Carbon and nitrogen cycling in a tropical Brazilian soil cropped with sugarcane
and irrigated with wastewater
Rafael Marques Pereira Leal
a,b,*, Lilian Pittol Firme
b, Uwe Herpin
b, Adriel Ferreira da Fonseca
c,
Ce´lia Regina Montes
b,d, Carlos Tadeu dos Santos Dias
e, Adolpho Jose´ Melfi
b,faLaborato´rio de Ecotoxicologia, Centro de Energia Nuclear na Agricultura – CENA/USP, P.O. Box 96, 13400-970, Piracicaba (SP), Brazil
bNu´cleo de Pesquisa em Geoquı´mica e Geofı´sica da Litosfera (NUPEGEL), Universidade de Sa˜o Paulo (USP), P.O. Box 09, 13418-900, Piracicaba (SP), Brazil cDepartamento de Cieˆncia do Solo e Engenharia Agrı´cola (DESOLO), Universidade Estadual de Ponta Grossa (UEPG), 84030-900, Ponta Grossa (PR), Brazil dLaborato´rio de Ana´lise Ambiental e Geoprocessamento, Centro de Energia Nuclear na Agricultura (CENA), USP, 13400-970, Piracicaba (SP), Brazil eDepartamento de Cieˆncias Exatas, Escola Superior de Agricultura Luiz de Queiroz (ESALQ), USP, 13418-900, Piracicaba (SP), Brazil
fDepartamento de Cieˆncia do Solo, Escola Superior de Agricultura Luiz de Queiroz (ESALQ), USP, 13418-900, Piracicaba (SP), Brazil
1. Introduction
Treated sewage effluent (TSE) is increasingly used as a source of water and nutrients (mainly nitrogen) in crop production in Brazil (da Fonseca et al., 2007a). However, the country lacks a long-term tradition on the agricultural use of such residues, so that national legal regulations and general guidelines for its sustainable application are still in development. For these reasons, scientific experimentation concerning the major implications of effluent irrigation under the local tropical conditions is highly required, in order that TSE may be used as part of a rational and sustainable agricultural planning.
Carbon (C) and nitrogen (N) cycling in agro-systems can be altered by TSE irrigation, mainly in the long-term (da Fonseca et al.,
2007a). Several studies have shown increased total carbon (TC) and total nitrogen (TN) contents in the soil due to C and N input by TSE irrigation (Friedel et al., 2000; Ramirez-Fuentes et al., 2002). Other studies have found decreased contents of soil TC and TN (Speir et al., 1999; Snow et al., 1999), mainly attributed to enhanced mineralization and nitrification processes under effluent irrigation (da Fonseca et al., 2007a).
Of greater concern, increasing concentrations of nitrate (NO3 )
in soil solution due to TSE irrigation have often been reported (Polglase et al., 1995; Smith and Bond, 1999), representing one of the main challenges for the sustainable land application of effluents (Bond, 1998; da Fonseca et al., 2007a). Also increases in dissolved organic carbon (DOC) in soil solution were reported after effluent irrigation (Bhandral et al., 2007; Gloaguen et al., 2007). The DOC fraction represents an important indicator of soil quality (Silveira, 2005) and may play an important role as a carrier of metals and pollutants in the soil profile due to its high mobility (Ciglasch et al., 2004).
