Short-range spatial variability of nitrogen fixation by field-grown chickpea

Biological N fixation (BNF) by legumes under field conditions is known to vary widely across landscapes and between agroecosystems. A poor correlation between the N-15 Natural Abundance ((15)NA) and N-15 Enriched ((NE)-N-15) 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 (15)NA 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 r(2) of 0.12 and 0.02 for BNF at sites spaced I 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 (15)NA 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 (15)NA and (NE)-N-15 approaches for estimating BNF under field conditions will be unreliable.