Nitrogen Cycling in Tropical Plantation Forests: Potential Controls on Nitrogen Retention

The establishment and management of tropical plantations has the potential to significantly alter patterns in nitrogen (N) cycling relative to old-growth tropical forests, which are generally characterized by high N availability and large fluxes of nitrous oxide (N2O), an important greenhouse gas. We used 15N tracer additions to examine the effects of plantation establishment and management on gross N-cycling rates, N retention via microbial assimilation and dissimilatory nitrate (NO3−) reduction to ammonium (NH4+) (DNRA), and N losses via N2O emissions. In general, plantations had lower rates of N cycling and increased potential for N losses compared to old-growth forests, but there were few differences between very short (one-year) rotation and 10-yr-old uncut plantations. Gross N mineralization declined by almost 50% in the plantations compared to the old-growth forests, and much of the mineralized N was nitrified at all sites. Gross nitrification rates were more variable and did not differ between old-growth forests and unfertilized plantations; however, fertilization increased gross nitrification by a factor of 6 in short-rotation forests, signaling a potential mechanism for increased N losses via leaching and gaseous emissions. Old-growth forests had significantly higher microbial biomass N and NH4+ assimilation rates. No microbial N assimilation was measured in the plantation soils, nor was there evidence of gross NH4+ immobilization from estimates of NH4+ consumption and nitrification. Plantations and old-growth forests had similar DNRA rates (0.23 μg·g−1·d−1), which retains N in the ecosystem, and plantations had lower N2O emissions. Nitrous oxide fluxes from plantations were highly sensitive to reducing conditions, highlighting the potential for high rates of N2O losses. Our results show that plantation establishment can decrease rates of N cycling, but once forests are converted to plantations, internal N-cycling pathways and N2O fluxes are relatively resistant to disturbance associated with short rotation length.