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, rc, i.e. the litter N:C ratio at which net N mineralisation commences. We show here how rc can be estimated from time series of decomposition and use four large data bases to investigate how environment and litter properties determine rc. We find that rc 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 rc we find an increase in SOM N:C. SOM N:C increases more rapidly than rc, which can be explained if the decrease in substrate quality during decomposition is taken into account.