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

Abstract

Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0-5 cm) but faster in subsurface soils (15-120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N -fixing trees into this P-limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate-biogeochemical models should attempt to represent C-N-P imbalances that may arise following vegetation change.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

“…"Anthropogenic land-use changes" are heterogeneous, but they tend to be associated with changes in soil N and P concentrations and N:P ratios Urbina, Grau, Sardans, Ninot, & Penuelas, 2019;Wang et al, 2014;Zhao et al, 2015;Zhou, Boutton, & Wu, 2018a, 2018b. For example, invasion by shrubs on grassland previously grazed by livestock is frequently associated with changes in soil-plant N and P concentrations and N:P ratios (Bui & Henderson, 2013;Urbina et al, 2019).…”

“…For example, invasion by shrubs on grassland previously grazed by livestock is frequently associated with changes in soil-plant N and P concentrations and N:P ratios (Bui & Henderson, 2013;Urbina et al, 2019). These changes go in parallel to a transition from rapid nutrient cycling, with high concentrations of N and P in the plant-soil system, to slower N and P cycling, with lower concentrations of N and P in the system, and higher accumulations of N and P stocks in the higher aboveground shrub biomass (Urbina et al, 2019;Zhou, Boutton, & Wu, 2018a, 2018b) that has a larger capacity to obtain nutrients from deep soil layers (Blaser, Shanungu, Edwards, & Venterink, 2014). These trends, however, vary with the traits of the shrub species (Eldridge et al, 2011;Knapp et al, 2008;Zhou et al, 2018b).…”

“…These changes go in parallel to a transition from rapid nutrient cycling, with high concentrations of N and P in the plant-soil system, to slower N and P cycling, with lower concentrations of N and P in the system, and higher accumulations of N and P stocks in the higher aboveground shrub biomass (Urbina et al, 2019;Zhou, Boutton, & Wu, 2018a, 2018b) that has a larger capacity to obtain nutrients from deep soil layers (Blaser, Shanungu, Edwards, & Venterink, 2014). These trends, however, vary with the traits of the shrub species (Eldridge et al, 2011;Knapp et al, 2008;Zhou et al, 2018b). Shifts in soil N:P ratios during processes of habitat transition may vary with soil layer, but soil N:P ratios tend to increase in the upper layers (Feng & Bao, 2018;Zhou et al, 2018aZhou et al, , 2018b.…”

“…These trends, however, vary with the traits of the shrub species (Eldridge et al, 2011;Knapp et al, 2008;Zhou et al, 2018b). Shifts in soil N:P ratios during processes of habitat transition may vary with soil layer, but soil N:P ratios tend to increase in the upper layers (Feng & Bao, 2018;Zhou et al, 2018aZhou et al, , 2018b.…”

A Three-Dimensional Assessment of Soil δ13C in a Subtropical Savanna: Implications for Vegetation Change and Soil Carbon Dynamics

“…Herbaceous vegetation is largely absent beneath tree/shrub canopies in clusters and groves. Grasslands, clusters, and groves are each distinct ecosystems characterized by different plant species (Table S2) and soil biogeochemical properties and processes [19,29,59,60], and they represent the most common elements of upland landscapes in this region [61].…”