Effects of Organic and Conventional Management of Sugar CaneCrop on Soil Physicochemical Characteristics andPhosphomonoesterase ActivityLuiza L. A. Purcena,*,† Maria Carolina B. Di Medeiros,† Wilson M. Leandro,‡ and Katia F. Fernandes†

†Departamento de Bioquımica e Biologia Molecular, ICB II, e ‡Escola de Agronomia e Engenharia de Alimentos, UniversidadeFederal de Goias, Samambaia Goiania, Brazil

ABSTRACT: Soil enzymes play an important role in agriculture and particularly in nutrient cycling. They are also involved inthe degradation, transformation, and mineralization of organic matter and availability of nutrients in soil. It is believed thatorganic agriculture causes fewer losses to soil quality and is less aggressive to the environment than conventional management. Inthis study, the effects of conventional (CM) and organic management (OM) on phosphomonoesterases, an important enzymefor soil fertility, were evaluated and compared to those results from native Cerrado (Brazilian Savanna) soil (NS), because theyare 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 affectedby land use. In the cultivated areas there was a reduction of almost 50% of AcP activity respect to native unexploited soils andthere was no significant difference between organic and CM, demonstrating that independent of the management chosen, therewas an impact of land use on AcP activity. Principal component analysis indicated that characteristics related to pH such as alkalisaturation (V%), aluminum saturation (M%), Al3+, soil total acidity (H+Al), and Ca2+ are the main factors that permitdistinguishing NS from OM and CM.

KEYWORDS: phosphomonoesterases, Cerrado, soil, principal component analysis, management

■ INTRODUCTION

Soil is a complex ecosystem in which the combination ofinsects, nematodes, annelids vegetation, microorganisms, andanthropogenic modifications will determine its characteristics.Management of agricultural systems also contributes to soilcharacteristics and can affect soil quality in the long term bymodifying soil physical, chemical, and biological characteristicsat a rate that is largely dependent on climate conditions andfarming practice.1

Cerrado, also known as the Brazilian Savanna, soils are acidand 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 Pfixation capacities.2

In addition, Cerrado soils present crop productionlimitations: 5−6 month dry season; dry spells of 1−3 weeksduring the rainy season, generally associated with highevapotranspiration rates; low water-holding capacity, even inclayey soils; limited rooting depth of many crops as a functionof aluminum toxicity and/or calcium deficiency in subsurfacesoil layers.3

Despite all of these problems, a breakthrough in agriculturaldevelopment has taken place in the area during recent decades,mainly involving food crops, pasture, and coffee. Yield levels ofsome of these crops exceed national averages.3

The overexploitation of soil is a consequence of the currentneeds to sustain the high demand for food and the search foralternative fuel sources such as ethanol. The ethanol producedfrom sugar cane monoculture has become very importantworldwide, because it is a viable alternative to vehicle fossil

fuels,4 especially considering the increasing prices of oil as aconsequence of the risk of shortage. Additionally, in contrast tooil, ethanol is a renewable energy source and reduces thegreenhouse effect of gas emissions. Moreover, gas emissionproduced from sugar cane ethanol is much less aggressive thanthat from corn because it decreases the use of fossil fuels byusing biomass.4

Several authors have reported that monoculture plantingcauses the loss of soil fertility because of the nutrientsconsumed by plants and pesticide use.5−7 Then, in an attemptto lessen agricultural impacts of monocultures, several manage-ments have been proposed, including organic management(OM), in which only organic fertilizers are used and there is nouse of pesticides or synthetic fertilizers. Some papers havealready described the benefits of OM1,8 by preserving somebiochemical characteristics such as the global, organic, and basalcarbon (C). Nevertheless, the impact of this management onsoil enzymes has not been evaluated yet.Biomass and enzymes present in soil are responsible for

several chemical reactions such as nutrient cycling and thephysicochemical characteristics of this environment.9 Theactivities of enzymes in soil have been correlated with plantgrowth and are reported as a useful tool to measure soilfertility,10,11 making them great candidates to measure soilquality. In addition, enzymes are sensitive to environmentalchanges and are more effective than microorganisms as

