Understanding the Effects of Grazing System on Soil Quality

Background

Globally, beef cattle are estimated to contribute 41% of livestock greenhouse gas emissions.  However, beef cattle often spend much of their life grazing on agricultural grasslands and therefore, also contribute to carbon sequestration in the soil, but this is not currently considered in greenhouse gas assessments of the beef industry.  There is a need to consider the impacts of grazing cattle on soil carbon capture to better account for the ability to ‘offset’ greenhouse gas emissions.  In particular, adaptive multi-paddock (AMP) grazing, a type of rotational grazing, where cattle are moved often from paddock to paddock depending on plant growth stage, and forage is allowed sufficient rest periods between grazing cycles, has been shown to increase soil C capture compared to continuous grazing. This has been demonstrated in a few places in North America, however, results can vary based on climate and soil type. In this project, we explored whether AMP grazing can increase soil organic carbon in Eastern Canada and also explored how stable that carbon might be.  Quantifying carbon sequestration under different grazing management systems, should help provide a more accurate assessment of net greenhouse gas emissions from the beef industry. 

Objectives

• Determine the influence of adaptive multi-paddock grazing on soil organic carbon concentrations and soil organic matter quality indicators in Ontario cow/calf and/or backgrounding operations.

What they will do

Sites for the study were selected after an initial screening of beef operations in southern Ontario identified as practicing AMP grazing.  Five sites that were practicing AMP for at least 10 years were selected for further study and were compared with nearby non-AMP pastures.  AMP farms on average kept cattle in one paddock ≤ 2 days.   Soil sampling was completed at all sites between July and September 2021.  Deep cores were collected to a depth of 60 cm or as deep as possible with a modified post pounder. Ultimately, only the top 45 cm of soil from the cores were included in the analysis due to difficulty in reaching 60 cm consistently at all sites. Soil cores were divided into 15 cm segments for further analysis.  Soil was analysed for soil organic carbon (SOC) and total nitrogen.  Soil from the surface layers was separated into particulate and mineral associated organic matter (POM and MAOM, respectively) to help assess soil organic matter quality/stability. Soil microbial communities of the surface soil and speed of water infiltration at the sites were also assessed. 

What They Learned

Higher SOC stocks were found under AMP grazing compared to non-AMP grazing when measured based on an equivalent soil mass (120 Mg C/ha vs. 107 Mg C/ha, respectively). Total nitrogen, which is highly correlated with SOC, was also significantly higher in AMP pastures.  When analysed by depth, SOC stocks were higher in AMP than non-AMP pastures in the 0-15 cm depth and in the 30-45 cm depth (Fig. 3), indicating some ability of AMP grazing management to increase carbon at depth. When SOM was separated into its mineral and particulate associated fractions, greater mineral associated organic matter (MAOM) carbon was found in the surface layer of pastures under AMP management. This indicates greater SOC stability under AMP grazing. This is consistent with the results of other researchers (eg. Mosier et al., 2021), who also found greater MAOM-C under AMP grazing. Abundances of soil microorganisms (detected by phospholipid fatty acid analysis (PLFA) were significantly higher in AMP soil samples for all microbial categories.  These results, along with greater mineral associated organic carbon (which has a microbial origin), point to a strong role of the microbial community in SOC increases in AMP grazing.  While our NMR results did not detect major differences in the quality of soil organic carbon, trends were apparent that support the potential for AMP management to contribute to more stable SOC.  On average, water infiltration was 40% greater in AMP compared to non-AMP pastures, however, differences were not statistically significant as variability was high.

What It Means

This research supports that in southern Ontario temperate pastures, AMP grazing should be encouraged over continuous grazing to increase SOC stocks and carbon stability. Through AMP grazing, the mineral associated organic matter fraction was increased, which is linked with more stable soil organic carbon. It is probable that AMP grazing management contributes to maintaining a healthy root system of pastures because overgrazing is avoided and adequate rest periods for forage recovery are provided.  Greater root abundance would feed the soil microbial population, which is supported by the observed greater microbial abundances under AMP. This project provides critical scientific evidence enabling government policies and programs relating to grazing management and carbon sequestration to be put forward with greater confidence and impact.  Greenhouse gas emission analyses /life cycle assessments of the beef industry should aim to account for carbon sequestered due to grazing in order to better represent net emissions.