Abstract

The objective of this study was to monitor the degradation and obtain the mineralization fraction of anaerobically digested sludge, also known as digestate, under field conditions, when applied to the surface or incorporated into the soil. Sludge was applied to a dystrophic Inceptisol at a dose of 500 kg ha–1 yr–1 of total nitrogen, where the monitoring period of the mineralization process lasted 131 days. Samples of the soil-residue mixture were collected for analysis of the total organic carbon (TOC) and easily oxidizable organic carbon (OOC), total, ammonia, nitrate and organic nitrogen (ON). The annual mineralization fractions of the digestate, estimated based on the difference between the initial and final contents of TOC, OOC and ON in samples of the material collected, were 99.5 and 100%, respectively, when incorporated with the soil or applied to the soil surface.

INTRODUCTION

Sewage sludge production has increased in Brazil due to the increase in the number of installed sewage treatment plants, making it urgent to develop new technologies for treatment and final disposal of this waste that are safe and present minimal environmental impact.

As a final destination, the sludge incorporation in soil is the alternative with greatest potential because this material improves the chemical, physical and biological properties of soils, promoting an increase in agricultural productivity, and reducing costs of soil recovery (Ferrer et al. 2011). Agriculture has long recognized the benefits of waste materials as a nutrient source and as an amendment to improve the physical and chemical properties of soils (Singh & Agrawal 2008; Alvarenga et al. 2015).

Land application of sludge provides an opportunity for recycling of nutrients and reducing the amount of sludge disposed in landfills. According to Boeira et al. (2009), after application of organic wastes and sewage sludge to soil, some of these materials are oxidized and transformed into carbon dioxide and water, remaining organic matter fraction (humus), which has proven agronomic benefits since it improves the quality and productive potential of agricultural soils. Sewage sludge application to agricultural land can supply needed nutrients, but they are not fully available to plants (Gilmour et al. 2003). However, the organic bound nutrients are slowly mineralized by microorganisms which make them less susceptible to losses through leaching and surface runoff. Therefore, one must consider the availability and transportation costs, as well as application of the residue to agricultural areas and the proper rate to prevent pollution of water resources and even the soil itself.

The usual criteria for setting the annual application rate of sewage sludge in agricultural soils is based on the mineralization of this residue, evaluating mainly the nitrogen availability (Hernández et al. 2002; Gilmour et al. 2003; Doublet et al. 2010). The annual mineralization fraction of the sewage sludge, in turn, depends on the soil type in which it was incorporated, treatment and rate applied (Hernández et al. 2002; Gilmour et al. 2003; Parnaudeau et al. 2004). Sánchez-Monedero et al. (2004) concluded that stabilization of organic wastes through composting before soil application is advisable for the lower perturbation of soil equilibria status and the more efficient C mineralization or higher mineralization fraction.

Some of the processes that stabilize organic matter in the sewage sludge are aerobic digestion, anaerobic digestion, composting, chemical stabilization and thermal stabilization, and liming, all examples of sanitation processes. In CONAMA Resolution No. 375/2006 (Brasil 2006), the established mineralization fractions (percentage of the organic material mineralized in a year) are 40% for untreated primary and secondary sludge, 30% for aerobically digested sludge (DS), 20% for anaerobically DS (digestate) and 10% for composted sludge.

Determination of the organic waste mineralization fraction is, however, complex and influenced by many factors affecting the dynamics of C and N in the soil, such as edaphoclimatic conditions, characteristics of the residue and how it is spread on the soil (Paula et al. 2013). Although the application method is important in defining the mineralization fraction, in CONAMA resolution No. 375/2006 (Brasil 2006) this is not considered.

The general objective of this work was to monitor the degradation and obtain the mineralization fraction of organic material from anaerobically digested sewage sludge (digestate), under field conditions, when applied to the soil surface or incorporated into the soil.

