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Journal of soil science and plant nutrition

On-line version ISSN 0718-9516

J. Soil Sci. Plant Nutr. vol.12 no.3 Temuco Sept. 2012  Epub Aug 31, 2012 

Journal of Soil Science and Plant Nutrition, 2012, 12 (3), 441-450


Effects of the application of biosolids on some chemical, biological and physical properties in an Andisol from southern Chile


I. Salazar1*, D. Millar2, V. Lara2, M. Nuñez2, M. Parada2, M. Alvear1 and J. Baraona2

1 Departamento de Ciencias Químicas y Recursos Naturales, Facultad de Ingeniería, Ciencias y Administración, Universidad de La Frontera, Casilla 54-D, Temuco, Chile.
* Corresponding author:
2Departamento de Ciencias Agronómicas y Recursos Naturales, Facultad de Ciencias Agropecuarias y Forestales, Universidad de La Frontera, Casilla 54-D. Temuco. Chile.


An important issue for soil conservation and waste disposal is the use of biosolids in agriculture and for land reclamation. Most biosolids contain valuable nutrients that could be used to improve soil fertility. The aim of this work was to establish the effects of the application of biosolids on some chemical, biological and physical properties in an Andisol from southern Chile. This study was carried out under farm conditions during the wheat (Triticum aestivum) growing cycle. Five land plots of 10 x 4 m each were prepared: control soil (TAT), chemical fertilisation (TAF) and applications equivalent to 30 ton biosolids/ha (TA30), 60 ton biosolids/ ha (TA60) and 90 ton biosolids ha-1 (TA90). The soil samples of both treatment and non-treatment groups were collected after the application of biosolids, after sowing and before the wheat harvest. The evaluated chemical soil parameters were modified by the application of biosolids. Base levels of organic carbon, pH and P increased. Biological activity measured by the evolution of C-CO2 followed the sequence TA90>TA60>TA30>TAF>TAT. The physical properties evaluated, namely water infiltration rate, accumulated infiltration and water retention capacity, all increased with the increasing dose of biosolids. The results show a strong correlation between the biosolids dose applied and the change in the soil properties studied.

Keywords: Biosolids, Organic matter, Biosolids application, Organic amendment.

1. Introduction

Agricultural practices often lead to a gradual decrease in the organic matter content of the soil and result in a decrease in soil fertility because of impoverishment of its physical, chemical and biological properties (Perucci et al., 1997). These agricultural practices have an impact on the soil microflora, which is important for understanding the biological and biochemical processes that contribute to soil fertility.

Biological activity is influenced by the transformation of organic matter, soil degradation and soil structure along with other factors. Biological activity was evaluated through CO2 evolution, and a high correlation was shown between respiration, microbial biomass and the carbon content of the soil (Witter et al., 1993).

Humans have used organic waste since ancient times to fertilise the soil used for agriculture and livestock production (Castro et al., 2007). Water purification produces large amounts of waste or sludge, the disposal of which is important because some of its components may be converted into nutrients or pollutants that can have a positive or negative impact, respectively, on the environment.

Agricultural use is a satisfactory alternative that can provide a double benefit:

1) elimination of urban organic waste from the environment without adverse effects on the ecological balance, and

2) incorporation of the organic matter and its nutrients into commercially cultivated soil (Celis et al., 2006).

Sludge and other waste biosolids are rich in organic matter that contains sufficient nitrogen and phosphorus to make them potentially useful as fertilisers and sources of organic matter (Mazzarino, 2003). They can also improve the physical properties of the soil, such as the bulk density, the porous structure and water retention, all of which may be reflected in an increase in crop yields (Barbarick et al., 2004). It is known that the practice of overworking cultivated areas causes a progressive loss of organic matter in the soil with damaging consequences: diminished fertility and topsoil, reduced nutrient retention capacity and erosion. These may become irreversible in some zones with serious short and long-term environmental consequences potentially affecting their future viability.

This research provides information about the effects of the application of biosolids to the soil at different doses (30, 60, and 90 ton biosolids/ha) on the soil's biological, physical and chemical properties in one wheat crop cycle.


2. Materials and methods

Experimental plots of land measuring 10 x 4 m and with slopes <2% were established in Los Prados Serie Andisols, 38° 58'21" South and 72° 7'21.7" West. The land had been meadowland for at least four years. The approximate average temperature was 10.5° C. The climate range was between moderately warm and rainy with 1,200 mm annual precipitation.

