Difficulty in quantifying rates of biological N fixation (BNF), especially over long time scales, remains a major impediment to defining N budgets in many ecosystems. To estimate N additions from BNF, we applied a tree-scale N mass balance approach to a well-characterized chronosequence of woody legume (Prosopis glandulosa) encroachment into subtropical grasslands. We defined spatially discrete single Prosopis clusters (aged 28–99 years), and for each calculated BNF as the residual of: soil N (0–30 cm), above- and below-ground biomass N, wet and dry atmospheric N deposition, N trace gas and N2 loss, leaching loss, and baseline grassland soil N at time of establishment. Contemporary BNF for upland savanna woodland was estimated at 10.9 ± 1.8 kg N ha−1 y−1, equal to a total of 249 ± 60 kg N ha−1 over about 130 years of encroachment at the site. Though these BNF values are lower than previous estimates for P. glandulosa, this likely reflects lower plant density as well as low water availability at this site. Uncertainty in soil and biomass parameters affected BNF estimates by 6–11%, with additional sensitivity of up to 18% to uncertainty in other scaling parameters. Differential N deposition (higher rates of dry N deposition to Prosopis canopies versus open grasslands) did not explain N accrual beneath trees; iterations that represented this scenario reduced estimated BNF estimates by a maximum of 1.5 kg N ha−1 y−1. We conclude that in this relatively well-constrained system, small-scale mass balance provides a reasonable method of estimating BNF and could provide an opportunity to cross-calibrate alternative estimation approaches.
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We thank Shauntle Barley and Chase Brett for assistance with sample collection, Kimberlee Sparks for technical support, and David and Stacy McKown for field logistics. This work was supported by the Cornell University Betty Miller Francis’47 Fund for Field Research and the Cornell University Program in Cross-Scale Biogeochemistry and Climate (supported by NSF-IGERT and the Atkinson Center for a Sustainable Future).
FMS and JPS formulated the original idea and developed methodology, FMS conducted field sampling, performed sample processing and statistical analysis, FMS and JPS interpreted data, and FMS and JPS wrote the manuscript.
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Soper, F.M., Sparks, J.P. Estimating Ecosystem Nitrogen Addition by a Leguminous Tree: A Mass Balance Approach Using a Woody Encroachment Chronosequence. Ecosystems 20, 1164–1178 (2017). https://doi.org/10.1007/s10021-016-0100-1