Phosphorus levels in topsoils under conifer plantations in Canterbury high country grasslands

Abstract

Topsoils under planted conifer forests in the Canterbury high country have high levels of 0.5 M H₂S0₄-P (inorganic P) compared to published records from topsoils of grasslands on soils of the same high country soil groups. Group means under conifers were from 16 per cent to 140 per cent higher than under grassland but such differences did not appear to be consistently related to moisture class or natural fertility. Concentrations of 0.5 h Olsen-P in conifer topsoils were also compared with topsoil records for unimproved grassland, semi improved grassland, and improved pasture for the same soil sets and soil groups. For forest topsoil samples, inorganic P and Olsen-P values were correlated, especially within the naturally more fertile groups of drier and younger soils. Both forests and grasslands exhibited a similar but small decline in Olsen-P values with increasing precipitation. For most soil taxa. variability in Olsen-P was high under any vegetation cover, but Olsen-P was clearly greater under conifer plantation than under grassland. Soils from the three development classes of grassland had similar mean Olsen-P levels whereas the average forest Olsen-P levels were generally 2 to 4 times higher than the average grassland level for the corresponding soil. The data presented indicate a significant enhancement of "plant available" topsoil phosphorus by conifer plantations in the montane zone across a wide precipitation range. Despite the high variability, this enhancement appears to be of similar magnitude along. the whole precipitation range sampled, a situation apparently different from that outlined for "inorganic P". This general situation is discussed in terms of possible processes suggested from earlier research.