Nutrient addition affects net and gross mineralization of phosphorus in the organic layer of a tropical montane forest
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In tropical ecosystems with highly weathered soils, transformation of organic phosphorus (P) to bioavailable inorganic P plays a crucial role for the nutrition of organisms. In these ecosystems, P is suspected to be growth-limiting and might therefore be affected by atmospheric nutrient deposition occurring even in remote areas such as montane rainforests. We assessed effects of P and nitrogen (N) addition on net and gross P mineralization rates, and microbial P immobilization in the organic layer along an altitudinal gradient of a tropical montane rainforest in Ecuador. Net P mineralization rates amounted to 1.8 ± 0.9 (at 1000 m a.s.l.), 3.7 ± 0.6 (at 2000 m) and 2.0 ± 0.4 (at 3000 m) mg P kg−1 day−1. Altitudinal differences led to an increased microbial P immobilization at 1000 m compensating for a higher gross P mineralization. P addition increased net P mineralization rates at 2000 and 3000 m, suggesting a higher P demand at 1000 m. At higher altitudes, P likely was released as a by-product during organic matter decomposition. Gross P mineralization, determined by means of the isotopic dilution approach, could not be calculated for control and N treatments due to rapid microbial P immobilization. For P (+N) treatments, gross P mineralization rates were lowest at the 1000 m site towards the end of the long-term incubation period. Atmospheric P deposition in the tropics might lead to P fertilization effects through direct input as well as through acceleration of P release from organic matter, thereby increasing P availability for organisms.
KeywordsEnvironmental change Nutrient cycling Organic phosphorus mineralization Phosphorus availability Tropical montane forest
Financial support was provided by the Deutsche Forschungsgemeinschaft (DFG PAK 823) and the University of Tübingen. We thank the Ministerio de Ambiente del Ecuador for the research permit and Nature and Cultural International (NCI) in Loja and the San Francisco research station. We would like to thank the Laboratory for Radioisotopes (LARI), Georg-August-University Göttingen, for providing technical support and working space. We appreciate very much the support of Emmanuel Münch and Elena Spoeri who kindly assisted in the soil sampling.
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