Long-term fertilization affects microbial necromass accumulation by regulating nutrient and enzymatic stoichiometry in a calcareous Fluvisol

Abstract

Microbial necromass accumulation through rhizodeposition plays a critical role in increasing the stable soil organic carbon pool, which is greatly affected by fertilization. However, the legacy effects of long-term fertilization on microbial necromass accumulation remain unclear. Here, we selected a calcareous Fluvisol treated with no fertilizer (Ctrl), P and K fertilizers (PK), N and K fertilizers (NK), and N, P and K fertilizers (NPK) for 34 years and conducted a 12-week microcosm experiment with simulated daily rhizodeposition to disentangle the legacy effects of long-term fertilization on microbial necromass accumulation. We measured soil C, N and P contents and C-, N- and P-acquiring extracellular enzyme activities and analyzed phospholipid fatty acids (PLFAs) and amino sugars to indicate microbial biomass and necromass, respectively. We found that compared with the Ctrl and NK treatments, the PK and NPK treatments significantly decreased the C:N and C:P ratios and the net accumulation of microbial necromass but significantly increased the ratios of C- to N- and C- to P-acquiring enzyme activities and the fungal PLFAs abundance in soils. The net accumulation of microbial necromass was positively correlated with soil nutrient stoichiometry (mainly the C:N and C:P ratios) but negatively correlated with soil enzymatic stoichiometry (mainly the ratios of C- to N- and C- to P-acquiring enzyme activities) and the fungal to bacterial PLFA ratio. Taken together, these results suggest that long-term fertilization has legacy effects on microbial necromass accumulation and that fertilization-induced changes in nutrient and enzymatic stoichiometry regulate microbial necromass accumulation through rhizodeposition in the calcareous Fluvisol.