From cd64149d79a944c39946836442dc6b71bfd79e9a Mon Sep 17 00:00:00 2001
From: David LeBauer <dlebauer@gmail.com>
Date: Tue, 29 Sep 2026 13:57:46 -0700
Subject: [PATCH] Fix zero-carbon nitrogen transfer

---
 docs/CHANGELOG.md                             |  3 ++
 src/sipnet/nitrogen.c                         | 18 +++++----
 .../sipnet/test_modeling/testNitrogenCycle.c  | 40 +++++++++++++++++++
 3 files changed, 53 insertions(+), 8 deletions(-)

diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index cdc5b4a6..9a83762e 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -15,6 +15,9 @@ sections to include in release notes:
 
 ### Fixed
 
+- Prevent zero carbon transfers from producing non-finite nitrogen fluxes when
+  litter or soil carbon pools are empty.
+
 ### Changed
 
 ### Removed
diff --git a/src/sipnet/nitrogen.c b/src/sipnet/nitrogen.c
index 739eb290..21fea522 100644
--- a/src/sipnet/nitrogen.c
+++ b/src/sipnet/nitrogen.c
@@ -49,14 +49,16 @@ static void calcNPoolFluxes(void) {
 
   // for both litter and soil, mineralization is calculated as heterotrophic
   // respiration divided by the C:N ratio of that pool.
-  double litterMin = fluxes.rLitter / litterCN;
-  double soilMin = fluxes.rSoil / soilCN;
+  // A zero carbon transfer carries no nitrogen, including from an empty pool.
+  double litterMin = fluxes.rLitter == 0.0 ? 0.0 : fluxes.rLitter / litterCN;
+  double soilMin = fluxes.rSoil == 0.0 ? 0.0 : fluxes.rSoil / soilCN;
 
   // Adding soil carbon saturation functionality so organic N fluxes to soil
   // and litter are proportional to respective carbon fluxes dependent on
   // soil carbon saturation
-  double soilNInputs = fluxes.litterToSoil / litterCN +
-                       fluxes.fineRootLoss / params.fineRootCN +
+  double litterNToSoil =
+      fluxes.litterToSoil == 0.0 ? 0.0 : fluxes.litterToSoil / litterCN;
+  double soilNInputs = litterNToSoil + fluxes.fineRootLoss / params.fineRootCN +
                        fluxes.coarseRootLoss / params.woodCN;
   // saturationFraction capped between zero and one
   double saturationFraction =
@@ -67,10 +69,10 @@ static void calcNPoolFluxes(void) {
   // The litter org N flux is determined by the carbon fluxes from wood and leaf
   // litter (modified by leaf N resorption), and N loss due to mineralization.
   // N added via fertilization is handled elsewhere.
-  fluxes.nOrgLitter =
-      fluxes.leafLitter / params.leafCN - fluxes.leafOffNResorption +
-      fluxes.woodLitter / params.woodCN - litterMin -
-      fluxes.litterToSoil / litterCN + (soilNInputs * saturationFraction);
+  fluxes.nOrgLitter = fluxes.leafLitter / params.leafCN -
+                      fluxes.leafOffNResorption +
+                      fluxes.woodLitter / params.woodCN - litterMin -
+                      litterNToSoil + (soilNInputs * saturationFraction);
 
   // soil
   // The soil org N flux is determined by the carbon flux from the litter pool,
diff --git a/tests/sipnet/test_modeling/testNitrogenCycle.c b/tests/sipnet/test_modeling/testNitrogenCycle.c
index 95e419c0..d5c96cf5 100644
--- a/tests/sipnet/test_modeling/testNitrogenCycle.c
+++ b/tests/sipnet/test_modeling/testNitrogenCycle.c
@@ -502,6 +502,45 @@ int testOrganicN(void) {
   return status;
 }
 
+int testZeroCarbonNitrogenTransfer(void) {
+  int status = 0;
+  logTest("Running testZeroCarbonNitrogenTransfer\n");
+
+  for (int organicNPresent = 0; organicNPresent <= 1; organicNPresent++) {
+    resetState();
+    envi.litterC = 0.0;
+    envi.soilC = 0.0;
+    envi.minN = 2.0;
+    envi.litterN = organicNPresent ? 0.0211 : 0.0;
+    envi.soilOrgN = organicNPresent ? 127.7255 : 0.0;
+
+    double initialTotalN = envi.minN + envi.litterN + envi.soilOrgN;
+    double initialLitterN = envi.litterN;
+    double initialSoilOrgN = envi.soilOrgN;
+
+    calcNPoolFluxes();
+    updateNitrogenPools();
+
+    if (!isfinite(envi.minN) || !isfinite(envi.litterN) ||
+        !isfinite(envi.soilOrgN)) {
+      logTest("Zero-carbon pools produced non-finite nitrogen stocks\n");
+      status = 1;
+    }
+    status |= checkFlux(fluxes.nMin, 0.0, "Zero-carbon mineralization");
+    status |= checkFlux(fluxes.nOrgLitter, 0.0, "Zero-carbon organic litter N");
+    status |= checkFlux(fluxes.nOrgSoil, 0.0, "Zero-carbon organic soil N");
+    status |= checkFlux(envi.minN, 2.0, "Zero-carbon mineral N pool");
+    status |=
+        checkFlux(envi.litterN, initialLitterN, "Zero-carbon litter N pool");
+    status |= checkFlux(envi.soilOrgN, initialSoilOrgN,
+                        "Zero-carbon soil organic N pool");
+    status |= checkFlux(envi.minN + envi.litterN + envi.soilOrgN, initialTotalN,
+                        "Zero-carbon total N conservation");
+  }
+
+  return status;
+}
+
 /////
 // Organic N with leafOffNResorption
 int testOrganicNWithResorption(void) {
@@ -841,6 +880,7 @@ int run(void) {
   status |= testFertilization();
   status |= testNLeaching();
   status |= testOrganicN();
+  status |= testZeroCarbonNitrogenTransfer();
   status |= testLeafOnNFromC();
   status |= testNFixation();
   status |= testNLimitation();