Annual grass invasion in a semi-arid shrubland affects seasonal N retention but not N mineralization patterns

Background/Question/Methods

Globally, semi-arid regions are experiencing grass- shrubland conversions. In Southern California wide-spread annual grass invasion and encroaching human development threaten formerly extensive native Coastal Sage Scrub (CSS) communities. Like in other Mediterranean ecosystems, water and temperature constrain plant community and soil nutrient dynamics in S. California. Invasive grasses begin to germinate following the first rain events of the season, in the fall. In contrast, native shrubs grow primarily in the spring. The success of invasive grasses is attributed in part to their ability to take advantage of soil N, prior to CSS N uptake in spring. The grasses and shrubs differ in phenology, litter quality, growth form, root dynamics and soil microbial community composition. As a result it is expected that grass invasion will alter the rate and timing of N mineralization.

Seasonal and plant community effects on net nitrogen mineralization, NO₃^- and NH₄⁺ pools and loss were measured under shrubs and grasses in an invaded CSS community. Soils were collected monthly from December 2009 to June 2010. Net N mineralization was determined using 15cm deep root exclusion cores which were left in the soil for a month, with resin bags at the base to capture N loss.

 Results/Conclusions

Seasonal changes in soil temperature, moisture and plant activity, but not plant community composition, affect N dynamics. However grass invasion dramatically reduces N loss, particularly during winter.

N mineralization is low in winter when soils are wet and temperatures low. As temperatures rise and plant activity increases from March to April, a large mineralization peak is observed, followed by high immobilization rates from April to May. At the end of the growing season, as soils dry, a slight increase in mineralization is observed.

The effects of plant community composition are most evident in NO₃^- retention. NO₃^- loss is high under shrubs from December to March but low under grasses throughout the season. From March to June NO₃^- loss does not differ between shrubs and grasses.

            Seasonal changes in moisture and temperature are the primary driver of soil N dynamics while invasion has a more subtle effect. Changes in N retention under grasses may be due to differences in microbial activity and C:N ratios rather than plant uptake, since N is lost from root exclusion cores. Further investigation of N movement between plants and microbes would shed light on the effect of grass invasion on N dynamics in CSS.