Perennial Forage Polycultures and Organic Amendments Drive Soil Carbon Sequestration and Organic Matter Stabilization: Results After 90 Years of Management at the Breton Plots
Creators: Achtymichuk, Jamin N.
Publisher: name: University of Alberta Library
Year: 2024

<p>Increasing agricultural intensification threatens to strain already degrading global soil resources while simultaneously escalating anthropogenic greenhouse gas emissions. Loss of soil organic matter (SOM) from agroecosystems is a major cause of soil degradation and a source of considerable carbon (C) emissions. By corollary, agricultural practices that increase SOM can improve soil productivity and sequester atmospheric C. However, SOM responses to land management can take decades to become apparent, necessitating long-term experiments to accurately identify them. This project aimed to quantify how diverse agricultural management practices impact soil health and agroecosystem sustainability and, more specifically, SOM accumulation and persistence and, in turn, soil C storage. To address these questions, soils from various long-term agricultural treatments from the Breton Plots (est. 1929) were evaluated, including manure amendment, conventional fertilizer application, alternating fallow, continuous and rotational cropping with and without perennials and legumes, no-till, and green manuring. This project explored the impacts of these practices on soil C stock and microbial necromass, along with various physicochemical properties important to soil health and agroecosystem sustainability. Soil C stocks down to 90 cm below the soil surface were evaluated and contrasted between different management practices. Further, the monetary values of C stock differences were quantified using the Government of Canada’s current minimum price on C pollution ($80 CAD per tonne carbon dioxide equivalent). Following up on this, soil microbial necromass in the top 7.5 cm of soil was estimated, and relationships between necromass and soil C and nitrogen (N) pools, along with various soil physicochemical properties in three soil particle size and density fractions, were examined. Fallow was found to cause severe soil degradation in both the topsoil (i.e., 0-15 cm depth) and subsoil (&gt; 15 cm depth) through C and N loss in all measured soil fractions, as well as SOM destabilization from a reduction in the proportion of fungal necromass and generally limited microbial necromass accumulation. Conventional fertilizer application consistently increased soil acidification and bacterial necromass C accumulation. Fertilization also increased topsoil C and N when paired with continuous cropping but not when paired with fallow. In contrast, manure application consistently raised soil pH and N levels while increasing C stored in mineral-associated organic matter fractions and all necromass pools. Similarly, perennial grass-legume forages increased C and N stored in all soil fractions and necromass pools in direct proportion to how long forages were consecutively grown within a given cropping system, with soil C enrichments reaching well into the subsoil. No-till generated an extremely C-rich uppermost soil layer enriched in fresh plant C residues, with a high proportion of SOC stabilized in fungal necromass. On the other hand, legume green manuring appears to have contributed to C accrual in both the topsoil and subsoil while favouring bacterial necromass accumulation in the surface soil layer. In total, compared to the annual cash crop system lacking fallow, the continuous forage system and the green-manure-inclusive eight-year rotation led to more than $5000 CAD/ha of additional C storage, while fallow reduced C storage by more than $11,000 CAD/ha. Finally, microbial necromass was found to be a strong predictor of overall soil organic C, with fungal necromass contributing more to predictive power than bacterial necromass. These findings demonstrate that soil health, fertility, and SOM accumulation and stability benefit from increased organic matter inputs, whether from increased crop biomass inputs or organic amendments like manure. Further, these benefits translated into considerable monetary sums under current C valuation practices.</p>