Point stresses during reproductive stage rather than warming seasonal temperature determine yield in temperate rice.

Climate change is predicted to shift temperature regimes in most agricultural areas with temperature changes expected to impact yields of most crops, including rice. These temperature-driven effects can be classified into point stresses, where a temperature event during a sensitive stage drives a reduction in yield, or seasonal warming losses, where raised temperature is thought to increase maintenance energy demands and thereby decrease available resources for yield formation. Simultaneous estimation of the magnitude of each temperature effect on yield has not been well documented due to the inherent difficulty in separating their effects. We simultaneously quantified the magnitude of each effect for a temperate rice production system using a large data set covering multiple locations with data collected from 1995 to 2015, combined with a unique probability-based modeling approach. Point stresses, primarily cold stress during the reproductive stages (booting and flowering), were found to have the largest impact on yield (over 3 Mg/ha estimated yield losses). Contrary to previous reports, yield losses caused by increased temperatures, both seasonal and during grain-filling, were found to be small (approximately 1-2% loss per °C). Occurrences of cool temperature events during reproductive stages were found to be persistent over the study period, and within season, the likelihood of a cool temperature event increased when flowering occurred later in the season. Short and medium grain types, typically recommended for cool regions, were found to be more tolerant of cool temperatures but more sensitive to heat compared to long grain cultivars. These results suggest that for temperate rice systems, the occurrence of periodic stress events may currently overshadow the impacts of general warming temperature on crop production.

Nutrients and sediments in surface runoff water from direct-seeded rice fields: implications for nutrient budgets and water quality.

Nutrient losses from rice fields can have economic and environmental consequences. Little is known about nutrient losses in surface runoff waters from direct-seeded rice systems, which are common in the United States and increasingly more so in Asia. The objectives of this research were to quantify nutrient losses from California rice fields in surface runoff waters and to determine when and under what conditions losses are greatest. Research was conducted in 10 rice fields varying in residue (burned or incorporated) and water management over a 2-yr period. Concentrations of NH-N and NO-N in runoff water across sites, seasons, and management practices averaged <0.1 mg N L. Runoff water PO-P concentration averaged 0.14 mg L and was not affected by season or straw management practices. However, P fluxes were higher in the winter when rice straw was burned (2.59 kg ha) as opposed to incorporated (0.44 kg ha). Average seasonal runoff water K concentrations did not vary with season and straw management, although they were highest at the onset of the winter season. Average total suspended solids (TSS) concentrations did not vary by season but were highest during the winter in the straw-incorporated fields (46 mg L). Rice fields were sinks for K (4.9 kg K ha) during the growing season. Fields were not significant sources of nutrients or TSS during the growing season; however, during the winter fallow they could be sources of NH-N, P, K, and TSS, especially as water fluxes from fields increased.