An integration of soil parameters characterizing a Danish agricultural soil

The complexity and interplay of soil processes are still investigated extensively. Continuous focus on this field of research is important since soil properties such as nitrate reductive capacity has a great influence on groundwater quality. Here, we try to give insight into the dynamics of a vadose zone soil under agricultural management.

A field of study was selected in Darum in Southwestern Jutland, Denmark. The site is situated in an old periglacial terrain on meltwater-deposited sand. The field has been under maize (Zea mays) monoculture for the past 20 years. Prior to this period it had been kept with C3 plants only.  Soil sampling was accomplished in three replicates of 1.6 m.

The bulk soil samples were analyzed for total C and N, δ¹³C and δ¹⁵N. Dissolved organic matter (DOM) and NO₃^- were recovered from cold-water extractions of the soil samples. Extractions were analyzed for their UV-Vis absorption spectra.

Incubation experiments were performed on bulk soil portions in order to assay the activity and isotopic imprint CO₂ respiration. The soil were also incubated under anoxic conditions with substrate amendments (KNO₃ and C additions). The resulting N₂O releases were assigned to biologically driven nitrate reduction. Ultimately, principal component analyses (PCA) were carried out on the results.

The C and N concentrations were highest in the Ap horizon and decreased with soil depth. The respiratory and nitrate reductive capacity also declined with depth, but were evident in all of the analyzed soil depths. All individual depths responded statistically significant to substrate addition by increase in the N₂O production.

The isotopic results showed that the main pool of maize-derived C were also found in the plough layer. However, the respiratory isotopic results evidenced the presence of C4 plant derived C throughout the soil profile, after 20 years of monoculture.

The UV-Vis absorption spectra gave insight into the quality of the DOM pools. The parameter E253/E203 is associated with functional groups on aromatic rings and increases with composting time. The soil had an overall increase in this parameter with depth. The integrated magnitude of distinct wavelengths (270-300 nm, 300-380 nm and 380-500 nm) is an index of protein-, fulvic-, and humic like substances. Surprisingly, no substantial discrepancies in the distribution between these pools was found with depth. However, the overall pattern was declining steeply with soil depth, emphasizing the importance of dilution when assessing DOM availability and quality.

The PCA could explain >55 % of the variance by the first principal component. The PCA showed that the C and N concentrations were positively correlated. Alongside were the ambient N₂O activity to the indexes of protein-, fulvic and humic like substances. The inherent NO₃^- concentration, the N₂O activity (KNO₃ amended) and the respiratory CO₂ production were also positively correlated – however negatively correlated with the E253/E203 parameter.
Therefore, respiratory and nitrate reductive capacities of the Darum soil, depends notably on the presence of less degraded DOM, on the concentration of protein-, fulvic and humic like substances, and finally on the inherent soil NO₃^- concentration.