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 (Nopt) 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 Nopt: the higher the yield, the smaller the response and Nopt. 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.