In general, the application of organic amendments had a positive effect on the soil's enzymatic activities. The highest enzyme activities were generally found in April, which might be because the April samples have better soil water and temperature conditions to stimulate activity. Total activity decreased for all enzymes in August. The decrease in activity may reflect a more conducive environment for activity. Table 1 shows the treatments’ chemical properties with compost and biochar having the highest organic matter as well as CEC values.
Table 1
Selected soil chemical properties for the study.
Treatments | pHs | EC dS m− 1 | %OM | P Bray I mg/kg | Ca mg/kg | Mg mg/kg | K mg/kg | CEC cmolc/kg |
Compost | 6.9 | 10.3 | 4 | 80.6 | 2872.0 | 764.6 | 527.3 | 12.2 |
Biochar | 6.8 | 9.23 | 4.2 | 88.8 | 2914.8 | 814.2 | 593.4 | 11.8 |
Compost + Biochar | 6.9 | 12.6 | 4.85 | 197.5 | 4998.4 | 1120.8 | 1120.8 | 12.4 |
Biochar + Compost Tea | 6.9 | 13.4 | 4.3 | 85.1 | 4013.0 | 695.6 | 695.6 | 11.7 |
Control | 6.65 | 1.7 | 2.75 | 27.7 | 5712.2 | 588.5 | 588.5 | 11.3 |
Fescue | 7.1 | 3.3 | 3.15 | 2.7 | 3830.2 | 263.1 | 263.1 | 11.3 |
Table 2 shows the mean soil pH values for all treatments for three sampling occasions, with no statistically significant differences observed between treatments. These pH values influence soil enzyme activity, resulting in changes in nutrient dynamics.
Table 2
Mean soil pH values of the study treatments for three sampling periods
Treatments | April | June | August |
Compost | 6.18 | 6.24 | 6.61 |
Biochar | 6.64 | 6.61 | 6.71 |
Compost + Biochar | 6.75 | 6.78 | 6.77 |
Biochar + Compost Tea | 6.51 | 6.69 | 6.76 |
Control | 6.36 | 6.56 | 6.74 |
Fescue | 6.55 | 6.66 | 6.78 |
Table 2
Mean soil pH values of the study treatments for the three sampling periods.
Treatments | April | June | August |
Compost | 6.18 | 6.24 | 6.61 |
Biochar | 6.64 | 6.61 | 6.71 |
Compost + Biochar | 6.75 | 6.78 | 6.77 |
Biochar + Compost Tea | 6.51 | 6.69 | 6.76 |
Control | 6.36 | 6.56 | 6.74 |
Fescue | 6.55 | 6.66 | 6.78 |
β- glucosidase enzymes are essential components of cellulase enzyme complexes that help in the breakdown of cellulose, a complex carbohydrate found in plant cell walls. These enzymes work together to degrade carbohydrates into their constituent glucose molecules and release bioactive compounds from glycosidic precursors which can then be used as an energy source (Ejaz et al., 2021). The β- β-glucosidase activity showed no differentiation in the last two sampling periods (June and August) for the treatments (Fig. 1). However, in the first sampling period (April) there was a clear increase in the activities of all treatments relative to the other sampling periods. There was an increase in β- glucosidase activity in August for all the treatments except in the control. The control soil consistently had the lowest activity values in the last month of analysis, which can be attributed to low organic matter content as it is directly related to enzyme activity factors that include both oxidative and hydrolytic responses (Sinsabaugh et al., 2005) and the inability of the soil to degrade recalcitrant materials left in the soil (García et al., 1996). A report by Perez de Mora et al. (2005) found increased β-glucosidase activity in municipal compost compared to biochar-treated soil. Conversely, this study found no significant difference between the compost treated, biochar, the compost biochar, or biochar compost tea in all sampling times except the control which decreased in the last month. This indicates similar substrate availability in each of the treatments, regardless of application rate or amendment source. β-glucosidase plays a major role in microbial metabolism by releasing low molecular weight sugars that serve as energy sources and help in global carbon cycling and it is also involved in the final step of cellulose degradation and is sensitive to residue management (Bandick and Dick, 1999; Acosta-Martinez et al., 2003; Zang et al., 2018). The carbon cycle transformation, soil organic matter composition, and cycle are all correlated with β-glucosidase. This enzyme serves as a marker for changes in soil agrotechnical parameters as well as some changes in soil biological activity (Rachwal et al., 2021).
