Microalgae biomass as a conditioner and regulator of soil quality and productivity

doi:10.21203/rs.3.rs-3604829/v1

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Microalgae biomass as a conditioner and regulator of soil quality and productivity

https://doi.org/10.21203/rs.3.rs-3604829/v1

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Characteristics of an acid soil cultivated with Uruchloa brizantha cv. Marandu were evaluated in relation to two types of fertilization: a conventional one, chemical based on nitrogen and potassium, and a biological one, based on microalgae biomass. The results were compared among three treatments: control, conventional, and biological fertilization, with seven replications each. The study evaluated microalgae community, total carbon and nitrogen contents, mineral nitrogen, and enzymatic activity. Chlorella vulgaris showed the highest organism density, which can be explained by its rapid growth and high resistance. The highest species diversity was detected in the control 1,380,938 org.(cm³)⁻¹ and biological 1,841,250 (org.(cm³)⁻¹ treatments, with the latter showing a higher density of cyanobacteria, especially Pseudanabaena limninetica with 394,554 org. (cm³)⁻¹. The soil treated with chemical fertilization showed higher nitrate (9.14 mg.NKg^− 1 NO₃⁻-N) and potassium (52.32 mg.dm^− 3) contents. The highest levels of sulfur (21.73 mg.dm^− 3) and iron (96.46 mg.dm^− 3) were detected in the biological treatment. The chemical treatment showed higher activity of the enzymes acid phosphatase, acetilglucosaminidase, and sulfatase, while α-glucosidase and leucine-aminopeptidase stood out in the biological treatment. Soil properties were not significantly affected by the treatments. The use of microalgae biomass derived from wastewater treatment from milking parlors was evaluated and presented as a promising fertilizer for agriculture, following the line of recovering nutrient-rich wastes. In this sense, although many challenges need to be overcome, the results suggest that microalgae-based fertilizers could lead to low-impact agriculture.

microalgae

nutrient cycling

organic matter

soil microorganisms

fertilization

agriculture

The current agricultural management practices for increasing food production have driven to soil fertility degradation, salinization in irrigated areas, groundwater depletion, increased resistance to pests and weeds and loss of biodiversity, which has driven to 25% of land resources in highly degraded (Tillman et al. 2002; FAO, 2011). Between the need to reduce the polluting load generated in agricultural processes, the depletion of mineral sources, and the continuous and increased demands for food and fertilizers, an important effort is now driven to understand, adopt and validate environmentally viable solutions to address agricultural sustainability (Valizadeh and Hayati 2021; Testani et al. 2023).

Soils when not properly managed or used beyond their support capacity, suffer losses in the quantity and quality of organic matter and consequently reduce the productivity and physical stability (Ziech et al. 2014; Mandal et al. 2021; Gholkar et al. 2022). Between the need to reduce the pollution load generated in agricultural processes, the depletion of mineral sources, and the continuous and increased demands for food and fertilizers, an important effort in research is now driven to understand and seek environmentally viable solutions to address these issues.

Soil fertility and physical-chemical properties of soil depend on biological processes and in the biodiversity of soil microbial community, which implies to introduce management practices that improve biological activity, long-term productivity, and crop health, preventing at the same time soil pollution and degradation (Singh et al. 2011; Li et al. 2021). In this sense, to ensure the maintenance of productivity in agricultural lands, conservation of soil resources is undoubtedly one of the most important aspects of the current conception of agricultural production (Hobbs et al. 2008).

Application of new agricultural practices based on conservation systems aimed to reduce soil degradation and increase the sustainability of agriculture, with a focus on microorganisms that act in the decomposition of organic matter, directly participating in the biogeochemical cycle of nutrients and, consequently, mediating their availability in the soil, as well as degrading toxic substances (Singh et al. 2011; Mandigan et al. 2016; Paz-Ferreiro and Fu 2016; Cui et al. 2018; Bhattacharyya et al. 2022).

Increasing the organic matter content of soils requires a sustained effort that includes returning organic materials to soils and rotations with high-residue crops and deep- or dense-rooting crops (Bot and Benites 2005; Bhattacharyya et al. 2022).

