Patterns of microbial processes shaped by parent material and soil depth in tropical rainforest soils

Abstract

Abstract. Microbial processes are one of the key factors driving carbon (C) and nutrient cycling in terrestrial ecosystems, and are strongly driven by the equilibrium between resource availability and demand. In deeply weathered tropical rainforest soils of Africa, it remains unclear whether patterns of microbial processes differ between soils developed from geochemically contrasting parent materials. Here we show that resource availability across soil depths and regions from mafic to felsic geochemistry shape patterns of soil microbial processes. During a 120-day incubation experiment, we found that microbial biomass C and extracellular enzyme activity were highest in the mafic region. Microbial C limitation was highest in the mixed sedimentary region and lowest in the felsic region, which we propose is related to the strength of contrasting C stabilization mechanisms and varying C quality. None of the investigated regions and soil depths showed signs of nitrogen (N) limitation for microbial processes. Microbial phosphorus (P) limitation increased with soil depth but was similar across geochemical regions, indicating that subsoils in the investigated soils were depleted in rock-derived nutrients and are therefore dependent on efficient biological recycling of nutrients. Microbial C limitation was lowest in subsoils, indicating that subsoil microbes can significantly participate in C cycling and limit C storage if increased oxygen availability is prevalent. Using multivariable regressions, we demonstrate that microbial biomass C normalized to soil organic C content (MBCSOC) is controlled by soil geochemistry and substrate quality, while microbial biomass C normalized to soil weight (MBCSoil) is predominantly driven by resource distribution. We conclude that due to differences in resource availability, microbial processes in deeply weathered tropical rainforest soils greatly vary across geochemical regions which must be considered when assessing soil microbial processes in organic matter turnover models.

Organic matter cycling along geochemical, geomorphic and disturbance gradients in forests and cropland of the African Tropics – Project TropSOC Database Version 1.0

“…(ii) Landform, since topography may influence water and soil fluxes, particularly erosional soil loss on slopes and soil deposition in valleys. (iii) Vegetation and land cover, since it may control the input to and extraction of organic matter from soil, and Conducted in one of the hotspots of Global Change, the Central African Congo Basin and African Great Lakes region the database described here is the foundation for several manuscripts published as a part of the 2021 special issue "Tropical biogeochemistry of soils in the Congo Basin and the African Great Lakes region" in SOIL Journal (Bukombe et al 2021, in review;Kidinda et al 2020;Summerauer et al 2021 in review;Reichenbach et al 2021 in review;Wilken et al 2020 in review).…”

The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils

“…The most important variables for explaining m/s + c ratios were found to be soil depth, solid-phase mineralogy and the chemistry of the soil solution, which could in total explain 44 % of m/s + c variance (Table 2). We interpret the high m/s + c ratios in topsoils as indicative for the formation of stable microaggregates promoted by the higher abundance of C, which functions as a binding agent (Denef and Six, 2005) and the generally more fertile conditions in tropical topsoil compared to subsoil favoring microbial activity (Kidinda et al, 2020). The abundance of pedogenic oxides further promotes aggregation by providing reactive mineral surfaces (Oades, 1988).…”

“…However, climate-driven factors can also influence SOC dynamics indirectly through the inter-action with soil factors (Doetterl et al, 2015b). For example, C-depleted tropical subsoils contain small but metabolically active microbial communities contributing to C cycling (Kidinda et al, 2020;Stone et al, 2014). Low soil pH in combination with high clay content dominated by pedogenic oxides can stabilize enzymes on mineral surfaces, which will affect microbial C acquisition (Dove et al, 2020;Allison and Vitousek, 2005;Liu et al, 2020).…”