If you are a citizen of a foreign country, it is possible that you need a Schengen Visa. You will require a visa if you are a national of one of these countries. You can apply for a Schengen Visa at the Dutch mission (embassy or consulate) in the country where you live. You can find more info about applying for a visa here. If you need an invitation letter from Rhizosphere 4, please follow the directions in the 'Visa Invitation Letter' tab in the registration drop-down menu.
As a national of any other country then the ones listed in the PDF-file above, you will only need a valid passport to enter the Netherlands.
Safe travels and we are looking forward to see you in Maastricht!