SIP370 Add guard to check for leach+vol -> negative min N

[Alomir]

Summary

• What: Guard against leaching + volatilization causing negative mineral N
• Motivation: This is an edge case, but still needs to be addressed

Details

• Added guard in limitations.c
• Guard function called right before general N limitation check
• Unit test points added

How was this change tested?

• Test points added for each case of excessive volatilization and leaching
• All other unit tests pass unmodified
• Smoke tests pass unmodified

Related issues

• Fixes Guard nitrogen limitation against negative available mineral N #370

Checklist

• Related issues are listed above. PRs without an approved, related issue may not get reviewed.
• PR title has the issue number in it ("[#] <concise description of proposed change>")
• Tests added/updated for new features (if applicable)
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)

[Copilot]

Pull request overview

Adds a defensive guard in SIPNET’s nitrogen-cycle limitation flow to prevent combined volatilization + leaching from over-drawing the mineral N pool (and cascading into negative growth-reduction factors), with regression tests to cover the edge case.

Changes:

• Add a mineral-N loss guard that scales volatilization/leaching when their combined timestep loss would exceed available mineral N.
• Add unit test coverage for “excess volatilization” and “excess leaching” scenarios.
• Add a release-note entry for the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/sipnet/limitations.c	Introduces and wires in a guard to bound volatilization + leaching against available mineral N before running the general N limitation logic.
tests/sipnet/test_modeling/testNitrogenCycle.c	Adds regression tests validating the guard’s scaling behavior and end-of-step mineral N behavior.
docs/CHANGELOG.md	Notes the new guard in the Unreleased changelog.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[dlebauer]

Looks great, fixes #370.