Resource allocation theory reveals sulfur shortage for microbes under phosphorus amendment in tropical forests with divergent land use history

Sulfur (S) availability plays key roles in the growth and health of microbes and plants, but little is known about how land use change and phosphorus (P) addition affect the soil S cycle. Based on a 10-year field nitrogen (N) and P addition experiment in three tropical forests (a primary forest, a rehabilitated forest and a disturbed forest), this study investigated the effects of land cover change, N addition and P addition on soil enzymes involved in carbon (C), N, P and S cycles. Results showed that the two secondary forests had lower S availability and higher microbial investments in arylsulfatase than the primary forest, indicating S deficiency in the two secondary forests. Correlation and path analysis showed that forest succession theory could explain the changes of absolute arylsulfatase activity while resource allocation theory explained the changes of specific arylsulfatase activity across the primary and secondary forests. Phosphorus addition had larger effects on soil properties and microbial biomass and activity compared to N addition. Phosphorus and NP addition significantly decreased extractable S by 27%∼43% in the three forests, resulting in increased absolute arylsulfatase activity (51%∼90%) and specific arylsulfatase activity (28%∼57%) in the primary and disturbed forests, but had little effect on absolute and specific arylsulfatase activity in the rehabilitated forest. Moreover, P and NP addition increased enzymatic stoichiometry of arylsulfatase to C and N-acquiring enzymes in the primary and disturbed forests, while tended to decrease ratios of arylsulfatase to C and N-acquiring enzymes in the rehabilitated forest. These results indicate that P fertilization with and without N addition can decrease soil S availability and induce S shortage for microbes. Results from this study advocate that we need to pay attention to S availability beyond N and P under the background of the decrease in atmospheric S deposition.