Phosphorus budgets in forest soils of differing diversity and land use


Previous project phase (2014 - 2017): DYNPHOS III

Scientific investigators:

Prof. Dr. Yvonne Oelmann

Elisabeth Sorkau

(Uni Tübingen)


Phosphorus (P) is an essential nutrient for photosynthetic carbon assimilation and besides nitrogen (N) the major element limiting terrestrial primary productivity. Because of N deposition and the associated shift in N:P ratios in biomass P has become increasingly important for plant growth and biodiversity. Despite knowledge on the importance of P for forest nutrition, mechanisms of P release have not yet been uncovered completely. Particularly in forests, decompositions of organic matter as a biologically-mediated P release process plays an important role in P nutrition because of the great organic P (Po) stocks in the organic layer. Because of the limited availability of inorganic P (Pi) in most soils, plants and soil organisms actively contribute to the release of Pi from soil organic matter by exudation of enzymes.

Results of the preceding DYNPHOS phase showed that microbial biomass P (Pmic) concentrations differed among study regions and were closely linked to P fractions in soil both in forests and grasslands. While land-use intensity had significant effects on several P fractions, Pmic seems to be insensitive to both forest and grassland management. Comparing high and low plant species richness, we already showed higher P exploitation by highly diverse plant communities. Therefore, particularly in ecosystems of high plant diversity the competition for P between plants and microbes in soil plays a role for plant P nutrition. Furthermore, Pmic correlated positively with total Po concentrations in both forest and grassland soil indicating the importance of organic matter as substrate for microorganism metabolism.


Objectives and Hypotheses

Management measures in grasslands and forests need to be adjusted to achieve tight P cycles by minimizing P losses from ecosystems while maintaining plant P nutrition. The evaluation of the effectiveness of management measures requires knowledge on the main mechanisms contributing to ecosystem P nutrition (short-term), the quantification of how long these mechanisms can guarantee P supply (medium-term) and the long-term perspective considering input and output fluxes of P as measure of tight P cycling in ecosystems.



Therefore, we want to answer the following questions:
(1)    How do management and biodiversity affect short-term, in-situ biological P release in forest soils?
(2)    Does the management and biodiversity effect on P exploitation and medium-term biological P release observed in grasslands hold for forests?
(3)    Which implications can be derived for sustainable P management from long-term P input/output budgets of ecosystems under different land use and of differing biodiversity?


Methods and Expected Outcomes

We will assess short-term biological P release by means of application of glacier water that differs in the isotopic signature as compared to soil water of the Exploratories. The enzyme-mediated hydrolysis of ester-bond P in soil organic matter is associated with an exchange of oxygen (O) atoms in phosphates with O atoms from ambient water. Therefore, the kinetics of the incorporation of glacier water (enriched in 16O) into released phosphate molecules might serve as a tool for assessing actual enzyme activity.

We will assess P exploitation based on yearly uptake of P by the vegetation related to the potentially available P pool. In addition to P exploitation, we will focus on medium-term biological P release.

We will calculate the long-term P budget:

considering atmospheric deposition and all types of fertilizer as input and harvested P, leaching and erosion as output. Preliminary calculations showed that the P budgets tend to be negative, so that we will calculate the time until the available P stocks will be exhausted. One important yet missing output is forest P stocks finally removed by felling trees. We will receive wood samples of tree species sampled by subproject Neighbor in the preceding funding phase which will be complemented by additional coring activities of our subproject in case of insufficient sample material. Another pathway of phosphorus loss for both forests and grasslands is in particulate forms, therefore, erosion will be estimated as well.


Results of the preceding phases

ALT F (2012): The Phosphorus Cycle in Grassland and Forest Ecosystems of different Biodiversity and Management. Diss. Eberhard Karls Universität Tübingen.

ALT F, OELMANN Y, HEROLD N, SCHRUMPF M, WILCKE W (2011): Phosphorus partitioning in grassland and forest soils of Germany as related to land-use type, management intensity, and land use-related pH. J. Plant Nutr. Soil Sci., 174, 195-209.

ALT F, OELMANN Y, SCHÖNING I, WILCKE W (2013): Phosphate Release Kinetics in Calcareous Grassland and Forest Soils in Response to H+ Addition. Soil Sci. Soc. Am. J., 77, 2060-2070.

in cooperation with SCALEMIC: REGAN KM, NUNAN N, BOEDDINGHAUS RS, BAUMGARTNER V, BERNER D, BOCH S, OELMANN Y, OVERMANN J, PRATI D, SCHLOTER M, SCHMITT B, SORKAU E, STEFFENS M, KANDELER E, MARHAN S (2014): Seasonal controls on grassland microbial biogeography: Are they governed by plants, abiotic properties or both? Soil Biol. Biochem., 71, 21-30.

in cooperation with SOILAGG: BARTO KE, ALT F, OELMANN Y, WILCKE W, RILLIG MC (2010): Contributions of biotic and abiotic factors to soil aggregation across a land use gradient. Soil Biol. Biochem., 42, 2316-2324.

