The UK’s total nitrogen budget from 1990 to 2020: a transition from source to sink?

This study estimates the annual total nitrogen balance of the UK from 1990 to 2020. The following inputs of nitrogen are considered: inorganic fertilizer, atmospheric deposition; food and feed imports; and biological nitrogen fixation. The outputs considered compose: atmospheric emissions; direct losses of sewage and industrial effluent to the sea; fluvial losses at source; food and feed exports; and terrestrial denitrification. It is shown that: (1) Inputs of inorganic fertilizer declined significantly over the study period with both atmospheric deposition and food and feed imports significantly increasing. (2) Outputs of total N also significantly declined with all output pathways decreasing except for atmospheric emissions and terrestrial denitrification to N2. (3) The UK was a net source of total nitrogen in 1990 of approximately −1941 ± 224 kilotonnes N/year (−8 tonnes N/km2/year; inter-quartile range of ±0.9 tonnes/km2/year). However, by 2012, this net nitrogen source had decreased to about −1446 ± 195 kilotonnes N/year (−5.9 tonnes N/km2/year). The future total N balance of the UK is being driven by declines in outputs rather than changes in inputs. The largest declines are in the atmospheric emissions of reactive nitrogen (Nr) and the fluvial flux of N such that by 2020 to the total N budget is predicted to be −1042 (±246) kilotonnes N/year (−4.2 tonnes N/km2/year) and by 2031 the UK would be a net sink of total N

A 50‐Year Record Of Nitrate Concentrations In The Slapton Ley Catchment, Devon, United Kingdom

Slapton Ley, a coastal lake, is the largest natural body of fresh water in south‐west England. There was concern in the 1960s that the lake was becoming increasingly eutrophic. To quantify inputs of water, sediment and nutrients into the lake, Slapton Ley Field Centre initiated a programme of weekly water quality sampling in September 1970. Of all the chemical properties which have been measured over the decades, the nitrate record has been the subject of more research than any other. The weekly monitoring has been supplemented by research projects aimed at understanding aspects of processes and patterns of nitrate delivery to the stream network. Three aspects of the nitrate record are reviewed: short‐term process dynamics; the annual cycle of influent streams and the lake itself; and long‐term trends. In the first two decades of monitoring, there was increasing concern about a trend of rising nitrate concentrations, an issue in most lowland rivers in the UK at the time. In the 1990s, nitrate concentrations levelled off and then have fallen steadily in recent years. In relation to eutrophication, there are clear signs of improvement in the influent streams, but concerns remain about water quality in the lake itself

Stream water quality in the Slapton catchments: a meta-analysis of key trends since 1970

A weekly programme of water quality monitoring has been undertaken by the FSC's Field Centre at Slapton Ley since 1970. Samples have been collected from the main streams draining into the Ley and from the Ley itself. The main purpose of this paper is to make available an online archive but in so doing the opportunity has been taken to update previous publications. Not surprisingly, given the ongoing eutrophic status of the Ley, most attention has been paid to the nutrients nitrate and phosphate; this review broadens the scope to include other water quality records. In relation to eutrophication, there are clear signs of improvement in the influent streams, but concerns remain about water quality in the Ley itself

Evidence for nitrogen accumulation: the total nitrogen budget of the terrestrial biosphere of a lowland agricultural catchment

Several national-scale studies have shown that reactive N is accumulating in developed countries even when only the terrestrial biosphere is considered. However, none of these studies was able to consider the total N budget and so any discrepancy in budgets could be dismissed as being accounted for by N2 exchange. This study considered a large (9,948 km2), mixed agricultural catchment where records of N flux, land use, climate and population go back at least to 1883. The N inputs were: biological nitrogen fixation, food and feed transfers, atmospheric deposition and inorganic fertilizers. The N outputs were atmospheric emissions (NH3, N2O, NO, N2), direct waste losses and fluvial losses at the soil source. The results showed that, prior to the large-scale use of inorganic fertilizers, the total N budget of the catchment was at steady state with only a small net loss of total N. After the widespread introduction of inorganic fertilizers, the balance of the catchment shifts in favour of the net accumulation. Even accounting for losses to groundwater, the catchment was found to have accumulated 315 ktonnes N (315 tonnes/km2) at a rate of 5.5 tonnes N/km2/yr (55 kg N/ha/yr) over 35 years since 1973. We propose that the accumulation of N could be occurring in subsoils of the catchment

The fluvial flux of particulate organic matter from the UK: the emission factor of soil erosion

