Exploration and modeling of the transport of mobile organic substances in the topsoil-subsoil-aquifer continuum
Final Report Abstract
The fate of organic matter in soils is strongly linked to the (trans-)formation, release and transport of its mobile fraction. Commonly, mobile organic matter is referred to as dissolved organic matter, an operationally defined fraction that passes a 0.45µm filter. In this study, it was clearly shown that under certain and favourable climatic (snow melt, heavy rains) and transport conditions (preferential release and transport through biopores) even particles up to a size of some tenths of micrometers can be translocated in soils. Release, transport and transformation of mobile organic matter were studied with an experimental approach that comprised field lysimeter. New to this approach was a novel design of the tension-controlled lysimeters that allowed collection of particles up to 10 µm from undisturbed topsoil and subsoil in natural bedding. During the four year monitoring period, the water movement was strongly controlled by preferential flow. This was enforced by the markedly higher bulk density due to compaction and a strongly compacted primary plough pan 20cm below the soil surface. During the vegetation period, which is different between wheat and maize (longer), effectively no seepage occurred and in turn no MOM export was observed. The large amount of biopores, in particular earthworm burrows (up to 60/m2) seem to play an important role both for the water flow and the matter transport during heavy and extreme events: They bypass the soil matrix and short-circuit the soil surface but in particular the plough layer with deeper soil horizons and even the aquifer. Snow melts followed by rain events produced high seepage with infiltration far below the plough pan. Only one steady rain event in summer 2013 produced deep infiltration in the summer period. Snow melts and heavy rain result in an enlarged OM relocation. This was observed for both observation depths in 35cm (topsoil-subsoil boundary and 65cm (below main rooting zone) and for both variants, wheat and maize. The released material comprised a vast variety of mineral, organomineral, and organic substances in the dissolved, colloidal and particulate fraction. Materials identified were clay minerals and secondary short range order oxides, composite subunits of microaggregates, organo-mineral associations and a considerably amount of cells, cell fragments, and even competent microorganism, although the fraction of biotic materials (mobile biomass) seems to be rather low in the MOM. This was specifically explored in a joint experiment with SP MicLink, were we found that in particular microorganisms that originated from the rhizoplane were translocated in the soil profile (see Dibbern et al. 2014). The average proportion of mobile matter larger than 0,45µm amounted to 20% of the total MOM pool. Under extreme conditions, e.g. combined snow melt and lasting rain events, suspended particles made up to 90% emphasizing the role of these conditions for the propagation of particulate matter larger than 0,45µm in greater soil depths. As essentially now seepage occurred during the vegetation period (exemption: Lasting rain event in precipitation in summer 2013), no seepage was observed and export of MOM in turn was zero in both the wheat and maize variants. Interestingly, the heavy rain events in summer did not led to larger OM fluxes below the plough horizon. From this study, we can conclude that in particular the colloidal and larger fractions of the MOM continuum are an important component to understand the biogeochemical cycling of carbon and other elements. The release and translocation of soil micro-organisms seem to play an important role in the inhabitation of pristine surfaces and habitats in the downstream compartments of the subsurface including the aquifers.
Publications
- (2012) Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems. Pedobiologia 55, 111-119
Kramer, S., Marhan, S., Ruess, L., Armbruster, W., Butenschoen, O., Haslwimmer, H., Kuzyakov ,Y., Pausch, J., Scheunemann, N., Schoene J., Schmalwasser, A., Totsche, K.U., Walker, F., Scheu, S., Kandeler, E.
(See online at https://doi.org/10.1016/j.pedobi.2011.12.001) - (2014) Selective transport of plant rootassociated bacterial populations in agricultural soils upon snowmelt. Soil Biology & Biochemistry 69, 187-196
Dibbern, D., Schmalwasser, A., Lueders, T., Totsche, K.U.
(See online at https://doi.org/10.1016/j.soilbio.2013.10.040)
