“…N and P commonly limit key ecosystem functions (e.g., primary production, Vitousek, Porder, Houlton, & Chadwick, 2010) and services (e.g., C sequestration, Hessen, Agren, Anderson, Elser, & de Ruiter, 2004), but their supply is fulfilled in different ways. In many dryland ecosystems, woody encroachment is often facilitated by the colonization of N 2 -fixing tree legumes (e.g., Prosopis in North America and Acacia in South Africa), which have the potential to add N to the system to overcome possible N limitation and enable rapid accumulation of C in vegetation and soils (Blaser, Shanungu, Edwards, & Olde Venterink, 2014;Boutton & Liao, 2010;Soper & Sparks, 2017). Unlike N, P is derived mainly from weathering of parent materials (Walker & Syers, 1976), and thus ecosystems begin their existence with a fixed amount of P (Vitousek et al, 2010).…”