N isotopes and N cycle in the TieShanPing subtropical forest ecosystem, Southwestern China

Nitrogen is essential for forest growth and forest stand development. It is commonly a limited factor for forest productivity. We examined δ ¹⁵N values in soils and plants by studying the sources of N used by vegetation and cycles of N in a 43-year-old plantation of the TieShanPing forest ecosystem in southwestern China, dominated by massone pine (Pinus massoniana). The N concentration of plant materials ranges from 1.1% to 2.2%. The nitrogen concentration of P. massoniana was 1.3% while soils showed the concentration of 0.04-0.15%. Regarding natural abundance of ¹⁵N, large significant variation (-6.0[per thousand] to -3.8[per thousand]) in δ ¹⁵N values was observed among shrub and tree leaves. δ ¹⁵N values were also significantly varied from -4.7[per thousand] to -3.8[per thousand] among the pioneer species in the plantation. Soil δ ¹⁵N values (3.1-6.3[per thousand]) were significantly enriched compared to those values in plant samples. Despite the negative δ ¹⁵N values of the vegetation cover, the high δ ¹⁵N values in the topsoil indicate that return of N to soils by litter-fall is minimal on TieShanPing and the present forests do not change very much the soil ¹⁵N signals at the surface layer. The positive δ ¹⁵N values may also indicate large N losses from the soil system vial leaching, volatilization and plant uptake.