Abstract. Vigorous Eucalyptus plantations produce 105 to
106 km ha−1 of fine roots that probably increase carbon (C) and
nitrogen (N) cycling in rhizosphere soil. However, the quantitative
importance of rhizosphere priming is still unknown for most ecosystems,
including these plantations. Therefore, the objective of this work was to
propose and evaluate a mechanistic model for the prediction of rhizosphere C
and N cycling in Eucalyptus plantations. The potential importance of
the priming effect was estimated for a typical Eucalyptus plantation
in Brazil. The process-based model (ForPRAN – Forest Plantation Rhizosphere
Available Nitrogen) predicts the change in rhizosphere C and N cycling
resulting from root growth and consists of two modules: (1) fine-root growth
and (2) C and N rhizosphere cycling. The model describes a series of soil
biological processes: root growth, rhizodeposition, microbial uptake,
enzymatic synthesis, depolymerization of soil organic matter, microbial
respiration, N mineralization, N immobilization, microbial death, microbial
emigration and immigration, and soil organic matter (SOM) formation. Model
performance was quantitatively and qualitatively satisfactory when compared
to observed data in the literature. Input variables with the most influence
on rhizosphere N mineralization were (in order of decreasing
importance) root diameter > rhizosphere thickness > soil temperature > clay
concentration. The priming effect in a typical Eucalyptus plantation
producing 42 m3 ha−1 yr−1 of shoot biomass, with assumed
losses of 40 % of total N mineralized, was estimated to be 24.6 % of
plantation N demand (shoot + roots + litter). The rhizosphere cycling
model should be considered for adaptation to other forestry and agricultural
production models where the inclusion of such processes offers the potential
for improved model performance.
Modeling rhizosphere carbon and nitrogen cycling in <i>Eucalyptus</i> plantation soil