title: Sustainable management of organic matter in a Swiss arable soil - carbon sequestration dynamics under various farming systems
Creators: name: Mayer, Marius Thomas
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 540 Chemistry
subject: 550 Earth sciences &amp; geology
subject: 570 Life sciences; biology
subject: 910 Geography &amp; travel

description: Soils, often overlooked and underestimated, are intricate ecosystems that support a diverse array of organisms, contribute to global biogeochemical cycles, and play a pivotal role in food production. In addition to being the largest reservoir of terrestrial carbon (C) and a vital regulator of the global climate, soils provide a multitude of ecological services that are essential for the functioning of natural environments and fundamental for human well-being. The ability of arable soils to act as long-term C sinks through the process of C sequestration, directly influences the extent to which human-induced increases in atmospheric carbon dioxide (CO2) can be offset and thus the extent to which climate change can be mitigated. However, the intensive and unsustainable use of arable soils has led to significant losses of soil organic matter (SOM), the storage medium of C in soil, which was accompanied by the loss of soil fertility. This prompted efforts to restore and increase SOM contents in soils through sustainable management, including economically incentivized C farming practices. The overall objective of this thesis was to improve our understanding of how various farming systems influence the dynamics of SOM within the soil. Particular emphasis was given to acquiring detailed insights into the mechanisms that govern SOM stability within different soil fractions. This would consequently allow the evaluation of different soil management strategies in terms of their effectiveness to contribute to climate change mitigation. To achieve this objective, the DOK long-term farming system comparison trial in Switzerland served as an experimental platform for the observation of SOM dynamics. The focus was on four farming systems with different fertilizer types and quantities. A purely minerally fertilized system (CONMIN), a mixed-fertilized system receiving mineral and organic fertilizer as stacked farmyard manure (CONFYM) and a purely organically fertilized system receiving composted manure (BIODYN) were compared with an unfertilized control (NOFERT), over a period of 36 years (1982-2017).
In the first study, physical fractionation was used to separate functionally distinct SOM fractions from archived soil samples. The findings revealed that no additional soil organic carbon (SOC) was sequestered in the finest separated soil fraction (<6.3 μm) under any of the farming systems over the whole observation period. The focus on this specific fraction is driven by its significance for long-term C sequestration, as the association of SOM with fine minerals (referred to as MAOM, mineral-associated organic matter) ensures the highest level of stability. The increase of bulk SOC in BIODYN (+13%) and CONFYM (+5%) (CONMIN: -8%; NOFERT: -20%), indicates that additional C accumulation was only recorded in the form of highly labile particulate organic matter (POM), more specifically, occluded particulate organic matter (oPOM), which undergoes initial stabilization via incorporation into aggregates. Interestingly, BIODYN showed the highest oPOM-C, although OM inputs via organic fertilizers were 20% lower compared to CONFYM. This indicates that the qualitative differences of the organic manures are vital for C sequestration and thus for increasing soil fertility. However, this labile fraction was prone to rapid losses within a few days. Considering that POM serves as a precursor to the highly stabilized MAOM, POM losses should be urgently avoided, as this impedes the potential for subsequent long-term C sequestration. Overall, these findings highlight the substantial dependence of soil fertility on the consistent application of organic fertilizers. Furthermore, in the context of climate change mitigation, it is very concerning that even under sustainable management practices over several decades, additional long-term C sequestration was insufficient. This finding was particularly surprising, especially considering that the saturation of the MAOM fraction could be ruled out as a contributing factor. In an effort to fully comprehend this observation, the second study was dedicated exclusively to the detailed examination of the MAOM fraction.
Consequently, the second study focused around the elucidation of the turnover dynamics within the MAOM fraction. MAOM samples from 1982 and 2017 were subjected to specific surface area (SSA) measurements before and after removal of OM with sodium hypochlorite (NaOCl). SSA results indicated best conditions for MAOM-C stabilization under organic fertilization and different sorption mechanisms in MAOM between farming systems with and without organic fertilization. In addition, 14C radiocarbon analysis of the MAOM fraction and subsequent estimation of its mean residence times (MRT = measure for SOM permanence in the soil) were performed, using a model that takes into account ‘bomb 14C’ and radioactive decay. Based on the findings of the first study, which indicated unchanged MAOM-C contents in BIODYN and CONFYM, we expected that long-term organic fertilization would enhance MAOM stability, resulting in longer MRTs. However, the findings demonstrated that continuous organic fertilization led to substantially higher MAOM-C turnover rates and significantly shorter MRTs (BIODYN: 140 ± 19 yrs, CONFYM: 138 ± 18 yrs) in comparison to non-organic fertilization (CONMIN: 195 ± 27 yrs, NOFERT: 238 ± 40 yrs). This indicated that MAOM is much more active under organic fertilization. This apparent contradiction, where increased MAOM-C turnover appeared to contribute to constant MAOM-C contents, could however be explained with the concept of ‘dynamic stability’.
The results of this thesis provide compelling evidence that the targeted efforts towards increased long-term C sequestration under agricultural management are insufficient. This dampens the high expectations for soil as a potent solution to bring about mitigated climate change. However, these findings fully align with the gradual and increasingly pronounced shift within the scientific community towards prioritizing soil management practices that promote soil fertility, rather than solely focusing on soil management for C sequestration as a means to address climate change. This thesis demonstrated that sustainable organic soil management, accompanied by consistent organic fertilizer inputs, emerges as the most favorable approach in this regard. Specifically, in the case of BIODYN, despite solely increasing C contents in the labile SOM fractions, this can have a beneficial impact on climate, as long as this management approach is sustained.