In this context, an interest in eco-friendly and sustainable agricultural practices arises as a tool to maintain soil fertility avoiding negative environmental effects of chemical inputs (Chaudhary et al. 2022). Biofertilizer containing efficient microorganisms, improve plant growth in many ways compared to synthetic fertilizers, enhancing crop productivity without environmental negative effects (Singh et al. 2011). Other types of fertilizers (biological, organic manure, and biocontrol agents) have been researched and re-emerge as a promise for nutrient replenishment for arable soils, as well as linking the nutrient recovery process with the circular bioeconomy strategy (Uysal et al. 2015; Kumar et al. 2022; Chaudhary et al. 2022).

Biofertilizers are bio-based organic fertilizers that either could be from plant or animal sources or from living or dormant microbial cells that have the potential to improve the bioavailability and bioaccessibility of nutrient uptake in plants (Chaudhary et al. 2022). Concerning biofertilizers, research focused on producing those based on microalgae deserves special mention. Microalgae and cyanobacteria impact critical agricultural needs such as biological nitrogen fixation, soil phosphorus cycling, effects on soil microorganisms, plant growth promotion either by soil nutrient cycling and/or phytohormones or root associations, biocontrol, soil stabilization and xenobiotic biodegradation (Abinandan et al. 2019; Kholssi et al. 2019, 2022; Alvarez et al. 2021). As photosynthetic microorganisms, microalgae and cyanobacteria are important in introducing organic matter into the soil (Li et al., 2015; Muñoz-Rojas, 2018; Paz-Ferreiro and Fu, 2016) and also stimulating heterotrophic microbial activity (Marks et al. 2017, 2019). Microalgae also help in the mineralization and solubilization of primary macro and micronutrients in the soil, which are important for plant growth (Coppens et al. 2016; Yilmaz and Sönmez 2017), besides being directly involved in the assimilation of atmospheric CO₂ in the biomass through photosynthesis (Renuka et al. 2018). In addition, the exopolysaccharides secreted by many cyanobacteria and chlorophytes can provide organic carbon for the development of beneficial microbiota for agriculture, leading to the formation of useful biofilms and bioflocs in the plant rhizosphere (Renuka et al. 2018).

Algae-based fertilizers are already available in the market, which, besides being a source of nutrients, also have a growth-promoting effect due to the presence of regulators of this function such as cytokinins and auxins, polyamines, putrescine, spermine (Dmytryk and Chojnacka 2018; Kholssi et al. 2022). In this context, the aim of this study was to assess the characteristics of an acid soil cultivated with Uruchloa brizantha cv. Marandu that received different types of fertilization by determining its chemical properties, macro and micronutrient contents, and enzymatic activities.

Production and characterization of microalgae biomass

Treatment and microalgae biomass production from cattle wastewater was carried out over a year (from February 2017 to 2018) at the experimental unit of the Sanitary and Environmental Engineering Laboratory (LESA) of the Federal University of Viçosa (UFV, Brazil).

The dairy cattle wastewater was produced at UFV in a milking parlor that uses free stall tracks. The wastewater was conducted to two tanks, where it was stored and then used for irrigating the pastureland that provides feed for the university's milk production research system. This stored wastewater was used to produce microalgae biomass in high-rate algal ponds (HRAPs) with the following characteristics: width 1.28 m, length 2.86 m, total depth 0.5 m, useful depth 0.3 m, surface area 3.3 m², and useful volume 1 m³. Built from fiberglass, the HARPs have six-blade stainless paddlewheels powered by a 0.5 hp electric motor. Frequency reducers (WEG series CFW-10) coupled to the motor were used to control the rotation of the paddlewheels to ensure a horizontal liquid velocity of approximately 0.10 to 0.15 m s^-1. The HRAPs were operated using an organic load rate of 150 kg BOD ha^-1 day^-1 and a hydraulic retention time (HRT) of 12 days. Clarified wastewater form the tanks was introduced manually with a flow rate of 65 L day^-1 flow rate (twice a day). Before using HRAPs, wastewater had a primary settling pretreatment process. After the wastewater treatment, the suspended biomass grown in the HRAP was harvested through gravitational sedimentation. Collected wet biomass was applied to the soil.

