Greenhouse gas fluxes derived from agricultural practice in forage crops in the atlantic area

Resumo

The overall aim of this thesis was to evaluate the impact of the agricultural practices for dairy farming on losses of GHG from soils under the Atlantic climatic conditions in Galicia (NW Spain), in order to make recommendations for practices that reduce emissions and increase crop yields. Large inter-annual and seasonal variations in the N2O emissions from grasslands with dairy cattle grazing management and mineral fertilization were observed. These losses were driven by climatic and soil variables but also affected by differences in N management. Under a scenario of future climate change in Galicia, predicted by a global climatic model, better grazing management could help to reduce annual N2O losses derived from N-management. Dry-wetting episodes can significantly contribute to increased annual N2O emissions from soils and therefore justifies further research to indentify the exact mechanisms in Galician soils. In grasslands, the use of cattle slurry as fertilizer did not significantly increase N2O emissions with respect to mineral fertilization. Similar conclusions were obtained during forage maize cropping when organic fertilizers (injected cattle and pig slurries) were compared with mineral fertilization. The soils used in these experiments were C-rich so this factor did not limit N2O production, and the large soil mineral N contents stimulated the losses of this gas. Modelling the effect of slurry application technique on emissions from grasslands showed that large proportions of mineral N are lost from slurries by ammonia volatilization when surface broadcast compared to when injected. When using slurries as fertilizers, shallow injection is recommended rather than surface broadcast application in order to mitigate indirect losses of N2O and increase the fraction of mineral N available for plant N uptake. In terms of emission factors, the fraction of N lost as N2O was lower than 1% proposed by IPCC when fertilizer applications coincided with dry weather conditions and mineral N was limited in soils. However, under the typical climatic conditions of the Atlantic area, that led optimal WFPS values for denitrification, losses of N2O derived from fertilization can reach values beyond 1%, especially when soil mineral N levels are large. Grassland soils in the Atlantic area were sinks of CH4 and mineral fertilizer applications did not modify soil capacity to CH4 uptake. Similar conclusions were obtained for slurry applications, if the CH4 emissions observed immediately after slurry application that resulted from the release of the dissolved CH4 in the slurry were not considered. That denoted that the N applications to soils that received N for many years did not modify methanotrophy activity of the microorganisms present in those soils. Slurry applications did not cause an overall effect in the ecosystem respiration compared to non-treated or mineral fertilized grassland soils even when the resulting high CO2 emissions observed immediately after slurry applications were considered. In grassland soils, mineral fertilizers and cattle slurries caused similar total CO2 equivalents to produce the same yields, so both fertilizers could be used. However, if the costs of purchasing mineral fertilizers is considered, using injected slurries as fertilizer would be more beneficial for dairy farmers as animal wastes produced on farms would be recycled and milk production costs would be reduced. In forage maize soils, organic and mineral fertilizers resulted in similar yield-scaled N2O emissions (expressed as dry matter or N uptake) . However, they failed to efficiently increase crop yields and caused high losses of N2O. Thus, the initial soil N contents at the moment of the N application and the dynamics of soil organic matter mineralization must be considered to adapt N rates to efficiently meet crop demands, especially in the period between sowing and top dressing application when demands are small.