In most Brazilian cities sewage sludge is dumped into sanitary landfills, even though its use in forest plantations as a fertilizer and soil conditioner might be an interesting option. Sewage sludge applications might reduce the amounts of mineral fertilizers needed to sustain the productivity on infertile tropical soils. However, sewage sludge must be applied with care to crops to avoid soil and water pollution. The aim of our study was to assess the effects of dry and wet sewage sludges on the growth and nutrient cycling of
Sewage sludge resulting from the treatment of urban liquid residue, channeled to treatment stations through the sewage system, is a residue rich in organic matter. This sludge corresponds to only 1% of the volume of sewage waste, but the treatment and final disposal represents 20 to 40% of the operational costs of a treatment station [
Applying organic matter at the soil surface to improve its fertility is a traditional practice. Sewage sludge use in agricultural production systems has become an interesting alternative to discarding it, as it may increase overall crop production [
Studies have been carried out worldwide from the early 1970s to assess the effectiveness of applying organic waste residues to forest areas [
Previous research suggests that the application of sewage sludge might significantly improve the economic performance of forest plantations due to increases in wood production [
Litterfall production is one of the main ways of nutrient transfer within the biogeochemical cycle in forest ecosystems and feeds the nutrient stocks in the litter layer accumulated in adult plantations [
The chemical composition of sewage sludge depends on the source from which it has been generated, such as industrial or residential facilities and the processes used in sewage treatment stations. The São Paulo Sanitation Company (Companhia de Saneamento Básico—SABESP, Brazil) started in 2002 an experimental process to thermally dry the sewage sludge produced in the São Paulo metropolitan area. Thermal drying is an expensive operation, but it significantly reduces the water content in the sludge, thus reducing transportation and field application costs. Furthermore, drying the sewage improves the quality of the sludge as it eliminates pathogenic microorganisms. Such aspects are important when the dried sludge is aimed at agro forest systems [
The objective of our study was to gain insight into the effects of the application of sewage sludge on the growth and nutrition of eucalypt plantations over the first half of the rotation. The effects of sludge applications (wet sewage sludge or dry sludge) on biomass production, leaf litterfall, and nutrient cycling were compared to mineral fertilizer applications (standard forestry practice of the region) and to a control treatment without nutrient addition.
This study was conducted at the Itatinga Experimental Station, University of São Paulo, São Paulo State, Brazil (23°02′ S, 48°38′ W and 830 m altitude). The natural vegetation of the region is an arboreal savannah (Cerrado). The climate is Cfa according to the Köppen classification, the average annual precipitation is 1370 mm, and the average temperature is 19.2°C (1990 to 2004). The relief is typical of the São Paulo Western Plateau, with a topography ranging from flat to hilly.
The study area is characterized by very deep (>12 m) Ferralsols [
Soil analysis of the experimental area before planting.
Exchangeable cations | ||||||||||||
Depth (cm) | P(1) | pH | K | Ca | Mg | CEC | S- | B(2) | Cu(2) | Fe(2) | Mn(2) | Zn(2) |
Mg g−1 | CaCl2 | mmolc kg−1 | mg kg−1 | |||||||||
0–5 | 3 | 4.0 | 0.2 | 1.7 | 1.3 | 27 | 24 | 0.08 | 0.27 | 35 | 0.9 | 0.27 |
5–10 | 2 | 4.0 | 0.2 | 1.0 | 0.7 | 21 | 30 | 0.07 | 0.20 | 25 | 0.4 | 0.13 |
10–20 | 2 | 4.0 | 0.2 | 0.8 | 0.7 | 17 | 34 | 0.06 | 0.23 | 18 | 0.2 | 0.10 |
20–50 | 2 | 3.8 | 0.0 | 0.8 | 0.4 | 14 | 9.6 | 0.05 | 0.22 | 14 | 0.2 | 0.12 |
(1)Resin extraction; (2)extractable amounts [
The experiment was set up in a former
The previous 6-year-old
Chemical analysis of wet and dry sewage sludge applied in the experiment.
