Breaking the Ice – Consequences of northwards extending agriculture for soil organic matter cycling under a changing climate
Final Report Abstract
Climate change is expected to foster agricultural development in the global North, which is widely covered by pristine ecosystems such as boreal forests and tundra. Subarctic soils are often permanently frozen, which currently preserves large amounts of soil organic carbon (SOC) from mineralization by microbes. Those soils are expected to become hot spots of greenhouse gas emissions in the near future due to permafrost thaw. Also, potential land use changes might catalyze permafrost thaw and SOC loss by changes in microclimate. The interaction of permafrost abundance and land use change, as well as mechanistic drivers of SOC stock changes, such as microbial physiology, have hardly been investigated. In the project Breaking the Ice, a total of 18 farms and adjacent pristine subarctic forests in the Canadian Yukon Territory were sampled and analysed for changes in SOC stocks and fractions, permafrost depth, soil temperature, microbial carbon use efficiency (CUE), microbial community composition and further soil properties to obtain a detailed understanding of processes involved in SOC cycling after land use change in subarctic soils. As hypothesized, the abundance of permafrost had a significant effect on the magnitude of SOC losses upon deforestation for both, subsequent cropland and grassland use. On average, soil temperatures increased by 2°C just by deforestation, which also significantly affected litter decomposition. Most severe SOC losses were found in wetter permafrost soils, in which the underlaying ice acted as a water-logging layer so that deforestation had a strong drainage effect. Large amounts of labile carbon were mineralised, while at the same time mineral-associated carbon tended to increase after deforestation. This indicated a shift in the importance of SOC stabilisation pathways. Indeed, also microbial CUE was increased in agricultural soils, suggesting that the in-vivo pathway of SOC stabilisation was fostered by agricultural activity. The most important drivers of CUE were C:N ratio and soil pH, suggesting that agricultural management affected microbial physiology via changes in abiotic soil properties. In soils without permafrost abundance, we partly observed increases in SOC stocks which might in fact be related to changes in CUE in combination with increased net primary production and external C inputs. No clear land use dependency of CUE temperature sensitivity was found. Altogether, the project provided evidence that permafrost agriculture needs to carefully consider initial site conditions for achieving a climate-smart land use transformation of the global North, which is likely to happen soon in certain areas. Due to personnel changes and partly insufficient management data, planned model refinement to better predict SOC changes upon deforestation in subarctic soils was not possible. However, we identified certain key gaps that should be addressed in process models to better capture SOC dynamics upon land use change, such as the change in soil temperature. Apart from important insights regarding the central hypotheses of the project, several unforeseen insights were gained: First of all, a global farmers survey about important features of Northern agriculture, farmers’ motivation, perception of climate change and their interaction with nature was conducted to obtain a better understanding of high latitude farming in general. Furthermore, a geothermal soil warming gradient (up to +7°C) in the Yukon Territory was sampled and monitored. In this way, we were able to quantify long-term warming-induced SOC losses and changes in organic matter quality. Before the main sampling campaign, a pre-study was conducted to test the influence of sample pretreatment on microbial CUE in forest and agricultural soils to develop a sampling and storage strategy for the central physiological measurements. Sample pre-treatment, including drying, freezing and fresh storage, did not significantly affect microbial CUE. All mentioned additional studies were published in high-ranked journals. Finally, a microcosm translocation experiment with labelled litter was started at three different farms. This will be used to better understand microclimatic effects on the fate of C inputs and SOC cycling in general. Part of the TV documentary “Bodenlose Zukunft? Wenn der Acker verschwindet”; Leschs‘ Kosmos, ZDF, 29.9.2020 Part of the TV documentary “Wenn die Kältesteppe taut“; Planet e, ZDF, 7.11. 2021 Part of the TV documentary “Wirschaftsträume im Permafrost“, MAKRO, 16.11.2021 Radio documentary “Landwirtschaft auf tauenden Permafrostböden”, Wissenswerte, rbb, 28.01.2022 Part of the article “Subarctic climate change: Agricultural opportunities and threats” in Growing for Market, March 2022
Publications
- (2019) Farmers’ Perspective on Agriculture and Environmental Change in the Circumpolar North of Europe and America. Land 8, 190
Poeplau C, Schroeder J, Gregorich E, Kurganova IJL
(See online at https://doi.org/10.3390/land8120190) - (2020): Soil organic carbon stocks along a geothermal gradient in North-West Canada. EGU (Online)
Tino Peplau
(See online at https://doi.org/10.5194/egusphere-egu2020-9182) - (2021) Impact of common sample pretreatments on key soil microbial properties. Soil Biology and Biochemistry, 160, 108321
Schroeder J, Kammann L, Helfrich M, Tebbe CC, Poeplau C
(See online at https://doi.org/10.1016/j.soilbio.2021.108321) - (2021) Long-term geothermal warming reduced stocks of carbon but not nitrogen in a subarctic forest soil. Global Change Biology, 27, 5341-5355
Peplau T, Schroeder J, Gregorich E, Poeplau C
(See online at https://doi.org/10.1111/gcb.15754) - (2021): Impact of conversion from boreal forest to agricultural land on microbial carbon use efficiency and community structure, Eurosoil (Online)
Julia Schroeder
- (2022) Subarctic soil carbon losses after deforestation for agriculture depend on permafrost abundance. Global Change Biology
Peplau T, Schroeder J, Gregorich E, Poeplau C
(See online at https://doi.org/10.1111/gcb.16307) - (2022) Unexpected microbial metabolic responses to elevated temperatures and nitrogen addition in subarctic soils under different land uses. Biogeochemistry
Schroeder J, Peplau T, Gregorich E, Tebbe CC, Poeplau C
(See online at https://doi.org/10.1007/s10533-022-00943-7)
