Effects of Organic and Conventional Management ... - ACS Publications

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Effects of Organic and Conventional Management of Sugar Cane Crop on Soil Physicochemical Characteristics and Phosphomonoesterase Activity Luiza L. A. Purcena,*,† Maria Carolina B. Di Medeiros,† Wilson M. Leandro,‡ and Kátia F. Fernandes† †

Departamento de Bioquı ́mica e Biologia Molecular, ICB II, e ‡Escola de Agronomia e Engenharia de Alimentos, Universidade Federal de Goiás, Samambaia Goiânia, Brazil ABSTRACT: Soil enzymes play an important role in agriculture and particularly in nutrient cycling. They are also involved in the degradation, transformation, and mineralization of organic matter and availability of nutrients in soil. It is believed that organic agriculture causes fewer losses to soil quality and is less aggressive to the environment than conventional management. In this study, the effects of conventional (CM) and organic management (OM) on phosphomonoesterases, an important enzyme for soil fertility, were evaluated and compared to those results from native Cerrado (Brazilian Savanna) soil (NS), because they are the most common phosphatases in soils. The results showed that there were both acid (AcP) and alkaline (AkP) phosphatases in all soils tested and that AcP activity was higher than that of AkP. In contrast to AkP, AcP had its activity affected by land use. In the cultivated areas there was a reduction of almost 50% of AcP activity respect to native unexploited soils and there was no significant difference between organic and CM, demonstrating that independent of the management chosen, there was an impact of land use on AcP activity. Principal component analysis indicated that characteristics related to pH such as alkali saturation (V%), aluminum saturation (M%), Al3+, soil total acidity (H+Al), and Ca2+ are the main factors that permit distinguishing NS from OM and CM. KEYWORDS: phosphomonoesterases, Cerrado, soil, principal component analysis, management

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fuels,4 especially considering the increasing prices of oil as a consequence of the risk of shortage. Additionally, in contrast to oil, ethanol is a renewable energy source and reduces the greenhouse effect of gas emissions. Moreover, gas emission produced from sugar cane ethanol is much less aggressive than that from corn because it decreases the use of fossil fuels by using biomass.4 Several authors have reported that monoculture planting causes the loss of soil fertility because of the nutrients consumed by plants and pesticide use.5−7 Then, in an attempt to lessen agricultural impacts of monocultures, several managements have been proposed, including organic management (OM), in which only organic fertilizers are used and there is no use of pesticides or synthetic fertilizers. Some papers have already described the benefits of OM1,8 by preserving some biochemical characteristics such as the global, organic, and basal carbon (C). Nevertheless, the impact of this management on soil enzymes has not been evaluated yet. Biomass and enzymes present in soil are responsible for several chemical reactions such as nutrient cycling and the physicochemical characteristics of this environment.9 The activities of enzymes in soil have been correlated with plant growth and are reported as a useful tool to measure soil fertility,10,11 making them great candidates to measure soil quality. In addition, enzymes are sensitive to environmental changes and are more effective than microorganisms as

INTRODUCTION Soil is a complex ecosystem in which the combination of insects, nematodes, annelids vegetation, microorganisms, and anthropogenic modifications will determine its characteristics. Management of agricultural systems also contributes to soil characteristics and can affect soil quality in the long term by modifying soil physical, chemical, and biological characteristics at a rate that is largely dependent on climate conditions and farming practice.1 Cerrado, also known as the Brazilian Savanna, soils are acid and have low availability of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), molybdenum (Mo), and zinc (Zn). They are high in aluminum (Al) saturation and possess high P fixation capacities.2 In addition, Cerrado soils present crop production limitations: 5−6 month dry season; dry spells of 1−3 weeks during the rainy season, generally associated with high evapotranspiration rates; low water-holding capacity, even in clayey soils; limited rooting depth of many crops as a function of aluminum toxicity and/or calcium deficiency in subsurface soil layers.3 Despite all of these problems, a breakthrough in agricultural development has taken place in the area during recent decades, mainly involving food crops, pasture, and coffee. Yield levels of some of these crops exceed national averages.3 The overexploitation of soil is a consequence of the current needs to sustain the high demand for food and the search for alternative fuel sources such as ethanol. The ethanol produced from sugar cane monoculture has become very important worldwide, because it is a viable alternative to vehicle fossil © 2014 American Chemical Society

