Research Highlights

Study Finds Nitrogen Mineralization From Crop Residues May Not Be Enough

By Carol Brown

In the agricultural environment, the nitrogen cycle is a complicated system with many moving parts as well as factors that affect its movement. Soil microbes, water, temperature, nutrients, and the crops themselves all have a role in how nitrogen cycles through the soil and air.

Because of these changing components, studying how nitrogen (N) mineralization and immobilization affect crop growth and yield is challenging. North Dakota State University (NDSU) Soil Science Professor Larry Cihacek and colleague Dr. Rashad Alghamdi are focusing on nitrogen usage in crop residue from soybeans and other common crops in no-till systems of the region through a research project funded by the North Dakota Soybean Council.

“In North Dakota, there has been a lot of push toward no-till,” Cihacek explains. “But we have a cool climate and not a long growing season, so crop residue has been accumulating over time on some of our long-term no-till fields. One item of concern is whether farmers are getting N mineralization from the residues. We’re finding out there isn’t a lot. Immobilization is probably more enhanced rather than mineralization.”

Nitrogen mineralization is the process where microbes decompose nitrogen from manure, organic matter, and crop residues to ammonium, which is then converted to nitrate for plant uptake. Immobilization is the reverse of this. Ammonium and nitrate are taken up by soil organisms, thus not available for crops. Long nitrogen story short — nitrogen mineralization is beneficial for crops and immobilization is not.

“Soybeans and most legumes are scavengers of nitrogen. If it’s available, they’ll take it up. If not, they’ll make it,” Cihacek says. “Contrary to what many people believe, soybeans don’t put nitrogen back into the soil.”

The research project, which is part of Alghamdi’s post-doctoral research, looked at mineralization in crop residues from two-year rotations — corn-soybean, wheat-soybean — up to six-year cycles. They studied residues that had accumulated after fall harvest and in the following spring just before planting to observe residue quality.

“It’s difficult to measure the transfer of nitrogen into the soil from the decomposition or mineralization process,” Cihacek says. “But we’re not really getting much mineral N out of post-harvest residue, regardless of whether it was corn, wheat or soybeans. The nitrogen has been taken off the field in the grain. Because of the cooler climate, the residue is not decomposing and nitrogen is not mineralizing, which is sort of an enigma. We’re just scratching the surface in our knowledge on this.”

Cihacek and Alghamdi also looked at identifying contributions to N mineralization from different parts of the soybean plant, including leaves, stems, and roots, between senescence and harvest. They tracked total carbon (C), N, and carbon to nitrogen (C:N) ratios in each plant section.

“In one study, we found the C:N ratios were really high at harvest from the stems and roots. To compensate for all the carbon in the residue, we were going to have to add 50 to 100 pounds of N,” Cihacek comments. “Generally, NDSU fertilizer recommendations give a 40 lb. per acre nitrogen credit for soybeans.”

Because of these low nitrogen levels found in the soil as a result of this study, additional nitrogen amounts need to be considered for crops grown in combination with soybeans. Testing for soil organic matter and soil N is a good place for farmers to start. But more studies need to be conducted before making changes in nitrogen credits across the state.

The project will continue this crop year and the pair of researchers are adding some elements to the study including daikon radish — a common cover crop and a “drastic” nitrogen supplier. The radish root accumulates and stores N, then releases it quickly when the plant dies. This may or may not help succeeding crops depending on soil and weather conditions.

“The radish also has a narrow C:N ratio and we’re using that as a marker to see if we can offset some of the N immobilization and capture what’s remaining at the end of the season,” Cihacek says.

Alghamdi’s research confirmed that incorporating radish at the 15 percent rate, in a controlled environment, did not offset immobilization during the growing season, but could potentially mitigate immobilization if the cover crop incorporation rate increased.

The ongoing project will include tracking N mineralization in parallel laboratory studies as well as small field plots of radish residue alone and mixed with wheat and soybean residue. The effects of contrasting tillage, cover crop rates, and fertilizer application will be evaluated in conjunction with soil moisture temperature monitoring to assess the potential mineralization or immobilization response to daily and seasonal fluctuations.

Published: Nov 8, 2021