Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils

Background/Question/Methods Secondary forests and young forest plantations frequently have high rates of tree growth and NPP – higher even than mature forests in similar situations.  The nutrients required to sustain this rapid growth are derived from the soil and by external inputs such as rainfall or, in the case of nitrogen, by biological N fixation.  In our study of the effects of tree species within replicated experimental plantations in a moist, lowland tropical environment in Costa Rica, high rates of biomass accumulation and productivity were coupled with high rates of N accumulation and cycling.  We applied extensive sampling through time within a mass-balance approach to address the question “Where does all the nitrogen come from?”

Results/Conclusions Rates of N uptake by the vegetation in these plantations were extraordinarily high, even by tropical forest standards, reaching 412 kg N ha^-1 yr^-1.  Rapid decay coupled to tight nitrogen cycling may have provided large amounts of available N for plant uptake, but do not explain the large quantities of N that accrued in the vegetation, up to 1075 kg N/ha over 16 years.  Surface soil organic matter stocks in the plantations increased by as much as 320 kgC/ha in surface soils, but soil nitrogen varied differently.  Soil N stocks to 1-m depth were depleted by an average of 2119 kg/ha relative to the mature forest.  Thus, mineralization of soil organic nitrogen could have supplied the N that accrued in biomass over the 16-yr period, but this apparently occurred without concomitant net loss of soil C.  The C:N ratios of soil organic matter (SOM) in the plantations indicated either replacement of SOM with more C-rich detritus or selective removal of N from existing SOM.  Regardless, high productivity in these plantations apparently was supported in part by mining of soil nitrogen. Species varied, however. Depletion of soil N stocks was only 219 kg/ha under Vochysia guatemalensis, in which 1075 kgN/ha had accrued. Asymbiotic N fixation is the next most plausible mechanism for supplying plant-available N, and may be enhanced in stands with high availability of recently produced photosynthates.