One of the original "citizen science" networks are the bird watching community. Audubon Society is one place they tend to meet up and exchange information:
http://www.audubon.org/
Another big one are astronomers, new asteroids and comets are still often discovered by citizens with telescopes, because the sky is just very, very big and even big telescopes don't have the funding or time to watch it all. Many areas have amateur astronomy societies, or planetariums or universities that may sponsor events. Nasa's Night Sky Network is one formal project space:
https://nightsky.jpl.nasa.gov/
University extension services (USDA) are often more conservative (following conventional dogma, largely funded by agri-biz), but can still be a good resource for getting soil tests,
water tests, and connecting with current, local resource in crops, climate, and alternative agriculture. If you combine regular, conventional soil testing with your own experiments in
sustainable, regenerative amendments, you can document your soil-building practices over time.
(You might also want to look into alternatives such as brix testing, to document the quality of produce not just soils. Conventional soil tests are more about mineral/chemical balance, and may not measure all the life-giving qualities of the soil as well as some organic/biodynamic/permie people might wish.)
Farmers' markets are also a good place to network to find farmers who share your interest in scientific method, and look for ways to join in ongoing experiments in seed and plant breeding, local "land races" or climate-tolerant varietals, optimal soil building practices and soil amendments for your local conditions, etc.
International
Rose Test Gardens can be found in a lot of urban areas, growing out plots of prize-winning roses to test for climate tolerance. Other Test Gardens are often sponsored by garden societies, universities, or
city parks.