Jouni K. Nieminen

Influence of carbon and nutrient additions on a decomposer food chain and the growth of pine seedlings in microcosms

Abstract Because of N deposition plant production is becoming increasingly limited by other nutrients in boreal forests. At the same time more C is suggested to become available for below-ground food webs because of enhanced CO2 fixation. We studied the effects of carbon and nutrient addition on a fungus–nematode food chain and on the growth of mycorrhizal or nonmycorrhizal pine seedlings (Pinus sylvestris L.) in microcosms with N-rich mineral soil and a humus layer. The role of the food chain and mycorrhizal fungi for pine growth was tested in a smaller set-up. The total pine biomass was not N-limited at the scale of two experimental growing seasons. In accordance with established knowledge on fertile soils, the mycorrhizal fungi did not affect pine growth. The fungus–nematode food chain reduced both above- and below-ground pine biomass. The abiotic manipulations had strong effects on the total fungal biomass and nematode abundance. Both carbon and nutrient addition increased fungal biomass in the nonmycorrhizal, but not in the mycorrhizal systems. Fertilization reduced fungal biomass in mycorrhizal microcosms, but not in combination with carbon addition. Nematode abundance was higher in the absence of mycorrhizal fungi both in humus and in mineral soil, and increased after C addition in the nonmycorrhizal systems. Although the strong below-ground effects of resource manipulations were not reflected in total pine biomass, C addition reduced the ratio of stem dry mass to stem length significantly. The results illustrate that although decomposers may become redundant for plant growth in nutrient rich soil, carbon enrichment to a redundant decomposer food chain is not necessarily reflected in plant growth. Two hypothesis (changes in fungal biomass quality and allelopathy) are discussed as possible explanations for the resistance of pine biomass to changes in the decomposer food chain. In addition, the results demonstrate that mycorrhizal fungi can regulate below-ground responses to resource additions other than rhizodeposition even when they are redundant in terms of plant growth.

Combined effects of loose wood ash and carbon on inorganic N and P, key organisms, and the growth of Norway spruce seedlings and grasses in a pot experiment

The effects of wood ash on the growth of Norway spruce seedlings and grasses, nitrogen and phosphorus leaching, and soil fauna were investigated at two levels of carbon availability in a greenhouse experiment simulating harvested boreal forest. While sucrose-C amendment reduced grass biomass regardless of wood ash by 88%, the shoot and root biomass of spruce seedlings increased by 38% and 370%, respectively. Despite the large variation in above-ground biomass, C addition did not alter the concentration of water extractable ammonium nitrogen in humus, but it counteracted the ash-induced increase in soil phosphate concentration. C addition reduced the proportion of bacterial-feeding nematodes in the nematode community. Wood ash reduced enchytraeid size, but their biomass was not affected. Carbon treatment was crucial for enchytraeids probably because amended pots were moister than controls. Small compensatory grass growth following harvest implied that soil fauna made little nitrogen available to plants in one growing season. The results support the hypothesis that C availability may be an important determinant of nutrient retention, and has the potential to control plant competition in intensively harvested forests.

Modelling the interactions of soil microbes and nematodes.

Six different soil food webs, assembled from a bacterium, a bacterial-feeding nematode, a fungus and a fungal-feeding nematode, were established in replicated laboratory microcosms. Glucose was supplied as the sole carbon source for the microbes. Biomasses of the organisms and the concentration of dissolved organic carbon (DOC) were measured ten times during 20 weeks. A discrete dynamic model based on the material flow between system components was fitted to the experimental data. Bacterial-based food chains were largely inactive in the absence of fungi, but mutual facilitation was observed in the systems with both fungus and bacterium. The population dynamics of a fungal-feeding nematode was adequately described by the models, but the model failed to describe DOC dynamics. The quality of fungal biomass appeared to be a key parameter in the system. Model performance was improved by letting fungal parameters vary with time and food web structure. Because fungal dynamics could not be explained by a trophic-dynamic model with rigid parameters, it is suggested that non-trophic effects of fungal-feeding nematodes on fungi may be more important in microcosms.

Effects of sewage sludge addition to Norway spruce seedlings on nitrogen availability and soil fauna in clear-cut areas.

Anaerobically digested and composted sewage sludge (CSS) has been suggested to be a slow-release fertilizer in forestry and an alternative to quick-release inorganic fertilizers. The effects of CSS with or without added carbohydrate on inorganic nitrogen availability and on soil animals were tested in two Norway spruce plantations. Half of the seedlings were individually fertilized with CSS, and the rest were left as controls. Solid sucrose was added to half of the fertilized and untreated seedlings. Soil samples were taken in the autumn in the first and the second year after the treatments. CSS increased soil NH4-N (2100%), the proportion of soil NO3-N, and the N concentration of spruce needles. CSS greatly reduced the abundances of enchytraeids, tardigrades and collembolans, but increased the proportion and abundance of bacterial-feeding nematodes irrespective of carbohydrate addition. A better stabilization method needs to be developed before CSS can be used as a forest fertilizer.

Wood Ash Effects on Soil Fauna and Interactions with Carbohydrate Supply: A Minireview

Wood ash effects on soil animals in a boreal forest ecosystem are reviewed focusing on recent results on interactive effects of wood ash and organic amendments, and laboratory microcosms as a tool to understand soil food webs are discussed. Loose wood ash can reduce the populations of enchytraeids, collembolans and mites, but increase nematode populations particularly in experimental laboratory ecosystems with little or no primary production. Recent studies indicate that the repressive effect on enchytraeids depends on carbon availability. Carbohydrate supply seemed to alleviate the negative wood ash effect on enchytraeid body size and abundance. The fact that carbon alleviated wood ash effects on enchytraeids without any change in pH supports the view that wood ash effects on soil animals are partly indirect consequences of altered food resources. Experimental evidence suggests that the negative wood ash effect on enchytraeids is partly linked to increased bacteria-to-fungi ratio after wood ash application, and that this may be counteracted by carbohydrate addition.

Dead-wood effects on enchytraeids and nematodes in thinned and unmanaged Norway spruce forests.

Abstract The effects of dead wood on enchytraeids and nematodes were studied in thinned and uncut Norway spruce forests in two experiments. Fifteen pairs of small spruce logs (one enclosed in polyethylene sheet and another untreated control) were returned to the forest floor in a complete randomized block design after thinning. Soil under the logs and at distances of 0–6 cm and 6–12 cm from each log was sampled after one growing season, and enchytraeids and nematodes were extracted and forest floor properties measured. Log enclosure increased enchytraeid length irrespective of the distance from the log. Soil moisture or pH were not affected by enclosure, but organic matter content was reduced, suggesting that log enclosure indirectly enhanced organic matter decomposition and enchytraeid growth rate. Soil samples were taken beneath fallen large-diameter spruces and at three distances from tree trunks at one thinned and one uncut site. Enchytraeids were significantly smaller beneath the trunks than in the surrounding soil, although forest floor properties were similar at all distances from the trunk. It is possible that the enchytraeid size distribution reflects past rather than present forest floor properties. Nematode abundance was not affected by the distance from tree trunk or log enclosure.