Responses of organic and inorganic phosphorus fractions in brown earth to successive maize stover and biochar application: a 5-year field experiment in Northeast China

To compare the effects of maize stover and its biochar on soil P fractions and investigate the effects of different stover incorporation practices (direct return or pyrolysis) on the transformation of soil P fractions after a long-term field experiment. Total P, Olsen-P, inorganic P fractions, and organic P fractions were analyzed via a 5-year field experiment growing maize. The treatments were as follows: CK0 (no fertilizer application or amendment), CK (application of mineral NPK fertilizer), ST (application of maize stover at 7.5 t ha−1 along with mineral NPK fertilizer), and BC (application of biochar at 2.63 t ha−1, a rate approximately equivalent to a maize stover biomass of 7.5 t ha−1; the biochar was charred according to a 35% output ratio in the factory). All treatments received N mineral fertilizer at 120 kg N ha−1, P mineral fertilizer at 26 kg P ha−1, and K mineral fertilizer at 50 kg K ha−1 annually. Relative to the initial fractions in 2013, biochar generally increased the inorganic P fractions (Ca8-P, Ca10-P, Al-P, and Fe-P), and maize stover generally increased the organic P fractions (MLOP, MROP, and HROP), while both maize stover and its biochar significantly decreased O-P in the soil. After 5 years (in 2017), relative to CK treatment, amendment with maize stover and its biochar increased total P and decreased Olsen-P. The total inorganic P, Ca8-P, and Fe-P were significantly increased by biochar. Ca2-P and O-P were significantly decreased by maize stover and its biochar. Total organic P was increased with maize stover and biochar amendment. LOP and HROP were significantly increased under biochar application, and all organic P fractions were significantly increased under maize stover amendment. The application of maize stover and its biochar affected both organic and inorganic phosphorus fractions; biochar had a more advantageous effect on the soil inorganic P fractions, while stover had a greater effect on the content of organic P fractions.