Soil water content drives spatiotemporal patterns of CO<sub>2</sub> and N<sub>2</sub>O emissions from a Mediterranean riparian forest soil

Abstract

Abstract. Riparian zones play a fundamental role in regulating the amount of carbon (C) and nitrogen (N) that is exported from catchments. However, C and N removal via soil gaseous pathways can influence local budgets of greenhouse gas (GHG) emissions and contribute to climate change. Over a year, we quantified soil effluxes of carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) from a Mediterranean riparian forest in order to understand the role of these ecosystems on catchment GHG emissions. In addition, we evaluated the main soil microbial processes that produce GHG (mineralization, nitrification, and denitrification) and how changes in soil properties can modify the GHG production over time and space. Riparian soils emitted larger amounts of CO 2 (1.2-10 g C m −2 d −1 ) than N 2 O (0.001-0.2 mg N m −2 d −1 ) to the atmosphere attributed to high respiration and low denitrification rates. Both CO 2 and N 2 O emissions showed a marked (but antagonistic) spatial gradient as a result of variations in soil water content across the riparian zone. Deep groundwater tables fueled large soil CO 2 effluxes near the hillslope, while N 2 O emissions were higher in the wet zones adjacent to the stream channel. However, both CO 2 and N 2 O emissions peaked after spring rewetting events, when optimal conditions of soil water content, temperature, and N availability favor microbial respiration, nitrification, and denitrification. Overall, our results highlight the role of water availability on riparian soil biogeochemistry and GHG emissions and suggest that climate change alterations in hydrologic regimes can affect the microbial processes that produce GHG as well as the contribution of these systems to regional and global biogeochemical cycles.

Riparian forest transpiration under the current and projected Mediterranean climate: Effects on soil water and nitrate uptake

“…Concentrations were similar to those in Mediterranean forested catchments (0.15–3.20 mg N─NO 3 − l −1 ; Butturini et al, ; Butturini, Bernal, & Sabater, ) and far lower than those in catchments with >70% of land in agriculture, whose concentrations can reach 15–35 mg N─NO 3 − l −1 (Clément et al, ; Hefting et al, ). Although the model was not calibrated for N─NO 3 − mass balances, its predicted soil N─NO 3 − concentrations were similar to field measurements from other studies at the site (0.09–2.34 mg N─NO 3 − l −1 ; Lupon, Sabater, et al, , Poblador et al, ).…”

“…Although microbial denitrification is considered the main N─NO 3 − removal process in riparian areas (Hill, ; Pinay et al, ), it contributes little in Mediterranean areas due to drier conditions and non‐anoxic soil limitations (Bernal et al, ; Poblador et al, ). Denitrification measured in Font del Regàs was low (<4 μg N kg soil −1 day −1 ; Poblador et al, ) compared with that measured in temperate regions (up to 2 mg N kg soil −1 day −1 ; Clément et al, ). As a riparian area with low human impact, our study site did not have high groundwater N─NO 3 − concentrations (0.39–2.67 mg N─NO 3 − l −1 ).…”

“…In contrast, the near‐stream zone had low N─NO 3 − concentrations throughout the year due to a stronger connection between saturated and unsaturated soil. Denitrification, four times as high in the near‐stream zone as in the other riparian zones (Poblador et al, ), could also have removed N. Despite plant uptake, annual mass balances suggest an ~1 g N─NO 3 − m −2 increase in all riparian soils. This might lead to N‐enrichment of riparian soil, because soil denitrification rates at the site ranged from 0.32 ± 0.23 g N m −2 year −1 (Poblador et al, ).…”

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

“…Based on existing published data, we forecast that denitrification is not significant in riparian zones along low stream order streams under Mediterranean, arid or arctic climate due to low water availability and/or short residence time of water and solutes (Jones et al, 2005;Bernal et al, 2007;Mu et al, 2017;Poblador et al, 2017). Therefore, we forecast that in Mediterranean and arid regions, denitrification rates are consistently low compared to more humid catchments (Holmes et al, 1996;Bernal et al, 2007;Harms and Grimm, 2010;Lupon et al, 2015;Poblador et al, 2017). Yet, the capacity for denitrification can rapidly increase once waterlogged conditions develop in riparian zones (Harms et al, 2009).…”

“…Under temperate conditions, strong seasonal shifts in rainfall and soil moisture patterns entail a decrease in denitrification activity in riparian zones along larger streams (Pinay et al, 1995(Pinay et al, , 2000Tabacchi et al, 1998). Based on existing published data, we forecast that denitrification is not significant in riparian zones along low stream order streams under Mediterranean, arid or arctic climate due to low water availability and/or short residence time of water and solutes (Jones et al, 2005;Bernal et al, 2007;Mu et al, 2017;Poblador et al, 2017). Therefore, we forecast that in Mediterranean and arid regions, denitrification rates are consistently low compared to more humid catchments (Holmes et al, 1996;Bernal et al, 2007;Harms and Grimm, 2010;Lupon et al, 2015;Poblador et al, 2017).…”