Field scale agronomic and environmental consequences of overlapping N fertilizer application by disc spreaders

Abstract Application of mineral nitrogen (N) fertilizer is not uniform at the field scale when applied with disc spreaders. Some of the distribution variation is due to potentially predictable mechanisms, but the variation is also influenced by stochastic events and multiple interactions, making each combination of field, spreader and application event a unique case. This may be one of the reasons why the agronomic, economic and environmental consequences of uneven fertilizer distribution at the field scale remains poorly understood and insufficiently quantified. Previous studies have addressed the problem statistically or empirically in regularly shaped experimental plots or a subsection of a field. While these efforts provide valuable contributions to our understanding, they neglect critical areas of the field with regards to fertilizer application: Wedges, interfaces between the in-field and the headland and in-field obstacles. In this modelling study, we present an attempt to describe the variation in fertilizer application at the field scale and assess the consequences on winter wheat grain yields, grain N yields and N losses to the environment. By combining GIS and agroecosystem modelling, we assess yield and environmental effects of two spreaders with different working widths (24 and 48 m) in four field polygons selected to represent a relevant span with regard to size and geometry for Danish conditions. The effects of the combinations were assessed for two soils, a coarse sandy soil and a sandy loam. Both accuracy (average N input rate relative to target) and precision (evenness of distribution) was found to decrease in small (4−6 ha) and geometrically irregular fields compared to large (40−50 ha) and regular fields. Increasing the working width from 24 to 48 m increased the variation in small fields, but not in large. Grain yields were negatively affected by distribution variation, while there was a poor correlation with average applied N rate. In contrast, grain N yields were insensitive to distribution variation, but showed strong correlation with average N input rate. N leaching was affected by both the amount and distribution of applied N. Across all field x spreader x soil scenarios, field scale grain yields decreased between 0–877 kg DM/ha and annual N leaching increased by up to 9 kg N/ha when fertilizer application was simulated with a spreader rather than even application. In general, the agronomic results from the large and regular field resembled the results from previous studies much better than results from the smaller and/or irregular fields. If not accounting for the impact of field size and shape distribution in the landscape, the effects of uneven N application, and thereby the potential gains by improving spreader performance, may therefore be underestimated.