Three long-term trials end with a quasi-equilibrium between sil C, N and pH: an implicaton for C sequestration

The aim of this study was to assess the long-term changes in some key soil chemical properties at the completion of three long-term trials in south eastern Australia and the relationship between those soil properties. From a soil organic matter perspective, the build up of %C requires an accumulation of %N, and the build up of %C and %N fertility comes at the cost of soil acidity. Rotation, tillage and stubble practices combine to alter the quantity, quality (C:N) and the depth distribution of organic matter in a soil, but the three soil chemical properties reported here seem to also be in quasi-equilibrium at the three long-term sites. The consequence is that if the build up of soil organic matter leads to soil acidification, then the maintenance of agricultural production will require liming. The emission of CO2 when limestone reacts with soil acids, plus the C cost of limestone application, will negate a proportion of the gains from C sequestration as organic matter in soil. Such cautionary information was doubtless unforeseen when these three long-term trials were initiated.