Phosphorus, nutrient, carbon and water interactions. Jason Neff Geosciences Dept, CU Boulder

What controls primary productivity? N P P P Ca, Mg Fe H2OH2O H2OH2O H2OH2O Short vs. long term limitations

Limits to Production Light CO 2 Temperature Moisture Nutrients Linked by physiology/biochemistry… Tradeoff water for CO 2 etc. Different set of constraints… Commonly viewed as a separate type of limitation but…

Nutrients as a limitation Justus von Liebig: “Liebig’s law of the minimum” ~1850: Yield (growth) is proportional to the amount of the most limiting nutrient, whichever nutrient it may be. Start of the green revolution & clearly linked plant growth rates to the amount of mineral nutrients present.

The physiology of nutrient limitation Redfield Ratios: Alfred Redfield (Woods Hole) Basic Observation: Biologically active tissues tend to have relatively constant proportions of elements. Observation based on oceans but “generally” holds in terrestrial systems as well C : N : P : Fe = 106 : 16 : 1 : 0.01

Early Succession – abundant rock-derived minerals (e.g. P, Mg, Ca etc). Very little N.

Mechanisms for N limitation I N fixers dominate in early succession N fixation is energetically expensive Fixers can’t control the flow of N (some ends up in soils) Other plants take advantage of added fertility and outcompete fixers for light, other nutrients (P) Alder thicket – interior Alaska One reason why N limitation is so common in temperate ecosystems

P i apatite P i occluded P i non-occluded PoPo P total Time g P / unit area profile From Walker and Syers 1976 Bedrock weathering can cause different limitations to develop

Long term P changes Chadwick et al, 1999 N Limitation or NP colimitation P limitation

Crews et al, 1995

Is P limitation pervasive in the tropics? Carl Bern et al – uplift regenerates soils and nutrient supply Steve Porder et al – Erosion and downcutting regenerates soils Volcanic Tropical soils can be very nutrient rich

Why aren’t all terrestrial systems P limited? NO 3 - ? X NO NH 3 N 2 O DON

Does it make sense to talk about single- element limitation? Metals 7 MG ions 2 CL ions 30 FE ions 2 MO ions N fixation is linked to other elements (including P) Nitrogenase enzyme Microorganisms can be P limited even when plants are N limited (Cory Cleveland et al)… So what does that mean?

Nutrients vs. Water Primary Production (g m -2 ) Annual Rainfall (mm) Actual Production Potential Production From Penning de Vries and Djiteye, 1982 Nutrient Limitation

Nutrients vs. Water N availability and loss are tied to moisture N delivery (deposition) and fixation are tied to moisture Changes in precipitation can influence production through the N cycle, particularly following perturbations

Summary N is not the only element that limits productivity. P in freshwater, oceans and tropical systems Metals (Fe) in oceans Base cations in some tropical settings Water-Nutrient interactions are complex There isn’t a clear distinction between a nutrient limited vs. water limited ecosystem Changes in water often influence ecosystems via the N cycle

Nutrient input/output balance Dust coming off the SaharaChadwick et al, 1999

P in Dust Dust contains mineral aerosols and often is generated in deserts (lots of CaPO4) Once deposited, dust can weather and contribute P to a soil. In very old systems, this is probably the only source of P to the ecosystems

Dust and the Mountains of the Western US

Dust Deposition in San Juan Mountains

5.3X 5.7X Sediment loading increased ~ 200 years ago

Dust Provenance

~200 years ~3000 years

~200 years ~3000 years

Sed. Rate (g m -2 sediment yr -1 ) Porphyry Tarn : Senator Beck Tarn Areal Flux (g m -2 yr -1 ) Porphyry Tarn : Senator Beck Tarn K Contemporary Historic 8.09 : : : : : : Mg Contemporary Historic 2.42 : : : : : : 0.04 Ca Contemporary Historic 3.11 : : : : : : 0.04 P Contemporary Historic 0.60 : : : : : : 0.01

How do changes in non-limiting nutrients affect ecological function? What does it mean that P deposition has increased 7X relative to background rates? What role does the deposition of base cations play in soil development and stream chemistry?

Summary There are human perturbations to most of the biologically-relevant element cycles on earth. For many of these perturbations (e.g. P, Ca, Mg etc), we lack basic information on the magnitude of change and the potential biological impacts.