Nitrogen dynamics of grass as affected by N input regimes, soil texture and climate: lysimeter measurements and simulations

Abstract

Nitrate leaching is often low from grasslands, primarily due to their long period of N uptake compared to arable crops. In the present paper we explore the combined effects of N input regime, soil type and climatic conditions through a combination of field lysimeter studies and simulation modeling of temporary grassland. A lysimeter consisting of eight 10 × 4 × 1 m individually drained cells was constructed in SW Norway, a region with a cool and wet marine climate. Six cells were filled with silty sand and two cells with coarse sand. The lysimeters were cropped first with barley for two years, followed by five years of grassland. Treatments included various combinations of N input (fertilizer, manure or both), and the results were analyzed by means of two coupled dynamic simulation models (CoupModel: a heat- and water transport model, and SOILN_–NO: a soil nitrogen model). The parameterized models were further used to assess a scenario with a more continental climate (somewhat cooler and drier). All treatments resulted in a net export of N, with N amounts removed at harvest ranging between 121 and 139% of that applied. Measured N yield from the treatment receiving manure only was almost as high as that from the treatment receiving fertilizer only, even though it received on average about 80 kg ha⁻¹ less inorganic N. Nitrogen losses through leaching were in the range of 5–23% of the N input, and soil type had a greater effect than source of N input. The inorganic N fraction of the leachate was 71–82% of the total N, and 98% of this was nitrate. The models gave reasonable simulation of N yields as well as of the timing and magnitude of nitrate leaching from the different treatments. They also clearly illustrated that the low nitrate leaching from the system is primarily attributable to a high plant N uptake. The scenario using weather data from a cooler and drier region showed a large decline in plant uptake of N outside the main cropping season, but simulated nitrate leaching was nevertheless significantly lower. With this scenario, precipitation was only 50% of that at the actual experimental site, and the lower temperatures during autumn and winter reduced net mineralization of soil organic N significantly. Thus, the reduction in precipitation and net mineralization of soil organic N apparently more than outweighed the effects of shorter growing season in the continental climate scenario.