METHODS

The experiment was conducted in an area of dystrophic Inceptisol, in the Experimental Area for Treatment of Urban Waste, Department of Agricultural Engineering, Federal University of Viçosa, Viçosa, Minas Gerais, Brazil; and all analyzes were carried out in the Laboratory of Soils and Solid Wastes, of the same department.

The Inceptsols are a soil order in the USDA soil taxonomy found in humid and sub-humid regions and are characterized by altered horizons that have lost bases or iron and aluminum, but which retain some weather-resistant minerals. Then, these soils are poor (dystrophic) in nutrients (Ca, Mg, and K) essential to plants, requiring fertilizing.

The anaerobically DS was collected at the ETE Arrudas, located in the city of Belo Horizonte and administered by the Minas Gerais Sanitation Company, where the sewage is treated in an activated sludge system, and the sludge generated is sent to an anaerobic digester.

Collected DS samples were analyzed chemically and physically, and subsequently the material, without having undergone any treatment or drying, was spread on the surface or incorporated into the soil. To characterize the receiving soil, the concentrations of total organic carbon (TOC) were quantified, calculated from the concentration of total volatile solids (TVS); easily oxidizable organic carbon (OOC), by oxidation with potassium dichromate in sulfuric medium; total nitrogen (TN), by the modified Kjeldahl method; total concentrations of phosphorus (P), potassium (K) and sodium (Na), extracted using the Mehlich-1 solution and quantified using spectrophotometer and flame photometer; exchangeable contents of calcium + magnesium (Ca+Mg), aluminum (Al3+), extracted with a 1 mol L–1 KCl solution, potential acidity (H+Al), extracted by 0.5 mol L–1 calcium acetate and quantified using titrimetric methods; as well as measures of the pH index and electrical conductivity (EC) of the suspension (ratio 1:2.5 for soil:water), using the potentiometric method (Matos 2012). Also analyzed was the granulometric composition of the soil by the pipette method, the standard method for soils, in Brazil (Matos 2012). For characterization of the sewage sludge, analyses were performed to determine the pH, water content, TOC and TN by the same methods already mentioned; NH4-N by the Kjeldahl method; NO3-N using the colorimetric method; total contents of P, K, Ca, Mg, sulfur (S) and sodium (Na) after nitric-perchloric digestion of the samples; in addition to the total solids (TS), total volatile solids (TVS) and total fixed solids (TFS), as defined in the APHA/AWWA/WEF (2005) and in Matos (2015). Table 1 presents the characterization of the soil and sludge used in the experiment.

Table 1

Chemical and physical characterization of the soil and the anaerobically DS (digestate)

