Molybdenum at German Norway spruce sites: contents and mobility

Molybdenum plays an important role in the nitrogen turnover of ecosystems. However, very little is known about Mo availability in forest soils. We measured the oxalateextractable Mo concentrations of acid forest soils, the Mo, nitrate, phosphate, and sulfate fluxes from the organic forest floor into the mineral soil using resin tubes and the Mo concentrations of the tree needles at 28 different Norway spruce (Picea abies (L.) Karst.) sites in southern Germany. The supply of oxalate-extractable Mo varied from 51 to 3400 g·ha-1, with the lowest values occurring in sandstone-derived soils (370 ± 212 g·ha-1; mean ± SD). Molybdenum concentrations of current-year needles were in the range of 5 to 48 ng·g-1. The Mo needle concentrations and oxalate-extractable Mo of soils did not correlate. However, Mo fluxes (6-60 g·ha-1·a-1) from the organic forest floor into the mineral soils were correlated to needle concentrations and to the NO3 fluxes. We conclude that Mo turnover within forest ecosystems is governed by Mo plant availability of mineral soils as well as by plant Mo uptake. In addition, Mo cycling strongly affects Mo distribution within soil profiles and Mo fluxes out of the organic layer.