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NMSU helps pecan growers decrease spring air pollution, find sustainable use for pruned wood

Doņa Ana County residents have noticed there's less smoke in the air during the spring. Thanks to New Mexico State University College of Agricultural, Consumer and Environmental Sciences researchers and the New Mexico Pecan Growers' Association, alternatives to burning the annual pruned pecan tree branches have been studied and implemented.

Pecan growers now use chipping machines to shred the wood pruned each year in their orchards. Traditionally this wood was burned, but with 27,289 acres in Doņa Ana County planted in pecans, burning is impacting the air quality of the region. After the wood is shredded it is worked back into the orchards' soil. NMSU researchers have also studied potential ways to generate revenue from the pruning byproduct. (NMSU photo by Jane Moorman)

With a third of the Elephant Butte irrigation district now in pecan orchards, the environmental impact of burning the pruned branches was becoming an air quality issue. In recent years, southern Doņa Ana County in New Mexico and El Paso County in Texas have failed to meet the national ambient air quality standards for one or more of the seven pollutant criteria, including ozone and particulate levels.

"Historically, pecan growers have burned the branches. But from a good neighbor standpoint it made sense to look for alternatives before they were regulated," said John Mexal, NMSU Plant and Environmental Sciences assistant department head. "They expressed concern that the enforcement of air quality standards and the banning of agriculture burning occurring in California would possibly be replicated in the Mesilla Valley as part of a state implemented plan under the federal Clean Air Act."

Two NMSU Plant and Environmental Sciences Department research projects focused on the question: What can we do with all this wood?

"Pushing the branches into a pile and burning is the cheapest way to dispose of pruned wood," said Bill Lindemann, NMSU Plant and Environmental Sciences professor. "One alternative would be to send the branches to the landfill, but that would be costly."

Another alternative is chipping the branches and returning the shredded wood fiber to the soil in the orchard. While this method was two to three times as expensive as burning the wood, Lindemann said the growers "saw the writing on the wall and stepped up to do it."

But a concern was the nutrient impact of returning the wood fiber to the soil.

"They were not concerned about the nutrients in the wood, but those not in the wood," Lindemann said. "When you add material with a lot of carbon, but no nutrient values, the problem is that the microorganisms that degrade the wood take the nutrients from the soil and use it for energy, depleting the soil of nutrients, particularly nitrogen, that are necessary for tree growth."

Research conducted by Lindemann and graduate student Mohammed B. Tahboub from 2002 through 2004 determined that incorporating the shredded wood into the soil is beneficial to the physical and biological properties of the soil and has no adverse effect on the fertility and chemical properties of the soil.

"The wood material has very little nutrients for either the trees or microbes," Lindemann said. "At the typical rates of application and typical size of wood material, the shredded wood degrades too slowly to pose a problem. In order for the microbes to pull in the nutrients, mainly nitrogen, from the soil they have to be vigorously working on the wood fiber. The pecan wood fiber is not ideal food for them. They decompose it but they don't do it quickly."

The researchers found that adding the wood fiber did increase the organic matter content of the soil, which improves the water-holding capacity of the sandier soil types.

While this method of disposing the wood eliminates the air quality concerns, it is another expense and a lost revenue opportunity for the grower. The NMSU research team of graduate student Jeff Kallestad, now with Washington State University at the Puyallup Research and Extension Center, Mexal and Ted Sammis, NMSU Plant and Environmental Sciences professor, wanted to determine if there were any value-added opportunities for the pruned wood.

During the 2006 pruning season, samples were weighed from seven orchards on 248 acres throughout the Mesilla Valley. Kallestad determined that orchards that had undergone maintenance pruning averaged 3.5 green tons per acre, while 8.4 green tons per acre were harvested from orchards that had undergone corrective structural pruning. The annual biomass estimated by the scenario-based model ranged from 12,791 to 41,523 bone dry tons.

One potential customer for the biomass would be electrical generation plants that burn wood alone or combine wood with other fuels. Burning wood has a zero carbon footprint. While the process produces carbon dioxide it is considered equal to the amount of carbon dioxide the trees fixed during the process of photosynthesis.

With 27,289 acres in Doņa Ana County planted in pecans, according to the USDA National Agriculture Statistics Service 2007 Agriculture Census, Mexal says the area is on the verge of being able to provide enough for a biomass power plant, but it would have to be within 25 miles of the orchards because of prohibitive transportation costs.

"We are at the tipping point of having enough biomass to support a power plant. A plant could be supported in the future, because it is estimated that half of the valley will eventually be planted in pecans," Mexal said.

But at this time it is not feasible.

"Given the biomass power plants currently operating in California consume approximately 18,000 bone dry tons per megawatt of power produced, and that the smallest plant still operating has a capacity of 4.5 megawatts and consumes 35,000 bone dry tons of lumber production waste per year, we concluded that commercial power production based solely on pecan prunings from the Mesilla Valley would not be feasible currently," the researchers reported in Research Report 764, which can be found at http://aces.nmsu.edu/pubs/research/horticulture/RR764.pdf. "However, as fuel prices increase and interest in renewable energy changes, technical obstacles associated with procuring, stockpiling and processing pecan biomass may be overcome to enhance the feasibility."

However, like the pecan shells that used to be considered waste and were given away but are now a cash byproduct, it is probable that in the future a use for the biomass will help pecan growers offset their pruning costs.