Challenges to incorporating spatially and temporally explicit phenomena (hotspots and hot moments) in denitrification models

Abstract

DenitriWcation, the anaerobic reduction of nitrogen oxides to nitrogenous gases, is an extremely challenging process to measure and model. Much of this challenge arises from the fact that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of the denitriWcation activity that occurs in both terrestrial and aquatic ecosystems. In this paper, we describe the prospects for incorporating hotspot and hot moment phenomena into denitriWcation models in terrestrial soils, the interface between terrestrial and aquatic ecosystems, and in aquatic ecosystems. Our analysis suggests that while our data needs are strongest for hot moments, the greatest modeling challenges are for hotspots. Given the increasing availability of high temporal frequency climate data, models are promising tools for evaluating the importance of hot moments such as freeze-thaw cycles and drying/rewetting events. Spatial hotspots are less tractable due to our inability to get high resolution spatial approximations of denitriWcation drivers such as carbon substrate. Investigators need to consider the types of hotspots and hot moments that might be occurring at small, medium, 50 Biogeochemistry (2009) 93:49-77 123 and large spatial scales in the particular ecosystem type they are working in before starting a study or developing a new model. New experimental design and heterogeneity quantiWcation tools can then be applied from the outset and will result in better quantiWcation and more robust and widely applicable denitriWcation models.

Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest

“…Limit of detection (LoD) of the chamber measurements was 3.6 mg CO 2 -C m −2 h −1 , 9.2 μg CH 4 -C m −2 h −1 and 10.1 μg N 2 O-N m −2 h −1 , respectively (Parkin et al 2012). Because N 2 O fluxes from temperate forest soils are known to be highly variable in time and space, with high fluxes during Bhot moments^such as drying-rewetting or freeze-thaw events, and low fluxes during the rest of the year (Groffman et al 2009). Therefore, in the present study fluxes below the LoD were not excluded from the calculation of average fluxes over the study period, because this would have caused a bias towards higher emissions.…”

Nitrogen budgets of urban lawns under three different management regimes in southern California

“…6). Denitrification is usually concentrated in ''hot spots'' and ''hot moments'' due to the high spatial and temporal heterogeneities of its required conditions: organic C, NO 3 --N, and anaerobic environments (Groffman et al 2009a). Therefore, the maximum peak of denitrification in each sampling period might represent the ''hot spots'' of most concentrated substrates and preferable conditions combined.…”

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

“…Emission factors (N 2 O emissions per unit N addition) have been reported to vary between 0.1 and 7% of the N applied (Skiba and Smith 2000). Since multiple processes and drivers are involved, N 2 O emissions are highly variable and often associated with ''hotspots'' (high emissions from small areas) and ''hot moments'' (high emissions for brief periods), making measuring, modeling, and up-scaling challenging (Groffman et al 2009;Reay et al 2012). High variability in the response of N 2 O to N-inputs indicates nonlinearity of the response function (Hoben et al 2011).…”

“…The insulating properties of snow can maintain soil temperatures sufficiently high to allow root growth, microbial respiration, and other biotic activities to continue (Groffman et al 2009). Soils devoid of a snowpack are more vulnerable to freezing and hence to physical and chemical changes, including the death of fine roots, cell lysis, and the alteration of soil microbial processes (Tierney et al 2001;Christopher et al 2008;Shibata et al 2013).…”

“…In the UK, winters when NO 3 -leaching to remote surface waters have been strongly associated with negative excursions in the winter North Atlantic Oscillation (NAO) Index, most likely due to low winter temperatures, lower than average rainfall and larger contributions from more polluted air masses originating from the European continent (Monteith et al 2000;George et al 2004). However, as soil temperature is a dominant driver, opposing inter-annual patterns in NO 3 -leaching may be observed in regions normally blanketed by snow in winter when they lack snow cover, since snow cover insulates the uppermost soil layer from the atmosphere (Groffman et al 2009;Makoto et al 2013). Consequently, relationships between NO 3 -leaching and the NAO index in the UK and in northern Europe may vary regionally (George et al 2004;Blenckner et al 2007;de Wit et al 2008).…”