Table 3.
Indicators for ecosystem P-acquisition and P-recycling, method of calculation of indicator values and justification of indicator (from Lang et al. 2017)
| Variable | Calculation of indicator values | Assumed underlying process |
|---|---|---|
| P acquiring indicators | ||
| N1: P-enrichment in topsoil | P stock in the upper 50% of soil fine earth mass divided by total soil P stock (up to 1 m) | Spatial redistribution induced by the P pumping of trees in the long term: root uptake of P in the subsoil, P deposition with litter at the topsoil and adsorption after mineralization |
| N2: Proportion of nonstable P in profile | Stock (up to 1 m) of non-stable P. (i.e., sum of Hedley P minus PHCl conc and Presidual) relative to total Hedley P | Chemical redistribution due to biological mobilization of P from primary minerals. Nutrient demand had been discussed as the reason for root induced weathering |
| N3: Phosphate exchangeability between 1 min and 1 day | Concentration of isotopically exchangeable P between 1 min and 1 day of topsoil horizons as described in methods chapter | P exchangeability based on physicochemical processes; indicator for P availability |
| P recycling indicators | ||
| N4: Accumulation of P in forest floor | P stock in the forest floor related to total P stock (up to 1 m mineral soil depth) | Forest floor pathways as short cut for plant P uptake without passing of P through the fixing mineral soil |
| N5: Concentration of fine-root biomass in forest floor | Total fine root biomass in forest floor and upper 0–5 cm mineral soil in relation to total fine root biomass (up to 1 m mineral soil depth) | Peak concentrations of fine roots in the forest floor have been assumed to favor tight P cycling in acid temperate forest ecosystems. Results are clearer when the 0–5 cm increment of the mineral soil is added |
| N6: Enrichment of diester-P | Diester-P/monoester-P ratio in the topsoil horizon as calculated from NMR spectra | Increased proportions of diester P were observed in acid soils and explained by changes in enzyme activity, and decreased accessibility of diester P for microbial decay due to accumulation within large organic molecules |
| N7: Mean residence time of forest floor SOM | Forest floor mass related to the mass of annual litter fall | Limited decay of soil organic matter enhances tight P recycling by providing forest floor P-pathways for tree nutrition |