Sustainable Agriculture Student Research Project

Introduction

What are Peatlands?

Peatlands cover only 3-4% of Earth’s land surface but store up to one-third of the world’s soil carbon. Peatlands are a type of wetland that have accumulated a thick layer of peat. Peat is partially decomposed organic matter formed over millennia. For a wetland to qualify as a peatland it must have a layer of peat at least 40 centimeters thick. 
 

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How do Peatlands Function?

Peatlands are waterlogged for most of the year, which creates an anoxic (oxygen-poor) environment below the soil surface. Decomposition happens very slowly under these conditions, which allows peat to accumulate. The rate of carbon accumulation by photosynthesis outpaces the rate of decomposition in this ecosystem. In other words, peatlands capture carbon dioxide (CO₂) faster than they release CO₂. As long as peatlands remain waterlogged, they act as carbon sinks.

Carbon Dioxide Emissions from Peatlands

Degraded peatlands are estimated to emit 2,000 megatons of CO₂ per year, about 4% of anthropogenic greenhouse gas emissions. Peatlands are drained for industry, agriculture, and urban development. Once the waterlogged conditions are disrupted, peat begins to decompose, releasing CO₂.

Microbial Oxidation

Microbial oxidation is the process driving CO₂ emissions. Drainage exposes peat to oxygen, enabling microbes to consume soil organic carbon to create energy through aerobic respiration. This process takes sequestered soil organic carbon and produces CO₂ and water:

Soil Organic Carbon + O₂ → CO₂ + H₂O

In undisturbed peatlands, carbon sequestration by photosynthesis surpasses carbon loss by microbial oxidation. However, once peatlands are drained, the rate of microbial oxidation overtakes photosynthesis, and peatlands begin to emit more CO₂ than they capture. In other words, they become a carbon source rather than a carbon sink. Protecting the massive amounts of carbon sequestered in peatlands is imperative to meeting global climate targets.

Soil Respiration

Soil respiration is a measure of CO₂ emitted from the soil surface. Soil respiration is a proxy for the decomposition of organic matter through microbial oxidation. For peatlands, it is an indicator of the rate of carbon loss.

• High soil respiration suggests fast decomposition and higher CO₂ emissions
• Low soil respiration suggests slow decomposition and better conservation soil carbon

By measuring soil respiration, researchers can monitor and assess the effectiveness of carbon sequestration efforts in peatlands and evaluate management practices that aim to reduce emissions.

Agriculture on Peatlands

When peatlands are drained and cultivated for agriculture, they make easy-to-work soils called muck soil. Muck soil is ideal for growing high value vegetable crops like carrots, onions, and lettuce. Unfortunately, agricultural management practices (drainage, cultivation, fertilization) accelerate microbial oxidation on former peatlands, leading to increased CO₂ emissions.

Most agricultural soils are mineral soils. Mineral soils contain less than 20% organic matter. In contrast, organic soils like peat, are composed of 75% organic matter or more. There is a lot more organic matter for soil microbes to consume in organic soils. Field measurements have shown that soil respiration does not seem to plateau or stop with increased disturbance, even as heavily degraded peat approaches the organic matter content of mineral soil.

Previous research suggests that covering peat with a layer of mineral soil could lower CO₂ emissions by protecting the peat below. This approach limits O₂ exposure to the native peat and allows it to remain waterlogged year-round without disrupting agricultural productivity.
 

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Peatlands in Richmond

In Richmond, a large portion of agricultural land is on former peatlands. The Agricultural Land Commission (ALC) has approved applications for farmers in Richmond to place soil fill to raise fields and improve poor drainage. In such cases, the ALC and City of Richmond require topsoil salvage. In the salvage process, topsoil is stripped, fill is placed, and the topsoil is redistributed over the fill. This strategy exposes peat to O₂ and creates the conditions for CO₂ emissions. This research evaluates whether an alternative management strategy, leaving native peat beneath fill soil, could lower CO₂ emissions.

