Evaluating the Effects of Aquaculture Wastewater Irrigation with Fertilizer Reduction on Greenhouse Tomato Production, Economic Benefits and Soil Nitrogen Characteristics

The utilization of aquaculture wastewater as irrigation is an effective way to recycle and reuse water and nitrogen fertilizer resources because it contains numerous nutrients. However, it is still unclear that the pattern of substituting aquaculture wastewater irrigation for fertilizer supplementing is conducive to improving the soil nitrogen status, fruit yield and water-fertilizer use efficiency for tomato production. In this context, the experiment was intended to establish the appropriate irrigation regime of aquaculture wastewater in tomato production for freshwater replacement and fertilizer reduction to ensure good yields. Pot experiments were conducted with treatments as farmers accustomed to irrigation and fertilization used as control (CK), 1.75 L aquaculture wastewater with base fertilizer (W1), 2 L aquaculture wastewater with base fertilizer; and 2.25 L aquaculture wastewater with base fertilizer (W3). We examined the effects of aquaculture wastewater irrigation on soil nitrogen distribution, N-related hydrolases, tomato yield, and economic benefits. The results showed that the control treatment had the highest N input, about 24.68% higher than the W3 treatment, while the yield was only about 7.81% higher than W3. This indicated that the overuse of chemical fertilizer was present in the current tomato production. Although the reduction of fertilizer in aquaculture wastewater irrigation caused a decrease in tomato production, this economic loss can be compensated by cost savings in the wastewater disposal. Among aquaculture wastewater treatments, the W3 treatment had the highest overall benefit, achieving 62.63% freshwater savings, 37.50% fertilizer input reduction, and an economic return of approximately 19,466 Yuan per hectare higher than the control. Additionally, increasing the irrigation volume of aquaculture wastewater could provide more available nutrients to the soil, which were more prevalent in the form of organic nitrogen. The lower soil nitrate reductase activities (NR) under aquaculture wastewater treatments after harvesting also proved that this pattern was beneficial to reduce soil nitrate nitrogen residues. Overall, the results demonstrate that aquaculture wastewater irrigation alleviates the soil nitrate residues, improves nutrient availability, and results in more economic returns with water and fertilizer conservation for the greenhouse production of tomatoes.