Using fish protein as activator to stimulate straw decompositionand enhance organic carbon accumulation in soil

Objectives Straw returning amount to field affects the decomposition, greenhouse gas emission and the growth of following crops. We studied the suitable returning amount of rice straws and the effect of fish protein as activator to promote straw decomposition, from the points of greenhouse gas emission and organic carbon accumulation.

Methods An indoor incubation experiment was conducted, three variables were included, as: straw returning with high and low amount (7500 and 10500 kg/hm²), organic activator source (fish protein and pig manure), and the percentage of organic activators (50% and 100%) in the total N required for rapid straw decomposition (regulating C/N ratio to 35∶1), and not regulating C/N ratio as the control for each returning amount, composing a total of 10 treatments, and the incubation lasted 60 days in total. During the incubation, the CO₂ and N₂O emission and cumulative emissions were monitored. At the end of incubation, soil nutrients, bacterial and fungal abundance and enzyme activities were measured.

Results High straw returning amount resulted in higher total CO₂ emissions and unit carbon input CO₂ emissions than the moderate straw retuning, although the total organic carbon accumulation in soil was still high. The cumulative emissions during the first 13 days accounted for more than 40% of the total emission, F50 treatments (fish protein replacing 50% of the total activator N) resulted in the highest CO₂ emissions under both straw retuning amount. Both F50 and P50 (pig manure replacing 50% of the total activator N) treatments were observed lower CO₂ emissions than F100 and P100 (fish protein and pig manure provide 100% of the total activator N), but higher N₂O emissions than F100 and P100. F50 and P50 treatment resulted in higher organic carbon accumulation in soil than F100 and P100 did. F50 was monitored higher CO₂ emissions at early period of incubation, lower cumulative CO₂ emissions at the end of incubation, and significantly higher organic carbon accumulation in soil than P50, while P50 resulted in higher soil nutrient contents, inducing higher nutrient availability of soil. High straw returning resulted in lower abundance of soil bacteria and fungi than moderate returning amount, and both F50 and P50 were recorded higher soil enzyme activities than the other treatments under the same straw returning amount.

Conclusions Suitable straw returning amount is beneficial to the organic carbon accumulation per unit carbon input. Replacing 50% of N required for regulation of straw C/N ratio with fish protein could increase early decomposition of straws, decrease the total CO₂ emissions, and enhance the organic carbon accumulation in soil, so is recommended for efficient straw returning practice.