The influence of site factors on nitrogen mineralization in forest soils of the Mpumalanga escarpment area: South africa

Synopsis

The complex biophysical environmental conditions that occur in South Mrican forestry plantations prompt appropriate guidelines for nutrient management applications. Nutrient budget studies in plantation ecosystems indicated a negative nitrogen (N) balance on many sites. Empirical fertilizer trails have shown that, although responses are highly variable, the application of inorganic fertilizers remain an efficient means of improving tree growth. This study was carried out to investigate the influence of variable site conditions on N mineralization, as well as the effect of N mineralization on forest productivity. Data were collected from 31 sample plots, covering a wide range of site conditions in the study area. N mineralization was monitored using multi-week in situ incubations, and compared to a wide spectrum of site factors recorded at each sample plot. The phase of plantation canopy closure (age 6-10 years) was used as the reference age.

A significant positive correlation was found between NH₄ + mineralization during summer and the growth of Pinus patula ( r = 0, 77, p < 0,001). This process of the N cycle can therefore be considered as the rate limiting step influencing tree growth. N mineralization is controlled by a complex interaction of various site factors, and numerous correlations and interactions between NH4 +mineralization (summer) and site factors are discussed. Statistical models were developed through multiple regression techniques, to predict N mineralization from independent site variables. The in situ rates of N mineralization could be predicted with an acceptable degree of accuracy by making use of the natural log of mean annual temperature, geological substrate and total N levels in the topsoil (r² = 0, 71). These models are suitable for incorporation into existing plantation decision support systems. The significance of the response in tree growth, following fertilization with N in existing mid-rotation forest nutrition trials, was evaluated against the predictions ofN mineralization using the proposed models. An inverse linear model ( r² = 0,89, p < 0,0001) was found to best describe this relationship. Additional research on the process of N mineralization in South Mrican plantations is required to provide accurate recommendations on nutrient management for the various forest management practices currently in use.

Keywords:

• N mineralization
• forest site
• Pinus patula
• nutrient management