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

Background/Question/Methods: Land cover/land use changes have dramatically altered Earth’s major biogeochemical cycles and induced imbalances between carbon (C), nitrogen (N) and phosphorus (P) in many ecosystems. Soil C, N and P cycles are strongly interlinked by biological processes such as primary production and decomposition, while the P cycle is further controlled by geochemical processes such as dissolution and precipitation. Geographically widespread woody encroachment into grasslands and subsequent changes in abiotic factors along the soil profile could exert different degrees of control on these biological and geochemical processes, potentially leading to disproportional changes in soil C, N, and P storage, although this has not been assessed. In this study, we evaluate how woody encroachment affects the stoichiometric relationships between soil organic C (SOC), total N (TN) and total P (TP) in 3-dimensional soil space across a landscape. To accomplish this, 320 spatially georeferenced soil cores (depth = 1.2 m) were collected across a 160 m × 100 m landscape in a subtropical savanna that has undergone encroachment by N₂-fixing Prosopis glandulosa in southern Texas. Geostatistics, structural equation modeling, and a scaling approach were used to quantify spatial patterns of SOC, TN, and TP and their relationships.

Results/Conclusions: SOC and TN were strongly coupled and increased proportionally in response to woody encroachment. In addition, spatial patterns of SOC and TN were identical throughout the soil profile. Spatial patterns of SOC and TN were related to vegetation cover in upper portions of the profile (0-15 cm), but not at deeper soil depths. In contrast, TP increased slower than SOC and TN in topsoil (0-5 cm), but faster in subsoil (15-120 cm). Spatial patterns of TP throughout the entire profile were strongly related to vegetation cover, with higher TP beneath woody patches compared to grasslands. This imbalanced relationship implies that net changes in SOC, TN, and TP were regulated by different factors. Structural equation models indicated that fine root mass explained large proportions of the variances in SOC, TN, and TP in topsoil. In subsoil, clay and silt content emerged as major factors controlling variation in SOC and TN, while soil pH was related to soil TP. Results suggest that soil TP is decoupled from SOC and TN following woody encroachment, and efforts to incorporate the effects of vegetation change into integrated climate-biogeochemical models should consider this imbalanced relationship.