Tryggve Persson

Effects of acidification and liming on carbon and nitrogen mineralization and soil organisms in mor humus

The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism biomass. Intact soil cores from F/H layers in a Norway spruce (C:N=31) and a Scots pine (C:N=44) stand in central Sweden were treated in the laboratory for 55 days with deionized water (control), weak H2SO4 (successively applied as 72 mm of acid rain of pH 3.1), strong H2SO4 (applied as a single high dose of pH 1), and lime CaCO3. Strong acidification reduced C mineralization and increased net N mineralization in both soils. Weak acidification resulted in similar but less pronounced effects. Liming initially stimulated C mineralization rate, but the rates declined, indicating that an easily available C source was successively used up by the microorganisms. Liming also increased net N mineralization in the C:N=31 humus, but not significantly in the C:N--44 humus. Strong acidification generally affected the amounts of FDA-active fungal hyphae, nematodes and enchytraeids more than the other treatments did. The increases in net N mineralization after acidification and liming could only partly be explained by the decreases in biomass N in soil organisms. Mineralization of biomass N from killed soil organisms could at the most explain up to about 30% of the increase in net N mineralization after strong acidification. Most of the effects on N mineralization seemed to depend on the fact that acidification reduced and liming increased the availability of C and N to the microorganisms. Furthermore, acidification seemed to reduce the incorporation of N from dead organisms into the soil organic matter and, thereby, make the N compounds more readily available to microbial decomposition and mineralization.

Nitrous oxide production in a forest soil at low temperatures – processes and environmental controls

Recent investigations have highlighted the relative importance of the winter season for emissions of N(2)O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N(2)O formation at temperatures below 0 degrees C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N(2)O formation and subsequent emission. Net N(2)O production rates at -4 degrees C equaled those observed at +10 to +15 degrees C at moisture contents >60% of the soils water-holding capacity. The source of this N(2)O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 degrees C. Furthermore, both net nitrogen-mineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N(2)O formation in soils at low temperatures.

Long-term effects of logging residue addition and removal on macroarthropods and enchytraeids

The long-term effects of logging residue addition and removal on soil macroarthropods and enchytraeids were examined in a Scots pine Pinus sylvestris L. stand in central Sweden. The study was performed 15-18 years after the treatments had been applied after clear-cutting in 1976. In comparison with plots receiving roughly twice the normal amount of residues, removal of logging residues (above-ground whole-tree harvesting) resulted in decreases in the total numbers of Collembola (springtails), gamasid mites, spiders, predatory insects and dipterous larvae, whereas no significant effects on enchytraeids and diplopods could be detected. Few effects on single species of Collembola and Gamasida were found. The composition of the soil fauna community, as well as food web structure, were significantly affected by whole tree harvesting, but the effects were quantitative rather than qualitative-most organism groups decreased, but the relative importance of different groups did not change markedly. Over the 4 years studied, community predictability (specifically, constancy) at the levels of higher taxa, functional groups and species did not differ substantially between the treatments. Predictabilities of higher taxa and functional groups were higher than predictability of species of Collembola and gamasid mites. Values of community predictability were similar to those found in other studies of forest soil fauna. It is concluded that whole tree harvesting may result in long-term decreases in the abundances of many soil animal groups. The possible impact of decreased abundances of fungivores and predatory arthropods on nutrient cycling and site productivity is discussed. It is argued that the direct effects of these changes on nitrogen mineralization are likely to be small. However, the possibility that the soil fauna may be involved in a positive feedback loop towards lower site productivity means that the observed long-term decreases in several organism groups should be of concern, at least on sites dominated by internal nutrient dynamics.