Received: Revised: Accepted: Published: 1456

July 22, 2013 January 21, 2014 January 29, 2014 January 29, 2014 dx.doi.org/10.1021/jf403141w | J. Agric. Food Chem. 2014, 62, 1456−1463

Journal of Agricultural and Food Chemistry

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The area under NS is a preserved Cerrado area in the sugar cane mill property, presenting the features of the vegetation of this biome, and has not been explored for any agriculture or livestock. All of the managements in the study area were under rotation with Crotalaria juncea every 5 years. Physicochemical Analyses. Physicochemical analyses were performed at the Laboratory of Soil Analysis of the School of Agronomy at Universidade Federal de Goiás according to methodology described by EMBRAPA.18 The analyses included the determination of clay, silt, and sand percentage (soil granulometry), soil organic matter (SOM), pH using CaCl2 determined by potenciometer, saturation of acids (M %), saturation of alkalis (V %), potential acidity total (H++Al3+ cmolc/ dm3) determined by Schoemaker, McLean, and Pratt buffer (SMP),19 aluminum (Al3+ cmolc/dm3) extracted with 1 mol/L KCl and determined by volumetric titration, inorganic phosphorus (P+ cmolc/ dm3, extracted by Mehlich 1 and determined by colorimetry), potassium (K+ cmolc/dm3) extracted by a Mehlich 1 extractor and determined by flame photometry, magnesium (Mg2+ cmolc/dm3) and calcium (Ca2+ cmolc/dm3) extracted with 1 mol/L KCl and determined by atomic absorption spectrophotometry, and cation exchangeable capacity (CEC), Ca/K, Ca/Mg, Mg/K, Ca/CEC, K/ CEC, and Mg/CEC. All tests were carried out in triplicates and are shown as the mean ± standard deviation. Optimum pH and Enzyme Assay. To determine the optimum pH for the measurement of enzyme activity, the reaction was performed at pH 2.0, 3.0, and 4.0 (0.1 M glycin buffer), pH 5.0 and 5.5 (0.1 M acetate buffer), pH 6.0 (0.1 M sodium citrate buffer), pH 7.0 and 8.0 (0.1 M TRIS buffer), and pH 9.0 and 11.0 (0.1 M glycin buffer) using samples from NS because it presents the natural conditions for native phosphatases from Cerrado. The assay was performed following the standard methodology for phosphatase measurement described by Eivazi and Tabatabai15 with modifications. The reaction was conducted using 16 mg of soil, 0.4 mL of the respective buffer, and 0.1 mL of 2.5 mM p-nitrophenyl disodium phosphate. The mixture reacted at 55 °C for 1 h. Then, 0.1 mL of 0.5 M Na2CO3 was added followed by the addition of 0.4 mL of 0.1 M NaOH to stop the reaction. The mixture was centrifuged and the supernatant measured at 400 nm using a UV−vis spectrophotometer. A control was performed by adding NaOH followed by Na2CO3 to stop the reaction immediately after addition of the substrate. One enzyme unit (U) was defined as the amount of enzyme that released 1 mg of p-nitrophenol (pNP) after 1 h of reaction per gram of soil. After determination of the optimum pH for acid (AcP) and alkaline (AkP) phosphomonoesterase, the enzyme activity was tested in the different soil samples according to the description above. AcP was measured using 0.1 M acetate buffer, pH 5.0, for all samples or with 0.1 M citrate buffer, pH 5.6 for CM and pH 5.7 for OM. AkP was measured in pH 8.0 using 0.1 M TRIS buffer, pH 8.0. Statistical Analyses. All experiments were conducted at least in triplicate, and the results were processed by central tendency (mean) and dispersion (standard deviation) measurements. Statistica software 7.0 (StatSoft Inc., Tulsa, OK, USA) was used to perform multivariate analysis (ANOVA) followed by correlation, as well as Tukey’s test, to determine the significant differences among the means. The p level of significance used was 0.05. Principal component analysis (PCA) was performed to evaluate the variables that would interfere in AcP activity according to soil management. Only principal components with eigen values that explain 10% of the total variance were retained. A Varimax rotation was performed to enhance the interpretability of the unrelated components.20

