Patterns of nitrogen and citric acid induced changes in C-turnover and enzyme activities are different in topsoil and subsoils of a sandy Cambisol

- N-addition reduced SOC-mineralization in the topsoil.
- No N-effect on C-turnover in subsoils.
- Enzymes were differently affected in top- and subsoils by citric acid and N.
- Priming effect in the topsoil was rated as “apparent priming”.
- Priming effect in both subsoil layers was rated as “real priming”.

Studies on factors controlling C-stability in subsoils are very scarce. Recent results suggest a lack of labile C substrates and N limitations in subsoils as a reason for suppressed C-turnover. The catalytic activity of soil enzymes plays an important role for the decomposition of organic matter in soils and can be a powerful tool to shed further light on substrate and N-limitation as a hypothesized controlling mechanism for C-stability in subsoils. Therefore, we studied the impacts of 14C-labelled citric acid and of NH4NO3 on changes in soil organic carbon (SOC)-mineralization and enzyme activities of dehydrogenase and 9 extracellular enzymes involved in C-, N-, P- and S-cycle. For this approach, we sampled a sandy Cambisol at three different depths (2-12, 35-65 and 135-165 cm) and conducted a laboratory incubation experiment for 63 days at 10 °C. N-addition reduced SOC-mineralization in the topsoil layer by 43%, while no N-effect was observed in both subsoil layers. In the topsoil samples, dehydrogenase-activity also decreased after the incubation with N additions. Further, the activity of extracellular enzymes involved in P- and N-cycling was differently affected in top- and subsoils, indicating that microorganisms in different soil depths have different demands for N or P after adding inorganic N. Additions of citric acid increased SOC mineralization by about 1.9- and 2.2-fold in the upper (35-65 cm) and lower subsoil (135-165 cm) samples, but only by about 32% in the topsoil samples (2-12 cm). The observed priming effect in the topsoil samples was not accompanied by an increased enzyme activity which indicates “apparent priming”. In contrast, priming effects in both subsoil layers were rated as “real priming” indicated by increased enzyme activities and continuously higher SOC-mineralization rates throughout the incubation compared to the controls.