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Implementation of soil fauna effects into the forest ecosystem model ANAFORE

Periodic Reporting for period 1 - ANAFAUNA (Implementation of soil fauna effects into the forest ecosystem model ANAFORE)

Reporting period: 2019-09-01 to 2021-08-31

Ecosystem models such as ANAFORE are models that dynamically simulate pools and fluxes of carbon, water, nitrogen and other nutrients through an ecosystem. These models originally served as tools to predict plant production and were therefore highly plant-oriented. Increasingly, the recognition of other ecosystem services has created the demand for more complex process-based soil models. Only models that account for all key interactions between climate, plants and soil can become versatile tools to predict the effect of different management or future global changes on ecosystem services. Increasingly, the active role of microbiota has been recognized in models as an important driver for many soil processes. Furthermore, implementing soil fauna effects such as bioturbation, aggregation, fragmentation, biopore formation, and soil foodweb interactions into ecosystems has been suggested as the next step. The KEYLINK model steps up to this challenge and simulates soil fauna effects both on soil organic matter dynamics and hydrology, with the final goal of serving as a soil module for the next generation of ecosystem models that allow simulating effects of climate change and management on ecosystems, including its soil functions. However, the development of KEYLINK and other similar models has been impeded by the lack of empirical data available to inform such models. The overall objective of ANAFAUNA was to carry out empirical studies that contribute to the development of KEYLINK. Such studies allow i) to test specific assumptions made in first version of KEYLINK and ii) to validate the current version of the model as a whole.
This dedicated effort was carried out in two different systems: (i) a field mesocosm experiment (REGIMESHIFT) simulating climate change towards more persistent precipitation regimes, (ii) a laboratory mesocosm experiment (EARTHWORM) testing the effect of earthworms on soil structure and C cycling. These matched each other well because the former system contained plants but not earthworms while the latter contained earthworms but not plant roots allowing to disentangle these two sources of macroporosity. Each system was used to test selected assumptions made in the current KEYLINK model.
While several assumptions of the model were supported by the experimental results, some others were not. The results highlight that soil structure is dynamic but cannot be simply estimated from other soil properties such as total or microbial C, roots, fungal or earthworm biomass. Instead, cumulative C inputs or earthworm activity over time might be more important and could be a way forward in modelling aggregation or burrowing in KEYLINK. Furthermore, some interesting and potentially important patterns were observed that will be implemented in KEYLINK, namely the changes in soil water repellency and its impact on infiltration. Altogether, these findings will result in an improved version of KEYLINK. Furthermore, the project brought important evidence of the detrimental effect of persistent precipitation regimes on soil.
In REGIME SHIFT, an array of soil properties was measured both in the beginning of the experiment as well as at the end of the 18-month experiment in order to track the effect of persistent precipitation regimes on the soil. Above that, PLFA and ergosterol were measured also after four months of the experiment; a pilot sampling and analysis regarding root biomass, total C and N and bulk density were carried out after 12 months of the experiment. After four months in REGIMESHIFT microbial biomass of both fungi and bacteria were affected negatively by the 60-day treatment with a tendency of fungal biomass and fungi:bacteria ratio to peak under intermediate weather persistence. After 18 months of the experiment, more persistent precipitation regimes led to lower root and fungal biomass, aggregation, water retention and density of some groups of mesofauna (oribatid mites and unpigmented springtails) while increasing bulk density and favoring other groups of soil fauna (pigmented springtails). Furthermore with longer drought, studied soils became increasingly hydrophobic and this reduced initial infiltration rates. Of the KEYLINK assumptions, data supported the assumed positive relationship between bacterial porosity and aggregation and partly between aggregation and fungal biomass, but not between aggregation and other pore size classes. Aggregation could be explained slightly better by fungal biomass than by total C but neither of these, nor other variables as root or microbial biomass represented a good proxy for aggregate stability.
In the EARTHWORM experiment, pF water retention curves, earthworm biomass, litter biomass and texture were measured. After four months of the experiment, all litter in treatments with earthworms disappeared from the soil surface confirming the significant influence of earthworms on the carbon cycle. In contrast to the hypothesis, no increase in macroporosity was found using the pF measurements in mesocosms with earthworms. A possible explanation is that the relatively high soil moisture content may have promoted settling. In fact, endogeic earthworms were found to decrease macroporosity and increase bacterial pores, and overall they had a compacting effect on the soil suggesting that their horizontal burrows may facilitate settling more than the mostly vertical burrows created by anecics.
The results of the project were disseminated via three poster presentations at the Biology Research Day (University of Antwerp), World Soil Day (Prague, Charles University) and AGU Fall meeting (San Francisco), one live presentation with virtual display at EGU General Assembly 2021 and one talk at the PLECO research group seminar. Furthermore, two manuscripts for peer reviewed scientific journals are in preparation.
Significant contributions beyond the state of the art have been made within the project. Firstly, assumptions of the KEYLINK model were tested regarding the relationships between the biomass of soil biota and soil structure, specifically aggregation and pore size distribution, as well as the mutual relationship of the two latter. Secondly, a study was carried out that concurrently tracks plant production, soil biota, soil structure and biogeochemistry in the same controlled system yielding comprehensive datasets that are suitable for testing KEYLINK and other similar complex models. Last but not least, the impact of climate change towards more persistent precipitation regimes on several soil variables was studied for the first time in a globally unique experimental setup using the ANTWERP FATI AnaEE platform (https://www.anaee.eu/) confirming that such climate change may have detrimental effects on certain soil properties such as aggregation, bulk density or certain groups of soil fauna. These findings together represent a step further towards our ability to predict the effect of future global change on ecosystem services.
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