Slope-based and ratio-based approaches to determine fertiliser-derived N in plant tissues for established perennial plants

• Published in: The journal of horticultural science & biotechnology 2007, Vol.82 (4), p.641-647
• Main Authors: Righetti, Timothy L., Dalthorp, Dan, Sandrock, David, Strik, Bernadine, Banados, Pilar, Zhou, Zhong-Wei
• Format: Article
• Language: English
• ISSN: 1462-0316; 2380-4084
• DOI: 10.1080/14620316.2007.11512285

Two approaches for estimating the amount of N in plant tissues derived from labelled fertiliser were evaluated. In the first, atom percentage values obtained by mass spectrometry were converted to the percentage of total N derived from the fertiliser (%NDFF). In the second, the slope of the regression line for the relationship between labelled fertiliser N and total N was used to represent the incremental increase in fertiliser N for each unit increase in total N. These two approaches were applied to data collected for a blueberry (Vaccinium corymbosum L.) field trial, where the effects of N rate and plant spacing on fertiliser accumulation were evaluated. Since varying degrees of biological scaling can occur, and many perennial plant tissues have an initial biomass, regression equations for different tissues produced both positive and negative y-intercepts. Log 10 fertiliser N vs. log 10 total N plots produced slopes that did not equal 1.0 (from 0.40 to 1.65, depending on tissue). When either non-zero y-intercepts for linear regression lines, or slopes not equal to 1.0 for log-log plots occur, %NDFF is dependent on the size (total N) of the tissue or plant. Depending on the tissue evaluated, the %NDFF can be unrelated, or negatively or positively related to plant or tissue size. Furthermore, increased nitrogen application alters the relationship between %NDFF and plant size. For tissues in which %NDFF declines with increasing total N, the relationship weakens as more N is applied. For tissues in which %NDFF increases with increasing total N, the relationship strengthens as more N is applied. An analysis of the slopes and y-intercepts of labelled fertiliser N vs. total N relationships produces a different interpretation than evaluations of the %NDFF for treatment means. Significant %NDFF differences for spacing treatments, and significant spacing tissue interactions were size-related rather than having other physiological causes. Increases in %NDFF associated with increasing N rate were directly related to differences in N accumulation. Difficulties associated with evaluating the ratio-based %NDFF in established perennial plants are even more problematic than the difficulties previously reported for plant systems with little initial N content. Similar scaling issues could be important when any exogenous substance is introduced into organisms or ecosystems.