Riparian forest zones are used for mitigation of agricultural and urban nonpoint source pollution. Phosphatase activity is ubiquitous in soil and sensitive to environmental perturbations, and hence, it may serve as an indicator of soil quality in riparian areas. We examined the relationship between phosphatase activity and selected soil physical and chemical properties and the spatial distribution patterns of phosphatase, organic matter, and moisture in a soil drainage catena within a riparian forest zone. Mean (%CV) phosphatase activity for the whole catena was 1.220 (40.6) μmol p-nitrophenol released/g/h (p-NP/g/h), with activity ranging from 0.152 to 2.544 μmol p-NP/g/h (n = 271). Poorly (PD), somewhat poorly (SPD), and moderately well drained (MWD) soil had significantly different (P < 0.01) mean (%CV) phosphatase activity of 1.516 (25.1), 1.327 (34.9), and 0.800 (39.9) μmol p-NP/g/h, respectively (n = 87-94). Phosphatase activity in each drainage class showed a significant, positive, and unique linear relationship with soil organic matter and moisture, and activity in MWD soil was also correlated positively with pH and water-extractable inorganic P levels. When data for the whole catena were considered, phosphatase activity was found to increase disproportionally with soil organic matter and moisture. Analysis of root mass distribution in a limited number of soil samples indicated that in SPD and PD soils roots constituted a significantly higher proportion of soil mass than in MWD soil. Furthermore, when mean phosphatase activity for each drainage class was considered, enzyme activity appeared to increase proportionally with the fraction of soil mass constituted by roots. Spatial structure of phosphatase activity was evaluated using spatial semivariance models. The fraction of total variance in phosphatase activity explained by the semivariance model, a measure of level of spatial structure exhibited by this variable, decreased in the order SPD>MWD>PD. The range value, representing the distance over which phosphatase was autocorrelated, differed markedly among drainage classes, with values of 12.2, 2.7, and 0.8 m for MWD, SPD, and PD soil, respectively. The range for organic matter and moisture was similar to that observed for phosphatase activity in SPD and PD soil; organic matter had a range similar to that of the enzyme activity only for MWD soil. Kriged maps of phosphatase distribution showed that spatial co-occurrence of phosphatase activity, soil organic matter, and moisture levels was most apparent in MWD soil, less so in SPD soil, and no spatial co-occurrence of these properties was apparent in PD soil. The spatial distribution of phosphatase activity in riparian forest soils appears to be partly controlled by position in the landscape, with organic matter and moisture as important factors in determining the distribution of phosphatase in moderately well and somewhat poorly drained areas, and less so in poorly drained areas. The distribution of roots may also be an important determinant of the spatial variability of phosphatase activity in riparian forest soils.

We investigated the effects of organic amendments (thermophilic compost, vermicompost, and coconut coir) on the bioavailability of trace heavy metals of Zn, Cd, Pb, Co, and Ni from heavy metal-spiked soils under laboratory conditions. To test switchgrass (Panicum virgatum) as a potential crop for phytoremediation of heavy metal from soil, we investigated whether the addition of organic amendments promoted switchgrass growth, and consequently, uptake of metals. Compost is a valuable soil amendment that supplies nutrients for plant establishment and growth, which is beneficial for phytoremediation. However, excess application of compost can result in nutrient leaching, which has adverse effects on water quality. We tested the nutrient leaching potential of the different organic amendments to identify trade-offs between phytoremediation and water quality. Results showed that the amendments decreased the amount of bioavailable metals in the soils. Organic amendments increased soil pH, electrical conductivity (EC), and soil nutrient status. Switchgrass shoot and root biomass was significantly greater in the amended soils compared to the non-amended control. Amended treatments showed detectable levels of heavy metal uptake in switchgrass shoots, while the control treatment did not produce enough switchgrass biomass to measure uptake. Switchgrass uptake of certain heavy metals, and concentrations of some leachate nutrients significantly differed among the amended treatments. By improving soil properties and plant productivity and reducing heavy metal solubility that can otherwise hamper plant survival, organic amendments can greatly enhance phytoremediation in heavy metal-contaminated soils.