Organic nitrogen components in soils from southeast China

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.

The role of Chinese Milk Vetch as cover crop in complex soil nitrogen dynamics in rice rotation system of South China

The effect of Chinese Milk Vetch (CMV) residues as a cover crop on the performance of rice plants and nitrogen mobilization and mineralization in paddy soil was evaluated in a pot-culture experiment. Three treatments were included in the trial, i.e. without exogenous-N (Control or CK), urea fertilizer as the sole N-source (N) and urea plus CMV (NM). The results revealed higher amounts of total-N, inorganic-N, acid hydrolysable-N and non-acid hydrolysable-N in the soil under NM, followed by N and CK treatments at tillering, booting and maturity stages of rice. A similar trend was observed for various soil ammonia oxidizing bacteria, aerobic and anaerobic bacteria. Correlation coefficients exhibited a negative correlation of residual exogenous-¹⁵N with only amino acid nitrogen (P < 0.01). Moreover, low abundance of the key functional gene amoA in rice was found in CK treatment. From the results it can be concluded that the nitrogen from organic source can improve the availability of total-N, inorganic-N and NH₄ in the soil, especially at the later stages of rice growth.