Effects of long-term phosphorus fertilization and straw incorporation on phosphorus fractions in subtropical paddy soil

Study on soil phosphorus (P) fraction is an important aspect in probing the mechanisms of soil P accumulation in farmland and mitigating its losing risk to the environment. We used a sequential extraction method to evaluate the impacts of long-term fertilization and straw incorporation on inorganic, organic, and residual P (P-i, P-o, and P-re) fractions in the plow layer (0-20 cm) of acidic paddy soil in southern China. The experiment comprised of six treatments: (i) no fertilizer control (CK); (ii) straw incorporation and green manure (SG); (iii) nitrogen and P fertilizer (NP); (iv) NP+SG; (v) NP+K fertilizer (NPK); and (vi) NPK+SG. The results showed that, compared to the initial total soil P content (TSP, 600 mg kg(-1) in 1990), long-term (20 years) combined continuous P fertilizer and SG significantly increased P accumulation (by 13-20%) while single fertilization (39:3 kg P ha(-1) yr(-1)) could maintain soil P status at the most. The average soil P fractions comprised of extractable P-i, P-o, and P-re by 51.7, 33.4, and 14.9% in total soil P, respectively. With comparison of no fertilizer addition (CK), long-term single fertilization significantly (P<0.05) increased the accumulation of NaHCO3-, NaOH-, and HCI- extractable P-i fractions accounting for two- to three-fold, while SG increased the accumulation of NaHCO3- and NaOH- extractable P-i and P-o accounting for 12-60%. Though the mobilization of P-re fractions was not significant (P>0.05), our data indicate that SG may partially substitute for fertilizer P input and minimizing soil P accumulation and subsequent environmental risk in the subtropical paddy soil.