Soil erosion has been identified as a potential global carbon sink since eroded organic matter is replaced at source and eroded material is readily buried. However, this argument has relied on poor estimates of the total fate of in-transit particulates and could erroneously imply soil erosion could be encouraged to generate carbon stores. These previous estimates have not considered that organic matter can also be released to the atmosphere as a range of greenhouse gases, not only carbon dioxide (CO2), but also the more powerful greenhouse gases methane (CH4) and nitrous oxide (N2O). As soil carbon lost by erosion is only replaced by uptake of CO2, this could represent a considerable imbalance in greenhouse gas warming potential, even if it is not significant in terms of overall carbon flux. This work therefore considers the flux of particulate organic matter through UK rivers with respect to both carbon fluxes and greenhouse gas emissions. The results show that, although emissions to the atmosphere are dominated by CO2, there are also considerable fluxes of CH4 and N2O. The results suggest that soil erosion is a net source of greenhouse gases with median emission factors of 5.5, 4.4 and 0.3 tonnes CO2eq/yr for one tonne of fluvial carbon, gross carbon erosion and gross soil erosion, respectively. This study concludes that gross soil erosion would therefore only be a net sink of both carbon and greenhouse gases if all the following criteria are met: the gross soil erosion rate were very low (<91 tonnes/km2/yr); the eroded carbon were completely replaced by new soil organic matter; and if less than half of the gross erosion made it into the stream network. By establishing the emission factor for soil erosion, it becomes possible to properly account for the benefits of good soil management in minimizing losses of greenhouse gases to the atmosphere as a by-product of soil erosion

The problem of self-correlation in fluvial flux data - the case of nitrate flux from UK rivers

This study proposes a general method for testing for self-correlation (also known as spurious or induced correlation) in comparisons where there is a common variable, e.g. the comparison of the fluvial flux of a component with water yield. We considered the case of the fluvial flux of nitrate from 153 catchments from across the UK for which there were at least 10 years of data. The results show that 66% of records (102 catchments) could be rejected as significantly self-correlated (P < 95%). Amongst the 51 catchments, which proved to be significantly different from the spurious, or self-correlated result, the response was variable with linear, convex, s-curve and mixed results proving the best description. There was no spatial pattern across the UK for the results that were and were not rejected as spurious; the most important predictor of not being self-correlated was the length of record rather than any catchment characteristic. The study shows that biogeochemical stationarity cannot be assumed and that caution should be applied when examining fluvial flux data

Assessment of sample frequency bias and precision in fluvial flux calculations – an improved low bias estimation method

Despite the importance of calculating the flux of solutes and particulates through the global fluvial network the number of studies that have considered the bias and precision of any method is limited. Furthermore, no study has, on the basis of the bias of the method, proposed new methods with a lower bias nor considered the implications of the bias estimation for existing published studies. Using 3 years of high frequency data (hourly) for dissolved organic carbon (DOC) this study systematically degraded the data and recalculated the flux for varying sample frequencies and considered a range of interpolation, ratio and extrapolation methods. The results show that: (i) Interpolation and ratio methods showed a consistent, small bias for sampling frequencies up to every 14 days, but bias rapidly increased for lower sample frequencies with the flux estimates being between 40% and 45% of the ‘‘true’’ flux at 31 day (monthly) sampling. (ii) The best ratio method was based upon correction against an unrealistic assumption that river flow was normally distributed. (iii) Extrapolation methods based on fixed sampling period monitoring proved to be erratic but no better than interpolation methods. Based upon the nature of the sources of variation within the flow and solute datasets we propose the following method for calculating the fluvial flux (F) of a solute: F ¼ KEðCiÞQtotal where: Qtotal = the total flow in a year (m3/yr); E(Ci) = the expected value of the sampled concentrations (mg/l); and K = a conversion factor. This new method preserved all the available flow information and had a bias of as low as 8% for monthly sampling. When the method was applied to DOC flux from Great Britain bias correction meant a 97% increase in the national flux over previous estimates

On the importance of very long-term water quality records

This overview is concerned with the value of long-term records of water quality in river basin management. In a world where change rather than stasis is increasingly the norm, monitoring is an essential way to discover whether there are significant undesirable changes taking place in the natural environment. The regular collection and processing of information involves systematic and purposeful observation, a deliberate plan of action in which the data have considerable value given the knowledge of their context in time and space. Long-term data reveal important patterns, which allow trends, cycles, and rare events to be identified. This is particularly important for complex systems where signals may be subtle and slow to emerge. Moreover, very long data sets are essential to test hypotheses undreamt of at the time the measurements were started. This overview includes long time series from UK rivers showing how water quality has changed over time—and continues to change. An important conclusion is the long time frame of system recovery, well beyond the normal lifetime of individual governments. At a time of increasing hydroclimatic variability, long time series of water quality observations remain critically important; continuity of observations is critical at key benchmark sites