Field-scale manipulation of soil temperature

by Alex Nuffer

Atmospheric CO2 has risen substantially due to an increase in fossil fuel combustion and the clearance of land for agriculture, significantly affecting global climate. Climate models predict that there will be an increase in temperature, as well as a change in precipitation patterns in the future. Variations in climate are likely to modify soil respiration and the soil carbon cycle, causing soil organic carbon to increase or decrease, which will either lead to a positive or negative feedback for atmospheric CO2. Poll et al. (2013) investigated the manipulation of various climate change factors on soil respiration and soil carbon cycle in an arable soil at field-scale in a temperate agricultural ecosystem. The experiment was established on an arable field, where temperature, precipitation amount, and frequency were manipulated to simulate various climate change scenarios. For two years CO2 efflux was measured weekly. Additionally, plant and soil microbial biomass were determined to accurately assess the effects of climate change factors on soil respiration. The results underlined the importance of soil water content to the response of ecosystems to climate change. There was a negative effect of increased soil temperature on soil moisture, which led to water limitation. Soil respiration and microbial biomass under increased soil temperatures were limited by water in the first year, but not in the second year. Altered precipitation showed only minor effects during the entire experiment. The study showed that the soil moisture regime under increased temperatures could determine whether soils are carbon sinks or sources.

The experiment was established in an arable field, and combined soil-warming and reductions in precipitation amount and frequency. Manipulations of climate factors were based on climate predictions for Germany. Soil temperature was increased by 2.5 °C to imitate predicted temperature increases until 2100. Precipitation amount was reduced by 25% during the summer and increased 25% during the winter. The frequency of precipitation events was extended during the summer by 50% to simulate future drought events. CO2 fluxes were measured weekly in closed chambers situated between the crop rows. The measured fluxes were a mixture of autotrophic and heterotrophic respiration due to the extension of the rooting system below the chambers.

The results indicated the importance of soil water content to the response of ecosystems to climate change. The experiment showed a negative effect of increasing soil temperature on soil moisture, which could have been attributed to the increase of evapotranspiration. In the first year, water limitation reduced microbial biomass in the soil, leading to a reduced effect on soil microorganisms from elevated temperatures. During the second year, water limitation did not affect microbial biomass and respiration because of lower soil moisture. Manipulation of precipitation amount and frequency did not show as significant effects as temperature on biomass and respiration. Climate change models for Germany predict an increase in temperature and decrease in precipitation. Based on the results from this experiment this change in climate will reduce soil moisture, therefore leading to a reduction of temperature sensitivity of soil respiration.

Water availability in soil plays a key role in the functions of ecosystems. Processes of an ecosystem, such as the carbon cycle, are of particular importance to the management of agriculture. The study highlights the importance of agricultural field experiments and the combined effects of different climate change factors on soil moisture. Understanding the effects of soil water content, and other key parameters of an ecosystem, will aid in agricultural adaptation to predicted variations in climate.

Poll, C., Marhan, S., Back, F., Niklaus, P.A., Kandeler, E., 2013. Field-scale manipulation of soil temperature and precipitation change soil CO2 flux in a temperate agricultural ecosystem. Agriculture, Ecosystems & Environment 165, 88-97

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