Effects of nitrogen and phosphorus addition on soil phosphatase activity in different forest types

Phosphorus( P) as a basic mineral nutrient is considered to constrain primary productivity in many tropical and subtropical forests. Soil phosphatase plays a very important role in P cycling in forest ecosystems because it catalyzes the hydrolysis of soil organic P compounds( e. g.,nucleic acids and phospholipids) into forms that are available to plants and soil microbes. Soil phosphatase activity is widely considered an effective indicator of the P demand of plants and microbes due to its ability to mediate plant and microbial nutrient acquisition from organic P compounds. In recent decades,increasing nitrogen( N) deposition due to human activity has been demonstrated to cause soil P deficiency and increase soil acid phosphomonoesterase activity( APA) in several tropical or subtropical forests. However,little is known about the effects of N deposition on soil APA in other forest types( e. g.,broadleaf forest and coniferous forest) or whether P addition may relieve soil P limitation in these forests. The present study investigated the responses of soil APA to N and P additions in a monsoon evergreen broadleaf forest( MEBF),a Pinus massoniana forest( PF),and a mixed broadleaf and pine forest( MF) in Dinghushan Mountain,Guangdong Province of southern China via a six- year fertilization experiment. The experiment used full factorial design,including four treatments: control( no fertilization),N addition( 150 kg N hm- 2a- 1),P addition( 150 kg P hm- 2a- 1),and combined N and P addition( 150 kg N hm- 2a- 1plus 150 kg P hm- 2a- 1).Each 5 m × 5 m plot was established with a surrounding buffer strip( 5 m wide). For each N and P application,NH4NO3 and Na H2PO4 solutions were applied below the canopy with a backpack sprayer,every other month from January 2007 to July 2013. In July 2013,soil samples were collected for analysis. Results showed that soil APA was significantly higher in MEBF(( 15. 83 ± 2. 46) μmol g- 1h- 1) than that in MF(( 10. 71 ± 0. 78) μmol g- 1h- 1) or PF(( 9. 12 ± 0. 38) μmol g- 1h- 1) soils,and a significant negative correlation existed between soil APA and soil available P contents in all forest types. N addition significantly increased soil APA in MEBF,while no statistical difference was found in MF or PF. P addition significantly decreased soil APA in MF and PF,but had no significant effect in MEBF. Combined N and P addition notably depressed soil APA in PF,but had no significant influence in MEBF and MF. Importantly,interactions between N and P additions were observed in MEBF. Based on our results,N deposition is expected to aggravate soil P deficiency in mature subtropical forest,while the N-induced P-limited state of these forests might be effectively relieved by P addition. In conclusion,the addition of P fertilizer may serve as an effective method for the sustainable future development of tropical and subtropical forests.