Macronutrient cycles: themed issue

Introduction

This themed issue focuses upon the key macronutrient cycles linking nitrogen (N), phosphorus (P) and carbon (C). N and P are required for biological metabolism, supporting both natural and anthropogenic systems. Disturbance of the interactions, or cycles, of these macronutrients, has had significant environmental and economic implications including soil nutrient enrichment, eutrophication of surface waters, reduced air quality, and loss of drinking water quality. These issues pose a threat not just to biodiversity, but also to public water supplies and public health.¹

Increases in nutrient fluxes have occurred under changing land use and population levels, due to combustion of fossil fuels, application of nutrient fertilisers, and burning of biomass² to a point where anthropogenic nutrient “footprints” can now be detected (Stevens et al. – DOI: 10.1039/c3em00690e). Nutrient dynamics have also changed under a perturbed climate system, however the influence of changing temperatures, precipitation patterns and altered CO₂ levels upon macronutrient cycles (MC) is not yet fully understood. Although national and inter-governmental measures have been implemented to initiate nutrient control processes, many focus upon a single nutrient,¹ and a greater understanding of nutrient interactions is required to ensure that feedback mechanisms do not undermine attempts to mitigate and adapt to changes. One such feedback involves increases in community respiration compared to photosynthesis, resulting in net CO₂ and N₂O release. Although in aquatic systems increases in N fixation will likely result in P limitation controlling in-stream plant growth, external organics generally support metabolics beyond that of the autochthonous alone, and thus aquatic ecosystem respiration will continue to increase, increasing CO₂ release to the atmosphere.³

Progress in critical science

Several research initiatives have begun to consider multi-component approaches to nutrient research, including the EU REFRESH programme, the DEFRA Demonstration Test Catchment Programme (McGonigle et al. – DOI: 10.1039/c3em00658a) and the NERC Macronutrient Cycles Programme (MNC). MNC spans aquatic (estuarine, freshwater and coastal), terrestrial, and atmospheric research and policy areas to address gaps in MC science and policy making, whilst developing important new modelling and monitoring strategies. This issue of Environmental Science: Processes & Impacts highlights pioneering developments in the critical research area of macronutrient interactions.

Problems associated with the atmospheric deposition of N have been acknowledged for decades, with recent international mitigation including the Gothenburg Protocol, which tackled sulphur, NO_x and VOC.⁴ There is also now a consensus amongst scientists and governmental bodies that increasing CO₂ concentrations in the atmosphere are a significant cause of global climate change. Only recently however has atmospheric deposition of P been considered more significant (Tipping et al. – DOI: 10.1039/c3em00641g). Results indicate that the majority of atmospherically transported P is natural in origin, where long range transport primarily involves fine dust from deserts and soils, which is important for tropical forests, large areas of peatland and the oceans. Local atmospheric redistribution from P-rich to P-poor ecosystems could also be significant however, involving relocation of soils from fertilised farmlands.

In recent decades the sensitivity of aquatic ecological communities to fluctuations in nutrient supplies has become an issue at the forefront of the environmental and political agenda, as loads of nutrients, faecal bacteria and pathogenic viruses to rivers, estuaries and coastal areas have increased through anthropogenic activities. Improvements in nutrient loading to rivers and reduction of their environmental impacts are clearly essential and directives have been introduced to improve water quality. An example of a multi-component directive is the European Water Framework Directive (WFD) which puts upper limits on concentrations of both N and P, based on the impacts of these nutrients on the ecological status of surface waters.⁵

Recent studies have demonstrated, however, that ecological response times to nutrients and flow can lag from 7–21 days (Snell et al. – DOI: 10.1039/c3em00680h). These findings are significant when considering interpretation of monitoring data, and implementation of the stringent controls on chemical and ecological river water quality imposed by the WFD. Although advances in technology and infrastructure have enhanced our ability to control nutrient additions through point sources (e.g. sewage treatment outlets), diffuse sources e.g. leaching and runoff from agricultural fields, and septic tank systems are notoriously more difficult to address. Mitigation strategies include the construction of artificial wetlands and buffer strips, and improvements to failing septic systems. Research suggests that the success of these strategies, specifically of septic system improvements, may depend heavily upon the scale and type of improvements made (Ockenden et al. – DOI: 10.1039/c3em00681f).

Finally, in terrestrial environments, the natural balance between MC inputs, storage and loss has been disturbed under these aforementioned changes in climate and land use. The relative limitation of N and P on plant productivity has been associated with soil age, where older soils are P limited (due to a primary natural P source of finite minerals gradually being consumed) and younger soils are N limited (due to time taken for atmospheric N sources to accumulate in soils).⁶ Recent research indicates however that these simple relationships may have been confounded by anthropogenic disturbance. In the UK, despite the relatively young age of soils, a strong association of plant productivity with P availability has been determined, suggesting a prevalence of P limitation attributed to increases in N pollution.

