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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with Treated Wastewater
Journal of Integrative Agriculture
August 2012
2012, 11(8): 1354-1364
RESEARCH ARTICLE
Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with Treated Wastewater XUE Yan-dong1, YANG Pei-ling1, LUO Yuan-pei2, LI Yun-kai1, REN Shu-mei1, SU Yan-ping1 and NIU Yongtao1 1 2
College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, P.R.China Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China
Abstract The reuse of treated wastewater in agricultural systems could partially help alleviate water resource shortages in developing countries. Treated wastewater differs from fresh water in that it has higher concentrations of salts, Escherichia coli and presence of dissolved organic matter, and inorganic N after secondary treatment, among others. Its application could thus cause environmental consequences such as soil salinization, ammonia volatilization, and greenhouse gas emissions. In an incubation experiment, we evaluated the characteristics and effects of water-filled pore space (WFPS) and N input on the emissions of nitrous oxide (N2O) and carbon dioxide (CO2) from silt loam soil receiving treated wastewater. Irrigation with treated wastewater (vs. distilled water) significantly increased cumulative N2O emission in soil (117.97 μg N kg-1). Cumulative N 2O emissions showed an exponentially increase with the increasing WFPS in unamended soil, but the maximum occurred in the added urea soil incubated at 60% WFPS. N 2O emissions caused by irrigation with treated wastewater combined with urea-N fertilization did not simply add linearly, but significant interaction (P<0.05) caused lower emissions than the production of N2O from the cumulative effects of treated wastewater and fertilizer N. Moreover, a significant impact on cumulative CO2 emission was measured in soil irrigated with treated wastewater. When treated wastewater was applied, there was significant interaction between WFPS and N input on N2O emission. Hence, our results indicated that irrigation with treated wastewater should cause great concern for increasing global warming potential due to enhanced emission of N2O and CO2. Key words: treated wastewater, nitrous oxide, carbon dioxide, water-filled pore space, urea
INTRODUCTION The concentration of carbon dioxide (CO2) and nitrous oxide (N2O) in the atmosphere has increased considerably over the last century and is set to rise further at a rate of approximately 0.4 and 0.25% per year, respectively. CO2 is the most important anthropogenic greenhouse gas. Its annual emissions have grown be-
tween 1970 and 2004 by about 80% (IPCC 2007). Compared with CO2 emission, N 2O emission is currently the single most important ozone-depleting emission and is expected to remain the largest throughout the 21st century (Ravishankara et al. 2009). With a long residence time in the atmosphere (120 yr), N2O is responsible for about 6% of the current greenhouse effect and it is approximately 300 times more potent than CO2 in absorbing terrestrial thermal radiation in
Received 10 June, 2011 Accepted 22 November, 2011 Correspondence YANG Pei-ling, Tel/Fax: +86-10-62737866, E-mail: [email protected]
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
the troposphere (IPCC 2007). Agricultural soil is the major source of N 2O, accounting for about 42% of the global annual emission (IPCC 2007). Enhanced N2O emissions from agricultural soils are mainly due to increasing N inputs by mineral fertilizers, manure application and low quality water such as sewage effluent and treated wastewater (Monnett et al. 1995; Zou et al. 2009; Fernández-Luqueño et al. 2010). Treated wastewater has become an important water source for agriculture in urban and suburban communities of the developing world (Rutkowski et al. 2007). Beijing, for example, is estimated to produce 1.0×109 m3 of treated wastewater per year by 2015, while the utilization of treated wastewater is expected to reach 75% based upon the “Key Technologies R&D Program of China during the 12th Five-Year Plan period”. In general, irrigation with treated wastewater could supply important nutrients such as inorganic N and organic matter, and thus has a potential for increasing the nutrient levels of soils (Yadav et al. 2002). Generally, however, treated wastewater differs from its fresh source water in its higher concentration of nutrients, salt, Escherichia coli, bacteria, fungi, and suspended solids after secondary treatment, and poses a risk both to crop growth and soil environment with additional environmental consequences, such as ammonia volatilization, nitrate leaching, and nitrous oxide emission (Neeteson et al. 2001). Recent studies have found both positive and negative effects on the biogeochemical cycling of C and N in long-term treated wastewater application sites (Meli et al. 2002; Ramirez-Fuentes et al. 2002; Chen et al. 2008). In fact, soil microbial biomass carbon and cumulative respiration are enhanced in soils under long term wastewater application (Meli et al. 2002). Furthermore, it has been demonstrated that enzyme activities involved in C and N cycling in soil are stimulated by reclaimed water irrigation (Chen et al. 2008). However, studies state that treated wastewater irrigation significantly increases the production of N2O (Master et al. 2003; Master et al. 2004; Zou et al. 2009), CO 2 emission and global warming potential (Fernández-Luqueño et al. 2010). One possible explanation for this phenomenon is that readily available C and N in treated wastewater could benefit nitrifying and denitrifying bacterial communities and their activi-
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ties in soils, thereby increasing N2O emissions (Ndour et al. 2008). Moreover, N mineralization is so low or absent that the losses of N could not be compensated through wastewater application. N fertilizer must then be applied to maintain the yield of crop production (Ramirez-Fuentes et al. 2002). Hence, this poses several concerns over how treated wastewater application may affect greenhouse gas emissions (GHG), what the intensity of this effect is, and how the combination of N derived from fertilizer and treated wastewater affects on N2O losses. Soil moisture content is very important in regulating gas emissions and the gas production processes (Skiba et al. 2000). Several laborary studies involving incubation of disturbed or undisturbed soil by different waterfilled pore space have found N2O emissions to be closely related to the dynamic nature of water-filled pore space (WFPS) (Hou et al. 2000; Dobbie et al. 2001, 2003; Del Prado et al. 2006; Ciarlo et al. 2007). It is a priority to study the N2O emission in order to quantitatively indicate the production from TWW (treated wastewater) treat soil incubated at specific WFPS. However, to our knowledge, it is hard to differentiate the effects of treated wastewater and increased soil water content compared with fresh water. We hypothesize that there is an interaction between WFPS and N input on N2O emission in the soil receiving TWW. To test our hypothesis, the objectives of this study are to: 1) quantify CO2 and N2O emissions affected by treated wastewater application; 2) assess the interaction between urea-N and TWW-N on N2O losses; and 3) provide some useful information about the influence of treated wastewater irrigation on greenhouse gas emissions from the utilization of sewage effluent in the North China Plain.
RESULTS N2O and CO2 emissions of soil incubated with treated wastewater Over the course of the experiment, the patterns of nitrous oxide emission rate in the unamended soils were generally similar and peaked at day 1 (Fig. 1). The N2O fluxes were very small (<3.0 μg N kg-1 d-1) in soils incubated at 40% WFPS (Fig. 1-A). As Fig. 1-B shows,
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Fig. 1 N2O emission rate in soil incubated at 40 (A), 60 (B), 80 (C), and 100% (D) WFPS. Values are means±SD. The same as below.
