Differential effects of nitrate and ammonium on three fen bryophyte species in relation to pollutant nitrogen input

Abstract

Summary• In Dutch fens, subjected to high nitrogen (N) deposition, Scorpidium and other brown mosses have declined markedly. A concurrent strong increase of Sphagnum and Polytrichum has promoted acidification. We measured nitrate (NO 3 -) and ammonium (NH 4 + ) availability in Dutch fens. We also tested preference for either N form of Scorpidium scorpioides , Sphagnum squarrosum and Polytrichum commune .• Ion exchange membranes were installed in the field. In a hydroponic experiment, plants were grown on 100 µ M N (reflecting concentrations in Dutch precipitation since 1980), provided as NO 3 -, NH 4 NO 3 , or NH 4 + .• NH 4 + availability in Sphagnum and Polytrichum stands and NH 4 + : NO 3 -ratio in Sphagnum stands were higher than in brown moss stands. In the experiment, Scorpidium performed best on NO 3 -. NH 4 NO 3 tended to decrease its growth, whereas NH 4 + was very toxic. N treatment did not significantly affect growth of Sphagnum and Polytrichum . Tissue pH and nutrient concentrations confirmed the growth patterns and indicated that Scorpidium was most sensitive to NH 4 + stress.• We conclude that high NH 4 + inputs pose a serious threat to the brown moss flora of rich fens.

Changes in nutrient availability from calcareous to acid wetland habitats with closely related brown moss species: increase instead of decrease in N and P

“…Mineral-rich wetlands not only have higher pH and base cation levels than mineral-poor wetlands, but are also generally considered to be more nutrientrich, because litter decay and nutrient turnover are faster. While there are indeed reports to support this (Bayley et al 2005), there are however also many studies which actually showed higher net Nmineralization in mineral-poor instead of mineralrich wetlands (Verhoeven et al 1988;1990;Scheffer et al 2001;Paulissen et al 2004). Also, Bridgham et al (1998) found a rapid turnover of the nutrient pools in ombrotrophic sites, despite low total soil N and P. However, many studies contrasted mineral-rich fens with Sphagnum-peatlands, which not only differ in mineral status, but at least in the bryophyte layer also in litter quality, which may be an important factor to decomposition and mineralization (Aerts et al 1999).…”

“…Even though based on laboratory incubation experiments, high net mineralization of N and P in mineral-poor fens is probably no artefact. Although Bayley et al (2005) reported that decomposition rates were significantly correlated with the mean daily net N mineralization rate, many studies actually measured high net mineralization in acid peatlands, and low values under mineral-rich conditions (Verhoeven et al 1988(Verhoeven et al , 1990Scheffer et al 2001;Paulissen et al 2004). Also, Bridgham et al (1998) found a rapid turnover of the nutrient pools in ombrotrophic sites, despite low total soil N and P. High instead of low net mineralization under acid conditions has also been reported for field and laboratory studies in forests (Zöttle 1960;Davy and Taylor 1974;Kooijman et al 2008) and coastal dune grasslands (Kooijman and Besse 2002).…”

“…If brown mosses prefer nitrate to ammonium, then N-availability may not actually have been higher at all. Rich-fen species such as S. scorpioides may generally be nitrate users, and sensitive to ammonium toxicity (Paulissen et al 2004). In mineral-poor fens, brown moss species have likely adapted to ammonium toxicity to some extent, but it is not known whether they have also shifted in preference from nitrate to ammonium.…”

“…Sphagnum spp. preferentially use ammonium to nitrate (Paulissen et al 2004), and generally have a much higher cation exchange complex than other bryophytes, which can absorb ammonium rapidly (Clymo and Hayward 1982). Given the general success of Sphagnum spp.…”

Sphagnum can ‘filter’ N deposition, but effects on the plant and pore water depend on the N form

“…In contrast, NO3 -uptake is 110 accompanied by hydroxyl ion (OH -) loss generated by nitrate reduction (Manninen et al,111 2011; Stevens et al, 2011). However, few manipulation studies have evaluated the form 112 of reactive N in wet deposition (Blodau et al, 2006;Paulissen et al, 2004;Sheppard et Science of the Total Environment Chiwa et al Revised MS al., 2014;Sheppard et al, 2013;Van den Berg et al, 2008 habitats through the northern hemisphere (Gore, 1983 of ca. 8 kg N ha -1 yr -1 and supplied 10 % additional rainwater (Sheppard et al, 2014).…”