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Restoration of the nitrogen cycle is an important step in the recovery of an ecosystem after mining. Carbon and nitrogen in rehabilitated lignite containing mine soils can be derived from plant material as well as from lignite inherent to the parent substrate. We assessed the use elemental and stable carbon and nitrogen isotope measurements to trace the orgin of soil nitrogen and applied these techniques to elucidate the origin of mineral N in the soil and the soil solution. The conceptual approach of this study included physical fractionation in addition to sampling of vegetation and soil from a lignite-containing mine site rehabilitated in 1985 with Pinus Nigra. We studied the elemental and isotopic composition of bulk samples as well as isolated fractions and soil solution. Our data indicate that the stable carbon and nitrogen isotopic composition of the soil samples are the result of mixing between plant material and substrate inherent lignite. delta15N isotopes may be used as indicators of nitrogen contribution from plants to solid samples as well as soil solution. N-isotope composition of ammonia shows low spatial and interannual variability, despite strong concentration changes. Plant-derived nitrogen contributes in higher amounts to the soil solution compared to the bulk mineral soil.

Stable isotopes of nitrogen (N) in plants are increasingly used to evaluate ecosystem N cycling patterns. A basic assumption in this research is that plant δN reflects the δN of the N source. Recent evidence suggests that plants may fractionate on uptake, transport, or transformation of N. If the dominant source of plant N is via roots, a difference in δN by tissue type would suggest fractionation on transport and assimilation of N. In order to evaluate differences between species and plant parts, we measured δN in root, stem, and leaf tissues of individual sugar maple ( Acer saccharum; SM) and American beech ( Fagus grandifolia; BE) plants ranging in age from germinants to mature trees at the Hubbard Brook Experimental Forest, New Hampshire (USA). For SM, root δN > stem δN > leaf δN; for BE seedlings, root δN > stem δN and root δN > leaf δN. These differences suggest that fractionation occurs during plant transport and assimilation of N. Beech δN (root, stem, and leaf) was consistently higher than SM δN for 1-7 year-old seedlings. At one site, we found no differences with age in foliar δN (range: 4.1-4.8 ‰) for seedlings, saplings, and trees which suggests that it may be possible to compare foliar δN of plants of different ages at some sites. However, at another site, foliar and root δN were higher for trees than 1-2 year-old seedlings. This study suggests that physiological differences in N assimilation and transport processes that differ by species likely control plant δN. [ABSTRACT FROM AUTHOR]

Abstract: There is concern that subsurface drainage, by destroying or by-passing active denitrification areas, may prevent nitrate retention processes and enhance nitrate contamination of surface water by agriculture. To address this question, we studied the flow and concentration signatures of drainage waters and their transformations in a series of 5 nested watersheds, from 1 to 100km2 area, in the Brie region near Paris (France). At all scales, nitrate concentrations are generally higher during the winter drainage season compared to the low flow periods (late spring to early fall). High nitrate concentrations characterizing drainage waters are visible at the 1st, 2nd and 3rd stream order but are “diluted” by surface runoff from forested zones and buffered by groundwater contributions. The analysis of nitrate chemographs and nitrate budgets established for the different nested watersheds show significant nitrogen retention. Isotopic measurements indicate that the nitrate pool is enriched in δ15N– as its concentration decreases. Direct estimation of benthic denitrification with benthic chambers allowed concluding that benthic denitrification is not the only retention mechanism and that “underground” denitrification, affecting nitrate on its way from the base of the root zone down to the limit of the river bed, may in fact dominate nitrogen retention processes even in this intensively drained watershed. [Copyright &y& Elsevier]