NITROUS OXIDE PRODUCTION FROM IRRIGATED MAIZE CROPPING IN THE MURRUMBIDGEE IRRIGATION AREA: IMPACTS OF CROP RESIDUE MANAGEMENT SYSTEMS
C.P. Meyer1, C.A. Kirkby2, I. Weeks1,
D.J. Smith2, S. Lawson1 A. Fattore2 and D. Turner3
1CRC for Greenhouse Accounting, CSIRO Atmospheric Research, PMB 1 Aspendale, Vic, 3195 mick.meyer@csiro.au
2CSIRO Land and Water, Private Bag 3, Griffith, NSW, 2680
3University of Melbourne and CRC for Greenhouse Accounting
Abstract
Irrigated summer crop production has been identified as a potential major emitter of nitrous oxide
(N
2O). The combination of high water content and high soil temperature favour high rates of denitrifica-
tion and therefore high rates of N
2O production. However, because of the absence of Australian data,
current greenhouse gas emissions inventory methodologies do not specifically account for these
potential hot-spots in emission sources. To address the issue, the N
2O emission rates from an irrigated
maize crop were measured through the complete yearly cycle from fallow. Key aspects of the study were the impacts on emissions of fertilizer application and stubble management options.
The study confirmed that irrigated maize production is a significant N
2O source. The average N
2
O
emissions during the fallow (3 ng N m-2 s-1) were similar to the background emissions from unfertilized pasture and natural soils, with no significant difference between stubble management and fertilizer application history. However, emission rates increased substantially following sowing. In the unfertilized (control) treatment emissions gradually declined through the cropping stage before falling below the rates observed during the fallow. In the fertilized crop, emissions increased by factors of approximately 5 to 100 (relative to the control) before and after irrigation, respectively. Emissions were greater and more persistent where stubble was burned rather than incorporated. Overall losses of fertilizer nitrogen were 1.5% of applied nitrogen where stubble was incorporated and 2.7% where stubble was burned. The large emission rates were associated with denitrification in the furrows where water filled pore space (WFPS) during the irrigation period exceeded 80% compared to the beds where WFPS ranged from 50% to 70%. The difference between beds and furrows was more pronounced in the stubble
incorporated treatment. High N
2O emission rates persisted for several weeks longer in the stubble
burned treatment compared to the stubble incorporated treatment. These emissions were correlated with the timecourse of soil nitrate concentration in the upper 75 mm of the bed. In the stubble incorpo-rated treatment, inorganic nitrogen concentration declined following the main fertilizer application, in contrast to the burned treatment.
Compared to the global average emission factor of 1 to 1.25% of applied nitrogen, this study suggests that irrigated maize is among the strongest emission sources of the crops and pasture systems investigated to date in Australia. However the study also indicated there is substantial potential for mitigation through improved soil porosity, and reduced availability of inorganic nitrogen in the surface soil that, in this study, was achieved by stubble incorporation. Alternative options for controlling both parameters are also possible and should be investigated.
Introduction
The major uncertainty for greenhouse gas emissions from fertilizer application to crops is the emission factor (EF). Recent reviews (e.g. Bouwman 2002) report that EFs vary from 0.01% applied N to more than 5% across a wide range of crops, climates and soil types around the world. However, despite the rapidly increasing number of published studies with more than 1000 studies reported in the scientific literature by 2002, it has not been possible to narrow the range of uncertainty; within similar crops, fertilizer type and even soil type a wide range of EF persists. The issue is particularly significant for Australia where there are less than 10 comprehensive studies of greenhouse gas emissions either completed or in progress.
Summer irrigated cropping produces a combination of soil environmental conditions including high soil moisture, high soil temperature and high inorganic nitrogen concentration that can lead to high rates of
denitrification and N
2O loss. Potentially, therefore, the Australian irrigated cropping industry could be a
major emitter of greenhouse gases. To determine whether in practice this was the case it was decided to measure the greenhouse gas emissions from an irrigated maize crop through all stages of the cropping cycle from harvest to harvest. Two crop management systems were investigated: a conven-tional system in which stubble is removed after harvest by burning, and a system where stubble is mulched and incorporated into the soil immediately following harvest. Both practices are commonly used in Australia.
