ORGN and AVLN abruptly drop to zero during future simulation

[amullen01]

During future scenarios, ORGN drops to zero and AVLN drops near zero and stays constant, beginning around the year 2045. This issue was observed with Caribou-Poker climate using Caribou-Poker Black Spruce and Birch calibrations, as well as the Murphey Dome Black Spruce and Deciduous calibrations (CMT1 & CMT3). You can see the issue with the grey line in the attached figure, which is the control run in which no fire occurs. I have not observed any other anomalous behavior preceding this abrupt drop in N. Labelling this issue with "bug" because it looks like buggy behavior, but not ruling out a calibration or climate issue.

[amullen01]

The issue seems to happen under very specific circumstances. When avlnlost < 0 (losses exceed total available nitrogen), the N mineralization is bumped up slightly to match the losses (lines 713 - 721, Soil_Bgc.cpp). The problem lies in line 719: del_soi2soi.netnmin[i] *=nminadj/totnetnmin;. In winter total net mineralization is 0. We turned this off a while back due to wintertime mineralization causing a runaway AVLN issue. So when both avlnlost<0 and totnetnmin == 0 , this causes a divide by zero error, causing ORGN to contain nan, and AVLN is not updated.

I have implemented a fix that does two things:

1. checks whether totnetmin==0 before adjusting netnmin
2. if netnmin is zero, adds n mineralization to layers where avln is extracted by root uptake to equal losses

This seems to fix the issue but I'm not sure if it is 100% scientifically accurate. I feel like instead of adding N mineralization to account for too big of plant uptake we should be limiting plant uptake. Should N uptake be zero when soil is frozen? Would be good to discuss further.