This paper presents a framed field experiment from China studying a spatially coordinated (SC) auction mechanism for the allocation of agri-environmental contracts, which pay farmers to change their agricultural practices to provide environmental benefits. The SC auction is designed to maximise a metric of environmental benefit that depends both on site-specific environmental values and benefits due to spatial coordination of conserved patches, subject to a budget constraint. We investigate whether auction performance can be improved by the introduction of agglomeration bonus (AB) and joint bidding (JB) mechanisms. The AB is a bonus payment awarded to neighbouring farmers who bid individually but receive agrienvironmental contracts simultaneously. The JB mechanism allows neighbouring farmers to bid jointly and provides a bonus payment for successful joint bids. We conducted experimental SC auctions with a total of 432 Chinese farmers randomly assigned to one of four treatments which differed in whether the AB and JB mechanisms were adopted, following a two-by-two full factorial experimental design. Our empirical results suggest that the SC auction has similar environmental performance regardless of whether an AB is provided, although cost-effectiveness is 1 slightly higher when AB is not provided. Moreover, introducing the JB mechanism into the SC auction leads to lower environmental performance and lower cost-effectiveness. Finally, the AB mechanism achieves higher environmental performance than the JB mechanism but has similar cost-effectiveness.