Sugarcane (Saccharumspp.) may represent an attractive crop for TSE irrigation in Brazil, because of the following characteristics: A R T I C L E I N F O
Article history:
Received 9 March 2009
Received in revised form 20 September 2009 Accepted 23 September 2009
Keywords:
Wastewater irrigation Water reuse
Saccharumspp. Nitrate
Dissolved organic carbon Tropical soil
A B S T R A C T
Carbon (C) and nitrogen (N) dynamics in agro-systems can be altered as a consequence of treated sewage effluent (TSE) irrigation. The present study evaluated the effects of TSE irrigation over 16 months on N concentrations in sugarcane (leaves, stalks and juice), total soil carbon (TC), total soil nitrogen (TN), NO3 -N in soil and nitrate (NO3 ) and dissolved organic carbon (DOC) in soil solution. The soil was
classified as an Oxisol and samplings were carried out during the first productive crop cycle, from February 2005 (before planting) to September 2006 (after sugarcane harvest and 16 months of TSE irrigation). The experiment was arranged in a complete block design with five treatments and four replicates. Irrigated plots received 50% of the recommended mineral N fertilization and 100% (T100), 125% (T125), 150% (T150) and 200% (T200) of crop water demand. No mineral N and irrigation were applied to the control plots. TSE irrigation enhanced sugarcane yield but resulted in total-N inputs (804– 1622 kg N ha 1) greater than exported N (463–597 kg N ha 1). Hence, throughout the irrigation period,
high NO3 concentrations (up to 388 mg L 1at T200) and DOC (up to 142 mg L 1 at T100) were
measured in soil solution below the root zone, indicating the potential of groundwater contamination. TSE irrigation did not change soil TC and TN.
ß2009 Elsevier B.V. All rights reserved.
* Corresponding author at: Laborato´rio de Ecotoxicologia, Centro de Energia Nuclear na Agricultura – CENA/USP, P.O. Box 96, 13400-970, Piracicaba (SP), Brazil. Tel.: +55 19 34294764; fax: +55 19 34294764.
E-mail address:fielpira@yahoo.com.br(R.M.P. Leal).
Contents lists available atScienceDirect
Agricultural Water Management
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a g w a t
(i) predominance of non-irrigated plantations with large potential for the adoption of irrigation practices; (ii) high demand of N (one of the main nutrient in TSE); (iii) perennial crop fitted to full mechanization, without direct contact to farm workers; (iv) major commodity for ethanol production in Brazil. These characteristics may result in agronomic, economic and environmental benefits, however, detailed studies on TSE irrigation in sugarcane agro-systems are lacking.
The purpose of this study was to evaluate the effects of short term TSE irrigation (16 months) on N uptake by sugarcane, N and C accumulation in soil and NO3 and DOC leaching.
2. Materials and methods
The experimental area is situated near the city of Lins, State of Sa˜o Paulo (longitude: 498500W; latitude: 228210S; average altitude of 440 m), Brazil, adjacent to the municipal wastewater treatment plant (stabilization ponds system). Mean annual temperature is 238C and precipitation was 1292 mm during the experimental period (16 months). Monthly rainfall and the amounts of TSE applied as irrigation throughout the experiment are presented in Fig. 1. The soil was classified as Typic Haplustox (Soil Survey Staff, 1999), sandy clay loam. The soil mineralogy of the area is dominated by quartz and kaolinite, and subordinately by hematite, magnetite and/or maghemite. The dominance of these minerals, commonly occurring in highly weathered and acid tropical soils, results in low cation exchange capacity (CEC) (Table 1).
The chemical and physical soil characteristics found at the beginning of the experiment (Table 1) are usual for most Brazilian agro-systems cropped with sugarcane. Available calcium (Ca), magnesium (Mg), potassium (K) and phosphorus (P) were found in adequate concentrations for sugarcane growth due to previous liming and addition of mineral fertilizers at the beginning of the experiment.
Sugarcane was planted in March 2005 with a distance of 1.4 m between the rows. TSE irrigation was carried out from May 2005 till August 2006 (16 months). After this period irrigation was stopped because sugarcane needs water stress before harvest in order to concentrate sugar. Plants were irrigated when
C
mwere less than 40 kPa. Harvest took place at the end of September 2006. Concerning mineral fertilizer application, all experimental plots received 15 kg ha 1 of N (ammonium nitrate, 50% ofrecommended mineral N), except the control plot; 52 kg ha 1of
P (simple superphosphate), and 66 kg ha 1 of K (potassium
chloride), distributed manually to the furrows during planting.
