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 CO 2 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.