Modelling soil nitrogen mineralisation in semi-mature pine stands of South Africa to identify nutritional limitations and to predict potential responses to fertilisation

Abstract
Key Message Estimates of available nitrogen (based on topsoil nitrogen mineralisation rates) in semi-mature stands of Pinus radiata (D Don.) as well as pure species and hybrids of Pinus elliottii (Engelm.) in South Africa display a strong, significant, negative relationship with growth response to applied fertilisers in field experiments. This inexpensive technique can be incorporated in a fertiliser decision support system to minimise wasteful fertiliser applications.
Context South African mid-rotation pine stands respond with high variability to nitrogen fertiliser supplements in the presence of phosphorus.
Aims We aimed to find a screening tool for stands that are responsive to fertilisation.
Methods The volume growth response relative to control plots (3 years after fertilisation with 200 kg ha−1 N in the presence of 100 kg ha−1 P) was determined across 14 pine fertiliser experiments in South Africa. These values were correlated to soil and other properties for each site and also to estimates of N availability and N mineralisation rates using the Soil Nitrogen Availability Predictor (SNAP) model.
Results The relative volume growth response to fertilisation ranged with a factor from 0.05 to 0.96. No single result from standard soil analyses showed a strong positive or negative correlation with this growth response. However, highly significant negative correlations were observed between an estimate of N availability based on topsoil aerobic nitrogen mineralisation rates and the relative volume growth responses to fertilisation.
Conclusion The estimate of N availability derived from incubation studies may assist us to predict the magnitude of the response to N fertilisation (in the presence of P) with a fair degree of accuracy. The accuracy can be improved if the N availability estimate from aerobic incubation is used with other variables (pH and soil C content) in a multivariate regression to predict relative yield response to fertilisation.

Keywords
Slash pine, Monterey pine, Nitrogen mineralisation, Site-specific fertiliser applications

Publication
Du Toit, B., Scheepers, G.P. Modelling soil nitrogen mineralisation in semi-mature pine stands of South Africa to identify nutritional limitations and to predict potential responses to fertilisation. Annals of Forest Science 77, 24 (2020). https://doi.org/10.1007/s13595-020-0923-y

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Data availability
The datasets generated and/or analyzed during the current study are available in the SUNScholarData repository, https://scholardata.sun.ac.za/https://doi.org/10.25413/sun.c.4824126

Collection
Forecasting Forests

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