Detailed information about irrigation and crop management are available in a previous work (Leal et al., 2009).
The experiment was arranged in a complete block design, with five treatments and four replications. The treatments consisted of: (i) control, without TSE irrigation and N fertilization; (ii) T100, T125, T150 and T200, with TSE irrigation supplying 100% (0% surplus), 125% (25% surplus), 150% (50% surplus) and 200% (100% surplus) of crop water demand, respectively. Total plot size was 280 m2(40 m
7 m) with a useful area of 126 m2.
The effects of effluent irrigation on carbon and nitrogen dynamics were evaluated by considering two application scenar-ios: (i) land application of TSE rates higher than 100% of crop water requirement (T125, T150 and T200) as a TSE disposal alternative, and (ii) TSE application according to crop water requirement (T100) to enhance agricultural production through the provision of water and nutrients.
Data about TSE constituents were taken from a previous detailed effluent characterization (da Fonseca et al., 2007b) because TSE quality has not changed considerably during the last five years (Leal et al., 2009). In average, TSE is characterized by: pH of 7.7; total-N concentration of 31.9 mg L 1; 22.4 mg L 1of NH
4+
-N (predominant -N form); 0.6 mg L 1of NO
3 -N; 65.28 mg L 1of
dissolved organic carbon (DOC); 301 mg L 1 of bicarbonate
(HCO3 ); 146 mg L 1of Na and a sodium adsorption ratio (SAR)
of 11.9 (mmol L 1)0.5.
The total-C and total-N inputs to the soil–plant system via TSE irrigation (16 months) were calculated for each treatment. As a result, 4500, 5690, 6830 and 8430 kg ha 1of total-C and 800, 1020,
Fig. 1.Monthly amounts of applied water (rainfall and wastewater irrigation) during the experimental period. Month 1 corresponds to May 2005 and month 16 to August 2006. T100, T125, T150 and T200: Treated sewage effluent irrigation (TSE) supplying 100% (0% surplus), 125% (25% surplus), 150% (50% surplus) and 200% (100% surplus) of crop water demand.
Table 1
Chemical and physical soil properties at the beginning of the experiment (March 2005).
Layer (m) pH H + Al (mmolckg 1)
Al
(mmolckg1) Ca (mmolckg 1)
Mg (mmolckg1)
K
(mmolckg 1) Na (mmolckg1)
CECa (mmolckg 1)
P (mg kg1)
TCb (g kg1)
TNc (g kg 1)
0–0.1 5.1 15.4 0.9 12.7 3.5 3.0 0.7 35.4 17.3 6.0 0.5
0.1–0.2 5.2 15.0 0.8 13.2 3.6 2.4 1.5 35.6 15.5 6.4 0.5
0.2–0.4 4.9 15.9 1.8 10.2 3.6 1.8 2.5 34.0 4.1 5.1 0.4
0.4–0.6 4.6 21.8 4.0 6.2 2.3 1.2 2.8 34.3 1.0 4.1 0.3
Layer (m) BSd (%)
ESPe (%)
B (mg kg 1)
Cu (mg kg1)
Fe (mg kg1)
Mn (mg kg 1)
Zn (mg kg1)
ECf (dS m1)
Sand (g kg1)
Silt (g kg 1)
Clay (g kg 1)
0–0.1 56.3 2.1 0.6 0.5 21.5 3.0 1.2 0.7 774 90 135
0.1–0.2 57.9 4.1 0.6 0.7 20.9 2.7 1.5 0.4 775 78 147
0.2–0.4 53.3 7.4 0.6 0.3 15.5 1.7 0.3 0.2 732 75 192
0.4–0.6 36.5 8.1 0.4 0.5 10.4 1.2 0.2 0.2 707 65 227
aCation exchange capacity at pH 7.0!CEC = Ca + Mg + K + Na + H + Al. bTC = total carbon.
cTN = total nitrogen.
