More Efficient Nitrogen Recycling or Less Efficient Carbon Use to Decompose Nitrogen-Poor Residues?

Nutrient limitation is widespread among microbial decomposers of plant litter, as they often feed on substrates with much wider carbon (C)-to-nutrient ratios than those of their biomass. How they cope with nutrient (especially nitrogen, N) limitation remains an open question. Possible responses include: upregulating N acquisition; respiring excess C and thus lowering microbial C-use efficiency (CUE, ratio of C used for growth over C taken up); or retaining N more efficiently in microbial biomass growing in N-poor litter. We tested these hypotheses using a minimal model of litter decomposition that includes preferential N acquisition, CUE, and N recycling efficiency as parameters, resulting in alternative model variants. Parameters encoding microbial responses to N limitation were fitted to more than 500 litter decomposition curves, which trace how litter N changes as a function of litter C in individual litter cohorts. The model indicates that preferential N acquisition is upregulated at high litter C:N ratios, but the performance of model variants with flexible N acquisition was worse than that of the other variants, suggesting that microbes might respond to N limitation by regulating CUE or N recycling. CUE decreased with increasing litter C:N when N was not recycled from senescent biomass and N recycling efficiency increased with increasing litter C:N when CUE was fixed. Both these model variants (flexible CUE or N recycling efficiency) had high fitting performance. Moreover, flexible N recycling supported higher microbial growth rates compared to flexible CUE. These model results indicate that flexible N recycling can explain patterns in N import and release in litter, while also being a physiologically more beneficial response to cope with N limitation than regulation of CUE.