Ammonium as a Driving Force of Plant Diversity and Ecosystem Functioning: Observations Based on 5 Years' Manipulation of N Dose and Form in a Mediterranean Ecosystem

Abstract

Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha−1 yr−1) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha−1 yr−1 as a 1∶1 NH4Cl to (NH4)2SO4 mixture, and 40 and 80 kg N ha−1 yr−1 as NH4NO3. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH4 +-N ha−1 yr−1 (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH4NO3-N ha−1 yr−1 and the % of bare soil increased under 40 kg NH4 +-N ha−1 yr−1. The treatment containing less ammonium, 40 kg NH4NO3-N ha−1 yr−1, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NHy availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NOx availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas.

On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition

“…Emissions reductions of SO 2 and NO x have caused deposition of SO 4 2− and NO 3 − to decline considerably across most of the U.S. (Vet et al, 2014), even in remote mountain regions (Heard et al, 2014), while increased production of NH 3 from agriculture and vehicles has caused an increase in chemically-reduced forms of N deposition (Thiruvengadam et al, 2016). Moreover, while the effects of N deposition on aquatic and terrestrial ecosystems are well known, these broad patterns are complicated by growing evidence that reduced forms of N cause more severe ecosystem responses than oxidized N forms (Dias et al, 2014;Glibert, 2010;Kleijn et al, 2008;Mur et al, 2017;Stevens et al, 2011;Van den Berg et al, 2005Verhoeven et al, 2011). These differential effects depending on N form may grow in importance as the proportion of N deposition in reduced forms continues to increase.…”

Plant tolerance of ammonium varies between co-existing Mediterranean species

“…One of the suggested hypotheses for the lack of stimulation of NRA by NO 3 − was that the summer semideciduous species were being inhibited, possibly by ammonium (NH 4 + ), as there appeared to be a threshold soil [NH 4 + ] above which no summer semi-deciduous species occurred . This exclusion of summer semi-deciduous species (but not of evergreen sclerophylls) from soil patches with higher [NH 4 + ] seems to occur despite the [NO 3 − ], as shown by Dias et al (2014) in a field N-manipulation experiment.…”

Evidence for differential effects of reduced and oxidised nitrogen deposition on vegetation independent of nitrogen load

“…In contrast, the highly buffered alkaline habitats base rich mires and calcareous grasslands, 34 showed no or even a negative relationship between grass:forb ratio and N deposition. These forms are correlated the variance that is explained by one N form, and which is factored out, 67 is likely to contain some degree of variance that in fact should be attributed to the other 68 form.…”

“…17 There are strong reasons, which have been recently reviewed by (Stevens et al 2011), for 18 expecting that there may be different effects of reduced and oxidised N deposition for each 19 of these mechanisms. For example, foliar uptake of gaseous NH3 is more likely to be directly 20 toxic than uptake of gaseous nitrogen oxides, while soil NH4 + is more likely to be toxic to 21 plant roots than soil NO3 - (Sheppard et al 2011, Sheppard et al 2014). Plant species also 22 differ strongly in their preference and tolerance for NH4 + or NO3 -uptake from soil solution 23 with species of acidic habitats generally more tolerant of higher soil ammonium 24 (Falkengrengrerup and Lakkenborgkristensen 1994).…”

“…For example, van den Berg et al (2008) showed that higher 31 NH4 + /NO3 -ratios in deposition to heathland mesocosms had significant adverse effects on 32 acid-sensitive species but not on acid-tolerant species that were also tolerant of high soil 33 NH4 + /NO3 -ratios. This effect was lost in limed mesocosms, suggesting that acidification at 34 higher NH4 + /NO3 -ratios was the key driving mechanism. By contrast, in Mediterranean 35 maquis vegetation, the application of both NH4 + and NO3 -increased biomass but not plant 36 diversity, while NH4 + alone increased plant diversity but not biomass (Dias et al 2014); these 37 effects can at least partly be explained by the different responses of individual species to 38 total N inputs or to reduced N deposition specifically.…”