Changes in soil organic carbon content affect plant available water more strongly in subsoil than in topsoil

With the increase of drought events due to climate change, agricultural production is under pressure to maintain yields. The subsoil (> 30 cm) often harbours unexploited water and nutrient resources that can meet the needs of plants when the topsoil has already dried out. The fertility of the soil is also closely linked to its organic carbon (OC) content. A higher soil organic carbon (SOC) content can improve the soil structure, which is associated with a higher water infiltration rate and a higher water retention capacity and thus can facilitate the adaptation of agriculture to a changing climate. However, the knowledge about quantity changes, especially in subsoils, is insufficient. Therefore, we analysed soils from eleven field sites to quantify the effects of SOC content on topsoil and subsoil plant available water (PAW), here defined as the water content between pF = 1.8 and pF = 4.2. Long-term experiments were set up at four sites, which we sampled after a duration of 57-68 years. In addition, four short- to medium-term trials with a duration of <1-10 years were sampled, as well as three treposols that were deep ploughed once 52-54 years prior to sampling. At all sites there is a management-related OC gradient over a wide range of clay contents (4-28%). Preliminary results of the long-term experiments showed that topsoil SOC contents increased on average by 43 % after the application of farmyard manure. PAW was also higher, with an average increase of 6 %, indicating a positive correlation between SOC content and PAW, although this was not linear. Significantly stronger effects on PAW were observed in the subsoil, where changes averaged 40 %. Our results therefore emphasise the importance of considering the subsoil when adapting agriculture to changing climatic conditions.