Silage maize cultivation is gaining importance in organic farming, and thus its environmental and climate impacts. The effects of digestate fertilization in combination with different catch crops and tillage intensities in maize cultivation are investigated in a long-term field experiment in southern Germany. The tested variants are (a) maize after winter rye, plowed, unfertilized and (b) fertilized with biogas digestate, (c) maize after legume-rich cover crop mixture, mulch seeding, fertilized with digestate, and (d) maize in a white clover living mulch system, fertilized with digestate. Over three years (2019 to 2021), crop yields and N balance were analyzed, N2O emissions were measured in high temporal resolution using the closed chamber method, and soil moisture, ammonium, and nitrate contents were continuously determined. Maize dry matter yields ranged from 4.2 Mg ha−1 (variant a, 2021) to 24.4 t ha−1 (variant c, 2020) depending on cropping intensity and annual weather conditions. Despite relatively high nitrogen fertilization with digestate, the N balances were negative or nearly balanced; only in 2021 did the N surplus exceed 100 kg ha−1 (variant b and c) due to low yields. In maize cultivation, relatively low N2O-N emissions (1.0 to 3.2 kg ha−1) were measured in the unfertilized variant (a), and very high emissions in variant b (5.6 to 19.0 kg ha−1). The sometimes extremely high N2O emissions are also due to soil and climatic conditions (high denitrification potential). The experimental results show that cover crops, living mulch, and reduced tillage intensity in silage maize cultivation can reduce N2O emissions, improve nitrogen balance and increase maize yields.
Agricultural ammonia (NH3) emissions can have serious environmental impacts, lower fertiliser nitrogen-use efficiencies, and cause economic losses. NH3 losses may not only occur directly from organic fertilisers such as biogas digestates when applied to crops, the crops themselves may also be a source of ammonia emissions. Wheat yields from 14 years of an organic small plot fertiliser trial fertilised with biogas digestate were analysed to determine if there was significant lateral N transfer between plots. A simple NH3 loss/gain model was developed to calculate possible N gains and losses via NH3 volatilisation from the applied digestate. This model was tested using NH3 volatilisation measurements. In addition, 15N isotope measurements of crop biomass were used to analyse plant N uptake. While digestate fertilisation increased wheat yields, yield patterns indicated that NH3 emissions from plots fertilised with biogas digestate affected yields in neighbouring unfertilised plots. Measurements of ammonia losses and gains in the field validated our modelling results, showing that 55% of digestate NH4+-N was volatilised. 15N isotope analysis indicated that crops took up as much as 30 kg ha−1 NH3-N volatilised from digestate, and that plots closer to fertilised plots took up more of this NH3-N than crops further away from fertilised plots. Our results imply that neither the results from the fertilised plots nor from the unfertilised plots are without bias. To avoid inadvertently introducing artefacts into fertiliser field trials, plot sizes need to be increased and treatments situated further apart.
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