when the soils were incubated at 60% WFPS, the patterns of N2O emission rates in soils without N application were different from soils added with urea, with considerably larger peaks observed at d 5 of the incubation for TWW (54.11 μg N kg-1 d-1) and CK (60.79 μg N kg-1 d-1). There were similar trends for N2O fluxes in soils incubated at 80% WFPS (Fig. 1-C). The largest peak fluxes were found from unamended soils (86.97 μg N kg-1 d-1) and added urea soils (53.89 μg N kg-1 d-1) incubated at 100% WFPS under treated wastewater irrigation (Fig. 1-D), respectively. Differences in cumulative N2O emissions were significant (P<0.01) for all main effects included in this experiment. Moreover, the interactions of water × WFPS, water × N fertilizer, WFPS × N fertilizer, and water × WFPS × N fertilizer also significantly contributed to the N 2O losses (Table 1). On average, cumulative N2O emission averaged over WFPS and N fertilizer in soils irrigated with TWW (117.97 μg N kg-1) was remarkably greater than that in soils receiving distilled water (96.71 μg N kg -1 ). In unamended soils, the production of N 2O under TWW irrigation were 33.8% larger than control treatment (CK). When fertilizer N was incorporated, cumulative N2O emissions were similar compared to the soils applied urea. Soil water-filled pore space had a significant impact on the emissions of N2O in this experiment (P<0.01). N2O emissions increased with the increasing water content in soils without fertilizer N application. Cumulative N2O emissions (<6.0 μg N kg-1) were very low in unamended soils associated at 40% WFPS, and showed a maximum (252.58 μg N kg-1) in unamended soils in-
cubated at 100% WFPS under TWW irrigation. To the contrary, the maximum of N2O loss in added urea soils was measured at 60% WFPS treated with TWW (178.81 μg N kg-1), which was 6.9 times larger than the minimum N2O loss in added urea soils incubated at 40% WFPS. When urea was added, N2O losses under TWW irrigation were 21% higher than that in soils without fertilizer. A larger range was measured from the added urea soils irrigated with distilled water, which had a 54% increase for N2O emission compared to unamended soils. In soil incubated at 40, 60, and 80% WFPS, the cumulative N2O emissions in soils amended with urea were greater than the unamended soils. In contrast, higher N2O production in unamended soils incubated at 100% WFPS was measured compared to soils applied with urea (Table 1). The mean CO2production rate was higher in soils irrigated with TWW compared to the distilled water incubation (P<0.05). The peaks of CO2 emission rate were measured at d 1, after which they decreased until d 7 except the CK and CK+urea treatments under 100% WFPS incubation, which peaked around d 3 and 5, respectively (Fig. 2). Statistical analysis showed that all three main factors significantly contributed to the CO2 emissions (P<0.05). However, there was no significant interaction between water source and fertilizer N application (Table 1). The cumulative CO2 production was larger in soils irrigated with TWW (2.21 mg C kg -1) compared to soils irrigated with distilled water (2.07 mg C kg -1). Considerably higher CO 2 emissions in unamended soils were detected at 80% WFPS compared with other WFPS treatments. However, the cumulative CO 2 losses showed a maximum at 60% WFPS
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
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Table 1 Effects of WFPS and urea on cumulative N2O and CO2 emissions1) WFPS
TWW Without N
CK With N
Without N
N2O emission (μg N kg-1 dry soil) 40% 60% 80% 100% Ave
5.55±1.77 40.5±1.77 108.52±24.38 252.58±17.85 101.79
23.44±2.86 178.81±15.36 124.64±2.00 161.70±8.59 122.15
CO2 emission (mg C kg-1 dry soil) 40% 60% 80% 100% Ave
1.47±0.36 1.94±0.14 2.26±0.11 1.39±0.13 1.77
2.40±0.16 3.66±0.17 2.93±0.39 1.62±0.22 2.65
1.62±0.55 1.94±0.04 2.14±0.19 0.93±0.08 1.66
P-level
F statistics 6.427 144.698 240.189 5.677 0.295 27.689 0.41
Source Source of water (S) WFPS (W) N fertilizer (N) S×W S× N W×N S × W× N
4.64±0.98 53.11±10.2 94.33±6.88 152.31±21.29 76.10
49.44±4.99 127.88±17.55 164.00±16.16 127.95±2.01 117.32 2.52±0.01 3.42±0.09 2.90±0.26 1.09±0.19 2.48 CO2 emission
N2O emission F statistics 18.295 311.507 74.473 24.528 8.546 108.994 7.307
With N
** ** ** ** ** ** **
P-level * ** ** **
ns **
ns
WFPS, water-filled pore space; TWW, treated wastewater. , significant at the 0.05 probability level; **, significant at the 0.01 probability level; ns, not significant.
1) *
Fig. 2 CO2 emission rate in soil incubated at 40 (A), 60 (B), 80 (C), and 100% (D) WFPS.
when N fertilizer was incorporated into soils. Generally, higher CO2 emissions were observed in soils incubated at 60 and 80% WFPS compared to that in the drier (40% WFPS) and wetter (100% WFPS) soils. The largest cumulative emission of CO2 (3.66 mg C kg-1) was measured from added urea soils incubated at 60% WFPS under TWW, and almost 1.6 times larger than the lowest CO2 emission in unamended soils incubated at 100% WFPS receiving distilled water (0.93 mg C kg -1). Under TWW incubation, adding urea enhanced soil CO2 emissions compared to unamended soils.
Interaction between WFPS and N input on N2O under TWW incubation As we expected, our results showed significant interaction between WFPS and N input on the emission of N2O, especially when treated with TWW irrigation. On the one hand, the direct effects of treated wastewater irrigation on N2O emissions might be mainly due to its complicated nutrient components (organic matter, organic N, and inorganic N). Enhanced N2O losses are also indirectly induced by increasing water content and low O2 availability thereby benefiting denitification in
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soils receiving TWW. Hence, to differentiate the direct and indirect effects from TWW irrigation, we postulated that the cumulative N2O emissions independently induced by treated wastewater should be equal to the difference of N2O losses of TWW minus CK (eq. 1). Here we presented an approximate equation to test the combined interaction between WFPS and N input on N2O emissions based on TWW+urea treatment (eq. 2). As the analysis of variance results shown, the production of N2O under TWW+urea was statistically lower (P<0.05) than the imaginary treatment of N 2 O TWW direct+N2OCK+urea (Fig. 3), indicating that there might be a more comprehensive impact on N2O emission when TWW and urea were applied together. Moreover, a highly positive correlation (R2=0.69) between TWW+urea and assumed treatment was measured in this case, suggesting that our assumption about this equation makes sense and could partially interpret the independent effect of treated wastewater irrigation. N2OTWW direct=N2OTWW-N2OCK (1) N 2 O TWW+urea =N 2 O TWW N2OCK+N2OCK+urea
direct
+N 2 O CK+urea =N 2 O TWW (2)
non-linear regression analysis (Table 2). N=Ce(aw+rw ) Where, C is an integral constant, r is b/2.
(3)
2
(1) Since W represents the WFPS, W [0, 1], a, b R. Therefore, N 0, the solution of eq. (3) is nonnegative and bounded in XOY. (2) The initial condition occurs as: N=CW=0 (4) The N should be equal to C when W is 0. Generally speaking, N should be equal to 0 (N represents the production of N2O by TWW incubation as driven factor in eq. (3). But C isn’t equal to 0 in Table 2 according to non-linear regression analysis, C changes in the range of 10-8-10-3; C is approximately equal to 0 in the practical application. (3) When the WFPS reaches the maximum, W=1 then eq. (3) will be: (5) N=Cea+r The eq. (5) is a monotonically increasing function, and consistent with the observed experimental data. (4) The intensity of the production of N2O driven by TWW could transfer from eq. (3):
Parameters and model discussion Eq. (3) is a numerical model for describing the net increase of N2O emissions in soils incubated at different WFPS under treated wastewater irrigation. According to the observed net increases of N2O emission from jars at d 1, parameters in eq. (3) were calculated by
(6) Where in the left of eq. (6) is the ratio of the rate of change (%) of TWW due to change rate of N2O emission, the ratio can be used to quantitatively characterize the driven strength of N2O emission by changing TWW. In our case, we calibrated the accuracy of the model using the data of net increase of N2O emission at d 3, 5, and 7, the parameters in eq. (3) were calculated by non-linear regression (Table 2). The results of the model calibration were shown in Fig. 4. Simulation of results were satisfactory for soil without adding urea (R 2=0.81), but poor for soil applied urea (R2=0.17) due to the huge N2O emission peak in soil incubated at 60% WFPS (Fig. 4-B). Based upon the continuous function relationship between N 2O and Table 2 The parameters of N 2O emission model under TWW incubation Fertilizer
Fig. 3 The combined interaction between WFPS and N input on N2O emissions compared to TWW+urea treatment.
