Does direct-seeded rice performance improve upon lime and phosphogypsum use?

Abstract

Changes in rice tillage systems have been taking place in the sense of seeking alternatives to reduce environmental impacts and promote improved yield. Direct-seeded rice technology reduces water consumption and methane emissions. However, in this cropping system, the natural effect of flooding on soil pH is lost because the area is maintained unflooded after seedling emergence until the crop reaches tillering stage. Consequently, it is unclear whether acidity in the topsoil and subsoil within this period affects nutrient availability and rice grain yield. Our study evaluated the effects of dolomitic lime and phosphogypsum applications on soil chemical attributes, plant nutrition, and grain yield in direct-seeded flooded rice. Two field experiments were conducted under different conditions of soil acidity. The main trial was conducted during three consecutive growing seasons from 2013 to 2016. A complementary trial was conducted in 2017 for only one rice harvest, in order to investigate the short-term effects of lime and phosphogypsum application. For both experiments, lime was incorporated into the topsoil at 0, 0.5-, 1-, and 1.5-times the recommended rate for flooded rice, according to SMP buffer method, and phosphogypsum was applied to the soil surface at 0, 2, 4, and 6 Mg ha−1. Liming alleviated topsoil acidity, causing an increase in soil pH and the exchangeable Ca2+ and Mg2+ contents, and a reduction in exchangeable Al3+ content. An increase in soil exchangeable Ca2+ content and a decrease in soil exchangeable Al+3 content also occurred at 0.20–0.40 m depth at 35 months after liming. Despite alleviating the soil acidity, lime application did not improve the rice grain yield. Phosphogypsum application increased Ca2+ and SO4-S availability throughout the soil profile (0–0.60 m), as well as the phosphorus, potassium, calcium, and sulfur levels of rice flag leaves and the grain yield. A promising short-term effect on rice yield was obtained, as well as a residual effect that lasted for at least three years after phosphogypsum addition. Overall, a phosphogypsum rate of ∼4 Mg ha−1 increased rice yield by 10–11%. Although a possible involvement in acid-subsoil amelioration on rice yield response to phosphogypsum is difficult to exclude entirely, it is more probable that yield benefits imparted by phosphogypsum resulted from increased SO4–S availability throughout the soil profile. We conclude that, although liming is of no advantage, the use of phosphogypsum improves the performance of direct-seeded rice.