LAUSR

Little is known about the effects of drying–rewetting cycles on phosphorus (P) speciation in paddy soils with organic amendments incorporated. In this study, the responses of microbial biomass P (MBP), alkaline phosphatase activity, inorganic pyrophosphatase activity, soluble P, available P (AP), organic P, total P (TP), organic carbon (C), and C:P ratio to two types of the organic amendment (Azolla compost and rice straw) under drying–rewetting cycles were evaluated in a 60-day incubation experiment. The experimental treatments follow: amendment factor at three levels of (i) unamended control (NT), (ii) soil amended with Azolla compost (AC), and (iii) soil amended with rice straw (RS); and irrigation regimes factor at three levels of (i) constant soil moisture in a waterlogged state, (ii) mild drying–rewetting (MDR) (moisture reduced to 100% water-holding capacity (WHC) at alternate periods), and (iii) severe drying–rewetting (SDR) (moisture reduced to 70% WHC at alternate periods). Soil drying–rewetting increased the contents of AP and soluble P and the activities of alkaline phosphatase and inorganic pyrophosphatase, but decreased contents of organic P and organic C. The contents of AP and TP in organic treated soils were significantly higher than those in non-treated soils (P < 0.01). Application of AC and RS amendments increased organic P content (45% and 46%, respectively) and also further intensified the increase in AP content (63% and 37%, respectively), soluble P content (3.7- and 2.7-fold, respectively), and MBP content (6.0- and 1.6-fold, respectively) compared to control. Rice straw addition combined with MDR also increased the C:P ratio 30 days after incubation. The MBP as a microbial factor had the highest correlation with other measured indices. The MDR along with organic matter (RS and AC) caused a continuous increase in the concentration of P species. In short, the best soil quality in terms of P supply in paddy soils was achieved by regulating soil drying–rewetting cycles along with the use of appropriate timing of organic amendments.