Wednesday, May 2, 2018

Results of our "Stoichiometry Experiment"

It's been quite a while since I've posted about our Stoichiometry Experiment. I explained a bit about that experiment in this post and this post. In this long-term experiment, we wanted to find out how soils in the McMurdo Dry Valleys will change when future climate warming creates a lot of ice melt. Along with that melted water will come a larger amount of important nutrients necessary for life: carbon, nitrogen, and phosphorus. What will the soil microbes and invertebrates do with these extra nutrients? Will the entire area of the Dry Valleys respond the same way, or does it depend on where you are in the Dry Valleys? Our goal in this experiment was to find out which of these nutrient elements might stimulate the soil organisms. Also, because different parts of the Dry Valleys might already have more nitrogen or more phosphorus in the soil than other places, we wanted to find out whether the stimulating nutrient changed from one location to another.

We did this experiment at two different places in the Dry Valleys: near Lake Fryxell and near Lake Bonney. Since the environment in both basins is so harsh for plant life, there's not a lot of carbon naturally in the soil. So, we hypothesized that both sites would be "carbon limited", meaning that if we add carbon to the soil, the soil organisms would be very excited to have the new food source and become a lot more active! Lake Fryxell and Bonney we think are both carbon limited, but we hypothesized that they would differ in whether nitrogen or phosphorus would stimulate the soil organisms. Lake Fryxell has a lot more phosphorus in the soil than Bonney, so we hypothesized that adding nitrogen would stimulate the soil biology more, because they already would have the phosphorus they need. Since Lake Bonney has more nitrogen than phosphorus, we hypothesized that adding phosphorus would stimulate the biology more, because they had the nitrogen they needed.

In the map at the upper right, the "B" marks Lake Bonney and "F" marks Lake Fryxell. The picture from Lake Fryxell shows how we added the nutrient treatments, using watering cans to sprinkle the salts dissolved in water over the plots. The cone helps prevent the wind from blowing away the water as we're pouring.
We started the experiment 10 years ago, during the 2006-2007 field season. Every year since then, we add water and nutrients to the soil. We added carbon (using a sugar called mannitol), nitrogen (using a salt, ammonium nitrate), phosphorus (using a different salt, sodium phosphate). They were added by dissolving those nutrients in water, and adding the solution to the soil. We also added carbon+nitrogen together, and carbon+phosphorus together. To make sure we weren't measuring just the effect of adding water, we had a control treatment where we only added water with nothing dissolved in it. So that gave us 7 different treatments: just carbon (C), just nitrogen (N), just phosphorus (P), carbon+nitrogen (CN), carbon+phosphorus (CP), water only (W), and a control where we didn't add anything at all (U... for "unamended"). Every year, we added these nutrients and water, and sampled the soil to see how the biology were responding. We measured respiration from the soil by measuring carbon dioxide coming out of the soil. I told you how we measured respiration on the soils back in this post. We extracted all of the nematodes, tardigrades, and rotifers living in the soil, and we measured nutrients in the soil, too.
Measuring respiration at the Lake Bonney experimental plots.

So what did we find? For 3 years, we didn't see any change in the soil at all. The biology didn't respond to the added water or nutrients. But, then, after 3 years, we started to see a difference! Soil respiration was greater in the plots where we added carbon+nitrogen. That means that the soil microbes were limited by both carbon and nitrogen, and having more of those allowed them to be more active. It just took a few years for them to be able to adjust to use the extra resources! It's also interesting, because Bonney Basin was also stimulated by carbon+nitrogen, even though it already had plenty of nitrogen (or so we thought!). We also noticed that the total number of invertebrates and total amount of microbes in the soil didn't change. Only their activity changes. It's similar to the way that having a lot of candy makes you more active and respire more, even though there's still only one of you.

h then leaves the soil. The graph on the left is for the Lake Fryxell soils, and on the right is Lake Bonney. You can see that all of the nutrient treatments are overlapping until year 3. Then, after year 3, the CN (carbon+nitrogen) treatment is higher than the others, meaning carbon and nitrogen together stimulated respiration. It's especially true in Lake Fryxell soils, but also Lake Bonney, where it was a smaller increase.

The increased activity with the nutrient additions means that future warming in the Dry Valleys that melts ice will have an impact on the soil organisms living there. But we also found that the stimulated activity was fairly short-lived, only lasting a few weeks, so it would take sustained pulses of water and nutrients to make a big impact on the soil organisms. A sustained pulse from melting ice is a likely scenario for the future of the Dry Valleys, so our experiment might tell us about the consequences of climate change for these soil organisms.

The results of this study are published in: Ball, B.A., B.J. Adams, J.E. Barrett, D.H. Wall, R.A. Virginia. 2018. Soil biological responses to C, N and P fertilization in a polar desert of Antarctica. Soil Biology & Biochemistry 122: 7-18. doi: 10.1016/j.soilbio.2018.03.025