Received: July 22, 2013Revised: January 21, 2014Accepted: January 29, 2014Published: January 29, 2014

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bioindicators because <1% of microorganisms can be isolatedfrom soil.12

Among soil enzymes, phosphatase is one of the mostimportant enzymes. It is involved in nutrient cycling,mineralization, and plant nutrition and provides soil organicphosphorus (P).13 The description of the action of theseenzymes on P cycling is fully described in the literature.14−17

Both acid and alkaline phosphomonoesterases can be found insoils according to the pH medium. Because phosphatases areinvolved in P cycling, which is essential for plant nutrition andsoil fertility, the maintenance, preservation, and understandingof the behavior and the factors affecting these enzymes on soilare very important to ensure soil quality, plant growth, andagricultural productivity.The aim of this study was to evaluate the effect of OM and

CM on acid phosphomonoesterase (AcP) and alkalinephosphomonoesterase (AkP) and which factors within thesesoils would contribute the most with the variation on thisenzyme activity on soil. For that, AcP and AkP activities weremeasured in soils under OM and CM, and the results werecompared with those found for the native soil (NS), withoutany agricultural practices. The main physicochemical character-istics of soils that may affect AcP activity were also assayed.

■ MATERIALS AND METHODSMaterials. p-Nitrophenyl disodium phosphate was purchased from

Sigma-Aldrich Co. (St. Louis, MO, USA). All other reagents were ofanalytical reagent grade and were used without further purificationunless otherwise specified. Solutions were prepared with distilleddeionized water.Site Description and Soil Sampling. Soil samples were collected

from areas under OM and CM sugar cane crop and NS field in Goias,Brazil. Samples of up to 15 cm of soil were collected on December 13,2011. This region is under tropical climate. For the season ofNovember 2011, the rainfall precipitation was 279.4 mm; on average,the temperature was minimum 20 °C and maximum 30.8 °C.There were three replications per land use type, and each replication

was composed of three soil sample cores. Fresh samples were sieved(<2 mm) after removal of debris and stored at −8 °C for AcP assay.The geographical coordinates for CM replications were as follows: CM1, 18.06806° S and 49.71139° W; CM 2, 18.06383° S and 49.71093°W; CM 3, 18.06384° S and 49.70392° W. For OM, the coordinateswere as follows: OM 1, 18.07656° S and 49.67653° W; OM 2,18.07394° S and 49.67653° W; OM 3, 18.07455° S and 49.68620° W.The coordinates for NS were as follows: NS 1, 18.07710° S and49.67603°W; NS 2, 18.07912° S and 49.68508°W; NS 3, 18.08009° Sand 49.68508° W.To standardize the amount of soil weighed in each test, moisture

was determined from weight loss after the samples had been dried at105 °C for 48 h.The area under OM has been treated with this management for 10

years. The cultivated area was fertigated with vinasse (average of 436m3/ha per year) and received 29.1 ton/ha of an organic compoundthat includes sugar cane bagasse and filter cake.The area under CM has been treated with this management for

more than 10 years. This area was fertigated with vinasse (average 306m3/ha per year). In addition, this area was also treated withcommercial pesticides and fertilizers. In the past three years, thechemical compounds used in CM were amonium nitrate, amoniumphosphate, (RS)-5-amino-1-(2,6-dichloro-α,α,α-trifluoro-p-tolyl)-4-tri-fluoromethylsulfinylpyrazole-3-carbonitrile, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-cyclohexyl-6-dimethylamino-1-methyl-1,3,5-tria-zine-2,4(1H,3H)-2′,4′-dichloro-5-(4-difluoromethyl-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl)methanesulfonanilide, limestone,gypsum, and a herbicide of the organoarsenic family.

The area under NS is a preserved Cerrado area in the sugar canemill 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 withCrotalaria juncea every 5 years.

