Geoderma 148 (2009) 421–427
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Nitrous oxide fluxes from Malagasy agricultural soils L. Chapuis-Lardy a,⁎, A. Metay a,b,1, M. Martinet b, M. Rabenarivo a, J. Toucet c, J.M. Douzet d, T. Razafimbelo e, L. Rabeharisoa e, J. Rakotoarisoa f a
UR SeqBio IRD, c/o LRI, Université d Antananarivo, BP 434, 101 Antananarivo, Madagascar ISTOM, 32 boulevard du Port, 95094 Cergy-Pontoise, Cedex 5, France UR SeqBio IRD, ENSAM, 2 place Viala, bât. 12, 34060 Montpellier Cedex 2, France d URP SCRiD CIRAD c/o SRR FOFIFA B.P. 230, Antsirabe, Madagascar e LRI-SRA, Laboratoire des Radio-isotopes, Université d'Antananarivo, Route d'Andraisoro, BP 3383, 101 Antananarivo, Madagascar f URP SCRiD FOFIFA c/o SRR FOFIFA B.P. 230, Antsirabe, Madagascar b c
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 11 September 2008 Accepted 14 November 2008 Available online 6 December 2008 Keywords: Tropical soil No-tillage N2O emissions Soil mineral N WFPS IPCC N2O emission factor
a b s t r a c t In Madagascar, no-tillage practices were developed since the early 90s to prevent soil erosion and improve soil fertility. Although such practices have helped to restore soil carbon in most cases, the impact on N2O emissions has not been investigated yet. The soil N2O fluxes and concentrations were measured during the growing season of an intercropping maize-soybean on a clayey soil of the Malagasy Highlands. Management treatments consisted of direct seeding mulch based cropping system (DMC) and traditional hand-ploughing after the preceding crop residues were harvested (HP), both with low N inputs (55–57 kg N ha− 1). No significant difference in N2O emissions was observed between treatments (DMC vs. HP). The N2O fluxes were weakly correlated to soil mineral N contents (R2 = 0.13; P = 0.03) while no relationship was emphasized with soil water filled pore space (WFPS). N2O concentrations in the soil atmosphere were correlated to fluxes at the soil surface and to soil WFPS. N2O emissions at the soil surface were low ranging from 0 to 8.84 g N-N2O ha− 1 d− 1, probably due to the low mineral N content of soil. The cumulative annual N2O emission was 0.26 kg N ha− 1 for both systems. The corresponding N loss as N2O-N was around 0.5% of applied N. This is in the uncertainty range of IPCC N2O emission factor (EF), but the IPCC EF mean estimate (1%) would overestimate true N2O emissions for the soil under evaluation. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the last few centuries, human activities such as industry, transport and agriculture have directly or indirectly contributed to the increase in concentrations of the major greenhouse gases in the atmosphere (Intergovernmental Panel on Climate Change, 2001). Three of the principal gases of interest are nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4). Both N2O and CO2 are emitted from the soil, whereas CH4 is normally oxidized by aerobic soils, making them sinks for atmospheric CH4 (Hütsch, 2001). Agricultural soils contribute about 60% of the global anthropogenic N2O flux, which is equivalent to a global warming potential of 2.8 Gt CO2 eq yr− 1 (Intergovernmental Panel on Climate Change, 2007). N2O is produced during numerous nitrogen transformations in soils (Robertson and Tiedje, 1987), but on most occasions denitrification and nitrification
⁎ Corresponding author. E-mail address:
[email protected] (L. Chapuis-Lardy). 1 Current affiliation: SupAgro, UMR SYSTEM (INRA-CIRAD-SupAgro), 2 place Viala, Bât. 27, 34060 Montpellier Cedex 2, France. 0016-7061/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2008.11.015
are the main sources. A synthesis of results on N oxide (N2O + NO) fluxes from natural or unfertilized systems in the humid tropics indicates that the fluxes are positively correlated with some measure of N availability (soil inorganic N) and with soil water-filled pore space (WFPS) (Granli and Bøckman, 1995; Verchot et al., 1999; Davidson et al., 2000). Theory suggests that the relationship between N inputs and N2O flux may be more complex, and in particular that N2O flux may exhibit a threshold response to N inputs and soil biota (Erickson et al., 2001). Nitrogen often limits both plant growth and N2O production in terrestrial ecosystems, so that where plants are competing with microbes for soil N, N2O production will be suppressed until plant N demands have been fully satisfied. The IPCC protocols calculate agriculture's contribution to atmospheric N2O loading as a simple percentage of total N inputs: 1% of added N is estimated to be lost as N2O based on fluxes from fertilized vs. unfertilized field plots (Intergovernmental Panel on Climate Change, 2006). While fertilization rate may be low in tropical countries, responses of N2O fluxes to low levels of N inputs are unknown for many tropical soils. The number of published measurements of N2O emissions from soils is increasing steadily at mid-latitudes (Europe and North America) but there are still few flux data from tropical and sub-tropical regions (Bouwman et al., 2002; Stehfest and Bouwman, 2006). The representation of tropical
