Perennial crops reduce inorganic recalcitrant P pools and increase reliance on active organic P relative to annual crops in long-term experiments at Rothamsted, U.K

Background/Question/Methods

In agriculture there are two widely-recognized avenues of phosphorus (P) loss that make it challenging to maintain adequate levels of soil available P: the intentional export of P from agroecosystems in crop harvests, and the unintentional losses of P via surface runoff and leaching.  A third avenue of P “loss”-- the reduction in available P due to “fixation” or high energy bonding by iron and aluminum oxides -- has historically not been considered a problem of agriculture per se, but simply a soil reaction with great consequences.  We investigated whether annual agriculture amplifies the proportion of P held in recalcitrant, inorganic pools by comparing how perennial pasture crops and annual wheat crops influenced the distribution of soil P in labile and recalcitrant as well as organic (Po) and inorganic (Pi) forms. We undertook detailed P fractionations and microbial P fumigation/extraction procedures on four soil depths spanning 0-92cm from the Broadbalk continuous wheat (annual since 1843) and Parkgrass hay meadow (perennial since 1856) experiments at Rothamsted Research in Harpenden, UK.  We also analyzed archived soils that were sampled from Broadbalk in 1893 and Parkgrass in 1876.  

Results/Conclusions

We found that both P-fertilized and unfertilized soils appear to maintain an inverse relationship between pool sizes of actively cycling Po (0.5M bicarbonate and 0.1M NaOH) and recalcitrant Pi (hot conc. HCl + final digest) with Po dominant in the perennial hay meadow and recalcitrant Pi dominant in the annual wheat.  Recalcitrant Pi was shown to be plant-available over time in both unfertilized wheat and hay meadow crops. Microbial biomass-P in the surface horizons of fertilized and unfertilized perennial hay meadow was an order of magnitude greater than in annual wheat.  These results and others suggest that compared to annual wheat, herbaceous perennials maintain a lower proportion of native and fertilizer P in recalcitrant inorganic forms and a greater proportion in relatively available organic forms.