1. Introduction
Soil quality is a determining factor in maintaining productivity in agricultural systems. Physicochemical properties are clearly useful in assessing the ecological state of soils; however, most processes that occur in soils are driven by their biota [
1]. In soils, enzymes are produced by microorganisms, secreted by plants or created by the decomposition of plant, animal and microbial biomass. Reactions catalyzed by these enzymes act in the cycle of nutrients essential for plant development, such as carbon, phosphorus, nitrogen and sulfur [
2]. Thus, the enzymatic activity is considered an indicator of biological balance, fertility and quality, being described as “biological fingerprints” of soil management [
3].
In soils, enzymes act as catalysts for biogeochemical cycles, related to the decomposition and synthesis of organic matter, cycling and availability of nutrients [
4]. Due to its indicative nature of soil ecological stress [
5], the enzymatic activity can be used to demonstrate early soil changes related to the production system adopted, such as, for example, the imbalance between organic matter levels and biological activity, when compared to chemical and physical indicators [
6,
7].
The construction of a more biologically active edaphic environment had as a fundamental milestone the expansion and continuous use of conservation management systems, such as the tillage system [
8]. The straw accumulated by cultural remains and cover plants from commercial crops provided a favorable environment for the recovery and maintenance of soil attributes [
9].
The main production systems present in the Cerrado region use soybean (
Glycine max L.) cultivation in the harvest and corn (
Zea mays L.) cultivated in the second harvest [
10], which can be planted in consortium with
Urochloa spp. In years of late planting or under conditions of water deficit, there is the alternative of millet (
Pennisetum glaucum) being cultivated in the second harvest due to its high biomass production and nutrient cycling, with greater tolerance to drought periods [
11]. It is important to emphasize that the cultivation of these grasses in the second harvest provides crop remains that decompose more slowly when compared to soybean [
12], which helps to preserve soil moisture and increases the stability of aggregates and organic carbon accumulation, directly influencing the activity of microbial communities in soils, enzymatic activity, cycling and release of nutrients [
13].
However, the expansion and long-term adoption of conservation management systems have allowed increases in crop productivity or the maintenance of production in the face of adverse environmental situations, which are often not explained by the results of chemical soil analyses [
14,
15,
16,
17]. This finding that chemically similar soils may present different performances demonstrates the need to include parameters related to the biological functioning of the soil (bioindicators) in routine analyses.
Arylsulfatases are enzymes that are present in the biochemical cycle of sulfur release (S), which are responsible for catalyzing the hydrolysis of sulfated esters to release SO
42−, since around 95% of S present in the soil is in the organic form [
18], thus making it available to plants and microorganisms [
19,
20] to be used in processes such as synthesis of amino acids (cysteine and methionine), coenzyme A, and chlorophyll. The activity of arylsulfatase may depend on the species of cultivated plants, type and use of soil, changes in pH, fertility management, and contamination with heavy metals [
21], which makes soil enzymology of practical importance as it highlights aspects inherent to this change [
3]. Soil enzymes have a direct relationship with the rate of SOM decomposition and nutrient cycling [
22].
β-glucosidase is an enzyme present in most soils, acting as a catalyst in the final stage of the cellulose decomposition process, hydrolyzing the cellobiose residue to release simple sugars at the end of the process. Free glucose molecules as a product are essential energy sources used for the growth and survival of many microorganisms that perform relevant functions in soils [
2]. Studies have demonstrated that the process of degradation of organic matter has a strong relationship with the activity of β-glucosidase [
23], and its activity is reduced depending on the availability of plant residues [
24]. Thus, due to its association with the carbon cycle (C), β-glucosidase is considered an indicator of soil quality; however, the challenge for the use of bioindicators in soil quality assessments consists in the interpretation of the analyzed values [
6].
These two enzymes have a close relationship with SOM, a basic parameter for soil quality and grain yield, a parameter that reflects the economic aspect of crops, which is fundamental for the sustainability of agricultural business [
25,
26,
27].
The Bioanalysis technology (BioAS) developed by the Brazilian Agricultural Research Corporation [
28] emerged as a bias to this problem and consisted of the analysis and interpretation of β-glucosidase and arylsulfatase activities, in addition to traditional fertility analyses [
8]. An important innovation linked to the use of BioAS was the creation of an index to evaluate soil quality (Fertibio IQS) based on the soil chemical quality index (chemical IQS) and the soil biological quality index (biological IQS), as areas with similar levels of nutrients can present different productivity, which highlights the importance of the biological component [
8]. Thus, the sum of physical, chemical and biological factors defines the potential of the soil’s capacity to maintain sustainability [
29].
