Study On Nitrogen And Phosphorus Loss From Different Croplands With Simulated Rainfall In Taihu Region

It was reported that N and P losses from cropping land into water system were mainly through runoff. Taihu Lake region is located at lowland plain with abundant precipitation and water-nets consisting of rivers, channels, ditches and flooding crop land. Under consisting raining period, which occur in winter or summer season, N and P loss through leaching may take place. Generally, N and P losses are depending on the losing procedures (runoff or leakage), the cropping lands (vegetable or rice field), the fertilizer rates and the soil nutrient concentrations. However, few works were done on N and P losses through runoff, leakage, from different cropland in Taihu Lake region. In this study, 12 site-specific rainfall simulation experiments were done in order to understand N and P losses through runoff, leakage and from different cropland. These experiments distributed in Shanghai and Zhejiang province and covered three different cropping land, i.e. rice filed, long-term vegetable land and vegetable land for less than 5 years. The main results were as follows.Under 80mm rainfall simulation, the results of 12 experiments showed that the amount of runoff water and leakage water in vegetable land were not different apparently, with 57.8% and 64.6% to the rainfall respectively. The amount of runoff water in rice field is higher than that of vegetable land, with an average of 89% to the rainfall. For vegetable land, the outflow lasting time after rainfall stopping under runoff is only 1/18 of that under leakage, with 1.7 minutes and 25.2 minutes respectively, but the velocity of runoff is 3 times faster than that of leakage, with 96.2 ml/min and 31.3 ml/min respectively.There are no apparent differences of total N loss through runoff among long-term vegetable land, vegetable land for less than 5 years and rice field, with 6.78,3.36å’Œ1.28 kg/ha ,respectively, and with average of total P concentration of 12.63,8.25,1.71 mg/L, respectively. And there are also no apparent differences of total N loss through leakage between long-term vegetable land and vegetable land for less than 5 years, with 78.83, 35.62 kg/ha, respectively,and with average of total N concentration of 164.32,56.32 mg/L, respectively. Analysis showed that NO₃^--N is the main form in leakage water, accounting for more than 70% of the TN, but that total dissolved nitrogen was the main form in runoff water. for that of runoff. The On the 8 sites of vegetable lands, analysis showed that total N losses (32.5 kg/ha) from leakage is significantly higher than that ( 5.4 kg/ha)of runoff.There are no apparent differences of total P loss through runoff among long-term vegetable land, vegetable land for less than 5 years and rice filed, with 0.93, 0.36 and 0.13 kg/ha,respectively, and with average of total P concentration of 1.83,0.78,0.17 mg/L. There are also no apparent differences of total P loss through leakage between long-term vegetable land and vegetable land for less than 5 years, with an average 2.06 , 0.23 kg/ha, respectively, and with average of total P concentration of 3.09,0.38 mg/L, respectively. Total dissolved P losses of runoff and leakage are both higher than 50% of total P losses. On the 7 sites of vegetable land, analysis showed that there are no apparent differences of total P between through runoff and through leakage, with an average of 0.71,0.62 kg/ha, respectively.Results from the runoff experiments showed that when soil NO₃^--N content is 31.2～472.9 and 33.2～171.1 mg/kg, there are no significant correlations between the content of 0⁵ cm or 0～20 cm soil NO₃^--N and concetrations of total N, total dissolved N, NO₃^--N, NH4+-N, particulate N in runoff water. But results from the leakage experiments showed that there were significant correlations between the content of 0⁵ cm or 0～20 cm soil NO₃^--N content and the concetrations or amount of total N, total dissolved nitrogen, and NO₃^--N. The depth of 0-20 cm was the main depth for leakage, and soil NO₃^--N content of this soil depth is the main factor for N loss in leakage. The effect of 0⁵ cm soil NO₃^--N content was higher than other soil depth.Results from the runoff experiments showed that when Olsen-P content is 3.9～59.6 and 2.4～45.0 mg/kg, there are no significant correlations between the Olsen-P content of 0⁵ cm or 0～20 cm soil depth and concetrations of total P, total dissolved P, dissolved reactive P, dissolved organophosphorus and particulate P. And results from the runoff experiments showed that when soil Olsen-P content is lower than 20 mg/kg, the total P losses is lower than 0.8 kg/ha. Results from the leakage experiments showed that when the content of 0⁵ cm soil Olsen-P is lower than 20 mg/kg, the total P concetration is lower than 0.53 mg/L, and the total P losses is lower than 0.46 kg/ha, but when the content of 0⁵ cm soil Olsen-P is higher than that value, the total P concetration is more than 1.57 mg/L, the maximum is 8.50 mg/L, and the total P losses is up to 6.61 kg/ha.Results from experiments showed that N loss through leakage was higher than through runoff on vegetable farmland under 80 mm rainfall simulation. That mean the risk of N loss was high under consisting raining condition. There were few difference between P loss through runoff and through leakage. With increase of NO₃^--N content of 0⁵ cm and 0²0 cm soil depth and increase of Olsen-P content of 0⁵ cm soil depth, N and P loss were obviously enhanced both through runoff and leakage. That mean NO₃^--N and Olsen-P content of 0⁵ cm could be well reflected the potential of N and P loss .