Transformations of inorganic-N in soil leachate under differing storage conditions

Incomplete recovery of N-15 often occurs in N-15 mass balance experiments, the unrecovered portion being assumed to be lost due to denitrification. De nitrification losses estimated in this way will be too high if any gaseous loss(es) of N occur during the sample collection, extraction or storage pro cedures. Leachate or KCI extracts of soil are mixtures of inorganic and org anic substances, so changes may occur during storage. The objective of this work was to assess the potential for inorganic N transformations and poten tial gaseous losses when a soil leachate was treated in various ways and st ored in glass bottles for 10 days at -20, 4 or 20 degreesC. The treatments were natural pH of 6.3, acidification to pH 4.5, and adjustment of the salt strength to 2 M in KCI. The NH4+, NO2- and NO3- concentrations were enhanc ed and differentially labelled with N-15. After 10 days, headspaces were an alysed to determine the fluxes of N2O and N-2, and solutions were analysed for inorganic-N concentrations and their N-15 enrichments. Concentrations c hanged least when the storage temperature was 4 degreesC or when the ionic composition was primarily 2 M KCI. Ammonium concentrations changed little, a small significant (P < 0.05) increase at pH 6.3 and 20 degreesC being due to mineralisation. Nitrate concentrations also changed little, a small sig nificant (P < 0.05) increase at pH 4.5 and -20 degreesC being due to oxidat ion of NO2-. Transformations of inorganic-N were dominated by chemical reac tions involving NO2-, particularly at pH 4.5 and -20 degreesC. Under these conditions, the NO2- concentration decreased from 8.6 to 1.3 mg N l(-1) par tly because of oxidation to NO3-, reduction to N2O, and reaction with NH4to form N-2. These processes accounted for 49.0, 0.2 and 14.2% of the NO2- initially present, respectively. Nitrous oxide production accounted for 3.2 % of the NO2- initially present at pH 4.5 and 20 degreesC with headspace co ncentrations reaching 125 mul l(-1) N2O-N. The potential importance of the inorganic-N transformations measured in this experiment depends on the rela tive inorganic-N composition. Deep-freezing of acidic or acidified soil lea chate which contains NO2- has the potential to not only increase NO3- conce ntrations but also decrease NO2- and NH4+ concentrations. Storing leachates or 2 M KCI extracts from soils at 4 degreesC without acidification is reco mmended for minimising N transformations of NO2- and NH4+ and avoiding the potential to overestimate NO3- concentrations. Future work should explore t hese gas loss mechanisms using sterilised samples in conjunction with more interim sampling to investigate the precise timing and rates of gas product ion. (C) 2001 Elsevier Science Ltd. All rights reserved.