Trenched Plots Research Articles

Belowground carbon allocation in unfertilized and fertilized red pine plantations in northern Wisconsin.

We estimated carbon allocation to belowground processes in unfertilized and fertilized red pine (Pinus resinosa Ait.) plantations in northern Wisconsin to determine how soil fertility affects belowground allocation patterns. We used soil CO(2) efflux and litterfall measurements to estimate total belowground carbon allocation (root production and root respiration) by the carbon balance method, established root-free trenched plots to examine treatment effects on microbial respiration, estimated fine root production by sequential coring, and developed allometric equations to estimate coarse root production. Fine root production ranged from 150 to 284 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Coarse root production ranged from 60 to 90 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Annual soil CO(2) fluxes ranged from 331 to 541 g C m(-2) year(-1) and were significantly lower for fertilized plots than for unfertilized plots. Annual foliage litterfall ranged from 110 to 187 g C m(-2) year(-1) and was significantly greater for fertilized plots than for unfertilized plots. Total belowground carbon allocation ranged from 188 to 395 g C m(-2) year(-1) and was significantly lower for fertilized than for unfertilized plots. Annual soil CO(2) flux was lower for trenched plots than for untrenched plots but did not differ between fertilized and unfertilized trenched plots. Collectively, these independent estimates suggest that fertilization decreased the relative allocation of carbon belowground.

Effect of superphosphate addition on N mineralization in some Australian forest soils

Pinus radiata stands are reported to accumulate increased N on addition of phosphatic fertilizers. Field and laboratory studies were initiated to determine if addition of superphoshate increases soil N mineralization in forest soils. In a field experiment, application of 200 kg P ha-1 as superphosphate to trenched plots in a Pinus radiata stand increased accumulated soil mineral-N contents by 122% and 82% above the control on two occasions. Application of 500 kg P ha-1 as superphosphate either alone or in combination with lime (10 Mg ha-1) increased in situ soil net N mineralization in a dry sclerophyll eucalypt forest from 20.7 (control) to 28.3 (+P) and 30.2 (+P+lime) kg N ha-1 yr-1 20 cm-1. Uptake by the vegetation accounted for all of the mineralized nitrogen. In a 180 day laboratory incubation using undisturbed soil columns (0-20 cm) from nine forest sites, seven soils showed a positive response to superphosphate (100 kg P ha-1) addition, with increases in N mineralization ranging from 14% to 117%. The response of N mineralization to superphosphate addition was not related to initial soil organic C, total N or P content in the 0-20 cm layer. However, soil pH and organic C combined to account for 76% of the variation in N mineralization response (P < 0.01). In the 0-5 cm layer of untreated soils, soil organic P content could explain 71% of the variation in net N mineralization. Addition of superphosphate appears to increase N mineralization in several Australian forest soils and the improved availability of N will enhance tree growth rates. The mechanisms underlying the response are still poorly understood and thus also our ability to predict its significance for tree nutrition on specific forest sites.

EFFECTS OF AMENDING SOIL WITH PROCESSED MUNICIPAL WASTE ON GROWTH AND YIELD OF TOMATO

The effects of amending soil with processed municipal waste (PMW), and the interaction of PMW with trenching, irrigation rates, and fertilizer rates on growth, and yield of tomato plants were tested. In a series of experiments, two rates of each of the following PMWs were incorporated into calcareous limestone soil: 1) Agrisoil (processed trash), 2) Daorganite (processed sewage sludge), 3) Eweson compost (processed trash and sludge), and 4) no PMW (control). In some experiments, secondary applications of PMW were applied to the beds at either a high rate, a low rate or not applied (control). There was no effect of secondary PMW applications on growth or yield. Generally, plants grown on trenched plots had greater growth and yield than plants on non-trenched plots. Plants grown in Daorganite had greater growth and yield than plants grown in the other PMWs. Plants in Daorganite tended to have higher photosynthelic and transpiration rates than plants in the other treatments. For all treatments, plants grown at one-half the standard fertilizer rate had less growth than plants receiving higher fertilizer rates. There was no interaction between irrigation rate and PMW for photosynthesis, growth, or yield. Plants grown in Daorganite had the greatest growth and tended to have greater yields, regardless of the fertilizer or irrigation rate. Processed trash composts (Agrisoil and Eweson) did not increase growth and yield, which may have been due to suboptimal application rates of these materials. Further studies are underway incorporating higher rates of these materials into the soil.

Insitu needle and fine root respiration in mature slash pine (Pinuselliottii) trees

Respiration of needles and surface fine roots was measured in a north central Florida slash pine (Pinuselliottii Engelm. var. elliottii) plantation. A controlled temperature chamber system was used to estimate respiration rates and Q10 values of insitu tissues over a range of 10 to 35 °C. Respiration rates did not differ significantly among seasons, fertilized versus unfertilized plots, or time of day in a diurnal time series (needles). Needle respiration from the lower canopy was less than that from the upper canopy. Fine root respiration measurements were consistent with previously made estimates based on soil CO2 partitioning and trenched plots.