The applied microalgae biomass is a polyculture with Chlorella vulgaris as predominant specie. and had the characteristics displayed in Table 1.

Table 1

Chemical properties of microalgae biomass.
TP (mgL^-1)	1.99	Trace Metals (mgL^-1)	As	0.33
TKN (mgL^-1)	1.66		Ba	43.14
TOC (mgL^-1)	67.38		Cd	0.33
Humidity (%)	97.34		Cr	2.94
pH	8.4		Cu	16.99
Chlorophyll-a (mgL^-1)	1.64		Hg	0.01
% Lipids	0.2		Mo	1.63
% carbohydrates	6.32		Ni	2.29
% carbohydrates	6.32		Pb	0.65
% protein	35.03		Se	0.65
% protein	35.03		Zn	0.1

TP - total phosphorus; TKN - total Kjeldahl nitrogen; TOC - total organic carbon.

As – Arsenic, Ba – Barium, Cd – Cadmium, Cr – Chromium, Cu – Copper, Hg – Mercury, Mo – Molybdenum, Ni – Nickel, Pb – Lead, Se – Selenium.

Analyzes were performed according to standards methods for wastewater (APHA 2012).

Site description

The agronomic experiment was carried out at the Agrostology facilities of the Department of Animal Science at UFV, in Viçosa-MG (20°46’23”S, 42°51’31”W), from April 2017 to March 2018. According to the Köppen classification, the municipality of Viçosa is classified as subtropical highland climate type (Cwa), with an average annual temperature of 20,4 ºC, rainfall of 1,221 mm, characterized by seasonal distribution, with well-defined dry and rainy seasons (Fig. 1).

Figure 1 - Maximum and minimum temperatures, and total rainfall during the experimental period.

The soil was classified as Lixisol (LX) according to FAO (IUSS 2022), with a loamy texture. Soil preparation was done with disc plow and harrow. Soil pH was corrected by applying 4.5 Mg ha^− 1 of dolomitic agricultural limestone (with a relative neutralization power of 76%) 60 days before sowing. Sowing on April 18th, 2017, was carried out in furrows approximately 3 cm deep and spaced 30 cm apart, using 8 kg ha^− 1 of Uruchloa brizantha cv. Marandu seeds. During planting, 90 kg ha^− 1 of P₂O₅ was applied for of all treatments, as P is the nutrient responsible for the initial development and vigor of the roots.

Experimental design

The Uruchloa brizantha cv. Marandu was planted, on April 18th, 2017, in 2.9 mx 0.9 m (2.61 m²) plots. The experimental design was randomized in blocks with three treatments (C - control, Q - chemical, and B - biological) and seven replications, totaling 21 plots of 2.9 x 0.9 m (2.61 m²). Treatment Q consisted of applying 40 kg/ha per cut of a blended fertilizer (N:P:K 20:0:20). Treatment B applied microalgae biomass in equivalent amount of 40 kg N per cut, calculated based on its N content (TKN content). Treatment C was the control and did not receive any fertilizer.

After seeding, the area was irrigated daily to ensure favorable conditions for seed germination. Cover fertilization was performed 88 days after germination in all treatments, changing the source of N among them. The standardization cut was made 159 days after seeding in plots of all treatments.

The criterion of cutting the grass was adopted whenever the height of the pasture reached 40 cm (de Góis Fontes et al., 2014; Ribeiro et al., 2016). The height of the plants was measured with a graduated ruler, at five points within each plot, at intervals of five days. After being cut with pruning shears, the plant was manually collected in a 0.25 x 0.25 m² area of the sampling square. Therefore, three cuts (219, 274, and 331 days after seeding) were made in the control treatment (C), four (209, 240, 291, and 341 days after seeding) in the biological treatment (B), and five (209, 240, 274, 319 and 353 days after seeding) in the chemical treatment (Q). New fertilization (chemical or biological, according to the treatments) was performed at the sowing line immediately after cutting.

