Current ozone levels threaten gross primary production and yield of Mediterranean annual pastures and nitrogen modulates the response

Abstract

Pastures are among the most important ecosystems in Europe considering their biodiversity and distribution area. However, their response to increasing tropospheric ozone (O3) and nitrogen (N) deposition, two of the main drivers of global change, is still uncertain. A new Open-Top Chamber (OTC) experiment was performed in central Spain, aiming to study annual pasture response to O3 and N in close to natural growing conditions. A mixture of six species of three representative families was sowed in the field. Plants were exposed for 40 days to four O3 treatments: filtered air, non-filtered air (NFA) reproducing ambient levels and NFA supplemented with 20 and 40 nl I -1 O3. Three N treatments were considered to reach the N integrated doses of "background", +20 or +40 kg N ha -1 . Ozone significantly reduced green and total aboveground biomass (maximum reduction 25%) and increased the senescent biomass (maximum increase 40%). Accordingly, O3 decreased community Gross Primary Production due to both a global reduction of ecosystem CO2 exchange and an increase of ecosystem respiration. Nitrogen could partially counterbalance O3 effects on aboveground biomass when the levels of O3 were moderate, but at the same time O3 exposure reduced the fertilization effect of higher N availability. Therefore, O3 must be considered as a stress factor for annual pastures in the Mediterranean areas.

Large but decreasing effect of ozone on the European carbon sink

“…This is primarily through a decrease in the size of the soil carbon pool as a result of reduced litter input to the soil, consistent with reduced GPP and NPP. Field studies show that in some regions of Europe, soil carbon stocks are decreasing (Bellamy et al, 2005;Capriel, 2013;Heikkinen et al, 2013;Sleutel et al, 2003). The study of Bellamy et al (2005), for example, showed that carbon was lost from soils across England and Wales between 1978 and 2003 at a mean rate of 0.6 % per year with little effect of land use on the rate of carbon loss, suggesting a possible link to climate change.…”

“…In recent decades much attention has focussed on the effects of rising atmospheric CO 2 on vegetation productivity (Ceulemans and Mousseau, 1994;Norby et al, 1999Norby et al, , 2005Saxe et al, 1998). The Norby et al (2005) synthesis of FreeAir CO 2 Enrichment (FACE) experiments suggests a median stimulation (23 ± 2 %) of forest net primary production (NPP) in response to a doubling of CO 2 .…”

Atmospheric concentration and deposition of reactive nitrogen in Spanish forests of Quercus ilex

“…In a monitoring study of undisturbed headwater catchments in N Spain, it has been described a deposition-driven increase of N exportation from perennial grasslands to aquatic ecosystems (Camarero and Aniz, 2010). (Sanz et al , 2015Calvete-Sogo et al, 2014). Fertilization experiments in mountain heathlands of the Cantabrian Range have described an increase in the abundance of arthropod herbivores in the short-term (Cuesta et al, 2008) and a significant increase in total plant richness in the long term, due to an increase in the number of perennial herbs without displacing the dominant woody species (Calvo et al,.…”

“…However, recent publications suggest that the addition and interaction of different stress factors (O 3 , N deposition, drought) can be affecting the growth of the trees Gerosa et al, 2015) and accompanying pastures (Calvete-Sogo et al, 2014. Thus, monitoring of nitrogen compounds such as NH 3 and HNO 3 should be incorporated into air quality monitoring networks (in most of them NO 2 and O 3 are already monitored).…”

“…The concentrations of N compounds seemed to be not high enough to directly affect vegetation but could be contributing through atmospheric N deposition to the eutrophization of these ecosystems. Moreover, although evergreen broadleaf Mediterranean woody species are assumed to be tolerant to air pollution due to their sclerophyllic adaptations, recent publications suggest that the addition and interaction of different stress factors (O 3 , N deposition, drought) can be affecting the growth of the trees Gerosa et al, 2015) and accompanying pastures (Calvete-Sogo et al, 2014). Thus, monitoring of nitrogen compounds such as NH 3 and HNO 3 should be incorporated into air quality monitoring networks.…”