Changes of Soil Phosphorus Fractions in Parent Material of a Mollisol at the Early Pedogenic Stage in Northeast China

Fertility degradation of Mollisols affects world food security. Understanding the degradation mechanisms of Mollisols is essential in order to restore the soils' productivity. A long-term field experiment was conducted to assess soil phosphorus (P) concentrations and fractions in a Mollisol at its early development stage under different cropping systems in Northeast China Parent material (PM) and mature Mollisol (OM) were used for comparison. Organic P (Po) concentrations under natural fallow (Pnat), alfalfa (Medicago sativa L.) (PA and soybean-maize (S-M) rotation without fertilizer application (PCS-F) increased by 68-74% after 10 years. S-M rotation with mineral fertilizer (PCS + F), organic matter amendment (PCS + F + OM), or crop straw return (PCS + F + BM) increased Po concentrations by 74.1-108 mg kg(-1), which is equivalent to 33% of total Po in the OM; and increased Ca-2-P, Ca-8-P, Al-P and Fe-P concentrations by 0.9-1.3, 7.1-8.8, 2.1-2.7 and 1.6-1.7 times, respectively. 67% of the applied fertilizer P was transformed to Pi for the treatments with chemical fertilizer. The results indicate that return of crop residue combined with chemical fertilizers enhances the accumulation of P in soils at their early development stage, which has important implications for the fertility restoration of a Mollisol in Northeast China and elsewhere in the world