Soil
Sewer sludge
VariableUnitValueVariableUnitValue
pH – 5.52 pH – 8.5 ± 0.0 
OOC (dag kg–1)* 0.88 OOC*** (g kg–1104.3 ± 11.3 
TOC (dag kg–11.06 TOC(v.s.)*** (g kg–1341.1 ± 2.1 
O.M (dag kg–11.82 TN*** (g kg–1127.8 ± 6.7 
TN (g kg–11.03 NO3--N*** (mg kg–116.2 ± 0.0 
EC (μS cm–1107.08 NH4+-N*** (g kg–1127.8 ± 27.0 
K (mg dm–357.2 C/N*** – 2.7 
Na (mg dm–3<0.01 Na*** (g kg–161.3 ± 0.0 
Ca+Mg (mmolc dm–328.6 K*** (g kg–19.6 ± 0.8 
P-avail (mg dm–352.58 P*** (g kg–120.5 ± 2.5 
Al3+ (mmolc dm–30.7 ST** (g L–17.8 ± 0.0 
H+Al (mmolc dm–374.1 TFS*** (g kg–1411.9 ± 3.6 
Clay (%) 43.0 TVS*** (g kg–1588.1 ± 3.6 
Silt (%) 12.0 MCwb*** (%) 99.2 ± 0.0 
Sand (%) 45.0 ρ** (kg dm–30.99 
ρ (g cm–31.07    
Soil
Sewer sludge
VariableUnitValueVariableUnitValue
pH – 5.52 pH – 8.5 ± 0.0 
OOC (dag kg–1)* 0.88 OOC*** (g kg–1104.3 ± 11.3 
TOC (dag kg–11.06 TOC(v.s.)*** (g kg–1341.1 ± 2.1 
O.M (dag kg–11.82 TN*** (g kg–1127.8 ± 6.7 
TN (g kg–11.03 NO3--N*** (mg kg–116.2 ± 0.0 
EC (μS cm–1107.08 NH4+-N*** (g kg–1127.8 ± 27.0 
K (mg dm–357.2 C/N*** – 2.7 
Na (mg dm–3<0.01 Na*** (g kg–161.3 ± 0.0 
Ca+Mg (mmolc dm–328.6 K*** (g kg–19.6 ± 0.8 
P-avail (mg dm–352.58 P*** (g kg–120.5 ± 2.5 
Al3+ (mmolc dm–30.7 ST** (g L–17.8 ± 0.0 
H+Al (mmolc dm–374.1 TFS*** (g kg–1411.9 ± 3.6 
Clay (%) 43.0 TVS*** (g kg–1588.1 ± 3.6 
Silt (%) 12.0 MCwb*** (%) 99.2 ± 0.0 
Sand (%) 45.0 ρ** (kg dm–30.99 
ρ (g cm–31.07    

OOC – easily oxidizable organic carbon; TOC(v.s.) total organic carbon, dry basis, equal to TVS/1.724; TN – total nitrogen; NO3-N – nitrate nitrogen; NH4+-N – ammonium nitrogen; C/NTOC/TN ratio; K – potassium; Ca – calcium; Mg – magnesium; Na – sodium; P – phosphorus; TS – Total solids; TFS – total fixed solids; TVS – total volatile solids; MCwb – moisture content on a wet basis; ρ – specific mass.

*Is the same as %.

**In relation to dry mass.

***In relation to fresh mass.

In the experimental area the soil was dug and removed from specific sites, forming pits, into which plastic pots were inserted measuring 0.20 m in height, 0.30 m in upper diameter and 0.20 m in lower diameter for a volume of approximately 10 L, perforated at the bottom and sides for contact of the soil contained in the pot with the surrounding soil. The soil removed for opening the pits was used to fill the pots buried in the soil. In these vessels two forms of DS application were adopted: incorporated, in which this material was homogeneously mixed with all soil contained in the pot, or spread on the soil surface, without incorporation. Each sludge application method was performed in five experimental units (buried pots). The DS dose applied to the soil was based on the application of 500 kg ha–1 of TN (3.81 Mg ha–1 or 3.5 L of DS per pot).

Samples were collected at 0, 14, 35, 66, 99 and 131 days after DS application to the soil, on the superficial layer 0–3 cm deep for the non-incorporated treatment, and at random points within the pots, at depths up to 20 cm, for the incorporated material. The samples were quantified with regards to concentrations of OOC, TOC, TN, NO3-N and NH4-N, and water content (Uwb), according to the methods described by Matos (2015). The ON was estimated by the differences of TN and NH4-N+NO3-N.

The soil temperature was monitored by measurements made with a digital thermocouple of the brand Incoferm, at 10 cm deep, in the treated soils to which the residue was incorporated, and by an infrared thermometer of the same brand in the treatment without incorporation of the residue.