Acid phosphatase enzymes are released by plants, fungi, and bacteria into the soil matrix and are optimal under slightly acidic (pH 6.5) soil conditions. They contribute significantly towards cycling phosphorus by breaking down organic forms of phosphorus, such as nucleotides, phytate, and other phosphate esters found in plant residue and soil organic matter in an ecosystem to release inorganic phosphorus for plant utilization (Park et al., 2022). Acid phosphatase is an indicator of the potential of soil to release PO43− from organic matter in soils. Due to high phosphorus loading rates in all amendments, there was an immediate difference in the phosphatase activities of the biochar compared to compost and compost biochar treatments. This may be due to phosphatases that were in the compost before incorporation into the soil. Activity levels showed a significant increase in the compost treatment compared to the biochar, for all three sampling periods. However, trends showed significant decreases over time (Fig. 2) with acid phosphatase activities at their lowest in August. The treatments showed no clear mean separation from the control in the final month. This suggests that the substrate that was lost had been stimulating the microbial community to produce phosphatase.
Alkaline phosphatase is generated by a wide range of microbes, plants, and even certain soil flora. Inorganic phosphate, which plants can absorb, is released when organic phosphates are hydrolyzed by the enzyme. This process promotes the availability of nutrients and boosts plant growth, both of which are crucial for maximizing soil productivity (Timofeeva et al., 2022). Activity levels showed a significant increase in the compost Biochar treatment compared to the others. However, trends showed significant decreases over time (Fig. 3). The treatments showed that there is no clear mean separation from the control in all studied times and were the lowest activity compared to the others. The differences in the application rates, and thus the amount of available N and P may affect the activity of phosphatase enzymes. The overall decrease in phosphatase activity may reflect the loss of P substrate due to microbial utilization, as well as inhibition from excess P inputs because of feedback inhibition.
Arylsulfatase is involved in the sulfur cycle and is an essential indicator of microbial activity and nutrient cycling within the soil ecosystem by breaking down sulfur-containing compounds to release sulfate ions (SO4²⁻), which are essential nutrients for plant growth as Sulfur is a critical component of amino acids, vitamins, and enzymes in plants, making it crucial for their development and overall soil fertility (Romillac et al., 2023). Looking into the activity of arylsulfatase, the control soil had significantly lower arylsulfatase activities in comparison with the rest of the treatments. However, two months after incorporation, there were no differences in the treatments, suggesting the loss of stimulation by compost and biochar (Fig. 4). Our results may be due to lower levels of organic matter in the control soil compared to the organic amended soils. Arylsulfatase activity was higher for the compost + biochar and compost biochar + compost tea treatments in comparison to all other treatments from April to August. Arylsulfatase activity significantly decreased by around 60% from April to August. Although this difference was significant compared to all other treatments, it was the only enzyme to have the highest values for the compost and compost biochar treatment. The control soil treatment, from June to August, showed no difference. This may be due to the loss of arylsulfatase substrates that stimulate the activity of this enzyme over time. Immediately after amendment incorporation, arylsulfatase activity was significantly higher in the first sampling time (April). Tejada et al. (2006) reported similar results with organic amendments of degraded soil. Similar patterns for arylsulfatase activity were found by Bandick and Dick (1999) during the given period following the introduction of manure into the soil. Arylsulfatase activity has been found in other studies to be susceptible to a range of different management approaches (Ndiaye et al., 2000; Knight and Dick, 2004). As an extracellular enzyme that catalyzes the hydrolysis of organic sulfate esters and releases plant-available sulfate (SO4 2−), Arylsulfatase is also important in that it may be an indirect indicator of fungi, because of the ester sulfate bond that is lacking in bacteria (Bandick and Dick, 1999).
The dehydrogenase enzyme is involved in the biological oxidation of organic matter and can be used to evaluate the overall metabolic activity of soil microorganisms. It contributes to the decomposition of organic matter, which improves soil structure by gluing soil particles for better structure, water retention, and nutrient availability (Wolińska and Stępniewska, 2012). The dehydrogenase in Fig. 5, shows that the highest dehydrogenase activity (0.78 µg of TPF formed g− 1 soil hr− 1) was observed for compost for the first sampling period (April). The highest dehydrogenase activity was observed for all soil treatments in that same period. The decrease in dehydrogenase activity between 60 and 120 days indicates the absence of active decomposition and confirms that the compost was mature. Activity levels showed a significant increase in the compost treatment compared to the biochar, for all three sampling periods, also the same for the control. This may be due to dehydrogenases that were in plots before incorporation into the soil. Dehydrogenase enzymes play a significant role in the biological oxidation of soil organic matter by transferring protons and electrons from substrates to accepters (Sánchez-Monedero, et al., 2008). Dehydrogenase provides correlative information on the biological activity and microbial population in soil and is considered to exist in soils as integral parts of intact cells (Gu, et al., 2009). Measurement of dehydrogenase activity represents the immediate metabolic activities of soil (Gu, et al., 2009).
