Abstract
Nitrogen (N) fertiliser is applied to perennial horticultural crops to increase yields, but subsequent N losses in subtropical plantations may be high due to intense rainfall and warmer temperatures. While legume cover crops could potentially contribute N to the tree crops and reduce fertiliser-N requirements, few studies have quantified potential fixed-N inputs from cover crops legumes in tropical or subtropical tree crop systems. To address this, we investigated growth and N fixation of summer-growing Pinto peanut (Arachis pintoi Krapov. & W. C. Greg cv. Amarillo) and winter/spring dominant white clover (Trifolium repens L. cv. Haifa) grown as a mixed species cover crop in two commercial subtropical avocado (Persea americana Mill. cv. Hass) plantations. Legume biomass was assessed prior to mowing of the inter-row (fortnightly in summer and every 6–8 weeks over winter) and N fixation was estimated using the 15N natural abundance technique. Biomass production was 7377 kg ha−1 (930 kg ha−1 for white clover and 6447 kg ha−1 for Pinto peanut) at the first site over the 14-month period from December 2014 to January 2016, and 4467 kg ha−1 (1114 kg ha−1 for white clover and 3353 kg ha−1 for Pinto peanut) at the second site over the same period. Estimation of N fixation was not possible at the first site, due to a lack of difference in isotopic discrimination between the legume shoots and the reference plant (kikuyu (Pennisetum clandestinum Chiov.)) material. While legume shoots accumulated 157 kg N ha−1 (38 kg ha−1 for white clover and 119 kg ha−1 for Pinto peanut) across the season at site 1, the % N derived from atmosphere (%Ndfa) in legumes was relatively low (50–60% in Pinto peanut during the warmer months and around 30% in autumn and early spring, and from 13 % in April to 69% in September for white clover). The low %Ndfa in the legumes may have been due to low rainfall or molybdenum (Mo) deficiency. Ultimately the legume cover crops contributed an estimated 50 kg fixed N ha−1, which could partially offset fertiliser N requirements of the tree crop. Our results demonstrate the need to quantify N fixation in legume cover crops to assess potential N benefits as opposed to relying on typical measurements of legume biomass and N accumulation.
Highlights
Inputs of nutrients, nitrogen (N), into tropical and subtropical fruit crops are essential to replace nutrient removed in harvested produce and maintain fruit yield and quality [1]
While legume shoots accumulated 157 kg N ha−1 (38 kg ha−1 for white clover and 119 kg ha−1 for Pinto peanut) across the season at site 1, the % N derived from atmosphere (%Ndfa) in legumes was relatively low
One published report on N fixation in tropical rubber (Hevea brasiliensis L.) plantations suggests that as much as 150 kg N ha−1 year−1 can be fixed by legume cover crops [10], which could make a substantial contribution to the N requirements of the cash crop
Summary
Nitrogen (N), into tropical and subtropical fruit crops are essential to replace nutrient removed in harvested produce and maintain fruit yield and quality [1]. Developing options to reduce the need for external N inputs may be a viable means of lowering the costs of production and minimising the environmental footprint of both conventional and organic subtropical fruit plantations. The incorporation of N2 -fixing leguminous crops into farming systems may be one means to reduce the need for external N inputs and decrease the environmental footprint associated with synthetic N fertilisers [6]. A number of studies have examined potential for leguminous ground covers to produce biomass and accumulate N in tropical plantations [8,9]; few published estimates of fixed-N contributions from cover crops in the tropics or subtropics exist. One published report on N fixation in tropical rubber (Hevea brasiliensis L.) plantations suggests that as much as 150 kg N ha−1 year−1 can be fixed by legume cover crops [10], which could make a substantial contribution to the N requirements of the cash crop
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