Global land use patterns and increasing pressures on water resources demand creative urban stormwater management. Strategies encouraging infiltration can enhance groundwater recharge and water quality. Urban subsoils are often relatively impermeable, and the construction of many stormwater detention best management practices (D-BMPs) exacerbates this condition. Root paths can act as conduits for water, but this function has not been demonstrated for stormwater BMPs where standing water and dense subsoils create a unique environment. We examined whether tree roots can penetrate compacted subsoils and increase infiltration rates in the context of a novel infiltration BMP (I-BMP). Black oak (Quercus velutina Lam.) and red maple (Acer rubrum L.) trees, and an unplanted control, were installed in cylindrical planting sleeves surrounded by clay loam soil at two compaction levels (bulk density = 1.3 or 1.6 g cm(-3)) in irrigated containers. Roots of both species penetrated the more compacted soil, increasing infiltration rates by an average of 153%. Similarly, green ash (Fraxinus pennsylvanica Marsh.) trees were grown in CUSoil (Amereq Corp., New York) separated from compacted clay loam subsoil (1.6 g cm(-3)) by a geotextile. A drain hole at mid depth in the CUSoil layer mimicked the overflow drain in a stormwater I-BMP thus allowing water to pool above the subsoil. Roots penetrated the geotextile and subsoil and increased average infiltration rate 27-fold compared to unplanted controls. Although high water tables may limit tree rooting depth, some species may be effective tools for increasing water infiltration and enhancing groundwater recharge in this and other I-BMPs (e.g., raingardens and bioswales).

Quantifying global soil respiration (R ) and its response to temperature change are critical for predicting the turnover of terrestrial carbon stocks and their feedbacks to climate change. Currently, estimates of R range from 68 to 98 Pg C year , causing considerable uncertainty in the global carbon budget. We argue the source of this variability lies in the upscaling assumptions regarding the model format, data timescales, and precipitation component. To quantify the variability and constrain R , we developed R models using Random Forest and exponential models, and used different timescales (daily, monthly, and annual) of soil respiration (R ) and climate data to predict R . From the resulting R estimates (range = 66.62-100.72 Pg), we calculated variability associated with each assumption. Among model formats, using monthly R data rather than annual data decreased R by 7.43-9.46 Pg; however, R calculated from daily R data was only 1.83 Pg lower than the R from monthly data. Using mean annual precipitation and temperature data instead of monthly data caused +4.84 and -4.36 Pg C differences, respectively. If the timescale of R data is constant, R estimated by the first-order exponential (93.2 Pg) was greater than the Random Forest (78.76 Pg) or second-order exponential (76.18 Pg) estimates. These results highlight the importance of variation at subannual timescales for upscaling to R The results indicated R is lower than in recent papers and the current benchmark for land models (98 Pg C year ), and thus may change the predicted rates of terrestrial carbon turnover and the carbon to climate feedback as global temperatures rise.

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Many bottomland tree species are tolerant of compacted soil and perform well in urban environments; however, the mechanism underlying this tolerance is unknown. Increased soil water content has been shown to alleviate some of the effects of soil compaction on plant growth, presumably because increasing soil water reduces soil strength. We hypothesized that tree species tolerant of very wet soils would have opportunities for root growth in compacted soil when high soil water contents reduced soil strength, whereas species intolerant of bottomland conditions would not. We tested this hypothesis on flowering dogwood (Cornus florida L.), a mesic species intolerant of inundation, and silver maple (Acer saccharinum L.), a bottomland species. Seedlings of both species were grown in pots for 21 and 30 days, respectively, in a growth chamber in native loam soil maintained at various combinations of soil strength and soil water tension. Downward root growth rate decreased in response to increasing soil strength in both species. At low soil strength (0.6 MPa), downward root growth rate of dogwood seedlings slowed when soil was either excessively wet or dry, whereas root growth rate of silver maple seedlings increased linearly with soil water content. In moderately compacted soil (1.5 g cm(-3) bulk density), silver maple seedlings had greater root growth rate, root length per plant, and ratio of root length to root dry weight in wet soil (0.006 MPa soil water tension) than in moist and dry soils (0.026 and 0.06 MPa, respectively), even though mean oxygen diffusion rate (ODR) was only 0.28 &mgr;g cm(-2) (SE = 0.05). No such effect was detected in highly compacted soil (1.7 g cm(-3) bulk density) in either species. Mean ODR showed a weak positive correlation with soil water tension (r = 0.40, P = 0.07), but was unrelated to soil strength. We conclude that silver maple roots can grow in moderately compacted soil when high soil water content decreases soil strength, whereas dogwood is unable to take advantage of this opportunity.

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Between 1960 and, the global soil respiration (R SG ) flux increased at a rate of 0.05 Pg C year À1 ; however, future increase is uncertain due to variations in projected temperature and regional heterogeneity. Regional differences in the sensitivity of soil respiration (R S ) to temperature may alter the overall increase in rates of R S because the R S rates of some regions may decelerate while others continue to rise. Using monthly global R S data, we modeled the relationship between R S and temperature for the globe and eight climate regions and estimated R SG between 1961and 2100 using historical and future (2015-2100) temperature data [Representative Concentration Pathways (RCP2.6 and RCP8.5)]. Importantly, our approach allowed for estimation of regional sensitivity, where respiration rates may peak or decline as temperature rises. Estimated historical R SG increase (0.05 Pg C year À1 ) was similar to the R SG increase of previous estimates. However, under the RCP8.5 scenario, which estimates approximately 3°C of warming globally, the forecasted acceleration of R SG increased to an average of 0.12 Pg C year À1 . Under RCP8.5, the temperature sensitivity of R S declined in the arid, winter-dry temperate, and tropic. These regional declines were offset by increased R S sensitivity and fluxes from the boreal and polar regions. In contrast, under RCP2.6 R SG decelerated slightly from current rates. If rising greenhouse gas emission remains unmitigated, future increases in R SG will be much faster than current and historical rates, thereby possibly enhancing future losses of soil carbon and contributing to positive feedback loops of climate change.

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