Internal nutrient transformations in the
soil-plant system – Work package 4.3

Treatment and seasonal effects on the transformation of N and P associated with the decomposition of the organic matter will be analyzed. The nutrient transformations (mineralization, release of DOC and DON, microbial N and P uptake and partitioning of released N and P between the soil and microbial pools) will be measured by the buried bag technique (e.g. Eno, 1960). The soil is incubated in a “buried bag” and the nutrient transformation rates are estimated as the pre- and post incubation differences in the various soil nutrient pools. Two sets of samples will be incubated in the spring 2006 (second treatment year). One set of samples will be retrieved at the end of the growing season for estimates of growing season nutrient mobilization, while the second set will be retrieved in late winter/early spring 2007 to account also for nutrient mobilization during the non-growing season. The results will be linked to the measurements in WP4.5. We will also consider running the traditional method together with a modification of it, by which plants are incubated together with the soil and the plant uptake is measured together with the other soil variables. We have used the method in one accepted publication (Jonasson et al. in press). It seems to give more realistic estimates of the nutrient transformations than the traditional method, because it accounts also for effects on mineralization by root exudation of labile C, possible plant microbial competition for nutrients and for N plant uptake of dissolved organic N (See WP4.5). Apart from the accepted manuscript, the modified method has also been used in another, submitted, manuscript, and it will also be presented at an international congress for comments in March. If it is favorably received throughout, we will use it in parallel with the traditional method.

Measurements will be carried out to determine treatment effects on P-availability and seasonal pattern in phosphatase activity. In WP3, changes in P availability and plant P pools and concentrations are monitored. If the plants are P limited or become P limited due to the manipulations, increased uptake of P by plants and microbes are likely to occur, which may obscure measurements of soil P availability. As a supplement to these measurements, phosphatase activity will be measured in plant roots and soil in the third treatment year as we expect this to be a more sensitive measurement of plant P limitation.