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agricultural systems in datasets really needs to be improved to provide additional information about driving factors of emissions and allow an improvement of statistical N-emission models and IPCC N2O emission factors. In Madagascar, no-tillage was developed in the early 90s in response to soil erosion and continuous declines in land productivity under conventional systems based on soil hand-ploughing. Adopting reduced- or no-till contributes to maintain or increase the soil organic carbon content, reducing the net CO2 emissions (e.g., Six et al., 2002; Razafimbelo et al., 2008). It may also affect N2O emissions but the net effects are inconsistent and not well-quantified globally (e.g., Cassman et al., 2003; Li et al., 2005). Some studies have reported higher N2O emissions from no-tillage than conventional tillage soils (e.g., Ball et al., 1999; Six et al., 2002), as a result of increased soil moisture content, water conservation and lower soil gas diffusivity, whereas other studies report no significant effects of tillage on N2O emissions (Choudhary et al., 2002; Liu et al., 2006; Metay et al., 2007; Jantilia et al., 2008). The objective of this paper was to quantify N2O fluxes from a cultivated tropical soil under conventional tillage and direct seeding mulch based cropping systems. 2. Materials and methods 2.1. Experimental site and design The experimental site was located at Andranomanelatra (19°47′S; 47°06′E; 1600 m above sea level) near the city of Antsirabe in the Malagasy Highlands. The area is under altitude tropical climate with a dry season from May to October and a humid one from November to April. Annual average rainfall and temperature are 1300 mm and 16 °C, respectively. The soil was developed on volcano-lacustrine alluvia (Raunet, 1981), and classified as a clayey Andic Dystrustept in the Soil Taxonomy (Soil Survey Staff, 2003) or a Ferralsol in the FAO classification (FAO, 1998). Upper soil layer (0–10 cm) contained 60% 1:1 clays, presented a pH of 4.9, an available (NaHCO3−extractable) P content of 13.9 mg kg− 1 soil, and iron and aluminium oxides contents of 47 and 17 g kg− 1 soil respectively. The study was conducted at an experimental station managed by the Malagasy National Institute of Agricultural Research (FOFIFA) in partnership with the Cirad (Centre of International Cooperation in Agricultural Research for Development, a French institute). The design consisted of completely randomised blocks established in 2002 on a long-term grassland dominated by Aristida species. It included tillage and direct seeding mulch based cropping (DMC) systems with various rotation and fertilization treatments. Our study focused on a soybean (Glycine max) and maize (Zea mays L.) intercropping cultivated between November 2006 and May 2007 after upland rice (Oryza sativa) the year before. Soybean was sown in 0.4 m spaced paired rows between 1-m spaced single rows of maize. Management treatments consisted of hand ploughing (HP) and direct seeding mulch based cropping (DMC), both replicated three times in 100-m2 plots. Preceding crop residues (rice straw) were left on the soil surface (0.27 kg dry matter m− 2) at harvest (May 2006) in direct seeding mulch based cropping (DMC) systems while traditionally exported from hand-ploughed plots. The organic C and N contents of straw were 446.1 and 9.2 g kg− 1 dry matter respectively corresponding to a C-to-N ratio of 48.5. No additional crop residue application was performed in both management treatments. Herbicides (Glyphosate and 2,4-dichlorophenoxy acetic acid) were spread before planting in DMC plots while weeds were removed by hand in tilled plots. Fertilization consisted of zebu manure (5 Mg ha− 1, i.e. 17 kg N ha− 1), NPK (11:22:16) fertilizer (300 kg ha− 1) and dolomite (500 kg ha− 1) applied on both management treatments with the seed in the planting hole on November 21, and urea (50 kg ha− 1) in a row-banded application 30 and 60 days after seeding.
