Abstract

Incorporation of rice straw into the soil just before flooding for water-seeded rice can immobilize mineral nitrogen (N) and lead to the production of acetic acid harmful to the rice seedlings, which negatively affects grain yield. This study aimed to evaluate the formation of organic acids and variation in pH and to quantify the mineral N concentration in the soil as a function of different times of incorporation of rice straw or of ashes from burning the straw before flooding. The experiment was carried out in a greenhouse using an Inceptisol (Typic Haplaquept) soil. The treatments were as follows: control (no straw or ash); incorporation of ashes from previous straw burning; rice straw incorporated to drained soil 60 days before flooding; straw incorporated 30 days before flooding; straw incorporated 15 days before flooding and straw incorporated on the day of flooding. Experimental units were plastic buckets with 6.0 kg of soil. The buckets remained flooded throughout the trial period without rice plants. Soil samples were collected every seven days, beginning one day before flooding until the 13th week of flooding for determination of mineral N- ammonium (NH4+) and nitrate (NO3-). Soil solution pH and concentration of organic acids (acetic, propionic and butyric) were determined. All NO3- there was before flooding was lost in approximately two weeks of flooding, in all treatments. There was sigmoidal behavior for NH4+ formation in all treatments, i.e., ammonium ion concentration began to rise shortly after soil flooding, slightly decreased and then went up again. On the 91st day of flooding, the NH4+ concentrations in soil was 56 mg kg-1 in the control treatment, 72 mg kg-1 for the 60-day treatment, 73 mg kg-1 for the 30-day treatment and 53 mg kg-1 for the ash incorporation treatment. These ammonium concentrations correspond to 84, 108, 110 and 80 kg ha-1 of N-NH4+, respectively. When the straw was incorporated on the day of flooding or 15 days before, the concentration of N-NH4+ in the soil was 28 and 54 mg kg-1, equivalent to an accumulation of 42 and 81 kg ha-1 of N-NH4+, respectively. There was formation of acetic acid in which toxic concentrations were reached (7.2 mmol L-1) on the 15th day of flooding only for the treatment with straw incorporated on the day of flooding. The pH of the soil solution of all the treatments increased after flooding and this increase was faster in the treatments with incorporation of straw, followed by the ash treatment and then the control. After 60 days of flooding, however, the pH values were around 6.5 for all treatments, except for the control, which reached a pH of 6.3. Rice straw should be incorporated into the soil at least 30 days before flooding; otherwise, it may immobilize part of the mineral N and produce acetic acid in concentrations toxic to rice seedlings.

Highlights

  • MATERIALS AND METHODSRice straw is the designation for the above ground parts of rice plants that remain in the field after harvest

  • Incorporation of rice straw just before flooding for water-seeded rice can lead to the production of organic acids that are harmful to seedling development (Sousa & Bortolon, 2002; Schmidt et al, 2007) and immobilize soil mineral N (Broadbent & Nakashima, 1970; Serpa et al, 2009), which can affect negatively grain yield (Camargo et al, 1995)

  • This study aimed to evaluate the formation of organic acids and variations in pH and to quantify mineral N in the soil as a function of different times of rice straw or ash incorporation before flooding

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Summary

Introduction

MATERIALS AND METHODSRice straw is the designation for the above ground parts of rice plants that remain in the field after harvest. Incorporation of rice straw just before flooding for water-seeded rice can lead to the production of organic acids that are harmful to seedling development (Sousa & Bortolon, 2002; Schmidt et al, 2007) and immobilize soil mineral N (Broadbent & Nakashima, 1970; Serpa et al, 2009), which can affect negatively grain yield (Camargo et al, 1995). Mutters (2009) observed that the transition from straw burning to incorporation coincided with a decrease in rice yield in that state. In Santa Catarina, many rice farmers do not incorporate the straw, due to operational difficulties and due to field observations related to better establishment and early development of seedlings when the straw is burned. In addition to N and C, rice straw contains other elements in significant quantities, such as P, K and S, which may be lost at least to a certain extent during the combustion process

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