15N natural abundance as a tool for assessing N 2-fixation of herbaceous, shrub and tree legumes in improved fallows

Abstract

Short-term legume–cereal rotation systems (referred to as improved fallows) with N 2 fixing leguminous species are being actively promoted to improve soil fertility in fallowed fields of smallholder farms in many parts of the tropics. Few estimates of N 2-fixation in deep-rooted woody fallow species are available due to methodological difficulties. We evaluated and developed the natural δ 15N abundance method for assessing N 2-fixation in herbaceous and woody legumes on a Kandiudalfic Eutrudox in western Kenya by (i) assessing isotopic discrimination during N 2-fixation and translocation, (ii) measuring variability of 15N with depth, (iii) comparing with an independent method (ureide assay) and (iv) using several non-fixing reference plants. Most tested tree/shrub legumes showed no 15N discrimination during N 2-fixation (i.e. whole plant δ 15N was close to 0‰). Significant 15N isotopic discrimination occurred during translocation of fixed N, which resulted in 15N depletion in shoots (up to −1.76‰ in Sesbania sesban) compared with roots and nodules which were 15N enriched. Soils were highly enriched in 15N (8.2–10.8‰) with little variation with depth to 2 m. δ 15N signatures of plant available N measured using non-fixing reference plants were lower than those of total soil N. δ 15N of the non-fixing reference species maize, Lantana camara and Tithonia diversifolia varied by 2.0‰ and resulted in corresponding variation of N 2-fixation estimates for respective species. 15N based estimates of N 2-fixation of pigeonpea and siratro were linearly related with those obtained using the ureide method ( R 2=0.80, slope=0.82) and confirmed the utility of the 15N natural abundance method. Field observations showed that under non-PK limiting growth conditions, the proportion of N 2 fixed ranged 75–83, 63–74, 55–67, 46–59, 36–54, 35–50, and 36–51% for Crotalaria grahamiana, Tephrosia vogelii, pigeonpea ( Cajanus cajan), S. sesban, Calliandra calothyrsus, siratro ( Macroptilium atropurpureum) and groundnut ( Arachis hypogaea). This resulted in average amounts of N 2 fixed of 142, 100, 91, 52, 24, 64 and 8 kg N ha −1, respectively, 9 months after planting. The amount of soil derived N ranged between 31 and 57 kg N ha −1 in woody species. The net N balance of woody fallows (after adjusting for N export in wood) was highest in Crotalaria due to high N 2-fixation and small amount of N exported in wood. Overall, partial N balances indicated that additional N derived from N 2-fixation constituted a major component of recyclable N of the system. We conclude that, in soil with sufficient and relatively uniform background 15N abundance and using appropriate, or a range of, non-fixing reference plants, the natural δ 15N abundance method is a useful tool for estimating the amount of N derived from N 2-fixation by field grown herbaceous and woody legumes.