Biochar, a carbon-rich product that is manufactured by thermal decomposition of organic material under a limited oxygen supply (pyrolysis) has a long tradition as a soil amendment in tropical agriculture. More recently it has attracted interest because of a number of hypothesized positive environmental impacts including efficient soil carbon sequestration and improved crop yields. A relatively recent albeit frequent observation relates to lowering nitrous oxide and partially also carbon dioxide emissions from biochar-amended soil.
The current debate on environmental benefits and possible drawbacks of biochar application in agriculture is still based on weak ground regarding the duration of experiments and their representativeness regarding framework conditions such as climate, agronomy, or soils. Still many of these findings are based on artificial, lab-scale experiments.
In this project we will test the following hypotheses on how biochar may reduce greenhouse gas (GHG) fluxes, particularly N2O, and influence crop yields:
- immobilization of mineral N reduces N availability for nitrifiers and denitrifiers
- increased plant N uptake and crop yield reduce N availability for N2O production
- increased soil pH leads to a systematic reduction in potential N2O emissions
The project is centered around two field experiments:
1 A lysimeter study with two different annual crops running over two years includes two soils with eight replicates each and will use biochar and double-labeled mineral fertilizer (15NH415NO3) to measure 15N2O and 15N in plants, soil, water, and biochar to determine biochar effects on N2O emissions, immobilization, plant uptake, and leaching. It will allow us to study if biochar changes mineral N availability and if this depends on soil type. Manual static chambers will be used for GHG measurements.
2 Biochar will be applied to an annual crop in the field on agricultural soil with pH 6.3 and GHG fluxes will be measured in comparison to a control and to plots limed to the same pH as the biochar plots. Automated chambers will be used for GHG measurement and arranged in a randomized block design. The experiment will allow to distinguish effects induced by biochar from those related to soil alkalinity alone. As the lysimeter study, it will also provide information on eventual changes in crop yield associated with biochar application.
In addition to GHG measurements biochar will be retrieved from soil samples during the experiment to study possible compositional changes and adsorption of labeled N.
The research will provide substantiated and quantitative understanding of important mechanisms involved in effects of biochar application on agricultural GHG emissions. It will evaluate whether results from lab-based experiments are applicable to field conditions. Most importantly it will help to decide whether biochar application is worth considering a GHG mitigation option in agriculture of temperate regions.