Surface and Root Inputs Produce Different Carbon/Phosphorus Ratios in Soil

Formation of soil organic C (SOC) is influenced by inputs. We applied organic amendments for five consecutive years at 250 g C m(-2) yr(-1). Seven years after the applications ended, the effects of biosolids and manure on SOC were greater than alfalfa (Medicago sativa L.), wood, wheat (Triticum aestivum L.) residue, sucrose, brassica residue, cotton (Gossypium hirsutum L.), wheat compost, or the unamended check. Soil C increases ranged from 3 to 49% of C applied and had changed little in the previous 28 mo. Plots sown to continuous winter wheat during the period of amendment had 10% more SOC than plots that had been fallow, an effect more than twice as large as most amendments. This suggests that the contribution of roots to SOC was more important than aboveground crop residues. The effect of amendments on SOC was highly correlated to their initial P content. In addition, for a similar available soil P and soil S content, SOC was significantly greater where wheat or perennial grass was grown. Wood caused an increase in the C/N ratio that persisted >3.5 yr but almost disappeared by the seventh year. This study indicated that continuous cropping or the addition of animal waste or municipal biosolids had the greatest impact on the formation of SOC, and this positive effect remained stable for many years after the end of the continuous treatment applications.