Bottom–up or top–down control in forest soil microcosms? Effects of soil fauna on fungal biomass and C/N mineralisation

A major question in soil ecology is whether soil food webs are regulated by resources or by predators, i.e. bottom–up (donor) or top–down controlled. We tested the hypothesis that meso- and macrofaunal soil predators can regulate fungivore populations and, thereby cause a top–down cascade effect on fungal biomass and decomposition/mineralisation processes in boreal forest soils. The study was performed as a microcosm experiment with two contrasting soils (humus layers), one poor and one rich in N, and with different combinations of fungivore and predator soil fauna added to “defaunated” soil. In comparison with control microcosms lacking mesofauna (but with nematodes and protozoans), the presence of a diverse Collembola and Oribatida fungivore community significantly reduced the FDA-active fungal biomass or tended to reduce the ergosterol fraction of the fungal biomass in the N-poor humus, but no clear effect could be detected in the N-rich humus. Fungivores as well as fungivores plus predators (a predator community consisting of gamasids, spiders and beetles or a subset thereof) reduced C mineralisation and increased net N mineralisation in both soils. The presence of predators (particularly gamasid mites) reduced collembolan numbers and alleviated the negative effect of fungivores on fungal biomass in the N-poor soil. In the N-rich soil, the presence of predators increased fungal biomass (ergosterol) in relation to the “defaunated” soil. Therefore, a top–down trophic cascade could be detected in the N-poor humus but not in the N-rich humus. Our results suggest that the degree of top–down control in soil fauna communities depends on resource quality and soil fertility.

Interactions Between the Carbon and Nitrogen Cycles and the Role of Biodiversity: A Synopsis of a Study Along a North-South Transect Through Europe

The NIPHYS/CANIF project of the EEC has provided the unique opportunity to examine forest ecosystem processes and diversity along a transect through Europe ranging from north Sweden to central Italy. The main objectives of this study were (1) to identify and quantify effects of N deposition on ecosystem processes, particularly the C cycle, by extending the range of observations across a deposition maximum in central Europe, and (2) to study feedback effects between ecosystem processes and biodiversity. The study resulted in a very comprehensive and consistent set of data on ecosystem processes over a 5-year period. However, in contrast to earlier large-scale ecosystem studies (IBP: Reichle 1981; Acid Rain Programme: Last and Watling 1991), the samples were collected and data generated by the same scientists at all sites. This assured comparisons of results on a broad geographic scale. In addition, key parameters were assessed by different methods, and integrating parameters were collected for different processes, in order to test and verify predictions made at higher and lower scales, ranging from physiological responses to ecosystem level processes.

Experimental sites in the NIPHYS/CANIF project

The NIPHYS/CANIF sites Fig. 2.1 were chosen to represent a latitudinal, climatic and deposition transect through Europe containing conifer and broadleaf stands. The main species are Norway spruce, Picea abies L. (Karst.) and European beech, Fagus sylvatica L. In southern France, Picea and Fagus are lacking at suitable sites and were replaced by maritime pine, Pinus pinaster Ait. (P. maritima Lam.), Aleppo pine, P. halepensis Mill. and white oak, Quercus pubescens Willd. for some of the studies. F. sylvatica has its most northern distribution in southern Scandinavia. Therefore, in northern Sweden, F. sylvatica was replaced with birch, Betula pubescens Ehrh.

Pools and fluxes of carbon and nitrogen in 40-year-old forest liming experiments in Southern Sweden

Soil samples were collected from litter, humus and mineral soil layers to a depth of 50 cm in 37–42 year-old limed and unlimed plots in one beech and three spruce stands in S Sweden for determination of carbon (C) and nitrogen (N) pools, C and N mineralization rates and nitrification rates. The samples were sifted while still fresh and incubated at a constant temperature (15°C) and soil moisture (50 % WHC) for 110–180 days with periodic subsamplings. The C and N pools in the uppermost soil layers were significantly lower in plots limed with 9–10 t CaCO3 ha−1 than in unlimed plots, whereas the pools in the deeper mineral soil did not differ markedly between the treatments. In the whole soil profile, the C and N pools had, on average, decreased by 16% (P<0.05) and 11% (P>0.05), respectively, after 40 yrs. The smaller reduction in N pools resulted in significantly lower C:N ratios and increased N immobilization in the limed spruce plots but not in the limed beech plot. C and net N mineralization rates were increased in some of the limed plots and decreased in others. This indicates that liming can still have a stimulatory effect after 40 yrs in some soils. The nitrification potential was increased in the limed plots. Liming did not increase tree growth in the stands investigated. We conclude that liming with high doses of CaCO3 is likely to reduce pools of soil C and possibly even soil N in relation to unlimed areas in spruce and beech forests in S Sweden. If trees in limed stands do not respond with better growth, the treatment will thus result in a net ecosystem loss of C and N in relation to unlimed areas. It was not possible to conclude whether the effects of low doses of lime would be similar to those of high doses.