Site/ Methods
The greenhouse gas emissions were measured on an experimental site established in 1999 to investi-gate the impacts of stubble incorporation (Kirkby 2006, this volume). The measurements were con-ducted at Commins Brothers property at Whitton, (34.5o S 146.2o E) in the Riverina of NSW. This region receives an average rainfall of 406 mm y-1, slightly more in winter than summer. The average monthly temperature maxima and minima range from 31.3o C and 16.4o C respectively in February to 14.3o C and 2.9o C respectively in July. The soil at the experimental site is a Mundiwa clay loam that consists of a brown sandy clay loam to 0.2 m, a dark reddish-brown heavy clay to 0.6 m and a mottled yellow red clay to 1.7 m.
The fluxes of N
2O and CO
2
were measured on three of the established treatments:
?zero N application and stubble removed by burning;
?329 kg N ha-1 stubble removed by burning; and
?329 kg N ha-1, stubble mulched an incorporated into the soil.
Comparison between the first two treatments yields the N
2O emissions associated with nitrogen
fertilizer addition while comparison between the second and third treatments yields the effect of stubble retention of soil nitrogen loss as N
2
O.
The CO
2 emissions from the soil do not constitute net greenhouse gas emissions in agricultural
systems because they are derived from a short-term biogenic source. However the rate of CO
2 produc-
tion indicates soil microbial and plant root metabolic activity in the soil which are closely associated
with nitrogen metabolism.
Emission rates were measured using an automated chamber system described by Meyer et al. (2001).
This system measures 24 h integrated fluxes continuously through the season and avoids the biases
and errors which can arise with intermittent manual flux measurements. Three flux chamber systems
were deployed, each comprising two chambers, four chamber bases, a controller/logger, a CO
2 analyser, and two sets of four Tedlar storage bags. Each system was powered by a 12V 120 amp-hour
battery. The battery capacity was supplemented by an 80 W solar panel that extended the period
between battery changeover to between 1 and 4 weeks. Soil temperature was measured using type K
thermocouples and soil moisture was measured using TDR (Theta Probes ML2x, Delta-T Devices Ltd,
Cambridge, UK). Both sets of sensors were logged by the controller/logger unit. Gas concentrations in
the Tedlar sample bags were measured off-site at Aspendale. N
2O was measured by gas chromatogra-
phy and CO
2 was measured using a Licor 6251 NDIR (Licor, Nebraska, USA). In addition CO
2
concen-
tration was also measured directly in the field using a Gascard_II (Edinburgh Instruments, Edinburgh, UK).
Chamber layout and the scheduling of the farm management events are presented in Beer et al. (2006, this volume). Key dates (days from 1/1/2004) are:
?Stubble incorporation day 110
?Sowing day 298
?Side dressing (200 kg N ha-1 )day 340
?Physiological maturity day 434
?Harvest day 495
?Stubble incorporation day 523
The mineral nitrogen content in the 0N-burn treatment also increased in this period to 93 mg N kg soil-Thus during the main crop growth period there was a very large pool of labile inorganic nitrogen avail-able for denitrification. This pool dissipated in the 300N-incorporate treatment in the next two months to 115 mg N kg soil-1 but remained high (503 mg N kg soil-1) in the 300N- burned treatment, in parallel with the observed pattern of N
2
O emission rates.
Therefore the main difference between the N
2O emissions from the stubble incorporated treatment,
compared with the burned treatment was the duration of the period of high N
2
O emissions, which, in
2
bed. During the irrigation period, and particularly in the month after the main fertilizer application (day
340), the furrows accounted for 85% to 95% of the total N
2O emission, however, when irrigation ceased
at crop maturity, the emissions were more evenly distributed across the profile. Averaged across the
growing season the furrows emitted 68%, 61% and 81% of the total N
2O nitrogen lost from the soil
(Table 1). Detailed measurements of N
2O flux across furrow/bed profile made 7 days after the main
fertilizer application confirmed that the emission originated from bottom of the furrow. Less than 5% of the total loss occurred from the plant line to the centre of the bed in both the 300N-burn and 300N incorporate treatments. There was no significant gradient across the profile of the unfertilized beds and furrows.
