Arboriculture & Urban Forestry 32(4): July 2006 185 Table 3. Soil organic matter pools from urban soils in Moscow, Idaho, and Pullman, Washington, in 2002 and 2003. Year 2002 Landscape type New mulch New residential Old mulch Old residential Park 2003 Street tree New mulch New residential Old mulch Old residential Park Street tree tSOM kg m−2 12.423 ab 8.574 c 10.448 bc 13.739 a 14.307 a 12.027 ab 13.529 ab 10.599 c 11.787 bc 15.740 a 15.942 a 13.830 ab maSOM kg m−2 7.458 b 6.277 b 8.229 a 10.259 a 10.955 a 8.320 a 9.055 b 7.927 b 9.501 a 12.301 a 12.828 a 10.608 a fPOM kg m−2 1.379 bc 0.941 c 0.980 c 2.033 a 1.978 a 1.741 ab 1.220 bc 1.047 c 1.018 c 1.900 a 1.777 a 1.492 ab cPOM kg m−2 2.606 a 1.000 b 0.853 b 1.053 b 0.985 b 1.319 b 2.216 a 1.197 b 0.886 b 1.073 b 0.913 b 1.173 b lSOM kg m−2 0.980 a 0.356 b 0.386 b 0.394 b 0.389 b 0.647 ab 1.038 a 0.428 b 0.382 b 0.466 b 0.424 b 0.557 ab fPOM C/N 15.5 a 17.9 a 20.5 a 16.3 a 15.7 a 20.5 a 15.9 a 19.8 a 20.4 a 16.6 a 16.1 a 20.5 a cPOM C/N 27.7 a 28.1 a 34.5 a 25.1 a 26.1 a 26.9 a 26.7 a 30.5 a 35.0 a 25.2 a 25.7 a 28.9 a 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). tSOM, total soil organic matter; maSOM, mineral-associated soil organic matter; fPOM, fine particulate organic matter; cPOM, coarse particulate organic matter; lSOM, and litter soil organic matter. but less than street, old residential, and park landscapes. Lit- ter SOM and coarse POM were significantly greater (34% to 66% and 41% to 68%, respectively) in new mulch compared with all urban landscapes except street sites, where litter SOM on street and new sites were not significantly different. To emphasize spatial differences of soil nitrogen, we focus on a comparison between new and old residential soils (Fig- ure 1). New and old residential landscapes had many soil- forming factors in common (i.e., climate, biota, relief, and parent material) (Jenny 1941). We suspect that the soil- Figure 1. A comparison of microbial biomass N, poten- tially mineralizable N, and fine particulate organic matter from old and new residential landscape soils of Moscow, Idaho, and Pullman, Washington, in 2003. Each value is the mean of seven sample dates, with four plots per type, two subplots per plot, for a total of 56. forming factor these landscapes differed mostly in was time since initial site disturbance. It is reasonable to assume that if spatial variability in soil nitrogen is evident when only time is different, additional variation can be expected when other soil-forming factors also vary among urban landscapes. Biologic measurements of nitrogen availability were sig- nificantly greater in old compared with new residential soils, MBN (71%) and PMN (83%). Potential carbon mineraliza- tion was greater (20%) in old compared with new residential soils. Negative values of PMN, indicating nitrogen immobi- lization, were measured in soils from new residential land- scapes on 9 of 16 sample dates, but not measured in soils from old residential landscapes. Nitrogen is more limited and tends to be immobilized when substrate C/N exceeds approxi- mately 25/1 (Brady and Weil 2002). The C/N ratios of the POM fractions from old residential landscapes (fine POM of 16.5 and coarse POM of 25.2) were less than on new resi- dential landscapes (fine POM of 18.9 and coarse POM of 29.3), likely contributing to the preference of nitrogen min- eralization over immobilization on old residential sites. Fine POM was significantly greater (50%) on old compared with new residential soils. The proportionate contribution to the total SOM of the fine POM fraction increased and the coarse POM fractions decreased in the older urban landscapes. Fur- thermore, the C/N ratios of the coarse POM fractions (mean of 28.4) were greater than the fine POM fractions (mean of 18.0). Because carbon is mineralized through microbial res- piration and nitrogen is conserved through microbial immo- bilization, the C/N ratio of the organic material will decrease with decomposition. Increases in finer-particle SOM and de- creases in the C/N ratio are indicative of soil organic stabi- lization and humification (Quideau and Bockheim 1996). ©2006 International Society of Arboriculture
July 2006
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