Field studies on perennial energy crops often do not measure emissions of N2O (e.g. Christian et
al., 2008; Christian and Riche, 1998; Dondini et al., 2009; Heaton, 2004; Price et al., 2004) perhaps in part due to issues of high spatiotemporal variation. Given the expectation of low N inputs, low N2O emissions may be expected and a reduction in N2O emissions compared to arable is generally
postulated (Kavdir et al., 2008; Powlson et al., 2005; Rowe et al., 2009). Since N cycling is highly complex, further consideration and field observation is warranted.
In a review of studies by Don et al. (2012) total N2O emissions and emissions as a proportion of N
inputs (emissions factor (EF)) were lower for SRC willow and Miscanthus when compared to annuals, with the exception of a one year study (Jørgensen, 1997) comparing Miscanthus and rye, in which emissions were similar without fertiliser, and EF for Miscanthus was three times that of rye. Additionally Kavdir et al. (2008) found higher emissions for SRC willow than for a rotation of annuals (rape/rye/triticale) where fertiliser was not applied to the perennials or the annuals . It is significant that these experimental conditions do not reflect conventional farming approaches, i.e. that annuals usually receive higher fertiliser applications than perennials. A study by Drewer et al. (2012) on loam over clay recorded much higher N2O emissions for wheat and OSR compared to
Miscanthus and SRC willow, which they attribute largely to differences in fertiliser regime, and state that Miscanthus only reduces N2O if little or no fertiliser is applied. Christian et al. (2008)
note that fertiliser inputs may be required to maintain yields on nutrient poor soils, and that warm moist soils may experience enhanced denitrification, hence a significant proportion of
46
observed losses of 16- 40% of N fertiliser inputs to Miscanthus plots in Christian et al. (1997) and (2006) may have been in the form of N2O emissions.
Hellebrand et al. (2008; 2005) and Kavdir et al. (2008) suggest that their finding of an EF for perennials to be around half that for annual crops is a result of the impacts of NT, whilst Don et al. (2012) attribute this difference to more efficient nutrient uptake by perennials. Drewer et al. (2012) observed similar rates of denitrification in response to fertiliser inputs for Miscanthus and OSR, however gaseous losses for the Miscanthus field had a higher N2:N2O ratio, which may
reflect higher pH or lower diffusivity due to the NT system. Hellebrand et al. (2010) record higher WFPS for SRC willow (38.4%) than annual crops (35.4%); since nitrification is dominant under these conditions, lower oxygen availability under SRC willow may reduce rates of N2O production.
Jørgensen et al. (1997) suggest that low emissions for their study were due to low precipitation and sandy soil; their finding of higher N2O emissions for Miscanthus is therefore attributed to NT
soil structure and residues increasing WFPS and associated levels of denitrification; emissions were higher from Miscanthus than rye, in spite of lower fertiliser inputs.
A range of modelling approaches have identified lower N2O emissions for perennial energy crops
e.g. (Davis et al., 2010; Hamelin et al., 2012; Tonini et al., 2012). Using the IPCC methodology Tonini et al. (2012) calculate N2O emissions of 5.8 kg N ha-1 a-1 for Rye, 2.3 kg N ha-1 a-1 for SRC
willow and 2.0 kg N ha-1 a-1 for Miscanthus according to assumed levels of of fertiliser input. Calculations elsewhere identified almost twice the N2O emissions associated with leaf litter
compared to direct fertiliser inputs for Willow with 100 kg N ha−1 ammonium sulfate added every three years (Keoleian and Volk, 2005). Modelling by Dufossé et al., (2012) of Miscanthus
cultivation over a 20,000 km2 region of France identified that N2O from Miscanthus could
contribute an average of 13-8.8 kg ha-1 CO2 equivalent per ha per year, depending on estimation
methods, with significant spatial variation in both yield and N2O emissions according to soil type;
the resultant variation in N2O emissions per unit energy contributes to relative benefits of
bioenergy. For land use change, existing N2O emissions should also be considered; for arable
usage of the same area, simulated emissions varied from 93 to 1744 kg ha-1 a-1 CO2 equivalents
due to site factors (Dufossé et al., 2012), meaning that spatial variation in potential emissions reduction is of even greater significance.
47 Prior to establishment of perennial energy crops, soils are often left fallow over winter, leaving them vulnerable to erosion and leaching, then tilled for planting in Spring, causing significant mineralisation loss of N from soil in the case of former grassland or otherwise undisturbed soil (Christian and Riche, 1998; Jug et al., 1999). Losses due to mineralisation from soil disturbance accounted for 24.7Gg of the 78.4Gg of UK soil N emissions in 1990 (Brown et al., 2002; Curley et al., 2009). A further important consideration which has not been well assessed is the risk of high emissions following an N pulse on ploughing in of the significant N storage in Miscanthus roots and rhizomes at the end of the crop lifecycle (Christian et al., 2006).
In general, N2O emissions losses appear to be lower for SRC willow and Miscanthus than for
arable crops to which they have been compared. However there is variation with site factors and N fertiliser inputs, and emission during site preparation and crop removal must be included in assessments. Comparisons are rarely made for grazed lands; emissions from these relate to compaction of soil by trampling of livestock and N inputs in manure and urine, leading to emissions (Mosier, 1998).
