Crop diversification supports nitrogen sequestration (but not carbon)

Diversifying rotations does not promote carbon storage but increases nitrogen availability with positive impact on soil and climate, a Iowa University study has found

by Matteo Cavallito

 

The use of longer, more diversified agricultural rotations and the use of manure as a natural fertilizer result in numerous environmental benefits but do not promote carbon sequestration. This is claimed by researchers at Iowa State University who, in a study published in Nature Sustainability, challenge some established beliefs with potential repercussions for some climate mitigation initiatives related to carbon markets.

The research is based on data collected in an experiment still underway at a field managed by the same university in which, since 2001, a traditional two-year corn and soybean rotation has been compared with other three- and four-year alternating systems that include alfalfa, clover and oats.

Carbon does not increase

“Diversified cropping systems offer a chance to mitigate environmental impacts of conventional agriculture, but effects on soil organic carbon (SOC) sequestration and nitrogen (N) dynamics remain debated,” the study explains. The authors, on the occasion, initiated the investigation from the field experiment by making laboratory measurements with three stable isotope-based mechanical models to examine organic carbon stocks and decomposition. In doing so, they compared three distinct systems: one conventional, with rotations of corn and soybeans, and two more diverse ones that also include small grains and legumes and are subject to manure inputs.

“Contrary to the prevalent hypothesis that diversified systems increase soil organic carbon, we found no differences,” they explain.

At the same time, “Diversified systems markedly increased N mineralization rates and decomposition of older SOC from previous corn inputs” the research continues. “Models revealed that increased C decomposition with residence times of months to years counteracted higher C inputs but increased N supply.” An that’s an important aspect.

The role of microbes

Taking a decisive role, once again, are microbes. The increased variety of crops and the addition of manure, in fact, increase the carbon input. The introduction of more organic matter into the soil, however, stimulates precisely microbial activity. This accelerates decomposition and causes an increase in CO2 emissions by offsetting the increase in sequestration of the element.

An important aspect, the authors recall, is the new survey method employed. This procedure, they say, would help better predict the variation in the element’s presence in the soil. “Isotopes improve our understanding of how long carbon can remain in soil,” explained Steven Hall, one of the study’s authors, in a note. “In a sense, we can ask the soil microbes what they had for dinner.”

But rotation remains a viable strategy

Diversified rotations, in any case, can still have a positive impact on climate. In soils characterized by more diversified rotations, in fact, organic nitrogen is converted to its inorganic form at a rate up to 70 percent higher in comparison with other soils.

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The findings, in short, and highlight the existence of a critical balance between carbon accumulation and nitrogen supply, “demonstrating that key climate benefits may arise from decreased N fertilizer use, not SOC sequestration.”

Increasing nitrogen availability in diversified cropping systems, in other words, allows manure to be used as a partial replacement for synthetic fertilizer. This reduces emissions of nitrogen compounds (which are known to be present in most chemicals) to a significant extent. “Actually, carbon levels in the soil didn’t change over 20 years,” explained co-author Wenjuan Huang, professor of ecology, evolution and organismal biology. However, “these regenerative management practices are still valuable in other ways.”