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Freshwater availability for irrigation decreases because of increasing demand from urban and industrial areas, degrading irrigation infrastructure, and water quality. The demanding for high production of rice with less water use is crucial for food supply. In this research, a field experiment was conducted during 2001 and 2002 to determine the effect of crop density on water productivity of rice crop. The study was carried out in a split-plot design with three plant spacings as subplots (20 cm × 20 cm, 15 cm × 15 cm, and 10 cm × 20 cm) and four different irrigation regimes (continuous submergence as the control and 100%, 75%, and 50% evaporation of pan) as main plots. To model the various water productivity components, the ORYZA2000 model was used. The comparison of model results with observed data was performed using different statistical methods. The results showed that the irrigation by 75% evaporation from pan evaporation and 20 cm × 20 cm crop size are the optimum irrigation method and crop density management.

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.

Paddy soils represent the largest anthropogenic wetlands on earth. Soil drying and rewetting that occurs annually inflict significant stress on soil microbial activities in paddy soils. An incubation experiment of 60 years of paddy soil was conducted to simulate the conditions of paddy fields (25 °C and 75% air humidity) during a 16-day incubation time. The effect of drying-rewetting [DRW, with 4 levels: (1) constant soil moisture (CSM), (2) one-stage drought stress (DRW1), (3) twostage drought stress (DRW2), and (4) three-stage drought stress (DRW3)] and how it evolves over 0, 4, 8. 12, and 16 days after incubation on the concentration of available phosphorus (AP), microbial biomass P (MBP) and microbial biomass C (MBC), and respiration rate (RES) was determined using repeated measures analysis (RMA). The results revealed that an increase in the number of drying-rewetting increases MBC and RES. Compared to CSM, frequent drying and rewetting caused an increase in RES, MBC and MBP by 88%, 38%, and 11%, respectively. Drying-rewetting increased microbial biomass C (MBC) and P (MBP) by 24-38% and 11-54%, respectively, during 8-16 days of incubation. Increasing the number of DRW cycles reduced AP concentration (except in DRW1). The decrease in available phosphorus is due to the increase in the intensity of immobilization under these conditions. Positive correlations were also observed between AP and MBP (r = 0.52), and between RES and MBC (r = 0.91). In general, the frequency of moisture in the paddy soil is favorable for increasing microbial activity.