Spatial Variability in a Successional Plant Community: Patterns of Nitrogen Availability

Abstract

We examined the spatial variability of N mineralization, nitrification, and denitrification at a resolution of 1 m over a 0.5—ha portion of an old field in southeast Michigan. Net mineralization and nitrification rates were estimated from changes in ammonium and nitrate during 45—d laboratory incubations of soil from >300 individual sample locations. Denitrification was estimated from nitrous oxide accumulation rates during 24—h incubations of intact cores (n = 252) under acetylene atmospheres at a pressure of 10 kPa. We used geostatistical procedures to characterize the spatial distributions of these and other soil variates. Semivariograms for all three N transformations showed a high degree of spatial dependence among points sampled within 1—40 m of one another. Nugget variances were 27—37% of structural variances, indicating that most of the variation within the sample populations for these rates could be attributed to spatial autocorrelation at a scale >1 m. Isopleths calculated using punctual kriging algorithms show a nonuniform distribution of these transformations across the field. High rates of all processes occurred in swales on the northern edge of the sample area, but also occurred elsewhere in the field on drier, more level sites. These results indicate that spatial characteristics of the measured nitrogen transformations in this old field are complex, and that only some of this complexity is associated with surface topography. Whether spatial complexity affects or mainly reflects plant community structure is not known, but this small—scale heterogeneity may influence existing plant and microbial population dynamics and should be considered by those attempting to understand community dynamics or to quantify ecosystem—level nutrient fluxes.

On the Nature of Environmental Gradients: Temporal and Spatial Variability of Soils and Vegetation in the New Jersey Pinelands

“…Furthermore, we have found that the scale of spatial patterning of an environmental property is itself variable over time. Other studies have documented a high degree of spatial variation in soil properties at scales of centimetres to lOs of metres (Quesnel & Lavkulich 1980;Grigal et al 1991;Goovaerts & Chiang 1993;Robertson et al 1988Robertson et al , 1997, and a few studies have shown that different nutrients can have different temporal patterns of availability in the same soil (Gupta & Rorison 1975;Federer 1983), but there has been no demonstration that the spatial patterns themselves change over time. Our data suggest that environmental variation is best conceptualized as four-dimensional, in which patterns extending over horizontal planes and simultaneously over depth in the soil profile are continuously changing in time.…”

“…In most models of vegetation structure, it is implicitly assumed that variation in species occurrence is the result of their responses to variation in 787 Environmental variability in the Pinelands resource availability. For example, in an old-field community the N mineralization, nitrification and denitrification rates were strongly patterned on a scale of 20-40 m, and because this was similar to the scale of pattern in the vegetation, it was suggested that there was a causal relationship (Robertson et al 1988). This implicit assumption underlies both direct and indirect ordination techniques (Jonglnan et al 1987).…”

“…In sites without strong topographic relief, however, topographic position and its correlate, soil moisture, may have little influence on spatial patterns ofN mineralization . Variations in both N content and rates of transformation have also been frequently demonstrated on smaller spatial scales of centimetres to metres (Quesnel & Lavkulich 1980;Courtin et al 1983;Arp & Krause 1984;Robertson et al 1988;Grigal et al 1991). In some of these studies, the intent has been to assess variability in soil nutrients as a basis for estimating the optimal number of samples needed to characterize forest soils.…”

“…studies by Courtin et al 1983;Robertson et al 1988;Schlesinger et al 1996). However, soil is a three-dimensional environment, in which variation with depth and across different soil horizons may have profound implicat,ions for root growth and nutrient availability.…”

Landscape and Regional Scale Studies of Nitrogen Gas Fluxes

“…The most important transformations that lead to gas flux are nitrification, an aerobic process, and denitrification, an anaerobic process. Given that these processes have complex regulating factors and high variability in time and space, N gas fluxes often exhibit extreme variation (Foloronuso and Rolston 1984, Parkin 1987, Robertson et al 1988. Moreover, it is difficult to measure gas fluxes without disturbing the physical soil environment and/or the biological transformations that produce the fluxes, leading to frequent concerns that observed results are artifacts of a particular method (Groffman et al 1999).…”