The effect of soil moisture content and K nutrition on denitrification measured by the acetylene inhibition method in short‐term assays was studied in pot experiments with wheat on a low‐nutrient sandy soil. Increasing the soil moisture from 60 to 80 or 100% water holding capacity (whc) increased denitrification which was further increased when the soil moisture was raised to 100% whc 24 h before the assay. This effect was not observed with unplanted pots. Denitrification increased with progressing plant age, provided sufficient nitrate was present. N2O production was particularly high after removal of the shoots as decaying root matter improved conditions for denitrification. Optimal plant growth reduced denitrification through more rapid nitrate uptake and lower soil moisture, whereas poor plant growth, induced by K deficiency, restricted NO3 uptake and sustained high soil moisture causing higher N2O production. In a separate experiment it was demonstrated that denitrification occurs mainly in the immediate vicinity of the roots.

Spring wheat was grown in nutrient solution culture to investigate the influence of aeration, different sources of nitrogen and discontinuation of potassium supply on root respiration and on the number of root bacteria. By definition root respiration included oxygen consumption of the excised roots and their microbial colonizers. Root respiration was subject to diurnal variations. It was low towards the end of the dark period and increased within two hours during the light period. Independent of the nutrient supply the respiration rate and bacterial number were considerably higher in unaerated than in aerated nutrient solution. Root respiration was lowest when using nitrate as a source of nitrogen and highest with ammonium nutrition. Intermediate respiration rates were obtained for mixed nitrogen nutrition. Respiration corresponded to the bacterial colonization of the roots. The discontinuation of potassium supply led in plants supplied with nitrate to an insignificant increase in root respiration, whereas a marked increase in respiration was observed in plants with mixed ammonium and nitrate nutrition. In contrast, Root respiration of plants supplied with ammonium declined when potassium was discontinued. Discontinuation of potassium supply caused an increase in root respiration and bacterial numbers in both aerated and unaerated media.

The direct measurement of gaseous N losses through denitrification requires a complicated methodology. Mostly alternative approaches are used to estimate these losses for a particular arable or grassland site. Evaluation of the literature shows that N losses as a result of denitrification ‐ determined by the 15N balance, by direct mass spectrometry of N2 + N2O and by the acetylene inhibition method, respectively ‐ may reach about 10 % of the fertilizer input on agriculturally used sites without irrigation or organic manure addition. The estimation of denitrification losses can be improved by considering site‐specific parameters affecting denitrification like total carbon content, pH, time of fertilizer application as well as soil temperature and soil moisture conditions.

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The influence of single nutrient stress on oxygen consumption of excised roots and on the number of root bacteria of spring wheat was studied by omitting N, P, K, Mg or Fe from the nutrient solution. In potassium and iron deficiencies the respiration first increased and then declined with increasing deficiency. It remained at the level of the control (complete nutrient solution) for up to 17 days after discontinuating the K supply, while it continued to fall off below the control in the case of Fe discontinuation. In the early stages of K and Fe deficiency the number of bacteria on the roots increased in agreement with the respiration rate. When nitrogen, phosphate and magnesium supplies were interrupted, respiration declined immediately. With continuing N and P deficiency respiration declined further but in the case of Mg deficiency it recovered slowly. The number of root bacteria went almost in parallel with the respiration rates.