Summer on a Shoestring


Soil core installation, summer 2013
July 23, 2013, 4:17 pm
Filed under: Roots

By Brannon Barr

 

Question:  How do you determine if a particular soil component has an effect on root growth?

Answer: Soil coring.

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After working under the direction of Melany Fisk and Shinjini Goswami, I got all the inside dirt on this procedure.  It is done by removing and amending a column of soil, reinserting it, waiting a specified length of time, and re-extracting the column to evaluate whether the amendment encouraged root growth.  We are interested in the conditions under which a mineral called apatite (Ca5(PO4)3(OH,F,Cl)) may serve as a source of calcium and/or phosphorus to plants, and so our amendment was the addition of apatite.  For the materials in apatite to be accessible to plants it must be chemically weathered, which plants cannot do on their own.  This task is performed by symbiotic fungi that colonize plant roots (known as mycorrhizal fungi).  In addition to being able to chemically weather rock, these fungi increase the water and mineral absorption capabilities of the plant roots.  In exchange for these services, plants supply the fungi with photosynthetically derived carbohydrates such as glucose and sucrose.  It may be predicted that when calcium and phosphorus are scarce in mineral soil plants will put more effort into mechanisms for acquiring these nutrients, such as the production of enzymes that break down phosphorus containing compounds in the soil and the growth of root structures to provide more area for mycorrhizal fungi to colonize.  If calcium and phosphorus are freely available in the soil, it is more advantageous for plants to invest their effort elsewhere.

Melany installing a soil core using a high tech "pusher outer."

Melany installing a soil core using a high tech “pusher outer.”

We collected soil by driving PVC pipes 20 centimeters into the mineral soil and then pulling them back out.  The soil was then pushed out of the PVC pipe with a smaller piece of PVC pipe (an “extruder,” to use the fancy term, or a “pusher-outer,” to use the really fancy term).  Forest floor was collected off the top, and the 0-10 and 10-20 centimeter depth portions were separated.  These were then sifted to remove rocks and roots.  In order to assure that the cores were all the same except for the addition of apatite, for each experimental plot all the 10-20 cm soil was combined and homogenized, as was all of the 0-10 cm soil and all of the forest floor.  The soil was reinstalled by adding 5 cm of the 10-20 fraction, lightly pressing with the extruder and adding a little water, another 5 cm, another light pressing and adding a little water, repeating with the 0-10 cm fraction, and topping it off with some forest floor.

Freshly extracted forest soil.  The upper half of the sample is noticeably darker due to higher organic matter content.

Freshly extracted forest soil. The upper half of the sample is noticeably darker due to higher organic matter content.

For every site in which we installed cores, two were enriched with apatite and two were left untreated to give us a baseline for root growth.  We installed cores in our calcium treated plots and their corresponding control plots (no treatment).  We will re-extract the cores in a year to evaluate the extent to which new roots colonized the soil we inserted.  Using a full factorial design, Melany and Shinjini will attempt to tease out to what extent and under what conditions apatite serves as a source of calcium, phosphorus, or both together.

Shinjini giving the soil a thorough mixing.

Shinjini giving the soil a thorough mixing.

This process was certainly not without its challenges.  I’m glad I did this before joining the rest of the crew with fertilizing, because fertilizing was a breeze in comparison.  It requires finding just the right spot; sometimes we would drive the PVC pipe in part way and it would come up against a root or a rock that is a complete impasse, so we would have to use the extruder to push it back into place, find another spot, and try again.  Of course, a power corer could cut right through these impediments, but for this study we would not want to cut through rock because it could alter the mineral content of the soil.  So we had to just do it manually, by pounding the PVC pipe corers into the ground with mallets.  About half the time, the pipes were very difficult to pull back out.  We could only twist them to loosen; side to side action would have distorted the hole.  At times I felt like I was trying to pull Excalibur out of the stone.  Also, there was no consistency with how much force it took to push the soil out.  If it was rather loose, the soil was liable to fly out into a big pile and getting all mixed up.  More often, though, it was very hard to push out, and sometimes we had to pound on the end of the extruder with a mallet, taking care to keep the other end still.  Great pains had to be taken to disturb the forest floor as little as possible, or we risked drawing Melany’s wrath (JK Melanie, you were quite pleasant to work with).  The humidity was intense on those days, and the mosquitoes were so awful it was like getting attacked by flying piranhas.  I’ve experienced some vicious mosquitoes in my life, but only in our C8 plot have I encountered mosquitoes that can bite me right through my jeans.  No joke, I had bites all over my lower half.  So I came out of the field feeling like I needed a transfusion, and I could have stuck my clothes to the wall.  The things we do for science!  I look forward to seeing the results.

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