Environmental effects of reactive nitrogen emissions from livestock farming

Resumen

Historically, the creation of reactive nitrogen (Nr: all nitrogen species except N2) has taken place through natural limiting processes and little Nr was accumulated in environmental reservoirs. However, in the last two centuries, this dynamic has been dramatically altered by human action, resulting in negative impacts on ecosystems and human health. In Europe, human perturbation of the nitrogen cycle is considered to be primarily driven by agricultural activities. In this sense, it is said that is the human use of livestock and the consequent need for large amount of animal feed, the dominant driver altering the nitrogen cycle. The present dissertation aims to assess environmental concerns related to Nr resulted from the livestock production sector. In the first three works, the impacts of nitrogen emission from livestock buildings on the adjacent oak forest have been assessed. Livestock farms are known as “hotspots” of ammonia (NH3) emissions. Consequently, downwind detrimental impacts of NH3 close to the farms are substantial. The studies were conducted in an oak forest in the vicinity of two livestock facilities in northern Spain. First of all, atmospheric NH3 and NO2, N deposition in throughfall (TF), soil N leached as well as lichens, bryophytes and forest ground vegetation were studied at increasing distance from the farms. Atmospheric NH3 decreased exponentially with the distance from the farms. Our results suggested an uptake of N in the closest plot to the farms which resulted in a nutritional imbalance that end up in a detrimental impact on foliage biomass. Furthermore, organic nitrogen deposition contributes from 23 to 49 % to N deposition, showing the importance of this fraction which is often overlooked in the characterization of nitrogen deposition. Emissions from the farms and N recirculation within the forest have been identified as potential sources of organic N. On the other hand, N saturation was observed in the closer sites to the livestock buildings, reflecting the long-term influence of the farms on the forest. Finally, the impact of the farms on vegetation species was evidenced. Secondly, the effect of atmospheric NH3 on soil microbial activity was assessed. To that end, the activity of soil extracellular enzymes involved in nutrient cycle and soil biological parameters in the vicinity of the livestock farms were studied. Our results suggested that NH3 concentration modulates the activity of extracellular enzymes involved in nutrient cycle and have a detrimental impact on soil microbial biomass. Furthermore, we found that soil metabolic quotient (qCO2) is a promising indicator of soil health in regards to NH3 pollution. In the fourth chapter of the present thesis the use of isotopic signatures to assess the impact of NH3 on the oak forest in the vicinity of the livestock farms was addressed. The analysis of N and C isotope ratios (15N/14N and 13C/12C) in plants and other ecosystem pools has been used to assess biogeochemical processes related to ecosystems. Our results suggest that soil 15N signature has been identified as a good indicator of the availability of N to plants. On the other hand, NH3 uptake by the foliage and mosses evidenced by their 15N signature as well as tissue N content caused photosynthetic impairments as reflected by their 13C isotopic signatures. This deterioration was especially important in mosses where the C content showed a remarkable decrease along the NH3 gradient. In addition to the evaluation of the impact of emissions from livestock buildings, Nr emissions from animal excreta as well as a mitigation strategy were evaluated in Chapter 5. Our results suggest that increasing HA concentration in animal urine had no impact on N2O emissions. Soil HA leaching under the permanent soil waterlogging conditions and sorption into clay under optimal soil pH may explain the lack of observed HA effect.