Effect of HNO₃ and H₂SO₄ on the Paddy Ecosystem: A Mesocosm Study with Exposure at PNEC and HC₅₀ Levels

Paddy mesocosms comprising of rice (Oryza sativa), snail (Pomacea canaliculata), and worm (Tubifex tubifex) were used to assess the damage caused by two acids (HNO₃ and H₂SO₄) at predicted no-effect concentration (PNEC) and hazardous concentration for 50% of species (HC₅₀) levels. In the fourth week, the fresh weight and shoot height of O. sativa at H₂SO₄-HC₅₀ were reduced by 83.2% and 30.3%, respectively. Wilted leaves (%) at HC₅₀ were approximately twice that at PNEC. No P. canaliculata and T. tubifex were recovered at HC₅₀. At H₂SO₄-PNEC, the length and weight of P. canaliculata were reduced by 7.4% and 25.9%, respectively, whereas fewer adult (46.5%) and juvenile (84%) T. tubifex were recovered. In the 20th week, rice growth and productivity were correlated with initial pH (pH_i) and nitrogen levels. Poor correlation with chlorophyll at the active tillering stage suggests the disturbance of nutrient uptake by roots. Partial least squares path modeling (PLS-PM) results further supported that the pH_i directly affects grain yield and quality, as well as plant growth. The indirect effect via intervening fourth-week-variables was also substantial. Therefore, it is important to measure initial pH upon acid spill to estimate the risk to the paddy ecosystem. Information on the change in soil properties associated with acidity will also aid in predicting the yield and quality of grain to be harvested.