Soil water controls on nitrogen oxide fluxes and N2O production and consumption along a rainfall gradient of tropical forests
Final Report Abstract
The patterns of gross N mineralization and gross nitrification across the five forest sites along the orthogonal precipitation and soil fertility gradients did not reflect the trends in soil moisture or precipitation, indicating that soil moisture or precipitation was not the proximal factor influencing the trends of soil N availability in these sites. Instead, these processes of mineral N production were paralleled by similar trends in microbial C or microbial N across sites and seasons. Microbial N, in turn, reflected the patterns of soil organic C, total N, moisture content and precipitation across sites, particularly in the wet season. Together, these suggest that microbial biomass was the proximal factor controlling the mineral N production and thus soil N availability in these sites, whereas soil moisture or precipitation was the distal factor controlling the differences in microbial biomass and its turnover time across sites. These site differences in microbial biomass and its turnover time reflect the long-term effects of organic matter (i.e. total N) and precipitation, whereas the differences between seasons signify the short-term effect of soil moisture. Across the five forest sites along the orthogonal precipitation and soil fertility gradients, annual trace gas fluxes ranged from: 8.0 to 10.2 Mg CO2-C ha-1 yr-1, -2.0 to -0.3 kg CH4-C ha-1 yr-1, 0.4 to 1.3 kg N2O-N ha-1 yr-1 and -0.82 to -0.03 kg NO-N ha-1 yr-1. Soil CO2 emissions did not significantly vary between the five sites, but did exhibit a clear seasonal pattern. All soils were CH4 sinks; within-site fluxes were largely controlled by soil moisture whereas fluxes across sites were correlated with soil fertility. Soil N2O fluxes were low throughout the study, but highest emissions occurred at a mid-precipitation site with high N availability. NO uptake was measured at all sites, with the highest uptake occurring at the site closest to Panama City; NO uptake was likely due to high ambient air NO concentrations from industrial sources. From the moist (2700 mm yr-1 annual precipitation) lowland forest and wet (5500 mm yr-1 annual precipitation) montane forest in Panama, denitrification is the dominant N2O source at the measured surface depths. Based on four years of repeated measurements of N2O concentrations and soil moisture contents in soil profiles of these forest sites, supported by seasonal isotopic measurements of δ15N2O natural abundance signatures, we deduced that denitrification process is probably also the main source of N2O down to 1.5-2-m depth, with diminished N2O-to-N2 consumption during dry season in the lowland forest. From four different ecosystems, spanning a wide range in soil physical and biochemical characteristics, we found that gross N2O emission and gross N2O uptake at the soil-atmosphere interface, measured by 15N2O pool dilution method, were consistently smaller than the actual gross N2O production and consumption in the soil, quantified by gas-flow core method. At present, these are the only two methods that are relatively easy to employ for repeated measurements of N2O-flux dynamics. However, in-situ measurement using gas-flow core method cannot be done because this instrument cannot be simply brought to the field. The 15N2O pool dilution techniques can, however, be applied to intact soil cores taken directly from the field and incubated in-situ. Thus, quantifying gross N2O production and consumption under actual field conditions may be attained by using 15N2O pool dilution techniques and deriving a conversion factor by calibration between the two methods.
Publications
- 2012. An in-depth look into a tropical lowland forest soil: How 9-11 years experimental nitrogen addition affected the contents of N2O, CO2 and CH4 down to 2-m depth. Biogeochemistry 111: 695-713. Erratum in 111: 715-717
Koehler, B., M. D. Corre, K. Steger, R. Well, E. Zehe, J. P. Sueta and E. Veldkamp
- 2013. Indications of nitrogen-limited methane uptake in tropical forest soils. Biogeosciences 10: 5367–5379
Veldkamp, E., B. Koehler, and M. D. Corre
(See online at https://doi.org/10.5194/bg-10-5367-2013) - 2014. Nitrogen-oxide emissions from tropical forest soils exposed to elevated nitrogen input strongly interact with rainfall quantity and seasonality. Biogeochemistry 118: 103-120
Corre, M. D., J. P. Sueta, and E. Veldkamp
(See online at https://doi.org/10.1007/s10533-013-9908-3) - 2015. Variation in canopy litterfall along a precipitation and soil fertility gradient in a Panamanian lower montane forest. Biotropica 47: 300- 309
Heineman, K. D., P. Caballero, A. Morris, C. Velasquez, K. Serrano, N. Ramos, J. Gonzalez, L. Mayorga, M. D. Corre, and J. W. Dalling
(See online at https://doi.org/10.1111/btp.12214)