Determinations | Wet sludge | Dry sludge |
---|---|---|
pH (0.01 M CaCl2) | 7.3 | 6.5 |
Bulk density | 1.03 g cm−3 | 0.97 g cm−3 |
Moisture | 77% | 7.4% |
Organic matter | 546 g kg−1 | 530.2 g kg−1 |
Nitrogen (N) | 32 g kg-1 | 35 g kg−1 |
C/N | 9.4 | 8.5 |
Phosphorus (P) | 14 g kg−1 | 17 g kg−1 |
Potassium (K) | 2.2 g kg−1 | 2.2 g kg−1 |
Calcium (Ca) | 25 g kg−1 | 24 g kg−1 |
Magnesium (Mg) | 4.9 g kg−1 | 3.9 g kg−1 |
Sulfur (S) | 6.6 g kg−1 | 6.8 g kg−1 |
Copper (Cu) | 0.6 g kg−1 | 0.7 g kg−1 |
Manganese (Mn) | 0.19 g kg−1 | 0.3 g kg−1 |
Zinc (Zn) | 2.4 g kg−1 | 3.2 g kg−1 |
Iron (Fe) | 39 g kg−1 | 45 g kg−1 |
Boron (B) | 0.009 g kg−1 | 0.002 g kg−1 |
Sodium (Na) | 0.6 g kg−1 | 0.9 g kg−1 |
Nutrients added in the treatments through the sewage sludge and mineral fertilizer application to the soil of the experimental forest stands.
Treatment | Age (days by implantation) | Input (per tree) | N | P | K | Ca | Mg | S | B | Zn |
kg ha−1 | ||||||||||
(1) C (Control) | no fertilization | — | — | — | — | — | — | — | — | |
TOTAL | — | — | — | — | — | — | — | — | ||
(2) MF (Mineral fertilizer) | −45 | 1.2 kg dolomite* | — | — | — | 440 | 160 | — | — | — |
7 | 160 g NPK 6 : 30 : 6 + 2% S + 0.5% Zn | 16 | 34 | 13 | — | — | 5 | — | 1.5 | |
90 | 70 g ammonium nitrate | 39 | — | — | — | — | — | — | — | |
90 | 50 g potassium chloride | — | — | 41 | — | — | — | — | — | |
90 | 8 g de Borax | — | — | — | — | — | — | 1.5 | — | |
180 | 180 g NPK 20 : 0 : 20 + 0.5% B | 60 | — | 50 | — | — | — | 1.5 | — | |
TOTAL | 115 | 34 | 105 | 440 | 160 | 5 | 3 | 2 | ||
(3) WS (10 t ha−1 wet sewage) | 7 | 26 kg wet sewage | 320 | 140 | 21 | 248 | 48 | 66 | 0.1 | 24 |
7 | 16 g potassium chloride | — | — | 13 | — | — | — | — | — | |
90 | 50 g potassium chloride | — | — | 41 | — | — | — | — | — | |
90 | 15 g de Borax | — | — | — | — | — | — | 2.9 | — | |
180 | 34 g potassium chloride | — | — | 28 | — | — | — | — | — | |
TOTAL | 320 | 140 | 105 | 248 | 48 | 66 | 3 | 24 | ||
(4) DS (10 t ha−1 dry sewage) | 7 | 6 kg dry sewage | 322 | 154 | 21 | 228 | 36 | 63 | 0.1 | 32 |
7 | 16 g potassium chloride | — | — | 13 | — | — | — | — | — | |
90 | 50 g potassium chloride | — | — | 41 | — | — | — | — | — | |
90 | 15 g de Borax | — | — | — | — | — | — | 2.9 | — | |
180 | 35 g potassium chloride | — | — | 29 | — | — | — | — | — | |
TOTAL | 322 | 154 | 105 | 228 | 36 | 63 | 3 | 24 |
*Dolomite was broadcast on the soil surface 45 days before planting.
