There’s a lot that’s been proven good about whole orchard recycling, putting those almond trees back into the soil where they stood. And there’s plenty to indicate that drawbacks from the practice can be addressed.
Presenters addressed the pros and cons during a recent workshop on fertility management at California State University-Fresno. Most made the case that the crux of the issue is addressing the ratio between carbon from wood left behind and nitrogen needs, and the importance of balancing those two elements.
Amisha Poret-Peterson, a U.S Department of Agriculture research microbiologist at the University of California-Davis, talked about the issues during the field day.
Orchard trees store carbon in woody biomass over decades, and that carbon leads to a reduction in carbon dioxide greenhouse gas emissions. Recycling is an alternative to burning and sending wood to cogeneration plants that are facing closures.
Recycling presents an opportunity for long-term storage of carbon in the soil and overall improvement of soil physical and biological properties.
Poret-Peterson pointed to established improved soil health benefits of whole orchard recycling that include increased soil organic matter, increased soil moisture and water retention, decreased soil compaction and higher microbial diversity.
She said those factors may lead to greater resilience to irrigation and water shortages and healthier trees.
Higher soil carbon
But whole orchard recycling leads to higher soil carbon to nitrogen ratios, and that reduces nitrogen availability and may negatively impact early tree growth and nutrition. Increased nitrogen fertilization can lower the carbon, nitrogen ratio.
But applying nitrogen fertilizer may increase emission of greenhouse gases such as nitrous oxide and other gases. It may also result in more leaching of nitrates into the groundwater.
Researchers continue to look at the short- and long-term impacts of whole orchard recycling on soil physical and chemical attributes and the effects on nitrous oxide, carbon dioxide emissions, nitrogen mineralization and denitrification.
Research sites include the Kearney Agricultural Research Center in Parlier, a site at Riverbend and Lincoln Avenue in Parlier, Fresno State University, Wonderful Orchards in in Kern County, Manteca and Chowchilla.
At the Riverbend-Lincoln site, buried bags with resin are used to study the capture of nitrate over the course of a season. Researchers also look at movement of nitrogen below the root zone.
Mae Culumber, UC farm advisor for Fresno County, talked of soil amendments that can be added to improve physical and biological properties that influence water retention, permeability, water infiltration, drainage, aeration and structure.
“The goal is to provide a better environment for tree roots,” she said.
Culumber said carbon stands at a whopping 50 percent when wood chips are applied at 45 tons per acre, while nitrogen is at just .31 percent at 45 tons per acre. That means a 160 to 1 carbon-nitrogen ration.
“Amending with eight tons per are of dairy manure will lower the carbon-nitrogen ratio to 136 to one.” Culumber said.
Applying one hundred pounds of urea ammonium nitrate will reduce the ratio to 58 to one.
Culumber said preliminary trials suggest early spring applications of between 39 and 48 pounds of nitrogen at a 15-15-15 rate improved nutrition early on.
Water and nutrient managemnt
Suduan Gao, a research soil scientist with the USDA San Joaquin Valley Agricultural Sciences Center in Parlier, talked of the potential of applying a biochar amendment to help in water and nutrient management.
Biochar is charcoal used as a soil amendment. It is made from biomass through thermal decomposition. Gao said it increases water use efficiency and nitrogen retention, reduces leaching, cuts nitrous oxide emissions and reduces ammonia emissions.
Gao said nitrogen dynamics are influenced by biochar and are highly affected by irrigation levels.
She said ammonia volatilization loss was substantially higher when fertilizer was applied only a few times in a larger amount than when it was applied more frequently in smaller amounts.
“Soil accumulates more nitrogen at lower irrigation levels than at higher irrigation levels,” Gao said. “The accumulated nitrogen, however, can be all leached during the rain system.”
She said there were no significant biochar effects on ammonia, nitrous oxide and soil nitrate concentration, but there was a significant interaction between biochar and irrigation.