Nitrification in organic and mineral soil layers in coniferous forests in response to acidity

In order to study the response of nitrification to changes in pH in the FH layer and the upper B horizon, sifted samples from three spruce sites in SW Sweden were treated with CaCO3, H2SO4, urea or nitrifying humus in the laboratory and incubated for 110–180 days. Acetylene was used to distinguish between autotrophic and heterotrophic nitrification. In the FH layer, net nitrification was stimulated by CaCO3 and tended to be reduced by H2SO4. In samples where nitrate production was not increased by CaCO3, inoculation with nitrifying humus had a stimulating effect. In the B horizon, there was always a net nitrate production, but addition of CaCO3 or urea had practically no effect. H2SO4 reduced net nitrification also in the B horizon. Acetylene inhibited most of the nitrification in both soil layers. The results showed that at these sites nitrification was more pH-dependent in the FH layer than in the B horizon. The most acid FH layer seemed to have small or inactive populations of nitrifiers that responded to increased pH only after several years. This shows that generalisations about nitrification in forest soils cannot be made from studies of the upper soil layers alone.

Effects of acidification and liming on feeding groups of nematodes in coniferous forest soils.

SummaryNematodes were sampled in untreated, acidified, and limed plots in a Norway spruce (Fexboda) and a Scots pine (Norrliden) stand. At Fexboda, the total number of nematodes was significantly reduced after the acidification. This reduction was probably due to a shock effect, because the samples were taken only 5 months after an application of 200 kg H2SO4 ha-1 to the forest floor. However, the root/fungal-feeding Aphelenchoides was not reduced, probably because it is more tolerant of high acid concentrations than most other nematodes. At Norrliden, where the samples were taken 7 years after the last application of H2SO4, no significant differences were found between the acidified and untreated plots. If the treatment with H2SO4 caused similar effects as at Fexboda, the results indicate a recovery of the nematode populations. Decreased predation from lumbricids rather than a recovery of microfloral populations probably allowed this recovery. No marked effect of lime, spread 2 (Fexboda) and 12 years (Norrliden) before the sampling on the numbers of any of the nematode feeding groups was found. This correlated with almost no change in bacterial biomass after liming, while the active fraction of fungal hyphae was unaffected by liming at Fexboda and reduced by liming at Norrliden. A tendency for decreasing numbers of all nematode feeding groups in the limed plots at Norrliden coincided with increasing numbers of lumbricids.

Effects of lumbricids and enchytraeids on nematodes in limed and unlimed coniferous mor humus

In a factorial laboratory experiment, specimens of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) were added to microcosms with unlimed (pH 4.5) and limed (pH 5.5) coniferous mor humus containing bacteria, fungi, protozoans, and nematodes. Effects on the nematodes were assessed after an incubation period of 207 days at 15°C and a soil moisture content of 60% water-holding capacity. When D. octaedra was absent, nematodes were significantly more abundant in the limed humus than in the unlimed humus. The presence of D. octaedra markedly reduced the number of nematodes in the limed humus but not in the unlimed one, where D. octaedra lost weight and probably did not feed. Most nematodes (92–97%) were bacterial-feeders. The presence of D. octaedra did not decrease the number or biomass of bacteria, indicating that the reduction in nematode numbers was not the result of competition for bacteria between D. octaedra and the nematodes. The presence of C. sphagnetorum had no effect on the nematodes in either of the treatments. We suggest that the reason why D. octaedra, but not C. sphagnetorum, reduced nematode numbers is that the former was more likely to inadvertently ingest the nematodes because of its much greater size. The results provide a possible explanation for the observation that liming sometimes enhances nematode populations, when lumbricids do not respond to the treatment, and sometimes causes decreases, when lumbricids increase in number.