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

Incomplete recovery of 15N often occurs in 15N mass balance experiments, the unrecovered portion being assumed to be lost due to denitrification. Denitrification losses estimated in this way will be too high if any gaseous loss(es) of N occur during the sample collection, extraction or storage procedures. Leachate or KCl extracts of soil are mixtures of inorganic and organic substances, so changes may occur during storage. The objective of this work was to assess the potential for inorganic N transformations and potential gaseous losses when a soil leachate was treated in various ways and stored in glass bottles for 10 days at −20, 4 or 20°C. The treatments were natural pH of 6.3, acidification to pH 4.5, and adjustment of the salt strength to 2 M in KCl. The NH4+, NO2− and NO3− concentrations were enhanced and differentially labelled with 15N. After 10 days, headspaces were analysed to determine the fluxes of N2O and N2, and solutions were analysed for inorganic-N concentrations and their 15N enrichments. Concentrations changed least when the storage temperature was 4°C or when the ionic composition was primarily 2 M KCl. Ammonium concentrations changed little, a small significant (P<0.05) increase at pH 6.3 and 20°C being due to mineralisation. Nitrate concentrations also changed little, a small significant (P<0.05) increase at pH 4.5 and − 20°C being due to oxidation of NO2−. Transformations of inorganic-N were dominated by chemical reactions involving NO2−, particularly at pH 4.5 and −20°C. Under these conditions, the NO2− concentration decreased from 8.6 to 1.3 mg N l−1 partly because of oxidation to NO3−, reduction to N2O, and reaction with NH4+ to form N2. 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°C with headspace concentrations reaching 125 μl l−1 N2O-N. The potential importance of the inorganic-N transformations measured in this experiment depends on the relative inorganic-N composition. Deep-freezing of acidic or acidified soil leachate which contains NO2− has the potential to not only increase NO3− concentrations but also decrease NO2− and NH4+ concentrations. Storing leachates or 2 M KCl extracts from soils at 4°C without acidification is recommended for minimising N transformations of NO2− and NH4+ and avoiding the potential to overestimate NO3− concentrations. Future work should explore these gas loss mechanisms using sterilised samples in conjunction with more interim sampling to investigate the precise timing and rates of gas production.