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A new on-line analytical setup for [15.sub.N] measurements of total dissolved nitrogen (TDN) has been developed through the coupling of a high-temperature catalytic (Ce(IV)[O.sub.2]) oxidation furnace, a Cu reduction furnace, and an isotope ratio mass spectrometer. The detection limit for accurate [sigma] [sub.15]N measurements is 20 mg of N [L.sup.-1]. For N-containing compounds dissolved in water, a standard deviation on N concentration measurements of 0.2 mg of N [L.sup.-1], independent of N concentration, has been found. Reproducibility on [sigma] [sup.15]N increased with increasing N concentration, with standard deviations varying from 0.8 to 0.1%o in the range of 20-100 mg of N [L.sup.-1]. Salt matrixes, in which the N species might be dissolved, could influence the analysis capacity and continuity, mainly at concentrations above 0.1 M. To our knowledge, this system is the first successful on-line setup capable of performing routine [sigma] [sup.15]N and N concentration measurements of the TDN pool. Potential applications are large and are believed to result in an increased insight in N cycling and dissolved organic nitrogen behavior in terrestrial and aquatic ecosystems.

Following mineralization of N-rich crop residues, large amounts of mineral N can be released into the soil. Manipulating N mineralization of crop residues may be an option to reduce N[O.sub.3.sup.-] in soil. The potential to manipulate the N release from vegetable crop residues by using organic wastes was tested under field conditions. At the start of the experiment, cauliflower (Brassica oleracea L. van botrytis L.) residues (~73 Mg flesh matter [ha.sup.-1]) together with an immobilizer waste (~5 Mg C [ha.sup.-1] of straw, green waste compost, sawdust, or paper sludge) were incorporated into a silt loam soil. After 154 d, a remineralizing waste (~1 Mg C [ha.sup.-1] of vinasse or dairy sludge) was incorporated. During the field experiment, the mineral N content in the soil was measured at regular time intervals, and net N release, N[O.sub.3.sup.-] leaching, and denitrification were simulated using a N mineralization-immobilization model coupled to a N[O.sub.3.sup.-] leaching model. Straw, green waste compost, and sawdust were able to immobilize between 54 and 68% of the N released by the cauliflower residues and reduced N[O.sub.3.sup.-] leaching by 56 to 68%. Paper sludge released an extra amount of N due to its low C/N ratio. No consistent remineralization of N could be found in any of the treatments, probably due to an unsuitable composition of the remineralizer wastes. Manipulating N release of N-rich crop residues by using organic wastes may be a suitable method to reduce N[O.sub.3.sup.-] leaching; however, stimulating remineralization of immobilized N by the start of the following spring may not be easy to achieve. Abbreviations: CD, coefficient of determination; CRM, coefficient of residual mass.

Highlights: [•] Litter quality is a major determinant of decomposition. [•] Mixed-species litter can give a negative and positive additive effect. [•] Stand type effect was observed for the forest litter mixture only. [•] Nutrient depletion can occur in long-term in Eucalyptus plantation. [•] Final decay rate is also essential to explain the remaining mass. [Copyright &y& Elsevier]

Apportionment of nitrate (NO3-) sources in surface water and classification of monitoring locations according to NO3- polluting activities may help implementation of water quality control measures. In this study, we (i) evaluated a Bayesian isotopic mixing model (stable isotope analysis in R [SIAR]) for NO3- source apportionment using 2 yr of δ15N-NO3- and δ18O-NO3- data from 29 locations within river basins in Flanders (Belgium) and five expert-defined NO3- polluting activities, (ii) used the NO3- source contributions as input to an unsupervised learning algorithm (k-means clustering) to reclassify sampling locations into NO3- polluting activities, and (iii) assessed if a decision tree model of physicochemical data could retrieve the isotope-based and expert-defined classifications. Based on the SIAR and d11B results, manure/sewage was identified as a major NO3- source, whereas soil N, fertilizer NO3-, and NH4+ in fertilizer and rain were intermediate sources and NO3- in precipitation was a minor source. The k-means clustering algorithm allowed classification of NO3- polluting activities that corresponded well to the expert-defined classifications. A decision tree model of physicochemical parameters allowed us to correctly classify 50 to 100% of the sampling locations as compared with the k-means clustering approach. We suggest that NO3- polluting activities can be identified via clustering of NO3- source contributions from samples representing an entire river basin. Classification of future monitoring locations into these classes could use decision tree models based on physicochemical data. The latter approach holds a substantial degree of uncertainty but provides more inherent information for dedicated abatement strategies than monitoring of NO3- concentrations alone. [ABSTRACT FROM AUTHOR]

