Soil scientist, Paige Stanley, has released the last chapter in her five-year research into adaptive multi-paddock grazing systems.
Her paper – Amping up soil carbon: soil carbon stocks in California rangelands under adaptive multi-paddock and conventional grazing management – was published last week in the International Journal of Agricultural Sustainability.
Stanley, from University of California, Berkeley, worked with Leslie Roche from University of California, Davis, and Timothy Bowles, also from Berkeley.
In their paper the researchers write that, “Adaptive multi-paddock (AMP) grazing is gaining attention for its potential to increase soil organic carbon, yet its efficacy on arid and semi-arid rangelands remains debated. Given the adaptive nature of AMP, on-ranch studies are essential for measuring its applied outcomes.”
To assess AMP’s impact on Mediterranean California rangelands, the team intensively sampled eight working ranches in northern California, collecting 1440 soil samples and 2,400 vegetation point intercepts, to evaluate ecological outcomes of AMP and conventional grazing systems in northern California.
This was the largest and most comprehensive single assessment of soil organic carbon (SOC) on California rangelands to-date.
The team incorporated many current methodological gold-standards (outlined in detail in the paper), including using improved equivalent soil mass methods to more reliably calculate soil organic carbon stocks compared to traditional bulk density methods, fractionating SOC into functionally distinct pools to assess SOC persistence, and incorporating deep soil sampling to more accurately capture SOC response to grazing at-depth.
Three AMP ranches had significantly greater soil organic carbon stocks in surface soils (17 per cent greater SOC at 0–10 cm), and two had greater SOC stocks to 100 cm (32 per cent greater), compared to conventional ranches.
The largest soil organic carbon differences occurred in the mineral-associated organic matter fraction, suggesting longer-term carbon storage.
While plant community composition did not differ significantly, AMP ranches, on average, had slightly less bare ground, greater live plant cover, and two sites had 82 per cent greater perennial grass cover.
The team writes that, “Based on our intensive sampling, our results suggest that AMP grazing systems may increase soil carbon stocks in certain contexts, especially in mineral associated organic matter (MAOM) fractions that persist in soil for longer than other forms of SOC; however, it is clear that other management and ecological factors play a large role in determining SOC given that differences were not consistent.
“Since both conventional and AMP ranches contain substantial stocks of SOC, research is needed to improve our understanding of how grazing impacts SOC over time and across systems, including across different ecological conditions.”
Stanley highlighted the key takeaways. She said that while AMP ranches generally do have more soil organic carbon than their conventionally grazed counterparts, the results varied by site. AMP grazing in California was most impactful on ranches with a larger SOC deficit to start.
She said that SOC differences were most prominent in surface depths, but several AMP ranches had greater SOC all the way down to 1 metre, which she said, highlighted the importance of things like deep perennial roots.
Several AMP ranches had significantly greater mineral associated organic matter – the kind of SOC that’s typically more persistent – though this wasn’t the case everywhere.
Stanley said that, “These data suggest that AMP grazing does hold promise for SOC accrual even in California rangeland systems, which have been historically thought to be too challenging for this to work because of precipitation variability, scale, etc.
“However, it seems like there are some site specific challenges that limit SOC accumulation from AMP grazing, such as soil texture. Interestingly, coastal vs inland didn’t seem to be a driving difference on the impact of AMP on soil organic carbon – so yes, it can work even in dry places.”
Stanley said that the work had led her to think that nitrogen dynamics could be a driving force on how AMP helps accrue soil organic carbon. She said, “AMP ranches in this study generally had more mineral associated organic matter carbon, more soil nitrogen in some fractions, and lower carbon/nitrogen ratio in the dissolved organic matter fraction – all of which are nitrogen-reliant ways of soil organic carbon accumulation.”
Stanley concluded that, “Like all work, there are limitations/caveats. This study was a point-in-time comparison, which limits our ability to ensure that differences were driven by grazing over time.”