The N ratio of nitrogen oxides (NO) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [δN(NO) = +5.7‰]. This high N abundance was used to estimate long-term NO dry deposition into a forest ecosystem by measuring δN in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO in a transect orthogonal to the highway. δN in the current-year needles of potted trees was 2.0‰ higher than that of the control after 4 months of exposure close to the highway, suggesting a 25% contribution to the N-nutrition of these needles. Needle fall into the pots was prevented by grids placed above the soil, while the continuous decomposition of needle litter below the autochthonous trees over previous years has increased δN values in the soil, resulting in parallel gradients of δN in soil and needles with distance from the highway. Estimates of NO uptake into needles obtained from the δN data were significantly correlated with the inputs calculated with a shoot gas exchange model based on a parameterisation widely used in deposition modelling. Therefore, we provide an indication of estimated N inputs to forest ecosystems via dry deposition of NO at the receptor level under field conditions.

Sampling Sites and MethodsThe 2. The seasonal change in the occurrence and strength of the surface inversion may contribute to the variability of all short-lived species in the boundary layer except locally produced sea-salt aerosol. As summarized by K6nig-Langlo et al.[this issue], the period from November to February shows a relatively weak or absent surface inversion at NM and DDU but generally stronger temperature gradients at NM (due to radiative cooling). Cyclonic activity, however, may regularly interrupt the isolation from higher atmospheric levels •' at both sites throughout all seasons.3. The surface'wind system is dominated by zonal flow at NM but by a meridional one at DDU and Mawson [Parish, 1988]. The latter two sites are therefore more directly influenced by air masses flowing down from the Antarctic plateau and by upper tropospheric air from midlatitudes eventually entrained in the downslope winds.In summary, the climatologies at DDU and Mawson are similar, whereas NM is characterized by its ice shelf position (leading to zonal winds) and by its relatively more frequent location inside the polar vortex. Palmer station stands out by its position on the Antarctic Peninsula and reflects mainly the subpolar maritime regime. Sampling and AnalysesDetails on the sampling procedure and analytical methods used for aerosol chemical analyses are summarized in Table 1 2.2.1. Assessing local contamination at DDU. The very small size of the island (Ile des P6trels) on which DDU is located makes it impossible to define a "clean air sector" there. In order to estimate the effects both of human activities (which confined to the summer period and to individual filters with very low sea-salt loads. On the other hand, the HNO 3 collection efficiency for the backup aerosol filter was found to be between 60 and 90%, allowing resampling of most of the evaporated nitrate. Again, inspection of the summer nitrate variability at NM did not reveal a systematic relationship to the sea-salt or acidity loads of the filters. In particular, the general nitrate trend during the summer season is not substantially influenced by the dramatic change in the aerosol acidity during that season. We cannot exclude, however, the possibility that some filters collected during the acidity peak in January under low sea-salt loads may underestimate the total atmospheric nitrate concentration. We expect that the same holds true for the total nitrate data reported by Savoie et al. [1992] from Mawson station since sea salt is lower there by a factor of-4 compared to NM and since nonhygroscopic polycarbonate fibre filters are used.In conclusion, total atmospheric nitrate records reported here are affected to a minor extent by year-round contamination events at DDU and by HNO 3 remobilization from summer filters at NM, respectively. None of these shortcomings occurring on individual filter samples could, however, substantially perturb the observed mean nitrate levels and their seasonal patterns. ResultsBecause of the large difference in the sampl...

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Summary In a long‐term field experiment started in 1956 on a clay loam soil at Uppsala, Sweden, changes of organic carbon in the topsoils receiving various organic amendments at the rate of 200 kg C ha'1 year'1 were studied to determine soil organic matter characteristics, variations of δ13C in the soil and to estimate a carbon balance. Fallow and mineral fertilizer without N led to a significant decrease of soil organic matter (SOM) in the soil, green manure maintained the SOM content, and animal manure and peat increased the SOM content significantly. The stable portion of the added organic materials after 37 years of continuous input was 12·8, 27·3, and 56·7%, for green manure, animal manure and peat, respectively. This was reflected by half‐lives of organic carbon originating from the amendments between 3·0 (green manure) and 14·6 years (peat). The isotopic composition of SOM changed both due to mineralization (continuous fallow) and the addition of amendments is topically different from soil humus (green manure, animal manure). The isotopic effect was used to calculate the percentage of carbon derived from animal manure present for the year 1993. This value (55·4%) was larger than that derived from the carbon balance, which indicated a priming effect of the animal manure on the initial soil humus. Mineralization of microbially available organic substances led to an increase in the degree of humification on plots not receiving organic amendments. Adding peat and animal manure resulted in a decrease of the humification index due to the continuous input of poorly humified material. The extinction ratio (E4/E6) and ratio of fulvic acid to humic acid changed considerably in the peat treated plots. Fourier transform infrared (FTIR)‐measurements of the extracts showed that peat characteristics can be detected in peat treated soils. The other amendments did not alter the characteristics of the extractable humic substances.

Summary Fundamental knowledge about the complex processes during the decomposition, mineralization and transfer of residue organic matter in soils is essential to assess risks of changes in agricultural practices. In a double tracer (13C, 15N) experiment the effect of maize straw on the mineralization dynamics and on the distribution of maize‐derived organic matter within particle size fractions was investigated. Maize straw (a C4 plant) labelled with 15N was added to soils (13.2 g dry matter kg–1 soil) which previously had grown only C3 plants, establishing two treatments: (i) soil mixed with maize straw (mixed), and (ii) soil with maize straw applied on the surface (surface). Samples were incubated in the laboratory at 14°C for 365 days. The size fractions (> 200 μm, 200–63 μm, 63–2 μm, 2–0.1 μm and < 0.1 μm), obtained after low‐energy sonication (0.2 kJ g–1), were separated by a combination of wet‐sieving and centrifuging. The mineralization of maize C was similar in the two treatments after one year. However, decomposition of maize particulate organic matter (predominantly in the fraction > 200 μm) was significantly greater in the mixed treatment, and more C derived from the maize was associated with silt‐ and clay‐sized particles. A two‐component model fitted to the data yielded a rapidly mineralizable C pool (about 20% of total C) and a slowly mineralizable pool (about 80%). Generally, the size of the rapidly mineralizable C pool was rather small because inorganic N was rapidly immobilized after the addition of maize. However, the different mean half‐lives of the C pools (rapidly decomposable mixed 0.035 years, and surface‐applied 0.085 years; slowly decomposable mixed 0.96 years, and surface‐applied 1.7 years) showed that mineralization was delayed when the straw was left on the surface. This seems to be because there is little contact between the soil microflora and plant residues. Evidently, the organic matter is more decomposed and protected within soil inorganic compounds when mixed into the soil than when applied on the soil surface, despite similar rates of mineralization.