Without urea Urea
Parameters C
a
3.47E-06 4.19E-05
19.43 14.04
b -18.76 -13.86
R2 0.96 0.94
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
WFPS, N2O emission should not change dramatically with a corresponding change of WFPS. In soil incubated at 60% WFPS, the N 2O production was an order of magnitude larger than other WFPS treatments. The system error can be calibrated due to the systematic variation. But if the data from soil incubated at 60% WFPS were lower by an order of magnitude, the simulated results became much better, as the determi-
Fig. 4 Model estimation for cumulative emission of N2O in soil receiving treated wastewater unamended urea (A), added urea (B), and added uera-corrected (C).
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nation coefficient (R2) was 0.83 (Fig. 4-C).
DISCUSSION The reuse of treated wastewater is an increasing trend for sustainable water resource utilization. Irrigation of crops with wastewater is already a common practice in urban and suburban farming communities of the developing world (Rutkowski et al. 2007). Urban wastewater, after secondary treatment to minimize health hazards, may provide organic matter, phosphorus and inorganic N content as valuable nutrients for soil and crops or alter in carbon-nitrogen cycle in soil, thereby inducing greenhouse gas emissions. The cumulative emissions of N 2O over averaged WFPS and fertilizer input in soil irrigated with TWW increased by 33.8% compared to CK, which agrees with past researches (Monnett et al. 1995; Master et al. 2004; Zou et al. 2009). Enhanced N2O emissions induced by TWW are mainly due to the improved C and N availability as substrate derived from TWW. Generally, elevated nutrient availability for soil nitrification and denitrification processes could benefit nitrifying and denitrifying bacterial activities and increase microbial biomass (Meli et al. 2002; Ramirez-Fuentes et al. 2002), and then result in larger N2O emission. In contrast, Master et al. (2003) reported that reclaimed effluent application did not increase the N2O emission due to low nutrients content (BOD5=100 mg L-1). Fernández-Luqueño et al. (2010) also stated that irrigation with wastewater had no significant effect on the emission of N2O compared with unamended soil. Treated wastewater used in this study had a relatively low level of BOD 5 (24.4 mg L -1), but the increased nitrous oxide emission data indicated that nutrient content is not a limiting factor for the scenario of treated wastewater irrigation. When urea was incorporated, there was no significant effect on N2O emission in soil receiving TWW (122.15 μg N kg-1) compared to added urea soil irrigated with distilled water (117.32 μg N kg-1), suggesting that irrigation water type might not be a dominating factor when a large amount of source of nitrogen was available without regard to the effect of water-filled pore space. However, this explanation was rejected due to the statistical result based upon assumed eqs. (1) and (2).
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Correlation analysis results indicated that N2O emissions caused by irrigation with treated wastewater combined with urea fertilization did not simply add linearly, but significant interaction (P<0.05) caused lower emission than the hypothetically constructed treatment. For this reason, we concluded that N from TWW and urea showed intense interaction on N2O emission. Emission of CO2 from soil is due to two primary processes: the production of CO2 (mainly respiration by plant roots and microbes) and gas transport through the soil which controls the movement of CO2 from the soil to the atmosphere and of O 2 in the opposite direction. In this study, CO2 emission was mainly from uncultivated soil due to microbial respiration. Our results demonstrated that cumulative CO2 emission was significantly influenced by TWW, which was in agreement with the findings of Fernández-Luqueño et al. (2010). Organic material added through treated wastewater will increase the emission of CO2 due to decomposition (Rosso et al. 2008). Furthermore, the use of secondary-treated municipal wastewater in irrigation has been shown to enhance the nutrients availability, microbial metabolism and enzyme activity in some field experiments (Brzezinska et al. 2006; Chen et al. 2008; Adrover et al. 2010), and microbial activities may be stimulated by the biodegradable organic matter and nutrients in reclaimed water, increasing the activities of soil enzymes (Chen et al. 2008). A tiny variation of soil moisture results in a significant impact on production and consumption of greenhouse gases (Smith et al. 2003; Del Prado et al. 2006). Among all WFPS treatments, the emission rates of N 2O in soil incubated at 40% WFPS remained low during the entire incubation period, which might be the gaseous product of nitrification (Stevens et al. 1997). Because there are few anaerobic zones existing in the soil, low soil moisture content reduces microbial activity through osmotic stress (Stark et al. 1995). N 2O evolution in unamended soil increased with increasing water content and showed a maximum at 100% WFPS. This observation agrees with the results of other laboratory incubations (Hou et al. 2000; Dobbie et al. 2001, 2003; Bateman et al. 2005; Ruser et al. 2006; Guo et al. 2010). When urea was incorporated, cumulative N2O
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emission in soil incubated at 60% WFPS was significantly for this phenomenon might be that we did not preincubate soil samples before the onset of the experiment with microbial activity kept unstable under substantial substrate and suitable temperature conditions, and thus induced an undesired peak of N 2O emission (Khalil et al. 2002). Our results are also in agreement with McSwiney et al. (2005), who observed that the magnitude of N2O fluxes was controlled by the interaction between soil water content and N availability. Soil water content is also an important factor that affects the rate of respiration (Xu et al. 2001). In this experiment, the CO2 emission rate in soil incubated at the medium range of WFPS (60 or 80%) reached a maximum. There is a wide range of water contents (40-70% WFPS) for the change of CO2 emission. In very wet soils (>80% WFPS), aeration is restricted because a large proportion of the pores are filled with water (Smith et al. 2003). Urea, one of inorganic fertilizers widely used in agriculture, is the main source of N2O emission from arable soil (Mosier et al. 1991; Hou et al. 2000; Khalil et al. 2002; Akiyama et al. 2003; Dittert et al. 2005). In our study, fertilizing with urea significantly increased CO 2 production, which agrees with the results from Silva et al. (2008). The addition of inorganic N fertilizer often stimulates the microbial activity and thereby CO2 emissions (Aarnio et al. 1996). Cumulative N2O emissions in added urea soil incubated at 40, 60, and 80% WFPS were significantly larger compared to that in soil without urea application. As we expected, cumulative N 2 O emission in TWW+urea treatment was supposed to be the largest due to higher available C and N when the soil was incubated at 100% WFPS. In contrast, the production of N2O in unamended soil was larger than that in added urea soil incubated at 100% WFPS, the reason might be that quite a number of available C and N source inhibits the emission of N2O (Maljanen et al. 2003) or accelerate the last step of denitrification (Bowman et al. 1998). So far, several N2O emission models have been set up in agro-ecosystem to evaluate the greenhouse gas emission and global warming potential. These include the process-oriented simulation models DNDC (Li
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
et al. 1992a, b) and DayCent (Parton et al. 1996; Grosso et al. 2000), and also some empirical models such as those relating flux to WFPS, temperature, and soil mineral N content (Conen et al. 2000; Del Prado et al. 2006). In our study, increases in WFPS also result in an exponential increase in N2O emission from eq. (3), this shows agreement with some observations (Keller et al. 1994; Smith et al. 2003). Nevertheless, in this model, water-filled pore space was conducted as a single factor, when actually it is a compound factor because: 1) treated wastewater is not only iwater, but contains nutrient elements (C and N) and organisms, which might be closely related with nitrous oxide emission; 2) treated wastewater is obviously one type of source of water, but increased soil moisture condition from any source has a remarkable influence on N2O emissions, which is also proved in soil irrigated with distilled water. Hence, our simple model can only describe the emission of N2O in soil receiving TWW under laboratory conditions, and still needs to be calibrated by field data in combination with the complex interactive effects of factors such as tillage, soil texture, and temperature.
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MATERIALS AND METHODS Sampling sites The field site was located at Bei Yechang Treated Wastewater Irrigation District (39°27´N, 116°24´E), Beijing, China, which was cultivated with tomato under treated wastewater irrigation since 2008. The mean annual air temperature is 11.5°C and the 50-yr average annual precipitation is 507.2 mm. In this study, soil samples (0-30 cm) in the field site (667 m2) were randomly collected by shovel and brought to the laboratory, and then air-dried for 1 wk and sieved through 2 mm mesh on April, 2010, visible roots and stones were removed. The air-dried soil had the following properties: sand 58.0%, silt 34.0%, clay 8.0%, total N 1.5 g kg-1, NO3--N 91.6 mg kg-1, NH4+-N 2.0 mg kg-1, total organic C 12.6 g kg-1, organic matter 23.2 g kg-1, pH 7.7, and EC 1.0 ds m-1. The chemical properties of treated wastewater used in this study were shown in Table 3.
