Contrasting Phosphorus Economies Under Annual Versus Perennial Crops After >150 Years of Harvests.

This research was motivated by growing interest in the further development and deployment of high-yielding herbaceous perennial crops—whether for biofuels in the short term or grain production further into the future.  In this work I investigated the degree to which soil P fractions and microbial biomass-P under perennial hay crops differed from those under annual wheat after more than 150 years of cropping.  Soils were sampled from the Broadbalk continuous wheat (annual crop) and Parkgrass hay meadow (perennial crop) experiments at Rothamsted Research in Harpenden, UK.   The Broadbalk and Park Grass experiments were initiated in 1843 and 1856, respectively, and the soil organic matter in the experiments appears to be very close to steady state.  At the two sites soils were sampled at 0-23, 24-46, 47-69, 70-92cm depths from plots that have never received nutrient amendments (nil plots) and plots that received 96 kg N ha^-1 y^-1, 35 kg P ha^-1 y^-1 and other fertilizer inputs.  The 2009 soils samples were matched with archived soil samples taken from the same nil and fertilized plots in 1893 from Broadbalk and 1876 from Park Grass.  I used the P fractionation procedure described by Tiessen and Moir (2007) with some modifications detailed by Blake and colleagues (1997).   P fractions in the soils under annual and perennial crops varied markedly.  In particular, the fertilized and unfertilized soils under annual wheat maintained higher concentrations of P with low availability (Hot conc. HCl + final digest) compared to soils under perennial hay.  In contrast, the soils under perennials consistently maintained >100% higher concentrations of organic fractions thought to be actively cycling (0.5M bicarbonate and 0.1M NaOH).  Microbial biomass in the surface horizons under fertilized and unfertilized perennial hay were an order of magnitude greater than measurements at the same depths under fertilized and unfertilized annual wheat.  These results and others suggest that herbaceous perennials may maintain a greater proportion of native or fertilizer-P in relatively available forms compared to annual wheat.