No effect of cropping system on the greenhouse gas N2O

Organic farming is comparable to conventional in terms of field emissions of the strong greenhouse gas nitrous oxide (N2O). Our study points to the need for increased yields in organic farming as measure to reduced emissions per unit of produce

Biologically fixed N2 as a source for the N2O production in a grass clover mixture, measured by 15N2

The contribution of biologically fixed dinitrogen (N2) to the nitrous oxide (N2O) production in grasslands is unknown. To assess the contribution of recently fixed N2 as a source of N2O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a 15N2-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass-clover pots were sampled for N2 fixation determination, whereas the remaining half were examined for emission of 15N labelled N2O for another eight days using a static chamber method. Biological N2 fixation measured in grass-clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m-2 d-1 at 16, 26 and 36 weeks after emergence, respectively. The drop in N2 fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m-2 d-1, which represented 1.7 ± 0.3 % of the N accumulated in grass shoots during the labelling period. Total N2O emission was 91, 416 and 259 μg N2O-N m-2 d-1 at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N2 was emitted as N2O on a daily basis, which accounted for 2.1 ± 0.5 % of the total N2O-N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N2O

Greenhouse gas emissions from cultivation of energy crops – is it important?

Replacing fossil fuel-derived energy with biomass-derived energy is commonly emphasized as a means to reduce CO2 emissions. However, our study highlights the risk of large greenhouse gas emissions when wastes from bioenergy production are recycled as fertilizer for energy crops. Crop management affects the magnitude of these emissions, which in some cases negate a considerable fraction of the global warming savings associated with biofuels

Stor forskel på energiafgrøders udledning af drivhusgas

Udledning af drivhusgas ved dyrkning af energi-afgrøder kan ophæve en betydelig del af den drivhusgas-gevinst, der er forbundet med biobrændsle

Drivhusgas fra økologiske sædskifter

Økologiske marker udleder lige så meget af drivhusgassen lattergas som konventionelle marker. Udfordringen er at undgå ophobning af mineralsk kvælstof i jorden

Production of N2O in grass-clover pastures

Agricultural soils are known to be a considerable source of the strong greenhouse gas nitrous oxide (N2O), and in soil N2O is mainly produced by nitrifying and denitrifying bacteria. In Denmark, grass-clover pastures are an important component of the cropping system in organic as well as conventional dairy farming, and on a European scale grass-clover mixtures represent a large part of the grazed grasslands. Biological dinitrogen (N2) fixation in clover provides a major N input to these systems, but knowledge is sparse regarding the amount of fixed N2 lost from the grasslands as N2O. Furthermore, urine patches deposited by grazing cattle are known to be hot-spots of N2O emission, but the mechanisms involved in the N2O production in urine-affected soil are very complex and not well understood. The aim of this Ph.D. project was to increase the knowledge of the biological and physical-chemical mechanisms, which control the production of N2O in grazed grass-clover pastures. Three experimental studies were conducted with the objectives of: I assessing the contribution of recently fixed N2 as a source of N2O II examining the link between N2O emission and carbon mineralization in urine patches III investigating the effect of urine on the rates and N2O loss ratios of nitrification and nitrification, and evaluating the impact of the chemical conditions that arise in rineaffected soil The results revealed that only 3.2 ± 0.5 ppm of the recently fixed N2 was emitted as N2O on a daily basis. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N2O. Furthermore, increased N2O emission following urine application at rates up to 5.5 g N m-2 was not caused by enhanced denitrification stimulated by labile compounds released from scorched plant roots. Finally, the increase of soil pH and ammonium following urine application led to raised nitrification rate, which appeared to be the most important factor explaining the high initial N2O emission from simulated urine patches. The results are discussed in relation to the national N2O inventory guidelines issued by the Intergovernmental Panel on Climate Change, and the environmental impact of organic farming practises are also considered. Suggestions for future research are outlined

Lack of increased availability of root-derived C may explain the low N2O emission from low N-urine patches

Urine deposition on grassland causes significant N2O losses, which in some cases may result from increased denitrification stimulated by labile compounds released from scorched plant roots. Two 12-day experiments were conducted in 13C-labelled grassland monoliths to investigate the link between N2O production and carbon mineralization following application of low rates of urine-N. Measurements of N2O and CO2 emissions from the monoliths as well as δ13C signal of evolved CO2 were done on day -4, -1, 0, 1, 2, 4, 5, 6 and 7 after application of urine corresponding to 3.1 and 5.5 g N m-2 in the first and second experiment, respectively. The δ13C signal was also determined for soil organic matter, dissolved organic C and CO2 evolved by microbial respiration. In addition, denitrifying enzyme activity (DEA) and nitrifying enzyme activity (NEA) were measured on day -1, 2 and 7 after the first urine application event. Urine did not affect DEA, whereas NEA was enhanced 2 days after urine application. In the first experiment, urine had no significant effect on the N2O flux, which was generally low (-8 to 14 μg N2O-N m-2 h-1). After the second application event, the N2O emission increased significantly to 87 μg N2O-N m-2 h-1 and the N2O emission factor for the added urine-N was 0.18 %. However, the associated 13C signal of soil respiration was unaffected by urine. Consequently, the increased N2O emission from the simulated low N-urine patches was not caused by enhanced denitrification stimulated by labile compounds released from scorched plant roots

Is organic farming a mitigation option? – A study on N2O emission from winter wheat

The objective of the study was to evaluate whether N2O emissions from cropping systems are affected by 1) organic versus conventional farming, 2) proportion of N2-fixing crops in the rotation and 3) use of catch crops

Nødvendig viden om mark-emissioner for valg af biobrændsel

Det bliver ofte fremhævet, at CO2-udledningen kan reduceres ved at erstatte fossile brændsler med brændsler produceret på plantebiomasse. Vores studie viser dog, at der er risiko for store drivhusgasudledninger, når restaffald fra bioenergiproduktion føres tilbage til marken som gødning. Afgrøde og dyrkningspraksis påvirker størrelsen af udledningen, som i nogle tilfælde ophæver en betydelig del af drivhusgas-gevinsten forbundet med biobrændsler

Consequences of agro-biofuel production for greenhouse gas emissions

The objective of the study was to examine the effect on N2O and CH4 emissions when residues from bio-energy production are recycling as organic fertilizer for a maize energy crop. The study showed that the N2O emission associated with the cultivation of the maize crop offset a considerable faction of the fossil CO2, which was avoided by producing the biofuels