Experimental procedure In this experiment, a factorial design was used to determine gas production by three factors: water quality (treated wastewater and distilled water), fertilizer (urea-N and none), and water-filled pore space (40, 60, 80, and 100%). Each factorial combination was replicated three times, with a total of 48 experimental units and 192 jars. 200 g air-dried soil was added to a 250-mL jar, moistened to 40, 60, 80, and 100% WFPS using a pipette to add water evenly with treated wastewater (TWW) or distilled water (CK). Additionally, 200 mg urea N kg -1 dry soil was added to half the soil samples as N input, the amount of which was decided from North China Plain cucumber fertilization guidance (Zhang et al. 2009). All the jars were sealed with a rubber lid, placed into a growth chamber and incubated at 25°C for 7 d. Twelve jars without soils were prepared as blanks to account for the CO2 and N2O in the atmosphere during each sampling period. The headspace of each jar was sampled and analyzed for CO2 and N2O.
CONCLUSION Our results demonstrated that applied treated wastewater to soil could enhance the emission of N2O and CO2. WFPS and N input were still the main driving factors for greenhouse gas emission from soil receiving treated wastewater. In general, cumulative N2O emission in unamended soil had a positive correlation with increasing WFPS, and the maximum production of N2O occurred in added urea soil incubated at 60% WFPS. However, appropriate soil moisture content such as 60 and 80% WFPS will be helpful for CO 2 emission from soil. Moreover, N inputs, derived from urea and treated wastewater, interact and thus involve in the production of N2O. Our simple numerical model showed a good result on the exponential relationship between N2O emission and WFPS.
Sampling and analysis Three jars were randomly selected from each treatment. Gas samples (200 mL) were collected from the headspace in each jar using an evacuated pump at d 1, 3, 5, and 7, and stored in the air pocket for determining net increases of
Table 3 Chemical parameters of treated wastewater used in this study pH
EC (ms m-1)
TN (mg L-1)
TOC (mg L-1)
BOD 5
CODcr
NH4+-N (mg L-1)
NO3--N (mg L-1)
8.7
145
33.1
10.7
24.4
34.9
0.18
31.4
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N2O and CO2 emissions within 7 d. 10 mL gas samples were collected from the air pocket by syringe, and then analyzed for CO2 and N2O by gas chromatograph (Agilent 7890A, Agilent Technologies Inc., USA) equipped with a 63Ni electron capture detector (ECD) for N2O analysis and a flame ionization detector (FID) for CO2 analysis. After gas samples were taken, fresh soil samples were destructively sampled from the jars, and soil water content was measured by oven drying method. Soil samples (10 g air-dried weight) were extracted with 50 mL of KCl (2 mol L-1) and shaken for 1 h, then filtered through Whatman no. 42 filter. Concentrations of extracted NH4+ and NO3- were determined by UV Spectrophotometer (UV-1200, MAPADA Inc., China).
Statistical analysis Cumulative N2O emissions were calculated by integrating measured data through linear interpolation between consecutive sampling dates. In the text, means±standard errors of means (SEM) were reported. Statistical analysis was conducted by the General Linear Model of the SPSS17.0 package. Significant differences were reported at the 0.05 and 0.01 probability levels. Non-linear correlation analysis was used to calculate the parameters of the eq. between net N2O increase and WFPS.
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In this experiment, the change of bulk density caused by the irrigation was very small due to adding water and fertilizer evenly into the soils, so this change was ignored. The range of WFPS was set from 0 to 1, the emitted nitrous oxide just response to a limited range of WFPS, so the effect of WFPS (>1.0) on N2O emission was ignored. As a compound input, the enhanced N2O emission may occur due to the elevated nutrient availability and water content, when treated wastewater was irrigated. Therefore, we assumed there should be interaction between these two factors on N2O emission. The incubation temperature kept at 25°C during the experiment, so the change of volume caused by temperature was ignored. Carbon had been regarded as an important factor when denitrification in the soil occurred (Groffman et al. 1988). The selected soil material had low carbon content (Table 3), and we presumed that the effect of carbon on rapid denitrification in the soil samples was consistent, so we ignored this effect. Thus, it is reasonable for the above hypotheses in this study. Construction of the model Here, the production of N2O was defined as net increase of headspace gas concentration which was conducted as the function in the numerical model. By taking WFPS and N inputs together into account, the numerical model for specific volume change of the soil applied with a certain amount of TWW at certain water content is constructed as: (7)
Model for soil N2O emission Interaction between WFPS and N input As mentioned above, WFPS is one of the most important soil physical properties, which is a soil physical parameter indicating whether nitrification or denitrification becomes dominant (Hofman et al. 2004). When the soil was irrigated with treated wastewater, increased water content and nutrient availability might highly affect the emission of N2O. Hence, there should be an interaction between WFPS and N input on N2O emission in the soil under TWW irrigation in our study. Hypotheses for the model (1) The WFPS-treated soil particles were homogeneous and isotropic, and the effect of WFPS treatment on soil bulk density was ignored. Therefore, WFPS of the soil samples kept constant through the course of the experiment. (2) The change of N2O emission only response to a limited range of changes of WFPS. (3) N elements and increased water content combined together involving in the formation of nitrous oxide. (4) At normal temperature, the effect of temperature on N 2O emissions was ignored, so the pore volume and O 2 availability change caused by temperature change was not taken into account. (5) The carbon content in homogeneous soil particles was relatively low, so the benefit for rapid denitrification was ignored in this case.
N, production of N2O from different sources of nitrogen; W, WFPS; a, b-constants:
Acknowledgements The study was funded by the National Natural Science Foundation of China (50979107). We also grateful to Liang Yuan, China Agricultural University, for lab analysis and gas sampling. Many thanks also to people in the Public Analysis and Test Center of Beijing Forestry University, China, for the timely analyses of our samples.