Soil analysis

Microalgae propagation in the soil was evaluated by collecting samples composed of 1 cm³ between the subplots. The collected soil was stored in tubes with a 9 mL solution of 4% formaldehyde for qualitative and quantitative analysis of the microalgae community in the biomass applied to the soil and in the soil used at the end of the experiment. The microalgae community was characterized at the genus level, and the species present were identified as the dominant genus. Quantitative analysis, based on the number of individuals, was carried out in a sedimentation chamber under an inverted microscope using the Uthermol (1958) method. The density of organisms was determined based on the criteria described in APHA (2012). According to Parra et al. (1982) and Komarek and Fott (1983), qualitative analysis was performed using an inverted microscope.

The chemical properties of soil were analyzed based on composite samples collected from each subplot at a depth of 20 cm using the methods described by Embrapa (1997). To analyze C and N, the dry mass of the soil was obtained by drying a representative sample for 72 hours at 70°C. This material was ground, and the total contents of C and N were measured from 0.2 g of this sample using an elemental analyzer (TrueSpec CN, LECO, Saint Joseph, USA). Were extract exchangeable Ammonium ion (NH₄⁺) and nitrate (NO₃^-) with 1 M KCl (1:5 w/v), and analyzed using a segmented flow analyzer (San++, Skalar). Enzymatic activity was evaluated based on fluorimetric assays according to Marx et al. (2001), using methyl-umbellipheryl (MUF) or amino-methyl-coumarin (AMC) fluorinated) substrates in 96-well microplates.

Statistical analysis

The data were statistically analyzed through the analysis of variance (ANOVA) after checking for normality and homogeneity of the data, and means were compared by means of Tukey's test at a 5% significance level, using IBM SPSS Statistics version 22 software. Descriptive statistics were used when there was a high variability of data.

As reported in Fig. 2, Chlorella vulgaris predominated in all treatments, with the biological treatment standing out due to a higher number of cyanobacteria, particularly Pseudanabaena limnetica, with a relative abundance of 21%.

Figure 2 - Density of organisms in the soil (C = Control treatment, Q = Chemical treatment, B = Biological treatment).

The chemical, macro, and micronutrient characteristics of the soil are presented in Table 2.

Table 2

Soil chemical properties, macro and micronutrient contents.
Chemical Characteristics	Initial Values	C	Q	B
pH (H₂O)	4.3	5.09a	4.99a	4.79a
EC (mSm^− 1)		207.63b	268a	221.9b
Sum of bases (cmolc dm^− 3)	1.68	3.58a	2.85ab	2.63a
CEC (cmolc dm^− 3)	7.13	8.97a	7.89a	8.05a
OM (g/Kg)	36	41,9a	36b	34,3c
P-rem (mg L^− 1)	15.2	23.24a	20.47a	15.86b
Total Carbon Organic (mg Kg^− 1)		22.91a	21.25b	20.31b
Total Nitrogen (mg Kg^− 1)		2.03a	1.97a	1.9a
Total Phosphorus (mg Kg^− 1)	2.4	2.59a	3.17a	4.23a
C:N		11.3a	10.96a	10.75a
NH₄⁺ - N (mg N Kg^− 1)		1.97a	2.76a	1.52a
NO₃⁻ - N (mg N Kg^− 1)		3.94b	9.41a	4.3b
Ca (mg dm^− 3)		544a	430b	402b
Mg (mg dm^− 3)		87.48a	63.18a	51.03a
K (mg dm^− 3)		37.28b	52.32a	29.28b
S (mg dm^− 3)		11.34b	13.60b	21.73a
Fe (mg dm^− 3)		91.17b	83.86c	96.46a

C – Control treatment, Q – Chemical treatment, B – Biologic treatment.

pH – hydrogen ion potential, EC – electrical conductivity; CEC – total cation exchange capacity, OM – organic matter, P-rem – Remaining phosphorus. C:N – carbon-nitrogen ratio, C:P – carbon phosphorus ratio. NH ₄ ⁺ - N – ammonium, NO₃^- - N – nitrate.

Means followed by the same letter in the row do not differ according to Tukey's test at 5%.

The sum of bases, organic matter (OM), and remaining phosphorus (P-rem) contents presented no statistical differences (p > 0.05). Regarding the total contents of C, N, and P, treatments affected only the soil carbon (p < 0.05). No differences were detected between the chemical and biological treatments for the C and N contents. The chemical treatment presented significantly higher NO₃⁻ content, while the NH₄⁺ values did not differ between treatments.

