Modeling of Phosphorus Dynamics in Dryland Ecosystems

Abstract

The availability of phosphorus (P) in soils can be one of the primary factors limiting growth for ecosystems located in both humid and arid climates (Chadwick et al. 1999; Wardle et al. 2004; Crews et al. 1995; Vitousek et al. 2010; He et al. 2014). While P and nitrogen (N ) can be co-limiting nutrients in some terrestrial ecosystems, P can be equally as limiting or more limiting in dryland areas especially those that have calcareous soils or soils with a high pH (Belnap 2011). Over the next century, the extent of P limitation will likely increase globally due to the projected increase in inputs of anthropogenic N (by ~120%) over the next 50 years. These changes could cause a shift from some ecosystems being primarily N limited to being limited by nutrients such as P (Galloway et al. 2004; Mahowald et al. 2008; Vitousek et al. 2010). However, it is not clear how an increase in global aridity over the twenty-first century will affect dryland areas (Chap. 21). For instance, Delgado-Baquerizo et al. (2013) examined how aridity affects the balance between C, N, and P in soils collected from 224 dryland sites across all continents except Antarctica. They found a negative effect of aridity on the concentration of soil organic C and total N but a positive effect on the concentration of inorganic P. They suggest that this is due to aridity being negatively related to plant cover, which may favor physical processes such as rock weathering, and oftentimes a major source of P to ecosystems, over biological processes such as litter decomposition that provide more C and N. It has been estimated that 42% of the world’s croplands are located within drylands (see Chap. 19). About 30–40% of global cropland is primarily P limited with large regions being located in semiarid Africa and Asian steppes (Runge-Metzger 1995; Von Uexkull and Mutert 1998; MacDonald et al. 2011; Fig. 12.1). In agricultural systems, P limitations can be overcome in the short term by utilizing fertilizer to stimulate crop yields; yet despite this, MacDonald et al. (2011) found that global agronomic inputs of P fertilizer (14.2 Tg of Py⁻¹) and manure (9.6 Tg of Py⁻¹) collectively exceed P removal by harvested crops (12.3 Tg of Py⁻¹). Moreover, the supply of rock phosphate needed to create P fertilizer is not limitless and could be depleted in as few as 50 years (Vance et al. 2003; Cordell et al. 2009). Thus, it is important to understand the controls on P availability in dryland ecosystems because of both the potentially expanding extent of P-limited systems and the finite supply of P for fertilizer, which is many times needed during the agricultural production process.