Microbiological mechanism of long-term organic fertilization on improving soil biological properties and double rice yields in red paddy soil

Objectives Based on the long-term organic fertilizer experiment set in Jiangxi Institute of Red Soil, the effects of long-term different fertilization on soil microbial biomass carbon, microbial biomass nitrogen, enzyme activities and rice yield were evaluated, the function of these soil microbiological properties as bio-indicators of soil fertilization was illustrated, and the key factor of improving yield and soil microbiological properties was explored in red paddy soil by applicating green manure, pig manure and straw.

Methods The long-term experiment was established in 1981. In the present study, six fertilization treatments were selected: CK (no fertilizer), NPK (chemical NPK fertilizers), M1 (green manure merely at early rice), M2 (double M1 rate at early rice), M3 (M1 at early rice and pig manure at late rice), M4 (M1 at early rice and straw returning at late rice). In addition, a certain amount of fertilizer (N 69 kg/hm², P₂O₅ 30 kg/hm², K₂O 67.5 kg/hm²) was applied to the organic treatments (M1, M2, M3 and M4) every season on the basis of the application of organic materials. Rice yield were investigated after each harvest. After harvesting early rice in 2018, topsoil (0–20 cm) sample was collected and soil chemical properties, soil microbial biomass carbon and nitrogen contents, urease and other soil extracellular enzyme activities involved in soil carbon, nitrogen, phosphorus and sulfur cycles were measured.

Results Long-term fertilization increased rice yield from 2009 to 2018, with the highest yields in M3 and followed by M2, and those in both of them were obviously higher than that in NPK. Soil microbial biomass and enzyme activities were both higher in organic treatments (M1, M2, M3 and M4), among which the soil microbial biomass carbon and enzyme activities in M3 were the highest, and the soil microbial nitrogen in M2 was the highest. The microbial quotients in organic amendments were higher than those in NPK and CK. Furthermore, there was significant positive relationship between soil microbiological properties and soil organic carbon, total nitrate and available phosphorus; 3) Soil organic carbon played a more profound influence than pH on increasing crop yield, soil microbial biomass and enzyme activities.

Conclusions Soil microbial carbon and nitrogen, enzymes activities can be bio-indicators reflecting the changes of soil fertilization. Soil organic carbon is the key factor of regulating soil microbiological properties and rice yield. Under the experimental condition, returning green manure at early rice and pig manure at late rice has been proved of the most efficient organic fertilization in increasing soil microbial biomass, enzyme activities and rice yields of the double rice system.