Update 2012: N Application Rates & Seedling Response

Growth, Photosynthesis & Water Relations of Pecan Seedlings in Response to N Fertilization Rates

Nitrogen (N) is important for developing healthy, productive pecan (Carya illinoinensis K. Koch) trees. However, current applications of fertilizer generally follow a standard practice, without consideration of spatial, temporal and climatic variability. Many growers believe high rates of N accelerate tree development and increase yields, but the over-application of N is not cost-effective and can be harmful to the environment.

To improve current N management practices during tree establishment, it is important to know when and how much N to apply.

Objective

• To evaluate above- and below- ground tree performance by measuring tree diameter growth rates, leaf photosynthesis, and root production following N application at 5 different rates.

Summary of the Experiment
 
This study was conducted at the Texas A&M Experimental Orchard in Burleson Co. In 2010, second-leaf, open-pollinated ‘Apache’ pecan seedlings and two-year old bare-root open-pollinated ‘Mohawk’ seedlings were included in the experiment. In additional 2010 and 2011 experiments, only two-year old bare-root open-pollinated ‘Mohawk’ seedlings planted one year prior to first N application were studied.

Nitrogen (NH4SO4) was applied to 40 trees at 5 rates: 0x, 1/4x, 1/2x, 1x, and 2x on two dates: May 21 & June 21 in 2010; June 13 & 16 in 2011.
x = recommended application rate for young tree establishment. ( x = 96 kg N / ha per growing season 48 kg N / ha per application for the “Apache” seedlings and  x = 229.5 kg N/ha per growing season for the “Mohawk” seedlings). Seedlings were irrigated once a week for 24 hours using Microsprinklers.

Sampling

• Base and leader diameters were measured throughout each year.
• Photosynthesis, transpiration and stomatal conductance were measured every 3-4 weeks (between 10:00 am & 1:00 pm) using a Li-Cor 6400 portable photosynthesis system (Picture 1).
• Predawn and midday water potentials were measured in 2011.
• Soil samples were collected 5 days after N application as well as 3, 9, and 16 weeks after initial application. Soil was then processed to determine NO3-N concentration.

Results

The amount of nitrate-N in soils was significantly higher in the 1x and 2x treatments than the other treatments 5 days after the first application (Fig. 1). There was no significant effect of N application rate on trunk diameter, pre=dawn water potential or photosynthetic activity (Fig. 2A). Although, for the “Apache” cultivar WUE (net photosynthesis/ transpiration) was significantly higher for the 2x treatment at the beginning of the season, (Fig. 2B), but this effect did not persist throughout the season.

In conclusion, preliminary data suggests that adding large amounts of N fertilizer does not stimulate photosynthesis, water use efficiency or diameter growth and thus recommended application rates could be reduced; however, more data needs to be evaluated to support this conclusion.

Fig. 2. Net photosyntesis measured in the Mohawk trees during 2010 and 2011 (left panel) and Instantaneous water us efficiency (WUE) from “Apache” trees under three fertilization rates during 2010 (right panel).

For more information see the presentations and publications under Outreach