The Possibility of Increasing Soil Carbon to Offset Methane and Nitrous Oxide Emissions from Agriculture
Concerns are being raised, especially by the agricultural sector, about the possible economic impacts of ratifying the Kyoto Protocol. Two potent greenhouse gases, methane and nitrous oxide, represent about 55 percent of New Zealand's emissions profile, and mainly come from agriculture. The sensitivity of the agricultural sector to potential negative impacts of ratifying has caused misunderstanding about how the emissions of these gases might be reduced or even offset altogether. Dr Surinder Saggar of Landcare Research reports on the chances of increasing soil carbon stocks through changes in land use and land management. This report is extended from his recent article in AgScience1.
Some farmers believe that by increasing pasture production, the humus content of soils (and hence the soil carbon content) will increase to outweigh methane or nitrous oxide emissions completely. If true, even small increases in soil carbon over time could strongly impact on our national carbon budget, because nearly half (13.61 million hectares) of our land is used for livestock farming. Research by Landcare Research in Palmerston North, however, shows that soil carbon levels in New Zealand's grazing lands are at, or near, steady state. This research has involved a review of published data from long-term sites, the comparison of soil carbon in archived and recent samples from the same sites, and the comparison of soil carbon in samples from trial sites under differing management2. The review supported the hypothesis that soil carbon levels in New Zealand's grazing lands are at or near steady state.
To understand this conclusion, it is useful to consider how carbon cycles in soil, and what influences the cycling processes.
Carbon enters the biosphere through photosynthesis, and leaves through respiration, principally from the soil. The degree of accumulation or loss of organic carbon in pastures depends on the relationship between carbon inputs to soil from roots and surface litter, and decomposition rates of soil humus. Shifts in this balance between gain and loss of carbon result from a complex interaction of ecosystem processes influenced by, for example, climate, soil type, landscape position, land use and soil management. As the rate of carbon
input approaches that of decomposition, soil carbon concentration stabilises.
Cultivation of soils under pasture or forest into cropping causes a significant loss (30-50%) of soil carbon and increases carbon dioxide emissions into the atmosphere. The reasons for soil carbon loss are an accelerated decomposition of the existing organic matter in soils on the one hand, and reduced input of plant material into the soil on the other.
The restoration of soil organic matter, depleted through cultivation of agricultural soils to near their original level, appears to be a realistic opportunity. Improvements of agricultural soils beyond the original level of soil organic matter through soil improvement may also be possible and can help to sequester more carbon. Among the options of restoring soil carbon in cropland soils are: adopting conservation tillage practices and crop residue management; intercropping; growing winter cover crops; green manuring; fallowing; establishing shelter belts and windbreaks; and retiring erodible lands from cultivation.
What should also be kept in mind are the additional requirements of fertilisers and fossil energy to achieve such soil carbon sequestration, and increased nitrous oxide emissions from nitrogen-enriched soils. Secondly, this carbon sink option is of limited duration because any subsequent poorer soil management may well cause another significant drop.
No-tillage management has the potential to sequester carbon. In some cases, soil carbon can be augmented up to 50 percent due to smaller carbon losses in crops grown under no-tillage than under conventional tillage. But nationally, the scope for sequestering carbon in New Zealand cropping soils is negligible, because of the small area covered (0.21 million hectares) and the commonly practised pastoral-cropping rotation. There is very little information on nitrous oxide emissions in New Zealand cropping soils. No differences in nitrous oxide emission between conventional and no-tillage were observed after five years of cultivation. However, the cropping land management effects on nitrous oxide emissions are insignificant in comparison with national estimates, which are mainly attributed to dairy grazed pastures.
Where no major disturbances such as soil erosion occur, pastoral farming impacts on soil organic matter are so slow they are obvious neither to the landowner nor to outside observers. Such changes in soil carbon are likely to be too small to measure. New Zealand has a large reservoir of carbon that can vary widely spatially and temporally. Soil carbon levels of grazed pastures represent different slope and aspect categories. Research by Landcare Research and others suggests that soil carbon levels under established pastures in New Zealand may have reached a near steady state.
If the farmer starts from an unimproved pasture (with low soil fertility), then there may initially be a small increase in soil carbon after the soil is ploughed out of an unfertilised, low-fertility pasture, sown to permanent ryegrass-white clover pasture, and fertiliser is applied. But this situation is not common these days _ little unimproved grassland is being broken in for pastoral farming. The baseline for most farmers in dairying, for example, is a high-fertility soil, and our research has shown that adding fertiliser generally has no detectable effect on soil carbon content. There is some evidence that soil carbon levels can at times decline slightly if already fertile pastures receive more fertiliser. Research, however, generally shows that fertiliser addition not only produces more pasture and adds more carbon to the soil, but also enhances soil carbon decomposition. The net result of fertiliser application is that only the quality of soil humus may improve without any quantitative increase in soil carbon.
The development of agriculture in New Zealand differed from that in most other countries: forest clearing after Polynesian and European arrival resulted mainly in pastoral farming rather than in arable cropping. New Zealand soils already had high carbon contents under forest, and the introduction of pastures tended to increase mineral soil carbon, so that opportunities for sequestering more carbon in pasture soils are limited unless the eroded land is re-vegetated. Our soils already have higher soil carbon levels than the world average. This historic build-up in soil carbon results partly from the predominance of pastoral use, which conserved and even increased carbon, and partly from the relatively slow decomposition of the soil carbon stored during millennia under forest.
At the national level, our pastoral systems are therefore unlikely to sequester more carbon into soils, and the scope for cropping soils is also negligible. Hence opportunities for increasing soil carbon stocks, at the national level, through changes in land use and land management, are small.
1Saggar S (2002) Can soil carbon be increased to offset methane and nitrous oxide emissions from pastoral agriculture? AgScience, Issue 6, March 2002 (in press).
2Saggar S, Tate KR, Hedley C, Perrott K, Loganathan P. Are soil carbon levels in our established pastures at or near steady state? New Zealand Soil News 49(4), 73-78.
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Dr Surinder Saggar SaggarS@LandcareResearch.co.nz Surinder is actively involved in the study of soil carbon and nutrient transformations in relation to greenhouse gas emissions and sustainable management of landuse systems. For over 29 years, he has studied soil issues related to cropping, pastoral land, tussock grasslands, and plantation forestry, and has been involved in the development of management strategies for sustainable land use. His current research features the development of a modified process-based model to estimate accurately nitrous oxide emissions for New Zealand soils, climatic conditions and pastoral systems. |
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