MAL21

The Amazon rainforest is an important sink for atmospheric CO₂ counteracting increased emissions from anthropogenic fossil fuel combustion and land use change storing large amounts of carbon in plant biomass and soils. However, large parts of the Amazon Basin are characterized by highly weathered soils (ultisols and oxisols) with low availability of rock-derived phosphorus (and cations), which are mostly occluded in soil or bound in organic matter. Such low phosphorus availability is thought to be (co-)limiting plant productivity. However, much less is known whether low phosphorus availability influences the activity of heterotrophic microbial communities controlling litter and soil organic matter decomposition and thereby long-term carbon sequestration in tropical soils.

In tropical soils high temperature and humid conditions allow overall high microbial activity. Over a larger soil phosphorus fertility gradient across several Amazonian rainforest sites, at low P sites almost 40 % of total P was stored in microbial biomass, highlighting the competitive strength of microorganisms and their importance as P reservoir. Across all sites soil microbial biomass was a significant predictor for soil microbial respiration, but mass-specific respiration rates (normalized by microbial biomass C) rather decreased at higher soil P. Using the incorporation of ¹⁸O from labelled water into DNA (i.e., a substrate-independent method) to determine microbial growth, we found significantly lower microbial growth rates per unit of microbial biomass at higher soil P. This resulted in a lower microbial carbon use efficiency, at a narrower C:P stoichiometry in soils with higher P levels, and pointed towards a microbial co-limitation of phosphorus and carbon at low soil P levels. Furthermore, data from a multi-year nutrient manipulation experiment in French Guiana and from short-term lab incubations suggest that microbial communities thriving at low P levels are highly efficient in taking up and storing added P, but do not necessarily respond with increased growth.

Soil microbial communities play a crucial role in soil carbon and phosphorus cycling in tropical soils as potent competitors for available P. They also play an important role in storing and buffering P losses from highly weathered tropical soils. The potential non-homoeostatic stoichiometric behavior of microbial communities in P cycling is important to consider in soil and ecosystem models based on stoichiometric relationships.

How to cite: Fuchslueger, L.: Investigating nutrient controls over microbial activity in tropical soils, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8289, https://doi.org/10.5194/egusphere-egu21-8289, 2021.

Formation of mineral-associated organic matter (MAOM) is a decisive process in the stabilization of OM against rapid microbial decomposition and thus in the soils’ role as global carbon (C) sink. Sorption experiments of dissolved OM (DOM) repeatedly showed that particularly mineral subsoils have a large sorption capacity to retain more C. However, there is also an increasing body of literature, revealing an increasing output of dissolved organic C (DOC) from soils. Here, we investigated into this paradox in forest soil under beech by a combination of a field labelling experiment with ¹³C-enriched litter with a unique DO¹³C and ¹³CO₂ monitoring, an in-situ C exchange experiment with ¹³C-coated minerals, and batch sorption experiments.

Within two years of ¹³C monitoring, only 0.5% of litter-derived DO¹³C entered the subsoil, where it was only short-term stabilized by formation of MAOM but prone to fast microbial mineralization. The ¹³C monitoring, sorption/desorption experiments in the laboratory, and also the in-situ C exchange on buried soil minerals revealed that there is a frequent exchange of DOM with native OM and a preferential desorption of recently retained OM. Hence, there appeared to be a steady-state equilibrium between C input and output, facilitated by exchange and microbial mineralization of an adopted microbial community. The remobilized OM was also richer in less sorptive carbohydrates. Along with transport of most of DOM along preferential paths, this further increased the discrepancy between laboratory-measured sorption capacities of subsoil and the actual C loading of minerals. Finally, the ¹³C labeling experiments revealed that input of fresh litter-derived OM into subsoil may even mobilize old-soil derived OM. Hence, in the field different biogeochemical constraints are acting that prevent that the laboratory-based C sink can be reached in the field.  We conclude, that forest subsoils can hardly be considered as additional C sink, even at management options that increase DOC input to subsoil.

How to cite: Guggenberger, G., Liebmann, P., Mikutta, R., Kalbitz, K., Wordell-Dietrich, P., Leinemann, T., Preusser, S., Bachmann, J., Don, A., Kandeler, E., Marschner, B., and Schaarschmidt, F.: Additional carbon stabilization in temperate subsoils impeded by biogeochemical and hydraulic constraints, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14580, https://doi.org/10.5194/egusphere-egu21-14580, 2021.

Environmental change, particularly the impact of climate change, is having a profound impact on humankind. Rising seas and temperatures are causing increasing flooding and melting of ice and permafrost soils. The impact of these processes on biogeochemical cycling of metals, carbon, and nutrients in soils and water is not well understood. For example, how do rising seas, which cause inundation of soils with saline water, followed by retrenchment, and salinization of groundwater, affect cycling of redox active elements such as arsenic and iron as well as nutrients such as phosphorus.  Complexation of carbon with iron-bearing minerals is a major mechanism for carbon retention. Under changing climatic conditions, how will carbon cycling be impacted, particularly in permafrost soils, which are sinks for a large portion of terrestrial carbon? This presentation will explore these questions, and others, over a range of spatial and temporal scales.

How to cite: Sparks, D.: Impact of Environmental Change on Biogeochemical Cycling in Soil Systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-435, https://doi.org/10.5194/egusphere-egu21-435, 2021.