Changes in soil total, microbial and enzymatic C-N-P contents and stoichiometry with depth and latitude in forest ecosystems.

Soil microorganisms and their extracellular enzymes are key factors determining the biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P). Relevant studies mainly focus on surface soils (0-20 cm), while deep soils (>20 cm) are often neglected, let alone comparing multiple ecosystems simultaneously. In this study, we studied the latitudinal (19-48°N) and vertical (0-100 cm) patterns of soil total, microbial and enzymatic C-N-P contents and ratios (stoichiometry) in eight temperate, subtropical and tropical forest ecosystems in eastern China. We found that the C-N-P contents and their stoichiometry in soil, microbial biomass and extracellular enzymes all varied significantly with depth and latitude. Soil total C, N and P declined with depth, as did microbial biomass and enzyme activity, while microbial and enzymatic C:N ratios showed increasing or no trend with increasing soil depth. Moreover, soil total and microbial C-N-P contents in surface soils (0-20 cm) showed positive correlations with increasing latitude, and such correlations tended to be weaker or disappeared in deep soils (>20 cm). Overall, changes in total, microbial and enzymatic C-N-P contents and ratios among latitudes suggested a shift from relative N limitation in the north to relative P limitation in the south.

Nitrogen deposition has minor effect on soil extracellular enzyme activities in six Chinese forests.

Soil extracellular enzymes play a key role in mediating a range of forest ecosystem functions (i.e., carbon and nutrients cycling and biological productivity), particularly in the face of atmospheric N deposition that has been increasing at an unprecedented rate globally. However, most studies have focused only on surface soils in a single ecosystem. In this study, we aimed to determine whether the effect of simulated N deposition on the activities and ratios of soil enzymes changes with soil depth across six forest ecosystems in eastern China. We collected soil samples from three blocks×four soil depths (0-10cm, 10-20cm, 20-40cm and 40-60cm)×three N treatment levels (control, 50 and 100kgNha-1year-1) at each of the six forest ecosystems. We measured the activities of seven soil enzymes involved in C-, N- and P-cycling. We found that 4-5years of N addition had no significant effect on the activities and ratios of these enzymes in most cases. The interactions among N addition, site and soil depth on soil enzyme activities were not significant, except that acid phosphatase activity showed site-specific responses to N addition. Our findings suggest that the activities of soil enzymes involved in C- and N-cycling generally do not track simulated N deposition in the six forest ecosystems. Further work on plant, soil and microbial characteristics is needed to better understand the mechanisms of soil enzyme activities in response to N deposition in forest ecosystems.