Variation in change in N2O emissions between sites according to current land usage, as well as site
and land management factors will affect the relative benefits of bioenergy cultivation.
1.4.2.6 Leaching
Leaching losses are expected to be lower for Miscanthus and SRC willow compared to annuals, due to low N inputs and efficient uptake (Curley et al., 2009; Powlson et al., 2005). The IPCC emission factor (EF) for leached N is twice that for applied N, hence indirect emissions of N2O
must be regarded a key component of the GHG balance (Groffman et al., 2000b; Nevison, 2000). Leaching also has ecological implications, polluting river systems and coastal waters, causing eutrophication (Brown et al., 2002; Kramer, 2006).
Haag and Kaupenjohann (2001) state that at a watershed scale, most applied N is retained, and often stored for decades, whilst Breemen et al. (2002) suggest retention can last from decades to centuries. This may explain why leaching can be traced to historic land management Koh et al.
48
(2010). Land use change may encourage either release or accumulation of N in catchment stores, making them a significant area for research. Differences between inputs and outputs in N budget calculation are often attributed to retention (Alexander, 2002), however other factors should be considered first (Galloway 1995). For example N budgets often overlook dissolved organic nitrate (DON) which contributes around 20% N leaching (van Kessel et al., 2009), as well as emissions from shallow aquifers (Heide et al., 2009) and flow interaction between surface and subsurface water systems (Baresel and Destouni, 2006) both of which remain poorly understood
The IPCC standard methodology used for governmental calculations assumes that 30% of fertiliser input is leached; calculations by Tonini et al. (2012) gave values of around 74 kg N ha-1 a-1 for Rye, 10 kg N ha-1 a-1 for SRC willow and 10 kg N ha-1 a-1 for Miscanthus, although these are dependent on assumptions about fertiliser application rate. The ultimate fate of N from fertiliser is poorly understood in agricultural studies, and use of a default value for leaching rates may compound inaccuracies (Nevison, 2000; Van Breemen et al., 2002). Variation in leaching rates is significant; an analysis of 16 US catchments by Breemen et al. (2002) found that 20-60% N inputs were lost as leached N, whilst Meisinger and Delgado (2002) suggested a value of 10-30% leaching for grain systems. A review by Alexander et al. (2002) of several catchment studies suggested that predictions of fluvial N export could be improved by mapping patterns of variation in processes controlling Nitrogen cycling. Other crucial factors include timing and amount of fertiliser
application, nitrates applied faster than they can be used by plants will remain in soil and can be easily leached in the event of rain (Christian and Riche, 1998). Precipitation and soil structure and drainage are also important controls, since good drainage and increased precipitation increase leaching and decrease in-field N2O losses (Basset-Mens et al., 2006; Christian and Riche, 1998;
Hutchins et al., 2010).
Mineralisation due to over-winter fallow and tillage pre-planting can leave soils rich in available N, leading to higher leaching during the first year of establishment; SRC willow leaching has been recorded at levels comparable with maximum values for arable crops (Mortensen, 1998), and Miscanthus leaching at significantly higher levels (Christian and Riche, 1998). Reduced
mineralisation under the NT system may mean lower levels of available soil N in subsequent years, and thus leaching is often much below that seen for annuals (Jørgensen and Schelde, 2001). Soil type affects pattern of nutrient loss during establishment; Mortensen (1998) found that leaching was reduced in year 2 for coarse sand and year three for loamy sand. Structural changes which develop in soil under NT systems may increase bypass flow, which avoids contact with N
49 stored in the soil matrix, thus reducing concentration of N in percolating water (Soane et al., 2012).
Energy crops such as SRC willow with high uptake have been recommended as riparian buffers to protect water quality by uptake of nutrients in throughflow (Delgado et al., 2010; Elowson, 1999; Jørgensen and Schelde, 2001). Growing season is a significant factor; Lesur et al. (2013) suggest that N uptake in autumn by perennial energy crops can reduce leaching in response to high rainfall inputs in autumn and winter, whereas Christian et al. (2008) note that Miscanthus root activity is delayed by a month compared to shoot activity, and suggest that period of root uptake is shorter than for arable crops. The year round cover provided by perennials also reduces soil and N losses from surface runoff (Jørgensen and Schelde, 2001).
Measurements by Behnke et al. (2012) show leaching of 8 kg N ha-1 a-1 for Miscanthus with no
fertiliser input, and 28 kg N ha-1 a-1 leaching with 120 kg N ha-1 a-1 fertiliser input. Even at slurry application rates of 180 kg N ha-1 a-1 Miscanthus N levels remain within safe drinking water limits, and at similar levels to annuals (Curley et al., 2009). A study in Denmark (Mortensen, 1998) found increased leaching with N input during the first year of SRC willow, but thereafter 75 kg N ha-1 a-1 was added without increased leaching and values around 10 kg N ha-1 a-1 were recorded, comparable with other studies. It is therefore likely that land use change to perennial energy crops could reduce N leaching compared to annual arable crops, although soil N and amount and timing of any fertiliser inputs will affect the overall outcome.