bioindicators because 0.05) (CM 2.9%; OM 2.9%; NS 3.0%). Except for Mg2+, there was significant difference of K+ and inorganic P+ contents (Table 2). The results showed higher K+ and inorganic P+ in OM (K+ 0.25 cmolc/dm3; P+ 11.3 × 10−3 cmolc/dm3) than in CM (K+ 0.13 cmolc/dm3, P+ 1.2 × 10−3 cmolc/dm3) and NS (K+ 0.17 cmolc/dm3; P+ 3 × 10−3 cmolc/ dm3). The amount of Ca2+ was also different in all tested soils, with higher values in OM (2.6 cmolc/dm3). The saturation of alkaline ions (V %) represents the capacity of a soil to exchange alkaline cations such as K+, Mg2+, and Ca2+. The amounts of K+, Mg2+, and Ca2+ adsorbed on clay were higher in OM (71%) and CM (54%) (Table 2). This means that soils used in agriculture present more cations with exchangeable capacity than NS (35%), which is a consequence of management practices such as fertilizer addition in OM and CM. The results showed that soil pH values in the cultivated areas (5.6 in CM and 5.7 in OM) were higher than in the NS (4.9). This pH variation among the studied soils occurred as a consequence of liming, which is also the reason for the higher amount of Ca2+ in cultivated soils. Cerrado soils are known for presenting a high amount of aluminum, which is toxic for many plants by causing plasmolysis on plant roots. This problem is overcome by the precipitation of Al3+ by addition of limestone, which is converted in the soil into Ca(OH)2, causing the precipitation of Al3+ by the formation of Al(OH)3. The results showed the presence of Al3+ in NS (0.7 cmolc/ dm3) and its absence in the cultivated soils. Optimum pH. Figure 1 depicts the activity of phosphomonoesterases in NS expressed as U (mg−1 g−1 h−1). The figure

shows AcP presented a considerably higher activity than AkP and that the optimum pH values for AcP and AkP activity were pH 5 (113.5 U) and pH 8 (53.3 U), respectively. Acid and Alkaline Phosphomonoesterase Activity in NS, OM, and CM. There was no difference in the AkP activity between the cultivated areas and NS, considering most of the samples (average of activity in CM 33.5 U, OM 31.5 U, and NS 39 U) (Figure 2). This result implies that sugar cane

Figure 2. Alkaline phosphomonoesterase activity in cropped (organic and conventional management) and native soils.

management did not affect AkP activity. On the other hand, AcP activity is quite different among cultivated areas and NS (Figure 3). The activity observed in the NS (average of 96 U) was considerably higher than in the soils with sugar cane agriculture (average of CM 47.3 U and OM 45.6 U). In the original pH of soil, CM presented an average enzyme activity of 61.6 U and OM, 49 U. Effects of Soil Physicochemical Characteristics on Acid Phosphomonoesterase Activity. An additional approach to estimate the relationship among physicochemical