Considering that, in this research, soil fertilization was carried out based on N levels, a balance of this nutrient was carried out to indicate the efficiency of fertilizer use and soil fertility. From the balance carried out (Table 3), the exported amounts of N were smaller than the initial values in the control treatment. In contrast, the exported amounts exceeded the initial ones in the chemical and biological treatments.

The treatments significantly differed between potassium, sulfur, and calcium (p < 0.05). In addition, Fe showed a significant difference between treatments (p < 0.05) among the micronutrients.

The enzymatic activity of soil hydrolases was evaluated (Table 4), considering that organic fertilization contributes to their increase and that these activities are related to the mineralization of essential soil nutrients (Moeskops et al. 2010; Dindar et al. 2015).

Table 3

Soil enzymatic activity activities for the different treatments.
Treatment/Enzymes	C	Q	B
	(µ mol MUF g^-1 min^-1)
AcPA	904,76 (± 124,26)	948,92 (± 124,26)	860,97 (± 124,26)
ϐGlu	173,74 (± 14,02)	152,42 (± 14,02)	151,83 (± 14,02)
αGlu	67,46 (± 8,38)	67,33 (± 8,38)	73,92 (± 8,38)
bNAG	1049,17 (± 323,16)	1574,49 (± 323,16)	1149,25 (± 323,16)
AS	47,58 (± 9,85)	65,58 (± 9,85)	60,25 (± 9,85)
	(µ mol AMC g^-1 min^-1)
LAMP	602,65 (± 70,77)	560,59 (± 70,77)	656,24 (± 70,77)

C – Control, Q – Chemical, and B – Biologic

AcPA – Acid phosphatase, βGlu – β-glucosidase, αGlu – α-glucosidase, βNAG – Acetylglucosaminidase, AS – Sulfatase, LAMP – Leucine-aminopeptidase.

Acid phosphatase (AcPA), N-acetylglucosaminidase (βNAG), and sulfatase (AS) showed higher values in the chemical treatment. In comparison, α-glucosidase (αGlu) and leucine-aminopeptidase (LAMP) presented higher values in the biological treatment. In addition, β-glucosidase (βGlu) was higher, compared to the other treatments, in the control treatment.

Microalgae biomass characterization showed approximately 6% carbohydrates and 40–60% crude proteins (Becker 2007). These values vary according to the species, culture medium condition, and method applied for extraction and quantification. It is known that proteins act as a storage medium to meet the growth and nutritional demands of developing plants (Rasheed et al., 2020) and participate in the stabilization of organic C and N in soil (Rillig et al., 2007). In this research, the plant's productivity presented in Lorentz et al. (2020) is attributed to the application of microalgae biomass, considering the report by Battacharyya et al. (2015), which associates the biochemical content of this biomass with precursor compounds of plant growth-promoting phytohormones.

Despite the wide variety of recognized species of algae that benefits in soil quality and crop improvement, the main attributes that confirm their effectiveness remain uncertain (Alobwede et al. 2019; Solomon et al. 2023). Living and/or dead cells containing beneficial microbiota have been applied to plants and soil to promote plant growth and to convert inaccessible mineral forms into available nutrient forms through nitrogen fixation, mineralization, and phosphate solubilization (Priya et al. 2014; Alvarez et al. 2021). Lin et al. (2013) point out that Anabaena, Calothrix, Lyngbya, Microcoleus, Nostoc, Oscillatoria, and Phormidium are common genera of cyanobacteria in soils. Chlorophytes such as Chlorella, Cladophora, and Klebsormidium, and diatoms including Hantzschia, Navicula, Nitzschia, and Pinnular are also frequently present (Lin et al. 2013). The present study was conducted in acidic soil, with pH values < 5 (Table 2). It was observed that regardless of the treatment, the chlorophyte Chlorella vulgaris stood out with the highest organism density. The lowest diversity of organisms was found in the chemical treatment. Zancan et al. (2006), working with corn crops subjected to long periods of intense fertilization, detected a reduction in species diversity and development of cyanobacteria, indicating an adverse effect of intensive chemical fertilization on the microalgae community in the soil, as found in the present study. The highest density of Chlorella vulgaris in the soil, as also found by Castro et al. (2017), can be explained by its rapid growth and high resistance to environmental stresses (Han et al. 2018). Results showed that cyanobaterial density was generally highest, and this may be one of the reasons why the biological treatment presents a good performance in nutrient recovery. To be more specific, these taxa are capable of fixing atmospheric nitrogen and then used to promote plant growth (Kholssi et al. 2022). The genus Pseudanabaena limnetica, considered rare in soil (Lin et al. 2013), presented 394,554 org. (cm³)⁻¹ of organisms in this treatment.

