Nutrient transfer from soil to surface waters:: Differences between nitrate and phosphate

Nitrate (NO3-) and soluble reactive phosphorus (SRP), the two major dissolved N and P species available to aquatic biota, respond differently to varying water discharge rates (Q) in agricultural drainage pipes and rivers (Fig. 1): SRP concentrations are positively related to Q, whereas NO3- concentrations decrease with increasing discharge rates. In addition, NO3-N concentrations exceed (in mass units) SRP concentrations up to 700-fold even though the liquid manure applied to agricultural fields has a N:P ratio equal to only about 5. Preferential flow of rainwater across the soil column and different affinities of the two nutrients for the soil matrix explain these differences in behaviour and mobility: i. Concentrations of substances that have a high sorption affinity for the soil matrix (such as SRP) tend to increase in drainage pipes and streams as water discharge increases. ii. Concentrations of species that are not retarded by sorption processes (such as NO3-) and, hence, do not accumulate in the topsoil, tend to be negatively related to discharge rate. Differences in the availability and pool size of NO3- and SRP in the topsoil explain the different hysteresis patterns if NO3- and SRP concentrations are plotted versus the corresponding discharge rate during precipitation events (Fig. 2).