Anastasia's Question

Anastasia said...
HI Kim
This study was pretty interesting. It never occurred to me that precipitation could affect carbon and nitrogen in the soil and in the plants themselves. Very cool.My question is why did woody plant invasion on drier sites increase the amount of soil organic carbon and soil organic nitrogen then on wetter sites? I would think that woody shrubs would increase the amount of nutrients in the soil because they would shelter the soil from the rain and grab nutrients before they are leached out.It seems like underground biomass would have probably affected the results. How do you think they would change the findings??Anyway, nice work Kim.

Here's my Reply:

Hi Anastasia

Thanks for your great question. The answer is a bit complicated, but I’ll do my best. (As it turns out underground biomass is very important. )

The change from grasses to woody plants causes a dynamic shift in the location and amount of carbon fixed by the plants due to their physiology and their ability to fix carbon at varying moisture levels.

Grasses are well adapted to grow in moister conditions and they fix carbon not through the growth of their leaves and stems, but rather through the input of the continual growth and die-off of their many fine roots. The root input into the soil is considered to be soil organic carbon, and isn’t something we usually see or harvest.

Woody plants and trees are well adapted to grow in drier conditions and fix carbon in the form of biomass (wood) in their stems and branches, but not as much in their roots. The carbon they fix as soil organic carbon from the growth and die-off of their fine roots, as well as the input of organic carbon from their fallen leaves or needles is relatively small compare to the carbon they fix as wood.

On wetter sites the grasses have ample water and can fix more carbon through root input into the soil than a tree could fix carbon by wood growth in the same conditions. On drier sites however, a tree can still fix carbon by growing wood, but grasses couldn’t produce the same volume of root input soil organic carbon because they don’t have enough moisture.

As far as nitrogen is concerned, woody plants would input more nitrogen on a dry site than grasses because the woody plants can input lots of nitrogen from their fallen leaves and needles. In dry conditions grasses can’t match the leaf inputs of woody plants.

Also, there probably won’t be much leaching away of nutrients on a dry site due to the low levels of precipitation.

I hope that answers your questions.

Kim P.