Background and Objective

The earth is warming and human population is growing. It is thus likely, that thawing permafrost soils under natural vegetation will be cultivated in the near future.  It is however completely unknown, how this will affect soil organic matter cycling in these soils. It is also not understood, how cultivation of such nutrient poor natural soils will change the temperature sensitivity of organic matter decomposition under ongoing climate change. Such knowledge is however crucial for predicting CO₂ fluxes between atmosphere and biosphere. Therefore the project aims to evaluate:

• Soil organic carbon stock changes after land-use change on frozen and unfrozen soils as a function of time.
• Changes in soil temperature, active layer depth and potential litter decomposition upon land-use change
• Evaluate changes in microbial carbon use efficiency, community composition and temperature sensitivity of microbial activity

Approach

The Yukon Territory in Northwest Canada has a more than 100 years old history of land-use changes on cold soils. This is unique and provides the opportunity to study soil organic matter dynamics after land-use change as a function of time in a chronosequential approach. An additional thermosequence exists via the temperature gradient from south (mostly permafrost-free soils) to the north (mostly permafrost soils) of the Yukon Territory. Microbial processes will be quantified in incubation experiments and results will be implemented in turnover models.

Results

• When permafrost-affected forest soils were used for agriculture, there was a significant loss of soil C (but not in the case of non-permafrost-affected soils). This was the case under both arable and grassland use. Permafrost could not be detected in any of the sampled agricultural soils (in the upper 80 cm). This was a clear indication that deforestation leads to increased thawing of soils.
• On average, agricultural soils were 2°C warmer than forest soils, and litter decomposition was significantly accelerated, at least in cropland soils. Irrigation may also have had an amplifying effect on the latter, as the effect of enhancing litter decomposition was not significant in the infrequently irrigated grasslands.
• Microbial use efficiency was significantly increased after reuse, which could be attributed to increased nitrogen availability and higher pH values. Warming had a positive effect on C use efficiency (increased microbial growth).
• Overall, it can be concluded that climate-smart agriculture in areas of discontinuous permafrost will only work if deforestation is limited to permafrost-free soils. Through improvements in soil conditions for microbial metabolism (as well as C imports), it may be in permafrost-free soils that agriculture has a positive effect on soil C stocks.

Links and Downloads