Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover under a warmer climate may cause soil freezing events to become more common in northern temperate ecosystems. In this experiment, snow cover was manipulated to simulate the late development of snowpack and to induce soil freezing. This manipulation was used to examine the effects of soil freezing disturbance on soil solution nitrogen (N), phosphorus (P), and carbon (C) chemistry in four experimental stands (two sugar maple and two yellow birch) at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. Soil freezing enhanced soil solution N concentrations and transport from the forest floor. Nitrate (NO3-) was the dominant N species mobilized in the forest floor of sugar maple stands after soil freezing, while ammonium (NH4+) and dissolved organic nitrogen (DON) were the dominant forms of N leaching from the forest floor of treated yellow birch stands. Rates of N leaching at stands subjected to soil freezing ranged from 490 to 4,600 mol ha(-1) yr(-1), significant in comparison to wet N deposition (530 mol ha(-1) yr(-1)) and stream NO3- export (25 mol ha(-1) yr(-1)) in this northern forest ecosystem. Soil solution fluxes of P-i from the forest floor of sugar maple stands after soil freezing ranged from 15 to 32 mol ha(-1) yr(-1); this elevated mobilization of P-i coincided with heightened NO3- leaching. Elevated leaching of P-i from the forest floor was coupled with enhanced retention of P-i in the mineral soil Bs horizon. The quantities of P-i mobilized from the forest floor were significant relative to the available P pool (22 mol ha(-1)) as well as net P mineralization rates in the forest floor (180 mol ha(-1) yr(-1)). Increased fine root mortality was likely an important source of mobile N and P-i from the forest floor, but other factors (decreased N and P uptake by roots and increased physical disruption of soil aggregates) may also have contributed to the enhanced leaching of nutrients. Microbial mortality did not contribute to the accelerated N and P leaching after soil freezing. Results suggest that soil freezing events may increase rates of N and P loss, with potential effects on soil N and P availability, ecosystem productivity, as well as surface water acidification and eutrophication.