Addition of `M horizon` as a permissible value to `SoilHorizonEnum`

[sujaypatil96]

Current term details Please supply the current details of the term that you would like to update:

Term name - soil_horizon
Term ID - [MIXS:0001082](https://genomicsstandardsconsortium.github.io/mixs/0001082/)

Suggested update(s) https://genomicsstandardsconsortium.github.io/mixs/SoilHorizonEnum/

permissible_values:
  SoilHorizonEnum:
    permissible_values:
      A horizon:
      B horizon:
      C horizon:
      E horizon:
      O horizon:
      M horizon:
      Permafrost:
      R layer:

Additional context As part of the experiments/studies in the NEON projects, there are some soil metagenome datasets, which we are trying to ingest into NMDC. There are soil samples in this study that have been retrieved from the M horizon layer of the soil surface, and we need to make sure that an appropriate permissible value exists on the SoilHorizonEnum so that it can be applied to NMDC and INSDC databases.

[turbomam]

Thanks, @sujaypatil96

NMDC includes this sample (two views provided)

• https://data.microbiomedata.org/details/sample/nmdc:bsm-11-06qrej20
• https://api.microbiomedata.org/nmdcschema/ids/nmdc%3Absm-11-06qrej20

Which seems like it may correspond to this NEON API call

https://data.neonscience.org/api/v0/samples/view?sampleTag=ABBY_004-M-20170605-COMP&sampleClass=sls_metagenomicsPooling_in.compositeSampleID

which does show a "sample_type" value of "M". Maybe we need to look at one of their protocol PDFs for more information?

https://data.neonscience.org/api/v0/documents/NEON_metagenomes_userGuide_vE.pdf

which contains the text

For soils, a sample represents a plot‐level composite sample of soils collected at 1‐3 randomly assigned, individual X, Y locations of a particular horizon type (Figure 2). NEON designates soil horizons broadly as either organic (O) or mineral (M).

[turbomam]

As one of our soil experts, @mslarae13, can you please comment on this request?

[turbomam]

I am becoming concerned that the only authorities that use the term "M horizon" are the USDA Soil Taxonomy (NRCS Soil Survey) and NEON, and that they have completely different meanings in those two contexts.

NEON uses "M horizon" to mean mineral soil, as opposed to organic soil

The USDA Keys to Soil Taxonomy introduced the M horizon (or M layer) in the 10th edition (2006) as a new master horizon designation for human-made materials in soils. In USDA usage, an M horizon is defined as a root-limiting, manufactured layer beneath the natural soil surface – for example, nearly continuous, horizontal human-made barriers like asphalt, concrete, geotextile liners, rubber, or plastic sheets

https://www.nrcs.usda.gov/sites/default/files/2022-06/Illustrated_Guide_to_Soil_Taxonomy.pdf#:~:text=M.%20Root,Soil%20horizon

If we do add a "M horizon" permissible value, it will be essential to annotate it with provenance and clarification. While, we're at it, we should do the same for the slot, the enumeration as a whole, and the other permissible values.

[turbomam]

@sujaypatil96 and others have pointed out that NEON's use of "M horizon" shares the same semantics as EnvO's ENVO:03600011 mineral horizon class

[mslarae13]

I think adding M horizon is fine. It was a weird one to me too when NMDC first added it for NEON https://www.soils4teachers.org/soil-horizons/

I agree with @turbomam that ensuring we have clear descriptions for the PV and source of description will be vital to ensure M horizon isn't confused