Liao Yulin

Study of Dynamics of Floodwater Nitrogen and Regulation of Its Runoff Loss in Paddy Field-Based Two-Cropping Rice with Urea and Controlled Release Nitrogen Fertilizer Application

Abstract The article deals with the effects of urea and controlled release nitrogen fertilizer (CRNF) on dynamics of pH, electronic conductivity (EC), total nitrogen (TN), NH4+-N and NO3−-N in floodwater, and the regulation of runoff TN loss from paddy field-based two-cropping rice in Dongting Lake, China, and probes the best fertilization management for controlling N loss. Studies were conducted through modeling alluvial sandy loamy paddy soil (ASP) and purple calcareous clayey paddy soil (PCP) using lysimeter, following the sequence of the soil profiles identified by investigating soil profile. After application of urea in paddy field-based two-cropping rice, TN and NH4+-N concentrations in floodwater reached peak on the 1st and the 3rd day, respectively, and then decreased rapidly over time; all the floodwater NO3−-N concentrations were very low; the pH of floodwater gradually rose in case of early rice within 15 d (late rice within 3 d) after application of urea, and EC remained consistent with the dynamics of NH4+-N. The applied CRNF, especially 70% CRNF, led to significantly lower floodwater TN and NH4+ concentrations, pH, and EC values compared with urea within 15 d after application. The monitoring result for N loss due to natural rainfall runoff indicated that the amount of TN lost in runoff from paddy field-based two-cropping rice with urea application in Dongting Lake area was 7.47 kg ha−1, which accounted for 2.49% of urea-N applied, and that with CRNF and 70% CRNF application decreased 24.5 and 27.2% compared with urea application, respectively. The two runoff events, which occurred within 20 d after application, contributed significantly to TN loss from paddy field. TN loss due to the two runoffs in urea, CRNF, and 70% CRNF treatments accounted for 72, 70, and 58% of the total TN loss due to runoff over the whole rice growth season, respectively. And the TN loss in these two CRNF treatments due to the first run-off event at the 10th day after application to early rice decreased 44.9 and 44.2% compared with urea, respectively. In conclusion, the 15-d period after application of urea was the critical time during which N loss occurred due to high floodwater N concentrations. But CRNF decreased N concentrations greatly in floodwater and runoff water during this period. As a result, it obviously reduced TN loss in runoff over the whole rice growth season.

脱甲河农业流域土壤沉积物氮素时空分布与N 2 O释放

为研究脱甲河农业小流域氮素输出特性，运用流动注射仪法和顶空平衡-气相色谱法于2015年4月—2016年1月对流域内4级河段（S1、S2、S3和S4）稻田-岸坡-河底沉积物土壤铵态氮（NH4+-N）、硝态氮（NO3--N）及水体溶存氧化亚氮（N2O）浓度进行了连续10个月的监测，并利用双层扩散模型法对水系N2O排放通量进行了估算.结果表明：脱甲河流域稻田-岸坡-河底沉积物NH4+-N含量逐渐升高，NO3--N含量逐渐降低，其中，岸坡及河底沉积物土壤中的氮主要以NH4+-N形式为主，均值分别为（7.38±0.62）mg·kg-1和（16.49±1.70）mg·kg-1；稻田土壤和脱甲河水体中的氮主要以NO3--N为主，均值分别为（7.40±0.81）mg·kg-1和（1.55±0.03）mg·L-1.水体溶存N2O浓度范围在0.005~7.37 μmol·L-1之间，均值为（0.54±0.05）μmol·L-1；扩散通量在-1.11~1811.29 μg·m-2·h-1之间，均值为（130.10±12.04）μg·m-2·h-1，每年向大气输出的N2O量为11.40 kg·hm-2.其中，在早稻生长初期和早晚稻收割、栽种交替时段N2O输出量达到高峰.空间上，N2O扩散通量表现为S1 < S4 < S3 < S2，S1级河段显著低于其他3级河段（p < 0.01）.相关分析表明，脱甲河表层水体N2O扩散通量与NH4+-N（r=0.87，p < 0.01）、NO3--N（r=0.80，p < 0.01）和水温（r=0.57，p < 0.01）呈显著正相关，流域内稻田-岸坡-河底沉积物及水体NH4+-N和NO3--N浓度间相关性不显著.脱甲河农业小流域氮素流失主要包括稻田-岸坡-河底沉积物中铵态氮、硝态氮及水体中N2O，在水稻栽种期间出现高峰，存在较大氮素流失风险，因此，开展农业小流域氮素流失研究对区域氮素周转及农业生产活动具有重要的指导意义.