Physicochemical Analyses. Physicochemical analyses wereperformed at the Laboratory of Soil Analysis of the School ofAgronomy at Universidade Federal de Goias according to method-ology described by EMBRAPA.18

The analyses included the determination of clay, silt, and sandpercentage (soil granulometry), soil organic matter (SOM), pH usingCaCl2 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/dm

3) extracted by a Mehlich 1 extractor anddetermined by flame photometry, magnesium (Mg2+ cmolc/dm

3) andcalcium (Ca2+ cmolc/dm

3) extracted with 1 mol/L KCl anddetermined by atomic absorption spectrophotometry, and cationexchangeable 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 optimumpH for the measurement of enzyme activity, the reaction wasperformed 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 7.0and 8.0 (0.1 M TRIS buffer), and pH 9.0 and 11.0 (0.1 M glycinbuffer) using samples from NS because it presents the naturalconditions for native phosphatases from Cerrado.

The assay was performed following the standard methodology forphosphatase measurement described by Eivazi and Tabatabai15 withmodifications. The reaction was conducted using 16 mg of soil, 0.4 mLof the respective buffer, and 0.1 mL of 2.5 mM p-nitrophenyl disodiumphosphate. The mixture reacted at 55 °C for 1 h. Then, 0.1 mL of 0.5M Na2CO3 was added followed by the addition of 0.4 mL of 0.1 MNaOH to stop the reaction. The mixture was centrifuged and thesupernatant measured at 400 nm using a UV−vis spectrophotometer.A control was performed by adding NaOH followed by Na2CO3 tostop the reaction immediately after addition of the substrate. Oneenzyme unit (U) was defined as the amount of enzyme that released 1mg 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 thedifferent soil samples according to the description above. AcP wasmeasured using 0.1 M acetate buffer, pH 5.0, for all samples or with0.1 M citrate buffer, pH 5.6 for CM and pH 5.7 for OM. AkP wasmeasured in pH 8.0 using 0.1 M TRIS buffer, pH 8.0.

Statistical Analyses. All experiments were conducted at least intriplicate, and the results were processed by central tendency (mean)and dispersion (standard deviation) measurements. Statistica software7.0 (StatSoft Inc., Tulsa, OK, USA) was used to perform multivariateanalysis (ANOVA) followed by correlation, as well as Tukey’s test, todetermine the significant differences among the means. The p level ofsignificance used was 0.05.

Principal component analysis (PCA) was performed to evaluate thevariables that would interfere in AcP activity according to soilmanagement. Only principal components with eigen values thatexplain 10% of the total variance were retained. A Varimax rotationwas performed to enhance the interpretability of the unrelatedcomponents.20

■ RESULTS

Physicochemical Characteristics. In regard to clay, silt,and sand, there were differences in the particle size distributionin the soils (Table 1). Even though Cerrado soils present a highpercentage of sand (CM 37%; OM 54%; NS 44%), they are

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considered loamy.18 The amounts of clay in the soils testedwere CM 39%, OM 34%, and NS 35%.Physicochemical analysis showed that there were no

statistical differences in the SOM content among the samplestested (p > 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/dm

3, P+ 1.2 × 10−3

cmolc/dm3) and NS (K+ 0.17 cmolc/dm

3; 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/dm

3).The saturation of alkaline ions (V %) represents the capacity

of a soil to exchange alkaline cations such as K+, Mg2+, andCa2+. The amounts of K+, Mg2+, and Ca2+ adsorbed on claywere higher in OM (71%) and CM (54%) (Table 2). Thismeans that soils used in agriculture present more cations withexchangeable capacity than NS (35%), which is a consequenceof management practices such as fertilizer addition in OM andCM.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 aconsequence of liming, which is also the reason for the higheramount of Ca2+ in cultivated soils.Cerrado soils are known for presenting a high amount of

aluminum, which is toxic for many plants by causingplasmolysis on plant roots. This problem is overcome by theprecipitation of Al3+ by addition of limestone, which isconverted in the soil into Ca(OH)2, causing the precipitationof Al3+ by the formation of Al(OH)3. The results showed thepresence of Al3+ in NS (0.7 cmolc/ dm

3) and its absence in thecultivated soils.Optimum pH. Figure 1 depicts the activity of phospho-

monoesterases in NS expressed as U (mg−1 g−1 h−1). The figure

shows AcP presented a considerably higher activity than AkPand that the optimum pH values for AcP and AkP activity werepH 5 (113.5 U) and pH 8 (53.3 U), respectively.

