Mineral-Associated Organic Carbon Promoted Phosphorus Accumulation in Organo-Mineral Complexes Under Long-Term Fertilization

Abstract

Soil phosphorus (P) pool is intimately connected to soil organic matter (SOM), especially mineral-associated organic carbon (MAOC). However, the relationship and mechanism between MAOC fractions and P fractions and their response to fertilization remain unclear. Based on a long-term (21- and 29-year) field fertilization trial in black soil, organo-mineral complexes (< 20 µm) were separated and the chemical properties, MAOC fractions, P fractions, P species and P sorption were analyzed. Compared to no-fertilizer control, chemical fertilizer resulted in soil acidification (soil pH 6.15–6.27), increasing the contents of the MAOC fraction bound to minerals by weak linkages in complexes. Straw addition maintained soil pH at 7.53–7.84, and the contents of some MAOC fractions (which were remaining water-soluble, bound by cations, encapsulated by resistant carbonate, and insoluble Humin) were significantly higher than that of chemical fertilizer alone. Fertilization increased total P contents in complexes, with chemical fertilizer mainly increasing highly labile P (Resin-Pi, NaHCO3-Pi, NaHCO3-Po, and NaOH-Pi) and straw addition mainly increasing Dil. HCl-Pi of moderately labile P. Correlation analysis showed that the increased and dominant MAOC fractions were significantly positively correlated with the increased P fractions. XANES spectroscopy further revealed that chemical fertilizer increased the proportion of AlPO4, suggesting that MAOC promotes the retention of labile P via association with Al under weakly acidic conditions. Straw addition increased the proportion of Ca3(PO4)2 and Ca5(PO4)3OH; moreover, it also increased the maximum P sorption capacity of complexes, suggesting that MAOC enhances P sorption via association with Ca under weakly alkaline conditions and that adsorbed P will transform into more stable Ca-associated P. Our findings demonstrate that MAOC promotes P accumulation via association with different P fractions, and these processes are mineral and pH-dependent. This information may be useful for managing and regulating P in agriculture.

Keywords: Phosphorus fractions, Mineral-associated organic carbon, Chemical fertilizer, Straw addition