<p class=MsoNormal><span style="mso-spacerun: yes">?????????????????? </span>Stable isotopic analysis of nitrate (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) was used to trace nitrate sources and cycling under undisturbed conditions and following harvest at the Turkey Lakes Watershed (TLW), located near Sault Ste. Marie, Ontario, Canada. <span style="mso-spacerun: yes">?? </span> <p class=MsoNormal><span style="mso-spacerun: yes">?????? </span><span style="mso-spacerun: yes">????????????</span>Bulk precipitation collected biweekly at the TLW from 1995 to 2000 had nitrate isotope values that ranged from +42. 4 to +80. 4&permil; for <span style='font-family:Symbol'>d</span>¹⁸O and -6. 3 to +2. 8&permil; for <span style='font-family:Symbol'>d</span>¹⁵N. <span style="mso-spacerun: yes">?? </span>An incubation experiment indicated that the isotopic composition of atmospheric nitrate was not compromised by collection methods whereby unfiltered bulk precipitation samples remain in the collector for up to two weeks. <span style="mso-spacerun: yes">?? </span> <p class=MsoNormal><span style="mso-spacerun: yes">?????????????????? </span>The first direct measurement of the isotopic composition of microbial nitrate produced <i>in situ</i> was obtained by eliminating precipitation inputs to three forest floor lysimeters and subsequently watering the area with a nitrate-free solution. <span style="mso-spacerun: yes">?? </span>Microbial nitrate had <span style='font-family:Symbol'>d</span>¹⁸O values that ranged from +3. 1 to +10. 1&permil; with a mean value of +5. 2&permil;, only slightly higher than values predicted based on the <span style='font-family:Symbol'>d</span>¹⁸O-H₂O of the watering solution used. <span style="mso-spacerun: yes">?? </span><span style='font-family:Symbol'>d</span>¹⁸O values of soil O₂ (+23. 2 to +24. 1&permil;) down to a depth of 55cm were not significantly different from atmospheric O₂ (+23. 5&permil;) and therefore respiratory enrichment of soil O₂ did not affect the <span style='font-family:Symbol'>d</span>¹⁸O values of microbial nitrate produced at the TLW. <span style="mso-spacerun: yes">?? </span> <p class=MsoNormal><span style="mso-spacerun: yes">?????????????????? </span>Nitrate export from two undisturbed first-order stream basins was dominated by microbial nitrate, with the contribution of atmospheric nitrate peaking at about 30% during snowmelt. <span style="mso-spacerun: yes">?? </span>Clear-cutting of catchment 31 in 1997 resulted in elevated nitrate concentrations, reaching levels that exceeded the drinking water limit of 10 mg N/L. <span style="mso-spacerun: yes">?? </span>Isotopic analysis indicated that the source of this nitrate was predominantly chemolithoautotrophic nitrification. <span style="mso-spacerun: yes">?? </span>The <span style='font-family:Symbol'>d</span>¹⁸O values of microbial nitrate in stream 31 progressively increased during the post-harvest period due to an increase in the proportion of nitrification that occurred in the summer months. <span style="mso-spacerun: yes">?? </span>Despite drastic alteration of nitrogen cycling in the catchment by the harvest, <span style='font-family:Symbol'>d</span>¹⁵N-nitrate values in shallow groundwater did not change from the pre-harvest. <span style="mso-spacerun: yes">???? </span>Denitrification and plant uptake of nitrate in a small forested swamp in catchment 31 attenuated 65 to 100% of surface water nitrate inputs following harvest, reducing catchment-scale nitrate export by 35 to 80%.