2.2. Local climatic data and soil temperature Soil temperatures were measured on each gas sampling date by probes installed 30 cm apart from the gas sampling design at 10, 20 and 30 cm depths. The rainfall and ambient air temperature were monitored using a meteorological station located in the experimental field. 2.3. N2O flux measurements 2.3.1. At the soil surface Soil N2O fluxes were measured on 18 dates from November 9, 2006 to April 12, 2007 using closed chambers. The minimum set of criteria suggested by Rochette and Eriksen-Hamel (2008) for reliable soil N2O flux measurement at the soil surface using non-flow through nonsteady-state chamber methods was applied to avoid large biases in the accuracy of chamber N2O data. Six replicate chambers (volume 5.04 L) were randomly placed per treatment in the interrow between soybean rows. Briefly, metal containers (cross section 630 cm2) with sharpened edges were inserted into the ground to a depth of 6 cm with as little disturbance as possible and kept in the same position throughout the season. The containers were closed with a gas-tight lid for a total gas sampling period of 90 min. At 30-min intervals (0, 30, 60 and 90 min), 7.5 ml gas samples were taken from the chamber headspace using gas-tight syringes. Samples were stored in 7 ml vacutainers (Venoject® tubes), transported to the laboratory, and analysed for N2O on a Varian 3800 gas chromatograph (GC) equipped with a 63Ni-electron capture detector. To minimise any effects of diurnal variation in gas emissions, samples were taken at the same time (morning) on each occasion. Hourly fluxes were calculated from the linear increase in gas concentration in the chamber headspace with time according to Metay et al. (2007) after being tested for nonlinearity. Estimated fluxes per year were calculated using monthly weighted averages and then converting it to an annual flux. The obtained values will be considered as rough estimates as the calculations contained a certain level of uncertainty. As our observations mainly covered the wet season which also corresponded roughly to the growing season (Nov–April), the flux value of April was used as an estimate of the monthly emission during the dry season. 2.3.2. At different soil depths N2O concentrations in the soil were measured simultaneously at 10, 20 and 30 cm depths for both management treatments on 9 dates. Soil-air samples were collected using a passive method (Metay et al., 2007) and analysed in the laboratory on the same instruments used for gases sampled at the soil surface. The equipment was installed into the soil more than 3 weeks prior to the first soil gas measurements (February 6). 2.4. Soil sampling and analyses 2.4.1. Soil total N and C contents Twenty cores (5 cm diameter) were taken before the growing season in each of the three plots per management treatment and pulled into a composite soil sample per depth (0–10, 10–20, and 20– 30 cm) and per plot. Carbon and nitrogen contents were determined by a CHN combustion analyser (Carlo Erba NA 2000). 2.4.2. Soil mineral N and water contents Soil samples were collected between soybean rows at each day of gas sampling to analyse soil mineral ammonium (NH+4) and nitrate (NO−3) contents and determine the soil moisture. Fresh soil (50 g) were extracted in 1 M KCl (soil:solution ratio 1:3.4) for the determination of NH+4 and NO−3 by colorimetry (Anderson and Ingram, 1989). Soil moisture was determined gravimetrically by drying the sample at 105 °C during 48 h and water filled pore space (WFPS in
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Table 1 Total carbon and nitrogen contents (g 100 g− 1) and bulk density (BD, g.cm− 3) in 0–10, 10–20 and 20–30 cm soil layers of DMC and HP treatments Treatment
Soil depth (cm)
C (g 100 g− 1)
N (g 100 g− 1)
BD (g.cm− 3)
DMC
0–10 10–20 20–30 0–10 10–20 20–30
4.27a 3.98a 3.43b 4.04a 3.96a 3.23b
0.31a 0.28a 0.24b 0.29a 0.28a 0.22b
0.90a 0.84a 0.89a 0.93a 0.86a 0.89a
HP
Means within one column and depth followed by a different letter are significantly different at P b 0.05.
