Is organic agriculture in line with the EU-Nitrate directive? On-farm nitrate leaching from organic and conventional arable crop rotations

Abstract

Sustainable intensive agriculture requires enhanced nitrogen use efficiency and reductions in nitrogen (N) leaching losses. To investigate if organic farming systems have potential to achieve this, a comparison between conventional and organic arable farming was carried out over a two-year period. Two differently managed specialized organic arable crop rotations (low N-intensive (Ol), semi-N-intensive (Os)), and a typical N-intensive conventional crop rotation (Ch) were studied. The study was done under commercial farm scale conditions covering close to 400 ha in total (5−23 ha field sizes) in a highly productive arable region on fertile soils in northern Germany. Nitrogen leaching losses were measured using ceramic suction cups. Two functional units were used as indicators for eco-efficiency: N losses per area and per unit of grain equivalent (GE) produced. Results revealed that organic arable crop rotations reduced the nitrate (NO₃-N) leaching loads per area significantly, with losses (kg NO₃⁻-N ha⁻¹ a⁻¹) of 22.0 in Ol, 24.4 in Os and 35.1 in Ch. In contrast, when using the functional unit ‘nitrate leaching per grain equivalent’ as an indicator for eco-efficiency the conventional system resulted in 50 % lower N leaching loads per GE. This was due to the much higher land use efficiency of the conventional system (115 GE ha⁻¹ vs 36 GE ha⁻¹ for Ol and 38 GE ha⁻¹ for Os), which resulted in N leaching losses (kg NO₃-N GE⁻¹) of Ch 0.32 for Ch, 0.64 for Ol, and 0.66 for Os. Regarding EU-Nitrate directive threshold for drinking water of 11.3 mg NO₃-N l⁻¹, all crop rotations exceeded the critical load. For the various main crop/preceeding crop sequences, the soil mineral N content at the end of the vegetation period and the autumn N uptake were the best predictors for N leaching, whereas for the aggregated effect across the various crop rotations within a farm system the N balance was the best descriptor. Considering N leaching loads our results show that both, specialized arable conventional and organic farming systems fail to meet the requirements of given environmental standards for water protection in the EU. This suggests that integrated approaches beyond such specialised systems are necessary, which are based on a balanced combination of forages (with two year leys), catch crops and cash crops.