Effects of increased nitrogen input on the net primary production of a tropical lower montane rain foest, Panama

Abstract

Increasing anthropogenic nitrogen (N) deposition from agricultural and industrial use, legume cultivation, combustion of fossil fuels, and biomass burning has until recently been a problem of industrialized countries in Europe, North America and East Asia. Consequently, most studies so far investigating the response of natural ecosystems to this threat originate from these temperate regions. With the ongoing development of economically-emerging countries, the most substantial increase in anthropogenic N deposition will occur in tropical regions of Asia and Latin America but knowledge about how tropical ecosystems will respond to this upcoming threat is greatly lacking. As net primary production (NPP) in many terrestrial ecosystems is N-limited and tropical rain forests generating one-third of global terrestrial NPP exert a considerable influence on the world s carbon (C) budget, human alterations of the N constraints on possibly N-limited NPP of some tropical ecosystems might have a drastic influence on the global C cycle. The present thesis assessed how N fertilization affected different aspects of NPP in a tropical lower montane rain forest in western Panama with the objectives 1) to identify differences among components of above-ground net primary production (ANPP; stem growth, litterfall), 2) to determine the response of fine root productivity and turnover, and 3) to estimate the potential of the vegetation to serve as a sink for N and C. An N fertilization experiment was set up with four control and N-fertilized replicate plots of 40 × 40 m, the latter receiving 125 kg urea-N ha-1 year-1 in four applications per year. Stem diameter growth was analyzed by diameter at breast height (DBH) classes and also for the three most abundant species (Oreomunnea mexicana, Eschweilera panamensis, Vochysia guatemalensis). Litterfall was collected every other week from four litter traps per plot. In three soil depths (organic layer, 0-10 cm and 10-20 cm mineral soil), fine root production and turnover were measured by sequential coring and fine root biomass allocation by the ingrowth core approach. Analyses of the N and C content of different tissues (fresh leaves, wood and bark, leaf litter, and fine roots) were used to estimate N and C sequestration by NPP. The responses of stem growth and litter production to N fertilization were highly variable as well within these components (DBH classes, species; litter categories) as in time, since the different ANPP components were not uniformly limited by N supply and subject to inter-annual climatic variation. N fertilization led to an increase in ANPP in the first year of the experiment driven by the response of its most important component which is litterfall. Total litterfall and leaf litterfall were higher under N fertilization also for the two years combined. Above-ground woody biomass was unresponsive to N addition as was stem growth of most DBH classes and species, the only exception being E. panamensis 10-30 cm DBH in the first year. The ability of a species to increase its stem growth in response to N addition seemed to depend on the N costs of stem growth expressed as wood C:N ratios. E. panamensis with a low wood C:N ratio was stronger limited by N than O. mexicana and V. guatemalensis with relatively high wood C:N ratios, and hence, a lower N demand for wood C sequestration. Fine root production and turnover were not affected by N fertilization. Fine root biomass allocated to the 10-20 cm mineral soil in the N-fertilized plots increased two-fold compared to the control, probably because the changed vertical distribution of mineral N allows fine roots to forage for other limiting nutrients, e.g. phosphorus, in the mineral soil without being constrained by the low N availability of the unamended mineral soil. N addition increased C sequestration in the first year. This increase can be attributed to an increased total NPP as tissue C concentrations did not change under N fertilization. Also the increase in C and N return to the forest floor with leaf litter is attributable to the increased leaf litter production. 16.5% of the added N were returned by this pathway. Leaf litter and fine root production were the most important C and N sinks. C and N sink strength of the vegetation is dependent on whether an increase in NPP will occur and also on the C:N stoichiometry of the responsive NPP component(s).