Posted Sept. 19, 2014
New research suggests that conventional corn-soybean systems, managed with synthetic fertilizer alone, do not produce enough plant material to sustain soil organic matter levels, reducing crop yield in the long term.
Iowa State University agronomist Michael Castellano, who led research on a recently completed project funded by the Leopold Center for Sustainable Agriculture, studied soil properties at four field sites in Iowa, each with continuous corn and corn-soybean rotations. Plots at each site have been maintained for 13 years under specific fertilizer regimes—ranging from 0 to 240 lbs. of synthetic nitrogen per acre, per year—including some maintained at the agronomic optimum of about 150 lbs. per acre.
Results of the two-year study show that when managed at optimum levels, continuous corn systems generally produce enough residue — about 3 tons per acre, per year — to maintain or increase soil organic matter. Soybeans, on the other hand, create half that much residue, so corn-soybean systems overall do not produce enough bulk matter to replenish soil organic matter stocks in the long term, even at optimum fertilizer levels. Moreover, increasing synthetic fertilizer inputs beyond optimum levels does not appear to impact soil organic matter because it does not increase residue inputs and is rapidly lost to the environment.
“The corn-soybean system in Iowa is losing organic matter when managed with synthetic fertilizer only, even in the absence of erosion,” says Castellano. “Our data highlights the importance of getting as much organic matter into that system as possible.”
Funded in 2011 by the Leopold Center’s Ecology Initiative, the project was conducted on sites with little or no slopes to control against erosion, otherwise the biggest factor in soil organic matter loss. The carbon and nitrogen content of soil samples collected in 1999 were compared against samples from 2009.
More detailed analysis also was done on the continuous corn systems to understand what percentage of the soil organic matter was chemically available for plants to take up, and what percentage formed stable aggregates resistant to breakdown by soil microbes. The soil aggregates, because they resist decomposition for many years — sometimes centuries — are critical for long-term carbon and nitrogen sequestration.
On corn-soybean operations, Castellano says the best way to increase soil organic matter, after controlling for erosion, is to return manures to the soil, since they contain carbon in addition to nitrogen.
Cover crops also may increase soil organic matter levels. At the Marsden Farm, where researcher Matt Liebman has run a diversified cropping systems project since 2002, Castellano collected soil data from a two-year corn-soybean rotation versus a three-year corn-soybean-oat + red clover cover crop system. Preliminary analysis confirms previous research suggesting that the three-year rotation has more nitrogen available for plant uptake, resulting in greater yields than those observed for two-year rotations.
Castellano hypothesizes that high-quality manure or crop residue benefits soil quality. He’d also like to test the hypothesis that soils with low organic matter levels have lower fertilizer and nitrogen use efficiency than well-managed soils with high organic matter.
Answering these new questions dovetails with Iowa Nutrient Reduction Strategy goals to reduce nutrient loading of waterways. While over-fertilization results in nutrient pollution to air and water, Castellano’s work shows that under-fertilization also is detrimental, since it reduces the amount of soil organic matter that acts as a reservoir for nutrients put onto the field. Meanwhile, the “optimum” level of synthetic nitrogen inputs is always variable, depending on factors including field site conditions, management goals and weather conditions.
“There are many sites that don’t respond to nitrogen fertilizer in one given year, and we don’t even know how to predict those,” says Castellano. “There’s always going to be uncertainty surrounding whether [farmers] are managing their organic matter optimally.”
Given all this variability, he says, manure inputs or cover crops provide a level of insurance against nutrient losses. By improving organic matter, they serve as a buffer against year-to-year uncertainties.
“Cover crops are particularly promising,” says Castellano, “because the Iowa Nutrient Reduction Strategy has identified those as a big tool to reduce nitrate loads. So if they also can increase or neutralize the loss of organic matter in a corn-soybean system, there are two factors that really can contribute to long-term sustainability of Iowa’s farmlands.”