Retention and hydrolysable fraction of atmospherically deposited nitrogen in two contrasting forest soils in Switzerland
2010
Morier, I. | Schleppi, P. | Saurer, M. | Providoli, I. | Guenat, C.
Nitrogen (N) from atmospheric deposition has been shown to be mainly retained in the organic soil layers of temperate forest ecosystems, but the mechanisms and the physico-chemical fractions involved are still poorly defined. We performed a hot-acid hydrolysis on ¹⁵N-labelled soil samples collected 1 week, 3 months and 1 year following a single in situ application of either ¹⁵NO₃⁻ or ¹⁵NH₄⁺ in two montane forest ecosystems in Switzerland: Grandvillard (beech forest on a calcareous, well-drained soil, 650 m above sea level) and Alptal (spruce forest on hydromorphic soil, 1200 m above sea level). After ¹⁵NH₄⁺ application, recovery rates in the soil were smaller in Alptal than in Grandvillard through a large rate of absorption by mosses. At both sites, the organic soil layers retained most of the tracers at all three sampling times between 1 week and 1 year. In Grandvillard, the hydrolysable fraction (hydrolysable N : total N) of ¹⁵N was on average 79% and thus similar to the hydrolysable fraction of native N. This similarity is probably because of the rapid incorporation of N into organic molecules, followed by stabilization of the recalcitrant N pool through organo-mineral bonds with soil minerals. In Alptal, the ¹⁵N hydrolysable fraction was greater than that of native N, particularly after ¹⁵NH₄⁺ application (¹⁵N, 84%; native N, 72%). At both sites, ¹⁵N and the fraction of hydrolysable native N remained constant between 1 week and 1 year. This shows that both the recalcitrant and the hydrolysable pools are stable in the mid- to long-term. We present arguments indicating that biological recycling through microbes and plants contributes to the stability of the hydrolysable N fraction.
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