The simple exponential model of first order chemical kinetics, proposed by Stanford & Smith (1972), was adopted to describe the mineralization of TOC (Equation (1)), OOC (Equation (2)) and ON (Equation (3)) in the soil:
formula
(1)
formula
(2)
formula
(3)
where TCO(min) is the concentration of degraded organic carbon at determined time t (dag kg–1 or %); TOC(0) is the initial concentration of mineralizable organic carbon in the soil (dag kg–1); Kc(T) is the mineralization coefficient of the TOC (d–1); Kc(eo) is the mineralization coefficient of the OOC (d–1); t is the time elapsed after incubation of the soil with the organic material (d); OOC(min) is the concentration of degraded mineralizable organic carbon at determined time t (dag kg–1); OOCo(0) is the initial concentration of mineralizable organic carbon in the soil (dag kg–1); ON(min) is the concentration of mineralized organic nitrogen at determined time t (mg kg–1); ON0 is the concentration of potentially mineralizable organic nitrogen in the soil (mg kg–1); and Kn is the mineralization coefficient of the ON (d–1).

The mineralization potential and mineralization coefficients were obtained after fitting of mathematical models by non-linear regression, using the software Sigma Plot 12.0.

The mineralization fraction of organic C and N was calculated by two methods (Paula et al. 2013), as described below.

Method 1: Mineralization fraction of organic C and N observed in the field (MF(ob)), calculated from Equations (4)–(6) which use the carbon and nitrogen concentration values quantified at the beginning and end of the process:
formula
(4)
formula
(5)
formula
(6)
where MFTOC(ob) is the mineralization fraction considering the TOC(i) of the sludge as a reference (%); TOC(i) is the total organic carbon of the sludge immediately after application to the soil (dag kg–1 or %); TOC(f) is the total organic carbon of the sludge at 131 days after application to the soil (dag kg–1); MFOOC(ob) is the mineralization fraction considering the OOC(i) of the sludge as a reference (%); OOC(i) is the easily oxidizable organic carbon of the sludge immediately after application to the soil (dag kg–1); OOC(f) is the easily oxidizable organic fraction of the sludge at 131 days after application to the soil (dag kg–1); MFON(ob) is the mineralization fraction considering the ON(i) of the residue as a reference (%); ON(i) is the organic nitrogen of the sludge immediately after application to the soil (mg kg–1); ON(f) is the organic nitrogen of the soil at 131 days after application to the soil (mg kg–1).
Method 2: Mineralization fraction of the adjusted organic C and N (MF(aj)), calculated by means of Equations (7)–(9), according to the carbon and nitrogen concentration values based on the adjusted exponential equations, considering the potentially mineralizable TOC(min), OOC(min) and ON(min) of the sludge samples as a reference:
formula
(7)
formula
(8)
formula
(9)
where FMTOC(aj) is the mineralization fraction calculated based on the adjusted exponential equations, considering the potentially oxidizable TOC of the sludge as a reference (%); TOC(min) is the accumulated concentration of TOC of the mineralized sludge during the 131 experimental days (dag kg–1 or %); TOC(0) is the potentially mineralizable TOC of the sludge (dag kg–1); MFOOC(aj) is the mineralization fraction calculated from the adjusted exponential equations, considering potentially mineralizable OOC of the sludge as a reference (%); OOC(min) is the accumulated concentration of OOC of the mineralized sludge after 131 experimental days (dag kg–1); OOC(0) is the potentially mineralizable OOC of the sludge (dag kg–1); MFON(aj) is the mineralization fraction calculated from the adjusted exponential equations, considering the potentially mineralizable ON of the sludge as a reference (%); ON(min) is the accumulated concentration of ON of the mineralized sludge after 131 experimental days (mg kg–1); ON0 is the potentially mineralizable ON of the sludge (mg kg–1).

For calculation of the mineralization fractions and adjusts of the experimental equations, the concentrations of OOC, TOC and ON measured in the field were adopted, after subtracting the concentrations of OOC, TOC and ON of the control soil, except in the situation where the residue was applied superficially. This is because the sample in the case of incorporated application contained both soil and waste. In all samples collected at 66, 99 and 131 days after application of the residues to the soil, also subtracted were the values of OOC, TOC and ON of the control soil. When DS was applied superficially the thickness of the sludge layer decreased with the degradation, and more soil was collected next to the sludge. For this, the soil:sludge ratio in the samples was considered in the calculations.

