Most cycad seeds germinate under the parent plant, and seedlings die before recruitment to the juvenile stage. Decomposition of the senesced organs releases the nutrients to influence nutrient cycling. The aim of this study was to quantify the soil nitrogen that accumulates from seedling turnover. Soil cores were collected beneath male and female trees of four Cycas species in five Philippine habitats from 2019 through 2025, with matching cores collected 5 m from the trees. Five to nine replications were employed depending on the habitat. One seedling was excavated beneath each tree in one location. Total nitrogen concentration was determined by dry combustion in soil and plant tissues, and total nitrogen content was calculated for seedlings. The soils beneath female trees contained more nitrogen than beneath male trees or away from cycad trees in every habitat. The highest nitrogen concentration within seedlings occurred in coralloid roots, but leaflets contained the most nitrogen pool, indicating rapid release of nitrogen during decomposition of the senesced seedling. No differences in rhizosphere nitrogen occurred in a 2017–2025 ex situ experiment using Cycas edentata, where seeds were sown beneath female and male trees. A second 2018–2025 experiment revealed that female trees provisioned with self-seeds did not differ in rhizosphere nitrogen compared with non-kin seeds. Nitrogen fixed by cyanobacteria endosymbionts of cycad seedlings and programmed seedling mortality combine to influence nitrogen cycling in soils beneath female trees over time.
Thomas E. Marler (Fri,) studied this question.