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
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and cultivated grasslands. Nature, 350, 330-332. Ndour N, Baudoin E, Guissé A, Seck M, Khouma M, Brauman A. 2008. Impact of irrigation water quality on soil nitrifying and total bacterial communities. Biology and Fertility of Soils, 44, 797-803. Neeteson J J, Carton O T. 2001. The environmental impact of nitrogen in field vegetable production. Acta Horticulturae, 563, 21-28. Parton W J, Mosier A R, Ojima D S, Valentine D W, Schimel D S, Weier K, Kulmala A E. 1996. Generalized model for N2 and N2O production from nitrification and denitrification. Global Biogeochemical Cycles, 10, 401-412. Ramirez-Fuentes E, Lucho-Constantino C, Escamilla-Silva E, Dendooven L. 2002. Characteristics, and carbon and nitrogen dynamics in soil irrigated with wastewater for different lengths of time. Bioresource Technology, 85, 179-187. Ravishankara A R, Daniel J S, Portmann R W. 2009. Nitrous oxide (N2O): the dominant ozone-depleting substance emitted in the 21st century. Science, 326, 123-125. Rosso D, Stenstrom M K. 2008. The carbon-sequestration potential of municipal wastewater treatment. Chemosphere, 70, 1468-1475. Ruser R, Flessa H, Russow R, Schmidt G, Buegger F, Munch J C. 2006. Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting. Soil Biology and Biochemistry, 38, 263-274. Rutkowski T, Raschid-Sally L, Buechler S. 2007. Wastewater irrigation in the developing world - two case studies from the Kathmandu Valley in Nepal. Agricultural Water Management, 88, 83-91. Silva C C, Guido M L, Ceballos J M, Marsch R, Dendooven L. 2008. Production of carbon dioxide and nitrous oxide
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in alkaline saline soil of texcoco at different water contents amended with urea: a laboratory study. Soil Biology and Biochemistry, 40, 1813-1822. Skiba U, Smith K A. 2000. The control of nitrous oxide emissions from agricultural and natural soils. Chemosphere Global Change Science, 2, 379-386. Smith K A, Ball T, Conen F, Dobbie K E, Massheder J, Rey A. 2003. Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes. European Journal of Soil Science, 54, 779-791. Stark J M, Firestone M K. 1995. Mechanisms for soil moisture effects on activity of nitrifying bacteria. Applied and Environmental Microbiology, 61, 218-221. Stevens R J, Laughlin R J, Burns L C, Arah J R M, Hood R C. 1997. Measuring the contributions of nitrification and denitrification to the flux of nitrous oxide from soil. Soil Biology and Biochemistry, 29, 139-151. Xu M, Qi Y. 2001. Soil-surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California. Global Change Biology, 7, 667-677. Yadav R K, Goyal B, Sharma R K, Dubey S K, Minhas P S. 2002. Post-irrigation impact of domestic sewage effluent on composition of soils, crops and ground water - a case study. Environment International, 28, 481-486. Zhang F S, Chen X P, Chen Q. 2009. China’s Major Crops Fertilization Guide. China Agricultural Univeristy Press, Beijing, China. (in Chinese) Zou J, Liu S, Qin Y, Pan G, Zhu D. 2009. Sewage irrigation increased methane and nitrous oxide emissions from rice paddies in southeast China. Agriculture Ecosystems and Environment, 129, 516-522. (Managing editor WANG Ning)
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2012, 11(8): 1354-1364
RESEARCH ARTICLE
Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with Treated Wastewater XUE Yan-dong1, YANG Pei-ling1, LUO Yuan-pei2, LI Yun-kai1, REN Shu-mei1, SU Yan-ping1 and NIU Yongtao1 1 2
College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, P.R.China Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China
Abstract The reuse of treated wastewater in agricultural systems could partially help alleviate water resource shortages in developing countries. Treated wastewater differs from fresh water in that it has higher concentrations of salts, Escherichia coli and presence of dissolved organic matter, and inorganic N after secondary treatment, among others. Its application could thus cause environmental consequences such as soil salinization, ammonia volatilization, and greenhouse gas emissions. In an incubation experiment, we evaluated the characteristics and effects of water-filled pore space (WFPS) and N input on the emissions of nitrous oxide (N2O) and carbon dioxide (CO2) from silt loam soil receiving treated wastewater. Irrigation with treated wastewater (vs. distilled water) significantly increased cumulative N2O emission in soil (117.97 μg N kg-1). Cumulative N 2O emissions showed an exponentially increase with the increasing WFPS in unamended soil, but the maximum occurred in the added urea soil incubated at 60% WFPS. N 2O emissions caused by irrigation with treated wastewater combined with urea-N fertilization did not simply add linearly, but significant interaction (P<0.05) caused lower emissions than the production of N2O from the cumulative effects of treated wastewater and fertilizer N. Moreover, a significant impact on cumulative CO2 emission was measured in soil irrigated with treated wastewater. When treated wastewater was applied, there was significant interaction between WFPS and N input on N2O emission. Hence, our results indicated that irrigation with treated wastewater should cause great concern for increasing global warming potential due to enhanced emission of N2O and CO2. Key words: treated wastewater, nitrous oxide, carbon dioxide, water-filled pore space, urea
INTRODUCTION The concentration of carbon dioxide (CO2) and nitrous oxide (N2O) in the atmosphere has increased considerably over the last century and is set to rise further at a rate of approximately 0.4 and 0.25% per year, respectively. CO2 is the most important anthropogenic greenhouse gas. Its annual emissions have grown be-
tween 1970 and 2004 by about 80% (IPCC 2007). Compared with CO2 emission, N 2O emission is currently the single most important ozone-depleting emission and is expected to remain the largest throughout the 21st century (Ravishankara et al. 2009). With a long residence time in the atmosphere (120 yr), N2O is responsible for about 6% of the current greenhouse effect and it is approximately 300 times more potent than CO2 in absorbing terrestrial thermal radiation in
Received 10 June, 2011 Accepted 22 November, 2011 Correspondence YANG Pei-ling, Tel/Fax: +86-10-62737866, E-mail: [email protected]
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
the troposphere (IPCC 2007). Agricultural soil is the major source of N 2O, accounting for about 42% of the global annual emission (IPCC 2007). Enhanced N2O emissions from agricultural soils are mainly due to increasing N inputs by mineral fertilizers, manure application and low quality water such as sewage effluent and treated wastewater (Monnett et al. 1995; Zou et al. 2009; Fernández-Luqueño et al. 2010). Treated wastewater has become an important water source for agriculture in urban and suburban communities of the developing world (Rutkowski et al. 2007). Beijing, for example, is estimated to produce 1.0×109 m3 of treated wastewater per year by 2015, while the utilization of treated wastewater is expected to reach 75% based upon the “Key Technologies R&D Program of China during the 12th Five-Year Plan period”. In general, irrigation with treated wastewater could supply important nutrients such as inorganic N and organic matter, and thus has a potential for increasing the nutrient levels of soils (Yadav et al. 2002). Generally, however, treated wastewater differs from its fresh source water in its higher concentration of nutrients, salt, Escherichia coli, bacteria, fungi, and suspended solids after secondary treatment, and poses a risk both to crop growth and soil environment with additional environmental consequences, such as ammonia volatilization, nitrate leaching, and nitrous oxide emission (Neeteson et al. 2001). Recent studies have found both positive and negative effects on the biogeochemical cycling of C and N in long-term treated wastewater application sites (Meli et al. 2002; Ramirez-Fuentes et al. 2002; Chen et al. 2008). In fact, soil microbial biomass carbon and cumulative respiration are enhanced in soils under long term wastewater application (Meli et al. 2002). Furthermore, it has been demonstrated that enzyme activities involved in C and N cycling in soil are stimulated by reclaimed water irrigation (Chen et al. 2008). However, studies state that treated wastewater irrigation significantly increases the production of N2O (Master et al. 2003; Master et al. 2004; Zou et al. 2009), CO 2 emission and global warming potential (Fernández-Luqueño et al. 2010). One possible explanation for this phenomenon is that readily available C and N in treated wastewater could benefit nitrifying and denitrifying bacterial communities and their activi-
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ties in soils, thereby increasing N2O emissions (Ndour et al. 2008). Moreover, N mineralization is so low or absent that the losses of N could not be compensated through wastewater application. N fertilizer must then be applied to maintain the yield of crop production (Ramirez-Fuentes et al. 2002). Hence, this poses several concerns over how treated wastewater application may affect greenhouse gas emissions (GHG), what the intensity of this effect is, and how the combination of N derived from fertilizer and treated wastewater affects on N2O losses. Soil moisture content is very important in regulating gas emissions and the gas production processes (Skiba et al. 2000). Several laborary studies involving incubation of disturbed or undisturbed soil by different waterfilled pore space have found N2O emissions to be closely related to the dynamic nature of water-filled pore space (WFPS) (Hou et al. 2000; Dobbie et al. 2001, 2003; Del Prado et al. 2006; Ciarlo et al. 2007). It is a priority to study the N2O emission in order to quantitatively indicate the production from TWW (treated wastewater) treat soil incubated at specific WFPS. However, to our knowledge, it is hard to differentiate the effects of treated wastewater and increased soil water content compared with fresh water. We hypothesize that there is an interaction between WFPS and N input on N2O emission in the soil receiving TWW. To test our hypothesis, the objectives of this study are to: 1) quantify CO2 and N2O emissions affected by treated wastewater application; 2) assess the interaction between urea-N and TWW-N on N2O losses; and 3) provide some useful information about the influence of treated wastewater irrigation on greenhouse gas emissions from the utilization of sewage effluent in the North China Plain.
