THE INTERNAL NITROGEN CYCLE
BETWEEN MICROORGANISMS, VEGETATION AND SOIL
T. ROSSWALL
Department of Microbiology, Swedish University for Agriculture, Forestry and Veterinary
Medicine, S-750 07 Uppsala 7, Sweden
ABSTRACT
The major portion of the global nitrogen cycle occurs between vegetation and soil, only minor ex
changes generally taking place with the atmosphere and the hydrosphere. Nitrogen conversions in soil
are briefly reviewed and a global flow chart for soil nitrogen is presented. The turnover times of nitro
gen in various components of this global system are compared with its turnover times in some selected
ecosystems. It is shown that 95 % of the nitrogen flow in the global terrestrial system is restricted
to the plant-microorganism-soil system; only 5 % of the total flow is concerned with exchanges to
and from the atmosphere and the hydrosphere.
INTRODUCTION
Annual nitrogen transfers between soil and vegetation by far exceed other global
nitrogen transfers. In the present paper the internal nitrogen cycle in the vegetation
?
soil system is compared with the external global nitrogen cycle (S?derlund & Svensson,
1976). In the estimates of the global cycle S?derlund & Svensson concentrated on the
exchange processes between the pedosphere, hydrosphere and atmosphere (inputs/losses
in Fig. 1), paying only limited attention to conversions in the soil ? mo