Estimating the critical N: C from litter decomposition data and its relation to soil organic matter stoichiometry

Carbon (C) and nitrogen (N) interact strongly during decomposition of litters with repercussions for stoichiometry of soil organic matter (SOM). A characteristic parameter describing this interaction is the critical N:C ratio, r_c, i.e. the litter N:C ratio at which net N mineralisation commences. We show here how r_c can be estimated from time series of decomposition and use four large data bases to investigate how environment and litter properties determine r_c. We find that r_c increases with increasing initial litter N:C and N fertilisation. The critical N:C responds also to large scale variations in environment but it is not clear which environmental variable drives this variability. With increasing r_c we find an increase in SOM N:C. SOM N:C increases more rapidly than r_c, which can be explained if the decrease in substrate quality during decomposition is taken into account.