Fig. 1. Air temperature (°C) and monthly rainfall (mm) from September 2006 to August 2007, and rainfall monthly averaged for the past 5 years, Andranomanelatra, Madagascar.
%) was calculated using bulk density determined by the core-sampling method at one date during the cropping period (6 cores of 460 cm2/ plot) (Linn and Doran, 1984).
season (December–April) was 173 mm (15%) greater than the 5-year mean, with January being the wettest month followed by December and February. Rainfall in January 2007 reached 700 mm, almost twice the normal rainfall, and with sometimes more than 95 mm in 1 day (Fig. 2). The monthly ambient air temperature ranged from 12.8 to 20.0 °C during the year. Soil temperature at 10 cm depth did not significantly vary between management treatments at one measurement date and ranged from 16.3 (in March, DMC) to 25.2 °C (in December, HP) during the investigation period.
2.5. Statistical analyses 3.2. Soil total C and N Data were analysed using Statistica software with differences found to be significant at probability level of 0.05. Prior to analysis of variance with date and treatment as main fixed effects, and t-tests between treatments, data were tested for normality and ln-transformed where appropriate. The relationships of N2O fluxes or concentrations and soil variables were assessed by linear regression analysis. Average N2O fluxes per date and treatment were tested for their difference from zero and discarded from the calculation of the cumulative fluxes when appropriated. 3. Results 3.1. Climatic data and soil temperature Total rainfall from September 2006 to August 2007 was 87 mm above the norm (1500 mm) (Fig. 1). Rainfall in the 2006–07 growing
Carbon and nitrogen contents ranged from 3.23 to 4.27 g C 100 g− 1 soil and 0.22 to 0.31 g N 100 g− 1 soil respectively (Table 1). Direct seeding mulch based cropping (DMC) tended to increase total C and N contents at all depths but the difference was not statistically significant. 3.3. Soil bulk density and water-filled pore space Soil bulk density was between 0.84 and 0.93 g cm− 3 and did not differ among depths and management treatments (Table 1). The water-filled pore space (WFPS) ranged from 25 to 80% in the 0–10 cm soil layer (Fig. 2). These values were significantly higher in direct seeding mulch based cropping (DMC) than in hand-ploughing (HP) systems in November and April while no difference appeared between treatments during the wettest months. During the wet period
Fig. 2. Daily rainfall (mm) and changes in the water-filled pore space (WFPS %) with time during the sampling period.
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Fig. 4. N2O fluxes (g N2O-N ha− 1 d − 1) measured at the soil surface from DMC and HP treatments from November 2006 to April 2007 (18 dates). Arrows indicate time of manure + NPK (dashed) and urea applications. The horizontal dashed lines represent very low (①) and low (②) emissions thresholds according to Scheer et al. (2008).
Fig. 3. Available NH+4-N and NO−3N in the 0–10 cm soil layer of DMC and HP treatments. Arrows indicate manure + NPK (dashed) and urea applications.