Table 1. Average N
2
O flux during the crop season
N2O flux (ng N m-2 s-1) Treatment Furrow Bed Total Fraction from furrow (%)
0N burned 3.8 1.8 5.6 68% 300N burned 31.6 20.2 51.8 61% 300N Incorporate 26.4 6.0 32.4 81%
In summary, the study has found that the annual emission of N
2O from irrigated maize grown in a
conventional management system of 300 kg fertilizer nitrogen, and stubble removed by burning is 2.7% of the applied nitrogen. When stubble is retained and incorporated into the beds by mulching, this loss rate declined to 1.5% of applied nitrogen. The emission largely takes place in the furrows where low porosity and high water filled pore space favour denitrification, however the duration of the period of high emissions occurs only when soil mineral nitrogen concentration of the bed remains high. This occurs for two to three months after fertilizer application in the burned treatment, but only for one month where stubble is incorporated. The measurements of soil respiration also point to substantial differences in soil microbial activity during the growing period between the conventionally managed soil and soil amended by incorporation of mulched stubble.
Discussion
Compared to the global average emission factor of 1 to 1.25% of applied nitrogen, this study suggests that irrigated maize is a significant source of nitrous oxide. Internationally, maize crops are generally high users of nitrogen with application rates typically in excess of 150 kg N ha-1 (Bouwman et al. 2002),
and they tend to be large emitters of N
2O. Bouwman et al. (2002) reviewed the majority of the interna-
tional literature on N
2O emissions from agricultural crops and pastures. The 70 maize experiments
reviewed in their study reported emission factors ranging from 0.2% to 8.2% of applied nitrogen with an average of 1.8% and a median of 1.5%. Our study therefore is entirely consistent with other reports. It is, however, among the highest emitters of the limited set of studies for Australian crops (Galbally et al. 2005).
The two main drivers of the N
2O emission appear to be soil porosity and soil inorganic nitrogen avail-
ability. Many studies (e.g. Dobbie and Smith 2003, De Klein and Logtestijn 1996, Rudaz et al. 1999)
have reported high rates of N
2O emissions where WFPS exceeds 70%. Under these conditions denitri-
fication dominates the soil nitrogen transformations and N
2O is the major product (Dalal et al. 2003). In
many cases, WFPS is found to be the principal determinant of N
2O emission rates in the short term
(Ruser et al. 2001), and also, in the longer term, of emission factor (Dobbie and Smith 2003). However
it is also clear that inorganic nitrogen availability (particularly nitrate) is a key determinant of N
2O
emission. McSwiney and Robertson (2005) found that N
2O emission factors were low and independent
of application rate up to the rate required for maximum crop yield. Above this rate, available soil nitrate
concentrations increased substantially accompanied by high rates of N
2O emission, particularly in wet
conditions where available inorganic soil nitrogen exceeded 100 mg (kg dry soil)-1.
Finally, the spatial distribution of soil porosity and available nitrogen appears to be a critical factor. In
the few published reports of spatial variation in N
2O emissions in row cropping systems, it is generally
observed that the furrows tend to have higher WFPS at all times during the growing season, and
generally dominate the N
2O emissions (Ruser et al. 2001) as was observed in our study. Also in our
study the CO
2 fluxes suggested that there was little difference between treatments in microbial activity
in the furrows (as might be expected because all the stubble was incorporated into beds), and therefore
N
2O emission potential would depend on the supply of nitrate moving out of the beds. In the beds,
however, the differences in CO
2 emission rate indicated substantial treatment effects that were not
reflected in N
2O emission differences, possibly due to the lower emission potential caused by lower
WFPS in the beds.
In summary, therefore, the rates of N
2O emissions and the mechanisms observed in our study are
consistent with the current understanding of nitrogen transformations in soil. What is new is the
demonstration that it is possible to move an agricultural soil system towards a domain where N
2O
emission rates are less likely. Incorporating stubble into beds increases soil porosity and lowers the
availability of inorganic nitrogen both of which reduce N
2O emission potential without diminishing crop
yield. Complementary studies conducted at the site suggest that stubble incorporation is also pro-foundly changing the soil system through soil physical properties, through soil organic composition, through the mobility of nutrients (particularly C and N) and through soil microbiology (Kirkby et al. 2006, this volume). Through an understanding of these changes, it may be possible to design and
optimize crop management options that are practical yet move the risks of N
2O emissions in a predi-
cable and quantifiable direction.
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