When your study organism fights back...

I explained back in this previous post about how important decomposition is for an ecosystem. Without the recycling of dead plants through decomposition, new plants wouldn't have enough nutrients to grow! It's such an important process that scientists have been studying it for decades. We want to understand what determines the speed of decomposition, and the way that the nutrients are recycled.

Of course, decomposition in the desert can be very different from other ecosystems, because it is very dry and hot, so biological processes (like decomposition) can be much slower. Another reason that desert decomposition can be different is that we have a lot of unique plants that become that "plant litter" that decomposes. Our plants are well-defended against herbivores, which means they are also (often accidentally) well-defended against decomposers! Decomposition studies don't often look at those unique plants. For example, there are only very few studies that have measured cactus decomposition. Even though cacti are an incredibly abundant plant in the Sonoran Desert, we don't know much about their life-after-death role in the ecosystem!

Students in my lab got curious about cactus decomposition, so we set up an experiment. 

Miranda and Ephraim building cactus decomposition cages.

The normal way to study decomposition is using litterbags, like I showed you here. But that's hard to do with a whole piece of cactus, because of the spines that stick out! It's hard to bag a plant that fights back! Ouch! So we built "cages" on the ground made out of the same material that is used for litterbags. These cages trap the cactus in place, so that we can refind each piece to measure its decomposition.

We put two different species of cactus into the cages. Both species are common in the Sonoran Desert. Prickly pear cacti have big, flat segments (called "clades") with large spines but are very juicy inside. Cholla clades are hard and cylindrical, and while they still have soft insides, they have a thicker skin on the outside that might make it hard for decomposers to eat through. So we hypothesized that these two species would decompose at different rates, and release different amounts of nutrients.

The big, flag cactus in the cages on the left are prickly pear, and the skinny, round cactus in the right-side cages are cholla.

Once our fresh cacti were in their cages, we let them decompose for a year. They went from looking like this at the beginning of the experiment...

The holes in these prickly pear clades are from cores that we used to measure their chemical properties at the start of the experiment.

...to this one year later!

Over the course of the year, we collected some of the cacti every few months so that we could track how much mass and nutrients had been released during decomposition. So, every few months, we brought the cacti back into the lab to measure their weight and chemistry:

Guillermo, Coby, and Chase breaking apart a decomposing cholla clade to measure its chemistry.

What did we learn about cactus decomposition? We learned that decomposing cacti recycle nutrients just as well as leaves from deciduous shrubs and trees. We also noticed that cacti recycle a LOT of calcium, way more than other types of plants (almost 10 times as much as other leafy plants!). That's because cacti have a lot of compounds called "calcium oxalates", which means there's a lot of calcium stored in their clades to be recycled when they die. 

We also learned that, despite the differences between prickly pear and cholla, their decomposition is pretty much the same... at least over the first year that we investigated in this study. There are a few differences between the species, though. Prickly pear released more water than cholla, and it also released potassium (an important nutrient for new plants!) faster.

There is still a LOT left to learn about cactus decomposition. We only looked at two species in one desert site, which is just a "drop in the bucket" for understanding how important cacti are in desert ecosystems. But what we do know is that cacti have an important role in nutrient recycling, so it is worth learning more about it!

The results of this study are published in: Bilderback, A.H., A.J. Torres, M. Vega, B. Ball. 2021. The structural and nutrient chemistry during early-stage decomposition and desiccation of in the Sonoran Desert. Journal of Arid Environments. 195: 104636. DOI:10.1016/j.jaridenv.2021.104636