RESULTS AND DISCUSSION

While conducting the field experiment, the water content and temperature at the soil surface varied, respectively, from 11 to 381 g kg–1 and from 24.2 to 42.2 °C. On the other hand, at the average soil depth in which the residue was incorporated, the water content and temperature ranged from 152 to 221 g kg–1 and from 21.0 to 36.5 °C, respectively. Although for short periods of time the surface temperature exceeded 40 °C and the water content was very low (11 g kg–1), it can be considered that in general the environmental conditions favored the degradation of organic material on the soil. According to Costa & Sangakkara (2006) for soil temperatures between 5 and 35 °C degradation is accelerated, provided that the temperature does not exceed 40 °C.

Figure 1 shows the accumulated concentration curves of mineralization as a function of the monitoring time. It is observed that the simple exponential model of first-order kinetics, proposed by Stanford & Smith (1972), adjusted well to the data obtained in the two treatments for a 5% minimum significance level of the coefficients.

Figure 1

Cumulative concentration of total mineralized organic carbon ((a) and (b)), mineralized easily oxidizable organic carbon ((c) and (d)) and mineralized organic nitrogen ((e) and (f)) of the DS (digestate), respectively, integrated with the soil ((a), (c) and (e)) and applied to the soil surface ((b), (d) and (f)), and adjusted equations for the 131 day monitoring period. Note: dag kg–1 is the same as %.

Figure 1

Cumulative concentration of total mineralized organic carbon ((a) and (b)), mineralized easily oxidizable organic carbon ((c) and (d)) and mineralized organic nitrogen ((e) and (f)) of the DS (digestate), respectively, integrated with the soil ((a), (c) and (e)) and applied to the soil surface ((b), (d) and (f)), and adjusted equations for the 131 day monitoring period. Note: dag kg–1 is the same as %.

Analyzing the adjusted equations, it can be verified that the DS applied to the soil surface presented mineralization coefficients for TOC, OOC and ON of 1.3, 1.9 and 2.5 times larger than the ratios obtained when it was incorporated, considering that the DS showed great mineralization potential due to the large concentration of OOC (Table 1), where its exposure to aerobic conditions on the soil surface facilitated its degradation.

Pereira et al. (2015) estimated values of Kc(eo) and Kn for peach palm residues incorporated into the soil under field conditions, which were approximately 2.7 and 2.3 times higher than the values obtained for the residue applied to the surface during 112 days of monitoring. These authors explained that the peach palm residue presents a high C/N ratio (51.8), its mineralization was strongly dependent on nitrogen in the soil, therefore with incorporation of this residue, the greater nitrogen availability in the medium resulted in more intense action of soil microorganisms for its degradation.

The values of mineralization coefficient of the OOC (Kc(eo)) obtained in this study (0.0359 d–1) are close to those reported by Torri et al. (2003), analyzing sewage sludge incorporated in typical argentine acid soils, obtained Kc(eo) of 0.030–0.035 d–1.

The mineralization coefficient of the ON (Kn) obtained in this study for the superficial mineralization (0.0367 d–1) are smaller than the value reported by Moore et al. (2004), from swine wastewater: 0.070 in the fall; 0.075 in the winter; 0.22 in the spring; and 0.36 d–1 in the summer.

Table 2 shows the parameters of the adjusted equations for TOC(min), OOC(min) and ON(min) in each form of DS applied to the soil, accumulated during 131 days of monitoring. Also shown are the estimated values of the mineralization fractions of TOC, OOC and ON calculated using Method 1 (Equations (4)–(6)), which is based on mean concentrations observed in the field, and those calculated using method 2 (Equations (7)–(9)), which is based on carbon and nitrogen concentrations estimated using the adjusted exponential equations, considering the mineralizable potential of OOC, TOC and ON of the sludge as a reference. As can be observed, the mineralization fractions of TOC and OOC for DS incorporated or applied to the surface, calculated by both methods in a merely descriptive analysis, may be considered similar to each other, however the mineralization fractions of ON, calculated based on the same methods, were different.

