Stoichiometric analyses of soil nutrients and enzymes in a Cambisol soil treated with inorganic fertilizers or manures for 26 years

The biogeochemical interactions between soil nutrients and microbial enzymes in agroecosystems remain poorly understood. The objective of this study was to determine effects of long-term (26 years) application of chemical fertilizers (NPK) or adding manure (M) on the elemental and the enzymatic functional stoichiometric characters in a brown soil. The concentrations of soil elements, i.e. carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) and the activities of soil hydrolases including alpha (beta)-galactosidase (alpha-GAL and beta-GAL), alpha (beta)-glucosidase(alpha-GLU and beta-GLU), urease, protease (PR), phosphomonoesterase (PM), Phosphodiesterase (PD) and arylsulfatase (AS) were investigated. Results showed that the NPK additions alone led to a decrease in soil pH and increased the acidification of the soils, while M additions could buffer the acidification. With doubling application rate of M, the concentrations of soil C, N, P and S, and soil enzymes activities increased significantly, but P concentrations increased stronger (doubled) than C and N contents (increased by 30%), leading to shifts in element stoichiometry in response to fertilizer and manure applications. The manure additions led to a relatively constant C:N ratio, a low C:P ratio and a high C:S ratio but lower functional ratios of In (beta-GLU + alpha-GLU + alpha-GAL + beta-GAL): In(urease + PR); In (GLU + GAL): (PM + PD); and In(GLU + GAL):ln(AS), which implied high availability of P and low availability of S. The comparison of element and enzyme stoichiometry indicated that N and S were limited in the manure plots, and C and P were limited in the NPK and No-NPK fertilizer treatment (WF) plots. The increase in enzyme activities in manure plots was caused primarily by the accumulation of soil nutrients with M treatment and not by a buffering effect on the pH.