The effect of land conversion from forest to agriculture on soil health indicators in rainy river, Ontario

Abstract

Due to changing climate and shifting crop zones, there is an opportunity for agriculture to expand in northern regions. This will require conversion of forest and scrubland to agriculture. Conventional methods of land conversion from forest to agriculture have resulted in a decrease in soil organic matter (SOM), which contributes to the maintenance of soil structure, water holding capacity, and a diverse microbial community within the soil. This degradation to the soil may result in a decline in soil health. Integrating wood mulch, which is woody material that would otherwise be removed from the site, might help reduce the degradation of soil health by retaining more SOM on site. The objectives of this study are to: 1. examine the effects land conversion has on soil health by measuring indicators identified as responsive to land management practices, and 2. determine if the integration of wood mulch during land conversion can mitigate any declines in soil health. Methods for measuring the soil health indicator ‘wet aggregate stability’ were also explored in terms of sensitivity, cost, training, and time to complete the analysis. In June 2024 agricultural fields (n=27) and forests (n=9) were sampled in Rainy River, ON, to investigate the effect of land conversion on soil health. Eighteen recently (<10 yr) converted fields (9 conventional and 9 integrated with wood mulch) and nine established fields (>50 yr; cleared conventionally) were sampled for this study. Soil health declined with land conversion, but the clearing approaches did not differ significantly from each other. Land conversion, regardless of timing of conversion, significantly reduced wet aggregate stability, carbon mineralization rate and soil moisture in 27 sampled agricultural fields compared to 9 referenced forest sites. Quantitatively, automated wet sieving was found to have the lowest coefficient of variation at 11.2%. This approach for quantifying wet aggregate stability did not show any significant effect of land conversion on aggregate stability. Volumetric aggregate stability tests (VAST) had a coefficient of variation of 17.5% and did show significant effect of land conversion. The SLAKES mobile application had a coefficient of variation of 15.4% and showed a significant effect of land conversion. Post-hoc test results differed for VAST and SLAKES. Qualitatively SLAKES was the most economical method with low training and time requirements. VAST was more expensive than SLAKES and required a similar amount of training as SLAKES but data collection was faster. Automated wet sieving was the most expensive and required more training and time to produce results. SLAKES is recommended for future wet aggregate stability measurements of agricultural soils due to its high sensitivity to changes in treatments of agricultural soils, affordability, speed in results, and low training requirements. Land conversion degraded soil health and the only difference between approaches for conversion was for potential carbon mineralization, which was lower in the mulched soils. Based on the findings of this study, land conversion will degrade soil health regardless of the tested approach taken to mitigate negative effects.