1220 and 1620 kg ha 1of total-N were added to the soil at T100,
T125, T150 and T200, respectively. Taking into account that the TSE applied presents a Na/total-N ratio of about five (da Fonseca et al., 2007b), its application also added high amounts of Na to the soil system, representing a major risk of sodification (Leal et al., 2009). Soil sampling for TC and TN was carried out in February 2005 (before planting) and in September 2006 (after sugarcane harvest and 16 months of irrigation). At each plot, twelve subsamples were randomly collected at the 0–0.1, 0.1–0.2 m soil layers to compound composite samples. At the 0.2–0.4 and 0.4–0.6 m soil layers six subsamples were taken to form composite samples. Soil samples were air-dried, sieved at 0.15 mm and soil TC and TN were determined by dry combustion (Nelson and Sommers, 1996). For initial soil chemical characterization (Table 1), sieved (2 mm) soil samples were analyzed according to standard procedures: (i) Embrapa (1999)for P, Ca, Mg, K, Al, and Na, and (ii)Van Raij et al. (2001)for pH, H + Al, B, Cu, Fe, Mn and Zn. Soil solution electrical conductivity (EC) was obtained using saturation extracts, accord-ing toVan Raij et al. (2001).
Nine months after planting (December 2005), sugarcane was in the stage of maximum vegetative growth. At that time, 25 leaves were randomly sampled within each plot for mineral nutrition analysis. The samples were rinsed in distilled water, oven dried at 608C, ground in a Wiley type mill, passed through a 0.85 mm sieve and stored in capped vials. For N analyses the samples were submitted to sulphuric acid digestion and determined by the semi-micro-Kjeldahl procedure proposed byMalavolta et al. (1997). In September 2006 (date of harvest), ten stalks per plot were randomly sampled for N analyses. The procedures used for stalks N analyses were similar to those described for the leaves. Juice extracted from stalk samples were kept frozen prior to N analysis and subsequently evaluated according to the same analytical procedures as mentioned above.
For soil inorganic N determination, fresh soil samples (10 g) from the sampling campaign in September 2006 were immediately extracted in 50 mL of 2 M KCl, according toPiccolo et al. (1994). After 24 h, the extracts were centrifuged and colorimetrically analysed using an automated flow injection system (Ruzicka and Hansen, 1981). For the estimation of mineral N stocks in the 0– 0.60 m soil layer at the end of the experimental period (Table 3), an average soil density of 1.5 kg dm 3was employed.
Nitrate and DOC concentrations in soil solution were monitored every 2 months, from May 2005 (start of the irrigation period) to June 2006 (approximately 45 days before irrigation was stopped). The samples were obtained using porous ceramic cups installed at soil depths of 1–3 m. Samples were collected after applying a tension of 50 kPa for 8 days to obtain a minimum amount of soil solution required for analysis (Surita et al., 2007). Subsequently the samples were filtered through a 0.22
m
m pore diameterester-cellulose membrane and frozen prior to NO3 determination.
Nitrate analyses were carried out with liquid chromatography (Dionex 500). Only mineral N in form of NO3 was quantified,
becauseGloaguen et al. (2007)found in effluent generated in the same treatment plant a mean [NO3 ]/[NH4+] ratio of 0.05 toward
396 in the soil solution due to fast nitrification of effluent-N in the soil (da Fonseca et al., 2007c). For DOC analyses, an aliquot of soil solution was filtered through a fiber glass filter (0.7
m
m) andpreserved with 30 mmol L 1mercuric chloride solution (HgCl 2).
Determinations were carried out using a Shimadzu TOC-5000 Analyzer. At the control plots without TSE application, no sufficient volume of soil solution for NO3 and DOC determinations could be
obtained during the sampled period.
