Effects of recent and accumulated livestock manure carbon additions on soil fertility and quality

Significant improvements in soil quality have been measured after repeated livestock manure additions at four experimental sites in England. The aim of this study was to determine whether these improvements were caused by the recent addition of fresh organic carbon (OC), or the long-term build-up of soil OC resulting from repeated additions. Livestock manures were withheld ('historic' treatment) from one half of each treatment plot and continued on the other half ('recent' treatment), and a range of soil properties measured 2 years later. Topsoil OC stocks and total nitrogen both increased by approximately 7%, with every 10 t ha-1 manure OC applied, with no difference between the recent or historic additions. Similarly, topsoil available water capacity and porosity increased by approximately 2 and 1% with every 10 t ha-1 OC applied, respectively. Bulk density and penetration resistance decreased, by approximately 1 and 3% with every 10 t ha-1 OC applied, respectively, with again no difference between the recent and historic treatments. Even microbial biomass N and potentially mineralizable N showed no differences between the recent and historic additions, increasing by 8 and 30% with every 10 t ha-1 OC applied, respectively. Only three of the wide range of soil properties measured showed a different response for the recent and historic additions. These were fungal biomass, which increased only with recent additions, extractable potassium, which was proportionally greater in the recent additions, and light-fraction OC, which increased at a greater rate with the recent additions. The results clearly demonstrate that improvements in soil quality and functioning after repeated additions of livestock manures can persist for more than 2 years after the cessation of applications. Overall, it was the total accumulated manure C inputs that had the greatest influence on soil properties, regardless of whether the source of C was from recent or historic (over 2 years old) applications.