D.J. Hunsaker

Leaf nitrogen concentration of wheat subjected to elevated [CO2] and either water or N deficits

Leaf N concentration is important because it is associated with the CO2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. Consequently, the productivity of both domestic and native animals in future global environments may be closely linked to possible changes in leaf N concentration of grasses. Since grasslands are frequently subjected to water-deficit or N-deficit conditions, it is important to investigate the interactive responses between elevated [CO2] and these stress conditions. Therefore, this 4-year research program was undertaken with wheat (Triticum aestivum L.) as a model system for forage grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions near Phoenix, AZ, USA, were subjected to elevated [CO2] and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, the elevated [CO2] treatment under optimum conditions resulted in little change in leaf N concentration. Therefore, no change in the nutritive value of forage from highly managed pastures would be expected. Further, water-deficit treatment had little influence on leaf N concentration. To some extent, the lack of response to the water-deficit treatment resulted because severe deficits did not develop until late in the growing seasons. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated [CO2], however, had a large influence on leaf N concentration. Low levels of applied N resulted in decreased leaf N concentration under both [CO2] treatments, but the lowest levels of leaf N concentration were obtained under elevated [CO2] through much of the growing season. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes.

CO2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency

Evapotranspiration (ET) and water use efficiency (WUE) were evaluated for two spring wheat crops, grown in a well-watered, subsurface drip-irrigated field under ambient (about 370 mmol mol 1 during daytime) and enriched (200mmol mol 1 above ambient) CO2 concentrations during 1995‐1996 and 1996‐1997 in Free-Air CO2 Enrichment (FACE) experiments in central Arizona. The enriched (FACE) and ambient (Control) CO2 treatments were replicated in four, circular plots, each 25 m in diameter. Two soil nitrogen (N) treatments, ample (High N) and limited (Low N), were imposed on one-half of each circular plot. Wheat ET, determined using soil water balance procedures, was significantly greater in High N than Low N treatments starting in late-March (anthesis) during both years. Differences in ET between CO2 treatments during the seasons were generally small and not statistically significant, however, there was a tendency for the ET to be lower for FACE than Control under the High N treatment. The reduction in the cumulative seasonal ET due to FACE averaged 3.7 and 4.0% under High N and 0.7 and 1.2% under Low N in the first and second years, respectively. However, WUE (grain yield per unit seasonal ET) was significantly increased for the FACE treatment under both soil N treatments. For the High N treatment, the WUE was 19 and 23% greater for FACE than Control and for the Low N treatment the WUE was 12 and 7% greater for FACE than Control in the 2 years, respectively. Published by Elsevier Science B.V.

Interrelations of yield, evapotranspiration, and water use efficiency from marginal analysis of water production functions

The study uses the concepts of marginal water use efficiency (MWLTE), and elasticity of water production (EWP) to reveal the dynamic interrelations of crop yield (Y), seasonal evapotranspiration (ET), and water use efficiency (WUE) based on the functional relation of an ET production function (ETPF). When the ETPF is linear, the changing trend of WUE with ET is directly affected by the intercept of the function, and the EWP will be numerically equivalent to a yield response factor (K-y) when ET reaches maximum ET (ETm). When the ETPF is quadratic, the ET needed to maximise WUE is less than the ET for maximum yield (Y-m), and the ET value that occurs at maximum WUE equals the arithmetic square root of the ratio of the intercept of the function to the coefficient of function quadratic term. The interrelationships of Y ET, and WUE are demonstrated using a quadratic ETPF developed for maize from data obtained in a field experiment

Cotton response to high frequency surface irrigation

High frequency irrigation with surface irrigation methods has been proposed as a means to increase cotton productivity in cases where drip irrigation or other pressurized systems are not economically justifiable. Field studies were conducted in 1993 and 1994 to evaluate the effects of surface irrigation frequency on the growth, lint yield and water use for a semi-determinate cotton cultivar in central Arizona. Cotton was grown in level basins on a sandy loam under three irrigation treatments defined as low frequency irrigation for the whole season (L), high frequency irrigation for the whole season (H), and low frequency irrigation until the initiation of rapid fruiting, high frequency during rapid fruiting, and low frequency after rapid fruiting (LHL). The treatments were governed by the percentage of allowable soil water depletion within the effective root zone, and the allowable depletion targets for low and high frequency irrigation were 55 and 30%, respectively. An irrigation scheduling program calculated the soil water depletion within the estimated cotton root depth on a daily basis for each treatment and was used to project the dates and amounts for treatment irrigations. In 1993, seven, 14, and 11 irrigations and in 1994 eight, 13 and 10 irrigations were given to the L, H, and LHL treatments, respectively. The total amount of water applied including rainfall differed among the treatments by 4% in 1993 and by 1% in 1994. Soil water measurements indicated that actual soil water depletion within the estimated cotton root depth immediately before treatment irrigations was close to the intended treatment allowable depletion targets for the majority of the growing season. Cotton growth and lint yields were maximized under the H treatment, and yields in this treatment averaged 15 and 21% more lint than the L treatment for the first and second seasons, respectively. The LHL treatment, although not as effective in increasing crop productivity as the H treatment, out yielded the low frequency treatment by an average of 10% in the two seasons. Crop evapotranspiration determined from the soil water balance was 8 and 9% higher for the H than the L treatment and 3 and 5% higher for the LHL than the L treatment in 1993 and 1994, respectively.

Nitrogen Management Affects Nitrous Oxide Emissions under Varying Cotton Irrigation Systems in the Desert Southwest, USA

Irrigation of food and fiber crops worldwide continues to increase. Nitrogen (N) from fertilizers is a major source of the potent greenhouse gas nitrous oxide (NO) in irrigated cropping systems. Nitrous oxide emissions data are scarce for crops in the arid western United States. The objective of these studies was to assess the effect of N fertilizer management on NO emissions from furrow-irrigated, overhead sprinkler-irrigated, and subsurface drip-irrigated cotton ( L.) in Maricopa, AZ, on Trix and Casa Grande sandy clay loam soils. Soil test- and canopy-reflectance-based N fertilizer management were compared. In the furrow- and overhead sprinkler-irrigated fields, we also tested the enhanced efficiency N fertilizer additive Agrotain Plus as a NO mitigation tool. Nitrogen fertilizer rates as liquid urea ammonium nitrate ranged from 0 to 233 kg N ha. Two applications of N fertilizer were made with furrow irrigation, three applications under overhead sprinkler irrigation, and 24 fertigations with subsurface drip irrigation. Emissions were measured weekly from May through August with 1-L vented chambers. NO emissions were not agronomically significant, but increased as much as 16-fold following N fertilizer addition compared to zero-N controls. Emission factors ranged from 0.10 to 0.54% of added N fertilizer emitted as NO-N with furrow irrigation, 0.15 to 1.1% with overhead sprinkler irrigation, and <0.1% with subsurface drip irrigation. The reduction of NO emissions due to addition of Agrotain Plus to urea ammonium nitrate was inconsistent. This study provides unique data on NO emissions in arid-land irrigated cotton and illustrates the advantage of subsurface drip irrigation as a low NO source system.