Global bounds on nitrogen gas emissions from humid tropical forests

Abstract

Denitrification and hydrologic leaching are the two major pathways by which nitrogen (N) is lost from the terrestrial biosphere. Humid tropical forests are thought to dominate denitrification losses from unmanaged lands globally, but there is large uncertainty about the range and key drivers of total N gas emissions across the biome. We combined pantropical measures of small watershed stream chemistry with ecosystem modeling to determine total N gas losses and associated uncertainty across humid tropical forests. Our calculations reveal that denitrification in soils and along hydrologic flow paths contributes on average >45% of total watershed N losses. However, when denitrification occurs exclusively in shallow soils, simulations indicate that gas emissions would exceed N inputs and render plants severely N limited, which contradicts observations of widespread N sufficiency in tropical forests. Our analyses suggest an upper bound on soil denitrification of ~80% of total external N losses beyond which tropical plant growth would be compromised.

Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

“…, Brookshire et al. ). Others have shown that terrestrial denitrification fluxes are underestimated by up to 98%, and that N gaseous export exceeds NO 3 − by a factor six (Fang et al.…”

“…Empirical estimations of N 2 production from old tropical forests were roughly four times higher than N 2 O production (Hedin et al 2003), and up to five times in process-based models (Bai and Houlton 2009). Moreover, recent isotopic studies and ecosystem models have pointed out that denitrification losses in humid tropical forests make up a large fraction of total ecosystem N losses (Houlton et al 2006, Brookshire et al 2017. Others have shown that terrestrial denitrification fluxes are underestimated by up to 98%, and that N gaseous export exceeds NO 3 À by a factor six (Fang et al 2015).…”

“…, Brookshire et al. ) and temperate (Perakis and Hedin ) forest ecosystems. Second, research results from both montane and lowland tropical forests have not been differentiated but rather just generalized.…”

“…However, the data at the very basis of the nitrogen paradox, i.e., observations of sustained N loss in tropical rainforests, are actually few and of varying quality. First, some of the export budgets have reported DIN losses only (Appendix S1: Table S1), leaving both dissolved organic and particulate organic N (DON and PON) unquantified, although these have proven to be of extreme importance in the overall N balance of some tropical (Taylor et al 2015, G€ ucker et al 2016, Brookshire et al 2017) and temperate (Perakis and Hedin 2002) forest ecosystems. Second, research results from both montane and lowland tropical forests have not been differentiated but rather just generalized.…”

Remotely sensed canopy nitrogen correlates with nitrous oxide emissions in a lowland tropical rainforest

“…, Brookshire et al. ), emission rates also vary widely by up to two orders of magnitude among sites, from 0.1 to > 9 kg N 2 O‐N ha −1 yr −1 , with a reported mean of 2.8 kg N 2 O‐N ha −1 yr −1 (Erickson et al. , Castaldi et al.…”