Spatial distribution of soil phosphatase activity within a riparian forest

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Riparian forest zones are used for mitigation of agricultural and urban nonpoint source pollution. Phosphatase activity is ubiquitous in soft 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 soft physical and chemical properties and the spatial distribution patterns of phosphatase, organic matter, and moisture in a soft 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) soft 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 soft organic matter and moisture, and activity in MWD soft 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 soft organic matter and moisture. Analysis of root mass distribution in a limited number of soft samples indicated that in SPD and PD soils roots constituted a significantly higher proportion of soil mass than in MWD soft. 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 soft, 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 soft. 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 soft, and no spatial co-occurrence of these properties was apparent in PD soft. The spatial distribution of phosphatase activity in riparian forest softs 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.

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Soil Science