The seedlings were planted between the rows of the previous plantation after subsoiling (depth 45 cm). Mineral fertilizer and wet and dry sewage sludge were applied manually on a 1 m-wide strip in the planting row (at the soil surface without incorporation) some days after planting. Weed and ant control were undertaken before and after planting. High mortality rates occurred within the first days after sewage sludge application and all dead seedlings were replaced 15 days after treatment establishment.
Circumference at breast height and height of eucalypt trees were measured at ages 12, 24, and 36 months, excluding 1 buffer row in each plot. Allometric relationships between tree size and above ground biomass were established at each age by sampling 10 trees distributed throughout the circumference range in each of 3 adjacent plots with application of 10 Mg ha−1 of wet sewage sludge, commercial mineral fertilization, and a control treatment. The trees were separated into components: leaves, branches, stemwood, and stembark. The stem of each tree was sawn into 1 m sections at age 12 months and 3 m sections at 24 and 36 months. Diameters, lengths, and weight were measured in the field. The foliage was collected from three different sections of the crown of the trees at each age. Subsamples of each component were dried at 65°C to constant weight and ground for chemical analysis. Allometric relationships were established for each component at ages 12, 24, and 36 months and applied to the inventory in each plot of the experiment.
Foliar nutrient concentrations were measured every six months in fully expanded young leaves collected from the upper third of the crown in eight central trees within each plot. Leaf samples were dried (65°C), weighed, and the concentrations of N, P, K, Ca, Mg and S were determined [
Leaf fall production and nutrient returns to soil were assessed over the first 36 months after planting. Leaf litter was collected monthly in 12 plots (4 treatments in 3 blocks) from six litter traps (0.25 m2 each one) systematically located in each plot to sample representative distances from the trees (a total of 18 traps per treatment). The samples were dried at 65°C and weighed. Composite samples were made for each season (summer, fall, winter, and spring) for chemical analysis to determine the concentrations of N, P, K, Ca, Mg, and S.
Leaf litter decomposition was measured using litter bags sprawled on the floor of the experimental areas. Nylon bags (20 cm × 20 cm) with a 5 mm mesh were filled with 10 g of air-dried leaf litter material. The mesh was chosen to interfere a minimum with the processes of leaf degradation by mesofauna, as well as to facilitate breakdown and leaching of nutrients from plant material. The collection of the remaining leaves contained in the sampled bags started when the trees were 18 months old, and the canopy of eucalypt stands was already closed, thus providing continuous shade in all the experimental plots. The litterbags were collected at 1.5, 3, 6, 9, and 12 months after set up in the plots. The leaves inside the bags were carefully removed, dried, separated from soil particles, weighed, and analyzed to determine the concentration of lignin.
Leaf-litter mass accumulated in the forest floor was collected every 3 months the third year after planting, from 6 collection points randomly located in each plot (18 positions per treatment). Square wooden frames with an area of 0.25 m2 were used. The samples were dried at 65°C, weighed, and ground for chemical analysis.
Nutrient retranslocations in senescent leaves were calculated multiplying the mass of leaves in litterfall by the difference in nutrient concentration in green leaves. A correction was made to take into account the decrease in leaf mass during senescence, from the ratio between the biomass of individual mature green leaves with the mass of the senescent fallen ones (approximately 18% in our experiment) [
The measurement variables were submitted to variance analysis (ANOVA) at a 5% confidence level. The analyses that showed a significant F test were submitted to multiple-comparison tests through a Tukey range test.
Treatment establishment led to significant differences in tree mortality the first days after planting. Whilst wet sewage sludge application led to a mortality of about 20%, tree mortality in the other treatments was <5%. Large losses of N by volatilization of ammonia have been reported by Robinson and Röper [
Wood biomass accumulation 12 months after planting was 9.2, 7.8, 7.0, and 3.2 Mg ha−1 in the dry sludge, wet sewage sludge, mineral fertilization, and control treatments, respectively, (Table
Aboveground tree biomass accumulated at 12, 24, and 36 months after planting.
