Douglas Schaefer

The failure of nitrogen and lignin control of decomposition in a North American desert

We measured mass losses of both buried and surface litter of six litter types: leaves of the perennial evergreen shrub, Larrea tridentata, leaves of the winter deciduous perennials Fluorensia cernua, Prosopis glandulosa and Chilopsis linearis (a desert riparian species), an evergreen monocot, Yucca elata, and a mixture of annual plants. These species differed in lignin content and carbon-nitrogen ratios. There was no correlation between rates of mass loss and percent lignin, carbon-nitrogen ratio, or lignin-nitrogen ratio. The leaves of F. cernua and the mixed annuals exhibited the highest rates of mass loss. Surface litter of Y. elata, the mixed annuals and C. linearis exhibited higher mass loss than buried litter of the same species. The patterns of mass loss of buried and surface litter differed with buried litter mass loss occurring as a negative exponential and surface litter exhibiting low rates in winter and spring and high rates in summer. There was no correlation between mass loss in surface bags that were field exposed for 1 month and actual evapotranspiration (AET) but there was a correlation between AET and mass losses in buried litter. A model relating mass loss to AET and initial lignin content underestimated mass losses in all species examined.

Throughfall Chemistry and Canopy Processing Mechanisms

Forest canopies receive chemical inputs from the atmosphere by rainfall, cloud droplet capture, and the accumulation of particles and vapors by dry deposition. These chemical inputs interact with surfaces in the canopy and are released to the forest floor primarily as throughfall (TF). A quantitative understanding of chemical fluxes in TF requires examination of the mechanisms of TF processing by forest canopy components. One such mechanism involves interactions between anthropogenic acidity inputs and the forest canopy, which may increase chemical fluxes in TF. The processes that control the inorganic chemistry of coniferous forest TF include atmospheric inputs from wet and dry deposition as well as physical, chemical, and biological processes that occur on forest canopy surfaces. A review of these processes suggests that dry deposition washoff, diffusion, uptake, and cation exchange control TF chemistry. In this chapter we use these processes to generate hypothetical patterns for ion-specific TF chemical fluxes. We compare these hypotheses to short-term sequential samples of the net ionic fluxes in TF and two coniferous forests that differ in atmospheric inputs. Substantial progress has been made toward the development of a general model of TF chemistry. Experiments are proposed to address the questions that remain for the development of such a model.

Labile carbon retention compensates for CO2 released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO2 may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of (13)C-labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO2 and priming of SOC decomposition over 6 months. Total and (13)C labeled CO2 were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose-C. Cumulative priming ranged from 1.3 to 5.5 mg C g(-1) SOC in the subtropical, and from -0.6 to 5.5 mg C g(-1) SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1-11.2 mg C g(-1) SOC or 41-56% of added glucose; tropical: 8.7-15.0 mg C g(-1) SOC or 43-75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO2 can increase C stocks in soil despite accelerated C cycling by priming in soils.

Effects of Hurricane Disturbance on Stream Water Concentrations and Fluxes in Eight Tropical Forest Watersheds of the Luquillo Experimental Forest, Puerto Rico

Stream water chemistry responds substantially to watershed disturb- ances, but hurricane effects have not been extensively investigated in tropical regions. This study presents a long-term (2.5-11 y) weekly record of stream water chemistry on eight forested watersheds (catchment basins) in the Luquillo Moun- tains of Puerto Rico. This includes a period before and at least 2 y after the disturbance caused by the 1989 Hurricane Hugo. Nitrate, potassium and ammo- nium concentrations increased after the hurricane and remained elevated for up to 2 y. Sulphate, chloride, sodium, magnesium and calcium showed smaller relative significant changes. Average stream water exports of potassium, nitrate and ammonium increased by 13.1, 3.6 and 0.54 kg ha -1 y -1 in the first post-hurricane year across all watersheds. These represent increases of 119, 182 and 102% respectively, compared to the other years of record. The increased stream outputs of potassium and nitrogen in the first 2 y post-hurricane are equivalent to 3% (potassium) and 1% (nitrogen) of the hurricane-derived plant litter. Effects of hurricanes on tropical stream water potassium and nitrogen can be greater than those caused by canopy gaps or limited forest cutting, but less than those following large-scale deforestation or fire.

