Can δ¹⁵N 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, R ² =0.86) than locations GF1, GF2, GO, GC, LD and CP (–2.4‰, P <0.001, R ² =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.