Soil and plant data were submitted to analysis of variance. The analysis presented a homogeneous variance, a necessary condition to carry out univariate statistical analysis for a complete block design, considering time (sampling date) as subplot. The variables
which showed significant F test (P<0.05) were submitted to multiple comparisons by Tukey’s test adjusted for descriptive level (
a
= 0.05). Mean comparisons of NO3 and DOC values of soilsolution were made with transformed data (square root transfor-mation for NO3 values and logarithmic transformation for DOC
values). However, all given means are original values. All statistical analyses were carried out using the SAS system, version 8.02 (SAS System, 1999).
3. Results and discussion
3.1. Nitrogen contents in plants and sugarcane yield
There were no differences between N concentrations in sugarcane leaves at the irrigated treatments (17.1–18.2 g kg 1)
compared to the control (19.7 g kg 1). At the irrigated plots, N
concentrations in leaves were close to the lower range limit of adequate values (18–25 g N kg 1 leaves) proposed by Van Raij
et al. (1996). Nitrogen concentrations in sugarcane juice (8.4– 11.7 g L 1) and stalks (7.2–8.6 g kg 1) were also not influenced by
the treatments. However, the accumulated N in stalks (2.30– 2.60 kg N per Mg of fresh stalks 1) was considerably higher than
the reference range (0.90–1.32 kg N per Mg of fresh stalks 1)
(Dematteˆ, 2005). Although N concentration in leaves were close to the lower range limit of adequate values, the high accumulated N in stalks and the high sugarcane yields suggest adequate conditions for crop development.
All TSE irrigated plots (T100 = 247 Mg ha 1; T150 = 233 Mg ha 1;
T200 = 232 Mg ha 1), except T125 (199 Mg ha 1), showed
signifi-cant higher yields than the control (153 Mg ha 1), probably due to the water supplied by TSE irrigation. However, despite increasing irrigation rates, similar (T150 and T200) or lower (T125) yields were found compared to T100. This indicates no positive effect of higher TSE irrigation rates (T125–T200) on sugarcane yield and is important to be considered for an adequate and sustainable land application of TSE. Sugarcane extracted significant amounts of N from the soil, from 345 kg ha 1at the control to a maximum of 597 kg ha 1(T200), with
no differences between irrigated treatments (463–597 kg ha 1).
However, the accumulated N in stalks was not sufficient to counterbalance the large N input via TSE irrigation, resulting in a low N recovery (maximum of 23%, given as the ratio between N in stalks and total N applied). Effluent induced N overfertilization may negatively affect the ability of the crop to resist disease and also to synthesize important compounds related to crop natural defenses (Hamilton et al., 2007), representing a relevant aspect to be considered in TSE irrigation management.
Different from the following productive cycles, the first sugarcane cycle is known to be not responsive to N fertilization, because the required N can be obtained by alternative sources such as biological fixation of atmospheric N and through the utilization of N reserves stored in the seedpiece (Carneiro et al., 1995). Therefore, it is suggested that mineral N application in the first crop cycle can be avoided because the effluent may entirely supply the required N. The high yields at the irrigated treatments, where 50% of the recommended mineral N fertilization was applied, also support this statement.
3.2. Total carbon (TC) and total nitrogen (TN) in soil
Effluent irrigation showed no effects on soil TC (Table 2) and negligible effects on soil TN contents (Table 2), which may be referred to the short term experimental period.da Fonseca (2005) found significant decreases in soil TC and TN concentrations after 18 months of effluent irrigation of Tifton 85 bermudagrass, and attributed it to the mineralization of native organic matter due to a high N (through mineral fertilization and effluent irrigation) input, a process known as ‘‘the priming effect’’.
Generally, changes in soil TC and TN contents were mainly found after effluent irrigation in long-term studies (da Fonseca et al., 2007a), showing both increases (Friedel et al., 2000; Ramirez-Fuentes et al., 2002) and decreases (Polglase et al., 1995; Falkiner and Smith, 1997). The increases in soil TC and TN were related to the significant input of N and C via effluent irrigation over time. On the other hand, decreases in soil TC and TN were mainly referred to the occurrence of permanent moist soil conditions due to irrigation and the low C/N ratio of the effluent. These factors might stimulate microbial activity enhancing soil organic matter mineralization (da Fonseca et al., 2007a).
