Long-term response of soil Olsen P and organic C to the depletion or addition of chemical and organic fertilizers

Soil degradation and water pollution could have resulted from inappropriate phosphorus (P) supply. Soil Olsen P is generally a good indicator to estimate bio-availability of P and environmental risk in alkaline soil. The change in Olsen P is always strongly affected by soil organic carbon (SOC) when different forms of inorganic and/or organic P are applied to farmlands with chemical fertilizer and/or manure. We related soil Olsen P to apparent P balance (APB) and SOC in alkaline soils at three 15-year (1991–2005) fertilization sites of northern China. Six treatments were examined: unfertilized control, chemical nitrogen (N), chemical NP, chemical N plus potassium (NK), chemical NPK, and chemical NPK plus animal manure (NPKM, same total N but 20–80% more P). Compared to the initial Olsen P in 1990, after 15 years Olsen P was increased under P fertilization but decreased under no-P fertilization. At the three 15-year fertilization sites, annual mean Olsen P was 4.9–12.3 times higher under NPKM, 1.9–2.8 times higher under NP and NPK, but only 28.3–84.8% under Control, N and NK. Annual mean percentage of Olsen P to total P was higher under NPKM (4.6–8.1%) than under other five fertilizations (0.4–2.9%). Change in Olsen P significantly positively correlated with accumulated APB under all fertilizations (r2 = 0.10–0.31, P < 0.05). At these three sites, Olsen P could be increased by 5.2, 5.8 and 12.1% respectively under NP, NPK and NPKM when 1 kg P/ha was in surplus, meanwhile Olsen P was decreased by 2.1, 4.0 and 6.4% under Control, N and NK when 1 kg P/ha was in deficit. Significantly higher SOC accumulation was under NPKM than under other five fertilizations when calculated by the same unit of P input. The combination of chemical P and manure P with organic C input is a better strategy to increase soil Olsen P and SOC accumulation on farmland with maize–wheat rotation in northern China.