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 eve nts to become more common in northern temperate ecosystems. In this experim ent, snow cover was manipulated to simulate the late development of snowpac k and to induce soil freezing. This manipulation was used to examine the ef fects of soil freezing disturbance on soil solution nitrogen (N), phosphoru s (P), and carbon (C) chemistry in four experimental stands (two sugar mapl e 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 th e dominant N species mobilized in the forest floor of sugar maple stands af ter soil freezing, while ammonium (NH4+) and dissolved organic nitrogen (DO N) were the dominant forms of N leaching from the forest floor of treated y ellow birch stands. Rates of N leaching at stands subjected to soil freezin g 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 wi th 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 rel ative to the available P pool (22 mol ha(-1)) as well as net P mineralizati on rates in the forest floor (180 mol ha(-1) yr(-1)). Increased fine root m ortality 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 en hanced leaching of nutrients. Microbial mortality did not contribute to the accelerated N and P leaching after soil freezing. Results suggest that soi l freezing events may increase rates of N and P loss, with potential effect s on soil N and P availability, ecosystem productivity, as well as surface water acidification and eutrophication.