Soil fertility promotes decomposition rate of nutrient poor, but not nutrient rich litter through nitrogen transfer

Background and aims Litter decomposition is a critical process in terrestrial ecosystems and understanding the effects of soil fertility on the litter decay rate is of great ecological relevance. Here we test the hypothesis that N transfer from soil to litter will promote the decay rate of N poor but not N rich litter types. Methods Ten organic substrates, encompassing a wide range of biochemical quality in terms of C/N and lignin/N ratios, were decomposed in microcosms over three soil types with different N content, but inoculated with the same microbiome. Organic substrates were characterized for mass loss, C and N content to assess N transfer from soil to litter. Results The decay rate response to soil fertility was related to their initial N content: positive for substrates with little initial N content and not significant for N rich plant residues. A significant N transfer, generally larger from N rich soil to N poor substrates, was found. Litter C/N and lignin/N ratios showed variable relationships with the litter decay according with the soil fertility gradient, with positive and negative correlations in N rich and N poor soils, respectively. Conclusions Our study demonstrated that the decomposition of N rich litter proceeded irrespective of soil fertility while the decay rate of N poor substrates, either lignin poor or rich, was controlled by soil fertility likely as a result of N transfer. Litter C/N and lignin/N ratios were reliable indicators of litter quality to predict their decay rate in N poor soil, but not in N rich soils.