In mango production systems, falling leaves, flowers, branches and fruit as well as end of season prunings constitute the tree litter and continue to be an under-rated source of nitrogen on the tropical orchard floor.

Trees are pruned after harvest to maintain canopy shape, control vigour and promote new growth.

Pruned material is usually mulched by driving a flail mower or slasher across it, then left on the orchard floor to decompose, the litter is often dismissed when calculating seasonal fertiliser inputs.

As part of a project on enhancing nitrogen use efficiency in horticultural systems, the team at the Department of Primary Industries and Resources, NT quantified annual tree litter and prunings in two commercial mango orchards in the Northern Territory and measured its nitrogen (N) content.

Falling litter and material mechanically pruned after harvest was collected in mature Calypso and Kensington Pride (KP) mango orchards for twelve months during 2017-2018, with the majority of natural litter drop occuring over approximately 100 days between flowering and fruit maturity.

While vigorous canopy growth was observed as expected during the wet season, differences were noted in litter accumulation between varieties and sites.

Over a year, researchers found suprisingly high tree litter dry weights, with Calypso generating 4 t/ha and KP 4.2 t/ha at a tree density of 250/ha.

Nutrient analysis showed that leaves and flowers contributed the most N. When combined, the annual quantity of N in litter and prunings of Calypso and KP orchards were similar, 18.8 and 22.2kg N/ha respectively. This equates to 75-90g of N per tree falling onto the ground.

While the amount of N in mango litter and prunings vary according to tree planting density and management practices, the results show they contribute a significant amount of N on the orchard floor that can be incorporated or mineralised in soil over time.

Researchers also know from recent work in the NT, that most mango leaf litter decomposes annually in tropical conditions according to research scientist, horticulture Dr Joanne Tilbrook.

“Mangoes require quite low N inputs compared to other tree crops and annual recommendations vary according to local conditions,” she said. “For one mature tree this can range from 3kg N/year in NSW and QLD to 700g N/year in WA.

“Practices vary widely in the NT. The differences reflect the diversity of soils and climates that mangoes are grown in and the need for broader, evidence-based N recommendations.

“It is well known that too much N results in green skinned ripe mangoes which is considered a post-harvest quality defect. Recent research in Katherine, NT shows that the risk of this defect increases when more than 25-30kg N/ha or 100-120g N/tree- is applied to KP in a year with commercial yields up to 20 tons/ha (KP tend to bear more heavily on a biennial basis).

“If we take the N uptake efficiency of about 40 per cent for soil applied N fertiliser into account (with 60 per cent lost by volatilisation and leaching), it means that N cycling in the litter/soil system is potentially providing more N for the crop than the fertiliser applied to the soil.

“Therefore, N from mango tree litter and prunings should be considered when calculating annual N fertiliser application in orchards in the tropics.”

Northern Territory commercial growers produce about 50 per cent of mangoes sold in Australia, with researchers saying the new information further helps us to understand N sources and cycling in tropical mango production systems.

For the industry, they say it will contribute to improved N use efficiency in orchards and help develop a set of evidence based N management guidelines to reduce production costs and environmental N losses.