From Mud to Mortar: 2021 Climate Intern Update

2021 Climate Intern, Paula Sternberg returns with a follow-up blog to tell us more behind the scenes science stories. This time, Paula shares the detailed process of going from a muddy field into a highly precise lab as part of her efforts to gather data on mangrove soils.


Versión en español al final.


Paula Sternberg grinds mangrove soil with a mortar and pestle.

The weight of a fly. That’s about how little soil is actually needed to determine carbon composition in mangrove forests. Alright, that’s a bit of an exaggeration, but for each sample taken, processed, and analyzed, only about 9 milligrams of the original sample are needed to determine the percent carbon in the soil. But, what exactly needs to happen before a computer regurgitates fancy numbers that tell me how much organic carbon is in my soil sample? Allow me to explain!

As soon as we’ve returned from the field, our soil samples must be fully dried in a specialized oven that is set to 60°C until constant weight is achieved. Once the samples are dry, we make sure to weigh the sample in its jar. The samples are then individually ground and sieved through a 500 micron mesh. This is one of the most time-consuming parts of the processing (Photo 1). Before returning the samples to their jar, the empty jar is weighed to determine the sample weight, which is just a simple subtraction (full jar - empty jar = sample weight).


Matthew Costa weighs the samples after the acidification and drying process.

For this study, we are only interested in percent organic carbon which—for our samples—is composed of peat, rooty material, leaves, and other plant residues. However, soil naturally contains both organic and inorganic carbon, the latter of which (at least for our samples) is mostly found as calcium carbonate in shell and mineral residues. Because of this, we must ensure that all inorganic carbon is removed prior to the soil analysis, as we do not want both organic and inorganic carbon to be reflected in the percent carbon results.


Titanium tins containing 9mg of sample are placed in a 96 well plate and are ready to be analyzed. Arrow points to a compacted tin with soil.

To remove the inorganic carbon we are not interested in, we must acidify our soil samples. First, we separate our sample into small vials of about 300 mg of soil + water, which are placed in a desiccator with hydrochloric acid (HCl). The samples are fumed for 72 hours in this acidic environment. After they are flushed with air to clear away any residue of HCl gas, we dry and weigh the samples once more (Photo 2). We have now successfully removed the inorganic carbon and are ready for the next step.


Finally, we can begin preparing our samples for the gas chromatography analysis. We begin by weighing about 9 mg of sample, which is placed in a 9 x 6 mm titanium tin. Then, we compact that small tin into a cube (Photo 3). After repeating this for every sample (52 times in my case), the samples are ready to be analyzed!


We shipped our samples to the UC Davis Isotope Analysis lab, which will soon tell us the organic carbon contents of our sampled soils.

 

El peso de una mosca. Esa es la cantidad de sedimento que se necesita para determinar la composición de carbono en los bosques de manglar. Bueno, eso es una exageración, pero para cada muestra que se toma, procesa y analiza, solo aproximadamente 9 miligramos de la muestra original son requeridos para determinar el porcentaje de carbono en la tierra. ¿Pero, exactamente qué debe ocurrir antes de que una computadora me regrese números complicados que me indican cuánto carbono orgánico hay en mi muestra de sedimento? ¡Déjame explicarte!