Timing of xylem sampling for ureide analysis of nitrogen fixation

Abstract

The measurement of N2 fixation by legumes is necessary for gaining an understanding of their contributions to the N economies of agricultural and forestry systems and for their management in those systems. We report research to determine whether N2 fixation of four of the commonly-grown ureide-producing legumes, soybean (Glycine max), cowpea (Vigna unguiculata), mungbean (V. radiata) and black gram (V. mungo), could be quantified from a single sampling and N-solute analysis of xylem sap. Data were derived from a previously-published experiment involving six genotypes of soybean at five field sites and from a second, irrigated experiment in which two genotypes of soybean, and one each of cowpea, mungbean and black gram were assessed in low- and high-nitrate soils for nodulation, yields of shoot and grain dry matter and N, and N2 fixation using xylem solute (ureide) and 15N methods. Regression analysis of the published soybean data set indicated that the early pod-fill (R3.5 and R4) samplings for xylem sap gave estimates of percentage of plant N derived from N2 fixation (%Ndfa) which agreed well with %Ndfa for the entire growing season obtained from 15N analysis of the shoots at R6-7. There was a marginal benefit in combining the R3.5 and R4 samplings and using the average of the two, with regression coefficients (r 2) increasing from 0.86 (R3.5 or R4 alone) to 0.92 (average of R3.5+R4). There was no additional benefit in combining R3, R3.5 and R4. In the second experiment, agreement between 15N-determined %Ndfa and either measured (R4 sampling) or calculated ureide-determined %Ndfa (R3.5 sampling) was also good (r 2 of 0.73 (R4) and 0.79 (R3.5)). We conclude that seasonal %Ndfa can be accurately estimated using the xylem solute (ureide) method from a single sampling of xylem sap during early pod-fill (R3.5) and that this simplification of the protocol of the technique may encourage expanded use.