N2o Emissions Hot Spots in Grazed Grasslands and Mitigation Measures

Relatively many papers published in the last decade were dealing with N2O sources and sites of emissions on grassland livestock systems (see e.g. Oenema et al., 1997; Kammann et al., 1998; Chadwick et al., 1999; Freibauer, 2003; Müller and Sherlock 2004; Saggar et al., 2004; Oenema et al., 2005), thus reflecting the fact that this type of ecosystem is responsible for significant proportion of N2O emissions worldwide. Recent estimates show that emissions of greenhouse gases from agriculture account for ca 22 % of all global emissions. Livestock production - including land use changes and deforestation, soy-feed production, energy used for grain production, processing and transport, soil organic matter decomposition followed by CO2 (and CH4) releases and a number of direct and indirect greenhouse gas sources in animal husbandry - accounts for nearly 80 % of agricultural emissions (McMichael et al., 2007). Emissions from agriculture are dominated by N2O and CH4 and agriculture accounts for 52 and 84 % of global anthropogenic emissions of N2O and CH4, respectively (Smith et al., 2007). Similar data apply for the Czech Republic; according to national greenhouse gas emission inventory, the major sources of N2O are agricultural soils and organic fertilizers followed by fossil fuel burning and industry. The estimates for 2005 show that in the country-level sectoral emission budget (expressed as CO2eq), 58 % emissions in agriculture is N2O from the soils (both fertilized croplands and grazed pastures), 31 % CH4 from enteric fermentation, 6 % N2O and 5 % CH4 both from organic fertilizers (Anonym, 2007).

The primary source of N2O emissions from animal production systems is urine and dung, and in smaller extent soils and sometimes also inorganic fertilizers. Therefore improvements in animal waste management are necessary if sustainable reduction of N2O emissions is required. Several types of environments or events characterized by potentially high N2O production and thus possible increased emissions (= emission hot spots) can be distinguished in the livestock farming system. These include:

i. hot spots in grazed grasslands (where combine effects of urine, dung, and compaction and other soil structure impacts occur resulting in creating conditions for high nitrification and denitrification rates), ii. hot spots in manure storage facilities (manure heaps, lagoons and tanks, where either anoxic and oxic conditions occur facilitating microbial transformations in conditions of surplus of nutrients), iii. hot spots in livestock (ruminants and others, mostly related to methane production by enteric fermentation; however N2O production can be also expected), iv. other hot spots (e.g. N2O emissions from burned dung collected after deposition in paddocks or outside areas)

v. hot spots generated or supported by management practices and policy decisions (decisions influencing occurrence, intensity and timing of grazing, quality of grasslands, numbers of animals, dietary aspects, etc.). For more details see Table 1.

In addition to other known hot spots listed in Table 1 we propose cattle overwintering areas, that is pastures where cattle is located in high stock densities for relatively long period during winter season, as potential hot spots of N2O (and also CH4) emissions. In these grassland areas severe damage of plant cover is common and the effects typical for the camping and overgrazed areas are even reinforced.

Table 1. N2O emission hot spots in animal production systems based on grazed grasslands including a newly proposed hot spot - an overwintering area

Area

Hot spots

grazed grasslands

camping areas drinking sites feedlots shade areas footpaths dung (deposited) urine patches overwintering area

manure storage, handling and application

manure heaps tanks and lagoons soil after application

livestock

intestine

other direct

dung use as biofuel

indirect: management and policy

grazing intensity grass stand productivity fertilizer management feeding practice dietary additives

Damage of vegetation takes place in the same time of large depositions of nutrients; consequently lower utilization of deposited N by plants in a following period is a prerequisite for higher rates of microbial transformations and subsequent gaseous losses from the system.

There are three mechanisms to lower N2O emissions from agriculture in general as identified by Smith et al. (2007), which can be adopted to grazed grasslands, too. These include:

i. reduction of existing emissions: Mitigation options include better management of nitrogen flow through the system, increased efficiency of nutrient utilization and thus lowering losses.

ii. enhancement of removals: Nitrous oxide produced in soil, dung, manure and other hot spots can be further reduced to N2 before escaping the site of origin. Gross production of N2O can be thus substantially reduced resulting in decrease of net N2O fluxes.

iii. elimination of N2O production: Management and policy decisions which can eliminate the possibility for N2O production in grazed ecosystems.

As pointed out by Smith et al. (2007) many practices were suggested and introduced to mitigate N2O emissions through above mechanisms, focusing on the hot spots listed in Table 1. A number of publications and extensive reviews on mitigation options for N2O emissions from agriculture have been published in last years (e.g. Beauchamp, 1997; Oenema et al., 1997; Senevirante, 1999; Senevirante, 2001; Six et al., 2004; de Klein and Ledgard, 2005) including several recent papers (Clough et al., 2007; McMichael et al., 2007; Schils et al., 2007; Smith et al., 2007; Vergé et al., 2007; Noble and Christmas, 2008; van Groenigen et al., 2008). It is therefore not the aim of this brief review to repeat information already widely available. Instead we will focus on the above mentioned newly proposed hot spot, cattle overwintering areas, which represent potentially significant sources of N2O emissions.

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