Plant carbon allocation, soil nutrient availability and the mycorrhizal business
The soil market: plants spend some of their carbon on buying nutrients from mycorrhizae. Similar to the way that we take up carbon (C) through the food we eat, plants absorb C in the form of CO2 from the atmosphere through photosynthesis, and use it to build sugars (Fig. 1). After having a good snack, our body decides where to send the C it has just gained: should I use it to grow muscles (or fat), or do I need to burn it in order to work out, now? If we burn this C, we breathe it out as CO2 in respiration. Similarly, plants also decide how to allocate their sugary C, and their ‘duties’ also include growth and respiration.
Whilst we get all the building blocks that we need to grow and live from food, plants have to ‘shop for’ nutrients, such as nitrogen and phosphorus in soil, and this also costs them C. However, nutrients are sometimes unavailable in local shops. One-way plants then obtain the less available nutrients is by making long-term deals with so-called mycorrhizal fungi (mycorrhizae) in the soil. Again, the currency for this transaction is C.
The fraction of C allocated to different processes is referred to as C partitioning. This is expressed as a fraction of gross primary production (photosynthesis), and is a key process that affects the growth of individual plants, as well as whole terrestrial ecosystems. It determines how much of the CO2 taken up by plants will be released back into the atmosphere as CO2, as well as where and how much of it will be sequestered and stored in the ecosystem. This is important for the pathways through which plant C reaches the soil (e.g. as litter, dead roots, root exudates, mycorrhizal biomass) which can strongly influence its fate. Despite its central role in terrestrial C cycling, C partitioning and its relationship with nutrient availability (i.e., the availability and cost of nutrients in the local shops), remains poorly understood. In some ecosystems, it has been shown that only 40% of the photosynthesis ‘budget’ is invested in plant growth, whereas elsewhere it can be up to 70% . Why this variation occurs is still unknown, but evidence is growing that nutrient availability plays a key role here, and business done with mycorrhizae might be a critical determinant of the price of nutrients.
And that is where our research came in. We added some phosphorus to some corn and measured a bunch of things to quantify C partitioning to unravel the role of mycorrhizae in determining the price of nutrients.
Although, phosphorus addition stimulated photosynthesis and plant growth in both experiments, only in the first experiment did we find what we expected. When phosphorus availability was low, plants invested a greater percentage of their C budget in dealing with mycorrhizae, and less in their own growth (i.e., biomass production efficiency often called BPE, decreased). Unexpectedly in the second experiment, biomass production efficiency did not significantly change with phosphorus availability. Instead, plants under lower phosphorus availability invested greater part of their C into root growth and less into aboveground respiration. These contrasting results can be explained by greater C partitioning to mycorrhizae (much more “plant/mycorrhizae business” going on) in the first experiment. In the second experiment, mycorrhizal abundance was so low that we were unable to measure the C allocated to it. Since mycorrhizae were not available for business – probably due to a pH decrease from 7 to 5.8 – plants needed to invest a greater percentage of their budget in root growth to increase their phosphorus range (to reach more distant shops).
Advances in Crop Science and Technology Journal