Nitrogen balances and yields of spring cereals as affected by nitrogen fertilization in northern conditions: A meta-analysis

Nitrogen (N) fertilization decisively influences the yield of arable crops as well as increases N surplus, thereby leading to N losses to the environment. The study aimed to estimate the potential to reduce both N balances and N inputs without economic loss for agriculture in northern conditions. To that end, the source of variation in yield and N balance responses was studied and, taking into consideration the yield response variability, precise economically optimal N rates (N-opt) were estimated. In the present study, 61 Finnish N fertilizer experiments with spring cereals (barley, wheat and oats) cultivated on clay and course-textured mineral soils were gathered from the years 1940-2004 in order to integrate studies by means of research synthesis and meta-analysis. With increasing N rates, N balances increased more steeply in moderately acidic (MA) soils (pH 4.9-5.7) than in slightly acidic (SA) soils (pH 5.8-6.9). An increase of 10 kg N ha(-1) in fertilization was associated with a 7.4 and 5.0-6.6 kg ha(-1) year(-1) increase in the N balances for MA and SA, respectively. This indicates that applying increasing amounts of N to fields with low pH may pose a greater risk to the environment, Moreover, the yield without added N strongly governed both the yield response and N-opt: the higher the yield, the smaller the response and N-opt. Ignoring this fact may lead to overuse of N, as demonstrated for the present Finnish recommendations. In contrast, taking into account the difference in response would reduce N input by 20-75 kg ha(-1) and N balances by 10-40 kg ha(-1) year(-1) with no economic loss. The current Finnish fertilizer recommendations based on yield expectancy (the higher the expected yield, the higher the N fertilizer recommendation) should be reconsidered to take into account the assessment of yield without added N in farmers' fields. This would make it possible to combine both economic profitability and environmental protection. (C) 2012 Elsevier B.V. All rights reserved.