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C : N ratios and carbon distribution profile across rooting zones in oilseed and pulse crops

Y. T. Gan A D , B. C. Liang B , L. P. Liu C , X. Y. Wang A and C. L. McDonald A
+ Author Affiliations
- Author Affiliations

A Agriculture and Agri-Food Canada, Semiarid Prairie Agricultural Research Centre, Gate #3, Airport Road E., Swift Current, SK, S9H 3X2, Canada.

B Greenhouse Gas Division, Environment Canada, 9th Floor, Fontaine Building, 200 Sacré-Coeur, Gatineau, Québec, K1A 0H3, Canada.

C Department of Plant Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada.

D Corresponding author. Email: yantai.gan@agr.gc.ca

Crop and Pasture Science 62(6) 496-503 https://doi.org/10.1071/CP10360
Submitted: 11 November 2010  Accepted: 30 June 2011   Published: 7 July 2011

Abstract

Knowledge on the C : N ratio of crop residues is of great importance for modelling carbon (C) and nitrogen (N) dynamics of agricultural systems. This study determined (i) the C : N ratios in the seed, straw, and roots of selected broadleaf crops and (ii) the root C and N distribution in the 0–100 cm rooting zone. Three oilseed (Brassica napus canola, Brassica juncea mustard, Linum usitatissimum flax), three pulse crops (Cicer arietinum chickpea, Pisum sativum dry pea, Lens culinaris lentil), and spring wheat (Triticum aestivum L.) were grown under field conditions with low- (rainfall only) and high-water (rainfall plus irrigation) availability. Root C mass decreased substantially with rooting depth, with ~58% of root C mass in the top 20 cm of the soil, 78% in the top 40 cm, and 94% in the top 60 cm. Significant differences in root C mass between crop species occurred in the top 20 cm with canola, mustard, and wheat allocating 66% of their root C total, compared with 55% for dry pea, lentil, and flax, and 41% for chickpea. Root N mass followed a similar response to root C. Seed C : N ratios ranged between 6 and 17, whereas straw C : N ranged between 14 and 55, and root C : N between 17 and 75. Under low-water conditions, canola and mustard had a straw C : N of 33, lower than that of flax (38) and wheat (41). Under higher-water availability, however, mustard and wheat had straw C : N ratios at 55, greater than canola, mustard and flax (47). Three pulses had an average straw C : N ratio of 17, significantly lower than 41 for the oilseeds and 32 for wheat. On average, canola, mustard and wheat had greater root C : N ratios (44) than chickpea (33) and lentil (29), with dry pea having a smallest root C : N ratio (18). Root C : N ratios did not change with soil depth. These detailed measurements on the vertical distribution of root C and N as well as C : N ratios for various crops will assist in improving estimates of inputs for C and N cycling studies.

Additional keywords: oilseed, grain legumes, root carbon, C : N ratio, rooting depth, rooting zones, soil organic matter, soil nitrogen.


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