Table 2. Values of Ions and Alkali Saturation (V %) on Cropped and Native Soilsa management extractable P (cmolc/dm3) extractable K (cmolc/dm3) extractable Ca (cmolc/dm3) extractable Mg (cmolc/dm3) V% CEC (cmolc/dm3)

conventional

organic

native soil

1.2 × 10−3b ± 0.0005 0.13b ± 0.02 1.5b ± 0.3 0.4a ± 0.1 54a ± 8.2 4.6a ± 0.3

11.3 × 10−3a ± 0.008 0.25a ± 0.02 2.6a ± 0.4 0.6a ± 0.1 71a ± 9.2 3.4b ± 0.7

2 × 10−3b ± 0.0007 0.17b ± 0.05 0.5c ± 0.2 0.5a ± 0.2 35b ± 10.2 3.3b ± 0.5

Results are means ± standard deviation of three determinations. Data followed by the same letters in the same row are not significantly different (p > 0.05). a

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dx.doi.org/10.1021/jf403141w | J. Agric. Food Chem. 2014, 62, 1456−1463


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Table 3. Principal Component Loadings after Varimax Rotationa principal componentsb measurement eigen values explained variance % rotated loading on two retained componentsc AcP clay silt sand SOM pH extractable P+ extractable K extractable Ca extractable Mg H+Al extractable Al CEC M V Ca/Mg Mg/K Ca/K Ca/CEC Mg/CEC K/CEC

Figure 3. Acid phosphomonoesterase activity in cropped (organic and conventional management) and native soils in pH 5.0 (a) and in the natural pH for OM (5.7), CM (5.6), and NS (5.0) (b).

factors and AcP activity and to allow the distinction concerning these data, PCA was carried (Figure 4). PCA often reveals previously unexpected associations among variables and thereby allows interpretation that would not be possible otherwise.21

PC 1

PC 2

10.4 49.5

5.1 24.5

0.8 0.5 0.1 −0.4 −0.01 −0.9 −0.3 −0.5 -0.9 −0.7 0.9 0.7 −0.8 0.7 −0.9 −0.9 −0.2 −0.8 −0.9 0.1 −0.2

0.4 −0.6 −0.8 0.8 0.26 −0.3 0.3 0.7 0.06 0.4 0.05 0.5 0.2 0.5 −0.02 −0.2 −0.4 −0.4 −0.1 0.4 0.8

The soil parameters are grouped according to the maximum fittings to principal components (correlation coefficients > 0.50; n = 21). b Only principal components with eigen values >1 and those explaining >10% of the total variance were retained. cCorrelations with absolute values >0.50 are in bold. a

K+ (mg/dm3). Factor 1 is related to pH and CEC, and factor 2 is related to the soil’s granulometry. From the results of PCA (Figure 4), for AcP activity, it can be seen that this enzyme presentd a greater relationship with NS than with OM and CM and that the main factors related to AcP were those influencing soil pH, which is demonstrated by the high values and proximity of the loadings of aluminum and H++Al3+. The correlation matrix showed positive correlations between AcP and H+Al (r = 0.8), Al3+ (r = 0.9), and M % (r = 0.9). However, negative correlations with AcP were found for pH (r = −0.8), Ca2+ (r = −0.8), and V % (r = −0.8). According to PCA, granulometry also seems to be related to AcP activity. In fact, Table 1 shows a higher amount of sand in OM than in CM and NS and clay appears to be more related to AcP.

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DISCUSSION Soil Physicochemical Characteristics and Acid Phosphomonoesterase Activity. Cerrado soils present color variations from red to yellow and are classified as haplustox dystrophic.18 They are deep and well-drained during the major part of the year, making them vulnerable to leaching, contributing to their low CEC (Table 1). This characteristic is related to the granulometry of soil particles, especially the sand content (Table 1). Even though Cerrado soils present a high percentage of sand, they are considered loamy because clay appears in a percentage >20% (Table 1). Loamy soils are

Figure 4. Principal component analysis of the soil physicochemical characteristics and acid phosphomonoesterase activity from organic and conventional management and native soils.

