Calcium and Nitrate Limitation

Tom W. Baribault
Michigan State University,
East Lansing, MI USA

Aboveground net primary production (ANPP) in temperate forests could be regulated by multiple soil resources, but many studies have focused on solely nitrogen limitation. We evaluated a broad suite of resources in three major categories—nitrogen (nitrate, ammonium, N-mineralization), cations (calcium, magnesium, manganese, potassium, phosphorous, acidity), and water availability—as predictors of ANPP across a natural fertility-productivity gradient in the post-glacial landscape of northwestern Lower Michigan. Within the context of effects of atmospheric deposition on soil nutrients, our goals were to: 1) evaluate the strongest predictors of productivity within and among resource categories; and 2) test for soil resource effects on wood versus leaf production.

We measured annual ANPP, including wood and leaf increments, and soil resource availability at 13 sites across the fertility gradient. We fit models of four functional forms to each combination of resource and ANPPwood, ANPPleaf, and ANPPtotal. We used Akaike’s Information Criterion to assess relative empirical support of different resources and functional forms as predictors of ANPP. Including all sites, ANPPtotal and ANPPwood were best predicted by sigmoid functions of exchangeable Ca, whereas ANPPleaf was best predicted by a Michaelis-Menten function of soil nitrate.

ANPPtotal and ANPPwood reached an asymptote at relatively low Ca levels. Across the nine least fertile sites, Ca again was the best predictor of ANPPtotal and ANPP wood, though as a linear function. However, ANPPtotal and ANPPwood were positively, but not significantly, correlated to nitrate at sites with elevated Ca, suggesting sequential nutrient limitation by first Ca and then nitrate. Under low fertility conditions, ANPPleaf constitutes the majority of ANPP total, but ANPPwood becomes the major component as soil fertility increases. To assess the potential for direct Ca limitation, we calculated an approximate annual calcium budget for each site. Current exchangeable Ca could supply ~3 to 10 years of Ca demand, suggesting the potential for direct calcium limitation at the lower fertility sites if weathering and mineralization rates are low.

Atmospheric deposition of Ca is insufficient to meet tree physiological demands, but would exert some influence on productivity at low-fertility sites. Both acidic deposition and nitrogen deposition can contribute to leaching of Ca; if Ca is limiting, then productivity may be reduced over time even at high-productivity sites. The strength of these correlative findings suggests limitation by Ca and nitrate, and we are assessing these results with experimental nutrient amendments