Temporal variations in potential nitrification dynamics in soil related to differences in rates and types of carbon and nitrogen inputs

We studied the effects of a combination of C and N supplements on potential nitrification rates (PNR) in an arable soil over a 22-month period. The supplements were chosen to represent a qualitatively varied range of organic compounds with different C:N ratios, and were applied at a range of concentrations varying from those analogous to those found in root exudates to those required for maximal microbial respiration responses. PNR in these soils showed large temporal variations over the study period. The supplements generally resulted in significant changes in PNR, ranging from an 88% decrease to a 654% increase compared to the control (no supplement) treatments. The nature of these responses were variable as the control PNR changed. In many instances, and particularly when control rates in the non-supplemented soils were relatively low, the addition of a C and N source resulted in significant increases in PNR. There was a tendency for no supplement effect when PNR in controls exceeded 100 μg NO2− -N kg−1 soil h−1. The form of supplement also had a marked impact on the measured responses. The highest concentrations of glucose, analogous to those used during a substrate-induced respiration assay, tended to result in a depression in potential nitrification rates, irrespective of supplementation with extra nitrogen. Soya peptone, alanine and asparagine often resulted in significant stimulation of potential nitrification, and such effects were often as pronounced when the lower concentrations (up to three-orders of-magnitude in relative terms) of compound were applied. These data suggest that subtle and temporally variable interactions between general soil heterotrophs and autotrophic nitrifiers may occur in arable soils, possibly stimulated by the added C and N.