From Missing Source to Missing Sink: Long-Term Changes in the Nitrogen Budget of a Northern Hardwood Forest

Abstract

Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the “missing” N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from ∼4 to ∼1 kg of N ha–1 year–1, despite chronically elevated atmospheric N deposition (∼7 kg of N ha–1 year–1) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying ∼8 kg of N ha–1 year–1. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha–1. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.

Carbon fluxes and interannual drivers in a temperate forest ecosystem assessed through comparison of top-down and bottom-up approaches

“…In closed-canopy forest stands, the pools of primary importance are live and dead woody biomass, as well as soil carbon. Here we focus on changes in woody carbon stocks and assume that changes in soil carbon stocks were minimal as was found from measurements at mature stands in nearby Hubbard Brook Experimental Forest (Yanai et al, 2013). Changes in soil carbon stocks would be very difficult to detect over a 13 year study period (Vadeboncoeur et al, 2012).…”

Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

“…Such results often lead to fertilizer overapplication in agricultural systems (Liu et al 2010) and then to N losses from leaching and denitrification. While gross rates of mineralization are higher and may better correlate with plant N uptake (Osterholz et al 2016), they are rarely measured at the temporal and spatial scales that are needed to resolve the complex processes that regulate bioavailable N. In addition, many natural ecosystems exhibit a ''missing sink'' phenomenon wherein soils store more N than can be accounted for by N budgeting (Bernal et al 2012;Yanai et al 2013;van Groenigen et al 2015). This suggests there are other pathways of N storage and production that are not adequately represented by standard laboratory measures of N availability.…”

Modelling water, nutrients, and organic carbon in forested catchments: a HYPE application

“…() reported that 0–70% of atmospheric N was retrieved in runoff from forest catchments in Europe and North America. In other investigations, part of the atmospheric N was accumulated in trees, denitrified, or lost through runoff (Nadelhoffer et al ., ; Yanai et al ., ), or stayed in the soil (e.g. 70% reported by Nadelhoffer et al ., ).…”