Phytoplankton community characteristics in rice paddy fields under different nitrogen fertiliser applications

Community characteristics of phytoplankton in the surface water of paddy fields under four nitrogen fertiliser (NF) treatments were studied in the Yangtze River Delta, China. The results were as follows: (1) in total, 65 species, belonging to six phytoplankton phyla and 50 genera were detected; their average total density was 1.25 x 10(6)/L. Number of species and diversity of phytoplankton were higher in the control treatment (0 kg N/ha; CK) than in the NF treatments at later experimental times; (2) there was a significant positive correlation between phytoplankton density and total N; however, total density of phytoplankton was higher in CK than in NF treatments; (3) during the transplanting and rehabilitation stage, under high N treatments, phytoplankton mainly comprised Chlorophyta, among which the dominant species was Volvox aureus, while under low N treatments, the main phyla were Chlorophyta and Cryptophyta, with Chlamydomonas reinhardtii and Chroomonas acuta forming the dominant species. During the rice tillering stage, the dominance of Chlorophyta was partially replaced by Cryptophyta and Bacillariophyta in all NF treatments, while the proportion of Cyanobacteria in CK increased significantly; (4) species and density of Cyanobacteria were higher in CK than in NF treatments, accompanied by the dominance of nitrogen-fixing species such as Anabaena spp. at later experimental times. The results indicate that NF application changed the phytoplankton community. Low N conditions can promote competition for photosynthesis and nutrients among phytoplankton in paddy fields, while low N application rates can induce growth of some algae with nitrogen-fixing abilities.