长江下游“玉米-花菜”轮作模式下旱地降雨产流过程及氮磷输出特征研究

农田氮(N)、磷(P)的流失是水体富营养化的主要原因,然而农田径流N、P浓度与流失量受到前期土壤含水率(AMC)、降雨量、翻耕等多种因素影响,尚缺乏田间监测方法。本研究以位于长江下游的崇明岛旱地为研究对象,建立自动取样方法,在天然降雨条件下对农田径流量和水质进行高频率取样与连续监测,分析"玉米-花菜"轮作模式下降雨产流过程及N、P输出特征,采用径流曲线数(CN)表征土壤的持水能力,研究10、30、50 cm深处土壤AMC(AMC_(10)、AMC_(30)、AMC_(50))对CN的影响,并利用电导率(EC)来反映径流离子浓度的变化趋势以及降雨的稀释效应。结果表明:农田径流峰值滞后于降雨峰值,CN值取值范围为37～88,且与AMC_(10)线性关系最好(y=293.40x-39.41,R~2=0.790 5,P0.01),与AMC_(50)无显著相关关系;秸秆还田后的翻耕导致总磷(TP)和溶解性总磷(DTP)浓度升高,夏季翻耕显著增加了P浓度,但N的浓度没有增加;径流N浓度随着径流速度的增大而减小,但P浓度相反。EC与径流速度呈现出完全相反的变化趋势,降雨量增加了径流流量并产生了稀释效应,导致EC下降,且EC可以反映总氮(TN)、NO_3~--N的浓度变化,EC的变化可充分反映N的输出特征,而P的输出与N输出不同,当径流速度较大且EC较低时,P的浓度可能较高;NO_3~--N、DTP分别是农田N、P输出的主要形式,径流流量和养分浓度差异影响N、P的输出负荷,但养分浓度差异对NH4_~+-N、TP、DTP输出负荷的影响要大于径流流量对其的影响。

Runoff production process and nitrogen and phosphorus output characteristics from farmlands in the lower reaches of the Yangtze River under cauliflower and corn rotation

Nitrogen(N) and phosphorus(P) losses from farmlands are major causes of water eutrophication. The N and P nutrient runoff rates are affected by various factors, such as antecedent moisture content(AMC), rainfall levels, and tillage management, of which on-site monitoring methods are still unavailable. This study established an automatic sampling method to achieve high-frequency sampling and continuous monitoring of nutrient runoff loss under natural rainfall conditions. The study was conducted at an upland on Chongming Island, located in the lower reaches of the Yangtze River. The characteristics of rainfall runoff and N and P output under the corn-cauliflower rotation cropping system were analyzed. The soil water holding capacity was indicated by the CN, and the effect of AMC on CN at soil depths of 10, 30 cm, and 50 cm(AMC₁₀, AMC₃₀, AMC₅₀) was studied. The change in nutrient output and the dilution effect of rainfall were studied using electrical conductivity(EC). The results showed that the peaks of the runoff velocity lagged behind that of the rainfall intensity. The CN of the field ranging from 37 to 88 was most significantly correlated with AMC₁₀(y=293.40x-39.41, R²=0.790 5, P<0.01), but was not significantly correlated with AMC₅₀(P>0.05). Plowing led to an increase in total P(TP) and dissolved total P(DTP) concentrations but had no obvious influence on the N concentrations. N concentrations in runoff decreased when runoff velocity increased, but P concentrations exhibited the opposite trend. The changes in EC and runoff velocity showed a completely opposite trend. Higher rainfall resulted in an increase in runoff flow, and the consequent dilution effects resulted in a decrease in EC. The changes in EC could reflect the concentration changes of total N(TN) and NO₃^--N, and the output characteristics of N. However, the output of P was different from the output of N. During events with higher runoff rates and lower EC, the concentration of P might be higher.