Exotic grasses alter controls over soil nitrogen dynamics in a Hawaiian woodland

Exotic invasive grasses and fire have altered plant species composition in the seasonal submontane woodlands of Hawaii Volcanoes National Park, These changes have affected both structural and functional aspects of the plant community, which could have consequences for soil nitrogen (N) dynamics and N availability to plants. To determine if, when, and how soil N dynamics were altered by grass invasion, we measured net and gross N mineralization and nitrification during wet and dry seasons across three vegetation types: (1) experimental grass removal plots within unburned woodland created to simulate the native ecosystem that may have existed prior to invasion; (2) woodland invaded by grasses; and (3) invaded woodland converted to grassland by fire. Grass invasion into woodland shifted the timing, but not the amount, of N available. After conversion to grassland, N-cycling rates were 3.4 times greater. The wet season accounted for 35% of annual net N mineralization in the grass removal treatment, 75% in the grass-invaded woodland, and 90% in the grassland. Soil transplant experiments showed that this shift was caused by an interaction between season and the effects of vegetation type on controls over soil N transformations. The effects of grasses on soil organic matter (SOM) composition enhanced net N mineralization during the wet season, whereas their effects on microclimate depressed net mineralization during the dry season. During the wet season, higher net rates in the grassland were caused by higher rates of gross N mineralization, which were negatively correlated with SOM carbon:N ratio. During the dry season, lower net N mineralization rates in the presence of grasses were due to a larger proportion of gross mineralization being immobilized, which was positively related to soil moisture. These results indicate that changes in plant species composition can alter soil N availability through effects on microclimate as well as more frequently studied effects on SOM composition. Both of these effects appear to have consequences for ecosystem function and should be considered in the design of restoration strategies for ecosystems impacted by exotic plants.