Environmental Monitoring and Assessment

, Volume 128, Issue 1, pp 121-131

First online:

Are Nitrogen-Fertilized Forest Soils Sinks or Sources of Carbon?

  • Helga Van MiegroetAffiliated withDepartment of Wildland Resources, Utah State University
  • , Robert JandlAffiliated withFederal Office and Training Center for Forests, Natural Hazards and Landscape (BFW) Email author 

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We developed a simple conceptual model that tracks nitrogen and carbon jointly through an N fertilized forest ecosystem. The stimulation of growth increases the litterfall and imports substrate for soil microorganisms. Microbial biomass forms according to the supply of C and N. The formation of microbial biomass is accompanied by respiratory C losses. The quantity of CO2 efflux depends on the C use efficiency of microbes. When excess N is available, the microbial activity is accelerated and the demand for substrate is high. Litterfall supplies an insufficient amount of C to the soil. In such a case, labile soil C is mineralized and the net effect of N fertilization is a loss of soil C. A strong N fertilization effect on the aboveground biomass can offset the soil C loss. In the case of a low N dosage or high N losses due to leaching or emission of nitrogen oxides, the soil C loss is small. The conceptual model was applied to a case study. The field data, collected over a time span of several decades, could not support sound conclusions on the temporal trend of soil C because the spatial and temporal variability of the chemical data was high. The conceptual model allowed to give an evaluation of the fertilization effect on soil C based on reproducible principles.


Forest soil Conceptual model C sequestration N fertilization