Acid and Alkaline Phosphomonoesterase Activity inNS, OM, and CM. There was no difference in the AkP activitybetween the cultivated areas and NS, considering most of thesamples (average of activity in CM 33.5 U, OM 31.5 U, and NS39 U) (Figure 2). This result implies that sugar cane

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 caneagriculture (average of CM 47.3 U and OM 45.6 U). In theoriginal pH of soil, CM presented an average enzyme activity of61.6 U and OM, 49 U.

Effects of Soil Physicochemical Characteristics onAcid Phosphomonoesterase Activity. An additionalapproach to estimate the relationship among physicochemical

Table 1. Amounts of Clay, Silt, Sand, and SOM and pH inthe Cropped and Native Soilsa

management

conventional organic native soil

clay % 39a ± 1.4 34b ± 2.8 35ab ± 2.8silt % 25a ± 0.9 16b ± 1.4 17b ± 1.1sand % 37b ± 1.2 54a ± 1.1 44b ± 7.6SOM % 2.9a ± 0.5 2.9a ± 0.6 3.0a ± 0.5pH 5.6a ± 0.1 5.7a ± 0.2 4.9b ± 0.1

aResults are means ± standard deviation of three determinations. Datafollowed by the same letters in the same rwo are not significantlydifferent (p > 0.05).

Table 2. Values of Ions and Alkali Saturation (V %) on Cropped and Native Soilsa

management

conventional organic native soil

extractable P (cmolc/dm3) 1.2 × 10−3b ± 0.0005 11.3 × 10−3a ± 0.008 2 × 10−3b ± 0.0007

extractable K (cmolc/dm3) 0.13b ± 0.02 0.25a ± 0.02 0.17b ± 0.05

extractable Ca (cmolc/dm3) 1.5b ± 0.3 2.6a ± 0.4 0.5c ± 0.2

extractable Mg (cmolc/dm3) 0.4a ± 0.1 0.6a ± 0.1 0.5a ± 0.2

V % 54a ± 8.2 71a ± 9.2 35b ± 10.2CEC (cmolc/dm

3) 4.6a ± 0.3 3.4b ± 0.7 3.3b ± 0.5

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

Figure 1. Effect of pH on the activity of phosphomonoesterase fromnative soil.

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

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factors and AcP activity and to allow the distinction concerningthese data, PCA was carried (Figure 4). PCA often revealspreviously unexpected associations among variables andthereby allows interpretation that would not be possibleotherwise.21

PCA (Table 3) showed that there were two factorsexplaining the variance of the data. Factor 1 explained 49.5%of the total of variance. The total of variance of this factor isdefined by AcP, pH, Ca2+ (cmolc/dm

3), Mg2+ (cmolc/dm3), H

+Al, Al3+ (cmolc/dm3), CEC (cmolc/dm

3), M %, and V %.Factor 2 explained 24.5% and is defined by silt %, sand %, and

K+ (mg/dm3). Factor 1 is related to pH and CEC, and factor 2is 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 withNS than with OM and CM and that the main factors related toAcP were those influencing soil pH, which is demonstrated bythe high values and proximity of the loadings of aluminum andH++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 inOM than in CM and NS and clay appears to be more related toAcP.

■ DISCUSSIONSoil Physicochemical Characteristics and Acid Phos-

phomonoesterase Activity. Cerrado soils present colorvariations from red to yellow and are classified as haplustoxdystrophic.18 They are deep and well-drained during the majorpart of the year, making them vulnerable to leaching,contributing to their low CEC (Table 1). This characteristicis related to the granulometry of soil particles, especially thesand content (Table 1). Even though Cerrado soils present ahigh percentage of sand, they are considered loamy becauseclay appears in a percentage >20% (Table 1). Loamy soils are

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

Figure 4. Principal component analysis of the soil physicochemicalcharacteristics and acid phosphomonoesterase activity from organicand conventional management and native soils.

