Carbon sequestration in soils: some cautions amidst optimism

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Abstract

A sink for atmospheric carbon (i.e., CO2) in soils may derive from the application of conservation tillage and the regrowth of native vegetation on abandoned agricultural land. Accumulations of soil organic matter on these lands could offset emissions of CO2 from fossil fuel combustion, in the context of the Kyoto protocol. The rate of accumulation of soil organic matter is often higher on fertilized fields, but this carries a carbon “cost” that is seldom assessed in the form of CO2 emissions during the production and application of inorganic fertilizer. Irrigation of semiarid lands may also produce a sink for carbon in plant biomass, but its contribution to a sink for carbon in soils must be discounted by CO2 that is emitted when energy is used to pump irrigation water and when CaCO3 precipitates in the soil profile. No net sink for carbon is likely to accompany the use of manure on agricultural lands.

Introduction

When fully implemented, the Kyoto protocol will require most nations of the world to reduce their net emissions of greenhouse gases by agreed, specified amounts by the year 2012. Despite the large pool of carbon (C) in soils, and large changes in soil organic matter (SOM) that often accompany human activities, changes in soil C are not explicitly included in the current version of the protocol (cf., Article 3.4). Agronomists have long recognized the benefits of maintaining and increasing SOM, which adds to soil fertility, water retention, and crop production. Increasingly, many soil scientists now suggest that the sequestration of atmospheric carbon dioxide, derived from fossil fuel combustion, should be added to this list of benefits, contributing to the Kyoto protocol (Bruce et al., 1999). Presently, the IPCC Panel on Land Use, Land Cover and Forestry is devising guidelines by which changes in soil C might be included in national carbon accounts.

In the US, lands set aside under the conservation reserve program (CRP) have been small sinks for atmospheric CO2, accumulating C at rates up to 110 g/m2/yr, or 17×1012gC/yr, during the past decade (Gebhart et al., 1994). Conservation tillage, including no-till, is also an effective process to sequester C in some agricultural soils (Rasmussen and Collins, 1991; Reeves, 1997; Lal, 1997; Paustian et al., 1998), although its success varies with soil texture (Needelman et al., 1999), and increases in SOM in the surface layers are sometimes matched by losses at depth (Angers et al., 1997; McCarty et al., 1998; Campbell et al., 1999; Six et al., 1999). By reducing the frequency of cultivation, conservation tillage also reduces the emissions of CO2 from fossil fuel use in the agricultural sector (Frye, 1984). Kern and Johnson (1993) calculate that the conversion of large areas of cropland to conservation tillage during the next 30 years could sequester all of the CO2 emitted from agricultural activities and up to 1% of the total annual fossil fuel emissions (at today’s levels) in the US. Similarly, improved management and alternative land use for agricultural soils in Europe could potentially provide a net sink for about 0.8% of the world’s current annual CO2 release from fossil fuel combustion (Smith et al., 1997).

Beyond conservation tillage, many of the other techniques recommended to increase C sequestration in soils contain hidden carbon “costs” in terms of greater emissions of CO2 and other “greenhouse” gases to the atmosphere. The objective of this paper is to examine several agricultural practices frequently recommended to increase C sequestration in soils and estimate how much we should “discount” their net contribution to soil C storage as a result of considering the ancillary C emissions associated with each practice.

Section snippets

Nitrogen fertilizer

Applications of nitrogen (N) fertilizer are often recommended to increase SOM, particularly on lands that have already experienced significant losses of SOM as a result of cultivation. Rasmussen and Rohde (1988) show a direct linear relationship between long-term N additions and the accumulation of soil organic C in some semiarid soils of Oregon. At 100% efficiency, the stoichiometry of the Haber–Bosch process for the industrial production of ammonia indicates an emission of 0.375 mol of C per

Greening the desert

Increasing the production of plants on marginal, semiarid lands is another method frequently proffered to increase the storage of C in soils. In most cases, increasing plant production on these lands will require irrigation, yet irrigation waters are potentially associated with large CO2 emissions to the atmosphere. From data presented by Maddigan et al. (1982) on the electricity used to pump irrigation waters, one can calculate that 22.5 g C/m2/yr are released during the irrigation of

The myth of manure

Since biblical times, farmers have returned animal wastes to farmland as a means of increasing crop yields (Luke 13:8). Applications of manure are often assumed to increase C sequestration in soils (Smith et al., 1997), but manure is not likely to yield a net sink for C in soils, as would be required by the Kyoto protocol. Buyanovsky and Wagner (1998) show increasing SOM as a function of increasing C input from residues and manure in the Sanborn plots in Missouri. Manure was applied at a rate

Conclusions

A small sink for C in soils may derive from the application of conservation tillage and the regrowth of native vegetation on abandoned agricultural land. Any accumulation of SOM on these lands would contribute to a net sink for CO2 that could offset emissions of CO2 from fossil fuel combustion and contribute to the Kyoto protocol. The rate of accumulation of SOM is often higher on fertilized fields, but this carries a carbon “cost” that is seldom assessed in the form of CO2 emissions during the

Acknowledgements

The genesis of this paper occurred while its author enjoyed the hospitality and facilities of the California Institute of Technology, where he was a visiting professor of biogeochemistry in 1998. It has benefited from discussions and data provided from Kris Havstad and Adele Morris, and critical comments by Laurie Drinkwater, Rattan Lal, Bill Parton, Pedro Sanchez, Pete Smith, Pieter Tans and Daniel Yaalon.

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