Can δ 15 N profiles in forest soils predict $$ {\rm{NO}}_3^ - $$ loss and net N mineralization rates?

Abstract

This paper studies changes of 15N signatures (δ15N, ‰) and total N (TN, %) in soil profiles among forest stands with different \( {\rm NO}_3^ - \) losses within the same climatic zone. An additional aim was to investigate whether the change of δ15N (Δδ15N) within the 0–10, 10–20 and 20–30 cm depths of the mineral layer could be linked to measured potential net N mineralization rates. Soil samples were collected from five forest stands in Belgium: three mixed deciduous forests (G, AE, LD), a homogenous deciduous (SB) and a coniferous stand (CP). At the G site, five locations were sampled: one at the forest edge (GE), two deeper in the forest (GF1, GF2), one clear-cut spot (GO) and one in coppice wood (GC). The δ15N and TN measurements were conducted for the litter layer, the fermentation + humus layer and the underlying mineral layers (0–30 cm, at 2 cm intervals). The δ15N values increased with depth, ranging from –12‰ to –1‰ for the forest floor and from –7‰ to +15‰ for the mineral layers. The overall enrichment factor was greater for locations GE, AE and SB (–5.2‰, P <0.001, R2 =0.86) than locations GF1, GF2, GO, GC, LD and CP (–2.4‰, P <0.001, R2 =0.93), possibly indicating NO3– losses. A significant linear regression model could be calculated between Δδ15N and potential net N mineralization rates (y =0.04x), explaining 65% of the variability of Δδ15N. Thus, δ15N profiles in forest soils might be useful as an indicator of \( {\rm NO}_3^ - \) loss and N mineralization behaviour, however, further research is needed to confirm our observations.