Is this nut burn?

No , nute burn is usually dead tips and margins. This is another issue could be ph out or overwatering.
 
I'd think it was nuteburn, but could be ph too as B suggests. Did you sow into soil for seedlings? - if not my guess would be nute burn.
New growth looks good, so hopefully it'll be good now.
 
New growth is supposed to be green, not dark dark green. When you overdo the nitrogen, it turns a dark green like that. If its overdone even more, that causes the burn. And burn can happen to one set of leaves if the roots hit a hotspot of nutes in the soil.
 
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