Moreover, prolonged TSE irrigation may increase pollutants (heavy metals) concentrations in soil (Friedel et al., 2000). In the long-term, this accumulation may provoke detrimental effects on soil microbial communities, affecting negatively C, N, P and sulphur (S) cycling processes (Kandeler et al., 1996).
3.3. Nitrate (NO3 ) in soil and in soil solution
Although the additions of effluent-N varied about 800 kg ha 1
at the irrigated treatments (T100–T200), inorganic soil N (NO3 -N)
concentrations were similar at the end of the experiment (Table 3).
However, nitrate concentrations at the irrigated treatments (minimum of 3.5 mg kg 1and maximum of 12.1 mg kg 1) were
high in comparison to the control (maximum of 3 mg kg 1)
(Table 3).
The higher levels of soil inorganic N stocks at the irrigated treatments compared to the control (Table 3) evidences the potential of N losses due to the low chemical affinity of nitrate anions to net negatively charged soil particles. Although soil inorganic N were only determined at the end of the experiment, higher nitrate levels in the irrigated soils are coherent with the high concentrations (>100 mg L 1) of the leached NO3 fraction measured between 1 and 3 m depth at T100–T200 over the experimental period (Fig. 2andTable 3). Although N recovery was not affected by effluent irrigation, the lost N, considered as the difference between total N input and N recovered at each treatment, was much higher at T200 (1383 kg) compared to T100 (626 kg), indicating a higher potential of N leaching at T200. Coherently, the leached NO3 was higher at T200 than at T100
(Table 3). At other depths, the concentrations of leached NO3 were
similar at all irrigated treatments.
Although chemically poor, Oxisols have generally good physical properties (porosity and permeability) and a very stable soil structure because of strong micro-aggregation (Driessen et al., 2001), which may favor N leaching throughout the soil profile. Because more than 50% of sugarcane roots are found in the 0–0.4 m layer (Faroni, 2004), we expect that NO3 at 1 m depth has already
passed through the root zone and unless further denitrification takes place it can reach and impact the groundwater table.
Although major concerns about groundwater nitrate contam-ination are present, higher mineral N stocks in the soil (Table 3) may represent an important nitrogen reservoir for crop nutrition in the following productive cycles, which may partially reduce
Table 2
Total carbon and nitrogen (g kg 1) concentrations in soil after application of different treated sewage effluent (TSE) irrigation rates (T100, T125, T150 and T200).
Treatmentsa Time after the beginning of irrigation (month)
0 16 0 16
Carbon Nitrogen
0–0.1 m layer (CVb= 9.1%) 0–0.1 m layer (CV = 8.6%)
Control 6.10 aAc 6.40 aA 0.52 aA 0.52 aAB
T100 5.82 aA 6.62 aA 0.52 aA 0.42 aB
T125 6.62 aA 6.25 aA 0.55 aA 0.55 aA
T150 5.75 aA 6.02 aA 0.50 aA 0.47 aAB
T200 6.00 aA 5.90 aA 0.52 aA 0.47 aAB
0.1–0.2 m layer (CV = 10.6%) 0.1–0.2 m layer (CV = 13.6%)
Control 6.22 aA 6.00 aA 0.57 aA 0.57 aA
T100 6.37 aA 6.02 aA 0.52 aA 0.55 aA
T125 7.10 aA 6.25 aA 0.60 aA 0.55 aA
T150 5.77 aA 6.12 aA 0.52 aA 0.60 aA
T200 6.50 aA 5.80 aA 0.55 aA 0.52 aA
0.2–0.4 m layer (CV = 10.2%) 0.2–0.4 m layer (CV = 12.8%)
Control 5.05 aA 5.12 aA 0.45 aA 0.45 aAB
T100 5.15 aA 5.10 aA 0.42 aA 0.42 aB
T125 5.22 aA 5.52 aA 0.47 aA 0.52 aAB
T150 4.97 aA 5.72 aA 0.42 bA 0.60 aA
T200 5.22 aA 4.77 aA 0.42 aA 0.47 aAB
0.4–0.6 m layer (CV = 8.5%) 0.4–0.6 m layer (CV = 17.4%)
Control 4.07 aA 4.02 aA 0.35 aA 0.32 aA
T100 4.25 aA 4.20 aA 0.32 aA 0.37 aA
T125 4.17 aA 4.12 aA 0.37 aA 0.42 aA
T150 4.10 aA 4.37 aA 0.35 aA 0.37 aA
T200 4.10 aA 4.20 aA 0.35 aA 0.42 aA
aTreatments: control (without TSE irrigation and mineral N fertilization) and T100, T125, T150 and T200 corresponding to TSE irrigation supplying 100, 125, 150 and 200% of the crop water demand, respectively.
