Impact of soil chemistry and diet on rates of calcite production by the earthworm Lumbricus rubellus.

Earthworms excrete tiny calcium carbonate granules (CaCO3, calcite) up to 2mm in diameter. Charles Darwin was one of the first people to notice earthworm granule production but due to the remarkably small amount of research work that has been done on granules reasons for their production remain a mystery. Regardless of why they form granules have great potential in environmental science. They are found almost ubiquitously in soils. Currently there are ongoing projects looking at their potential for carbon 14 dating (Canti has dated calcite granules and moss from a buried soil surface at Silbury Hill and obtained the same dates from both), amino-acid dating (Penkman, York University 'Amino-acid racemization in calcite: Dating the Pleistocene' Welcome Postdoctoral Fellowship) and U/Th dating (U replaces the Ca in the calcite) (Candy, Royal Holloway College 'A new approach to Quaternary terrestrial chronologies: U-series dating of earthworm calcite' IP/862/0505, NERC ISOTOPE GEOSCIENCE FACILITIES). Granules concentrate at soil surfaces so Canti has speculated granules could be used to identify fossil / buried soil surfaces in complex stratigraphic sections. He also estimated calcite production rates of c. 2.2 mg/day/earthworm, equivalent to 3% of the CO2 flux from soil. If these rates are correct and granules are long lived then granule production could constitute a newly recognized, highly significant part of the soil C cycle and even play a role in atmospheric CO2 regulation. How can we realize the incredible potential of earthworm calcite granules? At present we have lots of unanswered questions about their formation and longevity. We seek to answer the basic questions, what controls their production rate and how long do they last in soil? Our study will focus on Lumbricus rubellus. It is very common in the UK, occurring in soils ranging from acid (pH 3.8) to alkali (pH 8). Preliminary work shows this earthworm, together with L. terrestris, is responsible for producing the majority of granules in soils. To determine production rate we will keep earthworms in granule free-soil and extract and weigh granules after fixed time periods thus calculating production rates of mass of calcite produced / earthworm / time. We will use artificial soil in order to guarantee it is initially free from earthworm granules. The soil will be sieved to <250um so that we can extract granules >250um over the course of the experiment by sieving the soil again. The <250um material will be collected, made into thin sections, and examined with a petrographic microscope to quantify granules produced that are <250um. Soil chemistry will be altered to give high and low pH and Ca content to see how this effects production rate. Ca content of food will be controlled by feeding the earthworms on filter paper soaked in different strengths of Ca nitrate solution. Results from the variable soil chemistry and the variable food chemistry experiments will allow us to determine how soil chemistry and food chemistry affect granule production rate, how granule production rate varies between different earthworm individuals and how it varies over time (we will run our experiments for a year, sampling every 2 months). Once we have a collection of granules from these experiments we will investigate how long they last in soils. We will carry out flow through reactor experiments, dissolving granules in acid and monitoring pH change and increase in Ca concentration to determine dissolution rates. These rates will be compared to data in the scientific literature for calcite dissolution. We will also put granules in soil columns and monitor dissolution through the change in calcite granule weight over time. Results from these experiments will allow us to predict dissolution rates and longevity of the calcite granules to determine whether they are likely to be sufficiently long lasting for their isotopic analysis to be of use in palaeoenvironmental studies.