Evaluation of a non-destructive technique for inorganic soil N measurement

Abstract

Listen

Translate article

The use of ceramic discs for non-destructive estimations of soil nitrate (NO 3 −) may be an alternative to destructive soil samplings and enable reuse of soil cores. This could especially be of great interest when working with intact soil cores where high variability can be expected and several replicates are needed. Furthermore, the non-destructive method may be used for determination of initial soil nitrate in individual soil samples and thereby improve estimations of net mineralization compared with the use of an average value for initial nitrate content. In the present study, the use of the non-destructive technique for these purposes was evaluated on intact soil cores considerably larger than the soil samples used in the original test. Intact soil cores (100 cm 3) were sampled from a field with a naturally occurring textural gradient (11–45% clay). The non-destructive technique was tested in four experiments that were carried out prior to incubation studies on these cores. Included in the studies were effects of soil texture and matric water potential to show the relationship between nitrate determined destructively and non-destructively. The non-destructive technique was inapplicable for estimation of bulk soil nitrate when used on soil samples with large internal gradients in nitrate content. The diffusion of nitrate in the soil matrix was probably too low to achieve equilibrium in soil samples of this size. The non-destructive method could therefore not reduce the number of soil cores necessary for obtaining reliable data sets with intact soil cores. When the ceramic components were placed on soil with a more homogeneous distribution of nitrate and were allowed to equilibrate with the soil water for 4 h before drainage, the relationship between nitrate measured destructively and non-destructively was good ( r 2=0.83–0.96). It is concluded that the non-destructive technique for nitrate determination is a promising tool for improving the estimates of initial nitrate in soil samples in incubation studies, thereby enabling estimations of net nitrate mineralization. The technique is especially an improvement over the common correction method (where an average value of initial nitrate derived from destructive extraction of additional soil cores is subtracted) in incubation studies of short duration and in soil with high variations in initial nitrate. The non-destructive technique could not be used for ammonium (NH 4 +) determination probably because of a high cation exchange capacity in the ceramic components. However, this was not a problem for the soils studied due to the very low concentrations of ammonium.