Impact of drying–rewetting cycles and organic amendments on phosphorus speciation of paddy soil

Abstract

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.

Alternate Wetting and Drying Irrigation Reduces P Availability in Paddy Soil Irrespective of Straw Incorporation

“…These results confirmed our first hypothesis that AWD reduced paddy soil available P content compared with CF regardless of straw addition or not. However, Bagheri et al found that AWD increased paddy soil's available P content, contrary to our results [4]. This may be due to the different soil types used in the experiment.…”

“…Soil P fraction is affected by three main mechanisms: precipitation and dissolution, adsorption and desorption, and biotransformation [2]. Unlike dryland soils, the change in paddy water environment during rice production can easily affect soil P availability [3,4]. Therefore, water management in paddy plays a vital role in water use and soil nutrient availability.…”

Alternate wetting and drying water management can reduce phosphorus availability under lowland rice cultivation irrespective of nitrogen level

“…After air drying under both water management systems, the available phosphorus (P) content in fresh soil increased, which aligns with the ndings of a previous study (Bagheri et al, 2021). However, the increase in available P varied between the two water management regimes at different sampling times.…”

“…It is worth noting that the effects of water management on phosphorus availability can vary depending on soil type. Studies conducted on alkaline soil have shown different results, where AWD increased the available phosphorus content (Bagheri et al, 2021). This discrepancy is likely due to the different soil conditions and pH levels.…”

“…These mechanisms are affected by the state and condition of the soil, including water ux and oxidation-reduction state. The condition of the soil rhizosphere remains stable in dryland situations, whereas in the water environment of paddy elds, the changing condition of the rhizosphere signi cantly impacts P availability (Turner and Gilliam, 1976; Bagheri et al, 2021). Therefore, water management in paddy elds directly affects the availability of P for the crop.…”