Effects of the number of 15 N-injection needles on the estimation of gross N transformation rates using 15 N tracing tool including plant

It is not clear how the number of (15) N-injection needles affects the estimated results of gross N transformation rates in the presence of plants. To evaluate this, a (15) N tracing study of sandy loam soil and a silty loam soil in the presence of maize (Zea mays L., cv. Zhengdan 958) was conducted and the Ntrace(plant) tool was used to quantify gross N transformation rates. Our results showed that the number of (15) N-injection needles could significantly influence the estimated results of gross N transformation rates in the studied soil-plant systems. There was no discernible difference in the gross rates of N mineralization (Min), the oxidation of NH4+ to NO3- (O-NH4), the oxidation of recalcitrant organic-N to NO3- (O-Nrec), the NH4+ and NO3- plant uptake between 4-needle and 6-needle injections for both studied soils. However, for the 1-needle and 2-needle injections in the silty loam soil, Min was significantly lower than the 4-needle and 6-needle injections. While, in sandy loam soil, Min measured with 1-needle injection was significantly higher than the 4-needle and 6-needle injections. The O-NH4 and O-Nrec also varied for the 1-needle and 2-needle injections, compared to the 4-needle and 6-needle injections. The total plant N uptake rate declined with the increase in the number of injection needles. Based on these results and considering the simplicity of the experimental procedure, we suggest that the 4-needle injection could be used in the (15) N tracing studies conducted in pot (diameter = 6.4 cm in this study) experiments with plants, i.e., one needle for about 8 cm(2).

Automated summary

The number of (15)N-injection needles has a significant impact on the estimated results of gross N transformation rates in soil-plant systems. Different numbers of injection needles showed no discernible difference in N mineralization, NH4+ oxidation to NO3-, recalcitrant organic-N oxidation to NO3-, NH4+ and NO3- plant uptake, but there were variations in certain situations. Based on the experimental results, it is suggested that 4-needle injection should be used in (15)N tracing studies conducted in pot experiments with plants.