Above- and belowground nitrogen distribution of a red clover-perennial ryegrass sward along a soil nutrient availability gradient established by organic and conventional cropping systems
Belowground legume nitrogen (N) composed of roots and rhizodeposition is an important N input to soils, but published data of belowground N vary broadly, probably due to extrapolation from short-term experiments and dissimilar growing conditions. We quantified belowground N inputs of red clover (Trifolium pratense L.) during two consecutive years in a clover-grass sward along a soil nutrient availability gradient.
We established a red clover-perennial ryegrass (Lolium perenne L.) model sward in microplots located in field plots of the DOK experiment, which has a 33-year history of organic and conventional cropping, resulting in a soil nutrient availability gradient. Four treatments were examined: the zero fertilisation control, bio-organic with half and full dose manure application, and the conventional system with mineral fertilisation at full dose. We studied the development of clover aboveground and belowground N using multiple pulse 15N urea leaf labelling.
Belowground clover N increased over time and with rising nutrient availability and was proportional to aboveground clover N at all times. Belowground clover N amounted to 40% of aboveground clover N during two consecutive years, irrespective of the nutrient availability status. Belowground clover N development was initially dominated by fast root growth, followed by enhanced root turnover during the second year. Potassium availability limited clover growth and total N accumulation in treatments with low nutrient availability.
Belowground red clover N inputs could be estimated from aboveground N by a constant factor of 0.4, regardless of the nutrient availability and cultivation time. Root turnover led to a distinct absolute increase of N rhizodeposition over time. Hence, N rhizodeposition, with an 80% share of belowground N, was the predominant N pool at the end of the second year.
KeywordsRhizodeposition 15N leaf labelling Cropping systems Belowground to aboveground N ratio Nutrient availability
Belowground N, comprising physically recoverable root N at the time of excavation plus NdfR
Bio-organic treatment of the DOK experiment with half dose fertilisation
Bio-organic treatment of the DOK experiment with full dose fertilisation
Conventional treatment of the DOK experiment with full dose sole mineral fertilisation
Chloroform fumigation extraction
Long-term experiment comparing Bio-Dynamic, Bio-Organic, and conventional (K) cropping systems
Excess atom fraction
Labelled microplot, delimiting the 15N labelled plant-soil system (excavated after t months of sward cultivation)
Nitrogen derived from rhizodeposition
Unfertilised control treatment of the DOK experiment
Reference microplot, delimiting the unlabelled plant-soil system (excavated after t months of sward cultivation)
Time from planting of red clover and perennial ryegrass until microplot excavation in months
We warmly thank the Agroscope field team, especially their head Ernst Brack, Lucie Gunst and Monika Schnider from Agroscope for their versatile help in the DOK experiment and in the lab, the FiBL field team for their help in the DOK experiment, Stephano Bernasconi from the Geological Institute at ETH Zurich for isotopic analysis, Claude Renaux from the statistical consulting service of the seminar of statistics at the ETH Zürich, and Juliane Hirte from Agroscope for the final internal review of the manuscript. The work was funded by the Swiss National Science Foundation Grant 205321_132770 / 1.
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