Contrast of nitrogen contents around roots of mangrove plants

Abstract

Nitrogen is one of the most limiting elements for plant growth. Although nitrogen dynamics in mangrove forests are well understood at the ecosystem level, the nitrogen conditions in the root-zones of plants are less clear. The aim of this study was to analyze in detail the nitrogen conditions in the root zone of mangroves. We conducted a field survey in an area inhabited by three species of mangroves (Avicennia marina (Forsk.) Vierh., Rhizophora stylosa Griff. and Bruguiera gymnorrhiza (L.) Lam.). We collected soil and soil pore-water from the mangrove root zone, in which there were abundant fine roots, and from an area 10-cm away from the root zone (no-root zone). We determined soil organic nitrogen contents and concentrations of ammonium-N, nitrite-N and nitrate-N in soil pore-water along with soil temperature, NaCl and pH. We observed several common characteristics among the three mangrove species; in the root zone, pH was lower, soil organic matter content was higher, soil C:N was lower, and soil pore-water nitrite-N and nitrate-N were higher than their respective values in the no-root zone. These trends were observed in both summer and winter while soil temperature was about 10°C higher in summer than in winter. The fact that C:N was lower in the root zone than in the no-root zone suggests that bacterial nitrogen assimilation occurred near plant roots. The contrast in nitrogen content within a small scale (10 cm) between the root zone and no-root zone suggests that mangroves retain nitrogen source in their root zone. This property clearly works efficiently in intertidal habitats where tidal nitrogen export occurs naturally.