William F. J. Parsons

Root Gap Dynamics in Lodgepole Pine Forest: Nitrogen Transformations in Gaps of Different Size

Belowground responses to aboveground disturbance were studied in experimental gaps created in a 95—yr—old stand of Pinus contorta in southeastern Wyoming. One—, 5—, 15—, and 30—tree clusters were felled to create a series of gaps in the root mat, and solution—phase N was monitored over two consecutive snow—melt periods via tension—tube water collectors. We hypothesized that dissolved and extractable nitrogen concentrations would not exceed predisturbance levels until a threshold canopy gap size had been achieved. As predicted, NOx—N attained significantly higher solution N concentrations (2—5 mg/L) only with the death of 15 trees or more. However, dissolved organic nitrogen decreased gradually with increasing gap size. Net mineralization and nitrification were studied using 30—d in situ incubation assays in each gap. Extractable nitrate routinely was negligible until the 30—tree gaps had been attained. Predicting the effects of disturbance on nutrient cycling, including timber—harvesting practices, requir...

ON THE NATURE OF ENVIRONMENTAL GRADIENTS : TEMPORAL AND SPATIAL VARIABILITY OF SOILS AND VEGETATION IN THE NEW JERSEY PINELANDS

1 Environmental variability can occur over various spatial scales, ranging from small patches at the scale of individual plants to long gradients over hundreds of metres. 2 In the New Jersey Pinelands, different species in the diverse shrub understorey of pitch pine (Pinus rigida Mill.) forests are patterned at these various scales. 3 Soil moisture, extractable NH4-N and N mineralization rate vary in complex ways, with the scale of spatial patterning changing over time and with depth in the soil profile. Moisture in both mineral and organic horizons, and NH4-N in the organic horizon, have patterns that are more stable over time than the mineralization rate in either horizon, or the NH4-N concentrations in the mineral horizon. 4 Vegetation patterns, as captured in principal components analysis, were poorly explained by any of the soil properties. Only the more temporally stable properties showed any relationship with vegetation patterns. 5 These results suggest that environmental gradients reflect patterns of environmental variation in four dimensions. Variation in the vertical dimension and over time is as pronounced and important as variation in the horizontal dimensions. 6 Many methods used to analyse vegetation implicitly assume temporal and spatial stability of environmental properties. Our results suggest that a more complex, fourdimensional assessment of environmental variation should be incorporated into models of vegetation-environment relationships.

Nitrate limitation of N2O production and denitrification from tropical pasture and rain forest soils

Nitrous oxide production was measured in intact cores taken from active pasture and old-growth forest Inceptisols in the Atlantic Lowlands of Costa Rica. Following additions of aqueous KNO3 or glucose, or the two combined amendments, the cores were incubated in the laboratory to determine if N2O production rates were either N-limited or C-limited in the two land use types. Differences in rates of denitrification (N22O + N2 production) among amended forest and pasture soils were determined by addition of 10% C2H2.The forest soils were relatively insensitive to all amendment additions, including the acetylene block. Forest N2O production rates among the treatments did not differ from the controls, and were consistently lower than those of the pasture soils. With the addition of glucose plus nitrate to the forest soils, production of N2O was three times greater than the controls, although this increase was not statistically significant. On the other hand, the pasture soils were definitely nitrogen-limited since N2O production rates were increased substantially beyond controls by all the amendments which contained nitrate, despite the very low N level (5 mg N kg−1 soil) relative to typical fertilizer applications. With respect to the nitrate plus glucose plus acetylene treatment, denitrification was high in the pasture soils; N2O production in the presence of C2H2 was 150% of the rate of N2O production measured in the absence of the acetylene block. The results are discussed in relation to the effects of agricultural land use practices and subsequent impacts of disturbance on N2O release.

