Nitrogen transfer using ¹⁵N as a tracer in an integrated aquaculture and agriculture system

Abstract

Integration of aquaculture with agriculture appears to be an excellent way to save water, dispose of aquaculture effluents, and provide additional fertilizer to the agricultural crop. However, the amount of nitrogen transferred from fish effluent to plants has not been quantified. Therefore, I tested the feasibility of using ¹⁵N labeled fish feed, to produce a labeled effluent that could be used to study the fate of nitrogen in an integrated aquaculture and agriculture system. A pilot project showed that tilapia accepted labeled feed, that ¹⁵N ammonium sulfate was not hazardous to fish, and that labeled effluent could be detected in lettuce. Two experiments irrigating lettuce with the labeled effluent showed the pathways of nitrogen movement. Different combinations of chemical fertilizer and tap water and fish effluent were used to irrigate lettuce. Nitrogen transfer was quantified based on the percentage of ¹⁵N recovered from the fish effluent by plants. Lettuce that received fish effluent as the only source of nitrogen recovered practically all the available inorganic nitrogen. However, there was not sufficient nitrogen for optimal plant growth. Plants that received a combination of fish effluent and chemical fertilizer were more efficient at uptaking the nitrogen in the chemical fertilizer than in the fish effluent. Available inorganic nitrogen levels in fish effluent were generally too low to meet the nitrogen needs of plants. However, decomposition of organic material over time may improve long term soil fertility. Concentration of fish culture effluent might also increase nutrient content. The nitrogen budget for fish tanks used to produce effluent for these studies showed that fish contained the highest amount of nitrogen, followed by algae and effluent in descending order. Estimates of nitrogen assimilation based on ¹⁵N recovery from labeled fish feed did not agree with calculations based on recovery of total nitrogen. Therefore ¹⁵N recovery does not appear to be a good indicator of nitrogen assimilation by fish in short term experiments.