Treatment | Biomass (Mg ha−1) | |||||||||
Leaf | Bark | Wood | Branch | Total | ||||||
12 months | ||||||||||
Control | 1.1 | b | 0.6 | b | 3.2 | C | 1.5 | b | 6.5 | c |
Mineral fertilizer | 1.4 | b | 1.1 | a | 7.0 | b | 2.3 | a | 11.8 | b |
Wet sewage | 2.6 | a | 1.2 | a | 7.8 | ab | 2.3 | a | 13.9 | ab |
Dry sewage | 2.9 | a | 1.3 | a | 9.2 | a | 2.6 | a | 15.9 | a |
24 months | ||||||||||
Control | 4.1 | b | 2.4 | b | 13.2 | b | 4.3 | b | 24.0 | b |
Mineral fertilizer | 7.2 | a | 4.8 | a | 34.5 | a | 8.0 | a | 54.6 | a |
Wet sewage | 7.4 | a | 4.4 | a | 31.1 | a | 7.4 | a | 50.3 | a |
Dry sewage | 8.6 | a | 5.3 | a | 37.6 | a | 8.8 | a | 60.3 | a |
36 months | ||||||||||
Control | 2.1 | c | 4.2 | c | 41.4 | C | 4.0 | c | 51.7 | C |
Mineral fertilizer | 3.9 | a b | 6.6 | b | 76.8 | a b | 6.0 | a b | 93.3 | a b |
Wet sewage | 3.6 | b | 6.5 | b | 71.3 | b | 5.7 | b | 87.2 | B |
Dry sewage | 4.4 | a | 8.0 | a | 87.5 | a | 6.6 | a | 106.5 | A |
Significant differences (
Biomass production of all above ground tree components was about twice as high in the 3 treatments with nutrient addition than in the control treatment at 36 months of age (Table
Foliar concentrations showed large differences in tree nutrition between treatments (Figure
Nutrient concentrations within leaves sampled at 6, 12, 18, 24, 30 and 36 months after planting. Vertical bars represent the minimum significant value (Tukey—
The application of sewage sludge affected not only tree nutrition but also nutrient cycling within the ecosystem. Leaf litterfall amounted to 5.2–5.4 Mg ha−1 yr−1 in the plots with wet sewage sludge, dry sludge, or fertilizer applications. However, leaf litterfall was 35% lower in the control treatment (Table
Average nutrient concentrations in leaf litter and total amounts of dry matter and nutrients returning to the soil via litterfall from 12 to 36 months after planting.
Treatment | Dry matter | N | P | K | Ca | Mg | S | |||||||
Concentrations (g kg−1) | ||||||||||||||
Control | 10.2 | a | 0.4 | a | 0.9 | b | 6.1 | c | 1.7 | b | 1.4 | a | ||
Mineral fertilizer | 9.5 | b | 0.3 | a | 2.3 | a | 8.1 | a | 2.8 | a | 1.3 | a | ||
Wet sewage | 9.8 | a b | 0.4 | a | 2.1 | a | 6.7 | b c | 1.7 | b | 1.3 | a | ||
Dry sewage | 9.3 | b | 0.4 | a | 2.0 | a | 7.2 | b | 1.7 | b | 1.3 | a | ||
Annual amounts of dry matter and nutrients in litterfall | ||||||||||||||
Mg ha−1 year−1 | kg ha−1 year−1 | |||||||||||||
Control | 4.0 | b | 46 | a | 1.6 | b | 3 | b | 24 | b | 6 | c | 7 | a |
Mineral fertilizer | 5.2 | a | 54 | a | 1.6 | b | 10 | a | 41 | a | 14 | a | 7 | a |
Wet sewage | 5.3 | a | 56 | a | 2.2 | a | 10 | a | 35 | a | 9 | b | 7 | a |
Dry sewage | 5.4 | a | 55 | a | 2.3 | a | 10 | a | 37 | a | 9 | b | 6 | a |
Significant differences (
Nutrient concentration is an arithmetic average obtained in the analysis performed during the study (12–36 months), nutrient returned value is the sum of all periods.
