CARBON AND NITROGEN ISOTOPE RATIOS IN DIFFERENT COMPARTMENTS OF A HEALTHY AND A DECLINING PICEA-ABIES FOREST IN THE FICHTELGEBIRGE, NE BAVARIA

Natural carbon and nitrogen isotope ratios were measured in different compartments (needles and twigs of different ages and crown positions, litter, under-storey vegetation, roots and soils of different horizons) on 5 plots of a healthy and on 8 plots of a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), which has recently been described in detail (Oren et al. 1988 a; Schulze et al. 1989). The delta-C-13 values of needles did not differ between sites or change consistently with needle age,but did decrease from the sun- to the shade-crown. This result confirms earlier conclusions from gas exchange measurements that gaseous air pollutants did no long-lasting damage in an area where such damage was expected. Twigs (delta-C-13 between -25.3 and -27.8 parts per thousand) were significantly less depleted in C-13 than needles (delta-C-13 between -27.3 and -29.1 parts per thousand), and delta-C-13 in twigs inceased consistently with age. The delta-N-15 values of needles ranged between -2.5 and -4.1 parts per thousand and varied according to stand and age. In young needles delta-N-15 decreased with needle age, but remained constant or increased in needles that were 2 or 3 years old. Needles from the healthy site were more depleted in N-15 than those from the declining site. The difference between sites was greater in old needles than in young ones. This differentiation presumably reflects an earlier onset of nitrogen reallocation in needles of the declining stand. delta-N-15 values in twigs were more negative than in needles (-3.5 to -5.2 parts per thousand) and showed age- and stand-dependent trends that were similar to the needles. delta-N-15 values of roots and soil samples increased at both stands with soil depth from -3.5 in the organic layer to +4 parts per thousand in the mineral soil. The delta-N-15 values of roots from the mineral soil were different from those of twigs and needles. Roots from the shallower organic layer had values similar to twigs and needles. Thus, the bulk of the assimilated nitrogen was presumably taken up by the roots from the organic layer. The problem of separation of ammonium or nitrate use by roots from different soil horizons is discussed.