Cress-loach coculture for improving the utilization efficiency of biogas slurry in aquaponic systems

Potential limitations to the direct application of biogas slurry (BS) as a substitute for chemical fertilizer in agriculture include nitrogen (N) and phosphorus (P) losses. However, an aquaponic system might serve as a new way to utilize biogas slurry completely. The following hypothesis was tested: the cress-loach coculture system will improve the N and P use efficiencies of cress, enhance biogas slurry utilization efficiency and crop yield, for which there will be an optimal ratio of cress and loach. An outdoor mesocosm experiment was carried out to test these hypotheses. In comparison to the floating-bed cress, the cress-loach coculture boosted the ability of cress to accumulate N and P by 56.75∼84.53% and 148.54∼232.44%, respectively. The coculture also increased the relative growth rate (RGR) and biomass of the cress by 60.93∼61.87% and 97.64∼151.46%, respectively. Other important roles of the cress-loach coculture system were reducing the accumulation of nutrients at the bottom (sediment), driving the circulation of nutrients in the water, and promoting the accumulation of nutrients in the cress. Additionally, the coculture system could make the most utilization of BS when the planting density of the cress and breeding density of the loach were 1.08 ± 0.005 g m−2 and 0.405 ± 0.004 g m−2, respectively. Our findings imply that a cress-loach coculture is an affordable, efficient, and environmentally-friendly approach for treating livestock biogas slurry. This approach can be immediately applied to livestock biogas slurry and result in excellent crop yields.