We consider open IoT applications in which the IoT devices, i.e. sensor nodes and gateways, can be contributed by volunteer participants. Since the contribution of sensor deployments and their data is critical for the value of such IoT applications, we propose to account for each participant her donation of both the measured data and the hardware, and translate this information into rewards to create incentives. The system architecture we propose uses a blockchain-based backend and aims to extend properties of the blockchain to the IoT sensor layer in order to achieve the trusted accounting of each contribution. A potential use case that can benefit from this concept are environmental monitoring applications that need to integrate IoT sensor nodes from third parties for increasing their geographic reach.