Legume contributions to nitrogen nutrition in wheat
In a grain legume-wheat rotation, belowground residue consisting of roots, rhizodeposits, residual fertilizer and soil-N contributed 70 to 91% of wheat N uptake, with 66% of N uptake from the indigenous soil N pool. Wheat yield was statistically highest following lentil and pea, which were about 25% higher than wheat following wheat.
As an important rotational crop with cereals, legumes can contribute nitrogen (N) nutrition to the subsequent crop. However, research studies into the N contribution of chickpea, faba bean, lentil and field pea to a subsequent wheat crop on the Prairies were lacking.
A four-year study was conducted from 2014 to 2018. The objectives were to see how legume residue could affect fertilizer N recommendations and N uptake of the subsequent wheat crop; evaluate how different legume crops affect wheat yield; and measure N recovery from aboveground, belowground and fertilizer N in the subsequent wheat crop.
The research site was at the University of Saskatchewan Goodale Research Farm on a Dark Brown Chernozem loam soil. The research had two cycles of a legume-wheat rotation with the legume grown in 2014 and 2016, and wheat grown in rotation in 2015 and 2017. The legumes compared were field pea, lentil, faba bean and chickpea. For comparison, a spring wheat-spring wheat rotation with and without N fertilizer were also compared.
In the phase-one treatments in 2014 and 2016, each plot was subdivided into quadrants, and nitrogen-15 (15N) at a rate of 4.5 lbs/ac (5 kg/ha) was applied to one-quarter of each crop quadrant shortly after emergence. The use of 15N labeling was used to determine the contribution of above-ground crop residues (AGRP) and total below-ground residuals (BFRT) to N uptake by the following spring wheat crop. Belowground residuals included roots, rhizodeposits, residual fertilizer-N, and soil-N.
Labelled N was also applied to the wheat quadrants in phase 1 to determine the contribution of N fertilizer applied during the wheat phase. The fertilizer wheat treatments received additional non-labelled N at rate of 71 lbs N/ac (80 kg N/ha) in 2014 and 40 lbs N/ac (45 kg N) in 2016, as per soil test recommendations.
In the subsequent phase-two wheat treatments, N fertilizer was applied based on soil test recommendations, with application rates varying depending on the previous legume crop. A base application of phosphate and sulfur fertilizer was applied across all treatments. Labelled N was also applied to a quadrant (non-labelled the previous year) in the wheat plots. This was done to measure N uptake from fertilizer in the current cropping year.
The aboveground residue was lightly incorporated in the spring using a small rototiller.
Differences in residue volume and N returned from pulse crops
Total aboveground residue returned to the field from legumes were generally similar or greater than wheat residues returned to the field. Chickpea and faba bean typically had the most residue while pea and lentil the lowest.
Regarding total N returned to the field from aboveground residue, chickpea and lentil returned the statistically highest N in 2014, and chickpea, faba bean, and lentil had the highest N return in 2016. Wheat was statistically lowest in both years. For example, in 2014, chickpea returned an average of 96 lbs N/ac (108 kg N/ha) to the field while wheat returned 38 lbs N/ac (43 kg N/ha).
Nitrogen return to the soil from belowground residue was greater in 2014 than 2016. Chickpea yielded the highest amount of N returned to the soil while faba bean and lentil were intermediate, followed by pea and wheat returning the least N. Chickpea returned an average of 42 lbs N/ac (47 kg N/ha) in 2014 and 13 lbs N/ac (15 kg N/ha) in 2016. Chickpea also came out on top when combining above and belowground residues, totalling 139 lbs N/ac (156 kg N/ha) in 2016 and 65 lbs N/ac (73 kg N) in 2016. This compared to wheat at 53 lbs N/ac (59 kg N/ha) in 2014 and 17 lbs N/ac (19 kg N) in 2016.
These differences between years were due to higher crop residue and legume yields in 2014 compared to 2016.
Wheat yield highest after legumes
In the second year rotational wheat crop, wheat yield was statistically highest on lentil and pea stubble, which were about 25% higher than wheat following wheat. Wheat yield was about 6 to 7% higher following faba bean or chickpea compared to wheat stubble.
Labelled N tracked the sources of wheat N uptake in the wheat phase years of 2015 and 2017. Total belowground residue accounted for 70 to 91% of wheat N uptake from roots, rhizodeposits, residual fertilizer and soil-N. Of these components, soil-N was the main contributor, and was estimated to be 45 to 66% of N uptake from the indigenous soil N pool.
Fertilizer N contributed 5 to 21% of wheat N uptake while aboveground legume residue was responsible for 1 to 11% of wheat N uptake.
The researchers calculated the wheat uptake of 15N applied in the first year to be an average of about 6 to 16% of total 15N applied. Additional 15N recovered from the soil was 24 to 45% of total applied 15N. This means between 39 and 76% of applied 15N was unaccounted for at harvest of the second year wheat crop. Or put another way, 24 to 61% of total applied 15N was accounted for.
The researchers concluded that, “below-ground residue, particularly the soil, plays a pivotal role in regulating N supply to the succeeding wheat crop, overshadowing contributions from aboveground residue and fertilizer.”
Financial support for this project was provided by the Saskatchewan Pulse Crop Development Board, Saskatchewan Ministry of Agriculture – Agriculture Development Fund, and the Western Grains Research Foundation.
Liting Liu, J. Diane Knight, Reynald L. Lemke, Richard E. Farrell. Quantifying the contribution of above- and below-ground residues of chickpea, faba bean, lentil, field pea and wheat to the nitrogen nutrition of a subsequent wheat crop, Field Crops Research, Volume 313, 2024, 109412 OPEN ACCESS: https://doi.org/10.1016/j.fcr.2024.109412.
