Forms of C, N and P accumulation under different pasture soil carbon sequestration scenarios

Sequestering carbon and managing nutrients in soils are key issues in global agriculture. Understanding the quantities involved and the major processes of change are vital for improved utility of soils. Fertilised pasture soils potentially contain large pools of carbon, nitrogen and phosphorus. A knowledge of the amount and distribution of nutrients stored in such soils can have positive implications for sequestering carbon in order to mitigate anthropogenic climate change, reducing nutrient runoff and improving soil fertility. In this study, the soil carbon, nitrogen and phosphorus contents were determined for six fields of varying fertility, across two dairy farms in the Camden region of NSW. Three profiles were examined to 100 cm depth, to investigate the distribution and nutrient forms in the different soil layers. Profiles were described and sampled, and samples were analysed for total carbon, total nitrogen, organic and inorganic phosphorus. Ancillary data such as particle size distribution and bulk densities were also measured. Total quantities of C, N and P and nutrient ratios were calculated for each of the fields. Results showed large amounts of nutrients were contained in the topsoils, and also in the deeper 30-70 cm profile layers that are not commonly analysed. Total carbon contents ranged between 170,184 - 292,278 kg/ha, total nitrogen from 11,562 - 21,780 kg/ha and total phosphorus from 1,586 - 4,035 kg/ha. Large variations were found between the different fertility fields, particularly in the inorganic phosphorus content where some soils contained a possible P surplus. An average C:N:P ratio of 192:12:1 was calculated across all soils sampled. This is similar to values found elsewhere for pasture soils. Well managed pasture soils were shown to have potential to sequester large additional amounts of carbon. It is recommend that fertiliser application rates be based on regular soil testing, and appropriate management strategies be developed to effectively manage nutrient accumulation and distribution.