Maxwell Locke’s Research

Introduction

Boreal regions store approximately 32% of the carbon in the world’s forests, 60% of which is soil organic carbon (SOC). As agriculture expands Northward into boreal forests through land use change (LUC), marginal soils undesirable for agriculture will contribute to these new developments. Podzols are one such soil type and are acidic and sandy, making them naturally unproductive. These soils form as organic acids produced in the LFH layer (composed of partially decomposed plant litter) bind to metal oxides (i.e., iron and aluminum) in the mineral soil contributing to their translocation from the developing eluviated layer (Ae) and deposition in the underlying illuvial (B) horizon as organo-mineral complexes. Podzols are subclassified based on the dominance of metal oxides (i.e., iron and aluminum) over organic matter and the thickness of the diagnostic B horizon. Humo-ferric Podzols (pictured right) are the main soil for agricultural use in Happy Valley-Goose Bay (HV-GB), including the BioSoil North study sites, with low organic matter Bf horizons, developed on sandy glacial deposits and often bearing shallow cemented metal oxide hardpan layers.

Humo-ferric Podzol typical for HV-GB showing the distinct upper-profile horizonation.

The protocols for converting forest to agricultural land can differ significantly, which could lead to soils with variable initial workability, fertility and productivity, all of which are yet to be quantified in Podzols for agriculture. The first step in conversion is removing trees and other vegetation which occurs through clearcutting or bulldozing, resulting in the retention or removal of the LFH and Ae to variable extents (pictured left). Therefore, the amount and types of SOC in the evolving plough layer (Ap) initially reflect its constituent parent horizons, impacting post-conversion nutrient and SOC dynamics. Time, equipment, the amount and quality of land to clear and local market commitments are some of the factors driving the decision of how farmers convert their fields. Thus, there is a need to assess the diverse conversion and post-conversion management options to support this unstructured decision currently dependent on first-hand experience.

Preliminary assessment of carbon on BioSoil North sites

In 2022, with support from the John and Judy Bragg Family Foundation Applied Research Fund, a preliminary assessment of the BioSoil North study sites was employed to benchmark the status of SOC across a gradient of LUC from forest to agriculture typical for the HV-GB region. Samples were collected from 9 forest reference sites (LFH, Ae, Bf), 6 recently cleared fields (0-20 & 20-30 cm) representing a novel baseline before management and 3 actively managed fields (0-15 & 15-30 cm).

  • Sampling of fields converted about 1 year ago. Red circles are sites sampled from recently converted land and light blue triangles are forest reference sites sampled by horizon.
  • Locations of the mainly organically managed fields, converted about 12 years before sampling by retaining the organic layer. Samples were collected from 8 subplots per field, indicated by the red boxes.

Results summary

  • Approximately 78% of the SOC stock is lost from removing the organic layer (i.e., LFH) during conversion
  • Fields receiving organic fertilizers for 3-11 years (i.e., “managed” fields) have SOC stocks equivalent to the undisturbed forest reference sites
  • In the managed fields, subsoil total carbon and POXC are more than double that in the top and subsoil of recently cleared fields and the B horizon of the forest reference sites
  • Soil mass-corrected SOC stocks for the forest sites, recently cleared and actively managed fields.
  • Total carbon (TC) & permanganate oxidizable C (POXC) for the forest sites, recently cleared and actively managed fields.

Research questions

  • Does the application of diverse organic fertilizers impact SOC formation across differently stabilized pools? What management options favour recalcitrant SOC formation?
  • Will the application of biochar favour the formation of mineral associated organic carbon (MAOC), and does this differ with amendment/fertilizer and crop type?
  • Does the conversion of forest to agriculture impact SOC storage and stability, and how do these impacts differ with conversion mode and change with depth?
  • Does organic fertilizer application result in SOC leaching below the plough layer, with water extractable organic matter (WEOM) as a proxy for soluble C? Do these subsoils, sandy and low in SOC, accumulate WEOM with quality reflecting that of the topsoil with different organic amendments? Can biochar mediate these responses?
Close-up of soil from a recently cleared field
New Ap with B interspersed with Ae (scale in cm)

Hypotheses

  • Higher quality organic amendments favour the formation of MAOC over POC but do result in higher respiratory losses of C. This is reflected in burst respiration profiles, with higher quality amendments showing greater proportional respiration earlier during incubations.
  • However, with limited capacity to store SOC in the more stable SOC pool as MAOC, these efforts should be contextualized with crop nutrient budgeting to balance particulate organic carbon accumulation with more recalcitrant pools that may offer both long-term C storage and sustained fertility.
  • Biochar application, whose benefits to soil fertility have been calibrated for these soils, favours MAOC formation and leads to lower and delayed peak burst respiration.
  • Organic fertilization impacts WEOM amount and quality below the plough layer and changes in quality differ with amendment type. Furthermore, biochar may retain soluble C, thus its application may reduce the transfer of soluble C with depth while reducing the vertical dissimilarity in quality with ammendment and crop type.

Deep soil chronosequence in Cormack

Most SOC stock estimations use samples collected in the agronomically relevant layers (0-30 cm) despite more than half of SOC being stored below them; therefore, if changes occur in deep SOC pools, they often go unaccounted. Interest in these subsoil layers is growing as they might have the potential for long-term SOC storage under management, given their physical separation from more actively cycling surface layers and generally greater potential for mineral stabilization. The impact of LUC and long-term management on SOC in deeper soil layers of boreal Podzols is yet to be assessed, despite being the dominant soil type in these regions. Using a farm-level chronosequence approach (forest, 5, 34 and 60 years) across 0-70 cm soil cores, we evaluated the legacy effects of LUC and long-term management on SOC stocks, WEOM quantity and quality, assessed using absorbance and fluorescence spectroscopy, and burst respiration susceptible C.

More details coming soon…

  • Location of the sites and corresponding field age sampled for the chronosequence
  • Examples of the soil cores extracted from the sites with the time since conversion of each field at the time of sampling. Each photo represents a composite of three cores collected from one of the three plots sampled for each condition
  • Cutting soil cores into 10 cm increments

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