186 Scharenbroch and Lloyd: Particulate Organic Matter and Soil Nitrogen Availability Current nitrogen fertilization standards, 1000 to 2000 kg N Figure 2. R2 correlations for fine particulate organic mat- ter, microbial biomass N, and N mineralization with site quality index in urban soils of Moscow, Idaho, and Pull- man, Washington, in 2003. Each value is the mean of 16 sample dates, with four plots per type, two subplots per plot, for a total of 128. We found our site quality index to be significantly and positively correlated with fine POM, microbial biomass N, and N mineralization (R2 values of 0.86, 0.90, and 0.84, respectively) (Figure 2). Our site quality index incorporated management practices, but was mostly affected by landscape age. We suspect the differences of soil nitrogen pools among these urban landscapes are primarily derived from variances in time since initial site disturbances (Scharenbroch et al. 2005). Old residential, street, and park landscapes are older urban landscapes with mature trees (mean of 64 years old) and longer times since initial site disturbance; conversely, new residential, old mulch, and new mulch sites are younger urban landscapes (mean of 9 years old). As urban soils de- velop after initial site disturbances soil bulk densities de- crease, clay contents in the upper mineral horizons decrease as clay particles migrate lower in the soil profile, fine to coarse POM ratios increase, the C/N ratios of soil organic matter become narrower, and the microbial activity and bio- mass of soils increase (Scharenbroch et al. 2005). We suspect that these pedologic processes have led to differences in ni- trogen availability in these urban landscapes. ha−1 yr−1 (2 to 4 lb of N 1000 ft−2 yr−1), are not site-specific (ANSI A300 2001). However, our results show that soil ni- trogen is significantly different among urban landscapes. Ni- trogen mineralization over a 200-day growing season on old residential sites accounts for approximately 1500 to 3,000 kg Nha−1 yr−1 (3 to 6 lb of N 1000 ft−2 yr−1) and on new residential sites only 150 kg N ha−1 yr−1 (0.3 lb of N 1000 ft−2 yr−1). Therefore, the nitrogen supply on old residential land- scapes may be as much as 10 to 20 times that of new resi- dential landscapes. Using blanket recommendations across these sites could result in overapplication of nitrogen on old residential sites and possible underapplication on new resi- dential sites. These results stress the importance of consider- ing the spatial variability of microbial indices in urban soils when making nitrogen management recommendations. Al- though microbial biomass and activity did not vary so much that the spatial differences were hidden like mineral and dis- solved organic nitrogen, these measures did change through- out the growing season. Consequently, urban landscape man- agers would have to consider temporal changes in these mi- crobial indices to use them to assess nitrogen availability. Ideally, urban landscape managers would be able to assess nitrogen availability over one or two growing seasons with a one-time sampling. To address this goal, we examined cor- relations of relatively stable soil organic matter pools with microbial biomass and activity. According to data from Buyanovsky et al. (1994), coarse POM turnover is estimated to be less than 1 year and mineral associated SOM turnover to be greater than 400 years (Table 4). Consequently, coarse POM turnover is too quick to be used to assess seasonal nitrogen availability. Because ma- SOM includes recalcitrant organic matter held on clay and silt particles or within clay lattices, it turns over too slowly to be used to assess seasonal nitrogen availability. Fine POM turn- over times are estimated to be 1 to 2 years and thus would be ideal to relate seasonal nitrogen availability. Linear regressions were performed for all SOM fractions with potential N mineralization and microbial biomass N on each sample date. Of the soil organic pools, fine POM mea- surements most consistently correlated with MBN and PMN throughout both growing seasons. The quantity of fine POM determined by the LOI method consistently and accurately Table 4. Physical soil organic matter fractions and their respective turnover times. SOM fraction from this study Coarse POM 2.0 to 0.25 mm Organic component* Fine POM 0.25 to 0.053 mm Mineral-associated SOM <0.053 mm *Data from Buyanovsky et al. 1994. SOM, soil organic matter; POM, particulate organic matter. ©2006 International Society of Arboriculture Vegetable fragments 2.0 to 0.2 mm Vegetable fragments 0.2 to 0.053 mm Silt associated 0.053 to 0.002 Fine clay associated <0.002 mm Turnover time* 0.5 to 1 yr 1 to 2 yr Approximately 400 yr Approximately 1000 yr
July 2006
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