To identify different NO3 − sources in surface water and to estimate their proportional contribution to the nitrate mixture in surface water, a dual isotope and a Bayesian isotope mixing model have been applied for six different surface waters affected by agriculture, greenhouses in an agricultural area, and households. Annual mean δ15N–NO3 − were between 8.0 and 19.4‰, while annual mean δ18O–NO3 − were given by 4.5–30.7‰. SIAR was used to estimate the proportional contribution of five potential NO3 − sources (NO3 − in precipitation, NO3 − fertilizer, NH4 + in fertilizer and rain, soil N, and manure and sewage). SIAR showed that “manure and sewage” contributed highest, “soil N”, “NO3 − fertilizer” and “NH4 + in fertilizer and rain” contributed middle, and “NO3 − in precipitation” contributed least. The SIAR output can be considered as a “fingerprint” for the NO3 − source contributions. However, the wide range of isotope values observed in surface water and of the NO3 − sources limit its applicability. [Copyright &y& Elsevier]

Approximately 60% of Mexico (1.973 × 106 km2) is arid or semi-arid with vegetation dominated by N2 fixing shrubs, such as mesquite (Prosopis juliflora). Little is known, however, how these ecosystems contribute to production of nitrous oxide (N2O), a known greenhouse gas. We investigated this by adding inorganic fertilizer (20 mg ammonium (NH4+) N kg-1 soil) to soil at different water contents (40%, 60%, 80%, and 100% of water-holding capacity (WHC)) sampled under the canopy of mesquite and outside its canopy, while production of carbon dioxide (CO2) and nitrous oxide (N2O), and dynamics NH4+, nitrite (NO2-), and nitrate (NO3-)) were monitored. The CO2 production rate was 1.5 times larger for soil sampled under the canopy of mesquite compared to soil sampled outside the canopy of shrubs and lower in soil incubated at 100% WHC compared to the other treatments. Production of N2O increased with increased water content within the first day, but decreased in soil at 80% and 100% WHC thereafter. The production of N2O was 1.9 times larger for soil sampled under the canopy of mesquite compared to soil sampled outside its canopy. The addition of 20 mg NH4+-N increased production of N2O 1.8-fold. It was found that the cumulative production of N2O was larger from soil sampled under the canopy of mesquite than outside it when amended with inorganic fertilizer and increased with increased water content of the soil within a day. The relationship between the emission of CO2 and N2O, water-holding capacity are discussed. [ABSTRACT FROM AUTHOR]

Old-growth forests of southern Chile represent an important reserve of temperate (rain) forests in the world. Wetter and colder forest ecosystems appear to be more efficient in conserving and recycling N such that mostly non-plant available N species are lost, which could be indicated by more depleted δ 15 N values of the soil and plants. Hydrological N loss from the old-growth forests in southern Chile occurs mainly via dissolved organic nitrogen and not via dissolved inorganic N. Forest disturbances ( e.g . fire, clear-cutting or enhanced N deposition) cause (abrupt) changes in ecosystem N-cycling processes. In this study, we hypothesized that δ 15 N signatures of soil profiles under old-growth forests could be used as an integrator for ecosystem N-cycling, and changes of these δ 15 N profiles could be valuable to assess ecosystem resilience towards disturbances. Six old-growth forests were selected in the phytogeographical region of the Valdivian rain forest in southern Chile. One of the sites has been partly burned in February 2002. First, we observed that ecosystems with higher mean annual precipitation and lower mean annual temperature were relatively more depleted in 15 N. Secondly, we found that a forest fire caused a 100-fold increase of the nitrate export and induced an enrichment of the soil δ 15 N signal in the upper 20 cm. [ABSTRACT FROM AUTHOR]

Since the early 1990s, we have been carrying out experiments on N2O production, emission and sorption from a variety of soils and ecosystems. These laboratory studies revealed insights on the complex process of N2O formation in soils under contrasting conditions. Within Belgium, we also carried out numerous field experiments to measure in situ N2O emissions, using the closed chamber technique. The latter allowed us to suggest a region-specific N2O-N emission factor (2.66% of the applied N) for direct soil emissions and to assess the spatial and temporal variability of N2O emissions. Recently, we have been using these N2O field emission data to validate an adapted version of the DNDC process-based model to predict direct N2O emissions from agricultural soils of the Flemish region. Ultimately, it is our aim to develop, based on this validated DNDC model, scenario analyses of future direct N2O emissions from agricultural soils. [ABSTRACT FROM AUTHOR]