<p class=MsoNormal><span style="mso-spacerun: yes">          </span>Stable isotopic analysis of nitrate (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) was used to trace nitrate sources and cycling under undisturbed conditions and following harvest at the Turkey Lakes Watershed (TLW), located near Sault Ste. Marie, Ontario, Canada. <span style="mso-spacerun: yes">  </span> <p class=MsoNormal><span style="mso-spacerun: yes">    </span><span style="mso-spacerun: yes">      </span>Bulk precipitation collected biweekly at the TLW from 1995 to 2000 had nitrate isotope values that ranged from +42. 4 to +80. 4&permil; for <span style='font-family:Symbol'>d</span>¹⁸O and -6. 3 to +2. 8&permil; for <span style='font-family:Symbol'>d</span>¹⁵N. <span style="mso-spacerun: yes">  </span>An incubation experiment indicated that the isotopic composition of atmospheric nitrate was not compromised by collection methods whereby unfiltered bulk precipitation samples remain in the collector for up to two weeks. <span style="mso-spacerun: yes">  </span> <p class=MsoNormal><span style="mso-spacerun: yes">          </span>The first direct measurement of the isotopic composition of microbial nitrate produced <i>in situ</i> was obtained by eliminating precipitation inputs to three forest floor lysimeters and subsequently watering the area with a nitrate-free solution. <span style="mso-spacerun: yes">  </span>Microbial nitrate had <span style='font-family:Symbol'>d</span>¹⁸O values that ranged from +3. 1 to +10. 1&permil; with a mean value of +5. 2&permil;, only slightly higher than values predicted based on the <span style='font-family:Symbol'>d</span>¹⁸O-H₂O of the watering solution used. <span style="mso-spacerun: yes">  </span><span style='font-family:Symbol'>d</span>¹⁸O values of soil O₂ (+23. 2 to +24. 1&permil;) down to a depth of 55cm were not significantly different from atmospheric O₂ (+23. 5&permil;) and therefore respiratory enrichment of soil O₂ did not affect the <span style='font-family:Symbol'>d</span>¹⁸O values of microbial nitrate produced at the TLW. <span style="mso-spacerun: yes">  </span> <p class=MsoNormal><span style="mso-spacerun: yes">          </span>Nitrate export from two undisturbed first-order stream basins was dominated by microbial nitrate, with the contribution of atmospheric nitrate peaking at about 30% during snowmelt. <span style="mso-spacerun: yes">  </span>Clear-cutting of catchment 31 in 1997 resulted in elevated nitrate concentrations, reaching levels that exceeded the drinking water limit of 10 mg N/L. <span style="mso-spacerun: yes">  </span>Isotopic analysis indicated that the source of this nitrate was predominantly chemolithoautotrophic nitrification. <span style="mso-spacerun: yes">  </span>The <span style='font-family:Symbol'>d</span>¹⁸O values of microbial nitrate in stream 31 progressively increased during the post-harvest period due to an increase in the proportion of nitrification that occurred in the summer months. <span style="mso-spacerun: yes">  </span>Despite drastic alteration of nitrogen cycling in the catchment by the harvest, <span style='font-family:Symbol'>d</span>¹⁵N-nitrate values in shallow groundwater did not change from the pre-harvest. <span style="mso-spacerun: yes">   </span>Denitrification and plant uptake of nitrate in a small forested swamp in catchment 31 attenuated 65 to 100% of surface water nitrate inputs following harvest, reducing catchment-scale nitrate export by 35 to 80%.

Dissolved organic matter (DOM) is present in all forested catchments and can be important in binding metals, absorbing UV, and the transport of nutrients (C, N, S, P). DOM is extremely heterogeneous in time and space, making it difficult to characterize. New techniques have been developed to determine &#948;34S and &#948;18O in DOM. These techniques have been applied to samples from Harp and Plastic Lake catchments (45??23'N, 79?? 08'W, 45??11'N, 78?? 50'W) in order to obtain information about sources and sinks of DOM within forested catchments on the Canadian Shield. In conjunction with sulphate and DOC concentrations, this new data provides valuable insight into sulphur cycling and DOM alteration within these catchments. Data generated for &#948;34S-DOM and &#948;18O-DOM appears to be the first data reported in the literature for DOM. The inorganic (&#948;34S-SO42-) and organic S (&#948;34S-DOM) differs by environment in both catchments. The range of &#948;34S-SO42- is between 3. 3&#8240; and 10. 3&#8240;, and the range of &#948;34S-DOM is from 3. 4&#8240; to 8. 7&#8240;. Sulphate in the Harp Lake catchment in most samples is subject to some sort of cycling within the watershed, since &#948;34S-SO42- differs from precipitation. In the Harp Lake catchment, upland &#948;34S-SO42- is influenced by historical precipitation. The &#948;34S-DOM is derived from leaching and microbial activity of DOM from organic horizons in the soil. The &#948;34S-SO42- and &#948;34S-DOM of wetland streams is extremely variable, controlled by hydrology. The &#948;34S-SO42- provides information on oxidation-reduction dynamics in the wetland, and &#948;34S-DOM provides information about sources of DOS in the wetland. The &#948;34S-SO42- and &#948;34S-DOM are possibly related in Harp Lake. Mineralization of DOS as evidenced by &#948;34S-DOM and DOS concentrations could be a small input of SO42- into Harp Lake. It is possible &#948;18O-DOM could be an indicator of DOM alteration. The range of &#948;18O-DOM is between 8. 2&#8240; and 14. 4&#8240;. The &#948;18O-DOM in the Harp Lake catchment is highly correlated with relative molecular weight, which has been shown to decrease with increasing alteration. Wetland streams show the largest range in &#948;18O-DOM, while uplands, groundwater, and Harp Lake are the least varied. The highest &#948;18O-DOM values are from sources of DOM such as leaf leachates (representative of forest floor litter) and wetlands. The most depleted samples are from groundwater and Harp Lake which typically contain highly altered DOM. The &#948;34S-DOM and &#948;18O-DOM can provide valuable information on sources of DOM and DOM alteration within the catchment. The &#948;18O-DOM could also allow the separation of autochthonous and allochthonous DOM in lakes.