Table 3. Principal Component Loadings after VarimaxRotationa

principal componentsb

measurement PC 1 PC 2

eigen values 10.4 5.1explained variance % 49.5 24.5rotated loading on two retained componentsc

AcP 0.8 0.4clay 0.5 −0.6silt 0.1 −0.8sand −0.4 0.8SOM −0.01 0.26pH −0.9 −0.3extractable P+ −0.3 0.3extractable K −0.5 0.7extractable Ca -0.9 0.06extractable Mg −0.7 0.4H+Al 0.9 0.05extractable Al 0.7 0.5CEC −0.8 0.2M 0.7 0.5V −0.9 −0.02Ca/Mg −0.9 −0.2Mg/K −0.2 −0.4Ca/K −0.8 −0.4Ca/CEC −0.9 −0.1Mg/CEC 0.1 0.4K/CEC −0.2 0.8

aThe soil parameters are grouped according to the maximum fittingsto principal components (correlation coefficients > 0.50; n = 21).bOnly principal components with eigen values >1 and those explaining>10% of the total variance were retained. cCorrelations with absolutevalues >0.50 are in bold.

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generally considered fertile; however, Cerrado soils present lowactive clay, 1:1 of minerals proportion, making them less fertilesoils.It is important to consider a soil’s granulometry when soil

enzymes are studied, because they are often immobilized ontothese particles. Several authors reported that enzymeimmobilization on soil particles provides stability to thisbiocatalyst against unfavorable soil environment.22,23 Then,the smaller the particle size, the higher the area available forenzyme immobilization. Indeed, considering that Cerrado soilspresent a high amount of clay, it is possible to assume that thesesoils constitute a favorable environment for soil enzymes. AcPproduced by Heloma cylindrosporum and Suillus collinitus strainspresented more affinity for clay than the other soil sizefractions.24 In addition, it has also been reported that clay with1:1 minerals was able to retain more enzymes catalyticallyactive.25

In fact, results from PCA showed a relationship between AcPactivity and soil granulometry. Sand and silt are explained byfactor 2 and are more related with OM and CM. Indeed, Table1 shows a higher amount of sand on OM and a higher amountof 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’sgranulometry because the support for enzyme immobilization isresponsible for protecting enzymes against denaturation factorsand conditions.26−28 Several authors reported the influence ofsoil granulometry on enzyme activity. They report that soilgranulometry can affect enzymes in distinct ways.10,25,29,30

This finding is important because knowledge of howmanagements affect biochemical characteristics on soil mayallow agricultural techniques that reduce management impacton soil quality. In fact, the amount of sand and silt inagricultural soil is indicative of the possible reduction of AcPactivity.Not only clay but also the SOM content is important to soil

quality, because it also acts as a support for enzymeimmobilization 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 thisparameter is not the most relevant factor affecting AcP activity.This is an interesting result considering that the quality of theorganic matter is expected to change according to organic andconventional managements.31,32 In this sense, despite theprobable difference in the quality of SOM in the studied soils,this parameter did not significantly affect AcP activity. Differentamounts of SOM were reported by several authors according todifferent 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 ascalcium, magnesium, potassium, and some micronutrients. Inthis sense, these soils require amendments to allow sugar caneagriculture. The quantification of K+ and inorganic P+ amongthe tested soils showed that OM presented higher amounts ofthese 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 matterand nutrients, especially K+.33 The application of vinasse wasmore frequent in OM than in CM, because there wereapplications in 2008 (572 m3/ha) and 2010 (300 m3/ha),whereas in CM there was application only in 2010 (306 m3/ha).