Monthly litterfall production (a), and rainfall and mean temperature (b), from May 2004 to March 2006. Vertical bars indicate least significant differences between treatments (
Environmental factors, such as climatic variations, are likely to largely modify leaf fall and thus the nutrient cycling processes. The highest leaf fall occuring at times of water deficits may be a result of the eucalypts’ “strategy” to reduce water consumption through a sharp decrease in leaf area. Even though this behavior was observed in all the treatments the leaf fall was lower and occurred later in the control treatment, possibly due to less competition for water among the trees of the same stand and consequently less water stress. Tree foliage biomass in the control treatment was about half that in the other treatments for the first 3 years after planting (Table
Concentrations of N, K, Ca, and Mg in the leaf litterfall were affected by the treatments (Table
Nutrient retranslocation (%) in senescent leaves during 1 to 3 years after planting.
Treatment | N | P | K | Ca | Mg | S | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Control | 56.9% | b | 71.4% | a | 81.3% | a | −17.5% | a | 32.5% | a | 31.1% | a |
Mineral fertilizer | 62.5% | a b | 77.0% | a | 69.7% | b | −27.0% | a | 14.8% | b | 33.6% | a |
Wet sewage | 62.0% | a b | 70.1% | a | 70.7% | b | −26.6% | a | 29.7% | a | 32.8% | a |
Dry sewage | 64.0% | a | 70.2% | a | 68.7% | b | −21.2% | a | 31.4% | a | 37.3% | a |
Significant differences (
The highest retranslocation of K and Mg were found in the treatments with the lowest foliar concentrations. Other studies in eucalypt plantations have shown that the highest retranslocation of N and P occurs for the trees with the highest concentrations of these elements in fully expanded young leaves [
Mineral fertilizer and sludge applications led to large differences in returns to the soil by litterfall for P, K, Ca, and Mg (Table
Leaf decomposition rates in the forest floor, assessed through litter bags, tended to decrease over time (Figure
Dry matter (%) of leaf litter remaining after different times of decomposition within the litter bags distributed on the forest floor in the 4 treatments (time zero was October 2004). Vertical bars represent the minimum significant value (
Dry matter of leaf litter in the forest floor the third year after planting as well as N, P, and S contents were not significantly affected by mineral fertilizer and sludge additions (Table
Mean dry matter and nutrient amounts in the leaf litter accumulated on the soil surface in each treatment (average for the four seasons, the third year after planting).
Treatment | Biomass | N | P | K | Ca | Mg | S | |||||||
Mg ha−1 | kg ha−1 | |||||||||||||
Control | 2.9 | a | 37.4 | a | 1.3 | a | 1.5 | B | 18.9 | b | 5.7 | B | 2.5 | a |
Mineral fertilizer | 3.9 | a | 46.7 | a | 1.6 | a | 2.5 | A | 26.2 | a | 7.9 | A | 2.9 | a |
Wet sewage | 3.3 | a | 39.3 | a | 1.6 | a | 2.3 | a b | 19.6 | b | 4.6 | B | 2.2 | a |
Dry sewage | 3.7 | a | 44.0 | a | 1.9 | a | 2.8 | A | 24.7 | a | 5.5 | B | 2.7 | a |
Significant differences (
In this experiment the application of wet and dry sludge (supplemented with K and B) in the planting rows was a large source of nutrients for eucalypt trees and significantly increased the wood biomass production in comparison with the control treatment. Moreover, the application of wet and dry sewage sludge enhanced the biological cycling of nutrients, which was reflected by higher foliar nutrient concentrations and nutrient returns to the soil through litterfall over the first half of the rotation. Wet or dry sludge applications did not have significantly different effects on nutrient cycling in
The authors would like to thank all the coordinators and employees from the Itatinga Forest Science Experimental Station and from Applied Ecology and Chemistry, Cellulose, and Energy Labs from the Forest Science Department and Laboratory of Plant Stress from the Department of Biological Sciences of ESALQ/USP for their logistics support. They would also like to thank São Paulo Sanitation Company (SABESP) and National Research Council (CNPq) for permitting this research and the Forest Science Research Institute (IPEF) for technical and scientific support.