50 Years of Change In Total Soil C and N of Pastures Under Differing Irrigation Frequencies and P Fertiliser Application Rates.

Two common management practices that enhance production on intensively grazed pastures are the use of fertilisers and irrigation but the consequences of these practices on changes in soil carbon (C) and nitrogen (N) are poorly understood. At Winchmore, New Zealand, two long-term pasture trials were established in the 1950s designed to examine the effect of P fertiliser rates and irrigation frequencies on pasture production in a summer-dry environment. Both trials have been sampled for soils nearly every year since. We analysed archived soil samples from three P input treatments in the irrigated fertilizer rate trial (1952-2009) and from three irrigation treatments in the fertilized irrigation frequency trial (1959-2002). On the fertilizer rate trial, soil C increased linearly from 2.7 to 4.2% over 50 years with no difference in rate of increase in C or its absolute concentration for all P fertiliser treatments, despite measurements of higher aboveground production with added P. This lack of difference was attributed to mitigation of greater C inputs by stocking rate adjustments, and to the possibility that below-ground inputs (which differed less) were more important contributors to soil C. Soil N increases were also very similar for all treatments but the absolute %N was lower in the unfertilized treatment, which was attributed to lower clover dry matter production and estimated N fixation resulting from P limitation. In the irrigation treatments, % C was significantly greater in the unirrigated dryland treatment than the irrigated treatments throughout the majority of the 50 years, despite greater above-ground production in these irrigated treatments.  Irrigation must have increased C and N losses, possibly via increased respiration rates during seasonally dry periods decreasing organic matter storage or increased leaching of dissolved organic C and N from the surface soils.  The accumulation of N in the irrigated trials was constrained by the increase in %C as C:N ratios were around 10 throughout the trial. In contrast to many studies showing increased C content when improving pasture production, our study showed that P fertilizer application did not result in an increase in soil C and that flood irrigation resulted in a constrained increase in soil C content.