Characteristics of soil nitrogen mineralization in the rhizosphere of trees,shrubs, and herbs in subtropical forest plantations

Aims The objective was to explore the characteristics of soil nitrogen mineralization in the rhizosphere soils of trees, shrubs, and herbs in plantations and their variations among different species, forest types, and seasons.Methods The rhizosphere soils of trees, shrubs (Loropetalum chinense, Adinandra millettii, and Eurya muricate), and herbs (Woodwardia japonica and Dryopteris atrata) were sampled in the early growth season (April) and the vigorous growth season (July) in Cunninghamia lanceolata, Pinus massoniana, and Pinus elliottii plantations at Qianyanzhou Ecological Research Station, Taihe, Jiangxi. Net mineralization rate (N_min), net ammonification rate (N_amm), net nitrification rate (N_nit), soil chemical properties, and microbial properties were measured.Important findings The results found that, 1) Species, forest types, and sampling seasons significantly affected N_min, N_amm, and N_nit. Understory plants showed a higher seasonal sensitivity of rhizosphere soil N_nit than trees did in P. massoniana plantations and P. elliottii plantations. It means that rhizosphere soil N_min and N_amm of trees were significantly higher than those of most of the understory plants in April, but rhizosphere soil N_min and N_amm of the understory plants significantly increased and showed no difference with those of trees in July. This finding was consistent with the variance analysis of comprehensive scores by principal component analysis. Generally, rhizosphere soil N_min and N_nit in C. lanceolata plantation were higher than those in P. massoniana and P. elliottii plantations. Rhizosphere soil nitrogen mineralization in July was higher than those in April. 2) Soil ammonium nitrogen, nitrate nitrogen, soil total nitrogen concentration, and soil microbial nitrogen concentration were the main factors affecting net nitrogen mineralization of rhizosphere soil. Soil chemical properties contributed to 29.2% of the variation of rhizosphere soil nitrogen mineralization, which was significantly higher than soil microbial properties. Consideration of the seasonal variations of soil nitrogen mineralization in the rhizosphere of understory plants and their influencing factors will provide an important foundation for accurately evaluating nutrient cycling in the plantation ecosystem.