(December to February), WFPS was mostly above 60%. The coefficient of variation calculated per date and treatment ranged between 1 and 15%, indicating WFPS was relatively uniform through the study area. 3.4. Soil mineral N Ammonium and nitrate contents measured in the 0–10 cm soil layer were significantly different (P b 0.001) between treatments on day 1, for ammonium 8.4 and 4.5 mg N kg− 1 soil and for nitrate 35.9
and 55.6 mg N kg− 1 soil, in hand-ploughing (HP) and direct seeding mulch based cropping (DMC) treatments respectively (Fig. 3). The NH+4 and NO−3 concentrations in the 0–10 cm soil layer decreased afterwards being similar between treatments and ranged from 4.3 to 1.1 mg N kg− 1 soil for NH+4 and 15.4 to 0.6 mg N kg− 1 soil for NO−3, depending on the measurement date. The calculated coefficients of variation were similar for both NH+4 and NO−3 soil contents and ranged between 2 and 89%. Mineral N contents were measured in deeper soil layers (10–20 and 20–30 cm depth) from February to April (Table 2). NH+4 content remained similar or slightly decreased with soil depth, except for the measurement on April 12 (Table 2), when values in deeper soil layers were statistically higher than at 0–10 cm depth. NO−3 contents tended to be higher in depth in the DMC treatment, with the effect being especially notable in March and April, while concentration remained similar or decreased (on April 12) with depth in the HP treatment (Table 2). During the growing season, only 55 kg N ha− 1 was applied as manure (17 kg N ha− 1) or mineral fertilizer (33 kg N ha− 1 as NPK and 4.6 Kg ha− 1 as urea). These N applications done in the hole of planting or in the row according to the common local practices did not significantly affect the mineral contents of soil as sampled in the interrow. 3.5. N2O emissions
Table 2 Available NH+4-N and NO−3N contents (mg kg− 1 soil) in 0–10, 10–20 and 20–30 cm soil layers of DMC and HP treatments from February to April Date N–NH4 (mg kg− 1 soil) Feb. 6 Feb. 7 Feb. 21 Feb. 22 March 13 March 14 March 27 March 28 April 12 −1 N-NO3 (mg kg soil) Feb. 6 Feb. 7 Feb. 21 Feb. 22 March 13 March 14 March 27 March 28 April 12
DMC
HP
0–10
10–20
20–30 0–10
10–20
20–30
2.73a 2.15a 2.31a 2.51a 2.44a 1.87a 1.57a 1.61a 2.87ab 1.32a 1.10a 0.91a 1.22a 0.57a 0.84a 1.30a 2.40ab 4.81a
1.79abc 1.36a 1.63a 2.27a 2.43a 1.55a 1.45a 1.12a 1.78a 2.02a 1.54a 1.00a 1.52a 0.70a 1.85ab 2.36ab 2.25ab 5.43a
1.36bc 1.32a 1.43a 2.89a 1.95a 1.34a 1.13a 0.95a 3.49b 2.87a 4.05b 2.11a 2.15a 1.40a 3.39b 4.46b 4.16a 7.74b
2.02abc 1.46a 1.57a 1.95a 2.10a 1.29a 1.98a 1.47a 3.10b 1.48a 0.99a 0.92a 1.28a 0.73a 0.86a 1.04a 1.68b 2.39c
1.19bc 1.57a 1.14a 1.72a 1.53a 1.17a 2.61a 1.06a 1.86a 1.06a 1.08a 1.07a 1.43a 1.27a 1.60a 1.04a 1.68b 2.30c
1.24bc 1.31a 1.48a 2.74a 2.46a 1.28a 1.86a 1.06a 2.32a 1.16a 1.07a 0.80a 0.92a 0.63a 0.70a 1.07a 1.49b 5.05a
Values within a line with a different letter are significantly different for each other at P b 0.05.