Thus, the aim was to evaluate the activity of arylsulfatase and β-glucosidase enzymes in different succession systems and their correlation with Fertibio IQS. Therefore, under the study circumstances, the use of soil with soybean, corn, millet and brachiaria provided adequate sustainability conditions and high arylsulfatase and β-glucosidase levels.
4. Discussion
Enzymes have an effect on soil quality and health, performing fundamental functions in the biological process, whose supply of nutrients and catalytic reactions depend on the activity mediated by such enzymes, serving as a means of evaluating microbial activity, detecting changes in the soil environment, acting as soil quality indicators, being directly related to the organic matter content [
37].
Previous studies with bioindicators in the Brazilian Cerrado have demonstrated that these two soil enzymes can act as early warning indicators of soil change in response to different management practices [
28,
38]. Since 2019, it has been recommended in Brazil to assist producers in making environmentally and economically sustainable soil and crop management decisions [
28,
38].
Other characteristics that made the use of arylsulfatase and β-glucosidase advantageous are precision, coherence, sensitivity, simple analytical determination and reproducibility. Furthermore, the two enzymes are related to SOM cycling, are not influenced by the application of fertilizers and limestone and adapt to the Fertibio concept of soil sampling [
39]. These enzymes are also correlated with several other microbiological attributes (microbial carbon biomass, basal respiration, acid phosphatase, cellulase, dehydrogenase, etc.), which allowed the selection of just two indicators to express the functioning of the soil biological machinery.
Both soil quality indices and functions receive scores ranging from 0 to 1, and the closer to 1, the better the results. In the present study, arylsulfatase and β-glucosidase levels were classified between high and very high. These extracellular enzymes are used as soil quality bioindicators, aggregate to substrates such as clay and organic matter, acting as catalysts for chemical processes, decomposing these substrates and releasing mineral compounds to plants [
37]. According to studies [
8], such indices were calibrated in tests that aimed to correlate organic matter contents with enzymatic activities, as well as the accumulated yields of soybean and corn grains.
It was possible to observe that both enzymes showed greater activity in treatment 3 followed by treatment 4, treatments that also presented the highest OM content, which certainly increased the soil microbiota and, consequently, the enzymatic activity. Over time, this increase in biological activity can result in an increase in SOM, positively impacting the chemical (e.g., better storage of nutrients) and physical quality of the soil (e.g., better structure, reflected in storage and water infiltration). In the escalation of soil improvement, microbiological attributes are the first to be impacted. More biological activity means, over time, more SOM and, consequently, greater soil structuring and aggregation, which results in better infiltration and water retention.
The greater arylsulfatase activity in the succession system containing
Urochloa followed by millet (treatments 3 and 4) may be related to greater production and maintenance of biomass since these crops were used for this purpose in the off-season. Also, sampling was performed prior to the crop desiccation, which may have directly influenced the greater activity for
Urochloa since it is a perennial grass, allowing the maintenance of the humidity and temperature conditions necessary for the microorganism community, as described by [
40], who cite humidity, temperature, pH and type of vegetation present in the soil as factors that influence enzymatic activity.
Evaluating the Pearson’s correlation coefficient between arylsulfatase and S content (
Figure 2), a moderate and inversely proportional correlation (r = −0.64) is observed, i.e., as the S content increases, arylsulfatase activity tends to decrease. Similar results were found by [
41], where simple correlation analyses between S and arylsulfatase activity indicated a negative relationship between variables; however, with low correlation coefficients. In soil, S is predominantly found in the organic form, so the soil’s ability to meet the plant’s demand for nutrients is closely related to the organic matter content and its mineralization [
42].
Analyzing the correlation between arylsulfatase activity and organic matter content and organic carbon content and β-glucosidase activity, there was a strong correlation between these variables (r = 0.77 and r = 0.73, respectively), as well as the influence of soil organic matter on the activities of these enzymes. According to studies [
41], arylsulfatase activity decreases with the decrease in organic matter content since OM is the main reserve of sulfate esters, which are substrates for the enzyme. Similar results were found by [
22], who evaluated the relationship between pH, OM and soil texture with the activity of arylsulfatase and β-glucosidase enzymes [
6], aiming to develop a proposal for interpreting microbiological attributes, used three long-term experiments and different organic matter levels in soil, which showed a strong positive correlation with the soil microbiological attributes soil and organic matter content.
In a study of soil enzymatic activity in different production systems, ref. [
43] found that high organic matter values correlated with greater enzymatic activity in the areas under study since organic matter maintains soil enzymes in their active forms due to the interaction between humic substances and enzyme molecules. Increases in β-glucosidase activity in cultivated areas have also been reported by [
34] due to the biological peculiarities of the Cerrado soil, and the high values for the activity of this enzyme in agricultural areas can largely be explained by the lower complexity and lignification of residues present in agricultural systems.
