106 mol NO11x 137 x 1021l 41 x 1015molNO eqn 620

Fig. 6.31 Estimated nitrogen fertilizer use in the USA and mean nitrate-nitrogen concentrations* in the Mississippi River at St Francisville near Baton Rouge, Louisiana (A on inset map) between 1955 and 1995. Inset map shows the Mississippi drainage basin and the region of low oxygen that forms offshore of the Mississippi delta in the Gulf of Mexico. Data courtesy of United States Geological Survey. * Note on units: although we mainly use molar units in this book, in some applications results are reported in mass units. Nitrate is a case in point. Much nitrate data are reported in milligrams of nitrate per litre (mgNO-l-1). However, another common unit is milligrams of nitrogen from the nitrate per litre (mgNO--N l-1) as used in this figure. Conversion of the former to the latter requires division by the molar mass of NO- (62) and multiplication by the atomic mass of nitrogen (14.01).

Fig. 6.31 Estimated nitrogen fertilizer use in the USA and mean nitrate-nitrogen concentrations* in the Mississippi River at St Francisville near Baton Rouge, Louisiana (A on inset map) between 1955 and 1995. Inset map shows the Mississippi drainage basin and the region of low oxygen that forms offshore of the Mississippi delta in the Gulf of Mexico. Data courtesy of United States Geological Survey. * Note on units: although we mainly use molar units in this book, in some applications results are reported in mass units. Nitrate is a case in point. Much nitrate data are reported in milligrams of nitrate per litre (mgNO-l-1). However, another common unit is milligrams of nitrogen from the nitrate per litre (mgNO--N l-1) as used in this figure. Conversion of the former to the latter requires division by the molar mass of NO- (62) and multiplication by the atomic mass of nitrogen (14.01).

It is difficult to estimate the natural pre-human NO- concentration in rivers, since most have now been affected by human activities to some extent. Nitrate concentrations in the Amazon are about 20 |mmoll-1 and are probably close to natural levels. Multiplying this NO- concentration by the global river flux gives an estimate of the natural nitrate input to the oceans:

0 0

Post a comment