Carbon Sequestration and Greenhouse Gas Fluxes from Cropland Soils Climate Opportunities and Threats

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Pete Falloon, Pete Smith, Richard Betts, Chris D. Jones, Jo Smith, Deborah Hemming and Andy Challinor

5.1 Introduction

Globally, soils contain approximately 1500 Pg (1 Pg = 1Gt = 1015g) of organic carbon (C) (Batjes 1996), roughly three times the amount of carbon in vegetation and twice the amount in the atmosphere (IPCC 2001). The annual fluxes of carbon dioxide (CO2) from atmosphere to land (global Net Primary Productivity [NPP]) and land to atmosphere (respiration and fire) are of the order of 60 Pg C yr-1 (IPCC 2001). during 1990s, fossil fuel combustion and cement production emitted 6.3 ± 1.3 PgCyr-1 to the atmosphere, while land-use change accounted for 1.6 ± 0.8 PgCyr-1 (Schimelet al. 2001; IPCC 2001). Atmospheric C increased at a rate of

3.2 ± 0.1 PgCyr-1, the oceans absorbed 2.3 ± 0.8 PgCyr-1 and there was an estimated terrestrial sink of 2.3 ± 1.3PgCyr-1 (Schimel et al. 2001; IPCC 2001). The amount of carbon stored in soils globally is, therefore, very large compared to gross and net annual fluxes of carbon to and from the terrestrial biosphere, and the pools of carbon in the atmosphere and vegetation. Human intervention, via cultivation and disturbance, has also decreased the soil carbon pools relative to the store typically achieved under native vegetation. Historically, these processes have caused a loss of soil C between 40 and 90 Pg C globally (Paustian et al. 1998; Houghton et al. 1999; Lal 1999). Hence, increasing the size of the global soil carbon pool by even a small proportion has the potential to sequester large amounts of carbon, and thus help mitigate climate change.

The trace gases, like methane (CH4) and nitrous oxide (N2O), are also potent greenhouse gases and emitted from, and absorbed by soils. For this reason, soils also have a second role to play - reducing trace gas emissions to the atmosphere -in combating climate change. Nitrous oxide is formed primarily from nitrification

Met Office Hadley Centre for Climate Change, Fitzroy Road, Exeter, Devon EX1 3PB, UK e-mail: [email protected]

S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.5. © Springer-Verlag Berlin Heidelberg 2009

and denitrification processes and is a by-product of nitrification and an intermediate during denitrification. N2O fluxes from agricultural soils (0.53 Pg C equivalents yr-1) account for more than 50% of the global anthropogenic N2O flux (Robertson 2004). The majority of the CH4 flux from agriculture is associated with livestock systems, and arises from enteric fermentation in ruminants (Robertson 2004). The only significant soil source of methane in croplands is rice cultivation (0.25 Pg C equivalents yr-1), which accounts for 22% of agricultural emissions or 12% of total anthropogenic fluxes (Robertson 2004). There are also significant fluxes of N2O and methane from natural ecosystems. Since carbon storage and trace gas fluxes from natural ecosystems offer less greenhouse mitigation potential (with some exceptions - e.g. avoided deforestation) and are harder to manage, we focus mainly on the role of cropland soils in this chapter.

Climate change could also alter carbon storage and trace gas fluxes from cropland soils, since changes in temperature, precipitation and atmospheric CO2 concentration will affect net primary production (NPP), carbon/nitrogen inputs to soil, soil carbon decomposition rates and soil nitrogen cycling. Due to the large size of the soil carbon and nitrogen pools, they have considerable potential to drive large positive climate feedbacks, as increased atmospheric CO2, N2O and CH4 concentrations will enhance climate change (Cox et al. 2000; Friedlingstein et al. 2003,2006; Jones et al. 2003). Therefore, climate change could also act to reduce or alter soil carbon sequestration and trace gas mitigation actions in croplands, with associated impacts on soil quality and fertility. This chapter deals with:

(a) opportunities for greenhouse gas mitigation in cropland soils,

(b) the possible impacts of climate change on greenhouse gas fluxes from cropland soils,

(c) how climate change could impact future cropland greenhouse gas mitigation potential, and

(d) outline future challenges for scientists and policymakers.