PCA (Table 3) showed that there were two factors explaining the variance of the data. Factor 1 explained 49.5% of the total of variance. The total of variance of this factor is defined by AcP, pH, Ca2+ (cmolc/dm3), Mg2+ (cmolc/dm3), H +Al, Al3+ (cmolc/dm3), CEC (cmolc/dm3), M %, and V %. Factor 2 explained 24.5% and is defined by silt %, sand %, and 1459



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The higher amount of vinasse applied in OM might be due to OM requirements, because only the addition of organic fertilizers is allowed, and it would also be an attempt to overcome the lower productivity of OM due to the presence of high amounts of nutrients in this residue. In addition, filter cake, another organic residue from the industrial processing of sugar cane, is also added to OM soil as fertilizer. This residue is rich in P+, which contributes to its higher amount in OM. Then, the residual effect of vinasse and filter cake application over the years explains the high amount of some nutrients in soil, mainly K+ and P+. Potassium is a very important macronutrient for sugar cane and is the most required nutrient by this culture. Among the functions of K+ in sugar cane is the productivity effect. An adequate amount of K+ is important to the conversion of sugars in sucrose,34 to increase the amount of carbohydrates, oils, and proteins, and to promote sugar and starch storage. It also regulates water use in plants and increases resistance during dry periods. Additionally, an abundant amount of K+ may interfere in the absorption of Ca2+ and Mg2+ by the plant.35 The correlation matrix showed a positive correlation between K+ and Ca2+ (r = 0.6) and between K+ and Mg2+ (r = 0.7). As can be observed in Table 2, the amount of potassium was higher in OM, and there was no statistical difference in the amount of K+ between CM and NS (p > 0.05). Cerrado soil is poor in macronutrients such as potassium, and its vegetation is adapted to this condition. Haridasan36 observed a low amount of potassium content in the biomass of Cerrado vegetal species, corroborating this adaptive mechanism. On the other hand, sugar cane presents a high demand for potassium, and it is added to the soil by different fertilization strategies used in OM and CM. Shaviv et al.37 reported that continuous addition of fertilizers in cultivated soils may cause a saturation effect, which may reduce its ability to fix the potassium added by fertilization. Therefore, the addition of fertilizers does not solve the problem of low nutrient content in soils because other factors in the composition must be considered. In this scenario, the importance of soil particles is demonstrated by PCA. There was a positive correlation between K+ and sand (r = 0.9) and a negative correlation between K+ and clay (r = −0.7) and between K+ and silt (r = −0.7). Murashkina et al.38 studied the fixation of potassium in different soil size particles and reported that in soils with a predominant proportion of sand, this is the most important particle in the fixation of potassium. Because Cerrado soils present a high proportion of sand, especially in OM, this may be the explanation for the higher amount of potassium in this soil management. This result indicates that the presence of nutrients in soils depends on the characteristics of the cover, management, biochemical factors, and proportion of soil size particles. In regard to the amount of Ca2+, liming is the main factor that contributes to the increased amount of this ion in the managed soils. OM presented the higher values among the tested soils. The effect of liming is also observed in the values of soil pH, which were higher in the cultivated areas (5.6 in CM and 5.7 in OM) than in the NS (4.9). This result explains part of the reduction on AcP activity in the cultivated soil (Figure 3). Liming is the agricultural practice responsible for increasing soil pH, and it explains the negative correlation between AcP activity and Ca2+.