3.5.1. N2O fluxes at the soil surface The N2O emission rates at the soil surface were ranged from − 7.0 to 14.3 g N2O–N ha− 1 d− 1. The calculated coefficient of variation ranged from 30 to 249%, exhibiting a large spatial variability. The arithmetic mean values (n = 6 par date) ranged between −2.4 and 8.8 g N2O-N ha− 1 d− 1 without significant difference between HP and
Table 3 Cumulative N2O emissions (g N2O-N ha− 1) and amounts of N applied (kg N ha− 1) during the observation period (Nov 06–April 07), estimation of the N2O flux (g N2O-N ha− 1) and loss of N applied as N2O (emission factor, EF) on an annual basis in DMC and HP treatments Period Nov 06–April 07 (observation period)
Nov 06–Oct 07
Cumulative N2O emissions(g N2O-N ha− 1) N applied (kg N ha− 1) as NPK fertilizer Urea Manure Rice straw Total N application (kg N ha− 1) Estimated N2O emissions (g N2O-N ha− 1) EF (%)
DMC
HP
263 33 4.6 17 2.5 57.1 263 0.46
259 33 4.6 17 – 54.6 259 0.47
L. Chapuis-Lardy et al. / Geoderma 148 (2009) 421–427 Table 4 N2O concentrations (ppmv) in the soil atmosphere at 0–10, 10–20 and 20–30 cm depth in DMC and HP treatments from February to April Date N2O (ppmv)
Feb. 6 Feb. 7 Feb. 21 Feb. 22 March 13 March 14 March 27 March 28 April 12
DMC
HP
0–10
10–20
20–30
0–10
10–20
20–30
0.78a 0.83a 0.88ab 0.54a 0.72a 0.35a 0.78a 0.79a 0.69a
0.75a 0.90ab 0.78ab 0.54a 0.77a 0.34a 0.74a 0.86a 0.70a
0.80a 1.05ab 0.83ab 0.69a 0.77a 0.37a 0.84a 0.87a 0.73a
0.90a 1.03ab 1.02a 0.71a 0.73a 0.36a 0.73a 0.77a 0.75a
0.89a 0.97ab 0.82ab 0.70a 0.73a 0.34a 0.77a 0.76a 0.72a
0.90a 1.13b 0.66b 0.65a 0.67a 0.47a 0.71a 0.78a 0.75a
Values within a line with a different letter are significantly different for each other at P b 0.05.
DMC treatments. However, these fluxes were significantly different from zero (P b 0.05) only for Nov. 9 and 22 in both treatments, as well as for March 27 and 28 in DMC and January 25 in HP. When other values were discarded the N2O fluxes varied between 0 and 8.84 g N2O-N ha− 1 d− 1. Fluxes significantly increased after manure + NPK addition in November in both management treatments (Fig. 4). For the entire growing season, cumulative N2O emissions were 0.26 kg N2O-N ha− 1 in both treatments. As the fluxes in April were not significantly different from 0, the emissions during the dry season were assumed to be zero, so that the cumulative N2O emissions were unchanged when estimated on a year basis. The emission factors (EF), uncorrected for background emission, were 0.46 and 0.47% of the total amount of N applied to the plots in DMC and HP respectively (Table 3). 3.5.2. N2O concentrations in soil profile N2O concentrations in the soil atmosphere were measured at 10, 20 and 30 cm depth from February to April and ranged from 1 to 5 times the atmospheric natural concentration (Table 4). Calculated coefficients of variation ranged from 8 to 65%, exhibiting a spatial variability lower than those reported for surface measurements. ANOVA showed a strong effect of date, while treatment and/or depth did not significantly affect the N2O concentration in soil atmosphere. The lowest concentrations were observed in both treatments and at all depths on March 14 and the highest on February 7. 4. Discussion 4.1. N2O emissions from tropical agricultural soils and related factors The studied tropical agricultural soil mainly acted as a source of N2O. However, 44 events of negative fluxes at the soil surface have been reported out of 213 measurements, i.e. 20% of the data set. A large number of these negative events was very close to zero and was not considered as different from 0. But some of them were larger than the analytical error of the gas chromatograph and represented in absolute terms up to the half of the maximum positive flux (up to −7.0 g N2O-N ha− 1 d− 1). N2O uptake seems to be stimulated by low availability of mineral N (Chapuis-Lardy et al., 2007) which is the case for the studied soil, especially at the dates of occurring negative events. However, based on current knowledge, it is not