Slog: A soils blog
January 25, 2021: HOW WELL DO WE UNDERSTAND SOIL P?
I have been working on a manuscript this past week from our recent PhD graduate, Maryam Foroughi. Maryam's research in the Calhoun Critical Zone Observatory (CCZO) focused on P dynamics. Much of our previous work on P at the CCZO has focused on the long-term soil experiment (LTSE; Richter et al, 2006) but also some broader landscape comparisons (Richter and Markewitz, 2001). A common finding across this work at the Calhoun and others investigating land use change from agriculture to forest (e.g., Compton and Boone, 2000) is a legacy of surface soils (upper 30 cm) with elevated soil extractable P. In some cases this measurable difference in extractable soil P is evident even 80 years after afforestation (Falkengren-Grerup et al., 2006 ). Given these observations, as well as understanding of P biogeochemistry, it is presumed that P is strongly retained in the ecosystem due to P binding with Fe and Al. In the Ultisols of the Calhoun, with a thick (up to 2 meter) argillic horizon of Fe oxide coated clays, many that have been exposed due to a history of erosion, retention is reasonable.
A fascinating finding from Maryam's work, however, was that soil extractable P was not only elevated in the soil surface horizons but was evident in upper landscape positions across the CCZO through a full 2 m of the profile. This is counter to our statement above that P is strongly retained. If it is strongly retained it should not be leaching through the profile. In fact, much agricultural research focuses not on the retention of P but rather its loss in to nearby waterways. Eutrophication of waters due to P inputs is a global problem. In the Calhoun region with its clay rich Ultisols, P runoff is a similar problem, particularly relative to applications of P as poultry litter on to pastures.
So there is an interesting dichotomy: agricultural landscapes are losing P while afforesting landscapes appear to strongly retain P. We can hypothesize some mechanisms as to why this should be. For one, surface runoff and erosion are greater in agricultural landscapes so might fully explain high P loads to streams. Secondarily, P loads (poultry litter or fertilizer) are much higher during agriculture. On the other hand, afforestation may also increase P retention in vegetation and can augment evapotranspiration further limiting hydrologic transport. Maryam tried to address some of all these hypotheses but recognize the history of agriculture and afforestation in the Calhoun landscape has occurred over the last 100+ years so direct empirical evidence for all stages of land use is limited. Her approach was to sample what she could now (soil contents and water fluxes) and model the long sweep of forest time (Clawson 1979).
The result has been a creative effort to recreate P fluxes under different land use compositions from hardwood forest, to agriculture, to pine forest, along with the mixed land use forests in between. The results are validated against current observations of very low solution fluxes but elevated soil extractable P through 2 m of the profile. The outputs do find higher surface P losses during agriculture, although not sufficiently high to account for 100s of kilogram of P inputs that may have been input with fertilizers. Similarly, modeled vertical leaching of P into the soil profile can account for the increased content of extractable P (~40 kg) in soil with an agricultural history, although incorporating high rainfall events over time, such as hurricanes, makes the vertical fluxes more accurate.
Clawson, M. 1979. Forests in the long sweep of American history. Science 204:11-68-1174.
Compton JE, Boone RD (2000) Long‐term impacts of agriculture on soil carbon and nitrogen in New England forests Ecology 81:2314-2330
Falkengren-Grerup, U., ten Brink, D.J., Brunet, J., 2006. Land use effects on soil N, P, C and pH persist over 40–80 years of forest growth on agricultural soils. Forest Ecology and Management 225, 74-81.
Richter DD, Allen HL, Li J, Markewitz D, Raikes J (2006) Bioavailability of slowly cycling soil phosphorus: major restructuring of soil P fractions over four decades in an aggrading forest Oecologia 150:259-271
Richter DD, Markewitz D (2001) Understanding soil change : soil sustainability over millennia, centuries, and decades. Cambridge University Press, Cambridge