APSIM demonstrates the importance of rotations for simulating climate impact assessments.

In a recently published article Teixeira et al. (http://dx.doi.org/10.1016/j.envsoft.2015.05.012) used APSIM to assess the impact of different methods of representing the initial conditions of the soil.  In climate impact studies, weather data are commonly taken over a 20-30 year period to assess inter-annual variability of crop production.  Often, for simplification, individual crops (monocultures) are sown on the same date every year and soil water and nitrogen are reinitialised to default values prior to planting (re-initialised monoculture). However, in reality crops are often grown in a rotation and the soil conditions they encounter at planting are the result of the water and nitrogen balances of the preceding crops and fallow periods.  APSIM is able to construct realistic rotations and represent carryover effects of crop sequences. Teixeira et al. simulated a continuous wheat (grain) ® wheat (forage) ® kale (forage) ® maize (grain) rotation over a 30 years to compare with re-initialised mono-culture simulations.  The production, water use and soil nitrogen of simulated crops were all sensitive to the method of simulation (re-initialised mono-culture vs. continuous rotation) and the sensitivities were greatest when inputs (water and nitrogen) were lowest. This paper shows that greater emphasis should be placed on obtaining suitable initial conditions for simulating crop production, particularly for low intensity crop production systems.  It is difficult to achieve this in single crop simulations, which illustrates the benefit of representing the carryover of soil conditions across multiple crops grown in a sequence as performed with APSIM in this study.