Table 2

Parameters of the first order kinetic equations for degradation of total organic carbon (TOC), easily oxidizable organic carbon (OOC) and organic nitrogen (ON), the adjusting parameters of the equation (TOC(0), OOC(0), ON0) and mineralization coefficientsKc(T), Kc(e)andKn), as well as the adjusted (MFTOC(aj), MFOOC(aj)andMFON(aj)) and observed mineralization fractions (MFTOC(ob), MFOOC(ob)andMFON(ob)), in soil to which the anaerobically digested sewer sludge (digestate) was incorporated to the soil or applied to the soil surface and monitored for a period of 131 days

TOC 
 Application method OC(0) Kc(T) R2 TOC(min) MFTOC(aj) MFTOC(ob) 
(dag kg–1)a (d–1– (dag kg–1(%) (%) 
 Incorporated 1.2609 0.0256 0.96 1.22 96.5 71.6 
 Surface 9.5296 0.0335 0.94 9.41 98.8 99.9 
OOC 
 Application method OOC(0) Kc(eo) R2 OOC(min) MFOOC(aj) MFOOC(ob) 
(dag kg–1(d–1– (dag kg–1(%) (%) 
 Incorporated 0.7752 0.0359 0.99 0.77 99.1 91.1 
 Surface 3.7179 0.0691 0.98 3.72 100.0 99.8 
ON 
 Application method ON0 Kn R2 ON(min) MFON(aj) MFON(ob) 
(mg kg–1(d–1– (mg kg–1(%) (%) 
 Incorporated 435.9401 0.0147 0.96 372.39 85.4 63.1 
 Surface 2580.3836 0.0367 0.94 2559.31 99.2 88.0 
TOC 
 Application method OC(0) Kc(T) R2 TOC(min) MFTOC(aj) MFTOC(ob) 
(dag kg–1)a (d–1– (dag kg–1(%) (%) 
 Incorporated 1.2609 0.0256 0.96 1.22 96.5 71.6 
 Surface 9.5296 0.0335 0.94 9.41 98.8 99.9 
OOC 
 Application method OOC(0) Kc(eo) R2 OOC(min) MFOOC(aj) MFOOC(ob) 
(dag kg–1(d–1– (dag kg–1(%) (%) 
 Incorporated 0.7752 0.0359 0.99 0.77 99.1 91.1 
 Surface 3.7179 0.0691 0.98 3.72 100.0 99.8 
ON 
 Application method ON0 Kn R2 ON(min) MFON(aj) MFON(ob) 
(mg kg–1(d–1– (mg kg–1(%) (%) 
 Incorporated 435.9401 0.0147 0.96 372.39 85.4 63.1 
 Surface 2580.3836 0.0367 0.94 2559.31 99.2 88.0 

aEquivalent to mass/mass in percentage.

Considering the results obtained by monitoring the TOC, the mineralization fractions observed in the DS, when applied superficially or incorporated in the soil, after 131 days of degradation were respectively 99.9 and 71.6%, regarding the OOC were 99.8 and 91.1%, and when referring to the ON were 88.0 and 63.1%, values that are high. These results confirm that high rates of mineralization of organic materials should be expected in tropical climatic conditions, even in previously digested sewage sludge.

Pereira et al. (2015) found mineralization fractions of OOC and ON for the peach palm residue equal to 93.5 and 95.3% when it was integrated into the soil, and 59.8 and 62.7% when applied to the surface, respectively, after 102 days of monitoring. The authors credited the higher values of mineralization to the residue incorporating into the soil, which provided better contact with the soil, allowing greater degradation when compared with that obtained in the residue spread on the soil surface. However, Paula et al. (2013) found higher values of MFOOC and MFON for the anaerobic sludge when applied to the soil surface when compared to its incorporation into the soil. These authors observed that although the proportion of N was applied to the surface via the residues, this chemical element was more concentrated on the surface, which facilitated degradation of the residue. The authors attribute the overestimation of the mineralization rate to the difficulty of collecting samples containing only organic residues.

