Reconstruction of the historical changes in mycorrhizal fungal communities under anthropogenic nitrogen deposition

Abstract

Archived soil samples (1937^1999) and historic air quality data from the Los Angeles Basin were used for reconstructing the record of change between atmospheric NO x loads, soil d 15 N values and the diversity of arbuscular mycorrhizae (AM), which are ubiquitous plant^fungus mutualists that control plant community productivity. A tripling of atmospheric NO x loads between 1937 and the 1970s was paralleled by soil nitrogen enrichment (d 15 N 3.18). From 1975 onwards, atmospheric NO x declined, but soils became nitrogen saturated (d 15 N 74 and NO 3 -nitrogen 171mg kg 71 ). The shifts in the AM community followed 28 years of atmospheric nitrogen enrichment and coincided with the onset of soil nitrogen saturation. Such changes were manifest in the loss of AM productivity, species richness (one species per year), three genera (Acaulospora, Scutellospora and Gigaspora) in the spore community and Gigaspora within the roots. Nitrogen enrichment also enhanced the proliferation of potentially less mutualistic species of Glomus. Autoregressive models implied that such patterns will persist and be driven by soil nitrogen cycling patterns. Chronic nitrogen enrichment from air pollution thus alters the diversity and mutualistic functioning of AM communities, which, in turn, may in£uence the plant community.

Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis

“…Three previously common mycorrhizal genera disappeared from the mycorrhizal spore community in soil and one large-spored genus (Gigasopora) was no longer found in plant roots. N enrichment also enhanced the proliferation of potentially less-mutualistic species of small-spored genus Glomus, which may have implications for plant community succession in the face of chronic N deposition (Egerton-Warburton et al 2001).…”

“…& Arn. near Los Angeles was associated with dramatic changes in the mycorrhizal community (Egerton-Warburton et al 2001). Diversity, species richness, and productivity of the arbuscular mycorrhizal community had deteriorated severely by 1969.…”

Efficiency of partner choice and sanctions inLotusis not altered by nitrogen fertilization

“…Soil nitrogen varies because of natural and anthropogenic inputs into soils and since the industrial revolution atmospheric deposition has dramatically polluted some soils with reactive forms of nitrogen [35][36][37][38][39]. Assuming that host-control traits are costly to express [12,40], similar to other plant defences against bacteria [41,42], we predict that host control will be downregulated in nitrogen-rich soils, where hosts can gain nitrogen primarily from less costly mineral sources [43] rather than symbionts.…”

Nitrogen deposition decreases the benefits of symbiosis in a native legume

“…Most atmospheric N r deposition into terrestrial ecosystems likely occurs on historically nitrogen-limited soils (Vitousek et al 1997;Padgett et al 1999;Egerton-Warburton et al 2001). N r deposition and the resultant fertilization of soils can reduce plant species richness (Roem et al 2002;Carroll et al 2003;Maskell et al 2006;Clark and Tilman 2008;Maskell et al 2010) by altering outcomes of competitive interactions among plants, and by making the environment unfavorable for nitrogen-sensitive species (Bobbink et al 2010).…”