Plant response to fish farming wastes in volcanic soils

A three-month greenhouse experiment with ryegrass was conducted to determine plant availability of N and P from aquaculture wastes. Measured N and P uptake of ryegrass samples treated with different kinds of fertilizer indicate that living plants must be included in mineralization potential evaluations to avoid significant errors in the use of nutrient cycling models. These also emphasize the importance of organic amendments in ameliorating plant P availability in high P-fixing volcanic soils. Agricultural use of aquaculture wastes appears to be a sound ecological and economical means to improve soil fertility and to decrease the potential for adverse water quality impacts in South Argentina. A 3-mo greenhouse experiment with ryegrass (Lolium perenne L.) was conducted to determine plant availability of N and P from aquaculture wastes and compare results with previous laboratory incubations. Treatments included (i) wastes taken from sediments under cages in a 3-yr-old fish farm (EA) applied at 40 and 80 Mg ha-1; (ii) wastes taken from sediments under cages in an 8-yr-old farm (LM), applied at 10 and 20 Mg ha-1; (iii) the lowest waste rates amended with 50 kg ha-1 synthetic N; (iv) four synthetic fertilizer treatments including a single N rate of 80 kg N ha-1 and four P levels of 30, 40, 60, and 80 kg P ha-1, and (v) a no-fertilizer control. Although total N and P were higher in LM than in EA, and N mineralization rates in lab incubations were similar (13%), ryegrass yields and N uptake were much higher in EA than in LM. When mineralized N was estimated as the difference between added N and total N uptake, values were 50 to 63% for EA and 5 to 14% for LM. Ryegrass yields were approximately as follows EA > FI > LM > Control. Synthetic fertilizer treatments showed the lowest values of P utilization efficiency, residual Olsen-P in soil and plant P uptake in biomass, and the highest P retention in soils. The lowest waste rates with addition of N produced comparable ryegrass yields with the highest rates. Results emphasize (i) the need to include living plants in mineralization potential evaluations to avoid significant errors in nutrient cycling models, and (ii) the importance of organic amendments in ameliorating plant P availability in high P-fixing volcanic soils.