Introduction

One year ago we started Project Carbdown to generate more knowledge about the CO₂ sequestering effects of enhanced weathering with basalt/olivine dust on croplands (Announcement). Since March 2021 we have been running field experiments in Bremerhaven, Fürth (DE), Wageningen (NL) and Larissa (GR), together with extensive lab experiments and analyses - all with synchronized soil, rock materials and methods. The chemical/physical analyses are accompanied with hundreds of electronic in-situ sensors that take measurements every 20-60 minutes.

We recently published a long blog post with our learnings of the first year. These learnings lead to an additional, updated experiment that we will set up in early 2022, the

Carbdown XXL Lysimeter Project!

In year one we learnt that enhanced weathering actually works, but that the effect is very hard to measure in open nature when only 4 kg/m² of basalt dust is used because the “signal” we need to measure is relatively small compared to the “noise” that nature creates. With some effort it can be measured (e.g. extensive lab-analysis or isotope analysis) but our goal is to come up with a cheap method that can measure the effects in-situ right there on the field, preferably with electronic sensors. Only such a method would allow us to massively scale the experiments into hundreds or thousands of fields so we can quickly learn what speeds up or slows down the weathering.

The Problem

Our 2021 experiment setup could potentially lead to a situation where until the end of 2022 we might not have enough reliable data to understand enhanced weathering of basalt/olivine on croplands good enough to provide valid estimates of the actual climate effects. But this is crucial to understand e.g. the economical and chemical mechanics which will ultimately decide about the fate of enhanced weathering becoming an actual tool to fight against the climate crisis.

What we need

We need an additional experiment that conceptually sits between the “dead soil in the lab” experiments and the “maybe too much nature” experiments while still connecting both sides so learnings can be extended either way. We will be using synchronized soil, location, materials, methods, analyses as before, so relations can be drawn.

What we expect

Compared to lab experiments with “dead soil” (0,5%-1% of the CO₂ capturing potential per year, see our blog post), we expect up to an order of magnitude (5-10x) more weathering rate in soil under German conditions, because…

… in the upper 20 cm of the soil the CO₂ concentration in Germany should be at least 10x higher than in ambient air (0,4%) due to bioactivity of plants/roots etc.
… studies suggest that fungi, bacteria or worms can speed up the weathering process (due to faster disintegration of basalt rock particles).

What we want to achieve

Proof that EW works >5-10x better in soil with fungi/fauna/flora than in lab experiments with “dead soil” (and that we can measure this).
Coming up with measurement series (“Characteristic curve”) used later as base data for models of the CO₂ collection at x t/ha of basalt and various other metrics.
Proofing that electronic sensor measurements can act as a proxy for the chemical/analytical metrics from the lab.

The Experiment

We will custom-build 20 extra-large lysimeters (300 liters volume), we will fill them with homogenized soil and we will dig them into a field in Fürth, Germany, in spring 2022. They will remain there for at least 2 years.

For the first 6-8 weeks they will all remain untreated (while we wait for soil and sensors to settle down/accommodate). Then 5 lysimeters each will be treated with the equivalent of 100 t/ha, 200 t/ha, and 400 t/ha of Eifelgold basalt. The other 5 lysimeters will remain untreated controls. Together these data points should give us data for a CO₂ collection curve over 0/100/200/400 t/ha.

Every 2-4 weeks pore water samples from the bottom tank of the lysimeters will be collected and analyzed for TA, ion-balance, DIC, DOC, isotopes, etc. Electronic sensors will be measuring pH, electrical conductivity, moisture, temperature, on 3 levels inside the lysimeter and in the tank plus the tank’s water level, CO₂ in the tank’s air and environmental data (e.g. rain) every 60 minutes.

Based on the liquid levels in the tanks the lysimeters will be irrigated when necessary to make sure that there is always enough water to take samples every 2-4 weeks (during the last summer this was a problem with the non-irrigated, old lysimeters).

The XXL-Lysimeters

We had already built our own, smaller lysimeters last year. This year we have made them bigger, cheaper and easier to build, a great “Version 2”. Here is a concept drawing:

The lysimeters are based on simple, cheap water collector tanks/buckets that can hold about 300 liters of volume. The upper ~60 cm will be filled with soil from the place where the lysimeters are placed (we will homogenize the soil for all 20 lysimeters). In this process we will try to maintain soil horizons as well as roots, seeds, and fauna/flora as much as possible.

At the bottom of the lysimeter there is a tank (height about ~30 cm, volume about 100 liters) that collects the water after it goes through the soil (washing out the seeked-for weathering products on the way). In each lysimeter there will be multiple sensors transmitting measurements every 20-60 minutes wirelessly via LoraWAN to a central data processing and recording system in the cloud.

The electronic sensor measurements are:

Soil pH, soil EC (electrical conductivity), soil moisture in the center of the soil at depths of 30 cm and 60 cm (=4 hardware sensors, from vendor Dragino)
Water pH and water level in the bottom tank (same pH sensor and EC sensor as above, from vendor Dragino) plus a CO₂ sensor and a surveillance camera (in some lysis).
Weather station data (rain, solar radiation, wind, etc.) for the field and irrigation water volume.

4-6 weeks after planting the lysimeters into the soil we will add Eifelgold basalt from Basalt Union and mix it into the top 15cm of soil.

Chemical lab analyses

The local environmental lab AIR Analytik Institut Rietzler GmbH, Fürth, will collect and analyze the water samples every 14 days for - among others - pH, EC, total alkalinity, various anions and cations, and ion-balance. Additional lab analyses like isotope analysis will be performed by other laboratories.

Irrigation

We will use a consumer-grade drip-system from Gardena for irrigation. One or two drip pipes will be put on top of the soil of the lysimeter. The water will only go into the lysimeter. We will measure the total amount of water for each lysimeter so we can calculate the ratio between rainwater and tap water (higher pH). An irrigation computer will turn on/off the drip system in the early morning.

Cost Estimate

The building parts for each lysimeter cost about €50 (the bucket is less than €40), the local brewery Tucher donated the “Grünerla” beer crates for free (thanks!). The sensors are about €700 per lysimeter and the irrigation parts are about €30 per lysimeter. The most expensive part of this experiment is the lab cost of about €30,000 per year.

It’s exactly the lab costs that make such experiments so expensive and hard to scale. That’s why we want to get rid of lab analyses for future large-scale enhanced weathering experiments!

We will publish results here in the blog as soon as we have something to report! Follow us on Twitter!

First Test of Prototype

We put a surveillance camera into the protoype and could actually watch the first water dripping into the tank.

Introducing the Carbdown XXL Lysimeter Project