The effects of added nitrogen deposition on net nitrogen mineralization and microbial biomass in semi-arid shurblands

Background/Question/Methods

Anthropogenic activity has significantly increased the levels of N in the atmosphere that eventually deposit onto semi-arid shrublands. The increased N deposition has altered litter and soil organic matter quality (C:N ratio) , and presumably, levels net N mineralization and microbial biomass. We expect that semi-arid shrublands exposed to high levels of added N will have increased levels of net N mineralization and microbial activity, and we have been measuring in situ rates of net N mineralization in plots exposed to ambient and elevated (50 kgN ha^-1 yr^-1) N addition.  Soil samples in the upper 10 cm layer were collected monthly and incubated in perforated PVC tubes over monthly intervals beginning in December 2012. 

Results/Conclusions

Net N mineralization and nitrification were quantified as the change in NH₄ and NO₃, respectively over the monthly interval, and microbial biomass was measured using chloroform fumigation.  Previous results revealed that N addition significantly increased NH₄ and NO₃ availability; however, it was unclear whether net mineralization or microbial N uptake was affected by exogenous N.  These data will help determine how N addition alters N cycling dynamics in the field and how interactions between N availability and soil moisture alter microbial activity in semi-arid shrublands.