The higher amount of vinasse applied in OM might be dueto OM requirements, because only the addition of organicfertilizers is allowed, and it would also be an attempt toovercome the lower productivity of OM due to the presence ofhigh amounts of nutrients in this residue. In addition, filtercake, another organic residue from the industrial processing ofsugar cane, is also added to OM soil as fertilizer. This residue isrich in P+, which contributes to its higher amount in OM.Then, the residual effect of vinasse and filter cake applicationover the years explains the high amount of some nutrients insoil, mainly K+ and P+.Potassium is a very important macronutrient for sugar cane

and is the most required nutrient by this culture. Among thefunctions of K+ in sugar cane is the productivity effect. Anadequate amount of K+ is important to the conversion of sugarsin sucrose,34 to increase the amount of carbohydrates, oils, andproteins, and to promote sugar and starch storage. It alsoregulates water use in plants and increases resistance during dryperiods. Additionally, an abundant amount of K+ may interferein the absorption of Ca2+ and Mg2+ by the plant.35 Thecorrelation 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 theamount of K+ between CM and NS (p > 0.05). Cerrado soil ispoor in macronutrients such as potassium, and its vegetation isadapted to this condition. Haridasan36 observed a low amountof 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 isadded to the soil by different fertilization strategies used in OMand CM.Shaviv et al.37 reported that continuous addition of fertilizers

in cultivated soils may cause a saturation effect, which mayreduce its ability to fix the potassium added by fertilization.Therefore, the addition of fertilizers does not solve the problemof low nutrient content in soils because other factors in thecomposition must be considered.In this scenario, the importance of soil particles is

demonstrated by PCA. There was a positive correlationbetween K+ and sand (r = 0.9) and a negative correlationbetween K+ and clay (r = −0.7) and between K+ and silt (r =−0.7). Murashkina et al.38 studied the fixation of potassium indifferent soil size particles and reported that in soils with apredominant proportion of sand, this is the most importantparticle in the fixation of potassium. Because Cerrado soilspresent a high proportion of sand, especially in OM, this maybe the explanation for the higher amount of potassium in thissoil 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 themanaged soils. OM presented the higher values among thetested soils. The effect of liming is also observed in the values ofsoil pH, which were higher in the cultivated areas (5.6 in CMand 5.7 in OM) than in the NS (4.9). This result explains partof the reduction on AcP activity in the cultivated soil (Figure3). Liming is the agricultural practice responsible for increasingsoil pH, and it explains the negative correlation between AcPactivity and Ca2+.

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The saturation of alkaline ions (V %) represents the capacityof a soil to exchange alkaline cations such as K+, Mg2+, andCa2+. Soil with V % >50% is considered rich in nutrients oreutrophic, whereas V % <50% is poor in nutrients.39 Theamounts of K+, Mg2+, and Ca2+ adsorbed on clay were higher inOM and CM (Table 2). This means that managed soils presentgreater capacity of exchanging cations than NS.However, it is important to point out that the value of V %

alone does not guarantee soil quality, as it does not reflect thebiochemical conditions of soil. Indeed, this result indicates thateven though there are nutrients in soil, they may not beabsorbed by plant nor perform their functions, because nutrientavailability in soils depends not only on the presence of suchnutrients but also on other soil conditions such as enzymeactivities.Moreover, these applications include nutrients ready for

plant absorption. Then, if they do not aggregate with other soilfractions, they only attenuate the problem of nutrient losses bythe agriculture practices and do not contribute to themaintenance and preservations of soil structure, biological,and biochemical characteristics that are important to preservesoil fertility.Considering NS, the low value of V % is a consequence of

the higher amount of H+ and Al3+ sorbed on clay than thealkaline cations, which is characteristic of acid soils.The acidity of soils influences the amount of Al3+ in it and

affects its availability because an acid environment allowsdissolution of Al3+ present in the clay structure. This processoccurs especially in clay type 1:1 of minerals and the Al3+

released is toxic to plants, mainly to the Poaceae family. Theaddition of limestone to these soils decreases the Al3+

availability by the formation and precipitation of Al(OH)3,which is not soluble in pH >5.5.40,41

