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Th e loss of major nutrients can be high in rice (Oryza sativa L.) fi elds, particularly rainfed rice, where water fl owing from fi eld to fi eld during periods of high rainfall not only reduces the nutrient use effi ciencies but also has the potential for environmental degradation. We examined the infl uence of deep point placement of N, P, and K briquettes compared to broadcast incorporation of N, P, and K on fl oodwater nutrient loads aft er fertilizer application and on the performance of wet season rice in a Vertisol. Broadcast application of N as urea resulted in an average 10 times higher amounts of ammonium N in fl oodwater compared to deep placement of urea briquette. Th e broadcast application of single superphosphate resulted in 67 times higher amounts of P in fl oodwater than plots receiving deep placed P. Th e fl oodwater NH 4 + -N and P content in the deep placement treatments were negligible-similar to fl oodwater N and P content without fertilizer application. Th e fl oodwater K amounts were also significantly lower with deep placed N-P-K briquettes. Signifi cantly higher grain and straw yields, total N, P, and K uptake, and N and P use effi ciencies were observed with deep placement of N-P-K compared to broadcast application of N-P-K. Deep placed N-P briquettes gave signifi cantly higher grain yield, straw biomass, total P and K uptake, apparent P recovery, and agronomic N and P use effi ciencies when plant spacing was reduced from 20 by 20 cm to 20 by 10 cm. Closer plant spacing led to better utilization of P and K and provided opportunities for deep placement of N-P or N-P-K briquettes in soils with low available P. Combining site specifi c characteristics (high soil pH, low percolation rate, high rainfall and surface runoff s) with plant spacing and N-P-K briquettes prepared based on site-specifi c nutrient requirements off ers potential for higher yields, improved fertilizer use effi ciency, balanced fertilization, and reduced nutrient losses.

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Summary Effects of CO2 enrichment on leaf transpiration are well‐documented, but our understanding of how CO2 interacts with other variables to regulate evapotranspiration from plant communities is more limited. A series of weighing lysimeters in which tallgrass prairie species had been planted were exposed to a subambient to elevated gradient in CO2 in a field chamber. Lysimeters with intact monoliths of three soil types were represented along the CO2 gradient. We used regression analysis to determine how CO2 effects on evapotranspiration per unit of soil surface area (ETsoil) and per unit of leaf area (ETla) depended on variation in leaf area index (LAI) and diurnal changes in environmental variables during the initial 6 weeks of CO2 treatment. CO2 enrichment reduced ETsoil and ETla, and together with air temperature and LAI accounted for most of the variance in daily values of evapotranspiration explained by multiple regression models. The CO2 effect on ETsoil did not depend on values of other variables, but CO2 enrichment reduced ETla most at relatively low air temperatures and low LAI for all soils combined. Higher temperatures countered the CO2 effect by increasing ETla more at elevated than subambient CO2. Higher LAI countered the CO2 effect by decreasing ETla more at subambient than elevated concentrations. Plant (LAI) and environmental effects on ETla differed among soils, possibly because plant growth patterns and physiology differed among soils. Our results imply that the CO2 effect on evapotranspiration per unit of leaf area will vary with seasonal change in temperature and LAI, independent of seasonal shifts in leaf age and physiological activity.

No abstract