bCV = coefficient of variation.
cMeans followed by same small letters in the same row (different sampling data) and same capital letters in the same column (different treatments) do not differ by
mineral N fertilization additions. Our results suggest further approaches to evaluate effluent irrigation by supplying, e.g. different percentages of sugarcane N requirements via effluent which would provide more information about the potential of effluent irrigation to replace mineral N fertilization in sugarcane agriculture.
Nitrate leaching after wastewater irrigation have often been reported (Polglase et al., 1995; Speir et al., 1999). For example, after two cycles of maize and sunflower irrigated with TSE, nitrate leached up to a depth of 2 m, however, NO3 concentrations below
1 m did not exceed 35 mg L 1 (Gloaguen et al., 2007). The high
concentrations of NO3 found in the present study indicate the
importance of an adequate irrigation management to avoid N losses in soil.
Concerns related to excessive levels of NO3 in soil solution
have been stated due to its potential adverse effects on human health (when water is used for human consumption) and on the
ecological balance (when N-rich wastewater reaches surface waterbodies) (Bond, 1998; da Fonseca et al., 2007a). According toBond (1998), although NO3 leaching from effluent irrigated
areas is almost inevitably, the extent of possible negative effects depends on a range of factors such as: (i) depth of the watertable; (ii) quality of groundwater prior to effluent irrigation; (iii) extent of dilution of NO3 in the groundwater; (iv) size of the effluent
irrigated areas; (v) proximity of the effluent irrigation site to discharge zones and water supply wells.
3.4. Dissolved organic carbon (DOC) in soil solution
Considerable amounts of DOC in soil solution were measured between 1 and 3 m depth throughout the experimental period (Fig. 3andTable 3). With exception of T200 (26.5–31.9 mg L 1)
DOC concentrations were very similar (72–111 mg L 1) between
the other irrigated plots and remained high over time (Table 3). The measured DOC concentrations were much higher than the concentration ranges for other Brazilian tropical soils for which average values of 5 mg L 1(Lilienfein et al., 2001) and
<15 mg L 1 (Ciglasch et al., 2004) were reported. However, our values are consistent with data fromGloaguen et al. (2007), who reported for a two years experiment on maize and sunflower irrigated with the
Fig. 2.Temporal evolution of nitrate concentrations (6n9) (mean of depths 100, 200 and 300 cm,standard deviation) of soil solution in a tropical agro-system cropped with sugarcane and irrigated with treated sewage effluent (TSE). T100, T125, T150 and T200: Treated sewage effluent irrigation (TSE) supplying 100% (0% surplus), 125% (25% surplus), 150% (50% surplus) and 200% (100% surplus) of crop water demand.
Fig. 3. Temporal evolution of dissolved organic carbon (DOC) concentrations (5n9) (mean of depths 100, 200 and 300 cm) of soil solution in a tropical agro-system cropped with sugarcane and irrigated with treated sewage effluent (TSE). T100, T125, T150 and T200: Treated sewage effluent irrigation (TSE) supplying 100% (0% surplus), 125% (25% surplus), 150% (50% surplus) and 200% (100% surplus) of crop water demand.
