Nutrient addition experiment in a California grassland

In 1998 we established 4 experimental blocks of 22 m x 30 m along Figueroa Creek in Sedgwick Reserve for a long-term nutrient addition project. Each of these blocks consist of 42 plots of 4m x 2m and 1m aisles. The blocks are located in different habitats that include high natural native abundance, no native abundance, and restored agricultural field with high native abundance. Note: the block E904 was fenced with barbed wire for cattle exclosure (area = 50m x 40m), and had one time seed addition on 11/27/1998 of locally (on Sedgwick) collected native perennial seeds: Nasella pulchra (1000g), Lupinus bicolor & Escholzia californica (500g), Mellica sp.(1120g). The block E905 was set up in established native perennials (sown in 1997, see Reichman & Seambloom restoration/gopher experiment). The native annual seeds sown in 11/27/1998 are: Lasthenia californica (0.13 lb), Lupinus bicolor /pygmy (0.5 lb), Trifolium gracilentum ( 1 lb), Trifolium microcephalum (0.5 lb), Vulpia microstachys (1 lb), Orthocarpus purpurascens (0.25 lb).

The experiment was split in 3 phases: 1) nutrient addition, 2) nitrogen gradient, and 3) nutrient x water addition.

The first part of the nutrient addition experiment included the non-factorial treatments: control, nitrogen, phosphorus, cations, water, and all. In total, there was 6 replicates per block, and control has 12 reps, each at a rate of 10 g/m^2, 3 times per year, beginning 12/1998 and continuing through 3/2000. Water was applied 3x manually using 400 gal tank (treatment halted in 4/1999 due to impracticability). Measurements taken: percent cover by species, total biomass, light penetration, soil chemistry and water. The nitrogen gradient part of the experiment was done across all 4 blocks, in 24 plots per block. This experiment started in 1999 and finished in 2007 (excluding 2002 and 2004). In 2008, we only used blocks E902 and E903. Measurements taken: percent cover by species, total biomass, light penetration, and soil chemistry (LeBauer: roots, soil carbon). Moreover, we also tested Nutrient x Water addition in 96 plots using factorial treatments: N x P x K x Water (drip irrigation). Experiment started in 2000, ending in 2007 (excluding 2002, and 2004). Measurements taken: percent cover by species, total biomass, light penetration. Harpole, W. S., and D. Tilman. 2007. Grassland species loss due to reduced niche dimension. Nature 446:791-793.

Moreover, these plots were used in a community composition experiment of annual and perennial invasion in gopher enclosures. Treatments include: annual or perennial initial seeded composition crossed with either N addition or water addition, in 88 plots (48 plots in the nutrient experiment, and 40 in the water experiment). Measurements were taken in 1999 and 2001, including nitrogen concentrations, mineralization, percent cover, biomass, and soil moisture. Seabloom, E. W., W. S. Harpole, O. J. Reichman, and D. Tilman. 2003. Invasion, competitive dominance, and resource use by exotic and native California grassland species. Proceedings of the National Academy of Sciences 100:13384-13389.