Optimum pH. The optimum pH of a soil enzyme is notnecessarily the same as the soil’s. Nevertheless, enzyme activityis dependent on soil pH, because it affects the properionization, expression, enzyme stability, and release by soilmicroorganisms.13 In this sense, it is important to assay theoptimum pH for enzyme activity to establish the best catalysisconditions and to identify if the soil offers proper conditions forenzymatic catalysis.Aiming to evaluate the effect of pH on phosphomonoesterase

activity, tests were conducted on NS with pH ranging from 2 to11 (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 AcPand AkP, respectively. Despite the acid character of Cerradosoil, AkP were found in these samples. However, the AkPactivity is remarkably lower (53.3 U) than that in AcP (113.5U).It is important to note that although most reports in the

literature affirm that the optimum pH for assaying AcPs fromsoils is 6.5 or close to neutral15,42 acid phosphatases fromCerrado soil did not follow the same profile. The optimum pHfor AcP found in this study was 5.0, which is almost the samepH of the NS (pH 4.9).Acid and Alkaline Phosphomonoesterase Activity in

NS, OM, and CM. Another important result is that there wasno difference of AkP activity between the cultivated areas andNS. This result implies that management does not affect AkPactivity. The presence of AkP in acid soils such as soils fromCerrado is mainly from cell lyses of microorganisms and small

animals and was adsorbed on clay or other solid component ofsoil that preserved its activity.The lower activity of AkP is also explained because the

expression of acid and alkaline phophomonoesterases isinfluenced 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 toAkP. Therefore, AkP is not an effective enzyme to be used as abioindicator 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 thatthere was no difference between the two management practices,demonstrating that whatever the management is chosen, theenzyme activity is considerably affected.Previous studies showed that agricultural practices affected

biomass composition, enzyme activities, and organic andbiomass C and that phosphatase activity was significantlygreater in undisturbed soils compared to conventionalagriculture.43,44 Previous studies stated that a decrease inorganic soil disturbance could improve microbial activity and it,consequently, would increase the synthesis and secretion ofenzymes and that there were variations on enzyme activity dueto different groundcover crops and soil tillage, demonstratingthat enzymes are sensitive to soil disturbance.41,45

The activity found for AcP in NS from Cerrado (average of96 U g−1) was considerabley higher than that reported in theliterature. In the best conditions for AcP catalysis, themaximum 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 sandyUltisol soil (Parana, Brazil), the activity of AcP varied from 87to 176 U kg−1.41 AcP had its activity affected in differentmanagement practices with activity varying from 200 to 300 Ukg−1; in addition, in this study organic C also influenced AcPwith 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 givenin 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 literatureshowed that OM presented better chemical, biochemical, andbiological parameters compared to CM. Other studies havealready 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 permicroorganism biomass unity showed no difference betweenOM and CM. Moreover, biological parameters such as thepresence of arthropods sensitive to agriculture process washigher in CM than in OM.45

Another important observation is that the managed soils OMand CM did not favor any of the phosphatase forms (AcP andAkP) because the soil pH of OM and CM is higher than theoptimum pH for AcP and lower than the optimum pH for AkP,compromising enzyme synthesis in these soils.

Conclusions. All of the results presented showed theimportance of the identification of the physicochemicalcharacteristic changes by the sugar cane management affectingsoil enzymes. Reduction of AcP activity in both managementpractices, OM and CM, was observed. However, AkP activitywas not affected by management.

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Our results demonstrated that AcP is affected by soil pH andsoil particle size. This enzyme showed higher activity in lowerpH with optimum pH 5.0. Despite the acid characteristic ofCerrado soils, AkP was also found in the tested soil andpresented optimum pH 8.0. This finding highlights theimportance of research in soils because this ecosystem is stillnot well studied, especially with regard to the biological andbiochemical characteristics, which are highly influenced andhave 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 relatedwith the enzyme activity because these particles supportenzyme immobilization, contributing to the maintenance andprotection of enzyme structure and activity, which also explainsthe presence of AkP on these soils becausse they do not offerfavorable pH conditions for its secretion and hydrolysis.

■ AUTHOR INFORMATION

Corresponding Author*(L.L.A.P.) E-mail: luizaluanna@gmail.com. Phone: +5562-35211492.

FundingL. L. A. Purcena thanks FAPEGO and M. C. B. Di Medeirosthanks CAPES for fellowship support.

NotesThe authors declare no competing financial interest.

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