Nitrogen fertilization of grass leys: Yield production and risk of N leaching

The soil surface balance of nitrogen (N), calculated as the difference between N inputs and output (harvested yield), is a principal agri-environmental indicator that provides information on the potential loss of N to surface or groundwater. Relevant models of yield response to N fertilization could prove helpful in minimizing N balance and simultaneously maintaining high-yield production. For this purpose, we used meta-analysis to quantitatively summarize 40 Finnish N fertilization experiments on perennial grass leys. These experiments took place on mineral and organic soils at 17 sites over the past five decades. We assessed the effect of inorganic N fertilization on grass yields and N balances, and further estimated the potential to reduce N input and N balances. Since the relationship between N balance and N leaching is often complex, we estimated the relationship by using the COUP model (Coupled heat and mass transfer model for soil-plant-atmosphere systems) and by reviewing the 12 Nordic studies on N leaching experiments involving lysimeters and drained field plots. Nitrogen applications, together with the grass yield with no added N (N-0-yield), accounted for 80-95% of the variation in the yield response of perennial grass leys; with increasing N-0-yield, the yield response dropped considerably. The developed yield response models can serve to construct a dynamic tool for growers to adjust N applications for maximizing economic profitability. However, such a tool would require growers to estimate the magnitude of N-0-yields on their fields by, for example, leaving some representative areas unfertilized for a few years. The N balance of grass leys linearly correlated with N input in fertilizer (R-2 = 0.86-0.88), and an increase of 10 kg ha(-1) in fertilization was associated with a 4.8 and 6.4 kg ha(-1) increase in the N balances in mineral and organic soils, respectively. Otherwise N fertilization affected the N balance consistently across the studies. Evidently, adjusting fertilization to attain the economic optimum according to the developed models may reduce N fertilization, particularly when N-0-yield is high, and thus lower the N balances for perennial grass leys. However, concerns about the risk of N leaching losses when using only inorganic N fertilization seems less crucial due to its low level and weak association with N balances. Even vigorous drops in N input and N balance would result only in minor reduction of N leaching loss, by a maximum of 3 kg ha(-1) yr(-1) in mineral soils. (C) 2016 Elsevier B.V. All rights reserved.