A great follow-up question. There are definitely larger-system contexts that would make organic matter content vary. In paddy rice production systems, organic matter retention is much higher with tillage regardless of climate because the permanent flood over the rice cultivation helps retain the organic matter (but also causes more methane production than alternate wet-dry rice systems). And tillage makes rice cultivation so much easier, so I didn’t want to miss mentioning that. Farmer management practices should always be taken into account when promoting GMCCs.
I love a good case study! Soil type would make a huge difference in this scenario in Florida. If the fields have sandy soils, their organic matter content probably hasn’t changed very much in 1 to 2 years (assuming 1 cover crop over the summer months which is typical of South Florida). If they have muck soils or the rocky soils of the southern tip of Florida, they may see a jump of up to a percent or two (which is pretty great but also depends on the species they are using). GMCCs are a great low-cost tool for increasing soil organic matter over time, but results are slow, especially in degraded soils. Poeplau and Don (2015) analyzed 139 different GMCC studies and summarized that over 54 years, the studies showed an annual change of 0.32 Mg per ha (0.893 pounds per acre) of soil organic carbon (average soil depth sampled was 22 cm).
But for the sake of this example, the farmer who incorporated their cover crop most likely has less organic matter in their system compared to the farmer who left their residues on the soil surface (if the cover crop was productive). Residue decomposition and use are increased in hot, humid contexts when residues are tilled into the soil. Tillage also breaks up residues into smaller pieces which increases decomposition rates even faster. High nitrogen content of residues like what many leguminous cover crops have also increases microbial decomposition in soils though I’ve not seen many research articles that look into this component.
Something that maybe needs to be explained further so that this makes sense is the stability of carbon in agroecosystems. Carbon is divided into labial portions (easy to break down) and recalcitrant portions (difficult to break down). As carbon breaks down, some of it remains in the soil, some of it goes back into the atmosphere as CO2, some dissolves in water, and some is taken up into the bodies of organisms in the soil etc. So as we increase the decomposition rate of residues in hot, humid conditions, we also increase carbon use and cycling back into the surrounding ecosystems. I hope this makes sense. Here’s a graphic I made a while back.
Leaving residues on the soil surface is a “best practice” for hot, humid climates for long-term soil health. Also, as a fun fact, where ECHO is located in Florida has a spodic layer. This layer in the soil horizon is where the dry season water table resides and is where much of the organic matter settles as it percolates through our sandy soils. Here’s a picture of what that looks like on our farm:
Poeplau, C. and Don, A. (2015) Carbon Sequestration in Agricultural Soils via Cultivation of Cover Crops—A Meta-Analysis. Agriculture, Ecosystems & Environment, 200, 33-41.