Arylamidase plays an important role in assessing and influencing soil health. It is involved in the breakdown of organic nitrogenous compounds and serves as an indicator of microbial activity and nutrient cycling in the soil ecosystem. It works by breaking down the amides and in the process releases ammonia (NH3) and other forms of organic nitrogen. The process is crucial for nitrogen cycling in the soil, as it converts organic nitrogen into ammonium, which can be further transformed into forms that are accessible to plants (Ekenler and Tabatabai, 2004). The arylamidase enzyme demonstrates the highest (around 48 µg of p-naphthylamine g− 1 soil hr− 1) for the control in the last sampling period (Fig. 6). The highest arylamidase activity was observed for all soil treatments. The decrease in arylamidase activity between 0 and 60 days may indicate the absence of active decomposition. Activity levels showed a significant decrease in all treatments in the second sampling period compared to the others. This may be due to more rain during that period which resulted in low arylamidase activities in June. Arylamidase enzyme catalyzes one of the initial reactions in N mineralization because it is involved in the release of amino acids from soil organic matter. Arylamidase Indicator of N cycle, hydrolysis of an N-terminal amino acid from peptides, amides, and arylamides, an index of N mineralization in soils (Acosta- Martı́nez and Tabatabai, 2001).
Cellulase breaks down cellulose, it releases sugars into the soil, including glucose. For soil microorganisms, these sugars can act as a food source, fostering their activity and growth. In turn, this microbial activity aids in the mineralization of nutrients, releasing them for use by plants (Jayasekara and Ratnayake, 2019). Cellulase activity showed no statistical difference in the first two sampling periods (Fig. 7). However, by the last sampling period, there was a clear decrease in the activities of all treatments. The control soil consistently had the lowest activity values except for the last sampling period in the biochar treatment, which can be attributed to low organic matter content and the inability of the soil to degrade recalcitrant materials left in the soil. Conversely, this study found no significant difference between the compost treated with biochar, and the biochar treated with compost tea in the last two sampling periods. This indicates similar substrate availability in each of the treatments, regardless of application rate or amendment source. Cellulase is the most abundant organic compound in the biosphere comprising almost 50% of the biomass that is synthesized by the photosynthetic fixation of CO2 (Geisseler, et al., 2010). The growth and activity of soil microorganisms depend on the carbon source that is contained mainly as plant residues that occur in the soil, however, for carbon to be released as an energy source for use by the microorganisms, cellulose in plant debris must be degraded into high molecular weight oligosaccharides, cellobiose, and glucose by cellulase enzymes (Eriksson, et al., 2012). Since cellulase enzymes play an important role in the global recycling of the most abundant polymer in nature, more research should be done to understand the nature of this enzyme better, so that it may be used more regularly as a predictive tool in the assessment of soil fertility.
Urease enzymes catalyze the hydrolysis of urea, releasing ammonium ions (NH4⁺) and carbon dioxide (CO2) as byproducts via conversion of organic nitrogen. The process is essential for the mineralization of nitrogen in the form of ammonium that can be taken up by plants, contributing to their growth and overall nitrogen nutrition in soils. Adequate nitrogen availability is crucial for plant health and productivity (Cordero et al., 2019). Urease activity remained similar between all soil-amended treatments with slight differences except for the control which was different for the entire experiment (Fig. 8). Urease is a hydrolase that catalyzes the hydrolysis of urea to CO2 and NH4+, with the latter being available for uptake by microorganisms and plants (Bremner and Mulvaney, 1978).
In general, within the three-month sampling period, we observed that there was an increase in β- glucosidase activities in August for all the treatments except in the control. Both the acid and alkaline phosphatase activities decreased in August. Arylsulfatase activities decreased in August except for Biochar and compost tea. Dehydrogenase activities decreased in August except for Biochar. Arylamidase activities increased in August in all treatments whereas cellulase activities decreased in August. The urease activities increased in August in all treatments.