Nitrogen addition increases microbial necromass in croplands and bacterial necromass in forests: A global meta-analysis

Increasing anthropogenic nitrogen (N) input has changed the global soil carbon (C) stock, yet the contribution of microbial necromass (amino sugars) to soil C with N addition is poorly understood. Here, we conducted a meta-analysis of 32 publications and evaluated the responses of microbial necromass to N addition. Our results showed that the overall effects of N addition significantly increased the fungal (glucosamine, GluN) and bacterial (muramic acid, MurN; galactosamine, GalN) necromass but did not significantly affect the total microbial necromass (total amino sugars). N addition effects on amino sugars were contingent on ecosystem type. In particular, N addition increased the contents of GluN, MurN, GalN, and total amino sugars in croplands, but in forests N addition only increased the contents of MurN. In croplands, the effects of N addition on microbial necromass depended on whether N was added alone or in combination with phosphorus (P) and potassium (K). Specifically, N addition alone did not significantly affect the contents of bacterial MurN and GalN, fungal GluN, and total amino sugars, while NPK addition significantly increased the contents of all individual (i.e., GluN, MurN, and GalN) and total amino sugars. In addition, high N addition rates (>150 kg N ha−1 yr−1), and long-term N addition (>10 years) significantly increased the contents of all individual and total amino sugars in croplands, possibly because of high N addition rates and long-term N addition may have stimulated microbial growth. Our results demonstrate that N addition increases microbial necromass in croplands and bacterial necromass in forests, providing critical information to improve the sequestration of microbially derived C with continued global anthropogenic N input.