Yes, there are allometric equations, but not for all species. However, BGB is usually c. 25% of the total dry biomass

Book Carbon Inventory Methods Handbook for Greenhouse Gas Invento...

try to go through this article: Isometric scaling of above- and below-ground biomass at the individual and community levels in the understood of a sub-tropical forest

Dongliang Cheng,1,2 Quanlin Zhong,1 Karl J. Niklas,3 Yuzhu Ma,1 Yusheng Yang,1,4,* and Jianhua Zhang2,*Author information Article notes Copyright and License information Disclaimer

Oven-dry weight of net annual increment (s.b) of a tree, stand or forest plus oven-dry weight of annual growth of branches, twigs, foliage, top and stump. The term “growth” is used here instead of “increment”, since the latter term tends to be understood in terms of merchantable volume. www.fao.org/docrep/009/j9345e/j9345e12.htm

there are allometric equations used for this purpose but the location of the forest is a factor. Generally, BGB is about 20% of the AGB!

Be careful to use ratio around 20-30%, in many cases, especially in stress condition, the BGB can be higher than ABG.

You may want to have a look at recent work from Ledo et al. 2017 who determined the allometric for BGB as a funtion of AGB, DBH, and environmental stress. "Tree size and climatic water deficit control root to shoot ratio in individual trees globally". For that you need AGB from trunks.

You can find answer in the following article:

Michael A. Cairns á Sandra Brown Eileen H. Helmer á Greg A. Baumgardner. Root biomass allocation in the world's upland forests. Oecologia (1997) 111:1±11

Research Proposal Remote sensing techniques in agriculture: challenges for new...

this is suitable reference:

K.G. MacDicken. A Guide to Monitoring Carbon Storage in Forestry and Agroforestry Projects

I would add that it depends where the tree is growing as to how the the relationships hold up.If the tree is growing in an urban site, specifically in a curbside location, leaf area might be a better determinant of underground mass than DBH...

You can use root shoot ratio depending on your vegetation type. Generally the belowground biomass (BGB) is quantified as a proportion of AGB biomass using root shoot ratio . I recommend this work for you Article Root biomass allocation in the world's upland forests