Hi Rowan, sampling variability (sample depth, in-field variability, etc) could be an issue, but if you are running a lot of samples and seeing consistent results then that doesn’t fully explain it. Another common culprit is switching soil labs but I’ll assume that’s not the case.

If I had to guess I would say at 0.4% OM increase per year what you are seeing is a lot of particulate organic matter (residue in various stages of decomposition) building up near the surface. It’s really difficult to add enough biomass to the soil to sustain 0.4% increase year over year because only about ~10 – 15% of the biomass at most will end up as what we would call ‘organic matter’ on a soil test in the long run. The rest ends up consumed by microbes and gassed off as CO2. When you got the stretch of higher moisture, the microbes probably kicked into high gear and sped up the decomposition process and resulting CO2 loss increased. Just a guess but it might explain what you are seeing.

Rough math, there is ~2 million pounds of soil per acre in the top 6″. A 0.4% increase in OM is 8000 lbs of OM added per acre per year. Assuming 85% of the C from the added biomass is eventually lost as CO2, you would have to add over 50,000 lbs of biomass per acre per year (including root biomass) to achieve that. The actual number is lower because you are also adding OM from decomposing microbial biomass with your practices, but it’s still hard to achieve 0.4% per year in most cropping systems.