Besides soil microalgae, biomass application (biological treatment), derived from wastewater treatment in HRAPs, can provide entry into the system of other microorganisms (bacteria and fungi) present in the treatment process. Other studies have been moving towards its use not only as a fertilizer, due to its chemical composition, but also as a bio-stimulant, due to the composition of its microbial community, as some species perform functions that can contribute to soil improvement and consequently increase productivity (Ronga et al. 2019).

Considering the results of soil chemical characterization (Table 2) throughout the experiment and the lower plant productivity presented by Lorentz et al. (2020) associated with higher organic matter content in the control treatment, it is indicated that nutrient release through organic matter decomposition in this treatment was not sufficient for crop development as in the other two treatments. Regarding the biological treatment, the lower organic matter content is attributed to the productivity achieved by its degradation. Yilmaz and Sönmez (2017) evaluated the potential of different microalgae biofertilizers on soil organic carbon. They concluded that increasing the applied biofertilizer dose reflected positively on the increase in OM compared to control treatments. The positive reflection of the increase in OM pointed out by Yilmaz and Sönmez (2017) may be related to the initial fertilization with conventional chemical fertilizer N:P:K (15:15:15) in all treatments - except the control - associated with the doses in which the biological treatments were added with values up to 15 times higher than the chemical. In this research, only simple superphosphate (P₂O₅) was used at the seeding of Uruchloa in a single dose, while N:P:K fertilizer doses (20:0:20) were applied after the expected cuts.

Soil microorganisms use N as a source of energy and C in forming cells and tissues. The higher the C/N ratio, the lower the decomposition rate, mineralization, and N availability for plants (Dijkstra et al. 2009). In this way, the C/N ratio directly influences the degradation of residues and nitrogen cycling in soils. In this research, the C/N values were close to 11, corroborating the findings of Al-Maliki and Breesam (2020). Their results suggest that chlorophyte's applied microalgae biomass accelerated the mineralization of C, resulting in greater carbon decomposition and nutrient availability in the soil, promoting better plant development.

To become available to plants, soil N depends on the mineralization rate of organic matter (Pandey 2020). As well as, in its transformations, several reactions can occur, which lead to low efficiency of use, attributed to denitrification and ammonia volatilization, besides probable immobilization by the microbiota (Jílková et al. 2020). The treatments did not affect the total N contents, although the NO₃⁻ contents were higher in the chemical treatment. Wang et al. (2018), when comparing the application of organic and inorganic fertilizers for long periods, indicated increases in NH₄⁺ and NO₃⁻ values in soils that received inorganic fertilizers, certainly due to their ready availability in this type of fertilizer. Alobwede et al. (2019) detected, throughout all the experiment, an increase in soil total N with the presence of Chlorella sp. and Spirulina. In our experiment, in addition to the presence of Chlorella in all treatments, cyanobacteria were also detected in both the control and the biological treatment, which may explain the similar levels of total nitrogen (N_total) between treatments.

Some studies have further demonstrated investigating the potential of microalgae to increase the availability and absorption of micronutrients from the soil in the wheat crop, and highlighted that they become an organic carbon input (Renuka et al.,2018). Gougoulias et al. (2018) and Solovchenko et al. (2020) indicated that chlorophytes accelerated C mineralization, resulting in greater carbon decomposition and nutrient availability for plant development. In this research, the soil characteristics associated with the plant productivity results presented by Lorentz et al. (2020) between the treatments corroborate these previous results. Furthermore, C levels are generally linked to the amount of MO (Longo et al. 2011), which was once again confirmed in this research, given that the highest levels of MO and C were identified in the control treatment.

