No abstract

No abstract

Thirtyfour deciduous forest sites in southern Sweden, originally studied in 1949-1970, were resampled in 1984/85. The average pH change in the humus layer was-0.78 and-0.23 in soils originally studied 30-35 and 15-20 years ago, respectively. Cover changes in the field layer were measured and related to pH changes. The number of species had increased in spite of pH decreases, reaching a maximum at pH 4.0-5.0, while the total cover of the field layer was unchanged.Two groups of species showed no correlation with pH decreases in the humus layer. A large number of species had increased in cover on a majority of sites over the entire species specific pH interval, including nitrophilic species (Rubus idaeus, Chamaenerium angustifolium, Aegopodium podagraria, Stellaria nemorum). A few species had decreased in cover on a majority of sites (Polygonatum multiflorum, Pulmonaria officinalis, Dentaria bulbifera). Some species showed a covariation with pH changes, decreasing in cover in sites in the acidic part of the pH interval (Mercurialis perennis, Lamium galeobdolon, Galium odoratum, Oxalis acetosella, Luzula pilosa). Without excluding other factors, this paper suggests that the increased nitrogen deposition and the increased acidity in the humus layer might cause some of the cover changes in the presented species.

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2omega6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha(-1)y(-1) compared with 10 kg N ha(-1)y(-1)) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1omega5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1omega9, a17:0 and 18:1omega7, while some were negatively affected by pH, such as i15:0, 16:1omega7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1omega7c and 16:1omega9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2omega6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.

Three solution experiments were performed to test the importance of NH versus NO +NH to growth of 23 wild-forest and open-land species, using field-relevant soil solution concentrations at pH 4.5. At N concentrations of 1-200 μM growth increased with increasing N supply in Carex pilulifera, Deschampsia flexuosa, Elymus caninus and Bromus benekenii. Geum urbanum was the most N demanding species and had little growth below 200 μM. The preference for NH or NO +NH was tested also at pH 4.0; no antagonism was found between NH and H, as indicated by similar relative growth in both of the N treatments at both pH levels. Growth in solution with NH relative to NO +NH , 200 μM, was negatively related to the mean pH of the field occurrence of the species tested; acid-tolerant species grew equally well with only NH as with NO +NH (Oxalis acetosella, Carex pilulifera, Festuca gigantea, Poa nemoralis, Deschampsia flexuosa, Stellaria holostea, Rumex acetosella), while species of less acid soils were favoured by NO +NH (Urtica dioica, Ficaria verna, Melandrium rubrum, Aegopodium podagraria, Geum urbanum, Bromus benekenii, Sanguisorba minor, Melica ciliata, Silene rupestris, Viscaria vulgaris, Plantago lanceolata). Intermediate species were Convallaria majalis, Elymus caninus, Hordelymus europaeus and Milium effusum. No antagonism between NH and Ca, Mg and K was indicated by the total uptake of the elements during the experiment.