Role of soluble and exchangeable nitrogen pools in N cycling and the impact of nitrogen added in forest soil

Nitrogen (N) cycle in forest soils is altered by water, salt, or acid solutions, and its internal transfers to and from each existing inorganic pools are not known comprehensively. To evaluate the soluble and exchangeable N pools, bulk soil (B soil), water-extracted soil (W soil), and the 0.5 mol L⁻¹ K₂SO₄–treated soil (K soil) were incubated for up to 48 days to comprehend the dynamics of inorganic (NH₄⁺ and NO₃⁻) and soluble organic N (SON) in water-soluble, exchangeable, 2.5 mol L⁻¹ H₂SO₄ (labile pool I, LPI) and 13 mol L⁻¹ H₂SO₄ (labile pool II, LPII) pools. To test the N deposition effects, additional NH₄NO₃ solution was added to B, W, and K soils at amount of 40 mg N kg⁻¹ soil. The results showed that though there was more NO₃⁻ removed when W soil was prepared, the similar net nitrification rate in W soil to B soil and more than 20 mg N kg⁻¹ water-soluble NO₃⁻ were observed in W soil, which indicated that the loss of NO₃⁻ would be enhanced. In contrast, there was more water-soluble and exchangeable NH₄⁺ for K soil compared with B soil. The different dynamic of NO₃⁻ between W and K soil suggested that nitrifiers might dominate in the soil matrix rather than the soil solution. After incubation, each N form in the LPI decreased, which can be attributed to the allocation of remaining N into the recalcitrant pool, except the increase of NH₄⁺ for B soil and NO₃⁻ for K soil, and NO₃⁻ in LPII for B soil. Compared with control, N addition increased mineralization of exchangeable SON to promote nitrification regardless of soils, but weakened the immobilization of NO₃⁻. In addition, N in LPI and LPII pools have increased, which might be related to decomposition of recalcitrant organic matter induced by N addition to transform when the water-soluble and exchangeable N was removed. Therefore, the changes of soluble and exchangeable nitrogen pools impact the N cycling. Our findings can give some explanation for whole soil N transformation responses to N deposition.