Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration

Abstract

During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N-fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its effects upon pools of soil N, P, and carbon (C), and availabilities of N and P. We also evaluated whether these effects induced feedbacks upon the growth of understory vegetation and encroaching shrubs. Dichrostachys cinerea shrubs increased total N and P pools, as well as resin-adsorbed N and soil extractable P in the top 10-cm soil. Shrubs and understory grasses differed in their foliar N and P concentrations along gradients of increasing encroachment, suggesting that they obtained these nutrients in different ways. Thus, grasses probably obtained them mainly from the surface upper soil layers, whereas the shrubs may acquire N through symbiotic fixation and probably obtain some of their P from deeper soil layers. The storage of soil C increased significantly under D. cinerea and was apparently not limited by shortages of either N or P. We conclude that the shrub D. cinerea does not create a negative feedback loop by inducing P-limiting conditions, probably because it can obtain P from deeper soil layers. Furthermore, C sequestration is not limited by a shortage of N, so that mesic savanna encroached by this species could represent a C sink for several decades.We studied the effects of woody encroachment on soil N, P, and C pools, and availabilities of N and P to Dichrostachys cinerea shrubs and to the understory vegetation. Both N and P pools in the soil increased along gradients of shrub age and cover, suggesting that N fixation by D. cinerea did not reduce the P supply. This in turn suggests that continued growth and carbon sequestration in this mesic savanna ecosystems are unlikely to be constrained by nutrient limitation and could represent a C sink for several decades.

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment

“…This indicates that, prior to woody plant encroachment, grassland soils contained disproportionately less P than C, and are potentially P limited. For example, woody species have more extensive and intensive root systems compared to grasses, especially in the vertical dimension (Jackson et al, 1996;Schenk & Jackson, 2002), enabling them to mine P (i.e., deep acquisition) located below the depth of grass root systems (Blaser et al, 2014;Kantola, 2012;Scholes & Archer, 1997;Sitters et al, 2013). For example, woody species have more extensive and intensive root systems compared to grasses, especially in the vertical dimension (Jackson et al, 1996;Schenk & Jackson, 2002), enabling them to mine P (i.e., deep acquisition) located below the depth of grass root systems (Blaser et al, 2014;Kantola, 2012;Scholes & Archer, 1997;Sitters et al, 2013).…”

“…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).…”

“…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). Since woody proliferation can result in woody patches with aboveground biomass and primary productivity orders of magnitude greater than those of grasslands they replaced (Hibbard, Schimel, Archer, Ojima, & Parton, 2003;Hughes et al, 2006), a significant amount of P may become tied up in plant biomass, potentially leading to smaller pools of soil P. However, some field studies have shown that woody encroachment actually increases total P in surface soils beneath woody patches (Blaser et al, 2014;Kantola, 2012;Sitters, Edwards, & Venterink, 2013).…”

Soil properties and climate mediate the effects of biotic interactions on the performance of a woody range expander

“…This process has also gained attention due to its negative impacts on livestock ranching and on regional economies (Briggs et al 2002a, Limb et al 2011. At larger spatial and temporal scales, tree range expansion is predicted to modify nitrogen and carbon balance (Oechel et al 2000, Wheeler et al 2007, Neff et al 2009, Blaser et al 2014) as well as hydrogeological cycles , Hallinger et al 2010, Acharya et al 2017, with detrimental effects on ecological processes (Hoch et al 2002, Bond 2008, Limb et al 2010, Ward et al 2014.…”

Leaf anatomical traits of non-arboreal savanna species along a gradient of tree encroachment

“…Th is is only possible because some typical forest tree species can establish in savanna sites during fi re absence (Silva et al 2008;Hoff mann et al 2009;Geiger et al 2011). Th e recruitment of forest species contributes to canopy closure, decreasing the total amount of light radiation reaching the understory (Pinheiro et al 2016) and increasing the availability of nutrients (as P) in the superfi cial soils under encroachment (Blaser et al 2014;Pinheiro et al 2016;Rossatto & Rigobelo 2016). Changes in irradiance have been proven as a strong environmental filter (more than soil P) affecting the occurrence of nonarboreal savanna species, since its decrease drastically reduces species richness and diversity in encroached savanna sites (Ratajczak et al 2012;Pinheiro et al 2016).…”