A roadmap for sampling and scaling biological nitrogen fixation in terrestrial ecosystems

Abstract

1. Accurately quantifying rates and patterns of biological nitrogen fixation (BNF) in terrestrial ecosystems is essential to characterize ecological and biogeochemical interactions, identify mechanistic controls, improve BNF representation in conceptual and numerical modelling, and forecast nitrogen limitation constraints on future carbon (C) cycling.2. While many resources address the technical advantages and limitations of different methods for measuring BNF, less systematic consideration has been given to the broader decisions involved in planning studies, interpreting data, and extrapolating results. Here, we present a conceptual and practical road map to study design, study execution, data analysis and scaling, outlining key considerations at each step.3. We address issues including defining N-fixing niches of interest, identifying important sources of temporal and spatial heterogeneity, designing a sampling scheme (including method selection, measurement conditions, replication, and consideration of hotspots and hot moments), and approaches to analysing, scaling and reporting BNF. We also review the comparability of estimates derived using different approaches in the literature, and provide sample R code for simulating symbiotic BNF data frames and upscaling.4. Improving and standardizing study design at each of these stages will improve the accuracy and interpretability of data, define limits of extrapolation, and facilitate broader use of BNF data for downstream applications. We highlight aspects-such as quantifying scales of heterogeneity, statistical approaches for dealing with nonnormality, and consideration of rates versus ecological significance-that are ripe for further development. | 1123Methods in Ecology and Evoluঞon SOPER Et al.

Nodulation Alleviates Water Stress in Vachellia Sieberiana

“…If the plant has sufficient C reserves to maintain the increased rhizobia mutualism, this could increase the amount of plant available N for growth. This was contrary to what was predicted as we believed that the movement of rhizobia would be impinged by a lack of water (Fall et al, 2011;Swaine et al, 2007) and highlights remaining uncertainties to understanding N2fixation (Soper et al, 2021).…”

Impact of Nutrient Additions on Free‐Living Nitrogen Fixation in Litter and Soil of Two French‐Guianese Lowland Tropical Forests

“…For an accurate conversion from measured ethylene production to nitrogen fixation in litter samples, we used a conversion factor calculated using 15 N 2 calibrations performed in 2017 at the same time, location, and under comparable conditions as the ARA incubations of that year. Such an independently calibrated conversion factor is more accurate than using a theoretical conversion factor (Soper, Taylor, et al., 2021). However, because the ARA:BNF conversion factor can change over time (Saiz et al., 2019), mainly through differences in physical conditions such as, for example, water content (Liengen, 1999), the conversion factors would have been even more accurate when calibrated at each sampling time point, instead of only once.…”

Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems

“…If a net positive total N balance is calculated to occur between two points in time, then the N gain this is attributed to inputs of fixed N 2 Can potentially be applied to experiments and farmers’ fields N outputs through various loss processes are difficult and complex to quantify and often rely on assumptions rather than actual measurements Quantification of some N inputs (e.g. atmospheric deposition of N) can be challenging Quantification of changes in soil N pool is subject to large errors, substantial inputs from BNF are necessary to reliably quantify any increase in soil N As the method relies on many independent and unrelated measurements, each made with differing degrees of accuracy, the confidence in the final estimate of BNF can be low Use is limited to long-term studies for both non-symbiotic and symbitoic N 2 -fixing systems a Adapted and updated from information provided by Unkvoich et al (2008); Peoples et al (2009b) ; Chalk et al (2017) ; Soper et al (2021) . …”

“… a Adapted and updated from information provided by Unkvoich et al (2008); Peoples et al (2009b) ; Chalk et al (2017) ; Soper et al (2021) . …”

“…All these different approaches are technically challenging with their own unique strengths and limitations, and each are subject to specific potential sources of error when being applied to either symbiotic ( Minchin et al, 1983 , Giller, 2001 , Unkovich et al, 2008 , Peoples et al, 2009b , Chalk et al, 2016 ), or non-symbiotic systems ( Boddey, 1987 , James, 2000 , Santi et al, 2013 , Bellenger et al, 2014 , Unkovich et al, 2020 , Soper et al, 2021 ; Table 6 ). Consequently, putative evidence of BNF, even by nodulating legumes, should ideally not rely solely upon the application of a single measurement technique.…”