Do microbes regulate CUE or nutrient recycling to cope with nutrient limitation?

Nitrogen (N) limitation can have contrasting consequences on carbon (C) and N cycling in soils, depending on how soil microbes regulate their use of C and N. If microbes respond to N limitation by respiring or excreting more C (overflow hypothesis), C losses from the soil increase with decreasing N availability. In contrast, if under N limitation microbes use N more efficiently and rely less on the scarce available N, C and N can remain in the soil and possibly be stabilized. Efficient N use can be achieved by fungi via resorption of N from senescing mycelium, and in general via local recycling of N when cells die. Here we use a minimal model of litter decomposition to assess how microbes use C and N in litter types with contrasting N contents. The model is fitted to about 500 litter decomposition datasets to estimate microbial C use efficiency (CUE, defined as ratio of growth over C uptake) or N resorption efficiency. Model variants assuming that microbes regulate either their CUE or their N resorption can capture N accumulation and release well, but the latter variants have higher overall performance. This indicates that N resorption can be a fundamental mechanism to cope with N limitation. Moreover, N resorption efficiency as estimated from model fitting decreases with increasing initial litter N content or during decomposition as litter becomes enriched in N. This result implies that N resorption regulation can occur both across litter types with contrasting N contents, and during decomposition within a certain litter cohort. We conclude that N resorption is an ecologically more meaningful strategy to grow in N limited conditions compared to C overflow.