Effects of elevation on N cycling in tropical forests

Scientists predict that tropical regions will receive the most dramatic increase in nitrogen (N) deposition over the next decades. This is due to increased fertilizer use, legume cultivation, fossil fuel consumption and biomass burning. There is thus a need for a better understanding of N cycling in tropical forest ecosystems. In a recent study by Arnold et al. (2009) across an Andosol (young volcanic ash soil) toposequence in Ecuador (Equitorial South America), it was revealed that gross rates of N transformations, microbial N turnover time, and δ15 N signatures in soil and leaf litter decreased with increasing elevation, indicating a decreasing N availability across the toposequence. Accompanying the above-mentioned trend was a decreasing degree of soil development with increasing elevation as indicated by declining clay content, total C, total N, effective cation exchange capacity and increasing base saturation. The study also revealed that soil N-cycling rates and δ15 N signatures were highly correlated with mean annual temperature but not with mean annual rainfall. Microbial immobilization was the largest fate of produced NH4+ whereas nitrification activity was only 5-11% of gross NH4+ produced. A fast reaction of NO3- to organic N which suggests abiotic NO3- immobilization, was also observed.

Reference

Arnold J, Corre MD, Veldkamp E. 2009. Soil N cycling in old-growth forests across an Andosol toposequence in Ecuador. Forest Ecology and Management 257: 2079-2087.

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