Abstract. Repeated, grid-based forest soil inventories such as the
National Forest Soil Inventory of Germany (NFSI) aim, among other things, at
detecting changes in soil properties and plant nutrition. In these types of
inventories, the only information on soil phosphorus (P) is commonly the
total P content. However, total P content in mineral soils of forests is
usually not a meaningful variable with respect to predicting the availability of P to trees.
Here we tested a modified sequential P extraction according to Hedley (1982) to
determine the distribution of different plant-available P fractions in soil
samples (at depths of 0–5 and 10–30 cm) from 146 NFSI sites, encompassing a wide
variety of soil conditions. In addition, we analyzed relationships between
these P fractions and common soil properties such as pH, texture, and
soil organic carbon content (SOC). The total P content among our samples ranged from
approximately 60 to 2800 mg kg−1. The labile, moderately labile, and
stable P fractions contributed to 27 %, 51 %, and 22 % of the total P
content, respectively, at a depth of 0–5 cm. At a depth of 10–30 cm, the labile P
fractions decreased to 15 %, whereas the stable P fractions increased to
30 %. These changes with depth were accompanied by a decrease in the
organic P fractions. High P contents were related to high pH values.
While the labile Hedley P pool increased with decreasing pH in absolute
and relative terms, the stable Hedley P pool decreased in absolute and
relative terms. Increasing SOC in soils led to significant increases in all
Hedley P pools and in total P. In sandy soils, the P content across all
fractions was lower than in other soil texture types. Multiple linear
regression models indicated that Hedley P pools and P fractions were
moderately well related to soil properties (with r2 values that were mostly above
0.5), and that the sand content of soils had the strongest influence. Foliar P
contents in Pinus sylvestris were reasonably well explained by the labile and moderately
labile P pool (r2 = 0.67) but not so for Picea abies and Fagus sylvatica. Foliar P
contents in all three species could not be related to specific Hedley P
pools. Our study indicates that soil properties such as pH, SOC content, and
soil texture may be used to predict certain soil Hedley P pools with different
plant availability on the basis of large soil inventories. However, the foliar P
contents of tree species cannot be sufficiently well predicted by the soil
variables considered here.
Analysis of forest soil acidification processes in the area of NP 'Kopaonik'