Nitrogen source, transformation and fate within intensive dairy systems to inform sustainable intensification

Contamination and deterioration of natural water quality by nitrogen (N) from agricultural sources is a major threat to the environment. Globally, there is a societal expectation that sustainable food production should be achievable. The concept of sustainable intensification is based on to the equality between production and environmental targets. For this to become a reality, increased productivity must be accompanied by provision of clean water, air, habitats for biodiversity, recycling of nutrients and mitigation against climate change. Agriculture and food production rely heavily on external N inputs (e.g. fertilisers) and as agronomic systems generally have low use efficiency there is the risk of high N losses i.e. the leak of N excess to the environment. Agricultural landscapes contain many different soil/subsoil/bedrock typologies having heterogeneous N water attenuation capacities (intrinsic ability of soils to reduce contamination). Dairy farms represent complex environments, necessitating many techniques (isotopes, biogeochemical parameters, dissolved gases, bacterial gene abundances) used in combination, to provide a thorough characterisation of, examination of N source, transformation and fate along different subsurface pathways. These multiple techniques are currently seldom used in combination. In Ireland, 30% of milk production occurs in high rainfall conditions and heavy textured soil areas. For better grass growth, artificial drainage systems (shallow and groundwater systems) are installed. The role of land drainage in N transfer, transformation and fate is however relatively unexplored. These systems may reduce N transformation potential by, for example, creating unsuitable conditions for denitrification leading to greater nitrate (NO3--N) losses or by-passing zones of high soil N attenuation capacity further compromising sustainability targets. Indeed, the potential to use drainage systems as a monitoring tool, which covers large areas of contribution, has been neglected in terms of multiple techniques that could explore N transfer, transformation and fate. The concept of “sustainable intensification” includes all the aspects of agricultural productivity and environmental protection. The primary aim of this thesis was to examine this concept in terms of impacts and relationships of drainage systems installed at intensive sites on and with soil drainage classes, N transfer, transformation and fate and water quality to develop advises and a range of multiple techniques to improve and guide future management. Herein, this concept has been tested within a range of different contexts in terms of scale (farm, plot and laboratory), soil characteristics (from heterogeneous soils to heavy homogeneous types), drainage designs (from random to parallel and from single to multiple, from moles to piped systems) and techniques (gaseous emissions, biogeochemical parameters, isotopic signatures, gene abundances) in order to produce a more refined interpretation of artificial drainage systems and the role they play within the sustainable intensification framework. As agricultural landscapes contain many different soil types with heterogeneous nitrogen (N) attenuation capacity, a zone of contribution (ZOC) surrounding a borehole and an installed drainage system was used to interpret subsurface hydro-biogeochemical functional capacity within four hydrologically isolated plots. By using the drainage system as a monitoring tool in combination with multiple techniques, a disconnectivity and complexity of the system was highlighted in terms of contamination sources uncovered and separate water attenuation functionalities. This study showed that collating isotopic, dissolved gas and biophysical data from the drainage system and groundwater locations creates a clearer conceptual model of a site showing an interpretation of source and attenuation within these areas. Next, the study moved to five commercial farms where surface or groundwater gley soils were artificially drained (site specific designs) and monitored. This study aimed to investigate how drainage system design (e.g. shallow and groundwater) affected N transformation and how the multi-technique method could be broadened to rank commercial dairy farms in terms of their N attenuation capacity. These techniques showed the ability to divide sites into three distinct groups according to their respective water attenuation potential highlighting different sustainability for different drainage designs. A tool to compare or rank sites in terms of their N sustainability was created. From micro-plot and field this tool was then moved to farm scale on a heterogeneous soil landscape to infer further knowledge on attenuation within drained versus un-drained areas and future management to decrease N losses. The tool was able to divide the farm into several groups with different attenuation ability which was not disrupted by the imposed artificial drainage system. The identified groups and areas could be subjected to differential management to further move towards sustainability. The use of bacterial gene abundance was further tested as a tool to improve pour characterisation tool and lastly, an incubation experiment was conducted to examine more closely the effect of land drainage and saturation on an N problematic site and its gaseous phase component. Major findings of the present study include: • Techniques such as natural isotopic abundances, biogeochemical parameters, isotopomers, gaseous emissions, dissolved gasses, can be combined to elucidate sustainability of intensive dairy systems. • Drainage systems can be used, when analysed with the above techniques, to elucidate water quality but more interestingly can be used as a monitoring tool, in combination with groundwater monitoring networks, to interpret net N source, transformation and fate, over large areas, on agricultural landscapes. • Although surpluses of N were found to be uniform across intensive dairy sites on the present study, the soil water attenuation function and “net denitrification” varied considerably across sites. This means that there was considerable variation within dairy farms in terms of N sustainability, which will have consequences for sustainable intensification. • Drainage systems affect this water attenuation function differently depending on their design. This means that the presence of a drainage system on agricultural landscapes does not infer poor water quality, more importantly than absence/presence is the depth and type of drainage system present. • During this assessment the techniques used in combination with the present study worked well to characterise and rank sustainability.