Abstract. A unique long-term dataset from the UK National Ammonia Monitoring Network (NAMN) is used here to assess spatial, seasonal and long-term variability in atmospheric ammonia (NH 3 : 1998(NH 3 : -2014) and particulate ammonium (NH + 4 : 1999-2014) across the UK. Extensive spatial heterogeneity in NH 3 concentrations is observed, with lowest annual mean concentrations at remote sites (< 0.2 µg m −3 ) and highest in the areas with intensive agriculture (up to 22 µg m −3 ), while NH + 4 concentrations show less spatial variability (e.g. range of 0.14 to 1.8 µg m −3 annual mean in 2005). Temporally, NH 3 concentrations are influenced by environmental conditions and local emission sources. In particular, peak NH 3 concentrations are observed in summer at background sites (defined by 5 km grid average NH 3 emissions < 1 kg N ha −1 yr −1 ) and in areas dominated by sheep farming, driven by increased volatilization of NH 3 in warmer summer temperatures. In areas where cattle, pig and poultry farming is dominant, the largest NH 3 concentrations are in spring and autumn, matching periods of manure application to fields. By contrast, peak concentrations of NH + 4 aerosol occur in spring, associated with long-range transboundary sources. An estimated decrease in NH 3 emissions by 16 % between 1998 and 2014 was reported by the UK National Atmospheric Emissions Inventory. Annually averaged NH 3 data from NAMN sites operational over the same period (n = 59) show an indicative downward trend, although the reduction in NH 3 concentrations is smaller and nonsignificant: Mann-Kendall (MK), −6.3 %; linear regression (LR), −3.1 %. In areas dominated by pig and poultry farming, a significant reduction in NH 3 concentrations between 1998 and 2014 (MK: −22 %; LR: −21 %, annually averaged NH 3 ) is consistent with, but not as large as the decrease in estimated NH 3 emissions from this sector over the same period (−39 %). By contrast, in cattle-dominated areas there is a slight upward trend (non-significant) in NH 3 concentrations (MK: +12 %; LR: +3.6 %, annually averaged NH 3 ), despite the estimated decline in NH 3 emissions from this sector since 1998 (−11 %). At background and sheep-dominated sites, NH 3 concentrations increased over the monitoring period. These increases (non-significant) at background (MK: +17 %; LR: +13 %, annually averaged data) and sheep-dominated sites (MK: +15 %; LR: +19 %, annually averaged data) would be consistent with the concomitant reduction in SO 2 emissions over the same period, leading to a longer atmospheric lifetime of NH 3 , thereby increasing NH 3 concentrations in remote areas. The observations for NH 3 concentrations not decreasing as fast as estimated emission trends are consistent with a larger downward trend in annual particulate NH + 4 concentrations (1999-2014: MK: −47 %; LR: −49 %, p < 0.01, n = 23), associated with a lower formation of particulate NH
Understanding “soil change” at the national scale, in addition to soil status, is a key challenge for national scale soil monitoring programs and is essential if more sustainable use of this finite resource is to be achieved. We present results from the first national scale survey of soil change to be reported three times within Europe and perhaps globally, covering a 30‐yr time span. Countryside Survey is an integrated national monitoring program that makes measurements of vegetation; topsoil physical, chemical, and biological characteristics (0–15 cm); water quality; and land use across Great Britain (GB), thus recognizing their interdependence. Here we report on change in fundamental soil chemical characteristics. Soil pH and loss on ignition (LOI) were measured in 1978, 1998, and 2007 and soil total nitrogen (total N) concentration and C/N ratio in 1998 and 2007. Bulk density was measured in 2007. Mean soil pH increased significantly in less acidic soils from 1978 through 1998 to 2007. Mean pH increased significantly in more acidic, organic‐rich soils from 1978 to 1998 but not between 1998 and 2007, indicating spatial trends in both sulfur deposition reductions and soil sensitivity. There was a small increase (8%) in GB topsoil C concentration (calculated from LOI) between 1978 and 1998, a small decrease (6.5%) between 1998 and 2007, and no significant overall change between 1978 and 2007. The unresolved difference between these results and those from the National Soil Inventory of England and Wales that reported wide‐scale large decreases in soil C concentrations in 2005 are discussed. There were unexpected small but significant decreases in total N concentration in many broad habitats despite continuing atmospheric nitrogen deposition. In seminatural and woodland habitats, this was accompanied by an increase in the C to N ratio, indicating one possible explanation is dilution of the nitrogen signal due to high C/N litter inputs resulting from increased primary productivity as reported elsewhere due to a range of global drivers such as increased CO2, N, and temperature. In arable systems, comparable rates of loss of C and N suggest erosion losses or deep plowing are reducing soil condition. The results are discussed in relation to the influences on soil processes of key drivers of environmental change and the importance of considering habitat‐specific trends.
The impact of climate change on N leaching from hill land plant/soil systems was investigated using a transplant technique involving the movement of intact lysimeter cores of three contrasting soil types down an altitudinal gradient at Great Dun Fell, Cumbria. Air and soil temperatures and precipitation were monitored at four elevations down an altitudinal transect using automatic weather stations for a period of two years. The altitudinal sequence of air temperature followed the anticipated pattern, providing mean annual temperatures at the four locations of 3.4, 5.0, 6.3 and 8.1 °C. Lapse rates of both mean air and soil temperatures over the altitudinal range 171–845 m were 6.6 (1993) and 7.0 °C km–1 (1994). Soil monthly temperature gradients for a particular soil type for each of the two years showed a seasonal range of 6.0 and 7.4 °C km–1, respectively, and for air temperature of 4.3 and 3.1 °C km–1. Precipitation gradients showed the expected general increase with altitude, but were less predictable. Inorganic nitrogen leaching was studied in lysimeter leachates with climatic amelioration resulting in dramatic reductions in leachate nitrate concentrations and associated total concentrations of inorganic nitrogen. Decreases in leachate nitrate concentrations were observed for all three soil types studied. Soils receiving supplemented rainfall also showed decreased N concentrations, suggesting that temperature was the main controlling factor responsible for the observed reductions. Increased N uptake by the vegetation, in response to the increases in temperature, is considered to be critical in controlling soil solution chemistry at these sites.
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