The study included five treatments. There were two untreated soils: one control without the application of fertiliser (TAT) and the other fertilised with 735 kg ha-1 triple super phosphate and 920 kg ha-1 of potassium sulphate (TAF). The other three soils received doses of biosolids from the Water Treatment Plant, Vilcún, equivalent to 30, 60 and 90 ton dry matter ha-1 (TA30, TA60 and TA90, respectively). The plots were divided into experimental units of 13 m2 surface area. Treatments were performed in triplicate. The crop used was Triticum aestivum, var. Quijote. Three soil samples were taken from each plot before and after sowing and before harvesting in the summer period. In each sample, chemical, active biological and physical parameters were determined (Sadzawka et al, 2004). Chemical analyses of the soil, the biosolids and the soil under treatment were performed according to the regulations for the management of sludge generated at sewage treatment plants (Supreme Decree N° 004). Biological activity was evaluated based on the CO2 evolution (Stotzky, 1965) for 50 g of each soil sample (humidity 60%) incubated at 25± 2° C and trapped in 15 mL of NaOH 0.1 M. Carbonate was precipitated with BaCl2 0.375 M, and NaOH residue was titrated with HCl 0.1 M.

The global microbiological activity was determined based on the accumulated values of C-CO2 over a period of time according to the potential model used by Dommergues (1968). The evolution of organic C to CO2 is expressed according to the formula C = k*tm, where C is the loss in mg of organic C with CO2 accumulated over time for 100 g of dry soil; k is the first day constant emission in mg of C-CO2/100 g-1 of dry soil; t is the number of days; and m is the constant that determines the form of the curve. The values of k and m were obtained by applying the simple regression of the linear equation. The microbiological activity was calculated according to Levi-Minzi et al. (1990) and was the result of k*m.

The accumulated water infiltration in the soil and the infiltration rate were measured using an infiltrometer (a metal cylinder measuring 20 cm diameter by 30 cm length). The water retention in the soil at depths of 20 and 60 cm was determined every 15 days according to the gravimetric content. The field water capacity (FWC) was equal to retention of 1/3 atm, corresponding to the maximum water retention capacity of the soil. The permanent wilting point (PWP) was equal to 15 atm, corresponding to the minimum water content available for cultivation.


3. Results and discussion

Table 1 shows the results of the biosolids analysis used in this investigation. The total organic carbon (TOC) content is 25 ± 5 mg kg-1 and supplies organic C to the soil. The pH(H2O) of the raw sludge was 7.2 ± 0.8, which rose to pH 10.8 ± 0.8 with the addition of lime. The biosolids used in this trial were stabilised (with the addition of lime) according to Chilean law (Supreme Decree N° 004). This represents a 66.7% increase in the pH value because of the effect of the lime on the raw sludge. The lime sanitises the sludge while providing calcium to the soil under investigation (pH(HO) 5.7 ± 0.1) (Table 2). This assists the production of leaf biomass in the unit. The addition of biosolids to the soil with the exchange base contents, nitrogen, phosphorus, boron and heavy metals at the micronutrient level makes it economically interesting as a fertiliser supplement.

As shown in Table 2, the chemical parameters of the control soil (TAT), TOC and bases increase proportionally with the dose of biosolids from 30 to 90 ton ha-1. This contrasts with the fertilised soil (TAF), which shows no change except for an increase in the base phosphorus level. The increase in the base phosphorus level is attributable to the high content of this element in the biosolids. The reference ground (TAT) contained 5.3 ppm P, and the fertilised soil (TAF) had an increase in P of 2 ppm and applying fertiliser (TAF) P level increased by 2 ppm P, also At the end of the first year of the wheat cycle, the application of 30, 60 and 90 ton biosolids/ha resulted in increases of 16, 23 and 44 ppm P, respectively, compared with the control.

Figure 1 presents the C-CO2 evolution in soil fertilised with the three treatments: TA30, TA60 and TA90, 15 days after incorporation of biosolids, the soil fertilised with chemical fertiliser (TAF) and the control soil (TAT). There are no great differences compared to the TAT, indicating that the incorporation of the biosolids into the soil requires more time or that some effects may be outside the parameters measured. The system requires an initial stabilisation period because of the microbial stress produced by the change in pH and the new microenvironmental conditions affecting the native microbes. This was confirmed by the high levels of C-CO2 produced 15 days after initiation of the experiments in the five systems when no great differences were observed among them (Alvarez et al., 1999, Torri et al., 2003).


The condition of the treated soil system changed one month after treatment and after being sown, which helped biosolids-soil homogenisation.

