Soil was recognised as a significant carbon reservoir by the Kyoto protocol1 and the proposed EU Soil Directive2 would seek to enforce sustainable management practices. The soil carbon cycle is controlled by its chemistry, mineralogy and climatic environment. Warmer conditions encourage greater microbial activity in soil. Microbes help organic matter decay, releasing carbon. However, with the right soil chemistry, this carbon can remain locked in the soil. The findings of this study suggest that the composition of some Alpine soils do not favour carbon storage under a warmer climate.
The scientists analysed typical soils from eight sites (four south-facing and four north-facing) at 2100m high in two Italian Alpine valleys. The climate in each valley is cold (mean temperature below 8°C) and dry, with less than 90 wet days per year. Weathering rates and movement of minerals are high. Organic carbon content of the soil varies widely, depending on the plot, ranging between around 2-195g per kg. It is high in many cases.
Around half of the soil organic carbon was shown to be bound up with the dense mineral content of the soil. These 'organo-mineral complexes' protect the organic carbon from decay, so it can be stored stably in the soil for a long time. This was demonstrated by radio-carbon measurements, with ages of up to 17,000 years recorded, the oldest being found at the north-facing sites.
Total carbon content of north-facing soils was also higher, because they supported large amounts of weakly-degraded organic carbon which was not in mineral complexes. In south facing sites, this material decayed more rapidly. However, the north-facing soils did not support as much microbial activity, responsible for decay. This was mostly because the soils were acidic, but also because they received less sunlight.
The combination of soil acidity and high organic content also meant that north-facing valley soils were subject to greater 'podzolisation'. This is where partly decayed organic matter binds to iron and aluminium ions which are washed downward in the soil (along with clays) leaving an increasingly acid, nutrient-poor and quartz-dominated soil at the surface.
As a result of these environmental conditions, the soils studied currently act as a carbon sink, but many climate change models predict warming of the sub-alpine region in which they are found. These findings suggest that microbial activity, and hence the rate of decay of organic carbon, will increase during the transition to warmer conditions in the valleys. These soils will become a source of carbon during this time, rather than a sink.