Using the adjusted equations, the mineralization fractions of TOC, OOC and ON were estimated for 40, 131 and 365 days of degradation, and the results are shown in Table 3. Based on the data presented, the DS, independent of the application method, showed mineralization fractions greater than 50%, where the MFOOC(aj) at 40 days was greater than the MFTOC(aj) and MFONaj, i.e. the concentration of OOC decreased faster than that of TOC and ON at the end of 40 days. Boeira et al. (2002) found a mineralization fraction of 31%.

Table 3

Mineralization fractions of the TOC, OOC and ON, the anaerobically digested sewer sludge (digestate) either incorporated with the soil or applied to the surface, calculated using the audited equations for different degradation periods

TOC 
 Application method 40 days 131 days 365 days 
TCO(min) MFTOC(aj) TOC(min) MFTOC(aj) TCO(min) MFTOC(aj) 
(dag kg–1)a (%) (dag kg–1(%) (dag kg–1(%) 
 Incorporated 0.81 64.1 1.22 96.5 1.26 100.0 
 Surface 7.03 73.8 9.41 98.8 9.53 100.0 
OOC 
 Application method 40 days 131 days 365 days 
OOC(min) MFOOC(aj) OOC(min) MFOOC(aj) OOC(min) MFOOC(aj) 
(dag kg–1(%) (dag kg–1(%) (dag kg–1(%) 
 Incorporated 0.59 76.2 0.77 99.1 0.78 100.0 
 Surface 3.48 93.7 3.72 100.0 3.72 100.0 
ON 
 Application method 40 days 131 days 365 days 
ON(min) MFON(aj) ON(min) MFON(aj) ON(min) MFON(aj) 
(mg kg–1(%) (mg kg–1(%) (mg kg–1(%) 
 Incorporated 193.80 44.5 372.39 85.4 433.90 99.5 
 Surface 1985.90 77.0 2559.31 99.2 2580.38 100.0 
TOC 
 Application method 40 days 131 days 365 days 
TCO(min) MFTOC(aj) TOC(min) MFTOC(aj) TCO(min) MFTOC(aj) 
(dag kg–1)a (%) (dag kg–1(%) (dag kg–1(%) 
 Incorporated 0.81 64.1 1.22 96.5 1.26 100.0 
 Surface 7.03 73.8 9.41 98.8 9.53 100.0 
OOC 
 Application method 40 days 131 days 365 days 
OOC(min) MFOOC(aj) OOC(min) MFOOC(aj) OOC(min) MFOOC(aj) 
(dag kg–1(%) (dag kg–1(%) (dag kg–1(%) 
 Incorporated 0.59 76.2 0.77 99.1 0.78 100.0 
 Surface 3.48 93.7 3.72 100.0 3.72 100.0 
ON 
 Application method 40 days 131 days 365 days 
ON(min) MFON(aj) ON(min) MFON(aj) ON(min) MFON(aj) 
(mg kg–1(%) (mg kg–1(%) (mg kg–1(%) 
 Incorporated 193.80 44.5 372.39 85.4 433.90 99.5 
 Surface 1985.90 77.0 2559.31 99.2 2580.38 100.0 

aEquivalent to mass/mass in percentage.

