In this study, the dynamics of soil N and pH as affected by agricultural practices (e.g., N fertilization, flooding regime, cropping system) and climate change (simultaneous elevation of [CO2] and temperature) was investigated during the two consecutive rice seasons. Soil samples were collected at three different depths, before and after N fertilizations.
3.2.1. Seasonal changes of available N, non-exchangeable NH4+ and pH
3.2.1.1. Seasonal changes of available N
Available soil N varied significantly with years of study and date of sampling (p < 0.001) ( Table 3). Higher values of soil available N were observed in 2012 compared with 2011. At all sampling dates, before and after N fertilization, NH4+ was the dominant inorganic N form. The total available soil N content (NH4+ + NO3−) significantly rose after the basal dressing, from 5.3 to 10.2 mg N kg−1 at first sampling after this fertilization under aerobic conditions ( Table 2, Fig. 1). A significant decrease in total available N was observed after flooding till the topdressing (11 and 10 July, respectively in 2011 and 2012), in particular in NH4+ form. Immediately after topdressing and maintaining the flooding condition, the total available N significantly increased from 4.9 to 5.8 mg N kg−1, but decreased thereafter till the maturity. Overall, the total available N (NH4+ + NO3−) content at maturity was 1.7-fold lower than at seedling.