Nitrogen balance as an indicator of nitrogen leaching in Finland

When measured data of nitrogen (N) losses to the environment are not available, calculated N balances are often used to indicate losses. The aim of this study was to describe the relations between N surface balances and N leaching losses in two agricultural runoff fields. Data of runoff fields dominated by cereal and grass production were available for clay soil for 1980–2001 and for sandy soil for 1992–2000. The total sum of N leaching via drainage and surface runoff was 2–40 kg N ha−1 year−1 in the clay soil and 2–104 kg N ha−1 year−1 in the sandy soil. The annual N surface balances were from −44 to 145 kg N ha−1 in the clay soil and from −63 to 417 kg N ha−1 in the sandy soil. The highest annual N leaching losses were usually not caused simply by high N balances, but, rather, were mainly due to poor management. The highest annual N leaching losses occurred in the clay soil from bare fallow and in sandy soil after application of slurry to the frozen soil.Since the annual N balance did not predict the measured N leaching in the annual datasets, several smaller datasets were formed that consisted of similar management techniques and periods of 4–10 subsequent years. In the sandy soil, when the annual N balance (Nfertiliser + Nslurry + Nfixation − Nharvest − Nvolatilised from slurry) and the volume of subsurface drainage were used as predictors in a linear regression model, at most 56% of the variation in N leaching could be predicted. The clay soil received mineral N fertilisers only, and the annual N balance was calculated as Nfertiliser − Nharvest. When the N balance and the volume of total runoff or only subsurface drainage were used as predictors in a linear regression model, 55–60% of the variation in N leaching loss could be predicted. However, the average N balance calculated from 1980 to 1990 for the experimental clay soil plots, including bare fallows, predicted 71% of the variation in N leaching. Similarly, the average N balance calculated over 5 years for the slurry application experiment predicted 70% of the variation in N leaching in sandy soil. Average N balances were found to be useful indicators for N leaching when cultivation techniques included environmentally risky management, but when good agricultural practice was maintained, N leaching was not predicted by the balances.