RESULTS N2O and CO2 emissions of soil incubated with treated wastewater Over the course of the experiment, the patterns of nitrous oxide emission rate in the unamended soils were generally similar and peaked at day 1 (Fig. 1). The N2O fluxes were very small (<3.0 μg N kg-1 d-1) in soils incubated at 40% WFPS (Fig. 1-A). As Fig. 1-B shows,
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Fig. 1 N2O emission rate in soil incubated at 40 (A), 60 (B), 80 (C), and 100% (D) WFPS. Values are means±SD. The same as below.
when the soils were incubated at 60% WFPS, the patterns of N2O emission rates in soils without N application were different from soils added with urea, with considerably larger peaks observed at d 5 of the incubation for TWW (54.11 μg N kg-1 d-1) and CK (60.79 μg N kg-1 d-1). There were similar trends for N2O fluxes in soils incubated at 80% WFPS (Fig. 1-C). The largest peak fluxes were found from unamended soils (86.97 μg N kg-1 d-1) and added urea soils (53.89 μg N kg-1 d-1) incubated at 100% WFPS under treated wastewater irrigation (Fig. 1-D), respectively. Differences in cumulative N2O emissions were significant (P<0.01) for all main effects included in this experiment. Moreover, the interactions of water × WFPS, water × N fertilizer, WFPS × N fertilizer, and water × WFPS × N fertilizer also significantly contributed to the N 2O losses (Table 1). On average, cumulative N2O emission averaged over WFPS and N fertilizer in soils irrigated with TWW (117.97 μg N kg-1) was remarkably greater than that in soils receiving distilled water (96.71 μg N kg -1 ). In unamended soils, the production of N 2O under TWW irrigation were 33.8% larger than control treatment (CK). When fertilizer N was incorporated, cumulative N2O emissions were similar compared to the soils applied urea. Soil water-filled pore space had a significant impact on the emissions of N2O in this experiment (P<0.01). N2O emissions increased with the increasing water content in soils without fertilizer N application. Cumulative N2O emissions (<6.0 μg N kg-1) were very low in unamended soils associated at 40% WFPS, and showed a maximum (252.58 μg N kg-1) in unamended soils in-
cubated at 100% WFPS under TWW irrigation. To the contrary, the maximum of N2O loss in added urea soils was measured at 60% WFPS treated with TWW (178.81 μg N kg-1), which was 6.9 times larger than the minimum N2O loss in added urea soils incubated at 40% WFPS. When urea was added, N2O losses under TWW irrigation were 21% higher than that in soils without fertilizer. A larger range was measured from the added urea soils irrigated with distilled water, which had a 54% increase for N2O emission compared to unamended soils. In soil incubated at 40, 60, and 80% WFPS, the cumulative N2O emissions in soils amended with urea were greater than the unamended soils. In contrast, higher N2O production in unamended soils incubated at 100% WFPS was measured compared to soils applied with urea (Table 1). The mean CO2production rate was higher in soils irrigated with TWW compared to the distilled water incubation (P<0.05). The peaks of CO2 emission rate were measured at d 1, after which they decreased until d 7 except the CK and CK+urea treatments under 100% WFPS incubation, which peaked around d 3 and 5, respectively (Fig. 2). Statistical analysis showed that all three main factors significantly contributed to the CO2 emissions (P<0.05). However, there was no significant interaction between water source and fertilizer N application (Table 1). The cumulative CO2 production was larger in soils irrigated with TWW (2.21 mg C kg -1) compared to soils irrigated with distilled water (2.07 mg C kg -1). Considerably higher CO 2 emissions in unamended soils were detected at 80% WFPS compared with other WFPS treatments. However, the cumulative CO 2 losses showed a maximum at 60% WFPS
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Table 1 Effects of WFPS and urea on cumulative N2O and CO2 emissions1) WFPS
TWW Without N
CK With N
Without N
N2O emission (μg N kg-1 dry soil) 40% 60% 80% 100% Ave
5.55±1.77 40.5±1.77 108.52±24.38 252.58±17.85 101.79
23.44±2.86 178.81±15.36 124.64±2.00 161.70±8.59 122.15
CO2 emission (mg C kg-1 dry soil) 40% 60% 80% 100% Ave
1.47±0.36 1.94±0.14 2.26±0.11 1.39±0.13 1.77
2.40±0.16 3.66±0.17 2.93±0.39 1.62±0.22 2.65
1.62±0.55 1.94±0.04 2.14±0.19 0.93±0.08 1.66
P-level
F statistics 6.427 144.698 240.189 5.677 0.295 27.689 0.41
Source Source of water (S) WFPS (W) N fertilizer (N) S×W S× N W×N S × W× N
4.64±0.98 53.11±10.2 94.33±6.88 152.31±21.29 76.10
49.44±4.99 127.88±17.55 164.00±16.16 127.95±2.01 117.32 2.52±0.01 3.42±0.09 2.90±0.26 1.09±0.19 2.48 CO2 emission
N2O emission F statistics 18.295 311.507 74.473 24.528 8.546 108.994 7.307
With N
** ** ** ** ** ** **
P-level * ** ** **
ns **
ns
WFPS, water-filled pore space; TWW, treated wastewater. , significant at the 0.05 probability level; **, significant at the 0.01 probability level; ns, not significant.
1) *
Fig. 2 CO2 emission rate in soil incubated at 40 (A), 60 (B), 80 (C), and 100% (D) WFPS.
when N fertilizer was incorporated into soils. Generally, higher CO2 emissions were observed in soils incubated at 60 and 80% WFPS compared to that in the drier (40% WFPS) and wetter (100% WFPS) soils. The largest cumulative emission of CO2 (3.66 mg C kg-1) was measured from added urea soils incubated at 60% WFPS under TWW, and almost 1.6 times larger than the lowest CO2 emission in unamended soils incubated at 100% WFPS receiving distilled water (0.93 mg C kg -1). Under TWW incubation, adding urea enhanced soil CO2 emissions compared to unamended soils.
Interaction between WFPS and N input on N2O under TWW incubation As we expected, our results showed significant interaction between WFPS and N input on the emission of N2O, especially when treated with TWW irrigation. On the one hand, the direct effects of treated wastewater irrigation on N2O emissions might be mainly due to its complicated nutrient components (organic matter, organic N, and inorganic N). Enhanced N2O losses are also indirectly induced by increasing water content and low O2 availability thereby benefiting denitification in
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soils receiving TWW. Hence, to differentiate the direct and indirect effects from TWW irrigation, we postulated that the cumulative N2O emissions independently induced by treated wastewater should be equal to the difference of N2O losses of TWW minus CK (eq. 1). Here we presented an approximate equation to test the combined interaction between WFPS and N input on N2O emissions based on TWW+urea treatment (eq. 2). As the analysis of variance results shown, the production of N2O under TWW+urea was statistically lower (P<0.05) than the imaginary treatment of N 2 O TWW direct+N2OCK+urea (Fig. 3), indicating that there might be a more comprehensive impact on N2O emission when TWW and urea were applied together. Moreover, a highly positive correlation (R2=0.69) between TWW+urea and assumed treatment was measured in this case, suggesting that our assumption about this equation makes sense and could partially interpret the independent effect of treated wastewater irrigation. N2OTWW direct=N2OTWW-N2OCK (1) N 2 O TWW+urea =N 2 O TWW N2OCK+N2OCK+urea
direct
+N 2 O CK+urea =N 2 O TWW (2)
non-linear regression analysis (Table 2). N=Ce(aw+rw ) Where, C is an integral constant, r is b/2.
