Journal Article

Impact of elevated N input on soil N cycling and losses in old-growth lowland and montane forests in Panama

Arnold, Julia, Corre, Marife D., Wright, S. Joseph, Veldkamp, Edzo

2010

Overview

Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but changes in soil-N-cycling processes in tropical ecosystems under elevated N input are less well understood. We used N-addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. Our objectives were to (1) assess changes in soil mineral N production (gross rates of N mineralization and nitrification) and retention (microbial immobilization and rapid reactions to organic N) during 1- and 9-yr N additions in the lowland forest and during 1-yr N addition in the montane forest and (2) relate these changes to N leaching and N-oxide emissions. In the old-growth lowland forest located on an Inceptisol, with high base saturation and net primary production not limited by N, there was no immediate effect of first-year N addition on gross rates of mineral-N production and N-oxide emissions. Changes in soil-N processes were only apparent in chronic (9 yr) N-addition plots: gross N mineralization and nitrification rates, NO 3 − leaching, and N-oxide emissions increased, while microbial biomass and NH 4 + immobilization rates decreased compared to the control. Increased mineral-N production under chronic N addition was paralleled by increased substrate quality (e.g., reduced C:N ratios of litterfall), while the decrease in microbial biomass was possibly due to an increase in soil acidity. An increase in N losses was reflected in the increase in 15 N signatures of litterfall under chronic N addition. In contrast, the old-growth montane forest located on an Andisol, with low base saturation and aboveground net primary production limited by N, reacted to first-year N addition with increases in gross rates of mineral-N production, microbial biomass, NO 3 − leaching, and N-oxide emissions compared to the control. The increased N-oxide emissions were attributed to increased nitrification activity in the organic layer, and the high NO 3 − availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO 3 − leaching. These results suggest that soil type, presence of an organic layer, changes in soil-N cycling, and hydrological properties are more important indicators than vegetation as an N sink on how tropical forests respond to elevated N input.

• Publisher: Ecological Society of America
• Subject: abiotic N immobilization / Acid soils / Acidity / Altitude / ammonium compounds / Andisols / Animal and plant ecology / Animal, plant and microbial ecology / base saturation / Biological and medical sciences / Biomass / carbon nitrogen ratio / Denitrification / Ecosystem / ecosystems / Emissions / Forest soils / Forestry / Fundamental and applied biological sciences. Psychology / General aspects / General forest ecology / Generalities. Production, biomass. Quality of wood and forest products. General forest ecology / gross N mineralization / gross nitrification / hydrology / immobilization / Inceptisols / Leaching / litterfall / litterfall 15N signature / long-term nutrient manipulation experiment / lowland forests / Lowlands / microbial biomass / Mineral soils / Mineralization / montane forest organic horizon / Montane forests / Mountain forests / N leaching / N signature / N-oxide emissions / NH4+ immobilization / nitrates / Nitrates - chemistry / Nitrification / Nitrogen / Nitrogen - chemistry / Nitrogen - metabolism / nitrogen deposition / Nitrogen Isotopes / NO3 / Organic soils / Panama / Primary production / primary productivity / Quaternary Ammonium Compounds - chemistry / rain / Rainforests / Sandy loam / sandy loam soils / Sandy soils / Soil - analysis / Soil ecology / Soil microorganisms / soil pH / Soil types / stable isotopes / Trees / Tropical Climate / Tropical forests / tropical lowland forest / Tropical soils / Vegetation