Understanding Zn uptake dynamics is critical to rice grain Zn biofortification. Here we examined soil Zn availability and Zn uptake pathways as affected by genotype (high-grain Zn varieties IR69428 and IR68144), Zn fertilization and water management in two pot experiments. Results showed significant interactions (P < 0.05) between genotypes and Zn fertilization on DTPA (diethylenetriaminepentaacetic acid)-extractable soil Zn from early tillering to flowering. DTPA-extractable Zn in soils grown with IR69428 was positively correlated with stem (r = 0.78, P < 0.01), flagleaf (r = 0.60, P < 0.01) and grain (r = 0.67, P < 0.01) Zn concentrations, suggesting improved soil Zn availability and continued soil Zn uptake by IR69428 even at maturity. Conversely for IR68144, DTPA-extractable Zn was positively correlated only with leaf Zn uptake (r = 0.60, P < 0.01) at active tillering, indicating dependence on remobilization for grain Zn loading. Furthermore, the highest grain Zn concentration (P < 0.05) was produced by a combination of IR69428 and Zn fertilization applied at panicle initiation (38.5 μg g−1) compared with other treatments (P < 0.05). The results highlight that Zn uptake behavior of a rice genotype determines the fate of Zn from the soil to the grain. This has implications on overcoming Zn translocation barriers between vegetative parts and grains, and achieving grain Zn biofortification targets (30.0 μg g−1).

Rice straw incorporation is labor-intensive and influences greenhouse gas emissions but can increase soil organic carbon (C) and recycle nutrients. Rice straw contains about 80, 40, and 30% of the potassium (K), nitrogen (N), and phosphorus (P), respectively, taken up by rice and thus its incorporation can reduce the fertilizer requirement of the subsequent crop. However, because of rice straw's low quality, its decomposition is slow. So, the timing of this operation, in combination with water management, becomes important. Composting rice straw with the addition of farmyard manure can improve quality and nutrient supply. Similarly, biochar from thermal combustion of rice straw for energy production can be added to the soil to improve soil organic C. This chapter highlights the benefits derived from incorporating straw into the soil. Alternative forms of straw that can be used by farmers, depending on local situations, are discussed.

SUMMARYAs zinc (Zn) fertilizer and water management affect the expression of Zn-enriched grain traits in rice, we studied the effect of Zn fertilizer and water management on Zn uptake and grain yield of different biofortification breeding lines and the possible biases in selection for high grain Zn content. The first field experiment showed that longer duration genotypes had higher grain Zn uptake rate than shorter duration genotypes during grain filling. In the first greenhouse experiment, neither application of Zn fertilizer at mid-tillering nor application at flowering significantly increased the grain Zn concentration. In the second greenhouse experiment, application of alternate wetting and drying (AWD) significantly increased the available soil Zn and plant Zn uptake but not grain Zn concentration. Terminal drying (TD) did not increase the available soil Zn or grain Zn contents. The second field experiment confirmed that differences in TD were not important in understanding differences between genotypes. Zn application is not always necessary to breeding trials unless there is a severe Zn deficiency and there is no need to carefully regulate TD prior to harvest.

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