15N natural abundance as a tool for assessing N2-fixation of herbaceous, shrub and tree legumes in improved fallows

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 delta(15)N 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 N-15 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 N-15 discrimination during N-2-fixation (i.e. whole plant delta(15)N was close to 0parts per thousand). Significant N-15 isotopic discrimination occurred during translocation of fixed N, which resulted in N-15 depletion in shoots (up to -1.76parts per thousand in Sesbania sesban) compared with roots and nodules which were N-15 enriched. Soils were highly enriched in N-15 (8.2-10.8parts per thousand) with little variation with depth to 2 m. delta(15)N signatures of plant available N measured using non-fixing reference plants were lower than those of total soil N. delta(15)N of the non-fixing reference species maize, Lantana camara and Tithonia diversifolia varied by 2.0parts per thousand and resulted in corresponding variation of N-2-fixation estimates for respective species. N-15 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 N-15 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 N-15 abundance and using appropriate, or a range of, non-fixing reference plants, the natural delta(15)N abundance method is a useful tool for estimating the amount of N derived from N-2-fixation by field grown herbaceous and woody legumes. (C) 2002 Elsevier Science Ltd. All rights reserved.