Integration and potential nitrogen contributions of green manure inter-row legumes in coppiced tree cropping systems.

Highlights • Inter-row legumes contributed up to 90 kg fixed N ha−1 in coppiced tea tree crops at four sites. • Tea tree yield loss was observed with soybean intercrops at one site. • Further study needed to quantify potential N fertiliser savings when legumes are intercropped. Abstract Legumes play a key role in maintaining soil fertility in pasture systems and when used as a rotation crop or relay crop in annual cropping systems. Little is known about the potential to integrate green manure legumes into coppiced or ratooned perennial crop systems. We trialled the cultivation and green manuring of four annual legume species in the inter-row of coppiced tea tree (Melaleuca alternifolia) and assessed potential nitrogen (N) contributions using 15N natural abundance methodology. The yield and N uptake of tea tree crops with legumes was compared with tea tree crops amended with 5 t ha−1 (wet) poultry litter as per standard practice. At two field sites where warm-season legumes were sown in spring into coppiced tea tree stands, soybean (Glycine max) had a higher % N derived from the atmosphere (%Ndfa) than mung beans (Vigna radiata) (82 vs 68% at Casino and 65 vs 56% at Tweed Heads) and contributed more fixed N (shoots + roots) to the soil than mung bean at both sites (91 vs 24 kg fixed N ha−1 at Casino and 61 vs 33 kg fixed N ha−1 at Tweed Heads). Where cool-season legumes were sown into tea tree stands coppiced in winter, faba bean (Vicia faba) had a higher %Ndfa than chickpea (Cicer arietinum) at Rappville (88 vs 68%). Chickpea fixed less than 10 kg N ha−1 at Rappville, while faba been fixed 36 kg N ha−1 at Rappville and 92 kg N ha−1 at Leeville. At one site (Casino) tea tree yields in the legume plots were significantly (10–15%) lower than the poultry litter plots. The nutrient content of tea tree did not differ among treatments for any measured micro- or macronutrients, and the cause of the yield loss remains unknown. Ultimately, the highest fixed-N contributions of faba bean (92 kg fixed-N ha−1 at Leeville) and soybean (91 kg fixed-N ha−1 at Casino) equated to more than the amount of N in harvested shoot of a low yielding tea tree crop (67 kg N ha−1 at Leeville) and around 2/3 the N accumulated in harvested shoots of a high yielding tea tree crop (129 kg N ha−1 at Casino). Green manure legume crops have the potential to reduce the need for external N fertiliser inputs in coppiced tree crops, but further refinement is needed to integrate legumes into current weed control systems, optimise the timing of N release from decaying legume residues, and to minimise any competition between legumes and the tree crops for water, light or nutrients. [ABSTRACT FROM AUTHOR]