不同雨强和植被盖度对稻田径流及氮素流失的影响

阐明径流及养分流失特征对制定农田径流削减策略、降低面源污染发生风险具有重要意义。为明确稻田径流和氮素流失对雨强的响应，分别在水稻生育前期（低植被盖度）和后期（高植被盖度）选择3个降雨强度低雨强（SI），30 mm·h^-1；中雨强（MI），60 mm·h^-1；高雨强（LI），90 mm·h^-1]进行了田间降雨模拟试验。结果表明：稻田径流率均呈先上升后下降的趋势，且径流率峰值随雨强增大而增加。不同降雨强度下径流率峰值分别为72.58 （SI）、126.45 （MI）、234.90 （LI） m³·hm^-2·h^-1（低植被盖度）和41.94（SI）、70.02 （MI）、83.30 （LI） m³·hm^-2·h^-1（高植被盖度）。径流氮素浓度在初始产流期较高，不同植被盖度和雨强下径流氮素浓度随径流时间的变化均可以用对数函数方程进行描述Y=a-b×ln （X+c），P<0.01]。与浓度表现不同，受径流率影响，径流发生后的前40min内的氮素流失风险较高，特别是在径流发生后的20~30 min （流失率峰值时间）。低植被盖度下氮素流失率更易受降雨强度影响，两种植被盖度下氮素流失率峰值分别为0.07 （SI）、0.10 （MI）、0.27 （LI）kg·hm^-2·h^-1（低植被盖度）和0.05 （SI）、0.04 （MI）、0.06 （LI）kg·hm^-2·h^-1（高植被盖度）。因此，不同雨强下氮素流失负荷在低植被盖度条件下差异显著，且高降雨强度的氮素流失量（10.02mg·m^-2）显著高于中、低降雨强度，铵态氮（NH₄⁺-N）是稻田径流氮素流失的主要形态（占比约41%~52%）。氮素流失负荷与径流发生前期（0~20 min）和中期（20~40 min）的径流率及氮素浓度密切相关。结果表明，初始产流期是稻田氮素流失的高浓度风险期，而径流发生后的20~30 min内氮素流失最快，低植被盖度下径流发生更易受雨强影响。

Effects of different rainfall intensities and vegetation coverages on runoff and nitrogen loss from rice fields

To develop strategies to reduce the risk of non-point source pollution, it is important to elucidate the characteristics of runoff and nutrient loss. This understanding will inform the development of farmland runoff reduction strategies, which will reduce the risk of nonpoint source pollution. To understand the response of runoff and nitrogen loss from paddy fields to rainfall intensity, three rainfall intensitiessmall(SI), 30 mm·h^-1; middle(MI), 60 mm·h^-1; and large(LI), 90 mm·h^-1] were selected for the field rainfall simulation experiment. The experiment was conducted in the early(low vegetation coverage, LVC) and late(high vegetation coverage, HVC) rice growing stages. The results showed that the runoff rate in paddy fields first rose and then fell with runoff duration; the peak runoff rate increased with rainfall intensity. The peak runoff rates at different rainfall intensities were 72.58(SI), 126.45(MI) and 234.90(LI) m³·hm^-2·h^-1 under LVC, and 41.94(SI), 70.02(MI), 83.30(LI) m³·hm^-2·h^-1 under HVC. In addition, the nitrogen concentration in runoff water was the highest during the initial runoff period. The relationship between the nitrogen concentration and the duration of this concentration for different rainfall intensities under the two vegetation coverages was described using a logarithmic functional equation:Y=a-b×ln(X + c), P<0.01]. As opposed to the nitrogen concentration, the risk of nitrogen loss was higher during the first 40 min following the onset of runoff. This was influenced by the runoff rate, particularly during the initial 20~30 min after the runoff(time of peak loss rate). The nitrogen loss rate under LVC was more likely to be affected by rainfall intensity. The peak runoff rates at different rainfall intensities were 0.07 (SI), 0.10 (MI), 0.27 (LI) kg·hm^-2·h^-1 under LVC, and 0.05(SI), 0.04(MI), 0.06(LI) kg·hm^-2·h^-1 under HVC. As such, the loads of nitrogen loss with different rainfall intensities varied significantly under LVC, and nitrogen loss at high rainfall intensity(10.02 mg·m^-2) was considerably greater than that at moderate and low rainfall intensities. Here, ammonium nitrogen(NH₄⁺-N) was the major form of nitrogen loss(accounting for approximately 41%~52%). The nitrogen loss load was closely related to the runoff rate and nitrogen concentration in the initial(0~20 min) and middle(20~40 min) runoff periods. The results suggest that during the initial runoff period, the nutrient concentration peaks, whereas the nitrogen loss rate in paddy fields is the highest within 20~30 min after the onset of runoff. The runoff in the LVC scenario is more likely to be affected by rainfall intensity.