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 (A_e) 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 B_f 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 A_e to variable extents (pictured left). Therefore, the amount and types of SOC in the evolving plough layer (A_p) 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 link 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, A_e, B_f), 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 the start of the experiment 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 extant SOC and the formation of new SOC from these C sources across differently stabilized pools? What management options favour stable SOC formation?
Will the application of biochar favour the formation of the stable mineral associated organic carbon (MAOC), and does this differ with amendment type and conversion mode?
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?
If applying organic fertilizers results in SOC leaching below the agronomic layer, do these subsoils, sandy and low in SOC, accumulate MAOC with minimal positive priming? How do these responses differ with organic amendment and crop type and contrasting field histories?
Does organic amendment and plant biomass addition trigger negative priming of exogenous C or positive priming of native SOC, and is this more dependent on the quality of C inputs or field history?

Close-up of soil from a recently cleared field

New Ap with B interspersed with Ae (scale in cm)

Hypotheses

Given the low SOC post-conversion, adding organic fertilizers may lead to minimal respiratory loss of extant SOC through positive priming, i.e., the decomposition of native SOC triggered by the addition of exogenous C; initially, mineralization of added C primarily contributes to losses. Instead, organic fertilizer addition in such infertile soils, sandy and likely far from SOC saturation, may cause C accumulation through negative priming or adsorption of soluble C (conversion mode-dependent). However, with limited capacity to store SOC in the more stable SOC pool as MAOC, these efforts should be balanced with crop nutrient budgeting to avoid the buildup of SOC in the more vulnerable particulate organic carbon pool. Biochar application, whose benefits to soil fertility have been calibrated for these soils, may favour the formation of MAOC.

Deep soil work across BioSoil North sites

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. As an add-on to the current BioSoil North trials, a paired plot design was employed to determine the impact of conversion from forest to each of the contrasting field histories on SOC up to 1 m depth. In the final year of the project, the same sampling approach will be carried out within the existing experimental design to determine if the tested management practices have measurable effects on SOC pools and dynamics below the agronomic layer.