The Mediterranean Region as a Paradigm of the Global Decoupling of N and P Between Soils and Freshwaters

Abstract

Socioeconomic development and agricultural expansion are accompanied not only by larger inputs of N and P but also by a serious alteration of the water cycle (Grill et al., 2019). This coupling between water and nutrient resources underscores the need to address food security and water security as interrelated challenges. Agriculture accounts for 69% of annual water withdrawals worldwide (FAO Aquastat). Some of the countries currently making major efforts to expand irrigation systems are in arid and semiarid regions of the world, which entails intensive groundwater abstraction and the construction of dams and reservoirs to secure the access to water resources. Reservoir storage is also the classic solution applied to contend with water scarcity in areas experiencing urban and industrial expansion (Gleick, 2003). How human water management may affect the stocks and fluxes of N and P is a question that has received little attention so far. Studies that deal with water and nutrients at once are usually performed at the small-scale, analyzing specific agricultural patches (e.g., Bartoli et al., 2012; Törnqvist et al., 2015), and focus on (1) increasing crop yields without compromising water resources; or (2) agricultural practices as a means of reducing nutrient loads and shortcutting water eutrophication. Furthermore, the vast majority of these studies concern wet temperate systems, which are those for which the most data exist, but very few provide data specific to arid and semiarid conditions. Arid and semi-arid regions constitute a significant fraction of the Earth's terrestrial ecosystems, and are expected to increase in upcoming years (IPCC 2014), so filling this data gap is of concern in light of future climate scenarios. The Mediterranean basin offers a unique test-bed to assess the coupling of N, P with water management for several reasons. First, because it comprises a gradient of climatological regimes (Figure 1) and different

Ecosystem‐level effects of re‐oligotrophication and N:P imbalances in rivers and estuaries on a global scale

“…Changes in N:P may induce shifts among primary producers hence favoring the dominance of certain groups of phytoplankton (Romero et al, 2013 ). Given the high spatiotemporal variability of river ecosystems and natural and anthropogenic drivers of flow alteration (Romero et al, 2021 ; Sardans et al, 2012a ; Tong et al, 2020 ; Yan et al, 2016 ; Zhang et al, 2019 ), it is expected that this stoichiometric N:P imbalance has played a significant role in biological responses (Elser et al, 2009 ). Besides the widespread decrease in P due to management measures such as the installation of WWTP and the banning of detergents, and/or the increase in N due to an intensive use in N fertilizers, the increase in water N:P ratios can also occur naturally in rivers because of their high‐water turnover (Peñuelas et al, 2013 ).…”

Soil legacy nutrients contribute to the decreasing stoichiometric ratio of N and P loading from the Mississippi River Basin

“…Molecular forms of N are highly soluble and can convert to gaseous form through volatilization, nitrification, and denitrification processes, whereas P is less mobile and occurs largely in particulate form by attaching itself to clay surfaces, or combining with Fe, Al, Mg, and Ca (McDowell et al, 2004). The higher retention of P in soil than that of N decouples the N and P between soil and water, and the average N:P ratio of riverine export is usually higher than that of soil surplus (Romero et al, 2021). The mean residence time of P in the terrestrial biosphere was estimated to be over 3.5 times that of N (Schlesinger & Bernhardt, 2013; Wang et al, 2010).…”

Disclosing the effects of climate, land use, and water demand as drivers of hydrological trends in a Mediterranean river basin

“…Runoff coefficient in the Onyar has decreased by nearly half from 1961-1965 to 2011-2015, reaching values down to 0.13. These low runoff coefficients (R/P≤0.2) tend to occur under aridity conditions, or in basins located under extreme climatic conditions (Romero et al 2021). In this study case, human stressors, as stream discharge capture by groundwater withdrawal, must also be considered to explain such a low R/P ratio.…”