Soil N cycling in old-growth forests across an Andosol toposequence in Ecuador |
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| Authors: | Julia Arnold Marife D CorreEdzo Veldkamp |
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| Institution: | Buesgen Institute - Soil Science of Tropical and Subtropical Ecosystems, Georg-August University of Goettingen, Buesgenweg 2, 37077 Goettingen, Germany |
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| Abstract: | Nitrogen (N) deposition in the tropics is predicted to increase drastically in the next decades. The sparse information on N cycling in tropical forests revealed that the soil N status of an ecosystem is the key to analyze its reactions to projected increase in N input. Our study was aimed at (1) comparing the soil N availability of forest sites across an Ecuadorian Andosol toposequence by quantifying gross rates of soil N cycling in situ, and (2) determining the factors controlling the differences in soil N cycling across sites. The toposequence was represented by five old-growth forest sites with elevations ranging from 300 m to 1500 m. Our results provide general insights into the role of elevation-mediated factors (i.e. degree of soil development and temperature) in driving patterns of soil N cycling. Gross rates of N transformations, microbial N turnover time, and δ15N signatures in soil and leaf litter decreased with increasing elevation, signifying a decreasing N availability across the toposequence. This was paralleled by a decreasing degree of soil development with increasing elevation, as indicated by declining clay contents, total C, total N, effective cation exchange capacity and increasing base saturation. Soil N-cycling rates and δ15N signatures were highly correlated with mean annual temperature but not with mean annual rainfall and soil moisture which did not systematically vary across the toposequence. Microbial immobilization was the largest fate of produced NH4+ across all sites, and nitrification activity was only 5–11% of gross NH4+ production. We observed a fast reaction of NO3− to organic N and its role for N retention deserves further attention. If projected increase in N deposition will occur, the timing and magnitude of gaseous N losses may follow the pattern of N availability across this Andosol toposequence. |
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| Keywords: | 15N pool dilution technique Gross N mineralization NH4+ consumption Nitrification Soil and leaf litter δ15N Tropical forests |
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