Hello, I found a figure showing overall seasonal dynamics of total nitrogen (Ntot) in the soil with two periods of relatively stabile values of Ntot in the end of winter and summer. However I do not know source and reliability of the figure. Can someone direct me to the literature on seasonal dynamics of soil Ntot? If inter-annual comparisons of Ntot are to be made what is the best time for soil sampling?

Thanks a lot in advance, Libor