diff --git a/bld/CLMBuildNamelist.pm b/bld/CLMBuildNamelist.pm
index 9562cca9bd..8cc5b25187 100755
--- a/bld/CLMBuildNamelist.pm
+++ b/bld/CLMBuildNamelist.pm
@@ -3740,8 +3740,12 @@ sub setup_logic_c_isotope {
#
my ($opts, $nl_flags, $definition, $defaults, $nl) = @_;
- add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c13', 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'});
- add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c14', 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'});
+ add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c13',
+ 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'},
+ 'lnd_tuning_mode'=>$nl_flags->{'lnd_tuning_mode'}, ssp_rcp=>$nl_flags->{'ssp_rcp'} );
+ add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c14',
+ 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'},
+ 'lnd_tuning_mode'=>$nl_flags->{'lnd_tuning_mode'}, ssp_rcp=>$nl_flags->{'ssp_rcp'} );
my $use_c13 = $nl->get_value('use_c13');
my $use_c14 = $nl->get_value('use_c14');
if ( $nl_flags->{'bgc_mode'} ne "sp" && $nl_flags->{'bgc_mode'} ne "fates" ) {
@@ -3754,7 +3758,7 @@ sub setup_logic_c_isotope {
}
}
if ( &value_is_true($use_c14) ) {
- add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c14_bombspike', 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'});
+ add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c14_bombspike', 'use_c14'=>$use_c14 );
my $use_c14_bombspike = $nl->get_value('use_c14_bombspike');
if ( &value_is_true($use_c14_bombspike) ) {
add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'atm_c14_filename',
@@ -3768,7 +3772,7 @@ sub setup_logic_c_isotope {
}
}
if ( &value_is_true($use_c13) ) {
- add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c13_timeseries', 'bgc_mode'=>$nl_flags->{'bgc_mode'}, 'phys'=>$nl_flags->{'phys'});
+ add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'use_c13_timeseries', 'use_c13'=>$use_c13 );
my $use_c13_timeseries = $nl->get_value('use_c13_timeseries');
if ( &value_is_true($use_c13_timeseries) ) {
add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'atm_c13_filename',
diff --git a/bld/namelist_files/namelist_defaults_ctsm.xml b/bld/namelist_files/namelist_defaults_ctsm.xml
index 0498da2790..c043ff7255 100644
--- a/bld/namelist_files/namelist_defaults_ctsm.xml
+++ b/bld/namelist_files/namelist_defaults_ctsm.xml
@@ -115,11 +115,22 @@ attributes from the config_cache.xml file (with keys converted to upper-case).
.false.
.false.
.false.
-
-.true.
-.true.
-.true.
-.true.
+
+.true.
+.true.
+
+
+
+.true.
+.true.
+.true.
+.true.
lnd/clm2/isotopes/atm_delta_C13_CMIP6_1850-2015_yearly_v2.0_c190528.nc
@@ -141,6 +152,7 @@ attributes from the config_cache.xml file (with keys converted to upper-case).
.false.
.true.
+.true.
.false.
diff --git a/doc/ChangeLog b/doc/ChangeLog
index 6f5fb71011..a1a541b252 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -1,4 +1,101 @@
===============================================================
+Tag name: ctsm5.3.081
+Originator(s): erik (Erik Kluzek,UCAR/TSS,303-497-1326)
+Date: Wed 22 Oct 2025 05:31:52 PM MDT
+One-line Summary: Change defaults for when Carbon isotopes are turned on, and turn on irrigate for Sp/Bgc cases for clm6_0 historical transient cases
+
+Purpose and description of changes
+----------------------------------
+
+Only turn C13/C14 for clm6_0 and BGC, and also require the DATM forcing to be CRUJRA2024 or CAM7 and for the historical period. So for example SSP compsets won't have C13/C14 on, single point compsets won't, or for using older forcing (like CAM6, or GSWP3v1, Qian, or CRUv7). Also whenever use_c13 or use_c14 is on turn on the corresponding time series file.
+
+Also change default for irrigate to on for historical SP/BGC cases.