Dissolved organic matter (DOM) is present in all forested catchments and can be important in binding metals, absorbing UV, and the transport of nutrients (C, N, S, P). DOM is extremely heterogeneous in time and space, making it difficult to characterize. New techniques have been developed to determine &#948;34S and &#948;18O in DOM. These techniques have been applied to samples from Harp and Plastic Lake catchments (45º23'N, 79º 08'W, 45º11'N, 78º 50'W) in order to obtain information about sources and sinks of DOM within forested catchments on the Canadian Shield. In conjunction with sulphate and DOC concentrations, this new data provides valuable insight into sulphur cycling and DOM alteration within these catchments. Data generated for &#948;34S-DOM and &#948;18O-DOM appears to be the first data reported in the literature for DOM. The inorganic (&#948;34S-SO42-) and organic S (&#948;34S-DOM) differs by environment in both catchments. The range of &#948;34S-SO42- is between 3. 3&#8240; and 10. 3&#8240;, and the range of &#948;34S-DOM is from 3. 4&#8240; to 8. 7&#8240;. Sulphate in the Harp Lake catchment in most samples is subject to some sort of cycling within the watershed, since &#948;34S-SO42- differs from precipitation. In the Harp Lake catchment, upland &#948;34S-SO42- is influenced by historical precipitation. The &#948;34S-DOM is derived from leaching and microbial activity of DOM from organic horizons in the soil. The &#948;34S-SO42- and &#948;34S-DOM of wetland streams is extremely variable, controlled by hydrology. The &#948;34S-SO42- provides information on oxidation-reduction dynamics in the wetland, and &#948;34S-DOM provides information about sources of DOS in the wetland. The &#948;34S-SO42- and &#948;34S-DOM are possibly related in Harp Lake. Mineralization of DOS as evidenced by &#948;34S-DOM and DOS concentrations could be a small input of SO42- into Harp Lake. It is possible &#948;18O-DOM could be an indicator of DOM alteration. The range of &#948;18O-DOM is between 8. 2&#8240; and 14. 4&#8240;. The &#948;18O-DOM in the Harp Lake catchment is highly correlated with relative molecular weight, which has been shown to decrease with increasing alteration. Wetland streams show the largest range in &#948;18O-DOM, while uplands, groundwater, and Harp Lake are the least varied. The highest &#948;18O-DOM values are from sources of DOM such as leaf leachates (representative of forest floor litter) and wetlands. The most depleted samples are from groundwater and Harp Lake which typically contain highly altered DOM. The &#948;34S-DOM and &#948;18O-DOM can provide valuable information on sources of DOM and DOM alteration within the catchment. The &#948;18O-DOM could also allow the separation of autochthonous and allochthonous DOM in lakes.

Phosphorus (P) export from forest soils is mainly driven by storm events, which induce rapid flow processes by preferential flow bypassing large parts of the soil matrix. However, little is known about the dynamics, magnitude, and driving processes of P exports into surface waters. In this paper, we present the results of a monitoring study in a small forested catchment (21 ha) situated in the low mountain ranges of Saxony, Germany. During the fixed schedule-sampling (weekly to bi-weekly sampling frequency for a three-year period), a mean total-P concentration of 8 μg·L−1 was measured. However, concentrations increased up to 203 μg·L−1 during individual storm flow events. Based on the analyzed concentrations and continuously measured discharge we calculated mean annual export rates of 19 to 44 g·ha−1·a−1 for the weekly sampling frequency with different load calculation methods. If events are included into the annual load calculation, the mean annual export fluxes can be up to 83 g·ha−1·a−1 based on the different load calculation methods. Predictions of total-P export rates based on a sampling strategy which does not consider short-term changes due to factors such as storms will substantially underestimate P exports.

Phosphorus (P) export from forest soils is mainly driven by storm events, which induce rapid flow processes by preferential flow bypassing large parts of the soil matrix. However, little is known about the dynamics, magnitude, and driving processes of P exports into surface waters. In this paper, we present the results of a monitoring study in a small forested catchment (21 ha) situated in the low mountain ranges of Saxony, Germany. During the fixed schedule-sampling (weekly to bi-weekly sampling frequency for a three-year period), a mean total-P concentration of 8 μg·L−1 was measured. However, concentrations increased up to 203 μg·L−1 during individual storm flow events. Based on the analyzed concentrations and continuously measured discharge we calculated mean annual export rates of 19 to 44 g·ha−1·a−1 for the weekly sampling frequency with different load calculation methods. If events are included into the annual load calculation, the mean annual export fluxes can be up to 83 g·ha−1·a−1 based on the different load calculation methods. Predictions of total-P export rates based on a sampling strategy which does not consider short-term changes due to factors such as storms will substantially underestimate P exports.