THE EFFECT OF VARYING SOIL ORGANIC MATTER CONTENTS ON CURRENT AND FUTURE MOISTURE SUPPLY CAPACITIES: AN ITALIAN CASE STUDY

Increasing the soil organic matter content has been proposed as a significant climate mitigation measure with an additional benefit for farmers, such as increasing the soil moisture supply capacity (MSC) for plants during a growing season and the associated crop yield. Literature suggests that increasing % of soil organic matter (SOM) of a given soil does not significantly increase Available Soil Water Capacity (AWC) which is standard data in most soil databases and expresses soil water contents in the rootzone between field capacity (FC) and permanent wilting point (WP). The AWC has important implications when judging soil moisture supply to plants and potential for climate mitigation, but the standard values for AWC of -33 (FC) and -1500 kPa (WP) do not represent field conditions in many soils (real FC values vary between -10 and -50 kPa and WP values between -800 and -1500 kPa) and moreover, its static nature cannot represent the soil moisture supply capacity (MSC) that also reflects weather conditions. The latter is a dynamic concept that can be obtained by dynamic modeling of the soil-water-atmosphere-plant system, including a "sink-term" indicating a continuous relation between water uptake and negative pressure head of soil water and evaporative demand. Moreover, only models allow exploration of the effects of future IPCC climate scenarios on soil water availability for plants. The objectives of this contribution are to (i) explore the effects of an increase of % SOM on the MSC of different Italian soils (ranging from clay to sand) by applying the continuous SWAP model; (ii) compare these results with AWC values for the same soils to demonstrate their different character, and (iii) explore effects of increased soil organic matter contents on MSC values obtained by applying current climate data and the future RCP 8.5 climate scenario. The results showed that MSC values were: (i) on average 30% higher than the corresponding AWC; (ii) distinctly different for the investigated soils; (iii) affected by declines of 1-9% as a result of expected climate change under RCP 8.5; (iv) not significantly affected by increases of %SOM when considering climate change, except for the sandy soil.