Phosphorus cycling in grasslands and forests of differing diversity and land use
scientific investigators
(University Tuebingen)
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
- gross P mineralization,
- microbial biomass P, and
- 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.
Forests:
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.
Grasslands:
- 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.
Project DYNPHOS in phase 1 and 2:
scientific investigators
Prof. Dr. Wolfgang Wilcke
(University Bern)(University Tuebingen)
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
- Phosphorus fractions in soil
- Phosphorus cycling in soil
- 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
- Increasing land-use intensity probably associated with higher nutrient availability in soil
- results in a less strong relationship between biodiversity and P availability (organic and inorganic) in soil and
- will lead to a less strong relationship between biodiversity and P pools in plants.
- 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.
- 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.
- 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.