Table 3
Mean soil inorganic N concentrations (mg NO3 -N kg 1dry soil), nitrate (NO3 ) and dissolved organic carbon (DOC) concentrations in soil solution and mineral N stocks (kg ha 1) of a tropical agro-system cropped with sugarcane after application of different treated sewage effluent (TSE) irrigation rates (T100, T125, T150 and T200).
Depth (m) Treatmentsa
T100 T125 T150 T200 Control
Nitrate concentrations (mg L1) in soil solution
1 177.8 ab 183.1 a 193.6 a 264.6 a –
2 111.6 b 207.8 ab 204.2 ab 228.3 a –
3 183.0 a 235.3 a 192.1 a 270.2 a –
Dissolved organic carbon (mg L 1) in soil solution
1 110.4 a 78.3 a 99.3 a 26.5 b –
2 83.5 a 90.1 a 84.8 a 24.8 b –
3 111.0 a 72.0 a 76.4 a 31.9 b –
Nitrate concentrations in soil (mg NO3 -N kg 1dry soil)
0–0.1 4.54 AB 6.21 AB 7.95 A 6.99 AB 1.91 B
0.1–0.2 5.51 AB 8.09 A 7.86 A 7.18 AB 2.99 B
0.2–0.4 10.68 A 12.05 A 9.82 A 9.00 A 1.07 B
0.4–0.6 5.21 A 4.90 A 4.42 AB 3.45 AB 1.75 B
Mineral N stock (kg ha 1)
0–0.6 62.74 72.30 66.43 58.60 15.81
aTreatments: T100, T125, T150 and T200 corresponding to TSE irrigation supplying 100, 125, 150 and 200% of the crop water demand, respectively. Control: without TSE
irrigation and mineral N fertilization.
same TSE in the same area DOC concentrations from 7 to 131 mg L 1. High DOC concentrations in soil solution may reach
groundwater, where it may interact with organic and inorganic contaminants, fuel denitrification, leading also to potential problems for potable water treatment (Chomycia et al., 2008).
DOC concentrations in the soil solution are closely related to land management and microbial activity (Silveira, 2005). The high DOC values found in the present study may be attributed to the degradation of the added organic matter via effluent and to the organic components originated from higher microbial activity under irrigation conditions (Gloaguen et al., 2007). Due to its high mobility, the DOC fraction may play an important role as a carrier of metals and pollutants throughout the soil profile (Ciglasch et al., 2004). Moreover, DOC may enhance leaching of N, P and S, because these nutrients are also part of the chemical structure of dissolved organic matter (Ciglasch et al., 2004).
4. Conclusions
Although sugarcane extracted high amounts of N from soil, the quantities were insufficient to balance the large N input via effluent irrigation leading to nitrate leaching into the deeper soil layers. Also high concentrations of DOC in soil solution were identified. The supply of C and N by effluent irrigation had not affected soil total-C and total N concentrations possibly because of the short term experimental period. On the other hand, soil NO3
concentrations were higher at the irrigated treatments compared to the control at the end of the experiment.
Treated sewage effluent irrigation according to crop water requirement (T100) caused an excessive nitrogen input compared to the plant demand. The results indicated that crop full water supply by TSE irrigation should be avoided in the first crop cycle where sugarcane response to N fertilization is less pronounced than in the following cycles. Moreover, mineral N application can be avoided due to the N additions by effluent. Adequate plant selection combined with adapted irrigation management strate-gies are of future concern for an optimal and sustainable sugarcane production under effluent irrigation systems.
Acknowledgements
To FAPESP, CNPq and SABESP for financial support as well as to SABESP and the agroindustry EQUIPAV for logistic support. We also thank two anonymous reviewers for their meaningful contribution to the paper.
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