Boeira et al. (2009) reported mineralization fractions values between 12 and 19% and Andrade et al. (2005) between 7 and 22% for domestic sewage sludge, quantified in laboratory conditions without specification of the degradation time. In this study, for similar materials the mineralization fraction was greater than 63% in the monitoring period of 131 days. Oba & Nguyen (1982) obtained MFON 55.8% after incorporation and incubation of not limed sewage sludge for 30 days, in oxidic soil. Hattori & Mukai (1986) obtained MFTOC of 20–44%, while the MFON ranged from 15 to 28% during the sewage sludge incubation incorporated into the soil, for a period of eight weeks. Using sewage sludge processed in an extended aerobical sewage activation system and continuous flow, Giacomini et al. (2015) obtained, respectively, 45.3% and about 60% of the MFTOC and MFON, after 110 days of its incubation in a sandy soil. This value of MFON is twice as high as that defined by CONAMA (2006) for aerobically digested sewage sludge and it is at the upper limit of a wide range of mineralization indices (24–59%) determined in many different studies where aerobically treated sludge was used (Hernández et al. 2002; Gilmour et al. 2003; Doublet et al. 2010). Moretti et al. (2013), when analyzing the mineralization fraction of compost produced from a mixture of digested domestic sewage sludge with tree prunings, in the laboratory, incorporated into the soil at a dose of 14.3 Mg ha–1, obtained an adjusted organic nitrogen mineralization fraction of 11.7% after 147 days of material incubation. In this case, the low mineralization rates obtained are potentially attributed to the higher carbon:nitrogen relationship (C:N ratio) and the stability of the material used by these authors, in addition to the fact that they were quantified in laboratory conditions.

In estimates of the mineralization fraction at 365 days, values greater than 97% are obtained, independent of the reference used (TOC, OOC and ON) and application method of the organic residue in the soil. The highest mineralization fractions estimated in this study, as in that of Pereira et al. (2015), are due to several factors which occur in field conditions, such as: greater volume of soil in contact with the residue, being an open system which allows for free flow of solutes between the media, and greater interaction of the residue with the environment, allowing soil meso-organisms (insects, arthropods) to also act in the decomposition process, in addition to edaphoclimatic conditions such as rainfall and incident solar radiation.

The results obtained in this study differ from what is established in CONAMA Resolution No. 375/2006, in which it is stated that mineralization fractions of 20% should be considered for anaerobically DS, independent of the method in which it is applied to the soil. Although it is known that the dystrophic Inceptisol evaluated in this work is not a dominant class in Brazilian agricultural areas, it is understood that the results may be extrapolated to the dominant classes (Oxisols and Argisols), at least at the level of mineralization fraction ranges for the residue, since not even in legislation on soil classes is there a differentiating factor of this variable.

In several studies conducted in Brazil on the quantification of sewage sludge degradation, higher values were found than those defined in the referred legislation. The results estimated in this study are due to the occurrence of various field conditions, including: greater volume of soil in contact with the residue, being an open system which allows for free flow of solutes between the media, greater interaction of the residue with the environment, allowing soil meso-organisms (insects, arthropods) to also act in the decomposition process, in addition to edaphoclimatic conditions such as rainfall and incident solar radiation.

Pereira et al. (2015) obtained estimated values of Kc(eo) for the residue incorporated to the soil and applied to the surface of 30 and 20 times greater than the values obtained in laboratory conditions. With regards to Kn, it was estimated that the incorporated residue and that applied to the surface were, respectively, about 3.2 and 1.6 times greater than the same application methods under laboratory conditions. These results corroborate that it is necessary to estimate the mineralization fractions of organic residues in the field, since such fractions in the laboratory do not correspond to reality.

CONCLUSIONS

Based on the mineralization coefficients and mineralization fractions of TOC, OOC and ON, it was found that the anaerobically digested sewage sludge (digestate) mineralized faster when deposited on the surface than when incorporated into the soil. It is recommended that the values of digestate mineralization fractions, established in CONAMA Resolution No. 375/2006, are increased considerably, suggesting that a value of 97% is considered, independent of the application method, for the dystrophic Inceptisol.

ACKNOWLEDGEMENTS

The authors thank the ‘Conselho Nacional de Desenvolvimento Científico e Tecnológico’ (CNPq) and ‘Fundação de Amparo à Pesquisa de Minas Gerais’ (FAPEMIG) for financial support.

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