Above and below ground responses to N and P limitation in mycorrhizal spruce and pine saplings

Abstract

Trees in the boreal region provide enormous amounts of carbon storage in their above ground biomass as woody trunks, as well as below ground in their root systems. Mycorrhizal fungi also act as a carbon store in boreal forests and receive carbon from the host tree in the form of photosynthates. Partitioning of a tree's carbon above or below ground is highly dependent on other nutrient cycles such as nitrogen and phosphorus.

In this study, above and below ground biomass (plant and mycorrhizal) was measured in the common boreal species Pinus sylvestris and Picea abies. A mesocosm pot design was used and a nitrogen - phosphorus limitation gradient created through combinations of fertilization and an iron oxide soil amendment to occlude phosphorus. Ingrowth mesh bags (to capture extramatrical mycelium growth) of either pure quartz, or a phytic acid or apatite amendment were used to evaluate mycorrhizal growth to nutrient patches. Additionally, the nutrient concentration in the needles was measured and a nutrient comparison between metabolically active green needles and red needles was done.

Nitrogen limitation caused decreased shoot biomass, but did not show significant decreases in below ground biomass. Nitrogen fertilization was found to increase both shoot and thick root biomass in spruce and pine. Nitrogen fertilization plus iron oxide amended soil pushed spruce to phosphorus limitation yet this caused no significant differences in the below ground biomass except in the fungal root colonization.

None of the treatment or amendment combinations pushed the pine to nitrogen limitation or phosphorus limitation according to the nutrient concentration in the needles, however nitrogen additions significantly increased the above ground biomass. In spruce, needle nitrogen concentration was positively correlated with plant biomass, and negatively correlated with fungal biomass. In both spruce and pine, the N:P of the needles correlated positively with plant stem biomass, and negatively with fungal biomass. Needle phosphorus concentration did not show a significant correlation with plant or fungal biomass in either species.

The phosphorus mesh bag amendments did not show more mycorrhizal growth in seedlings that were evaluated to be phosphorus limited by needle concentrations. N+P fertilization showed a common trend of having much lower mycorrhizal biomass than other treatments. There were differences between red and green needles in the amount of nitrogen and phosphorus suggesting nutrient reabsorption after senescence. These differences were independent of the nutrient status of the plant.

This study gives insight to the mycorrhizal relationship in Pinus sylvestris and Picea abies on a nitrogen - phosphorus limitation gradient. The above and below ground allocation of that carbon influences long term storage capacity, their interactions between other members of the ecosystem, and the overall health of the forest ecosystem community. As N deposition becomes more common, it is vital knowledge to know how changing nutrient status affects carbon allocation, and therefore the ecosystem as a whole.

Popular Abstract

Trees and fungal symbionts in a changing environment

Trees in boreal regions provide huge amounts of carbon storage as a vital role in the global carbon cycle. Trees store carbon above ground as wood, as well as below ground in the wood-like tissues of the root system. A major determiner in how large these above or below ground portions are, is the availability of soil derived nutrients which are necessary for growth. Within the soil of a boreal forest lies a vast community of microbes living amongst the expanse of tree roots. A particular group of these microbes are fungi known as mycorrhiza, which live symbiotically in a tree's root system. The fungi are safely housed in the tree's roots, yet grow their own root-like structures which can effectively explore and mine the soil up to meters away. These fungi trade nutrients they’ve scavenged from the soil in exchange for sugars from the tree, maintaining a mutually beneficial relationship.

Through human activity, the nutrient composition of forests around the globe is rapidly changing. This can have an effect on the relationship between trees and mycorrhiza, since it is based on the exchange of nutrients. In turn, the growth of both parties can be affected as well. It is important to know how each side responds to these changing nutrients in terms of growth, to predict their adaptability to future conditions.

A greenhouse experiment was carried out to test how changing nutrient availability affects the growth of trees and mycorrhiza. The experiment involved spruce and pine, common boreal trees, which housed mycorrhiza in their root systems. The experiment consisted of groups of pots ranging from nutrient limitation to nutrient sufficiency. The researchers found that the most nutrient limited pots in fact housed more mycorrhiza in their roots, supporting their role as nutrient foragers on behalf of the host plant. They also found that pine were able to outcompete spruce under nutrient limited conditions, possibly implying they could outcompete spruce in a natural mixed forest setting. In order to better understand the mycorrhizal relationship, further experiments are needed which take into account the variation in traits among mycorrhizal species.

Master’s Degree Project in Biology
Department of Biology, Lund University

Advisor: Juan Pablo Almeida
Department of Microbial Ecology

• author: Harris, Molly
• supervisor: Juan Pablo Almeida
• organization: Degree Projects in Biology
• course: BION03 20192
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9041690
• date added to LUP: 2021-03-11 13:30:32
• date last changed: 2021-03-11 13:30:32