James H. McCutchan

Biogeochemistry of beetle-killed forests: Explaining a weak nitrate response

A current pine beetle infestation has caused extensive mortality of lodgepole pine (Pinus contorta) in forests of Colorado and Wyoming; it is part of an unprecedented multispecies beetle outbreak extending from Mexico to Canada. In United States and European watersheds, where atmospheric deposition of inorganic N is moderate to low (<10 kg⋅ha⋅y), disturbance of forests by timber harvest or violent storms causes an increase in stream nitrate concentration that typically is close to 400% of predisturbance concentrations. In contrast, no significant increase in streamwater nitrate concentrations has occurred following extensive tree mortality caused by the mountain pine beetle in Colorado. A model of nitrate release from Colorado watersheds calibrated with field data indicates that stimulation of nitrate uptake by vegetation components unaffected by beetles accounts for significant nitrate retention in beetle-infested watersheds. The combination of low atmospheric N deposition (<10 kg⋅ha⋅y), tree mortality spread over multiple years, and high compensatory capacity associated with undisturbed residual vegetation and soils explains the ability of these beetle-infested watersheds to retain nitrate despite catastrophic mortality of the dominant canopy tree species.

Uncertainty in the Estimation of Stream Metabolism from Open-Channel Oxygen Concentrations

The open-channel oxygen method for estimating stream metabolism avoids many of the problems associated with chamber techniques, but its uncertainty has not been rigorously quantified. Uncertainty in open-channel estimates of photosynthesis (P) and respiration (R) can be estimated by use of a Monte Carlo approach incorporating uncertainty in each of the terms (reaeration rate coefficient, range of temperature oscillation, midpoint temperature, travel time, metabolic rate, and precision of instrument calibration) affecting error in estimates of P and R. The distributions derived from the Monte Carlo simulations provide confidence limits for estimates of P and R. Use of this approach along with simulation of a range of stream conditions indicates that: 1) given equivalent metabolic rates and physical conditions, estimates of R are subject to greater uncertainty than are estimates of P, especially in high-gradient streams, and 2) uncertainty can be minimized by special attention to the precision of measurement for factors affecting the saturation concentration of oxygen. Reasonable confidence limits (95% CL within 30% of mean) can be achieved for estimates of P where daily photosynthetic rates exceed L-1d-1, but in turbulent streams (k20⚬=100/d), rates of R must be nearly 15 mg L-1d-1 to achieve similar precision.

Spatial and temporal patterns of denitrification in an effluent-dominated plains river

Denitrification, the microbial reduction of nitrate to gaseous forms (primari1y N2 but a1so N20), is an important mechanism for the remova1 of fixed N from aquatic systems. A1though denitrification rates tend to be higher in rivers than in other aquatic environments, rates of denitrification in rivers are high1y variab1e (PINA-ÜCHOA & ÁLVAREz-CosELAS 2006). Efficient denitrification in river sediments requires that sufficient nitrate and labi1e organic matter occur in combination with the proper redox conditions. Temperature a1so may 1imit the rate of denitrification, and seasona1 changes in denitrification rates often are driven by temperature (e. g., PFENNING & McMAHON 1996). Denitrification can occur over 1arge areas of a stream channe1 or may be 1imited to micro-sites that include the right combination of conditions. If any one of the requirements for denitrification (nitrate, organic matter, redox conditions, temperature) at a particular 1ocation i s insufficient, however, ra tes will be suppressed. Because the potentially 1imiting factors for denitrifying bacteria vary spatially and temporally within river networks, rates of denitrification can vary spatially and temporally, even over short periods of time and over short distances. Who1ereach estimates of denitrification are possib1e with isotopic tracers (e.g., MuLHOLLAND et al. 2004), but estimates with 15N have been 1imited to small streams due to the prohibitive cost of isotopic tracer additions in 1arge rivers. Who1e-reach estimates of denitrification also are possib1e through mass ba1ance oftransport and transformation rates (HILL 1981, SJODIN et al. 1997, PRIBYL et al. 2005), but accumu1ation of measurement errors can affect the precision for estimates of denitrification with this approach (CoRNWELL et al. 1999). Recently, an openchanne1 N2 approach has been deve1oped for the estimation of denitrification in running waters (LAURSEN & SEITZINGER 2002, McCuTCHAN et al. 2003). This method, which is analogous to the open-channe1 method for estimation of oxygen metabo1ism, has been tested extensive1y on the South Platte River in Co1orado (PRIBYL 2002, McCuTCHAN et al. 2003, PRIBYL et al. 2005). The open-channe1 method provides high precision and is well suited to the study of spatia1 and temporal patterns of denitrification at the reach scale. The purpose ofthis study is to describe the spatia1 and tempora1 patterns of denitrification in the South Platte River below Denver, Colorado. Although the open-channel N2 method has simp1ified estimation of denitrification, there are still relative1y few system-level estimates of denitrification for running waters (PINA-ÜCHOA & ÁLVAREz-CoBELAS 2006). Examination of the spatial and tempora1 patterns of denitrification in the South P1atte River may contribute to a better understanding of the controls on denitrification in running waters and may improve predictions of denitrification across a wide range of running waters.

Seasonal variation of phosphorus in precipitation at Hubbard Brook Experimental Forest

Nitrogen and sulfur are important components of atmospheric deposition and have received considerable attention in the last three decades (e.g. LIKENS & BORMANN 1995). Other elements, such as phosphorus (P), generally comprise a smaller fraction of atmospheric deposition and have been measured less frequendy. Atmospheric deposition of P can be an important source of nutrients to undisturbed aquatic ecosystems (MIGON & SANORONI 2000), and in ecosystems where P availability is low, seasonal variation in P inputs could potentially comribute to temporal variation in populations of biota or ecosystem processes that are P limited. Atmospheric deposition of P to ecosystems can occur in wet deposition and dry fallout. Forms of phosphorus in atmospheric deposition indude soi! partides, aerosols from sea spray, fine leaf fragments, pollen, fungal spores and microorganisms (GRAHAM & DucE 1979, NEWMAN 1995). Phosphorus deposition to a particular ecosystem can originate from local, regional and global sources. Heavy partides in atmospheric deposition (e.g. large soi! partides or leaf fragments) tend to originate locally, as they are not transported far in the atmosphere. Lighter partides (e.g. fine dust) can travel hundreds or thousands ofkilometers in air masses (e.g. AVILA et al. 1998). Phosphorus in deposition is usually measured in collectors that are placed at various locations across the ecosysrem being studied. In forest ecosystems, collectors are best placed in a large dearing so that inputs from local sources of phosphorus (e.g. pollen from local trees) can be minimized. Bulk deposition collectors (LIKENS et al. 1967) measure P in wet and dry deposition. Wet-only collectors are designed to measure P in wet deposition only (GALLOWAY & LIKENS 1976). The objectives of this study were to: (l) describe seasonal variation in the orthophosphate concentration in bulk deposition in the Hubbard Brook Experimental Forest (HBEF) during a 13-year period, (2) compare the P concentration between bulk and wet-only collectors to gain some insight into the relative contributions of dry and wet-only inputs, and (3) compare seasonal variation of P in bulk deposition to that of stream water from an experimemal watershed at HBER