It is noteworthy that the OM content did not differ between treatments (
Appendix A—
Table A3), and all showed high enzymatic activity values, indicating that the production systems and soil management practices used are adequate and sustainable. Soils with greater OM increment show increased microbiological activity, improving their structure and proving the resilience of biologically active soils compared to conventional cultivation [
43].
Another important innovation linked to the use of BioAS was the creation of an index to evaluate soil quality (FERTIBIO IQS) based on determinations of chemical fertility attributes (pH, H + Al, Ca, K, P, Mg and SOM) and the levels of arylsulfatase and β-glucosidase enzymes. This index integrates the results of BioAS Technology and soil fertility, generating a score (from 0 to 1), which expresses soil quality/health. The IQS calculations of BioAS technology (IQS Fertibio, IQS chemical and IQS biological) are based on the concept proposed by [
44], which takes into account the capacity of the soil to function to provide important environmental services. In BioAS, based on determinations of enzymatic activity and chemical properties, three functions related to the soil’s ability to promote plant nutrition are evaluated: (F1) the soil’s ability to cycle nutrients; (F2) the soil’s capacity to store nutrients; and (F3) the soil’s ability to supply nutrients [
35]. Like IQS, function scores also vary from 0 to 1, with the closer to 1 being, the better the function performance (function indices F1, F2 and F3). Similar to the strategy used to interpret enzyme activity values, all IQS and the scores of the three functions are calibrated in relation to grain yield and SOM [
35].
F1 aims to estimate the performance of biological activity and processes derived from it, directly or indirectly, such as nutrient cycling and the formation and decomposition of SOM. F2 aims to quantify the “reservoir” of soil nutrients, which is mainly related to texture, clay quality and SOM content and quality. F3 assesses the quality of the soil nutrient “reservoir” content, involving both aspects related to soil acidity and the soil’s capacity to make the main macronutrients available. The performance of these three functions is measured by indicators obtained from chemical and biological analyses of the soil, which are individually interpreted using algorithms defined according to the soil texture [
35].
Despite the difference observed, the biological IQS results presented values classified as high or very high on the proposed scale from 0 to 1 for indicator performance. This indicator estimates the performance of biological activity and of processes directly or indirectly derived from it, such as nutrient cycling and the formation and decomposition of organic soil matter. The highest arylsulfatase and β-glucosidase values, together with the high organic matter content, justify the behavior of this indicator. In a study that sought to evaluate the sustainability conditions of different succession systems, ref. [
45] found the greatest results in crop-livestock integration systems and permanent pastures.
Although no statistical difference was observed for chemical IQS between succession systems, this is an indicator that estimates the soil “reservoir” of nutrients, which is mainly related to texture, number of charges and OM present in the soil, as well as its acidity and ability to make the main macronutrients available.
For Fertibio IQS, it was observed that all succession systems are classified as very high, with values ranging from 0.81 to 1.0, which demonstrates stable patterns characteristic of use and management systems practiced over a long period of time in a given area, generally over 3 to 5 years, as described by [
8].
Fertibio IQS, when classified as high and very high, demonstrates conditions of stability and sustainability of systems adopted. It is worth highlighting that the knowledge of critical levels of quality indicators makes it possible to better understand soil functioning. The correlation between biological IQS and Fertibio IQS was high (r = 0.9), which indicates a strong correlation between the biological variable aspects in systems and the balance of the system used (
Figure 5).
Analyzing the nutrient cycling function, treatment 3 was similar to treatment 4 and differed statistically from treatments 1 and 2, which result is in accordance with what was observed for arylsulfatase, β-glucosidase and biological IQS parameters. To determine nutrient cycling, the enzymatic activity relating the biological aspect to mineralization processes is taken into account.
Regarding nutrient storage, no statistical difference was found between production systems. To determine this function, the values found for organic matter and CEC are considered in the analysis because organic matter acts to increase soil CEC when decomposed to a level that exposes its negative charges; consequently, increasing CEC increases nutrient storage capacity. As for the nutrient supply function, treatment 2 was statistically different from treatment 4. The nutrient supply considers the other analytes determined in soil analysis, such as exchangeable bases and P content; thus, the difference found for treatment 2 can be explained by the fertilizer used in corn cultivation.
These results indicate that in crop succession systems with cash crops, they tend to reduce the activity enzymatic of arylsulfatase and β-glucosidase in the medium to long term, which are important enzymes related to biological soil quality evidenced by the biological IQS, parameters that can infer the sustainability of production systems. Therefore, the inclusion of Urochloa or other species intercropped with corn can mitigate the deleterious effect on the biological quality of the soil in the long term.