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Phosphorus cycling in grasslands and forests of differing diversity and land use

Previous project phase (2011 - 2014)



Scientific investigators:

Prof. Dr. Yvonne Oelmann

Elisabeth Sorkau

(University Tübingen)

Phosphorus (P) is an essential nutrient for organism growth and for photosynthetic carbon assimilation and besides Nitrogen (N) the major element limiting terrestrial productivity. Particularly in managed ecosystems, the predicted P fertilizer scarcity calls for a comprehensive understanding of P transformation processes in soil. Results of the preceding DYNPHOS phase showed that increasing plant diversity decreased plant- available P concentrations in soil through increasing P exploitation at the Swabian Alb. It remains unclear if combined effects of land-use intensity and plant diversity on P transformation processes in soil will result in tight ecosystems P cycling and thus, requiring less fertilizer P while maintaining productivity.


Objectives and Hypotheses

The objective is to differentiate the effect of land-use intensity (LUI) and plant diversity on

  1. gross P mineralization,
  2. microbial biomass P, and
  3. dissolved P leaching (PO4-P and DOP)

in soil of all grassland and forest plots of the three Exploratories (n = 300).

SORKAU (2011), Swabian Alb
SORKAU (2011), Swabian Alb


LUI has contrasting implications for P cycling in forests and grasslands i. e., high removal of P in forests versus high input of P in grasslands under high land-use intensity. Therefore, hypotheses are postulated specifically for forests and grasslands. The following hypotheses will be tested.



 1a. Increasing LUI decreases tree growth, foliar P concentrations, and root exudation. Therefore, gross P mineralization rates are negatively correlated with LUI.

 1b. Increasing plant diversity results in balanced microclimate and increased decomposition-substrate diversity and thus, accelerated microbial activity. Therefore, gross P mineralization is positively related to plant diversity under comparable LUI.

 2a. Increasing LUI increases biomass removal and thus, reduces root mass and root exudation in soil. Therefore, microbial biomass P decreases with increasing LUI.

 2b. Plant diversity increases gross P mineralization and thus, microbial biomass. Therefore, plant diversity increases microbial biomass P under comparable LUI.

 3a. Increasing LUI decreases P supply in soil and thus, decreases PO4-P and DOP leaching.

 3b. Because of increased plant P uptake and increased microbial biomass P, plant diversity is negatively related to PO4-P and DOP leaching under comparable LUI.



WAHL (2011), Swabian Alb
WAHL (2011), Swabian Alb

 1c. Increasing LUI increases N (and P) input into soil thus, decreasing root mass and root exudation. These decreases microbial activity and thus, gross P mineralization rates.

 1d. Under comparable LUI, positive effects of microclimate and decomposition-substrate diversity result in increased gross P mineralization rates in highly diverse ecosystems.

 2c. The increased N (and P) input in case of high LUI results in decreased microbial biomass, Therefore, increasing LUI decreases microbial biomass P.

 2d. Under comparable LUI, increased gross mineralization rates in highly diverse ecosystems are related to increased microbial biomass. Therefore plant diversity increases microbial biomass.

 3c. Because of increased P input and decreased microbial biomass P, LUI increases PO4-P and DOP leaching.

 3d. Under comparable LUI, plant diversity leads to decreased PO4-P and DOP leaching because of increased plant uptake and microbial biomass P.

Therefore, we expect that plant diversity reduces P leaching under high LUI in grassland and we will focus on the relationship between biodiversity and the P cycle in soil accounting for management, site conditions and historic land-use of the studied plots.



Previous project phase (2008 - 2011): DYNPHOS


Scientific investigators:

Prof. Dr. Wolfgang Wilcke

(University Bern)

Dr. Fabian Alt (form. PhD student)

Elisabeth Sorkau

(University Tübingen)

Increasingly efficient nutrient exploitation with increasing plant diversity results in lower nutrient concentrations in soil solution. This was observed for nitrogen but not for other essential nutrients such as phosphorus. In managed ecosystems, diversity is closely linked to land-use intensity and history. To understand the controls of nutrient concentrations in soil, land-use and biodiversity effects must be disentangled. Therefore, DYNPHOS studies the effect of land use and biodiversity on phosphorus cycling in grassland and forest systems of the three Biodiversity Exploratories.

Our objective is to disentangle the effect of land use and plant diversity on

  1. Phosphorus fractions in soil
  2. Phosphorus cycling in soil
  3. Phosphorus storage in plants

Furthermore, we will assess the influence of land-use practices on phosphorus in soil by using novel isotope techniques, i.e. the determination of ?18O in phosphates.

With these goals we plan to test the following hypotheses

  1. Increasing land-use intensity probably associated with higher nutrient availability in soil

    1. results in a less strong relationship between biodiversity and P availability (organic and inorganic) in soil and
    2. will lead to a less strong relationship between biodiversity and P pools in plants.

    1. Intensive land use increases the proportion of fertilizer-derived easily-soluble P minerals. The concentration of P is to a large extent chemically controlled thereby reducing the effect of diversity on P concentrations in soil.
    2. At a given site fertility (within the same land use), increasing biomass production with increasing plant diversity results in an increasing size of the organic P pool and hence an increasing contribution of mineralized P to plant-available P. As a result the diversity effect on P availability will increase.

  2. The O isotope ratio in PO4 can be used to distinguish different P sources (dissolution of mineral P, desorption, mineralization).

We will determine P pools in soil and plant biomass, P release by dissolution and mineralization, and the isotopic signature (?18O) in phosphate extracted from soil.