Changes in soil moisture and its relationships with nitrogen cycle processes in a northern hardwood forest

Identifying which aspects of global environmental change are driving observed ecosystem process responses is a great challenge. Here, we address how long-term (10–25 years) alterations in soil moisture, and nitrogen (N) oligotrophication (i.e., decreases in soil N availability relative to plant demand), alter the production of plant-available N via net mineralization and nitrification in a northern hardwood forest. Our objectives were to determine whether soil moisture has changed over the past decade and whether N cycle processes have become less sensitive to soil moisture over time due to N oligotrophication. We used long-term datasets from several related studies to show (i) increasing winter soil temperatures and declining summer soil moisture from late 2010 into 2024, (ii) reductions in sensitivity of N cycling rates to soil moisture, and (iii) declining moisture-adjusted N cycle processes (the ratio of rate of N process:soil moisture) over time in both summer and winter. These changes suggest continued reductions in N availability to plants in these forests, with potential effects on forest productivity and response to disturbance.