+
+
+Significant changes to scientifically-supported configurations
+--------------------------------------------------------------
+
+Does this tag change answers significantly for any of the following physics configurations?
+(Details of any changes will be given in the "Answer changes" section below.)
+
+ [Put an [X] in the box for any configuration with significant answer changes.]
+
+[ ] clm6_0
+
+[ ] clm5_0
+
+[ ] ctsm5_0-nwp
+
+[ ] clm4_5
+
+
+Bugs fixed
+----------
+
+List of CTSM issues fixed (include CTSM Issue # and description) [one per line]:
+ Fixes #3527 -- irrigation settings for control vs historical cases
+ Fixes #3551 -- Turn on relevent Carbon isotope time-series when use_c13/c14 is on
+ Some work on #3346 -- CMIP7 Carbon isotope data
+
+Notes of particular relevance for users
+---------------------------------------
+
+Caveats for users (e.g., need to interpolate initial conditions):
+ This just changes the defaults for when Carbon isotopes are turned on, to make it less often
+ Users can still turn it on by hand for the cases that they want.
+
+Changes made to namelist defaults (e.g., changed parameter values):
+ Carbon isotope timeseries files are turned on whenever use_c13/use_c14 is turned on so it uses
+ the historical
+ Turn irrigate on for clm6_0 historical
+ Carbon isotopes only turned on by default for clm6_0 when CRUJRA2024 or CAM7 forcing is used
+ and only for historical cases and not SSP's
+
+Testing summary: regular
+----------------
+ [PASS means all tests PASS; OK means tests PASS other than expected fails.]
+
+ build-namelist tests (if CLMBuildNamelist.pm has changed):
+
+ derecho - OK (differences in namelists are all as expected)
+
+ python testing (if python code has changed; see instructions in python/README.md; document testing done):
+
+ derecho - PASS
+
+ regular tests (aux_clm: https://github.com/ESCOMP/CTSM/wiki/System-Testing-Guide#pre-merge-system-testing):
+
+ derecho ----- OK
+ izumi ------- OK
+
+If the tag used for baseline comparisons was NOT the previous tag, note that here: previous
+
+
+Answer changes
+--------------
+
+Changes answers relative to baseline: HistClm60 compsets and tests with the ciso testmod
+
+ Summarize any changes to answers, i.e.,
+ - what code configurations: HistClm60Sp/HistClm60Bgc now with irriage on
+ cases with use_c13/use_c14 now have c13/c14 isotope timeseries files on
+ - what platforms/compilers: all
+ - nature of change (roundoff; larger than roundoff/same climate; new climate):
+ Carbon Isotopes -- are diagnostic and only change fields on history files
+ Cases that now have C13/C14 isotopes on now use timeseries files rather
+ than the default pre-industrial so change answers for C13/C14 fields
+ Irrigate turned on for HistClm60Bgc/HistClm60Sp compsets will change answers
+
+Other details
+-------------
+
+Pull Requests that document the changes (include PR ids):
+(https://github.com/ESCOMP/ctsm/pull)
+
+https://github.com/ESCOMP/CTSM/pull/3549
+
+===============================================================
+===============================================================
Tag name: ctsm5.3.080
Originator(s): samrabin (Sam Rabin, UCAR/TSS)
Date: Thu Oct 16 13:41:08 MDT 2025
diff --git a/doc/ChangeSum b/doc/ChangeSum
index 5aba1a5c62..57cabc8687 100644
--- a/doc/ChangeSum
+++ b/doc/ChangeSum
@@ -1,5 +1,6 @@
Tag Who Date Summary
============================================================================================================================
+ ctsm5.3.081 erik 10/22/2025 Change defaults for when Carbon isotopes are turned on, and turn on irrigate for Sp/Bgc cases for clm6_0 historical transient cases
ctsm5.3.080 samrabin 10/16/2025 Merge b4b-dev to master
ctsm5.3.079 samrabin 10/06/2025 Update submodules to match versions in cesm3_0_alpha07e
ctsm5.3.078 slevis 10/03/2025 Merge b4b-dev to master