As an cultivated aquatic vegetable, the long-term continuous monocropping of water oat results in the frequent occurrence of diseases, the deterioration of ecological system and decreased quality of water oat. In this study, real-time quantitative PCR (qPCR) and Illumina high-throughput sequencing were used to determine the dynamic changes in bacterial and fungal communities in rhizosphere soil under continuous cropping of water oat for 1, 5, 10, 15 and 20 years (Y1, Y5, Y10, Y15 and Y20), and soil properties and enzyme activities were also determined. Results showed that the contents of soil organic carbon (SOC), total nitrogen (TN), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP) and the activities of four soil enzymes increased in Y5 and Y10 and then decreased in Y15 and Y20. Spearman correlation analysis identified SOC, TN, AP and AN as the main factors that affect the four enzyme activities. The qPCR results showed that there was no significant difference in bacterial abundance between the different planting years, while the fungal abundance first increased and then decreased. The long-term continuous planting of water oat (Y15 and Y20) significantly reduced the operational taxonomic unit numbers and the Shannon, Chao1, and ACE indices of rhizosphere bacteria and fungi. The bacterial and fungal community compositions were markedly affected by the continuous planting year. The relative abundances of Bacteroidetes and Firmicutes decreased significantly in Y10 and Bacteroidetes increased significantly in Y15. Relative abundances of dominated Mortierellomycota and Ascomycota phyla increased with the continuous cropping years, while Rozellomycota presented the opposite trend. The AK, AN, and SOC were the main factors that changed the bacterial community, while AK and AP significantly shifted the fungal community. Thus, long-term continuous planting of water oat resulted in the deterioration of soil nutrients and microbial communities. The results provided a reference for the remediation of soil under continuous water oat planting and sustainable development of water oat industry.

IntroductionThe application of controlled-release nitrogen fertilizer (CRN) has become an important production method to achieve high crop yield and ecological safety. However, the rate of urea-blended CRN for rice is usually determined by conventional urea, and the actual rate is still unclear.MethodsA five-year field experiment was carried out in the Chaohu watershed in the Yangtze River Delta to study rice yield, N fertilizer utilization efficiency (NUE), ammonia (NH3) volatilization and economic benefit under the four urea-blended CRN treatments with a 4:3:3 ratio applied at one time (60, 120, 180, 240 kg/hm2, CRN60, CRN120, CRN180, CRN240), four conventional N fertilizer treatments (N60, N120, N180, N240) and a control without N fertilizer (N0).Results and DiscussionThe results showed that the N released from the blended CRNs could well satisfy the N demand of rice growth. Similar to the conventional N fertilizer treatments, a quadratic equation was used to model the relationship between rice yield and N rate under the blended CRN treatments. The blended CRN treatments increased rice yield by 0.9-8.2% and NUE by 6.9-14.8%, respectively, compared with the conventional N fertilizer treatments at the same N application rate. The increase in NUE in response to applied blended CRN was related to the reduction in NH3 volatilization. Based on the quadratic equation, the five-year average NUE under the blended CRN treatment was 42.0% when rice yield reached the maximum, which was 28.9% higher than that under the conventional N fertilizer treatment. Among all treatments, CRN180 had the highest yield and net benefit in 2019. Considering the yield output, environmental loss, labor and fertilizer costs, the optimum economic N rate under the blended CRN treatment in the Chaohu watershed was 180-214 kg/hm2, compared with 212-278 kg/hm2 under the conventional N fertilizer treatment. The findings suggest that blended CRN improved rice yield, NUE and economic income while decreasing NH3 volatilization and negative environmental outcomes.

Cake fertilizer and dairy manure were used as experimental materials to carry out a 9-year (2012–2020) field experiment in the main rice production areas in the Yangtze River basin. Different fertilization modes were used (no fertilization, CK; chemical fertilizer application alone, HY; reduced fertilization with chemical fertilizer application, RF; cake fertilizer replacement of nitrogen fertilizer, CFR; and dairy manure replacement of nitrogen fertilizer, DMR). Changes in the total rice yield, yield components, absorption of nitrogen, soil pH, organic matter, total nitrogen, and soil bulk density under different fertilization treatments were analyzed. The results show that organic fertilizer replacement leads to a stable and high rice yield. The 9-year average rice yields of the CFR and DMR treatments were 60.0% and 61.5% higher than that of CK. The nitrogen uptake of the CFR and DMR treatments was also higher than that of the other treatments. The nitrogen recovery efficiency in the current season could be increased by 16.37–22.89%, and after 9 years of testing, the soil total nitrogen contents of CFR and DMR increased by 0.23–0.85 g·kg−1 compared to the other treatments. The available P and K contents of DMR increased by 30.17 mg·kg−1 and 22.02 mg·kg−1 compared with HY, respectively. The soil bulk density was reduced by 0.08 g·cm−3. Generally, the effects of dairy manure replacement were better than those of cake fertilizer. This is an important method that can be used to fertilize the soil and foster sustainable soil utilization in the rice-growing area of the Yangtze River Basin, as a long-term partial replacement for chemical nitrogen fertilizer.