Hi Dr Goitom Adhanom, 

May it's due to the column you used, where there is a preferential flow path through the wall of the column??? 

Hi Goitom

In addition to the suggestion given by Louadj, there are other possibilities. 

1) Did you measure concentration or total NO3 loss (just checking)

2) How long of a leaching event did you measure? and was the soil saturated at the high flow rate?  If it was a long leaching event (days)  this may have limited mineralization and nitrification rates reducing available NO3 to be leached..

John

Thanks John and Louadj

John: Yes I did measure nitrate-N concentrations at each of the pore volumes (PVs), where  a total of 9 PVs was collected at each flow rate. It was a continuous application that lasted few months (with an intermittent drying period between flow rates) and definitely the column was saturated especially during the high flow rate. 

Hi Goitom

Just checking. You did multiply the concentration of nitrate by the total volume of effluent to come up with the total (grams) loss of NO3 from each treatment?

John

Hi John

Yes, I did.

Hi John and Goitom, 

i suggest reading the paper from FADL entitled ' A modified permeameter for measuring hydraulic conductivity of soils , 1979) in the journal Soil science. and the first work in the field of solute transport are by Nielson and biggar in the 60's and also i suggest you reading this paper http://www.ars.usda.gov/sp2UserFiles/Place/20360500/pdf_pubs/P1292.pdf

i hope it will help 

one of the reason could be if columns are open, evaporation may condense the leachate to make high concentration in low flow rates (if  evaporation prolonged). 

Have you measured ammonium and organic nitrogen in the leachates? Considerable amounts of these forms of nitrogen may be leached at high flow rates. When the residence time of the effluent in the soil is higher (low flow rates), ammonium and organic nitrogen will oxidise to nitrate, provided the flow is not saturated or wetting and drying conditions alternate.