Submitted to: Forest Ecology and Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/2008
Publication Date: 1/13/2009
Publication URL: parking.nal.usda.gov/shortterm/21112_DijkstraetalForeEcolManage2009.pdf
Citation: Dijkstra, F.A., West, J.B., Hobbie, S.E., Reich, P.B. 2009. Antagonistic effects of species on C respiration and net N mineralization in soils from mixed coniferous plantations. Forest Ecology and Management 257:1112-1118. Interpretive Summary: When plants are growing together their effects on soil processes such as decomposition and nutrient mineralization are often greater or smaller than the sum of their effects when grown separately. These effects are called synergistic and antagonistic effects respectively. Synergistic and antagonistic effects on decomposition and nitrogen (N) mineralization have often been observed when fresh litter of different species is mixed together. Much less is known if synergistic or antagonistic effects on decomposition and N mineralization occur at later stages of soil organic matter decomposition. A better understanding of soil organic matter decomposition and N mineralization is important because it affects soil carbon (C) storage, tree productivity, and the global C cycle. We studied decomposition and N mineralization in soils from 10-year old tree plantations in northern Minnesota. Three common boreal tree species (black spruce, white pine, and tamarack) were planted as monocultures and as mixtures of two species. We observed strong antagonistic effects on decomposition and N mineralization when tamarack was present in the mixtures. Most likely, differences in litter chemistry between tamarack and the other two species caused these antagonistic effects. Our results indicate that in mixed forests decomposition and N mineralization can be much less than would be expected from the individual species alone. These antagonistic effects are important for predicting C and N cycling in boreal forests, particularly when changes in tree species composition (e.g., due to climate change) occur.
Technical Abstract: Mixtures of litter from different plant species often show non-additive effects on decomposition and net N release (i.e., observed effects in mixtures differ from predictions based on litter of the component species), yet mechanisms behind these effects remain unclear, with positive non-additive (i.e., synergistic effects) being most common. Compared with studies of litter mixtures, fewer studies have compared species monoculture vs. mixture effects on soil processes. We studied the interactive effects of black spruce (Picea mariana), tamarack (Larix laricina), and white pine (Pinus strobus) on soil organic matter decomposition and net N mineralization in a plantation in northern Minnesota, USA. The trees were planted in monoculture and in all three possible two-species combinations (mixtures). We sampled soil to 20-cm depth 10 years after the trees were planted and incubated samples in the laboratory. Soil organic matter decomposition and net N mineralization were significantly lower in mixtures with tamarack than would be predicted from the monocultures of the two component species. Possibly, mixing of lignin rich litter from black spruce or white pine with N rich litter from tamarack suppressed the formation of lignolytic enzymes or formed complexes highly resistant to microbial degradation. These antagonistic effects on soil C decomposition and net N mineralization in mixtures with tamarack did not result in reduced aboveground biomass in these plots after 10 years of growth. However, antagonistic effects on soil C decomposition and net N mineralization were large, and if persistent could potentially affect long-term forest productivity and dynamics in low tree-diversity boreal forests.