Does space-for-time substitution capture soil carbon recovery in restored dry grasslands of Canada?

Canadian grasslands are endangered ecosystems, with nearly two-thirds converted to cropland. Restoring cropland to grassland can help reintroduce biodiversity through the planting of native vegetation, increase soil carbon (С) storage, and reduce greenhouse gases through the establishment of perennial plants. However, grassland restoration is expensive and done primarily on private lands. Consequently, restoration must also benefit farmers by providing forage for livestock and creating healthy, resilient soils.

In the past, few grassland sites have been monitored long-term for soil C accumulation following restoration in the Canadian Prairie, despite the slow rate of change in soil C over time, and even fewer have examined deeper soils (beyond 30 cm).

This study addresses the following questions:

1) How do soil organic and inorganic C pools vary with depth and restoration age?

2) Do these sites also provide other important soil functions and support forage production?

To answer these questions, we sampled 18 restored grassland sites across southern Alberta and Saskatchewan, Canada, spanning a chronosequence from pre-restoration (0 years) to 24 years since seeding. Restoration practice involved a one-time seeding of a mix of native and agronomic plant species, along with exclusion from grazing during the early and mid-growing seasons (April-July) in the year following seeding. Sites were characterized based on local climate conditions and soil properties.

Soil samples were collected in May-June 2024 at depths of 0–15, 15–30, 30–60, and 60–100 cm. Soil samples were analysed for organic and inorganic C, moisture, texture, pH, and electrical conductivity. Plant surveys and biomass harvests were conducted in July 2024 to examine community composition and forage production. Climate variables were summarized using the annual heat-moisture index. Soil C and function, and vegetation responses to restoration were assessed using two complementary approaches: (1) chronosequence analysis to test space-for-time assumptions and assess temporal patterns in soil C pools, and (2) AIC_c-based model selection to quantify the relative influence of vegetation, climatic, and edaphic predictors.

Local climate and soil conditions played a dominant role in the rate of grassland restoration and C distribution within the soil profile, while established plant community composition was associated with changes in soil C storage and forage quality. This study provides a robust evaluation of space-for-time substitution for soil C recovery by examining organic and inorganic C responses across the one-meter soil profile using a large set of restoration sites, addressing limitations of previous studies. Together, these results improve understanding of soil and vegetation responses to restoration and provide new information for producers and policymakers supporting grassland restoration management.