Modest Gaseous Nitrogen Losses Point to Conservative Nitrogen Cycling in a Lowland Tropical Forest Watershed
- 308 Downloads
Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with characteristically large hydrologic and gaseous losses of inorganic N. However, emerging evidence suggests that some tropical ecosystems can exhibit tight N cycling, with low biologically available losses. In this study, we combined isotopic data with a well-characterized watershed N mass balance to close the N budget and characterize gaseous N losses at the ecosystem scale in a lowland tropical rainforest on the Osa Peninsula in southwestern Costa Rica. We measured δ15N and δ18O of nitrate (NO3 −) in precipitation, surface, shallow and deep soil lysimeters and stream water biweekly for 1 year. Enrichment of both isotopes indicates that denitrification occurs predominantly as NO3 − moves from surface soil down to 15 cm depth or laterally to stream water, with little further processing in deeper soil. Two different isotopic modeling approaches suggested that the gaseous fraction comprises 14 or 32% of total N loss (2.7 or 7.5 kg N ha−1 y−1), though estimates are sensitive to selection of isotopic fractionation values. Gas loss estimates using the mass balance approach (3.2 kg N ha−1 y−1) fall within this range and include N2O losses of 0.9 kg N ha−1 y−1. Overall, gaseous and soluble hydrologic N losses comprise a modest proportion (~ 25%) of the total N inputs to this ecosystem. By contrast, relatively large, episodic hydrologic losses of non-biologically available particulate N balance the majority of N inputs and may contribute to maintaining conservative N cycling in this lowland tropical forest. Similar patterns of N cycling may occur in other tropical forests with similar state factor combinations—high rainfall, steep topography, relatively fertile soils—such as the western arc of the Amazon Basin and much of IndoMalaysia, but this hypothesis remains untested.
Keywordsdenitrification nitrogen cycle nitrous oxide δ15N δ18O NO3− tropical forest
We thank W. Combronero-Castro for his invaluable assistance with fieldwork in Costa Rica. We thank M. Jimenez and the late H. Michaud of the Drake Bay Wilderness Camp for providing field access and logistical support and we also thank F. Campos Rivera, the Organizaciόn para Estudios Tropicales (OET) and the Ministerio de Ambiente y Energia (MINAE) for assisting with research permits and logistics in Costa Rica. This study was supported by a National Science Foundation GK-12 fellowship, the Andrew Mellon Foundation and an NSF DEB Award (# 0919080) to CC and NSF DEB Award (# 0918387) to SP.
- Buchs DM, Baumgartner PO, Baumgartner-Mora C, Bandini AM, Jackett S-J, Diserens M-O, Stucki J. 2009. Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin. Geol Soc Lon Spec Publ 328:411–56.CrossRefGoogle Scholar
- Chestnut TJ, Zarin DJ, Mcdowell WH, Keller M. 1999. A nitrogen budget for late-successional hillslope tabonuco forest, Puerto Rico. Biogeochemistry 46:85–108.Google Scholar
- Fang Y, Koba K, Makabe A, Takahashi C, Zhu W, Hayashi T, Hokari AA, Urakawa R, Bai E, Houlton BZ, Xi D, Zhang S, Matsushita K, Tu Y, Liu D, Zhu F, Wang Z, Zhou G, Chen D, Makita T, Toda H, Liu X, Chen Q, Zhang D, Li Y, Yoh M. 2015. Microbial denitrification dominates nitrate losses from forest ecosystems. Proc Natl Acad Sci USA 112:1470–4.CrossRefPubMedGoogle Scholar
- Ishizuka S, Tsuruta H, Murdiyarso D. 2002. An intensive field study on CO2, CH4, and N2O emissions from soils at four land-use types in Sumatra. Indonesia. Glob Biogeochem Cycles 16:22.Google Scholar
- Jenny H. 1941. Factors of soil formation: a system of quantitative pedology. New York: McGraw-Hill.Google Scholar
- Michalski G, Scott Z, Kabiling M, Thiemens MH. 2003. First measurements and modeling of Δ17O in atmospheric nitrate. Geophys Res Lett 30:1–4.Google Scholar
- Silver WL, Lugo AE, Keller M. 1999. Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils. Biogeochemistry 44:301–28.Google Scholar
- Uehara G, Gillman GP. 1981. The mineralogy, chemistry, and physics of tropical soils with variable charge clays. Westview Press.Google Scholar
- Wanek W, Drage S, Hinko N, Hofhansl F. 2008. Primary production and nutrient cycling in lowland rainforests of the Golfo Dulce region. In: Weissenhofer A, Huber W, Mayer V, Pamperl S, Weber A, Aubrecht G, Eds. Natural and cultural history of the Golfo Dulce Region, Costa Rica. Oberösterreichisches Landesmuseum, Biologiezentrum. pp 155–177.Google Scholar
- Wright SJ, Yavitt JB, Wurzburger N, Turner BL, Tanner EVJ, Sayer EJ, Santiago LS, Kaspari M, Hedin LO, Harms KE, Garcia MN, Corre MD. 2011. Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest. Ecology 92:1616–25.CrossRefPubMedGoogle Scholar