Soil organic matter is important as storage for carbon and nutrients, supporting soil
structure, and as a filter for pollutants entering the soil ecosystem. The recovery of soil
organic matter in depleted soils can take decades, or even hundreds of years. It has
been assumed that in non-eroding pasture, soil carbon levels either increase or not
change over time. However, some recent studies have suggested that fertiliser addition
to pasture soils may contribute to decreases in soil carbon content. My hypotheses
were:
1. As P fertiliser loadings increase the soil carbon content and C:N ratio will
decrease.
2. Changes in C pools will be greater in the more active pool (readily available
carbon, and microbial biomass carbon) within the soil total carbon
The study was undertaken at a long term fertiliser trial, established in 1980, at the
Whatawhata Hill Country Research Station west of Hamilton, New Zealand. The
fertiliser trial has P fertiliser application rates maintained since 1984. Olsen P, total C,
total N, labile carbon, respirable carbon, specific respiration rate, microbial biomass C,
microbial quotient, mineralised N, microbial biomass N, microbial N quotient, and
mineralised N per microbial biomass nitrogen, C:N ratio, and soil pH were measured
on soil samples collected from 12 paddocks with six P fertiliser loading (0, 10, 20, 30,
50, 100 kg P ha-1 yr-1).
As expected, the available P (Olsen P) increased significantly (P less than 0.001) with
increasing P fertiliser application rate. Total carbon, labile carbon, and total nitrogen
all decreased significantly (P less than 0.05) with increasing P fertiliser application. No
significant relationships were found between P fertiliser and respirable carbon,
microbial carbon, microbial (C) quotient, microbial specific respiration, microbial
nitrogen, microbial (N) quotient, mineralised N, or C:N ratio. The first hypothesis was
rejected as the C:N ratio did not change with increased P fertiliser application.
However, both C and N decreased with increased P fertiliser application. The second
hypothesis was, therefore, accepted in part because there was a decrease in labile
carbon (readily available carbon) and total carbon, with P fertiliser application, but no
relationship was evident for the respirable carbon and microbial biomass.
