184 Scharenbroch and Lloyd: Particulate Organic Matter and Soil Nitrogen Availability scapes (SAS Institute, Inc., Cary, NC). Linear regression analyses were performed to investigate soil measurements significantly correlated (P 0.05) with soil nitrogen avail- ability indices throughout the growing season across all sampled urban landscapes (SAS Institute, Inc.). RESULTS AND DISCUSSION Soil mineral nitrogen and dissolved organic nitrogen were not significantly different among the urban landscapes (Table 2). Mineral and dissolved organic nitrogen were found to vary erratically throughout the growing season on these urban landscapes, so much so that the time effect (time and time by treatment interaction P0.0001) confounded the differences among the environments. This is not surprising as these ni- trogen pools turnover quickly and repeatedly within the growing season (Khan et al. 2001; Mulvaney and Khan 2001). One-time sampling of mineral and dissolved organic nitrogen revealed minimal insight into the spatial variability of nitrogen availability of these urban landscapes. Conse- quently, we do not suggest assessing the nitrogen availability of an urban landscape with a one-time sampling of extract- able mineral or dissolved organic nitrogen. The interactions of time and treatments for microbial bio- mass and activity were orderly. Although the time and time by treatment interaction were significant (P 0.0001), the spatial differences were temporally consistent (Scharenbroch et al. 2005); thus, for these parameters, we present and dis- cuss spatial differences among these urban landscapes (Table 2). Microbial biomass nitrogen was found to be significantly greater (67% to 79%) on old residential and park landscapes compared with old and new mulch and new residential land- scapes. Street tree landscapes also had significantly greater (57% to 70%) microbial biomass nitrogen than mulched and 2002 Landscape type New mulch MIN (gNg−1 New residential Old mulch Old residential Park 2003 Street tree New mulch New residential Old mulch Old residential Park Street tree 10.3 a 8.0 a 5.2 a 5.9 a 9.4 a 10.0 a 6.0 a 4.4 a 4.6 a 5.8 a 7.8 a 7.5 a new residential sites, but less (16% to 34%) than park and new residential sites. Carbon mineralization was found to be greater (30% to 59%) on new mulch compared with old mulch sites. Carbon mineralization also tended to be greater (15% to 29%) on park, old residential, and street landscapes compared with new residential landscapes. Likewise nitrogen mineralization was significantly greater (62% to 97%) on park, old residential, and street landscapes compared with new residential landscapes. Nitrogen immobilization, de- tected by negative values of nitrogen mineralization, was found to occur only on new residential (9 of 16 sample dates), new mulch (4 of 16 sample dates), and old mulch (1 of 16 sample dates) landscapes. The time effects on the SOM fractions were not significant in 2002 (P > 0.14 for lSOM, cPOM, and fPOM) and signifi- cant but weak in 2003 (P 0.03 for all fractions). The interactions of time and treatments for the organic matter fractions were orderly (Scharenbroch et al. 2005), and spatial differences in organic matter fractions were significant among the urban landscapes (Table 3). Total SOM was sig- nificantly greater (23% to 46%) in old residential and park sites compared with new residential and old mulch sites. Min- eral-associated SOM on old residential and park sites was significantly greater (23% to 51%) compared with new resi- dential landscapes. Mineral-associated SOM on street sites was also significantly greater (5% to 41%) than new residen- tial sites. Fine POM was significantly greater (41% to 54%) on old residential and park compared with new residential and mulched sites. Fine POM from street tree landscapes was also significantly greater (30% to 40%) than from old mulched and new residential, but less than from old residen- tial and park landscapes. Fine POM from new mulch land- scapes was greater than from old mulch and new residential, Table 2. Soil nitrogen pools from urban soils in Moscow, Idaho, and Pullman, Washington, in 2002 and 2003. Year ) DON (gNg−1 3.0 a 1.2 ab 1.3 ab 1.9 ab 2.3 ab 0.8 b 1.7 a 2.0 a 2.1 a 2.3 a 2.1 a 1.8 a ) MBN (gNg−1 14.1 c 14.0 c 16.9 c 51.1 a 52.2 a 39.7 b 10.2 c 16.5 c 13.0 c 54.0 ab 60.5 a 42.9 b ) PMC (gCd−1 23.6 a 14.8 bc 12.5 c 19.6 ab 20.7 a 19.1 ab 17.8 a 15.2 ab 9.9 b 17.9 a 18.0 a 19.7 a ) PMN (gNd−1 0.38 cd 0.13 d 0.50 bc 0.60 bc 1.01 a 0.71 b 0.15 cd 0.03 d 0.18 cd 0.34 bc 0.67 a 0.56 ab Means with same letter are not significantly different at P 0.05. Each value is the mean of nine or seven sample dates (2002 and 2003, respectively), with four plots per type, two subplots per plot, for a total of 72 (2002) and 56 (2003). MIN, mineral N; DON, dissolved organic N; MBN, microbial biomass N; PMC, potentially mineralizable C; PMN, potentially mineralizable N. ©2006 International Society of Arboriculture )
July 2006
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