Litter decomposition in beech forests ? effects of excluding tree roots

The hypothesis proposed by Gadgil and Gadgil (1971) that mycorrhizal tree roots suppress the decomposition of forest litter was examined in trenching experiments. The tests were conducted in two beech forest sites in southern Sweden, one with mor and one with mull. The exclusion of tree roots had no apparent influence on litter mass loss at any of the sites, which differed markedly in soil fertility and root distribution. Nor did trenching have any significant effect on total organic matter over the following 6 years, although trenched plots tended to be wetter than undisturbed soil. The present results as well as results from other similar studies suggest that the suppression of litter turnover by mycorrhizal tree roots is not an important mechanism in northern temperate and boreal forests.

Soil CO2 evolution in Florida slash pine plantations. II. Importance of root respiration

Respiration of live roots was the single largest contributor to soil CO2 evolution in two mature slash pine (Pinuselliottii) plantations. Root respiration accounted for 51% of soil CO2 evolution at the 9-year-old plantation and 62% at the 29-year-old plantation. Additional estimates, calculated from data recorded from two small trenched plot sites at the 29-year-old plantation and based on possible variations in initial root biomass and subsequent decomposition rates, also averaged 62% of soil CO2 evolution. Specific root respiration averaged 0.40 g•g−1•year−1, varying from 0.34 to 1.70 g•g−1•year−1. Plots with larger proportions of fine roots had faster soil CO2 evolution rates.

Effects of trenching on soil processes and properties in a New Mexico mixed-conifer forest

Trenching was used to reduce root activity in treeless plots in a New Mexico mixed-conifer forest to examine the effects of plant roots on soil processes. Trenching led to increases in moisture content (104%), inorganic N concentration (115%), and mass loss from cellulose (196%). In laboratory incubations, trenched soils collected in the 1st and 2nd year after trenching evolved 52% and 115% more CO2, respectively, than control soils. Amending incubated trenched and control soils with moisture and inorganic N indicated that increased soil moisture content in trenched plots could explain the increased microbial activity. Trenching also had statistically significant but inconsistent effects on net N mineralization in incubated soils. The greatest effect of trenching was to increase net N mineralization under favorable temperature and moisture conditions. Irrigation of field plots increased both CO2 evolution and net N mineralization. Overall, these data are consistent with the hypothesis that plant roots reduced microbial activity by moisture uptake during the time of the study.

Responses of Understory Tree Seedlings to Trenching

The relative importance of aboveground and belowground competition to growth and establishment of understory tree seedlings was examined in the Piedmont region of North Carolina. First-year seedlings of three species-red maple (Acer rubrum L.), flowering dogwood (Cornus florida L.) and sourwood Oxydendrum arboreum (L.) DC]-were grown in trenched and untrenched plots under a forest canopy and within a windthrow. Survival and changes in height and leaf area were recorded. Nearly all maple seedlings survived. Those in windthrow plots were significantly larger than those in forest plots. Trenching had little significant effect on maple. Dogwood mortality was greater in untrenched than in trenched plots. Light (as estimated by canopy photographs) was significantly correlated with dogwood seedling size only in trenched plots. Availability of soil resourses, especially water, moderated the positive effect of decreased aboveground competition. Although all sourwood seedlings eventually died, those within the windthrow survived longer than those within the forest. Seedlings of this species are extremely scarce and apparently require both high light and mineral soil for successful establishment.

Water and Light in Relation to Pine Reproduction

Seedlings of certain species of pine often become established along the margins of forests (fig. 1) although they rarely become established within the forests. Seedlings growing at the margin must compete for water and minerals with the roots of trees comprising the stand but they are exposed to light of much higher intensity than if they were growing in the interior of the stand. These circumstances provide an opportunity to study the relative importance of light and water with respect to the establishment of pine seedlings. It is well known that in the Piedmont pine (Pinus taeda L., P. echinata Mill., P. virginiana Mill.) usually fails to reproduce under either pine or hardwood stands while many species of hardwoods grow under both pines and hardwoods. It was generally assumed at one time that failure of tree seedlings to survive under a forest canopy was the result of lack of light, and species failing to survive under such conditions were described as shade intolerant. Results of certain trenched-plot experiments performed during the past 40 years have been interpreted, however, as indicating that lack of water is more important than shade in the failure of seedlings to survive. It has been observed that if small areas under forest stands are protected from the root competition of the surrounding vegetation by trenching, seedlings of intolerant species sometimes become established. The only demonstrable environmental difference between such trenched plots and adjacent untrenched control plots is that the trenched plots usually contain more available soil moisture during periods of drought. It has therefore been argued that lack of available soil moisture rather than lack of light is responsible for the failure of intolerant species, such as many species of pine, to become established under a forest canopy. It seemed probable to the writers that the supply of water is as low in the marginal zone where pine seedlings survive as it is within the forest where they fail to survive. If this. were true it would suggest that pine seedlings are able to become established in the margin of a forest because they are exposed to higher light intensity than within the stand. In other words, possibly pine seedlings are more drought resistant in the open than in the shade. Therefore, it was decided to measure the moisture content of the soil along a transect extending from within the forest across the marginal fringe of pines into an adjacent open field to learn whether significant differences exist.