Research

Background

Integrating forages into crop rotations is known to have many benefits, including weed suppression, increasing productivity, and various soil quality impacts, but the short-term loss of cropland often makes this seem cost prohibitive. There has been little research that looks at the long term agricultural and economic benefits of forage inclusion in different rotations. The Breton Research Plots in west-central Alberta maintain and contrast 9 different long-term crop rotations and amendment treatments that differ from 1 to 8 years in duration and include variable amounts of perennial forage.

These plots date back to as early as 1929 and were established on Gray Luvisolic (grey wooded, low organic matter) soils to address issues of chronic low fertility and productivity. Unlike many other long-term studies where forages are examined as a single treatment, forages are a significant part of at least 5 of the long-term rotations at Breton. These rotational treatments, with comprehensive agronomic and soil data dating back 40 years, provide a unique opportunity to evaluate the effects of forage inclusion on soil properties, including carbon storage and stability, as well as net present values based on historical inputs and outputs.

Objectives

The objectives of this study are to:

1. Assess carbon stock, accrual and stability, across the long-term cropping system treatments of the Breton plots.
2. Evaluate soil microbial biomass and necromass, together with soil carbon respiration and mineralization, in relation to the long-term cropping treatments at Breton.
3. Quantify root biomass and annual carbon enrichment during the growing season in relation to the long-term crop rotation treatments at Breton.
4. Assess soil biomarkers associated with the long-term cropping treatments at Breton.
5. Quantify the net present value (NPV) of agronomic activities and carbon accrual among long-term treatments of the Breton plots containing variable forages.

What they Did

The Breton plots represent an important long-term research site situated on Gray Luvisolic soils in which soil fertility has been a limiting factor for agricultural productivity. This research team resampled 9 different treatments across 36 replicate plots at Breton in 2021 to evaluate differences in soil carbon stock, stability, and associated biological soil indicators. In addition, this team combined the most recent data with long-term soil and agronomic data tracked from individual plots for the period 1980 through 2021 to conduct a cost-benefit analysis of these different cropping systems. The core treatments included varying complexities of agricultural systems (e.g., wheat-fallow, continuous grain, 5 and 8 year-long rotational systems, and continuous forage), which included contrasting soil amendments (none, conventional fertilization, or manure) and perennial forage inclusion (none, 2 years out of every 5, 3 years out of every 8, and continuous forage).

What they Learned

Results of this study revealed a number of important findings. First, it provided clear evidence for the size of the soil carbon stock within Gray Luvisolic soils at the Breton plots, and the favorable impact of select agricultural management practices on soil carbon. More specifically, carbon stock was maximized, and extended to a greater depth in the soil, by an increasing use of perennial forages, relying on manure (livestock and/or green manure) as a soil amendment, and avoiding the use of fallow.

Second, the stability of soil carbon was generally greater in soils receiving these same practices, as represented by an increase in carbon within the more protected mineral-associated organic matter pool, together with carbon in the fungal necromass pool.

Third, several key insights were found on potential mechanisms that may explain the increase in soil carbon stock and stability. These included an increased root biomass at peak growth within plots having continuous forage, and favorable annualized carbon increases within the soil (7-9% from July through October) in plots having continuous cover, either as continuous forage, continuous grain, or the 8-year rotation that included manure and forage.

Fourth, the review of intermittent soil carbon data from the Breton plots revealed distinct increases in soil organic carbon over most or all of the last 40 years, which were most pronounced in the continuous forage and 8-year rotation.

Fifth, economic valuation of the treatments showed two important results. The value of standing soil carbon stocks in 2021 was markedly greater (by more than $5,000/ha) within the continuous forage and 8-year rotations than in the continuous grain (business-as-usual) treatment. Additionally, our net present value (NPV) analysis of annual historical input (cost) and output (return) data from the Breton plots indicated that the greatest annualized returns were consistently generated by the 8-year rotation. The continuous forage system provided generally positive returns, but these were closely tied to variation in carbon and commodity pricing.

What it Means

The net present valuation of the different cropping systems examined varied widely, depending on both the value of commodities produced (e.g., forage), and whether carbon valuation was included, and at what projected pricing. Results of this study provide compelling evidence for the importance and need to maintain agricultural practices that support soil organic matter inputs, in order to increase soil carbon stock and stability. Equally important, it highlights the significant economic value associated with soil carbon valuation in agricultural soils. In turn, this work lays a foundation supporting the need for novel future land-use policy changes that reward agricultural producers for the adoption of practices that increase soil carbon stocks and stability. These practices can include continuous cropping, use of manure as a rotational amendment, and the partial or prolonged use of perennial forages.

This project is also funded by the Beef Cattle Research Council, Results Driven Agriculture Research, Sustainable Canadian Agriculture Partnership, Western Grains Research Foundation, and the University of Alberta’s Breton Endowment