Nutrient cycling by the subterranean termite Gnathamitermes tubiformans in a Chihuahuan desert ecosystem

SummaryWe estimated the density of subterranean termites Gnathamitermes tubiformans at 800,000 · ha-1 for a standing crop biomass of 2 kg · ha-1 Predation losses were estimated to be 5,73 kg · ha-1 · yr-1 representing the major release of nutrients from termites to surficial soil layers. Nutrient fluxes from termites to predators amounted to 410g N·ha-1·yr-1, 33 g S · ha-1 · yr-1 and 19 g P · ha-1 · yr-1. These fluxes account for 8% of the litter N, 1.5% of the litter P and 2.9% of the litter S. The termites fixed an estimated 66 g · ha-1 · yr-1 atmospheric N and returned an estimated 100 g · ha-1 · yr-1 in the surface gallery carton. Since losses of elements from subterannean termites were greater than standing crops, we estimated an annual turnover of N at 3.5 times per year, P of 2.5 times per year, and S of 2.5 per times per year.Since surface foraging, predation and alate flights are pulse regulated by rainfall, nutrient flows through subterranean termites are episodic and releases of nutrients accumulated in termite biomass preceeds or is coincident with productivity “pulses” of some shallow rooted plants. We propose that subterranean termites are important as regulators in desert nutrient cycles.

Comparison of arboreal and terrestrial soil characteristics in a lower montane forest, Monteverde, Costa Rica

Summary In many tropical and temperate forests, live and dead components of canopy-held organic matter (COM) form communities that are distinct from terrestrially rooted plant and forest floor soil communities, but that interact with whole-forest processes. We quantified some of the soil characteristics of dead organic matter held within the canopy of mature trees in a tropical lower montane forest of Monteverde, Costa Rica, and compared them to soils from the upper horizons on the ground. The concentration of canopy organic matter was significantly higher than terrestrial soil, but similar for P and Ca. Canopy humus had very low pH compared to terrestrial soils. The terrestrial soil had a tenfold greater amount of extractable cations, but the C/N ratios and cation exchange capacity of COM and the upper soil horizon did not differ significantly. Canopy organic matter has rarely been considered in forest ecosystem studies due to its inaccessibility, the lack of rigorous sampling and extrapolation methods, and because its mass is small relative to total forest soil mass. However, in habitats where COM is large, a canopy root-humus mat occurs on branch and trunk surfaces, similar to that which occurs on the forest floor. Organic matter in the forest canopy may thus have more ecological importance than its mass implies, as the nutrient-retaining capacity of the root-humus mat layer could play an important role in nutrient conservation for the individual trees and epiphytes whose roots are imbedded within the mats, and for the forest ecosystem as a whole.

IDENTIFYING SOURCES OF SNOWMELT ACIDIFICATION WITH A WATERSHED MIXING MODEL

We used ionic tracers to estimate the volume of old (soil and ground) water interacting with snowmelt in eleven Adirondack, NY watersheds. The contribution of old water varied from 66 to 90%, with no general relationship between old water % and soil depth to till. This approach also discriminated between watershed retention and release of particular ions to lake outlet water during snowmelt. Most watersheds released NO3− during snowmelt, in addition to the snowpack NO3−. Nitrification of snowpack NH4+ explained part of the additional NO3− in lake out outlet water, but some NO3− was likely mineralized nitrogen from soil organic matter. All watersheds retained NH4+ as well. Nitrogen release was greatest in the acidic watersheds in the southwestern Adirondacks, a region thought to be impacted by anthropogenic deposition. During snowmelt, Ca2+ and Mg2+ ions (presumably from soil exchange sites) were also released from most watersheds. In watersheds with acidic (minimum pH<4.6) lake outlet water, Al was also released during snowmelt. Thus, lake outlet water acidification during snowmelt was both buffered by cation release, and intensified by NO3− release. If the soil exchangeable cation pools were not replenished prior to snowmelt, or NO3− mobilization were increased, acidification during snowmelt would intensify.