Land use is a particular focus of research as recent findings indicate that spatial variability in nutrients may be linked to soil and land cover (Cooper et al. – DOI: 10.1039/c3em00627a). Specifically, the relative degree of land management creates differences in nutrient concentrations which may partially regulate nutrient processes. For example, soil N concentrations regulate the extent of denitrification potential (Ullah et al. – DOI: 10.1039/c3em00693j). Moreover, land management can also affect interactions between soil organic carbon, pH, bulk density, water filled pore space and texture, which also exert controls on MC processes (Ullah et al. – DOI: 10.1039/c3em00693j).

The multiple impacts of changing drivers can be difficult to assess, and models provide the best method for tracking and quantifying sensitivity of nutrients through large scale atmospheric and land use changes. Additional research is required however to develop better tools as interactions between nutrients, microorganisms and particulates makes responses difficult to predict. Process-based models are especially useful in these cases, where they can be used to assess impacts of various climate, land use and mitigation scenarios. Couture et al. (DOI: 10.1039/c3em00630a) determined that although climate change has a significant impact on lake P loadings and chlorophyll concentrations, land use is the greatest driver. It is also however important to consider the impact of model parametric uncertainty upon scenario results, which can be used to provide more information to decision makers (Starrfelt et al. – DOI: 10.1039/c3em00619k).

In order to demonstrate the effectiveness of policies, and to improve model calibration, detailed data evidence is required. Models are being created to improve our estimates of nutrient loads, taking into account the sparseness of concentration data availability (Nedwell et al. – DOI: 10.1039/c4em00021h), however the question remains as to how much observed data is necessary. Multiple monitoring programmes have been conducted to address this, including catchment-scale efforts (Dunn et al. – DOI: 10.1039/c3em00698k) and more local assessments (Terry et al. – DOI: 10.1039/c3em00686g). Temporal lags have been identified between pollutant addition/removal and in-stream response (Dunn et al. – DOI: 10.1039/c3em00698k; Snell et al. – DOI: 10.1039/c3em00680h), and due to catchment recovery times more long-term monitoring programmes are suggested to help distinguish effects of climate and land use change upon the MC (Dunn et al. – DOI: 10.1039/c3em00698k). Where low frequency sampling typically undertaken during long-term monitoring programmes can fail to capture extreme fluxes in macronutrient concentrations (Bieroza et al. – DOI: 10.1039/c4em00100a) they can be complemented by short, high frequency monitoring programmes which provide more information on dynamic nutrient responses to a full spectrum of flow regimes (Bieroza et al. – DOI: 10.1039/c4em00100a).

Whilst previous research has focused on the individual cycles of N, P and C, there is now clearly a need to examine interactions and feedback mechanisms associated with changing land use, climate and population pressures. Research initiatives are beginning to adopt this multi-component approach to nutrient analysis, with programmes such as the MNC, DEFRA Demonstration Test Catchment, and the US Global Change Programme all working to build a knowledge base of interactions between multiple nutrients, land cover, climate and society. In order for research to develop into mitigation however, it is important for strong links to be developed and maintained between scientists, stakeholders, educators, and policy makers. The MNC research programme aims to bridge the gap in MC science by providing the means for macronutrient cycles knowledge base advancement through empirical research, and developing newly established modelling tools for provision of research to policy makers.

The themed issue

The guest editors of this themed issue form the directorate of the NERC Macronutrient Cycles Programme (see http://macronutrient-cycles.ouce.ox.ac.uk); Professor Paul Whitehead (Director) and Dr Jill Crossman (Assistant Director). The Directorate of the MNC help to ensure that projects are able meet their aims of quantifying the scales of N and P fluxes and nature of transformations through the catchment under a changing climate and perturbed carbon cycle.

Paul is a Professor of Water Science at the University of Oxford, and has over 35 years of experience of research on water resources, water quality and pollution, with a special interest in hydrologic and water quality modelling. Jill is a Postdoctoral Research Fellow at Trent University, Ontario, with a background in hydroecology, GIS and water chemistry, and specialises in modelling of climate, macronutrients, hydrology, and glacial mass balance.

References

1. P. G. Whitehead and J. Crossman, Macronutrient cycles and climate change: key science areas and an international perspective, Sci. Total Environ., 2012, 15(434), 13–17.
2. H. P. Jarvie, T. D. Jickells, R. A. Skeffington and P. J. A. Withers, Climate change and coupling of macronutrient cycles along the atmospheric, terrestrial, freshwater and estuarine continuum, Sci. Total Environ., 2012, 15(434), 252–258.
3. M. Trimmer, J. Grey, C. M. Heppel, A. Hildrew and A. Yvon-Rocher, River bed carbon and nitrogen cycling: state of play and some new directions, Sci. Total Environ., 2012, 15(434), 143–158.
4. UNECE, The 1999 Gothenburg Protocol to abate acidification, eutrophication and ground-level ozone to the 1979 convention on long-range transboundary air pollution, 1999.
5. European Parliament and Council, Establishing a framework for community action in the field of water policy, Directive 2000/60/EC, 2000, vol. 327, pp. 1–72.
6. P. M. Vitousek, S. Porder, B. Z. Houlton and O. A. Chadwick, Terrestrial phosphorus limitation: mechanisms, implications and nitrogen-phosphorus interactions, Ecol. Appl., 2010, 20(1), 5–15.

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