Phosphorus (P), one of the most essential and limiting nutrients for agricultural production (Badza et al. 2020), has low use efficiency, approximately 20% (Solovchenko et al. 2016). Its higher retention capacity is linked to the lower available P value. In contrast, higher values of this property are associated with higher amounts of organic matter (OM), which implies lower phosphate adsorption and consequent loss of this nutrient with the possibility of environmental pollution, such as eutrophication of water bodies and leaching into groundwater. Lower available P values in the biological treatment are related to its lower content of OM, which may be related to its use for higher plant productivity. The P from microalgae biomass, being organic, is slowly mineralized and released at a rate close to absorption by crops, resulting in lower losses of this nutrient by runoff, decreasing the risk of eutrophication (Solovchenko et al. 2020).

The highest calcium values in the control treatment may be associated with the highest OM content. The highest amount of K in the chemical treatment was already expected, as this received an N:P:K 20:0:20 mixture. Also, the highest amounts of sulfur and iron were detected (p < 0.05) in the biological treatment. These two nutrients are extremely important in the development of plants, the first for participating in N metabolism and the synthesis of proteins and amino acids, and the second for contributing to the adsorption process of the first, reducing losses by leaching (Uchida 2000). The high levels of S in the biological treatment can be explained by its content and form (elementary S) in the biomass, which has not yet been converted into SO₄⁻, the form preferred by plants. The Fe contents in all treatments were high (Uchida 2000). However, it was higher in treatment B. Fe is associated with the adsorption of pollutants (heavy metals), phosphorus fixation and acts as a cementing agent between soil particles, affecting their cation exchange capacity (Li et al. 2024).

Extracellular hydrolytic enzyme activities involved in nutrient cycling were evaluated. β-glucosidase, according to Guo et al. (2021), is an enzyme directly related to OM levels and decomposition processes that occur in the soil. Therefore, the highest value of this enzyme in the control treatment may be associated with the highest content and type of OM present in the soil.

Tian et al. (2022) evaluated maize cultivation with different fertilizations and found a higher activity of acid phosphatase during chemical fertilizer application, compared to organic fertilizer application, as in this study. Touhami et al. (2022), evaluating combined applications of N and P in pastures, researchers detected an increase in phosphatase activity, correlating this increase with the mineralization of organic P. The higher activity of arylsulfatase (AS) in the chemical treatment may be related to the mineralization of S, which is more readily available in this treatment.

The higher activity of LAMP in the biological treatment may be explained by some nutritional deficiency in the microalgae biomass, given that this enzyme is responsible for protein decomposition (Parvin et al., 2018) and amino acid addition - produced in response to some signal of need or stress based on N, C levels or some other stimulus (Jacobson Meyers et al., 2014).

Based on the results of Lorentz et al. (2020), organic fertilization allowed plant productivity close to conventional chemical fertilization. The authors assume that this result comes from the composition of the biomass applied in the biological treatment (several important nutrients for the development of the grass) and from the C/N ratio < 20, which favors the mineralization of the compounds (White 2006). The research's premise was that adding biomass would increase soil nutrient levels, which was not observed. However, the plant development was superior to that of the control treatment.

Final considerations

The use of microalgae biomass derived from wastewater treatment from milking parlors was evaluated and proved to be a promising fertilizer for agriculture following the recovery of nutrient-rich waste. The cost of chemical fertilizers, resistance to pesticides, and climate change have made microalgae biomass an opportunity for sustainable and resilient agriculture. However, further research needs to be conducted, considering longer evaluation times and aspects related to soil type, temperature, and humidity, among others, that may influence the results.

Acknowledgment

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors are also grateful for the financial assistance provided by the Minas Gerais Research Foundation - FAPEMIG (APQ-02527).

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Microalgae biomass as a conditioner and regulator of soil quality and productivity

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Materials and methods

Production and characterization of microalgae biomass

Site description

Experimental design

Soil analysis

Statistical analysis

Results

C – Control, Q – Chemical, and B – Biologic

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