Figure 2 shows the sequence of C-CO2 evolution: TA90>TA60>TA30>TAT>TAF. For time periods less than 6 days, the fertilised soil (TAF) presented less microbial respiration because of the presence of chemical fertilisers; in systems with added biosolids where there was additional organic matter available as a source of energy for microorganisms, microbial respiration increased.

Figure 3 shows that four months after the incorporation of biosolids into the soil and before harvesting, TA90, in particular, presented the highest level of microbial activity. The relationship shown in Figure 2 confirms the high availability of labile organic matter contributed by the biosolids. This activity is proportional to the amount of additional carbon.

The accumulated C-CO2 values over incubation time, according to the equation C = k*tm proposed by Dommergues (1968), were adjusted and linearized to obtain the values of k and m by simple regression and thereby permitting microbiological activity to be known. Table 3 shows the high CO2 emission on the first day of incubation, which is represented by the k values of the treated units: 537.6 mgC-CO2 /100 g dry soil for TA90; 555.3 mgC-CO2 /100 g dry soil for TA60; and 374.2 mgC-CO2 /100 g dry soil for TA30. These were followed by the TAT where the native microorganisms have not been altered by either positive or adverse environmental conditions. In contrast, the unit with added chemicals (TAF) presented lower C-CO2 emissions on the first day of incubation.

The global microbiological activity, calculated as the product of the parameters k and m, is presented in Table 4, which shows in numerical form the same information presented in the previous figures and in Table 3. The highest microbiological activity was observed in the treated units, followed by TAT. The least activity occurred in the fertilised units (TAF). TAF also presented strongly diminished activity before the harvest, most likely because of the exhaustion of the labile organic carbon available for microorganisms (García-Gil et al., 2004).

Regarding the physical properties evaluated, Moffet et al. (2005) observed that both the basic infiltration rate and cumulative infiltration increased at the highest dose of application of biosolids on degraded soils. This phenomenon coincided with the results obtained in this research, which showed significant differences (Figure 4) between the control and treatments of 60 and 90 ton biosolids ha-1.

The basic infiltration rate (steady state infiltration rate) (Figure 5) was higher in the treatments with higher biosolids doses, reflecting the facilitation of the entrance of water into the soil by the biosolids. The results also suggest a possible reduction in the risk of erosion in exposed soils.

There was no effect on the water retention properties of the soil in the first year of biosolids application. The values available in the literature for the parameters field water capacity and permanent wilting point (CIREN, 2002) are frequently high in Andisols in the Araucanía Region in Chile and become higher at greater depth (Table 5).


When the evolution of the gravimetric water content at 20 cm soil depth in dry conditions (rainfall only, without seasonal irrigation) is analysed based on the sample data (Figure 6), the treatments with biosolids tend to present greater humidity than the treated test units. Data from the latest sampling dates had values close to PWP and show the benefits of the application of biosolids because this application generates a hydric support condition for the plants and reduces the risk of water stress. The treatments TA60 and TA90 had water content superior to FW and close to saturation, which may indicate some risk of excess humidity in the surface zone of the soil where aeration is normally required for cultivation. The permanence of this system over time needs to be evaluated to provide further support for treatment with biosolids (Crush et al., 2005).

The FWC, PWP and available water content of the different treatments were statistically similar, indicating that the effect of the treatments on these variables would be longer-term.

There were significant differences between these parameters at 20 and 60 cm soil depth because of the higher water retention capacity, which is attributable to the higher percentage of clay at depth (19.1% clay at 20 cm depth and 24.1% clay at 60 cm depth).

The evolution of the water content at 60 cm depth (Figure 7) did not differ much with the various treatments, except on the latest sampling dates when the treated units were close to PWP. There is a marked tendency towards water contents close to saturation at 20 cm depth in treatments with a larger dose of bio-solids, which interestingly is not noted in this deeper layer.

4. Conclusion

The incorporation of biosolids into Los Prados Serie Andisols in the Araucanía Region, Chile shows changes in the biological, physical and chemical parameters of the soil, including increased nutritional content; greater biological activity; and an improvement in the physical parameters of the soil. The latter is expressed primarily in increased water retention and infiltrometric properties of the soil with benefits for cultivation. It should be noted that the possibility of excess water retention and risk of anoxia from the application of high doses of biosolids was not assessed.



This investigation was financed by the DIUFRO Project N°. 120620 and Empresa Aguas Araucanía, S.A. Temuco. It was also financed by Project DIUFRO N° IN-13-00 and DIUFRO N° 2004.



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