generally considered fertile; however, Cerrado soils present low active clay, 1:1 of minerals proportion, making them less fertile soils. It is important to consider a soil’s granulometry when soil enzymes are studied, because they are often immobilized onto these particles. Several authors reported that enzyme immobilization on soil particles provides stability to this biocatalyst against unfavorable soil environment.22,23 Then, the smaller the particle size, the higher the area available for enzyme immobilization. Indeed, considering that Cerrado soils present a high amount of clay, it is possible to assume that these soils constitute a favorable environment for soil enzymes. AcP produced by Heloma cylindrosporum and Suillus collinitus strains presented more affinity for clay than the other soil size fractions.24 In addition, it has also been reported that clay with 1:1 minerals was able to retain more enzymes catalytically active.25 In fact, results from PCA showed a relationship between AcP activity and soil granulometry. Sand and silt are explained by factor 2 and are more related with OM and CM. Indeed, Table 1 shows a higher amount of sand on OM and a higher amount of silt on CM. This result means that, apparently, the reduction of AcP activity on these soils is also affected by the size of the soil’s granulometry because the support for enzyme immobilization is responsible for protecting enzymes against denaturation factors and conditions.26−28 Several authors reported the influence of soil granulometry on enzyme activity. They report that soil granulometry can affect enzymes in distinct ways.10,25,29,30 This finding is important because knowledge of how managements affect biochemical characteristics on soil may allow agricultural techniques that reduce management impact on soil quality. In fact, the amount of sand and silt in agricultural soil is indicative of the possible reduction of AcP activity. Not only clay but also the SOM content is important to soil quality, because it also acts as a support for enzyme immobilization and can influence enzyme activity and stability. However, there was no statistical difference (p > 0.05) of SOM content in the studied soils, suggesting that this parameter is not the most relevant factor affecting AcP activity. This is an interesting result considering that the quality of the organic matter is expected to change according to organic and conventional managements.31,32 In this sense, despite the probable difference in the quality of SOM in the studied soils, this parameter did not significantly affect AcP activity. Different amounts of SOM were reported by several authors according to different locations, management, cover, and soil type. It is well-known that soils from Cerrado are acid, present aluminum toxicity, and are poor in essential nutrients such as calcium, magnesium, potassium, and some micronutrients. In this sense, these soils require amendments to allow sugar cane agriculture. The quantification of K+ and inorganic P+ among the tested soils showed that OM presented higher amounts of these cations (Table 2). This result indicates one of the effects of the application of vinasse, a residue from the fractional distillation of sugar cane, in OM and CM because this residue is rich in organic matter and nutrients, especially K+.33 The application of vinasse was more frequent in OM than in CM, because there were applications in 2008 (572 m3/ha) and 2010 (300 m3/ha), whereas in CM there was application only in 2010 (306 m3/ ha). 1460



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animals and was adsorbed on clay or other solid component of soil that preserved its activity. The lower activity of AkP is also explained because the expression of acid and alkaline phophomonoesterases is influenced by pH. Considering the acid pH of the soil samples, the microbial expression of AcP was induced in the tested soils, contributing to the higher amount of this enzyme compared to AkP. Therefore, AkP is not an effective enzyme to be used as a bioindicator of soil quality in acid soils. Samples from the NS showed markedly higher AcP activity when compared to OM and CM. Moreover, it is noticeable that there was no difference between the two management practices, demonstrating that whatever the management is chosen, the enzyme activity is considerably affected. Previous studies showed that agricultural practices affected biomass composition, enzyme activities, and organic and biomass C and that phosphatase activity was significantly greater in undisturbed soils compared to conventional agriculture.43,44 Previous studies stated that a decrease in organic soil disturbance could improve microbial activity and it, consequently, would increase the synthesis and secretion of enzymes and that there were variations on enzyme activity due to different groundcover crops and soil tillage, demonstrating that enzymes are sensitive to soil disturbance.41,45 The activity found for AcP in NS from Cerrado (average of 96 U g−1) was considerabley higher than that reported in the literature. In the best conditions for AcP catalysis, the maximum activity was 249 U kg−1 in a Kenyon loam soil (fine-loamy, mixed, mesic Typic Hapludoll) from Iowa,46 USA. In a study about the effect of permanent cover crop from sandy Ultisol soil (Paraná, Brazil), the activity of AcP varied from 87 to 176 U kg−1.41 AcP had its activity affected in different management practices with activity varying from 200 to 300 U kg−1; in addition, in this study organic C also influenced AcP with a maximum activity of 600 U1 kg−1 in the presence of 3% of organic C.43 Note that in these data, enzyme activity is given in U kg−1 and in this study AcP activity is expressed in U g−1 because of the high amount of this enzyme in Cerrado soils (NS). The most important finding of this study is that OM affected AcP activity as well as CM. Some reports in the literature showed that OM presented better chemical, biochemical, and biological parameters compared to CM. Other studies have already observed that organic C was higher in OM than in CM. They also showed that OM presented higher mineralized C, potentially mineralized C, and basal respiration than CM.1 However, soil microbial biomass and mineralized C per microorganism biomass unity showed no difference between OM and CM. Moreover, biological parameters such as the presence of arthropods sensitive to agriculture process was higher in CM than in OM.45 Another important observation is that the managed soils OM and CM did not favor any of the phosphatase forms (AcP and AkP) because the soil pH of OM and CM is higher than the optimum pH for AcP and lower than the optimum pH for AkP, compromising enzyme synthesis in these soils. Conclusions. All of the results presented showed the importance of the identification of the physicochemical characteristic changes by the sugar cane management affecting soil enzymes. Reduction of AcP activity in both management practices, OM and CM, was observed. However, AkP activity was not affected by management.

