Soil and plant nutrient status in temperate forests as affected by long-term drying-rewetting conditions

The increasingly warmer and drier climate will change the geographic distribution of tree species. Here, apart from direct effects of changes in temperature and soil moisture on tree physiology, indirect effects via interactions between soil environmental conditions and the bioavailability of nutrients may play an additional role. Specifically, this comprises effects of desiccation and rewetting on soil properties, and of drought and drought release on root-soil interactions in the rhizosphere. Such effects may be particularly strong for phosphorus, considering its low solubility and mobility in soils.

For this study, we considered data from 44 forest sites across Switzerland representing soil moisture gradients for each of the four major tree species beech (Fagus sylvatica L.), oak (Quercus sp.), pine (Pinus sylvestris L.) and spruce (Picea abies Karst). First, we explored the relations between the nutrient status of the soil (total N, hydrogencarbonate extractable inorganic and organic P, microbial P, exchangeable K, Mg, Ca) and soil environmental conditions during four years before soil sampling (average water potential and temperature, number of dry days and drying-rewetting [DRW] cycles as defined by a given water potential threshold). Second, we performed the same analysis using the nutrient status of mature trees (as indicated by nutrient concentrations in bark) instead of the soil nutrient status.

Results indicate a strong influence of DRW on the soil’s P status, whereas other nutrients in the soil are only little affected. On beech and oak sites, the N and P status of the trees increased with increasing moisture and decreasing temperature, and the P status was in addition negatively affected by the length and number of DRW cycles during which high drought stress levels were reached. By contrast, the Ca status of the trees increased with temperature and soil dryness.

Considering observations by others regarding positive effects of high plant nutrient status on a plant’s resilience to drought, our results call for more in-depth studies on the feedback-loops between soil water supply, soil nutrient availability, plant-physiology and root-soil interactions.