Methods in studying the effects of soil amendments on greenhouse gas emissions from cultivated peatland, and the effects on associated soil microbe communities

Background

Agricultural soils produce large quantities of greenhouse gases (GHG). Especially organic soils, such as peat, can act as a source of carbon dioxide (CO₂) and nitrous oxide (N₂O) when the natural water table height is lowered for agricultural use, allowing aerobic decomposition of the previously waterlogged organic matter. While organic soils, such as peat, make up approximately 13% of the total arable land area in Finland, CO2 emissions from cultivated peat constitute 40% of the total CO_2, and 22% of the N₂O emissions from agriculture. These emissions are the result of microbial activity related to carbon and nitrogen cycles, and according to current knowledge microbial activity is regulated by the pH and electrical conductivity of the soil. Soil amendments such as lime and wood ash are used to improve the alkalinity of cultivated soil and may influence microbial activity. Earlier experiments have also shown that wood ash addition can decrease the N₂O emissions from cultivated peat. Researching the extent to which it is possible to mitigate these GHG emissions with soil amendments is of vital importance in order to build sustainable land use practices and guidelines for agricultural use of peatlands.

 

Method

In our research we aim to study the effects of different soil amendments on GHG emissions from cultivated peatlands. The soil amendments that we study are wood ash, lime (calcium carbonate, CaCO₃), gypsum (CaSO₄* 2H₂O), and biochar. The soils we use are collected from four different cultivated peatland sites, and the effects were studied in bottle and core incubation experiments where the GHG emission rates were measured weekly. The soil was also sampled, and samples flash-frozen, before and after the incubation to allow for DNA and RNA extraction, for purposes of determining the soil microbe community structure and activity. We determine the soil microbial community by amplifying 16S rRNA-gene from the extracted DNA and sequencing the amplified DNA with MiSeq equipment. To further study the community structure and activity we determine the copy numbers of selected enzyme-coding genes (amoA, nirK, nirS, narG, nrfA) related to nitrogen cycling from both the extracted DNA and RNA using Quantitative PCR, and Quantitative Reverse Transcription PCR methods respectively. In addition to measuring the GHG emissions, we also measure the nitrous acid (HONO) and nitric oxide (NO) emissions from the soils during the experiment. Nitrous acid is precursor of atmospheric NO that depletes ozone, and hydroxyl radicals (OH) that can oxidize atmospheric methane (CH₄). Based on our initial results from the core incubations, we are also planning a follow up field experiment.