Helga Van Miegroet

Nitrogen Fertilization Strategies in a Short-Rotation Sycamore Plantation

Abstract This study evaluates the effect of different nitrogen fertilization regimes in an American sycamore (Platanus occidentalis L.) plantation on tree growth, fertilizer recovery by the trees, and nitrate leaching from the soil with the objective of determining the optimum application regime. A total of 450 kg N ha−1 was added as urea over a 3 year period at the following rates: a single dose shortly after planting (O1); 150 kg N ha−1 applied once every year (AE); applications of 50 kg N ha−1 three times per year (P); annual doses that increased with tree growth (AB: 50, 150, and 250 kg N ha−1). Aboveground biomass production, N accumulation in biomass, and soil solution chemistry were measured between 1989 and 1992 in three replicate plots per treatment and compared with those in unfertilized control plots. The O1 fertilization was inefficient because growth benefits were short-lived and were associated with excessive NO3 leaching losses in the first year. Multiple fertilizer applications generally resulted in better growth, while soil solution NO3 levels depended on the actual rate and frequency of fertilization. Nitrate leaching was greatest in the AE plots. The AB treatment, in which annual N additions increased commensurate with tree size, was optimum in terms of enhancing stem biomass production and reducing potential groundwater contamination. The reduction in fertilizer use efficiency and the increase in NO3 leaching in the third year indicate that the input of 250 kg N ha−1 exceeded tree N retention capacity and should have been reduced to approximately one-half that rate. Increasing the frequency of fertilization from once per year to three times per growing season also decreased NO3 leaching losses (i.e. improved N recovery) without measurable benefits to biomass production, and may therefore not be cost-effective in commercial operations.

Sulfur Cycling in Five Forest Ecosystems

The cycling and retention of sulfur were studied in five forest ecosystems: a chestnut oak and yellow poplar stand on Walker Branch Watershed, Tennessee; a mixed oak stand on Camp Branch Watershed, Tennessee; and a red alder and Douglas-fir stand at the Thompson site, Washington. Calculations from foliage sulfur turnover indicate that about one-half of total sulfur input was dry in the Tennessee sites, whereas only one-tenth was dry in the Washington sites. Atmospheric sulfur inputs exceeded forest sulfur requirements in all cases, but three sites (chestnut oak, mixed oak, and red alder) showed a net ecosystem retention of atmospherically deposited sulfur. Net ecosystem sulfur retention was consistent with laboratory-determined sulfate adsorption isotherms within a given location (Walker Branch, Thompson site) but not between locations because of differing deposition histories and consequent differing degrees of soil sulfate saturation. No consistent relationships between soil sulfate adsorption capacity and other soil properties (pH, base saturation, iron, and aluminum oxides) were found.

Changes in Soil Properties and Site Productivity Caused by Red Alder

Red alder (Alnus rubra Bong.) is well recognized as an effective host plant for the symbiotic fixation of N. While this fixation process leads to the rapid accumulation of N within the ecosystem, it also enhances nutrient accumulation in biomass and soil organic matter and increases nitrification and cation leaching. We hypothesized that changes in soil properties resulting from these processes would decrease site productivity for second rotation red alder. Adjacent stands of 55 yr old alder and Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) were studied at the Thompson Research Center on the Cedar River Watershed in western Washington, USA. The presence of red alder oaused the following soil changes: decreased soil solution pH, increased CEC, increased exchangeable acidity accompanied by a decreased soil pH and base saturation. This decreased soil and soil solution pH resulted in increased A1 concentration in the soil solution and on exchange sites as well as decreased P availability. To determine the effect of these changes on the productivity of the 2nd rotation alder forest, a species conversion experiment was initiated 5 yr ago. Results from this conversion study clearly indicated that the first rotation red alder forest has caused a relative decrease in the productivity of the second rotation red alder plantation. Compared to the growth of red alder on the former Douglas fir site, the second rotation red alder on the former red alder site exhibited 33% less height growth and 75% less aboveground biomass accumulation after 5 yr. Future research will focus on identifying those factors causing this lower productivity including P availability, soil acidity and Al toxicity, cation availability, and competition with other vegetation.

Cation distribution, cycling, and removal from mineral soil in Douglas-fir and red alder forests

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha−1 yr−1, 1 kg Mg ha−1 yr−1, and 0.3 kg K ha−1 yr−1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28–88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha−1 yr−1, 9 kg Mg ha−1 yr−1, and 11 kg K ha−1 yr−1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations, was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO3-driven cation leaching.

Contributions of Acid Deposition and Natural Processes to Cation Leaching from Forest Soils: A Review

Methods of quantifying the roles of atmospheric acid inputs and internal acid generation by carbonic, organic, and nitric acids are illustrated by reviewing data sets from several intensively studied sites in North America. Some of the sites (tropical, temperate deciduous, and temperate coniferous) received acid precipitation whereas others (northern and subalpine) did not. Natural leaching by carbonic acid dominated soil leaching in the tropical and temperate coniferous sites, nitric acid (caused by nitrification) dominated leaching In an N-fixing temperate deciduous site, and organic acids dominated surface soil leaching in the subalpine site and contributed to leaching of surface soils in several other sites. Only at the temperate deciduous sites did atmospheric acid input play a major role in soil leaching. In no case, however, are the annual net losses of cations regarded as alarming as compared to soil exchangeable cation capital. These results were used to illustrate the methods of quantifying the ...