5.2 Opportunities for Greenhouse Gas Mitigation in Cropland Soils

5.2.1 Mechanisms of Greenhouse Gas Mitigation in Cropland Soils

Cropland soils can act as both sources and sinks for carbon dioxide and other greenhouse gases. Whether soils act as a sink or source, but their sink/source strength depends critically on soil management (Janssens et al. 2003). Cropland soils emit carbon dioxide (CO2; through soil and root respiration/decomposition of soil organic matter), nitrous oxide (N2O; formed during nitrification and denitrification processes) and may either emit or remove (oxidise) methane (CH4) from the atmosphere. The greenhouse gas mitigation potential for cropland soils, therefore, depends on reducing these emissions, increasing carbon inputs or decreasing disturbance. This could be possible by alteration in greenhouse gas emissions - by manipulating the factors which control the inputs and outputs of the cropland soil carbon and nitrogen cycle.

CO2 mitigation involves either reducing the CO2 efflux from the soil or sequestering carbon in the soil. The major inputs to the soil carbon system are the amount and quality of plants (and any additional) carbon input, which, in croplands, are controlled by NPP (crop type and varieties) and management of organic residues. Losses of soil carbon via decomposition are mainly controlled by soil texture, moisture, temperature and oxygen availability, which (with the exception of texture) can also be modified by land management strategies. For instance, mulching and tillage can alter soil moisture and temperature regimes. Soil carbon sequestration can thus be achieved by either (a) increasing the net flux of carbon from the atmosphere to the terrestrial biosphere by increasing global NPP (thus increasing carbon inputs to the soil), (b) storing a larger proportion of the carbon from NPP in the longer-term carbon pools in the soil (e.g. altering root to shoot ratios), (c) adding additional C containing materials to the soil (such as manures or cereal straw), or (d) slowing decomposition. For soil carbon sinks, the best options are, therefore, to increase C stocks in soils that have been depleted in carbon, i.e. cropland soils and degraded soils (Lal 2004; Smith 2004a), since the capacity for increasing C storage is very high in these soils. It is also important to minimise further losses of soil carbon stocks by more judicious land management, for example avoiding land degradation and the drainage of peatlands (Bellamy et al. 2005).

N2O mitigation entails reducing N2O emissions. The main options for N2O mitigation in cropland soils involve altering organic and inorganic fertiliser applications, agricultural operations (especially tillage and compaction), the use of rotations and water management. The impact of these options is to either (a) reduce inputs of nitrogen (and thus reduce N2O emissions) or (b) minimise conditions suitable for N2O production (Smith et al. 2004).

For CH4, mitigation involves reducing CH4 emissions from rice paddy soils -(Guo and Zhou 2007) and maximising the methane oxidation potential of other soils. The greatest potential for CH4 mitigation from agricultural soils is in rice production. Rice crop management to reduce CH4 emissions includes yield improvement by well-managements, as high yield rice crops have significantly lower CH4 emissions (where more C is allocated to grain than to the rhizosphere where it can undergo methanogenesis). Other mitigation options in rice include residue management and irrigation scheduling (Robertson 2004).

5.2.2 Greenhouse Gas Mitigation Potential of Cropland Management Options

Cropland greenhouse gas mitigation options and indicative C sequestration rates are given in Table 5.l. Clearly, land management practices involving the greatest increases in C inputs to soils have the greatest C mitigation potential achievable

Table 5.1 Greenhouse gas mitigation potential of cropland management options

Management option

Indicative maximum soil C sequestration rate Likely

uncertainty

Additional C savings_

Impact on trace gas fluxes_Other impacts

Notes

Mechanical operations Zero/reduced tillage

Reduced compaction

0.02tCha_1 yr"1 overall reduced C costs (Increased C cost from extra herbicides, reduced fossil-fuel C costs via less farm machinery work)

Unclear

Impact uncertain Impact uncertain

General land use/management Set-aside/ Conservation < 0.3 8

Reserve Program Convert to permanent crops 0.62 Convert to deep-rooting 0.62 crops

Rotational changes >0

Very high

May reduce N2O

May reduce N2O

May reduce N2O May reduce N2O

May reduce N2O

Reduced soil erosion, improved soil fauna

Reduced surface runoff, improved root penetration

Biodiversity and amenity benefits

Interacts with soil physical conditions; Ploughing or deep ploughing may reduce trace gas emissions but decrease carbon sequestration; Conservation (reduced) tillage may be intermediate; no-till may increase nitrous oxide emissions

Impacts on crop production

Catch crops reduce bare soils (possible link to BNF), Amelioration crops (crop type i.e. deep rooting or shallow rooting)_

Table 5.1 (continued)

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