The saturation of alkaline ions (V %) represents the capacity of a soil to exchange alkaline cations such as K+, Mg2+, and Ca2+. Soil with V % >50% is considered rich in nutrients or eutrophic, whereas V % 5.5.40,41 Optimum pH. The optimum pH of a soil enzyme is not necessarily the same as the soil’s. Nevertheless, enzyme activity is dependent on soil pH, because it affects the proper ionization, expression, enzyme stability, and release by soil microorganisms.13 In this sense, it is important to assay the optimum pH for enzyme activity to establish the best catalysis conditions and to identify if the soil offers proper conditions for enzymatic catalysis. Aiming to evaluate the effect of pH on phosphomonoesterase activity, tests were conducted on NS with pH ranging from 2 to 11 (Figure 1). The presence of phosphatase activity is observed in both acid and alkaline ranges, with optimum at pH 5.0 and 8.0 for AcP and AkP, respectively. Despite the acid character of Cerrado soil, AkP were found in these samples. However, the AkP activity is remarkably lower (53.3 U) than that in AcP (113.5 U). It is important to note that although most reports in the literature affirm that the optimum pH for assaying AcPs from soils is 6.5 or close to neutral15,42 acid phosphatases from Cerrado soil did not follow the same profile. The optimum pH for AcP found in this study was 5.0, which is almost the same pH of the NS (pH 4.9). Acid and Alkaline Phosphomonoesterase Activity in NS, OM, and CM. Another important result is that there was no difference of AkP activity between the cultivated areas and NS. This result implies that management does not affect AkP activity. The presence of AkP in acid soils such as soils from Cerrado is mainly from cell lyses of microorganisms and small 1461



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Our results demonstrated that AcP is affected by soil pH and soil particle size. This enzyme showed higher activity in lower pH with optimum pH 5.0. Despite the acid characteristic of Cerrado soils, AkP was also found in the tested soil and presented optimum pH 8.0. This finding highlights the importance of research in soils because this ecosystem is still not well studied, especially with regard to the biological and biochemical characteristics, which are highly influenced and have particular characteristics according to the region, agricultural practice, and cultivated crop. The presence of AkP in the tested soils demonstrated also the importance of the soil particle size for AcP and AkP, because according to PCA clay size particles are closely related with the enzyme activity because these particles support enzyme immobilization, contributing to the maintenance and protection of enzyme structure and activity, which also explains the presence of AkP on these soils becausse they do not offer favorable pH conditions for its secretion and hydrolysis.

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AUTHOR INFORMATION

Corresponding Author

*(L.L.A.P.) E-mail: [email protected]. Phone: +556235211492. Funding

L. L. A. Purcena thanks FAPEGO and M. C. B. Di Medeiros thanks CAPES for fellowship support. Notes

The authors declare no competing financial interest.

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REFERENCES

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