biochar application in an acid soil. PeerJ. v. 7, e6346, 2019.); if applied alone, large amounts of biochar would be needed to have the same effect as liming and mineral fertilisers, which would make the use of carbonised material unfeasible over large areas of agricultural crops (MAROUŠEK et al., 2017MAROUŠEK, J. et al. Glory and misery of biochar. Clean Technologies and Environmental Policy, v. 19, p. 311-317, 2017.).

Other authors have pointed out the benefits of biochar in increasing the production of plant biomass due to infl uencing the expression of genes involved in the biosynthesis and regulation of the cell wall, resulting in increased levels of lignin and hemicellulose, and an increase in silica accumulation (MIAO et al., 2023MIAO, W. et al. Biochar application enhanced rice biomass production and lodging resistance via promoting co-deposition of silica with hemicellulose and lignin. Science of the Total Environment, v. 855, e158818, 2023.).

In relation to the root system, higher values for RDM were found in treatments with non-pelletised biochar, with an increase of 28.80% in relation to treatments with pelletised biochar (Table 5). Treatments that included liming were superior to treatments with no liming regardless of the use of mineral fertiliser, with values of 3.42 and 2.71 g pot^-1 for nonpelletised CMB and pelletised CMB, respectively.

The application of CMB, especially non-pelletised CMB, together with agricultural lime, contributes to root growth. Some authors have verified the effect of biochar on the root system, particularly in increasing the number of fine roots, which then favours an increase in nutrient absorption efficiency (TORRES et al., 2020TORRES, W. G. A. et al. Phosphorus availability in soil amended with biochar from rice rusk and cattle manure and cultivated with common bean. Science and Agrotechnology, v. 44, e014620, 2020.). Another important characteristic of biochar is its persistence in the soil, which increases the porosity, moisture content and nutrient availability, in addition to reducing the apparent density of the soil, preventing compaction, and increasing root penetration (AGBEDE; OYEWUMI, 2022AGBEDE, T. M.; OYEWUMI, A. Benefits of biochar, poultry manure and biochar-poultry manure for improvement of soil properties and sweet potato productivity in degraded tropical agricultural soils. Resources, Environment and Sustainability, v. 7, e100051, 2022.).

Compared to the control treatment, the accumulated quantities of macronutrients in the shoots, considering the five cuts, were greater in treatments that included CMB, regardless of pelletisation (Table 6). There was no significant difference between the treatments with or without pelletisation. On the other hand, the greatest macronutrient accumulation was seen in treatments with liming and the application of P and K mineral fertiliser (Table 6).

There was no difference in micronutrient accumulation in the shoots between the treatments with and without biochar pelletisation and the treatments with and without liming (Table 7). On the other hand, micronutrient accumulation was greater with CMB regardless of pelletisation compared to the control treatment. The highest values were obtained with the treatments that included PK mineral fertiliser (Table 7).

The combinations of CMB, liming and PK fertiliser led to a greater accumulation of macronutrients in BRS Paiaguás grass due to the greater availability of nutrients and greater biomass production in these treatments. Furthermore, the adsorption of micronutrients on the biochar particles may have reduced their precipitation reactions at higher pH values, contributing to their gradual release throughout the crop cycle, similar to chelates.

However, it is important to consider that the nutritional status of plants generally depends on the dosage and the level of nutrients available in the biochar, as well as on the species (Chagas; Figueiredo; Paz-Ferreiro, 2021CHAGAS, J. K. M.; FIGUEIREDO, C. C. de; PAZ-FERREIRO, J. Sewage sludge biochars effects on corn response and nutrition and on soil properties in a 5-yr field experiment. Geoderma, v. 401, e115323, 2021.).

CONCLUSIONS

1. Biochar from cattle manure improved the chemicalproperties of the soil, correcting acidity and increasing nutrient availability and the production of Urochloa brizantha ‘BRS Paiaguás’;

2. Cattle manure biochar enriched with phosphorus andpotassium behaved as a slow-release organomineral nutrient fertiliser inUrochloa brizantha ‘BRS Paiaguás’.

ACKNOWLEDGEMENTS

This research was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brasil), by the Programa Nacional de Cooperação Acadêmica da Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Brasil) and by the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG/Brasil).

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