Risk of acidification of the organic shrimp model at Tam Giang commune, Nam Can district, Ca Mau province

Abstract

This paper assesses the risk of acidification of the organic shrimp model certified by Naturland at Tam Giang commune, Nam Can district, Camau province. Pond water and sediment was sampled in 8 ponds in March, July and November 2015, dike soil and mangrove soils were sampled in March 2015. Pond sediment and mangrove soils are highly reduced (Eh of top sediment and soils to 60 cm depth range respectively from -299 – -1mV and -321 – -52mV). Pond water ranges from neutral to slightly alkaline (pH of pond water 7.01-8.82) and sediment from slightly acidic to slightly alkaline (pH of fresh sediment 6.05-7.64, pHH2O 6.63-7.78, pHKCl 6.35-7.43). Mangrove soils show a large pH range with very low minimums (pHH2O 3.72, pHKCl 3.05), reconfirming the presence of pyrite mineral in the soil profile. Mangrove soils deposited on the dikes (partly oxidized) are very acidic (pHH2O 2.51±0.72, pHKCl 1.81-2.14, exchange acidity 11.56±2.69 meq/100g). At the start of the wet season, pH of pond water reduces sharply as a result of reception of acidic components from the oxidized pyritic material (FeS2) on the dikes. The exchange acidity in pond sediment and mangrove soils are negatively correlated with organic matter, indicating that the decomposition of organic matter under reduced conditions contributes to exchange acidity in pond environment. Risk of acidification of the organic shrimp model is high due to managerial activities (mostly channel digging and disposal of pyrite-containing mangrove soils on the dikes).