Objective

• Assess effect soil fill placement on carbon CO2 from peatlands.

Materials and Methods

Study Site: Garden City Lands

This research was conducted at the Garden City Lands in Richmond, BC. Garden City Lands is a fragment of a historically extensive peatland, Lulu Island Bog. Ditching for urban development and agriculture have drained most of the water from this ecosystem, leaving the peat vulnerable to microbial oxidation.

Experimental Design

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• Randomized complete block design with four replicates
• Two factors:
  • Imported soil type: mineral or peat
  • Position: top or bottom
• Four fill treatments (layered over native peat):
  • 80 cm mineral soil
  • 40 cm mineral soil over 40 cm peat
  • 80 cm peat
  • 40 cm peat over 40 cm mineral soil

Layers of imported soil fill were simulated using modified buckets to contain the soil. The bottoms of the buckets were removed to leave them open to the soil below. Each experimental unit was made of two buckets stacked on top of each other.
 

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Measurements

Soil respiration was measured in kilograms of CO₂ emitted per square meter per second using the SRC-2 Soil Respiration Chamber on the CIRAS-3 Portable Photosynthesis System. Measurements were taken seven times between August and September.

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Statistical Analysis

Linear mixed model fit by REML to evaluate the effect of different treatments on soil respiration

• Dependent variable: soil respiration
• Fixed effects variables: position and soil type
• Random effect variables: date of measurement and block
  • Treated as cluster variables to take their effects into consideration

Results

Soil Respiration

• Highest respiration rate: native peat
• Second highest respiration rate: treatments with imported peat as the top layer
• Lowest respiration rate: treatments with mineral soil as a top layer

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Table 1. Mean soil respiration rates by treatment

Treatment	Soil respiration rate
Native Peat	5.40
80 cm Peat	3.59
40 cm mineral over 40 cm peat	1.20
80 cm mineral	1.02
40 cm peat over 40 cm mineral	3.22

Significance

The top layer (mineral or peat) significantly influenced soil respiration (p <0.001). The effect of the bottom layer was less significant (p<0.05). There was no significant interaction between layers (Table 2).

Table 2. Fixed effects omnibus tests for the influence of top and bottom layers on CO₂ emission

There was no statistically significant difference between treatments with the same top layer. There was a statistically significant difference between treatments with a different top layers (p<0.001).

Table 3. Tukey’s HSD post hoc comparison for the difference between treatments

Discussion

The results demonstrate that soil fill placement and soil fill type significantly affect CO₂ emissions from degraded peatlands.

• Exposed native peat: high soil respiration, supporting the notion that degraded peatlands emit CO₂
• Imported peat as a top layer: lower soil respiration than exposed native peat, suggesting it provided some protection
• Mineral soil as a top layer: lowest soil respiration, supporting previous research that found a layer of mineral soil can protect native peat and reduce CO₂ emissions

A mineral soil top layer over native peat effectively reduces CO₂ emissions compared to a peat top layer. The presence of either top layer decreases emissions relative to exposed, degraded native peat.

Conclusion

Strategies that reduce CO₂ emissions from peatlands are critical to meeting global climate targets. The carbon stored in peatlands is thousands of years old, and releasing into the atmosphere would certainly exacerbate climate change. Healthy peatlands are active carbon sinks. Conservation and restoration should be a priority.

Garden City Lands is surrounded by urban development. It is a multi-use site with space for agriculture and recreation. It has areas with potential for restoration. In areas that are dedicated to agriculture the best practice would be to layer soil fill over native peat.

The ALC and City of Richmond require topsoil salvage regardless of soil type, which means salvaged peat soils are placed on top of soil fill where they are vulnerable to microbial oxidation. Layering soil fill on top of native peat, especially mineral soil, significantly reduces CO₂ emissions. This approach offers a quick solution to reduce CO₂ emissions in Richmond.
 

Raw Data