yet possible to clearly define a set of conditions promoting N2O consumption (Chapuis-Lardy et al., 2007). N2O emissions at the soil surface averaged per date from 0 to 8.84 g N ha− 1 d− 1 (i.e. 0 to 36.8 μg N m− 2 h− 1) (Fig. 4) and may be considered as low regarding thresholds reported in the literature (Bouwman et al., 2002; Scheer et al., 2008). Low N2O emissions are commonly observed in tropical agricultural soils (e.g. Sanhueza et al., 1990; Levine et al., 1996; Jantilia et al., 2008). Metay et al. (2007) reported much lower fluxes (b1.5 g N-N2O d− 1 ha− 1) from a Brazilian oxisol under a rice-
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soybean rotation. Nitrous oxide fluxes averaged 57 μg N m− 2 h− 1 during the growing season of sorghum (Sorghum bicolor L.) cultivated on an oxisol of Puerto Rico. Fluxes measured in a nearby unfertilized pasture averaged 16 μg N m− 2 h− 1 (Mosier et al., 1998). Crill et al. (2000) found from a maize field in Costa Rica that N2O emissions from fertilized (122 kg N ha− 1/cropping season) versus unfertilized plots averaged 640 vs 120 μg N m− 2 h− 1. Likewise, Weitz et al. (2001) found that fertilized systems in Costa Rica had more than three times the N2O emissions of unfertilized systems. Stehfest and Bouwman (2006) summarized information from 1008 N2O measurements for agricultural fields, with respectively 14% and 13% located in sub-tropical and tropical climates. They identified the N application as a major control of N2O emissions. In our study, N fertilisation was quite low with a total of 55–57 kg N kg N ha− 1 (Table 3) and soil mineral N contents was weakly correlated with N2O fluxes. The soil nitrate content at 10 cm depth was weakly correlated to N2O fluxes at the soil surface (R2 = 0.13, P = 0.03) while no significant relationship linked NH+4 concentration to N2O fluxes. At least a higher variability is observed shortly after each fertilisation, which is consistent with the results of Nobre et al. (2001) under tropical conditions. The absence of accumulated soil N suggested that the N release by roots and by N mineralization occurred at a rate that did not exceed those of plant uptake and other soil N losses such as immobilization, denitrification and leaching. Moreover, N2O fluxes are low despite the presence of legumes that could stimulate N2O emissions by increasing N inputs into the soil, thus providing additional substrates for nitrification and denitrification. Indeed, Rochette et al. (2004) reported that there is considerable uncertainty relative to the emissions of N2O from legume crop fields. These results implied that the process of symbiotic N fixation per se does not stimulate N2O production or emission as suggested by Yang and Cai (2005). Fine particles of the studied clayey soil should improve soil capacity to retain water and limit O2 diffusion, thus favouring N2O production as suggested by Castaldi et al. (2004). A water-filled pore space (WFPS) value of 80% corresponded for clayey soils to field capacity (Granli and Bøckman, 1994). While no clear relationship appeared between N2O fluxes at the soil surface and WFPS the N2O efflux from the system was correlated to the soil gas concentration (R2 = 0.46, P b 0.05, N = 18 at 10 cm depth) which in turn was correlated to WFPS (R2 = 0.48, P b 0.05, N = 18 at 0–10 cm depth; R2 = 0.44, P b 0.05, N = 18 in the 10–20 cm soil layer, Fig. 5). During the study period, the WFPS of topsoil was mainly above 60%, value at which O2 diffusion is sufficiently reduced to allow for a sharp increase of N2O production (Davidson, 1991). In fact, N2O production by denitrification generally increases exponentially between 60% and 90% of WFPS, but also N2O production by nitrifiers improves as soil water content increases and aeration becomes restricted, with optimum values around 60% of
Fig. 5. Relationships between N2O concentration in soil atmosphere and WFPS in the 0– 10 cm and 10–20 cm soil layers (marks represent average per date and treatment; N = 18; P b 0.05).
