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DIVISION S-3 - SOIL BIOLOGY & BIOCHEMISTRY

Short-Range Spatial Variability of Nitrogen Fixation by Field-Grown Chickpea

^a Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, SK, S7N 0W0 Canada
^b Dep. of Agronomy and Range Science, Univ. of California-Davis, Davis, CA 95616

* Corresponding author (cvankessel{at}ucdavis.edu)

Biological N fixation (BNF) by legumes under field conditions is known to vary widely across landscapes and between agroecosystems. A poor correlation between the ¹⁵N Natural Abundance (¹⁵NA) and ¹⁵N Enriched (¹⁵NE) approaches for estimating BNF across the landscape has been observed by many. These observations led some to conclude that the two approaches are measuring different processes and can not be compared. Others argue that short-range spatial variability of BNF is very high, thereby obscuring any relationships between experimentally measured estimates of BNF. Our study, which quantifies spatial variability of BNF using the ¹⁵NA approach, provides evidence that short-range spatial variability of BNF is very high. In a field study, BNF of chickpea (Cicer arietinum L.) was measured at 0.3-m intervals on a 33-m transect, using wheat (Triticum aestivum ‘Katepwa’) as reference crop. Each crop was sampled at 110 points along the transect. Estimates of BNF in the grain varied from 36 to 70%, with a mean value of 55%. The variogram for BNF had a range of 3.2 m and a relative nugget effect of 71%. Using a simulation study, we calculated r² of 0.12 and 0.02 for BNF at sites spaced 1 m and 2 m apart, respectively. We concluded that short-range spatial variability of BNF across the landscape is the likely cause of reported discrepancies between the ¹⁵NA and ¹⁵NE approaches used in other studies. Moreover, if such a high spatial variability in BNF is the norm rather than the exception, comparisons between ¹⁵NA and ¹⁵NE approaches for estimating BNF under field conditions will be unreliable.

Abbreviations: BNF, biological N fixation • ¹⁵NE, ¹⁵N-enriched • ¹⁵NA, ¹⁵N natural abundance • %Ndfa, percentage of N derived from the atmosphere

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