Carbon-Nitrogen Interactions in the LM3 Land Model Stefan Gerber Department of Ecology and Evolutionary Biology Princeton University GFDL, March, 2010 Lars Hedin, Steve Pacala, Michael Oppenheimer, Elena Shevliakova, Sergey Malyshev, Sonja Keel, Jack Brookshire, Susana Bernal …

My 2 Zero Order Nitrogen Cycle Questions: 1.If the Fixation of N [conversion from N 2 to available N] has more than doubled during modern times, what has happened to the N cycle and N balances? 2. How do the nitrogen and carbon cycles interact and how does 1. influence current and future levels of atmospheric CO 2 ?

1.Roughly 90% of nitrogen was recycled every year in pre- industrial times. Losses were historically made up by natural nitrogen fixation [~100TgN/yr] 2.Humans now at least double these historic inputs by combustion and adding fertilizer [>100 TgN/yr]. Many land ecosystems now leak nitrogen. 3.Is the global Nitrogen cycle in Balance? Human Impact on the Nitrogen Cycle Midlatitude N Leaching

2050 Models Predict a Big Sink From CO 2 Fertilization

Uncertainty about the magnitude of CO 2 fertilization is the key factor determining whether vegetation is a net carbon source or sink No CO 2 fertilizationCO 2 Fertilization at 700 ppm Shevliakova et al Pg GFDL Slab-Ocean Climate Model SM2.1coupled to Dynamic Land model LM3V Atmospheric CO 2 concentration: 700 ppm Change in Vegetation Biomass, kgC/m Pg

CO 2 fertilization and N limititation: N supply does not support predicted CO 2 uptake Hungate et al., 2003

Nitrogen Cycling fertilizer combustion ?

CO 2, N 2, reactive N The coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition

Leaves ~30:1 Sapwood 150:1 Heartwood 500:1 Storage Nitrate and Ammonium Roots ~50:1 Tissue turnover Plant nitrogen limitation/sufficiency Specify C:N ratio in tissue as a parameters Storage is worth 1 year of tissue regeneration. Depletion of storage causes reduction in photosynthesis A sufficient large storage reduces plant N uptake 1

CO 2, N 2, reactive N The coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition 2

Litter Increasing N – demand for microbial growth Litter Decomposition Microbial N limitation This suggests that microbes are N limited when C:N of litter exceeds ~10 (for bacteria) or ~30 (for fungi). A solution is fast microbial turnover, so overall microbial mass is small and N saturation achieved quickly. 2

Response to N addition as a function of Litter Quality (and N content, Knorr et al., 2005) Litter bag experiments: Higher the initial N lower the decomposition. Mellillo et al., 1982 Litter Quality and Decomposition Rates are Complex Soil organic matter Litter and soil organic matter N might stimulate litter processing, but increase the stabilization of organic matter in soils. Li et al.,

CO 2, N 2, reactive N Internal N-Cycle and feedbacks on C-Cycle Litter Soil organic matter Mineral N Respiration Litterfall Stabilization (+) Mineralization Immobilization Uptake Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Leaching/Denitrification

Sinks of available N Immobilization / Uptake / Loss Available N Soil Immobilization and Stabilization Hydrological Leaching (and Denitrification) Plant Uptake Capacity (if N limited)

Primary succession experiment with fixed external N input: From bare soil to temperate forest Carbon only C-N C-N is N limitation in 1, 2, and 3 It takes much longer for C-N to reach equilibrium, but when reached, the system is not N limited. The system escapes N limitation because plants and soil retain any new N from deposition until they are saturated.

“Uncontrollable” losses Fire / Disturbance Organic losses via hydrological leaching

CO 2, N 2, reactive N A more fully coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition

Primary succession + fixed external N input + Dissolved Organic Nitrogen (DON) Carbon only C-N We now account for dissolved organic N losses. It takes much longer to reach steady state, and the system remains N limited, because DON losses scale roughly to biomass

Tropics Temperate Boreal time Ecosystem N-demand Early Succession Late Succession More Favorable Growth Conditions N fixers Non-Fixers A Powerful but Expensive Feedback from the C-Cycle on the N Cycle: Biological N fixation

Primary succession + fixed external N input and DON (previous experiment) Carbon only C-N

Primary succession + DON + biological N Fixation Carbon only C-N N fixation allows for faster biomass accumulation and steady state is reached much earlier.

N feedback on Net Primary Productivity (NPP) at Steady State: Relative change of Net Primary Productivity in a coupled C-N simulation vs. C only

Modeled Veg N [kg m -2 ] Global: 3.1 GtN (model) 3.5 GtN (obs/est.) Modeled Soil N [kg m -2 ] Global: 120 GtN (model) GtN (obs/est.) Reconstructed Soil N [kg m -2 ] (Global Soil Data Task Group, 2000)

Simulated soil N agrees well with reconstructed inventories in high-productivity regions but is low in low-productivity and low-latitude regions. This discrepancy is a direct result of the model’s temperature sensitivity during decomposition, which is higher than suggested by the gradients of the global inventory [Ise and Moorcroft, 2006]. The model is less capable of resolving variations in C:N ratios between biomes which are between 10 and 15 in warm zones and 15–20 in cooler regions: mean modeled C:N ratio in soils is 15 with little latitudinal variations. Modeled Soil Nitrogen: Details

Recapitulation of Important Points C-N interactions are most important during transient changes (primary succession and/or disturbance) At (quasi-) steady state, N limitation in most ecosystems is small Exceptions: Biomes with frequent disturbances Biological N fixation is a powerful feedback mechanism that is highly adaptive in tropical forests

Transient Behavior (Wind-Throw) – the N Perspective Tropical Site Temperate Site N fluxes N inventories as deviation from steady state

CO 2 fertilization and N limitation

Drivers Atmospheric CO 2 Recent climate (Sheffield et al., 2006) N deposition rates (Dentener, 2006) Land-use transition rates (Hurtt et al., 2006) Setup Start in year 1500 with potential vegetation Include/exclude C-N feedbacks Include/exclude Environmental Drivers Full Land Model Study

LM3 is designed to diagnose and predict the land use sink

Effect of Shifting Cultivation and Forestry on C-N dynamics The time scales depend on initial conditions (previous human disturbances), overall biomass, and turnover of plants biomass relative to litter/soil pools.

Terrestrial Uptake [PgC yr -1 ] Budget based on ocean models (Sarmiento et al., 2009, IPCC94)

Carbon Sink – C vs. C-N (PgC/yr)

Residual terrestrial sink 1800 to 2000 Effects of N cycle on residual sink (C-only minus C-N) Effects of anthropogenic N deposition cycle on residual sink (C-N minus C-N-Natural Deposition)

NPP changes for temperate and tropical forests

Conclusions Including the N cycle improves the terrestrial C-cycle model by constraining CO 2 fertilization The required nitrogen for CO 2 sequestration is supplied via: –Tropics: adaptive biological nitrogen fixation –Temperate/Boreal: anthropogenic nitrogen deposition The next step: add Phosphorus

Can the terrestrial C budget reconciled when the C only land model is coupled to N? Khatiwala et al., 2009

Land Use Only Ocean based range (Sabine et al., 2004) Dynamic Vegetation Target

N limitiation

Residual Sink - N deposition N deposition

DIN export at Hubbard Brook following Manipulation