Effect of soil texture and wheat plants on N2O fluxes: A lysimeter study
Introduction
Nitrous oxide (N2O) has a global warming potential 298 times that of carbon dioxide (Forster et al., 2007) and is a major ozone depleting substance (Ravishankara et al., 2009). Agricultural soils are a major source of direct and indirect N2O emissions (Mosier et al., 1998) accounting for 60% of anthropogenic N2O emissions (Syakila and Kroeze, 2011). Nitrification and denitrification are the main biological processes of N2O emissions (Firestone et al., 1989) although several other processes may also be responsible for N2O emissions (Butterbach-Bahl et al., 2013). Nitrification is the biological oxidation of ammonium (NH4+) to nitrite (NO2−) or nitrate (NO3−) under aerobic conditions, and denitrification is the reduction of NO3− to NO2−, nitric oxide (NO), N2O and molecular dinitrogen (N2) under anaerobic conditions (Bouwman, 1998).
In agricultural soils, the total N2O emissions in a season are heavily contributed to by the episodes of large N2O pulses observed after irrigation and rainfall events which are primarily derived from denitrification (Barton et al., 2013, Scheer et al., 2008, Trost et al., 2013). One of the main factors regulating the production of N2O by the denitrification process is soil is redox potential (Husson, 2013, Li, 2007) which is defined as the availability of electrons in soil and determined by the balance of the concentrations of reductants (e.g. organic matter) and oxidants (e.g. O2, NO3, SO4) present. Oxygen is used as the preferred electron acceptor until the redox potential drops to < + 300 mV below which nitrate, manganese, iron, sulphate and carbon dioxide are used as electron acceptors (Patrick Jr and Reddy, 1978). Factors that control the availability of O2 or the aeration of soil, namely O2 partial pressure in the gas phase, matric potential, soil water content (Bollmann and Conrad, 1998) and the interaction of these with the type and amount of organic matter, concentration of mineral N, temperature and pH, controls the availability of electrons in soils (Bremner and Shaw, 1958, Skiba and Ball, 2002) and hence, the process of denitrification and N2O emissions. The soil proxy that represents an integration of these parameters is soil texture (Corre et al., 1996). Through differences in air filled porosity (aeration), it is generally considered that fine textured soils provide conditions more favourable for denitrification and N2O emission at lower soil moisture contents than coarser textured soils which favour nitrification (Bollmann and Conrad, 1998, Parton et al., 1996b). Fine textured soils have small pores that become more easily anaerobic and remain that way for longer duration than coarser textured soils (Bollmann and Conrad, 1998, Groffman and Tiedje, 1991). It is important to recognize that soil aeration is not simply related to soil water content, but determined by transport and consumption rates of oxygen (Cook and Knight, 2003) which can be affected by environmental variables such as temperature and soil water content (Cook et al., 2013).
Plants are an integral component in agricultural systems and may also affect N2O emissions to the atmosphere. However, to date there is apparently no consensus as to whether plants promote or suppress N2O emissions. Plant roots may affect nitrification and denitrification processes which produce N2O in soil, by changing soil aeration, labile organic matter, water content and mineral N (Philippot et al., 2009). Plants are also known to directly produce N2O in their tissues (Hakata et al., 2003) and act as a conduit for the transport of N2O produced in soil to the atmosphere (Pihlatie et al., 2005). In a recent summary of published data for upland crops, planted soil was found to emit twice as much N2O as unplanted soil in upland crops (Hayashi et al., 2014). This stimulatory effect of plants is likely to be caused by increased denitrification rates which have often been observed to increase in the presence of plants (Prade and Trolldenier, 1988). Plant effects on denitrification are thought to be due to increasing O2 demand in the root zone and supplying labile organic matter from the fine roots and root exudates allowing denitrification to occur at lower water content (Cook et al., 2013, Smith and Tiedje, 1979). The N2:N2O ratios during denitrification may also be affected by the presence of plants caused by biogeochemical conditions created by the rhizosphere and phenology (Henry et al., 2008, Vinther, 1984). Conversely, other authors have considered upland crops may reduce N2O emissions by lowering soil N concentration and soil water content (Bouwman, 1996).
Most of the previous studies that have compared N2O emissions between different soil textures have been conducted in laboratory conditions (Balaine et al., 2013, Castellano et al., 2010, van der Weerden et al., 2012) due to the difficulty encountered in conducting comparative studies in the field caused by variation in weather, topography and crop responses at spatially diverse sites, even if sites can be chosen at the sub-kilometre scale (Gu et al., 2013). Lysimeters provide the ability to study a range of soil textures simultaneously under natural climatic conditions. They also enable the effect of plants to be considered when explaining the N2O emission response of soils that hold and drain water differently. Therefore, lysimeters provide a valid system for potentially identifying improved indicators that predict agricultural N2O emissions in soils of different textures and under different management. The objectives of this study were to investigate the effect of soil texture and wheat plant growth on total N2O emissions, denitrification-driven N2O emissions, leaching losses and evapotranspiration. To our knowledge, this is the first study in which surface fluxes of N2O, leaching losses of mineral N, water losses through leaching and evapotranspiration and plant N uptake were all measured simultaneously from three different soil types.
Section snippets
Lysimeter soil core collection and instrumentation
This study was conducted using a large experimental weighing soil lysimeter facility with underground access, located at Griffith, NSW, details of which have been described in earlier work (Jamali et al., 2015). The lysimeter was installed with 18 intact soil cores (diameter 0.7 m x height 1.2 m) comprising six cores from each of three soil types with a range of soil textures (Table 1). All soils were sampled from within the Murrumbidgee Irrigation Area (MIA), southern NSW, Australia (see Section
Seasonal N2O fluxes
In fallow cores, greater N2O emissions occurred in the fine textured soil compared with the freer draining soils in the order clay loam > loam > sand (Table 2). The differences in N2O emissions from planted cores were less pronounced and not significant among three soil types (Table 2). Nitrous oxide emissions were diminished in the clay loam and loam soils, planted to wheat compared with fallow soils but the effect was negligible in the sandy soil. (Fig. 2). The temporal dynamics of N2O emissions
Regulation of N2O fluxes in soils with variable texture
Soil texture predominantly determines the hydraulic conductivity, water-holding capacity, porosity and aeration of a particular soil type (Bollmann and Conrad, 1998, Saxton et al., 1986) which in turn regulate the soil redox potential and N2O emissions. The capillary pores in clay soils hold more water against gravity than sand and silt resulting in higher soil moisture content at field capacity (Saxton et al., 1986). This is confirmed by our data showing significant differences (p < 0.001) in
Conclusions
Knowing that organic carbon, pH and mineral-N are major drivers of N2O production through denitrification and often related to texture, the three contrasting soils used in this study were chosen for their similarity in these parameters. Soil texture played a key role in driving N2O emissions. The soil water dynamics used as a proxy for soil oxygen levels and governed by plant growth mediated ET and soil texture were the primary drivers of denitrification-driven N2O emissions in our study when N
Acknowledgments
This research was funded by the Australian Department of Agriculture through Filling the Research Gap (FtRG) funding and CSIRO’s Office of the Chief Executive (OCE). This study would not have been successful without the excellent technical support provided by Roy Zandona and Alison Fattore − technical officers at CSIRO, Griffith. We are grateful to Drs Ryan Farquharson and Henrike Mielenz, CSIRO, for their valuable feedback which helped improve the manuscript. We are also thankful to Bruce
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