Description: | MELNHE Overview
Although temperate forests are generally thought of as N-limited, resource optimization theory predicts that ecosystem productivity should be co-limited by multiple nutrients. These ideas are represented in the Multi-Element Limitation (MEL) model (Rastetter et al. 2012). To test the patterns of resource limitation predicted by MEL, we are conducting nutrient manipulations in three study sites in New Hampshire: Bartlett Experimental Forest, Hubbard Brook Experimental Forest, and Jeffers Brook in the White Mountain National Forest. Additional project information can be found at http://www.esf.edu/melnhe/default.htm
Site Description
At Bartlett, we have three replicate stands of three ages, young (~20 y), mid-aged (30), and mature (>100 years). We have identified stands at Hubbard Brook and Jeffers Brook that corresponds to the mid-aged and mature stands at Bartlett, for 4 more stands. Each of the 13 stands has four 1/4 ha (50 m x 50 m) treatment plots, treated annually beginning in spring 2011, with N (30 kg N/ha/yr as NH4NO3), P (10 kg P/ha/yr as NaH2PO4), N+P, or nothing (an untreated control). Five stands also have a Ca treatment plot (1150 kg Ca/ha in the form of CaSiO3).
We are monitoring stem diameter, leaf area, sap flow, foliar chemistry, leaf litter production and chemistry, foliar nutrient resorption, root biomass and production, mycorrhizal associations, soil respiration, heterotrophic respiration, N and P availability, N mineralization, soil phosphatase activity, soil carbon and nitrogen, nutrient uptake capacity of roots, and mineral weathering. This data set includes N mineralization data.
Sampling Design
In 2008 and 2009 (pretreatment), approximately 30, 2-cm diameter soil cores were collected throughout each plot and composited by horizon in order to best represent the average condition of each plot. Beginning in 2010 (also pretreatment), we established 4 soil sampling subplots in each plot in order to confine the disturbance associated with sampling and at the same time ensure that repeated sampling was always done in the same general area. In 2010 and thereafter 3-4 soil cores were collected in each soil sampling subplot and divided into horizons (Oe, Oa, 10 cm of mineral). All cores in one plot (12-16 total, from the 4 subplots) were mixed together within each horizon and homogenized for one composite sample per horizon per plot. The mineral soil horizon was not sampled in all years.
Laboratory Procedures
Samples were refrigerated until processing, and were processed within 4 days of collection (usually processed the day after collection). Samples were homogenized by gentle separation of organic fragments over a 4-mm sieve followed by thorough mixing. Fragments > 4 mm were not included. Soil moisture content was determined by drying subsamples at 60 degrees C to constant mass. Time-zero subsamples were extracted in 2M KCl, allowed to settle overnight (12-16 hr), and filtered. Paired subsamples were incubated in mason jars at 15-18 degrees C for approximately 21 days, and then were extracted in the same manner as time-zero samples. Time-zero and incubated soil extracts were analyzed for NH4+ and NO3- concentrations using a Lachat autoanalyzer.
Net N mineralization was calculated as the difference between incubated and time-zero NH4+ plus NO3- concentrations per g dry soil per day, and net nitrification was calculated as the difference between incubated and time-zero NO3- concentrations per g dry soil per day. |