Impact of Amendments on Microbial Biomass, Enzyme Activity and Bacterial Diversity of Soils in Long-term Rice Field Experiment

The long-term effects of soil management history on microbial communities are still poorly understood. Our objectives were to determine the impact of long-term application of soil amendments on microbial communities in rice paddy fields. The treatments selected were control where crops were grown without any nutrient application (CON); nitrogen-phosphorus-potassium (NPK); NPK plus compost (CNPK); NPK plus lime (LNPK); and NPK plus silicate (WNPK). The long-term addition of organic and inorganic amendments significantly changed soil chemical properties. The amount of organic carbon increased in the treatments with fertilizer and amendments over that in the soil without inputs. However, we could not observe the differences of bacterial population among the treatments, but the number of aerobic bacteria increased by the addition of amendments. Isolates from the rice paddy soils before irrigation were Dactylosporangium, Ewingella, Geobacillus, Kocuria, Kurthia, Kytococcus, Lechevalieria, Micrococcus, Micromonospora, Paenibacillus, Pedobacter, Pseudomonas, Pseudoxanthomonas, Rhodococcus, Rothia, Sphingopyxis, Stenotrophomonas, and Variovorax. Dominant genera were Arthrobacter, Kocuria, Kurthia, and Bacillus in the long-term field. Microbial biomass was the highest in the compost treatment (CNPK), and was the lowest in the CON. Dehydrogenase activity in soils treated with rice compost straw was the highest and the activity showed an increasing trend according to treatment as follows: CON < WNPK < NPK = LNPK < CNPK. These results demonstrate that soil management practice, such as optimal application of fertilizer and amendment, that result in accumulations of organic carbon may increase microbial biomass and dehydrogenase activity in long-term rice paddy soils.