Paradoxical differences in N-dynamics between Luxembourg soils: litter quality or parent material?

Abstract

To explore whether litter quality could alter differences in N-dynamics between soil types, we compared spruce and beech growing on soils with parent material sandstone and limestone, and beech and hornbeam on acid marl and limestone. We measured pH, organic matter content, C:N ratio, soil respiration and net N-mineralization of the organic layer and the mineral topsoil in a laboratory incubation experiment and estimated gross N-mineralization and immobilization with a simulation model. Species effects were restricted to the organic layer, but higher mass for low-degradable species was compensated by higher process rates for high-degradable ones, so N-dynamics per square metre did not differ. Also, the mineral topsoil was not affected by litter quality, which may have been overruled by soil conditions. Forest soils formed from different parent materials, however, clearly differed in N-dynamics, although different from expectations for net N-mineralization. Sandstone showed low respiration and gross N-mineralization, but net N-mineralization was higher than expected, probably due to low microbial N-demand. In contrast, limestone, and to some extent acid marl, showed high respiration and gross N-release, but lower net N-mineralization than expected, because microbial immobilization was also high. Simulated gross N-mineralization even showed a negative instead of positive correlation with net N-mineralization, probably due to the strong increase in immobilization when gross N-mineralization is high. The shift in microbial N-demand may in turn be related to a more general shift from bacteria to fungi over pH-gradients.

Do evergreen and deciduous trees have different effects on net N mineralization in soil?

“…This remarkable variation in the effects of species, genera, and plant functional types on net N min rates probably arises, in part, because the effect of plant traits on net N min depends on environmental factors and stand properties. For example, rankings for net N min rates among tree species can depend on stand age (Pare´and Bergeron 1996), soil horizon (Thomas and Prescott 2000), and soil type (Ste-Marie et al 2007, Kooijman andSmit 2009). The relative effects of different plant traits on net N min rates also are likely to depend on these and other factors.…”

Differences in activity and N demand between bacteria and fungi in a microcosm incubation experiment with selective inhibition

“…In the control treatment, the results further support the hypothesis that neutral soils have higher biological activity than acidic soils, but that net N-mineralization may nevertheless be lower due to higher microbial N-demand and immobilization. This could explain why net N-mineralization was lower in neutral than in acid soil in e.g., Zöttle (1960), Davy and Taylor (1974), Verhoeven et al (1990), Kooijman and Besse (2002), Kooijman et al (2008), Kooijman and Smit (2009) and Mettrop et al (2014).…”

“…Respiration was measured at the start and end of the incubation experiments, as preliminary experiments indicated that CO 2production during the experiment was more or less constant (Kooijman and Smit, 2009). Fresh or incubated material was placed in open glass jars during one night, with the equivalent of 5 g dry weight for organic and 10-15 g for mineral samples.…”

“…Efficiency of N-mineralization was calculated as net Nmineralization per unit C respired, which can be used as proxy for microbial immobilization, and the relative amount of N available to the vegetation (Kooijman et al, 2008;Kooijman and Smit, 2009). In this case, high N mineralization per unit respiration indicates low immobilization by micro-organisms, and a high efficiency of N-mineralization for the vegetation.…”

“…Yet, there are some problems with this scenario. More than a few studies report higher instead of lower net N-mineralization in acidic, fungal-dominated systems, compared to neutral, bacteriadominated soil (Zöttle, 1960;Davy and Taylor, 1974;Verhoeven et al, 1990;Kooijman and Besse, 2002;Kooijman et al, 2008;Kooijman and Smit, 2009;Mettrop et al, 2014). This may be explained by low immobilization in fungi, due to high C:N ratios and relatively slow life cycles (Hassink, 1994;Moore et al, 2005;Cleveland and Liptzin, 2007).…”