Isotopic studies of the nitrogen balance in a cracking clay. I. Recovery of added nitrogen from soil and wheat in the glasshouse and gas lysimeter

Abstract

The stable isotope 15N was used to study the fate of nitrogen fertilizers applied to a black earth growing wheat. In a glasshouse experiment using soil packed at 56 % moisture (pF 2) into pots, added nitrate was almost completely recovered (mean, 98.8 ± 2.3 %) by soil and plant analysis. This experiment was repeated using 15N-labelled ammonium as well as nitrate; mean recoveries at 15 weeks were 96.8 and 97% respectively (± 2.2% at P < 0.05). A series of pots with soil wetter than pF 2 (at 63% moisture) was also included in an attempt to favour denitrification. Again, virtually complete recovery of added 15N was measured, 97 and 96.8% at 56 and 63% moisture respectively. Fallow and planted systems were then studied in a gas lysimeter. During experiments lasting up to 14 weeks, gaseous losses as 15N-labelled denitrification products were less than the equivalent of 0.2 µg nitrogen/g soil (the lower limit of detection with a mass spectrometer). Although analysis of plant and soil from the lysimeter in two experiments showed virtually quantitative recovery (99.7 and 97.05%), small (0.16%) losses of labelled ammonia into the lysimeter atmosphere were detected. The significance of these results is discussed in relation to the common finding of large deficits in nitrogen balance studies.