Contrasting effects of nitrogen addition on soil respiration in two Mediterranean ecosystems

Abstract

Increased atmospheric nitrogen (N) deposition is known to alter ecosystem carbon source-sink dynamics through changes in soil CO fluxes. However, a limited number of experiments have been conducted to assess the effects of realistic N deposition in the Mediterranean Basin, and none of them have explored the effects of N addition on soil respiration (R ). To fill this gap, we assessed the effects of N supply on R dynamics in the following two Mediterranean sites: Capo Caccia (Italy), where 30 kg ha year was supplied for 3 years, and El Regajal (Spain), where plots were treated with 10, 20, or 50 kg N ha year for 8 years. Results show a complex, non-linear response of soil respiration (R ) to N additions with R overall increasing at Capo Caccia and decreasing at El Regajal. This suggests that the response of R to N addition depends on dose and duration of N supply, and the existence of a threshold above which the N introduced in the ecosystem can affect the ecosystem's functioning. Soil cover and seasonality of precipitations also play a key role in determining the effects of N on R as shown by the different responses observed across seasons and in bare soil vs. the soil under canopy of the dominant species. These results show how increasing rates of N addition may influence soil C dynamics in semiarid ecosystems in the Mediterranean Basin and represent a valuable contribution for the understanding and the protection of Mediterranean ecosystems.

Drivers of soil respiration in response to nitrogen addition in a Mediterranean mountain forest

“…Overall, previous and current results suggest a N-poor environment under pine, with predominantly oligotrophic microorganisms, whereas more copiotrophic microbes dominate under oak (Fernández-Alonso et al 2018a). Thus, the stronger N limitation in pine stands results in a higher response upon fertilization than in the oak ones, in line with studies reporting how, after N addition, the native N availability-derived from soil stoichiometryinhibits R S in N richer sites and enhances R S in N poorer ecosystems (Pregitzer et al 2008;Zhu et al 2016;Lo Cascio et al 2017).…”

“…In both forests, the displayed qCO 2 trend among N treatments (i.e. medium-fertilization C high-fertilization C control) suggests the existence of a threshold in the cumulative amounts of N that shift soil properties and functioning (Lo Cascio et al 2017). In these acidic soils, where the exchangeable base pool is presumably smaller than the total exchangeable acidity, the base cation losses with heavy N loads within several decades could lead to soil acidification (Reuss and Johnson 1986;Janssens et al 2010) and negative effects on microbial and plant activity (DeForest et al 2004;Sinsabaugh et al 2008).…”

“…Considering the background atmospheric N deposition (García-Gómez et al 2014), plots were receiving approximately 10-15, 20-25 and 50-55 kg N ha -1 a -1 . The high-N treatment falls out of the forecasted scenarios of N deposition for the Mediterranean region (up to 20 kg N ha -1 a -1 by 2030, Bobbink et al 2010), but it was included in order to encompass a wide range of N deposition rates, in line with similar experiments (Lo Cascio et al 2017;Forstner et al 2019). We used ammonium nitrate (NH 4 NO 3 34.5% N, Fertiberia S.A., Puertollano, Spain) as fertilizer.…”

“…Even if the availability of soil N increases, water scarcity and/or low temperatures may still limit plant nutrient uptake (Sardans et al 2011) or soil microbial transformations, and thereby R S (Almagro et al 2009;Fernández-Alonso et al 2018b). Following this argument, the seasonality of precipitations influences not only the R S rates per se, but also its response to N addition, which is suppressed during the dry period (Zhang et al 2014;Lo Cascio et al 2017). Likewise, the soil activity was limited in winter due to the low temperatures and the apparently higher soil moisture values observed in the oak stand after leaves senescence did not trigger a differential response upon rewetting (Figs.…”

(E)merging directions on air pollution and climate change research in Mediterranean Basin ecosystems

“…The impacts of atmospheric pollution and climate change can reverberate throughout the whole ecosystem and thus a multidisciplinary approach is needed, from soils and microbes to plants and soil fauna and from community-level analyses to physiological evaluations in response to pollutant exposure. A multidisciplinary approach that is patent in this special issue, which opens with two articles (Lo Cascio et al 2017;Ochoa-Hueso et al 2017b) that focus primarily on the response of Mediterranean soils and soil-inhabiting communities to N deposition, is followed by three articles that focus on the usefulness of cryptogams (mosses, lichens, and liverworts) as biomarkers of increasing aridity and toxic metal pollution (Basile et al 2017;Cogolludo et al 2017;Paoli et al 2017). The issue continues with two articles by Avila et al (2017) and Cabal et al (2017) about the patterns and effects of nitrogen deposition on woody plant-dominated ecosystems from Spain and finishes with three articles focused on the effects of tropospheric ozone on larch forests (Finco et al 2017), crops (Marzuoli et al, 2016), and pasture species (González-Fernández et al 2017).…”