Nitrous Waxcap
Neohygrocybe nitrata
Also called nitrous wax cap, nitrous hygrocybe
A drab grey-brown waxcap identified less by colour than by its sharp nitrous, bleach-like smell and fibrous, matt cap.
Overview
The Nitrous Waxcap is a member of Neohygrocybe, a small genus split from Hygrocybe for a handful of dull, fibrous-capped grassland species with strong smells. Where most waxcaps are glossy and brilliantly coloured, these are matt, brownish and easy to walk past.
Its defining character is the odour. Crushed flesh releases a sharp, chemical, nitrous or bleach-like smell, sometimes likened to ammonia or to a struck match, which is quite unlike anything produced by the coloured waxcaps growing alongside it.
The species is uncommon and, like other members of the genus, is confined to long-established, nutrient-poor grassland, so it is a valued record in grassland fungus surveys across its range.
How to identify it
Cap
The cap is 2-6 cm across, convex then flattening and often becoming slightly depressed or irregularly wavy. The surface is dry and matt with fine radial fibrils, never viscid, and coloured pale greyish-brown, mouse-brown or dull ochre-brown, sometimes paler at the margin.
Gills
Gills are thick and somewhat waxy but less obviously so than in true Hygrocybe, moderately spaced, adnate to slightly emarginate or with a short decurrent tooth. They are whitish to pale grey-buff, occasionally with a faint pinkish or brownish tinge in age.
Stipe
The stipe is 3-7 cm tall, cylindrical or tapering, dry, longitudinally fibrillose, whitish to grey-brown and often slightly paler than the cap. It is frequently compressed or grooved and lacks both ring and volva.
Flesh and spore print
The flesh is whitish to pale grey and may darken slowly to reddish-brown or greyish where damaged in some collections. The nitrous smell is strongest when the flesh is crushed. The spore print is white, distinguishing the species from the pink-spored grassland Entoloma that share the same habitat.
Where and when it grows
Nitrous Waxcap fruits in short, unimproved grassland, particularly on neutral to base-rich soils: old downland, closely grazed pasture, cemeteries, lawns of long-standing and occasionally dune grassland. It usually appears among a wider waxcap assemblage rather than alone.
The season runs from late summer into late autumn. The species is thinly distributed across Europe, with British and Irish grasslands supporting a significant share of known sites, and it is reported infrequently from North America. Records are patchy because the drab colours make it easy to overlook.
Ecological role
Like the rest of the waxcap group, Neohygrocybe nitrata lives in the moss and root layer of ancient turf, with nutritional evidence pointing to a biotrophic association with grassland plants or bryophytes rather than simple decay of dead plant material. It has never been reliably cultured on standard laboratory media, which is consistent with a dependence on a living partner.
Colonies persist for many years but are destroyed by ploughing, reseeding or fertiliser application. Fruiting occurs in autumn flushes after rainfall, and the species is scored as a strong indicator of grassland continuity in national survey protocols.
Similar species
Neohygrocybe ingrata is the nearest relative, sharing dull brown colours and a nitrous smell, but its flesh reddens distinctly where cut or bruised and its gills often flush reddish-brown, changes that are weak or absent in N. nitrata.
Porpolomopsis and dull Cuphophyllus species such as the Cedarwood and Grey Waxcaps have smooth or greasy caps and no chemical odour. Grassland pinkgills such as the Silky Pinkgill (Entoloma sericeum) are similarly drab and brown but have thin, crowded gills that become pink and give a pink spore print, plus a farinaceous rather than nitrous smell. Small brown Clitocybe species have thin, crowded, strongly decurrent gills and lack the fibrillose matt cap.