Effects of nitrogen fertilizer and returning straw on aquaculture water, soil nutrients, and enzyme activity in rice-crayfish fields during winter and spring

The integrated rice-crayfish breeding model is an ecological agriculture model that combines paddy field plantation and crayfish (Procambarus clakii) cultivation. This model both guarantees food production and increases economic benefits. At present, research relating to this model is focused on the efficiency of crayfish culture technology, while information regarding straw resource utilization under this model has not been reported. Rice straw is rich in nitrogen, phosphorus, and potassium. However, if improperly handled and directly immersed in water, its degradation process will cause deterioration of water quality and pollution of soil. Therefore, returning straw to the field not only effectively uses resources, but also avoids the risks caused by random straw disposal. In order to explore the optimal model for returning straw during winter and spring (when crayfish are breeding), we carried out a field experiment to study the effects of different C/N ratios of straw returning on cultured water, soil nutrients, and enzyme activity in Dec. 2017 to May 2018. The experiment consisted of five treatments: straw removal without fertilizer (CK), straw returned without N fertilizer (C/N ratio of 35:1, S), straw returned with N fertilizer (adjusted C/N ratio of 25:1, SN1), straw returned with N fertilizer (adjusted C/N ratio of 15:1, SN2), and N fertilizer without straw returned (N1). The results showed that total phosphorus (TP) content was the lowest in the CK treatment at the later stage of straw returning period. The total N content in each period was the lowest in CK treatment, followed by SN2 treatment. The ammonia nitrogen (NH₄^-) content in the treated water was lower than the safe tolerance limit of the crayfish. Additionally, the soil organic matter content increased under straw returning; the most significant result was obtained in SN2 treatment, with an increase of 56.1% compared to before the field experiment. Soil nitrate nitrogen (NO₃^-) content in SN2 treatment increased by 240% compared to that before straw returning. Returning straw reduced bulk density of soil. The SN2 treatment recorded the lowest bulk density overall. Finally, we found that N1 and SN2 treatments could increase soil polyphenol oxidase activity; activity was highest in SN2 treatment group. Among three straw returning treatments, soil urease activity in SN2 treatment was lower than that in other treatments only in the first month after returning to the field. N1 treatment showed higher enzyme activity in the early experimental stage and began to decrease in the later stage. Overall, the results suggest that an optimal C/N ratio for returning straw with N fertilizer was 15:1, in terms of soil nutrient content and enzyme activity.