(3)
2
(1) Since W represents the WFPS, W [0, 1], a, b R. Therefore, N 0, the solution of eq. (3) is nonnegative and bounded in XOY. (2) The initial condition occurs as: N=CW=0 (4) The N should be equal to C when W is 0. Generally speaking, N should be equal to 0 (N represents the production of N2O by TWW incubation as driven factor in eq. (3). But C isn’t equal to 0 in Table 2 according to non-linear regression analysis, C changes in the range of 10-8-10-3; C is approximately equal to 0 in the practical application. (3) When the WFPS reaches the maximum, W=1 then eq. (3) will be: (5) N=Cea+r The eq. (5) is a monotonically increasing function, and consistent with the observed experimental data. (4) The intensity of the production of N2O driven by TWW could transfer from eq. (3):
Parameters and model discussion Eq. (3) is a numerical model for describing the net increase of N2O emissions in soils incubated at different WFPS under treated wastewater irrigation. According to the observed net increases of N2O emission from jars at d 1, parameters in eq. (3) were calculated by
(6) Where in the left of eq. (6) is the ratio of the rate of change (%) of TWW due to change rate of N2O emission, the ratio can be used to quantitatively characterize the driven strength of N2O emission by changing TWW. In our case, we calibrated the accuracy of the model using the data of net increase of N2O emission at d 3, 5, and 7, the parameters in eq. (3) were calculated by non-linear regression (Table 2). The results of the model calibration were shown in Fig. 4. Simulation of results were satisfactory for soil without adding urea (R 2=0.81), but poor for soil applied urea (R2=0.17) due to the huge N2O emission peak in soil incubated at 60% WFPS (Fig. 4-B). Based upon the continuous function relationship between N 2O and Table 2 The parameters of N 2O emission model under TWW incubation Fertilizer
Fig. 3 The combined interaction between WFPS and N input on N2O emissions compared to TWW+urea treatment.
Without urea Urea
Parameters C
a
3.47E-06 4.19E-05
19.43 14.04
b -18.76 -13.86
R2 0.96 0.94
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
WFPS, N2O emission should not change dramatically with a corresponding change of WFPS. In soil incubated at 60% WFPS, the N 2O production was an order of magnitude larger than other WFPS treatments. The system error can be calibrated due to the systematic variation. But if the data from soil incubated at 60% WFPS were lower by an order of magnitude, the simulated results became much better, as the determi-
Fig. 4 Model estimation for cumulative emission of N2O in soil receiving treated wastewater unamended urea (A), added urea (B), and added uera-corrected (C).
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nation coefficient (R2) was 0.83 (Fig. 4-C).
DISCUSSION The reuse of treated wastewater is an increasing trend for sustainable water resource utilization. Irrigation of crops with wastewater is already a common practice in urban and suburban farming communities of the developing world (Rutkowski et al. 2007). Urban wastewater, after secondary treatment to minimize health hazards, may provide organic matter, phosphorus and inorganic N content as valuable nutrients for soil and crops or alter in carbon-nitrogen cycle in soil, thereby inducing greenhouse gas emissions. The cumulative emissions of N 2O over averaged WFPS and fertilizer input in soil irrigated with TWW increased by 33.8% compared to CK, which agrees with past researches (Monnett et al. 1995; Master et al. 2004; Zou et al. 2009). Enhanced N2O emissions induced by TWW are mainly due to the improved C and N availability as substrate derived from TWW. Generally, elevated nutrient availability for soil nitrification and denitrification processes could benefit nitrifying and denitrifying bacterial activities and increase microbial biomass (Meli et al. 2002; Ramirez-Fuentes et al. 2002), and then result in larger N2O emission. In contrast, Master et al. (2003) reported that reclaimed effluent application did not increase the N2O emission due to low nutrients content (BOD5=100 mg L-1). Fernández-Luqueño et al. (2010) also stated that irrigation with wastewater had no significant effect on the emission of N2O compared with unamended soil. Treated wastewater used in this study had a relatively low level of BOD 5 (24.4 mg L -1), but the increased nitrous oxide emission data indicated that nutrient content is not a limiting factor for the scenario of treated wastewater irrigation. When urea was incorporated, there was no significant effect on N2O emission in soil receiving TWW (122.15 μg N kg-1) compared to added urea soil irrigated with distilled water (117.32 μg N kg-1), suggesting that irrigation water type might not be a dominating factor when a large amount of source of nitrogen was available without regard to the effect of water-filled pore space. However, this explanation was rejected due to the statistical result based upon assumed eqs. (1) and (2).
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Correlation analysis results indicated that N2O emissions caused by irrigation with treated wastewater combined with urea fertilization did not simply add linearly, but significant interaction (P<0.05) caused lower emission than the hypothetically constructed treatment. For this reason, we concluded that N from TWW and urea showed intense interaction on N2O emission. Emission of CO2 from soil is due to two primary processes: the production of CO2 (mainly respiration by plant roots and microbes) and gas transport through the soil which controls the movement of CO2 from the soil to the atmosphere and of O 2 in the opposite direction. In this study, CO2 emission was mainly from uncultivated soil due to microbial respiration. Our results demonstrated that cumulative CO2 emission was significantly influenced by TWW, which was in agreement with the findings of Fernández-Luqueño et al. (2010). Organic material added through treated wastewater will increase the emission of CO2 due to decomposition (Rosso et al. 2008). Furthermore, the use of secondary-treated municipal wastewater in irrigation has been shown to enhance the nutrients availability, microbial metabolism and enzyme activity in some field experiments (Brzezinska et al. 2006; Chen et al. 2008; Adrover et al. 2010), and microbial activities may be stimulated by the biodegradable organic matter and nutrients in reclaimed water, increasing the activities of soil enzymes (Chen et al. 2008). A tiny variation of soil moisture results in a significant impact on production and consumption of greenhouse gases (Smith et al. 2003; Del Prado et al. 2006). Among all WFPS treatments, the emission rates of N 2O in soil incubated at 40% WFPS remained low during the entire incubation period, which might be the gaseous product of nitrification (Stevens et al. 1997). Because there are few anaerobic zones existing in the soil, low soil moisture content reduces microbial activity through osmotic stress (Stark et al. 1995). N 2O evolution in unamended soil increased with increasing water content and showed a maximum at 100% WFPS. This observation agrees with the results of other laboratory incubations (Hou et al. 2000; Dobbie et al. 2001, 2003; Bateman et al. 2005; Ruser et al. 2006; Guo et al. 2010). When urea was incorporated, cumulative N2O
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emission in soil incubated at 60% WFPS was significantly for this phenomenon might be that we did not preincubate soil samples before the onset of the experiment with microbial activity kept unstable under substantial substrate and suitable temperature conditions, and thus induced an undesired peak of N 2O emission (Khalil et al. 2002). Our results are also in agreement with McSwiney et al. (2005), who observed that the magnitude of N2O fluxes was controlled by the interaction between soil water content and N availability. Soil water content is also an important factor that affects the rate of respiration (Xu et al. 2001). In this experiment, the CO2 emission rate in soil incubated at the medium range of WFPS (60 or 80%) reached a maximum. There is a wide range of water contents (40-70% WFPS) for the change of CO2 emission. In very wet soils (>80% WFPS), aeration is restricted because a large proportion of the pores are filled with water (Smith et al. 2003). Urea, one of inorganic fertilizers widely used in agriculture, is the main source of N2O emission from arable soil (Mosier et al. 1991; Hou et al. 2000; Khalil et al. 2002; Akiyama et al. 2003; Dittert et al. 2005). In our study, fertilizing with urea significantly increased CO 2 production, which agrees with the results from Silva et al. (2008). The addition of inorganic N fertilizer often stimulates the microbial activity and thereby CO2 emissions (Aarnio et al. 1996). Cumulative N2O emissions in added urea soil incubated at 40, 60, and 80% WFPS were significantly larger compared to that in soil without urea application. As we expected, cumulative N 2 O emission in TWW+urea treatment was supposed to be the largest due to higher available C and N when the soil was incubated at 100% WFPS. In contrast, the production of N2O in unamended soil was larger than that in added urea soil incubated at 100% WFPS, the reason might be that quite a number of available C and N source inhibits the emission of N2O (Maljanen et al. 2003) or accelerate the last step of denitrification (Bowman et al. 1998). So far, several N2O emission models have been set up in agro-ecosystem to evaluate the greenhouse gas emission and global warming potential. These include the process-oriented simulation models DNDC (Li
© 2012, CAAS. All rights reserved. Published by Elsevier Ltd.
Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
et al. 1992a, b) and DayCent (Parton et al. 1996; Grosso et al. 2000), and also some empirical models such as those relating flux to WFPS, temperature, and soil mineral N content (Conen et al. 2000; Del Prado et al. 2006). In our study, increases in WFPS also result in an exponential increase in N2O emission from eq. (3), this shows agreement with some observations (Keller et al. 1994; Smith et al. 2003). Nevertheless, in this model, water-filled pore space was conducted as a single factor, when actually it is a compound factor because: 1) treated wastewater is not only iwater, but contains nutrient elements (C and N) and organisms, which might be closely related with nitrous oxide emission; 2) treated wastewater is obviously one type of source of water, but increased soil moisture condition from any source has a remarkable influence on N2O emissions, which is also proved in soil irrigated with distilled water. Hence, our simple model can only describe the emission of N2O in soil receiving TWW under laboratory conditions, and still needs to be calibrated by field data in combination with the complex interactive effects of factors such as tillage, soil texture, and temperature.
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MATERIALS AND METHODS Sampling sites The field site was located at Bei Yechang Treated Wastewater Irrigation District (39°27´N, 116°24´E), Beijing, China, which was cultivated with tomato under treated wastewater irrigation since 2008. The mean annual air temperature is 11.5°C and the 50-yr average annual precipitation is 507.2 mm. In this study, soil samples (0-30 cm) in the field site (667 m2) were randomly collected by shovel and brought to the laboratory, and then air-dried for 1 wk and sieved through 2 mm mesh on April, 2010, visible roots and stones were removed. The air-dried soil had the following properties: sand 58.0%, silt 34.0%, clay 8.0%, total N 1.5 g kg-1, NO3--N 91.6 mg kg-1, NH4+-N 2.0 mg kg-1, total organic C 12.6 g kg-1, organic matter 23.2 g kg-1, pH 7.7, and EC 1.0 ds m-1. The chemical properties of treated wastewater used in this study were shown in Table 3.
Experimental procedure In this experiment, a factorial design was used to determine gas production by three factors: water quality (treated wastewater and distilled water), fertilizer (urea-N and none), and water-filled pore space (40, 60, 80, and 100%). Each factorial combination was replicated three times, with a total of 48 experimental units and 192 jars. 200 g air-dried soil was added to a 250-mL jar, moistened to 40, 60, 80, and 100% WFPS using a pipette to add water evenly with treated wastewater (TWW) or distilled water (CK). Additionally, 200 mg urea N kg -1 dry soil was added to half the soil samples as N input, the amount of which was decided from North China Plain cucumber fertilization guidance (Zhang et al. 2009). All the jars were sealed with a rubber lid, placed into a growth chamber and incubated at 25°C for 7 d. Twelve jars without soils were prepared as blanks to account for the CO2 and N2O in the atmosphere during each sampling period. The headspace of each jar was sampled and analyzed for CO2 and N2O.
CONCLUSION Our results demonstrated that applied treated wastewater to soil could enhance the emission of N2O and CO2. WFPS and N input were still the main driving factors for greenhouse gas emission from soil receiving treated wastewater. In general, cumulative N2O emission in unamended soil had a positive correlation with increasing WFPS, and the maximum production of N2O occurred in added urea soil incubated at 60% WFPS. However, appropriate soil moisture content such as 60 and 80% WFPS will be helpful for CO 2 emission from soil. Moreover, N inputs, derived from urea and treated wastewater, interact and thus involve in the production of N2O. Our simple numerical model showed a good result on the exponential relationship between N2O emission and WFPS.
Sampling and analysis Three jars were randomly selected from each treatment. Gas samples (200 mL) were collected from the headspace in each jar using an evacuated pump at d 1, 3, 5, and 7, and stored in the air pocket for determining net increases of
Table 3 Chemical parameters of treated wastewater used in this study pH
EC (ms m-1)
TN (mg L-1)
TOC (mg L-1)
BOD 5
CODcr
NH4+-N (mg L-1)
NO3--N (mg L-1)
8.7
145
33.1
10.7
24.4
34.9
0.18
31.4
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N2O and CO2 emissions within 7 d. 10 mL gas samples were collected from the air pocket by syringe, and then analyzed for CO2 and N2O by gas chromatograph (Agilent 7890A, Agilent Technologies Inc., USA) equipped with a 63Ni electron capture detector (ECD) for N2O analysis and a flame ionization detector (FID) for CO2 analysis. After gas samples were taken, fresh soil samples were destructively sampled from the jars, and soil water content was measured by oven drying method. Soil samples (10 g air-dried weight) were extracted with 50 mL of KCl (2 mol L-1) and shaken for 1 h, then filtered through Whatman no. 42 filter. Concentrations of extracted NH4+ and NO3- were determined by UV Spectrophotometer (UV-1200, MAPADA Inc., China).
Statistical analysis Cumulative N2O emissions were calculated by integrating measured data through linear interpolation between consecutive sampling dates. In the text, means±standard errors of means (SEM) were reported. Statistical analysis was conducted by the General Linear Model of the SPSS17.0 package. Significant differences were reported at the 0.05 and 0.01 probability levels. Non-linear correlation analysis was used to calculate the parameters of the eq. between net N2O increase and WFPS.
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In this experiment, the change of bulk density caused by the irrigation was very small due to adding water and fertilizer evenly into the soils, so this change was ignored. The range of WFPS was set from 0 to 1, the emitted nitrous oxide just response to a limited range of WFPS, so the effect of WFPS (>1.0) on N2O emission was ignored. As a compound input, the enhanced N2O emission may occur due to the elevated nutrient availability and water content, when treated wastewater was irrigated. Therefore, we assumed there should be interaction between these two factors on N2O emission. The incubation temperature kept at 25°C during the experiment, so the change of volume caused by temperature was ignored. Carbon had been regarded as an important factor when denitrification in the soil occurred (Groffman et al. 1988). The selected soil material had low carbon content (Table 3), and we presumed that the effect of carbon on rapid denitrification in the soil samples was consistent, so we ignored this effect. Thus, it is reasonable for the above hypotheses in this study. Construction of the model Here, the production of N2O was defined as net increase of headspace gas concentration which was conducted as the function in the numerical model. By taking WFPS and N inputs together into account, the numerical model for specific volume change of the soil applied with a certain amount of TWW at certain water content is constructed as: (7)
Model for soil N2O emission Interaction between WFPS and N input As mentioned above, WFPS is one of the most important soil physical properties, which is a soil physical parameter indicating whether nitrification or denitrification becomes dominant (Hofman et al. 2004). When the soil was irrigated with treated wastewater, increased water content and nutrient availability might highly affect the emission of N2O. Hence, there should be an interaction between WFPS and N input on N2O emission in the soil under TWW irrigation in our study. Hypotheses for the model (1) The WFPS-treated soil particles were homogeneous and isotropic, and the effect of WFPS treatment on soil bulk density was ignored. Therefore, WFPS of the soil samples kept constant through the course of the experiment. (2) The change of N2O emission only response to a limited range of changes of WFPS. (3) N elements and increased water content combined together involving in the formation of nitrous oxide. (4) At normal temperature, the effect of temperature on N 2O emissions was ignored, so the pore volume and O 2 availability change caused by temperature change was not taken into account. (5) The carbon content in homogeneous soil particles was relatively low, so the benefit for rapid denitrification was ignored in this case.
N, production of N2O from different sources of nitrogen; W, WFPS; a, b-constants:
Acknowledgements The study was funded by the National Natural Science Foundation of China (50979107). We also grateful to Liang Yuan, China Agricultural University, for lab analysis and gas sampling. Many thanks also to people in the Public Analysis and Test Center of Beijing Forestry University, China, for the timely analyses of our samples.
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Characteristics and Driven Factors of Nitrous Oxide and Carbon Dioxide Emissions in Soil Irrigated with
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