Direct assessment of the fixation and release of nitrogen by pea, lentil, chickpea and faba bean
Date
2019-03-10
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-4909-2404
Type
Thesis
Degree Level
Masters
Abstract
Pulse crops make significant contributions to the economy and environmental sustainability of western Canadian cropping systems. Due to their ability to provide N and non-N benefits to subsequently grown crops, pulse crops play a key role in crop rotations. The aim of this study was to trace the fate of fixed N in a pulse–cereal crop sequence. The pulse phase of the study involved quantifying the amounts of fixed nitrogen in the above- and below-ground plant compartments of chickpea, faba bean, lentil and pea; and evaluate their contribution to soil nitrogen pools. Plants were labelled with 15N2 using a custom-built soil atmosphere labelling system. The amount of biologically fixed N in the above-ground biomass of the pulse crops was greatest for faba bean and chickpea (82%Ndfa), intermediate for lentil (70%Ndfa), and least for pea (28%Ndfa). Conversely, the percentage of fixed N in the below-ground biomass (i.e., roots) was greatest for the chickpea (58%Ndfa), intermediate for the faba bean (37%Ndfa) and lentil (32%Ndfa), and least for the pea (18%Ndfa). Total amounts of fixed N returned to the soil as crop residue increased in the order: pea (12 mg N pot-1) << lentil (193 mg N pot-1) << faba bean (375 mg N pot-1) < chickpea (556 mg N pot-1). The cereal phase of the study was carried out in a greenhouse to quantify the nitrogen contribution of the pulse residues to (i) the subsequent wheat crop, and (ii) nitrous oxide (N2O) emissions from the soils. The residue-amended soils were first frozen (-20C) and then slowly thawed at room temperature. Gas sampling was initiated during the thaw period, continued for 10 weeks, and was terminated two weeks prior to harvest—with the gas samples analyzed for total and 15N2O concentration. Wheat yields were about 63% greater when the previous crop was a pulse compared to when it was wheat. Recovery of fixed-N (i.e., residue-derived N) by the wheat ranged from 9% to 14% for the pulse crops, compared to recovery of only about 4% residue-derived N when the previous crop was wheat. Soils amended with pulse residue, but not wheat residue, exhibited significant N2O emissions (total and 15N2O) during the thaw period, which is indicative of greater (faster) turnover of the pulse residues. Residue-derived emissions also showed a strong positive correlation (r = 0.954; P = 0.012) with the amount of residue-N added to the soil.
Description
Keywords
nitrogen, pulse crops
Citation
Degree
Master of Science (M.Sc.)
Department
Soil Science
Program
Soil Science