Disentangling the impacts of soil management practices on nitrogen mineralization and inorganic nitrogen accumulation during soil drying events

The biogeochemical cycling of nitrogen (N) in drylands ecosystems depends strongly on the alternance dry-wet cycles caused by seasonal nature of rainfall pulses. While during the dry periods the soil organic matter (SOM) mineralization is limited and inorganic N (Ninorg) is accumulated in soil because the low diffusion of ions and low N uptake by plant and microbes, the rewetting enhances the diffusion of ions and causes a pulse of N transformations and N₂O emissions. Therefore, the understanding of the processes involved in the accumulation of Ninorg is crucial. In this study we combined a field and a laboratory study to disentangle the role of soil management practices on N mineralization and Ninorg accumulation in soils under drought conditions in Spain. First, we evaluated the N mineralization and Ninorg accumulation during summer fallow in a full-factorial field experiment comparing the effect of tillage (no-tillage (NT) vs traditional tillage (TT)) and liming. Second, we performed a soil drying experiment under controlled conditions using soil from the same field experiment to distinguish the impacts of management practices via changes in soil biogeochemical properties (mainly soil organic matter (SOM) and pH) and via soil microenvironmental conditions (soil water availability and temperature).

In the field experiment, Ninorg was accumulated in soils along the summer fallow (from May to July) while the evaluated enzymatic activities (β-glucosaminidase, Leucine aminopeptidase, BAA protease, Casein protease, L-glutaminase and Urease) and the abundance of chiA, pepA and apr genes were significantly reduced during the summer fallow. We observed a significant and positive interaction between NT and liming in the accumulation of Ninorg which may suggest higher risk of N losses upon rewetting. The higher Ninorg accumulation is linked to a similar synergistic effect of both practices on the activity of L-Asparaginase, L-glutaminase and Urease. The higher SOM, pH and less extreme microenvironmental conditions observed in soils managed by NT and liming can explain this synergistic effect. No effect of the treatments in the abundance of the evaluated genes was observed. In the soil drying experiment under controlled conditions where the differences caused by the microenvironmental conditions were excluded, we observed a positive effect of NT and liming on Ninorg accumulation along the drying experiment (29 days of drying). However, no interaction between NT and liming on the Ninorg accumulation was observed. This suggest that the synergistic effect observed in the field experiment was caused by the microenvironmental conditions rather than by changes in biogeochemical properties. Similar circumstance was observed in the analyzed enzymatic activities and chiA abundance (positive effect of NT and liming but without synergistic response) confirming the previous observation. In summary, our results suggest that the combination of NT and liming increases synergistically the accumulation of Ninorg in soil during summer fallow because of their positive effect on soil microenvironmental conditions rather than on soil biogeochemical properties