Answer: I think you are overwatering your new tree. Citrus trees do like moist soil, but it needs to dry out a bit between waterings or the saturated soil will suffocate the roots and one of the symptoms of that is browning leaves and dropping leaves. Water it thoroughly and then let the top half inch of potting soil dry out before watering again. Your tree should be putting out new growth now or within the next few weeks. When it does you can prune off the dead or dying leaves so it looks healthy and happy again.
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