Controls on nitric oxide emissions from tropical pasture and rain forest soils

In field studies, forest soils in the Atlantic Lowlands of Costa Rica emitted greater amounts of nitric oxide (NO) than soils from pastures that had been actively grazed for over 20 years following their conversion from forest. We measured NO production from intact soil cores from these land uses. Laboratory tests using ammonium(NH4+), nitrate (NO3−), nitrite (NO2−), water, and acetylene (C2H2) additions demonstrate a response consistent with field studies.Forest soil cores produced more NO than pasture cores regardless of treatment. In forest soil the response toNH4+solution was significantly greater than response to water or an ambient moisture control. Addition of 10 kPa C2H2 caused a marked decrease in NO production in forest soil cores. These responses suggest a nitrification-linked control over NO production. Large and rapid responses toNO2−additions suggest that chemical decomposition of this ion may contribute to NO production. Pasture soil cores did not show a significant response to any of the treatments including NO2−. Low porosity in the pasture soils may restrict emission of NO produced therein.

Above- and below-ground characteristics of persistent forest openings in the New Jersey Pinelands' 2

EHRENFELD, JOAN G., WEIXING ZHU AND WILLIAM F. J. PARSONS. (IMCS, Rutgers University, New Brunswick, NJ 08903). Above- and below-ground characteristics of persistent forest openings in the New Jersey Pinelands. Bull. Torrey Bot. Club. 122:298-305. 1995.-Openings in the pitch pine (Pinus rigida Mill.) forests of southern New Jersey may persist for decades. In five such openings, ranging in size from 22 to 223 m2, and their surrounding forest matrix, we characterized the vegetation, the forest floor, root biomass, soil chemistry and soil fungi. There was a much greater disparity between the openings and the forest matrix in the density of small ericad shrubs than in the number or basal area of the canopy trees, and this disparity was mirrored in the thickness of the litter and organic horizons, and in the total small root biomass. The matrix:gap ratios of total root biomass and of total soil fungal length were similar to the ratios for tree and large shrub densities. Extractable NH4-N, BrayP, pH, and soil moisture did not differ between the openings and matrix, but extractable N03-N was higher in the openings than the intact forest. We suggest that the shrubs may be an important factor in maintaining the differences between the openings and the matrix, by trapping litter, maintaining high root biomasses, and inhibiting decomposition. If the organic horizon is destroyed in a locally intense disturbance, the slow rate of invasion and growth of the shrubs into mineral soil may help perpetuate the opening for long periods of time.

The effects of live and dead roots on soil fungi in spodosolic soils of the New Jersey Pinelands

The effects of live and dead roots on soil fungi were investigated experimentally in a spodosolic soil of the New Jersey Pinelands. Field mesocosm plots were constructed to have a layer of either C- and N-rich organic soil or a vermiculite substitute overlying a layer of sandy mineral soil with a very low organic content. The plots were also supplied with live pitch pine and blueberry roots or dead pitch pine roots in varying quantities based on anturally occurring densities (half, same, and double the ambient quantities). All plots were sampled 1 year after construction (June 1991), and three more times in two subsequent years (November 1991, June 1992, June 1993). In the presence of live roots, fluorescein diacetate-determined (FDA-active) fungal hyphae, total fungal hyphae, and soil moisture decreased significantly in the organic material, while no change was associated with the dead roots. The FDA-active fungal length in the live-root plots ranged from 40 to 165 mg-1 soil, and from 55 to 335 mg-1 soil in the dead-root plots. While the total fungal length in live-root plots remained constant over time (∼3000 mg-1 soil), the total fungal length in the dead-root plots increased from an initial value of 3000 to >4000 mg-1 soil at the conclusion of the study. Fungal lengths in mineral soil were higher under organic material than under the vermiculite substitute. Soil moisture was higher in the presence of live roots in mineral soils, but this did not increase the fungal abundance. Inputs of dead roots did not alter the fungal abundance. Overall, we demonstrated that live and dead roots had different effects on fungal abundance in soils with contrasting qualities, and in a spodosolic forest soil, roots could have ecosystem effects very different from those in agricultural soils.