Our long-term monitoring and experimental manipulations are designed to determine the extent of N limitation in desert grassland and how shrub encroachment feeds back to alter the spatial distribution and quantity of soil nutrients.
I. Monitoring studies.
A. Ecohydrology. The island of fertility model (Schlesinger et al. 1990, 1996) describes a positive feedback loop between plant cover and resource accumulation, leading to patches (“islands”) of higher resource availability under plant canopies. Recent work at SEV suggests that differences between resource islands, such as microtopographic variation and the concentration of resources beneath plant canopies compared with open areas, decrease following fire and drought (Ravi et al. 2007, 2008). Thus, resource islands are more dynamic than previously thought, because prolonged drought and/or fire decrease spatial variability and alter patch structure (Ravi et al. 2009, 2010). One large-scale consequence of shrub invasion is a decrease in vegetation cover and an increase in surface runoff that transports nutrients and soil particles downslope (Turnbull et al. 2010a, b, c). The goal of our ecohydrological monitoring is to determine how rainfall pulses and fire affect nutrient, water, and soil particle transfers as shrub encroachment alters local surface hydrology and nutrient availability. Four 10m by 30m flumes were established across the grass-shrub ecotone to monitor nutrient and sediment content and amounts in runoff events triggered by monsoon precipitation events. Measurements include surface particle size distribution, soil organic matter, total N and C, and soil bulk density. Results to date show significantly higher transfer of soil particles, N and C in shrub- compared to grass-dominated areas (Turnbull et al. 2010a,c).
II. Experimental manipulations.
A. Nitrogen fertilization experiment (NFert). Nitrogen deposition is altering ecosystem structure and function globally (Stevens et al. 2004, Suding et al. 2005, LeBauer & Treseder 2008) and N availability is a key component of our TDND model. Specifically, the goals of NFert are to determine the effects of chronically elevated N availability on microbial community composition and structure, plant-fungal interactions, plant community composition and dynamics, and above- and belowground NPP. NFert contains 20 5x10m plots, 10 controls and 10 plots that receive 10 gN m-2yr-1 as NH4NO3. Minirhizotrons and root donuts (Milchunas et al. 2005) are installed in all plots (Corkidi et al. 2002, Johnson et al. 2003). Annual measurements include above- and belowground NPP and plant species composition (Ladwig et al. 2011). Periodic measurements include soil N availability, extracellular enzyme activities (Stursova et al. 2006, Zeglin et al. 2007), and comparative molecular analyses of fungal and cyanobacterial community composition (crusts), soil microbes, and fungal endophytes (Porras-Alfaro et al. 2008, 2009). Results to date show that soil, rhizosphere and endophyte microbial communities differ in response to N deposition (Porras-Alfaro et al. 2008). Also, aboveground NPP only increased in years of above average precipitation whereas belowground NPP had a lagged and multi-year pulse response to above average rainfall (Ladwig et al. 2011).
B. NutNet. SEV is a participant in the international Nutrient Network Collaboration (Firn et al. 2011, Adler et al. 2011). We have 5 replicates of each nutrient addition treatment (N, P, K singly and all combinations). Plots are 5mx5m. Soil nutrients are measured every three years, and plant community composition and aboveground NPP are measured twice per year.