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WFPS (Davidson, 1991; Castaldi, 2000). While WFPS is high enough to allow high rate of N2O production the low net amounts of available mineral nitrogen may limit N2O production (e.g., Ryden, 1983; Conen et al., 2000; Pinto et al., 2002), resulting in low losses of gaseous N from the ecosystem (Firestone and Davidson, 1989). N2O fluxes were circa 0.26 kg N ha− 1 when estimated on a year basis and represented in average N2O-N losses of 0.5% of total N applied (Table 3). Watanabe et al. (2000) reported N losses as N2O between 0.2 and 0.4 kg N ha− 1 from soils that received a 47–75 kg N fertilization during the maize growing season in Thailand. Khalil et al. (2002) reported a cumulative annual flux up to 0.75 kg N ha− 1 for an annual groundnut–maize crop rotation in fertilized Malaysian ultisols. The EF values calculated in the present study were inside the uncertainty range proposed by the IPCC (0.3 to 3% of applied N with 1% as average) for N2O emissions inventories purposes (Intergovernmental Panel on Climate Change, 2006). The use of the IPCC average value (1%) would overestimate N2O emissions in our case of tropical agricultural soils which received low N inputs levels (ca. 55–57 kg N ha− 1). 4.2. Effect of management practices The present experimental design was selected to cover management practices in use in the region but do not allow to separate tillage and residues effects. N2O emissions at the soil surface averaged from 0 to 8.8 g N ha− 1 d− 1 in both management treatments with no difference between direct seeding mulch based cropping (DMC) and handploughing (HP) systems. It is of particular interest as the method of cultivation was assumed to affect the magnitude and pattern of N2O emissions, presumably by varying the supply of organic C and N to micro-organisms, and by changing the soil water/aeration status around the residues (Aulakh et al., 1991). Some researchers have reported increased N2O emissions with crop residue applications (Baggs et al., 2000; Huang et al., 2004) and the flux depends on the amount of amendment introduced and its chemical composition (Aulakh et al., 1991; Baggs et al., 2006). However, Malhi et al. (2006) reported no significant influence of straw application on N2O emissions. The influence of residue management on N2O emissions is complicated by many factors including the timing of incorporation and fertilizer application (Hao et al., 2001) and environmental conditions. Except at the beginning and end of the rainy season when the water content did not favour denitrification, residue application did not significantly affect soil humidity. ANOVA analyses also revealed that management treatments did not affect soil mineral N contents despite residues returning to soil at harvest in DMC whereas exported in HP. It is possible that the pattern of N and C availability relative to the concurrent soil environmental conditions (e.g., soil-water content, soil temperature) in this region is such that straw retained after harvest in April has little impact on N2O loss potential. As usually observed in no-till management, the soil organic C and total N content under DMC were larger than in tilled systems (Table 1) but the difference is not significant probably because the systems have been settled only since 4 years (Six et al., 2004; Angers and Eriksen-Hamel, 2008). These DMC systems have the potential to store C in soil compared to conventional tillage as shown by Razafimbelo et al. (2008) for longer term systems in the region. The absence of higher N2O fluxes in response to keeping soil undisturbed in no-till systems compared to tillage was observed by Liu et al. (2006) and Jantilia et al. (2008) for temperate and subtropical systems, respectively. Choudhary et al. (2002) also reported no difference in N2O emissions between no-till and conventional tillage when soil and environmental conditions favoured high fluxes and Metay et al. (2007) for low flux conditions. 5. Conclusion The studied Malagasy agricultural soil mainly acted as a source of N2O. However, despite WFPS often larger than 60% in both treatments
(DMC vs. HP), N2O fluxes remained low (b12 g N ha− 1 d− 1) probably due to low soil mineral N contents that limited denitrification and N2O production. Applying the IPCC mean value for the emission factor (1%) would overestimate the N2O emission for the tropical agricultural soil under evaluation. The contribution of agriculture to N2O emissions and its impacts on global atmospheric chemistry remain to be expanded to the various soil types and cropping systems of the Tropics. N2O emissions were not influenced by management (tillage/ residue interaction) practices. From the data presented here it can be concluded that the use of direct seeding mulch-based cropping system did not significantly affect soil N2O emissions and did not offset the started benefits of C sequestration (trend to C storage in soil) when compared to the traditional maize-soybean system under handploughing. 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