Hi Thorsten, when plants grow they assimilate CO2 in their biomass ... that is why e.g. rain forests serve as huge CO2 sinks (obviously). Thus it makes sense, that in ecoinvent plants (during growth) have a negative CO2 impact. The respiratory emissions e.g. during digestion by animals only occur later in the supply chain and are not considered at the point of plant production. Thus, the (mathematically) negative environmental impact will only be canceled out if you consider the entire supply chain including the release of the plant(biomass) based CO2 back into the atmosphere. But again, only looking at the growth stage of plants I think it does make sense that the impact is negative.
I hope this helps.