Two major factors affecting the environment word-wide are the production of greenhouse gases (GHG), and soil loss due to erosion. Understanding how soil production and erosion processes occur on hillslopes is important not only for understanding soil transport but, less known to the average person, important for understanding global carbon cycles. As rock minerals are chemically weathered into soil, carbon dioxide is taken up in chemical reactions that produce the secondary minerals that soil is made of, effectively removing the carbon from the atmosphere. This occurs at the surface of the solid rock and the rate at which it occurs is influenced by temperature, the presence of water, and organic acids produced by plants and soil microbes. The thickness of soil above the rock influences its exposure to these weathering factors, and therefore lower rates of soil formation occur under thick soils. It was suggested long ago that production of soil from rocks in areas of high elevation has an influence on atmospheric carbon dioxide levels. This influence is a result of high rainfall rates and physical weathering processes that break apart rocks (such as the expansion of ice in rock fractures) at high elevations increasing erosion and exposing underlying rock to chemical weathering, and high rainfall rates and the presence of vegetation increasing chemical weathering and thus the uptake of carbon dioxide from the atmosphere.
Recent research involving Associate Professor Peter Almond, Dr Andre Eger and Dr Brendon Malcolm of Lincoln University, alongside Dr Isaac Larsen, Dr John Stone and Professor David Montgomery from the University of Washington in the United States, has shown that soil production on the western side of the Southern Alps is occurring at a rate far greater than initially thought. They found soil production to be occurring at a rate of 0.1 to 2.5 mm per year, which is up to ten times greater than soil formation rates reported elsewhere in the world, or even reported previously on the Southern Alps (see the graph below). “A couple of millimetres a year sounds pretty slow to anybody but a geologist”, stated Professor Montgomery, “Isaac measured 2 millimetres of soil production a year, so it would take just a dozen years to make an inch of soil. That’s shockingly fast for a geologist, because the conventional wisdom is [that] it takes centuries”. The high rates of soil production on the Southern Alps is thought to be a result of the high uplift rates caused by the active fault line beneath the Southern Alps, combined with the high levels of rainfall experienced along the western side of the alps. The Alpine Fault causes an uplift rate along the Southern Alps of up to 10 mm per year, exposing fresh rock to weathering processes, while rainfall of 10 m per year physically weathers rock and supports vegetative growth. This high level of rainfall and resulting increased vegetation at high elevation is thought to be the key contributing factor allowing such high rates of soil production.
A previous study of soil production and chemical weathering rates in the Southern Alps found an overall average weathering rate of just two times higher than the global average, with one type of chemical weathering occurring at a rate of up to five times higher than the global average. However, it was claimed that the high proportion of physical weathering relative to chemical weathering reduces the effect of this compared to more stable landscapes that have low levels of physical weathering, with stable landscapes having a higher overall carbon dioxide consumption. This study only looked at sediment transport and solute concentration in streams (where eroded material ends up) on a single occasion, while the scientists from Lincoln University and University of Washington compared data from streams, bare rock and soil production areas. This team found that while chemical weathering is less than 5% of weathering that occurs in the water catchment (the area which drains to a single point in the stream), it accounts for 16 to 32% of weathering that occurs on ridgelines. The difference between these two values is thought to be due to large amounts of unweathered rock being deposited in streams by landslides. Landslides occur infrequently enough on the Southern Alps that soil and vegetation is able to develop on the scar before another slip occurs. Given this infrequent occurrence (estimated to be several centuries between slips at any one point), it is likely that the majority of soil in the landslide material will be washed away far sooner than the bedrock fragments and thus were not detected on the single sampling date of the authors of the previous study.
The team of authors from Lincoln University and University of Washington summarized that the extremely high levels of soil production (and therefore carbon dioxide consumption) measured in the west of the Southern Alps is attributed to the high levels of rainfall experienced in the area. This rain and the vegetation that it supports physically weather rock, expose fresh rock to be weathered and enhance chemical weathering of rock, increasing soil formation and carbon uptake. Soil production rates measured in this study, combined with other research documenting the amount of carbon fixation resulting from chemical weathering of rock minerals, can be used to understand biogeochemical cycling, particularly the effects of mountains on the global carbon cycle. This information may then be used for carbon budgeting in accordance with the Kyoto Protocol and reporting of carbon emissions and accrual.
Here in New Zealand, a large proportion of our GHG production comes from the agricultural industry and due to this, our GHG emissions are notoriously difficult to reduce when compared to that of more industrial countries or worldwide with point-source emissions coming from factories or vehicles burning fossil fuels. Machinery and cars can be made electric or run on biofuel or renewable energy, but reducing agricultural emissions (such as belching by cows) is much more difficult. Though farming intensification has caused a net increase in GHG production since 1990, the agricultural industry has improved its efficiency regarding GHG emissions, and while research continues in this area, other means to balance emissions must also be investigated. Just as it is important to determine the sources of New Zealand’s GHG production, it is also important to understand and account for the carbon sinks to understand the true balance of New Zealand’s emissions and determine our overall contribution to climate change. Research into soil production in the Southern Alps has helped to shed light on how New Zealand’s climate indirectly acts both for and against our carbon balance by providing an environment that has allowed our agricultural industry to thrive (thus leading to associated GHG emissions) whilst providing conditions that facilitate high levels of carbon accrual via soil production. Big, bold and beautiful, the majestic Southern Alps are now a feature of international interest on an entirely different level.
The author Veronica Penny is a postgraduate student at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.
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