Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ¹⁵Nmeasurements

Abstract

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ(15) N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ(15) N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ(15) N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ(15) Nxylem sap and δ(15) Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ(15) N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ(15) Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ(15) N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ(15) N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone.

Greater root phosphatase activity in nitrogen‐fixing rhizobial but not actinorhizal plants with declining phosphorus availability

“…; Boddey et al . ; Soper, Boutton & Sparks ). Third, differences in plant rooting depths among N‐fixing plants and the non‐N‐fixing reference species in the field were not accounted for, but they might result in differential access of soil N sources with dissimilar isotopic ratios at different soil depths (Shearer & Kohl ).…”

“…Leaves of N‐fixing and non‐N‐fixing reference plants from each soil age were also collected from individuals that co‐occur within the same 10 m × 10 m plot to reduce variation of isotopic consistency of the soil N source (Shearer & Kohl ; Boddey et al . ; Soper, Boutton & Sparks ).…”

“…Only youngest fully expanded leaves from the same growing season were collected for analyses to reduce seasonal variation of leaf d 15 N (Shearer & Kohl 1986;Evans 2001). Leaves of N-fixing and non-N-fixing reference plants from each soil age were also collected from individuals that co-occur within the same 10 m 9 10 m plot to reduce variation of isotopic consistency of the soil N source (Shearer & Kohl 1986;Boddey et al 2000;Soper, Boutton & Sparks 2015).…”

“…As such, the isotopic consistency of the soil N source spatially may decrease due to differences in N cycling at a localised scale (e.g. Shearer & Kohl 1986;Yoneyama et al 1993;Boddey et al 2000;Soper, Boutton & Sparks 2015). Third, differences in plant rooting depths among N-fixing plants and the non-N-fixing reference species in the field were not accounted for, but they might result in differential access of soil N sources with dissimilar isotopic ratios at different soil depths (Shearer & Kohl 1986).…”

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment

“…Currently, upland landscapes consist of three landscape elements: grasslands, clusters (woody patches <100 m 2 ), and groves (>100 m 2 ; Appendix S1; Figure 1). The formation of clusters is initiated by the colonization of honey mesquite (Prosopis glandulosa; an N 2 -fixing tree legume, Zitzer, Archer, & Boutton, 1996;Soper, Boutton, & Sparks, 2015) in grasslands. Established honey mesquite trees then serve as nurse plants and facilitate the recruitment of numerous other woody species beneath their canopy (Archer et al, 1988).…”