Isotope geochemical determination of phosphorus weathering sources and fluxes in forest ecosystems

Phosphorus (P) plays a crucial role in life. It coheres the genetic information and delivers as adenosine triphosphate (ATP) energy to cellular metabolism. Hence, P is not only a macronutrient in ecosystems but also a life limiting element on our entire planet.

To feed the inexorably growing population on earth, P is often intensively used as fertilizer in agriculture on soils with low contents of bioavailable P. The production of P containing fertilizers is based on mining industry which extracts P, primary hosted in the mineral Apatite, from rock.

Over the past six years scientists and politicians recognized that primary P resources are limited on Earth. In 2008 Dana Cordell woke up the world’s widest society with her estimation that the peak of Phosphorus - related to the peak of oil - is likely to happen already around 2030. In 2009 Natasha Gilbert emphasized the potential P crisis in Nature titled with: “The Disappearing Nutrient”. Both caused a sensation in P research with the approach to better understand the P cycle in order to work out strategies for P recycling.

This project is part of the DFG funded Priority Program (SPP 1685) “Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources”. We estimate the rate at which P is released, that means becomes bioavailable, from primary bedrock at two ICP Level II Forest sites in South Germany (Black Forest and Bavarian Forest). Additionally, we fingerprint the source and depth at which plants take up P. Both approaches can be achieved with the application of innovative geochemical tools such as meteoric and in-situ produced 10Be, and traditional source tracers like radiogenic strontium isotopes and the elemental ratio Ca/Sr.

Our study sites lie in the Bavarian Forest as well as in the Black Forest and are underlained by gneissic bedrock. This means they contain quartz which enriches during chemical weathering in soil due to its relatively long resistance to dissolution. Topsoil erodes and becomes exported out of the catchment via a small creek. We sample bedload sediment at the outlet of the catchment, since it integrates over the entire catchment area. Furthermore, we measure in-situ 10Be concentrations from which we obtain a total denudation rate. Also, we determine total denudation rates on topsoil samples which are distributed across the whole catchment in order to check for spatial heterogeneity.

Once a total denudation rate is determined we apply chemical depletion fractions (CDF) (Riebe et al. 2003), which can be calculated from the concentration ratio of an insoluble element Xi, such as Zirconium or Titanium, of unweathered bedrock to its concentration of weathered soil (Eq. 1). CDF values, ranging between 0 (0 % mass loss) and 1 (100 % mass loss), provide the opportunity to divide total denudation into chemical weathering and physical erosion. In order to estimate P release fluxes we determine the degree of P depletion ( τP) in soils referenced to unweathered bedrock from Eq. 2 where, in this case, X refers to P. Having τP at hand we combine it with CDF and the total denudation rate, derived from in-situ 10Be concentrations.

Eq. 1
Eq. 2

In order to determine both CDF and tau values unweathered fresh bedrock is required. The most unaltered bedrock was sampled by 20 to 30 m deep drilling cores (Fig. 1). Both boreholes at the hillslope in the Black Forest and the Bavarian Forest was constructed to groundwater monitoring wells in close collaboration with the Department of Hydrology of the University of Freiburg. Additionally, we sunk a well at the ridge in the Black Forest to check for spatial heterogeneity and to avoid periglacial cover beds (Schaller et al. 2002).

 

Fig. 1 Drilling campaign in the Black Forest. Left: Drilling equipment. Upper middle: Drill pipe for first 4 m. Lower middle: Drill bitt. Right: 20 m deep drilling core. Each line represents 1 m (left: upper end of core, right: lower end of core).

 

First and preliminary results indicate that chemical weathering takes place deep (Fig. 2). How thick the regolith - by definition the sum of soil, saprolite and periglacial cover beds, respectively - is and how it is distributed across the area will be determined with seismology.

Fig. 2 First and preliminary chemical depletion (tau) profiles of the drilling core in the Black Forest. Horizontal line illustrates the regolith-bedrock boundary. Vertical line illustrates no chemical depletion.

 

The Phosphorus sources of plants, such as apatite hosted in primary bedrock, secondary soil minerals, organic litter or dust, will be fingerprinted with 87Sr/86Sr. Thus, we measure radiogenic strontium isotopes on unweathered bulk bedrock, mineral separates of unweathered bedrock, weathered bulk soil, easily extractable soil phases, soil, stream and groundwater and plant tissues like stem wood, leaves and needles. The same method will be used for the elemental ratio Ca/Sr and the isotopic ratio 10Be (meteoric)/ 9Be. On behalf of the latter, we leach the reactive soil phase with a sequential extraction method (Wittmann et al. 2012). By definition, the reactive soil phase consist of amorphous and crystalline oxides (Wittmann et al. 2012). Both isotopic ratios and the elemental ratio serves to disclose the depth of P uptake by plants. While the isotopic ratios doesn’t fractionate during nutrient uptake by plants at the soil-water-vegetation interface it does for the elemental ratio Ca/Sr due to the high affinity of Ca for plants. To overcome this issue we will determine a “foliar discrimination factor” (Blum et al. 2012).

With the previously described methods at hand we are able to proof the SPP’s 1685 main hypotheses (Fig. 3) which are:

 

  1. Phosphorus depletion of soils drives forest ecosystems from P acquiring systems (efficient mobilisation of P from the mineral phase) to P recycling systems (highly efficient cycling of P).
  2. Recycling systems are more sensitive to human impact than acquiring systems.

Fig. 3 Illustration of the SPP’s 1685 main hypothesis 1: Phosphorus depletion of soils drives forest ecosystems from P acquiring systems (efficient mobilisation of P from the mineral phase) to P recycling systems (highly efficient cycling of P).

 

References

Blum, J.D., Hamburg, S.P., Yanai, R.D., Arthur, M.A. (2012) Determination of foliar Ca/Sr discrimination factors for six tree species and implications for Ca sources in northern hardwood forests, Plant and Soil 356, 303-314

Gilbert, N. (2009) The Disappearing Nutrient, Nature, 461, 716-718 Riebe, C.S., Kirchner, J.W., Finkel, R.C. (2003) Long-term rates of chemical weathering and physical erosion from cosmogenic nuclides and geochemical mass balance. Geochimica Cosmochimica Acta 67, 4411-4427

Schaller, M., von Blanckenburg, F., Veit, H., Kubik, P.W. (2002) Influence of periglacial cover beds on in situ-producedcosmogenic 10Be in soil sections, Geomorphology 49, 255–267

Wittmann, H., von Blanckenburg, F., Bouchez, J., Dannhaus, N., Naumann, R., Christl, M., Gaillardet, J. (2012) The dependence of meteoric 10Be concentrations on particle size in Amazon River bed sediment and the extraction of reactive 10Be/9Be ratios, Chemical Geology, 318-319, 126-138

 

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M. Sc. David Uhlig
Geochemistry of the Earth's surface

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