Regulation of soil nitrogen cycling by shrubs in grasslands

• Published in: Soil biology & biochemistry 2024-04, Vol.191, p.109327, Article 109327
• Main Authors: Chen, Jie, Li, Fen-Can, Jia, Bin, Gang, Shu, Li, Yawen, Mou, Xiao Ming, Kuzyakov, Yakov, Li, Xiao Gang
• Format: Article
• Language: English
• Subjects: absorption; Alpine shrubby meadow; ammonification; biochemistry; China; Fabaceae; meadows; Net soil nitrification; nitrification; nitrogen; plant communities; Plant–soil δ15N values; Shrub expansion; shrubs; soil depth; Soil nitrogen cycling; Soil nitrogen storage; soil profiles; spatial variation; species; topsoil; total nitrogen
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2024.109327

Globally, grasslands are experiencing shrub expansion, but the effects of such plant community shifts on soil N cycling are seldom investigated. We explored the spatial heterogeneity of total soil N content by grid sampling down to a soil depth of 1.2 m, and analyzed net ammonification and nitrification rates in the top 0.2 m, during the growing season, in response to the patchiness of grasses and four shrub species in a grazed meadow on the Tibetan Plateau. The δ15N analyses of plant, soil, and mineral N (NH4+ and NO3−) revealed differences in N absorption strategies between shrubs and grasses. Heterogeneity in the total soil N distribution was common to at least a depth of 1.2 m. The lower δ15N values of the 1.2 m soil profile under shrubs were consistent with the lower δ15N values of leaves, aboveground litter, and roots of shrubs compared to grasses, reflecting the imprint of long-term N cycling in the soil as affected by shrubs. Increases in total soil N stock in the entire 1.2 m profile by 28%–32% under non-leguminous shrubs relative to grasses were ascribed to N accumulation in the lower 0.8 m. Consequently, deep sampling is essential for accurate estimation of soil N storage in shrubby meadows. The total N stock in soil under the leguminous shrub species was similar to that under grasses, which showed that the effect of shrubification on soil N storage depends on the shrub species. Nitrification rates in the topsoil under all shrubs were greater than under grasses, producing higher 15N-depleted NO3− contents under shrubs. Therefore, the lower δ15N signatures of shrub tissues compared to grasses reflected a preferential or adaptive uptake of NO3− by shrubs. The expansion of shrubs in grasslands creates high heterogeneity in N cycling and storage down to a considerable soil depth. [Display omitted] •Spatial variation of N stock is common up to a depth of 1.2 m in shrubby meadows.•Shrubs increase soil ammonification and nitrification compared with grasses.•Shrubs deplete 15N of mineral N pools by increasing nitrification relative to grasses.•Shrubs lower δ15N in tissues by uptake of 15N-depleted NO3− relative to grasses.•Shrubification creates significant heterogeneity in N cycling in grasslands.