Nitrogen biogeochemistry of three hardwood ecosystems in the Adirondack Region of New York

The biogeochemistry of nitrogen (N)was evaluated for three forest ecosystems[Woods Lake (WL), Pancake-Hall Creek (PHC) andHuntington Forest (HF)] in the Adirondackregion of New York, U.S.A. to evaluate theresponse of a range of N atmospheric inputsand experimental N additions. Bulk Ndeposition was higher at sites in the westthan those in the central and easternAdirondacks. These higher atmospheric N inputswere reflected in higher bulk throughfallfluxes of N (WL and PHC, 10.1 and 12.0 kg Nha−1 yr−1, respectively) in thewestern Adirondacks than at HF (4.6 kg Nha−1 yr−1) in the centralAdirondacks. Nitrogen was added to plots as(NH4)2SO4 at 14 and 28 kg Nha−1 yr−1 or as HNO3 at 14 kg Nha−1 yr−1. Litter decompositionrates of Fagus grandifolia and Acerrubrum were substantially higher at WL andPHC compared to HF but were not affected byexperimental N additions. Results usingmineral soil bags showed no effects of Naddition on N and C concentrations in soilorganic matter, but C and N concentrationincreases were less at WL and PHC compared toHF. Soil solution nitrate (NO3−)concentrations at 15-cm depth in the referenceplots were higher at PHC than at WL and HFwhile at 50-cm concentrations were higher atPHC and WL than at HF. The reference plots atthe two sites (WL and PHC) with the highestatmospheric inputs of N exhibited lower Nretention (53 and 33%, respectively) than HF(68%) in reference plots. The greatestincrease in NO3− loss in response tothe experimental treatments occurred at HFwhere the HNO3 additions resulted in thehighest NO3− concentrations andlowest N retentions. In contrast, at WL andPHC increases in soil water NO3−were not evident in response to experimental Nadditions. The results suggest that the twosites (WL and PHC) in the western Adirondacksdid not respond to additional N inputsalthough they have experienced elevatedatmospheric N inputs and higher N drainagelosses in reference plots than the HF site inthe central Adirondacks. Some of thesedifferences in site response may have alsobeen a function of stand age of WL and PHCthat were younger (24 and 33 years,respectively) than the HF (age ∼ 70).Highest NO3− fluxes in thereference plots across the sites correspondedto higher δ15N values in soil andplants. An experimental addition experimentat PHC found that the forest floor and themineral soil were the largest sinks forexperimentally added N.

Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum

Decomposition of organic matter (OM) in soil, affecting carbon (C) cycling and climate feedbacks, depends on microbial activities driven by C and nitrogen (N) availability. However, it remains unknown how decomposition of various OMs vary across global supplies and ratios of C and N inputs. We examined OM decomposition by incubating four types of OM (leaf litter, wood, organic matter from organic and mineral horizons) from a decay continuum in a subtropical forest at Ailao Mountain, China with labile C and N additions. Decomposition of wood with high C:N decreased for 3.9 to 29% with these additions, while leaf decomposition was accelerated only within a narrow C:N range of added C and N. Decomposition of OM from organic horizon was accelerated by high C:N and suppressed by low C:N, but mineral soil was almost entirely controlled by high C:N. These divergent responses to C and N inputs show that mechanisms for priming (i.e. acceleration or retardation of OM decomposition by labile inputs) vary along this decay continuum. We conclude that besides C:N ratios of OM, those of labile inputs control the OM decay in the litter horizons, while energy (labile C) regulates decomposition in mineral soil. This suggests that OM decomposition can be predicted from its intrinsic C:N ratios and those of labile inputs.

An observational study of the carbon-sink strength of East Asian subtropical evergreen forests

Relatively little is known about the effects of regional warming on the carbon cycle of subtropical evergreen forest ecosystems, which are characterized by year-round growing season and cold winters. We investigated the carbon balance in three typical East Asia subtropical evergreen forests, using eddy flux, soil respiration and leaf-level measurements. Subtropical evergreen forests maintain continuous, high rates of photosynthetic activity, even during winter cold periods. Warm summers enhance photosynthetic rates in a limited way, because overall ecosystem productivity is primarily restrained by radiation levels during the warm period. Conversely, warm climates significantly enhance the respiratory carbon efflux. The finding of lower sensitivity of photosynthesis relative to that of respiration suggests that increased temperature will weaken the carbon-sink strength of East Asia subtropical evergreen forests.