Freeze-thaw cycle amplitude and freezing rate effects on extractable nitrogen in a temperate old field soil

Freeze-thaw cycles can promote soil N losses as a result of microbial and root cell lysis; however, minimal freeze-thaw effects have typically been observed in studies that have imposed moderate temperature cycles. We conducted laboratory incubations on surface soil (top 3 cm) collected in a temperate old field from late fall through mid-winter to examine how variation in freeze-thaw amplitude, number, timing of collection, and freezing rate altered soil extractable N. We varied freeze-thaw amplitude by imposing minimum cycle temperatures of 0, -1, -2, -5, and -10A degrees C for a series of either one or two cycles and held control samples constant at 3A degrees C. We also examined the effects of freezing rates of 1, 3, and 30A degrees C h(-1). We hypothesized that extractable N would be highest for both the maximum freezing amplitudes and rates. While multiple freeze-thaw cycles at -10A degrees C and freeze-thaw cycles associated with artificially high freezing rates increased extractable N, freeze-thaw cycles representative of field conditions at our site had no effect on extractable N in late fall and early winter. By mid-winter there was a significant freeze-thaw cycle effect but, contrary to our prediction, less N was extracted from freeze-thaw treated samples than from the control samples, which remained thawed over the treatment period. Increased extractable N in control samples was driven by increased organic N rather than increased inorganic N. Our results suggest that freeze-thaw damage to soil organisms does not contribute substantially to N release in our system. Instead, soil extractable N may increase during mid-winter thaws as a result of increased soil proteolytic activity above freezing temperatures.