Nutrient limitation reduces land carbon uptake in simulations with a model of combined carbon, nitrogen and phosphorus cycling

Abstract

Terrestrial carbon (C) cycle models applied for climate projections simulate a strong increase in net primary productivity (NPP) due to elevated atmospheric CO<sub>2</sub> concentration during the 21st century. These models usually neglect the limited availability of nitrogen (N) and phosphorus (P), nutrients that commonly limit plant growth and soil carbon turnover. To investigate how the projected C sequestration is altered when stoichiometric constraints on C cycling are considered, we incorporated a P cycle into the land surface model JSBACH, which already includes representations of coupled C and N cycles. <br><br> The model reveals a distinct geographic pattern of P and N limitation. Under the SRES A1B scenario, the accumulated land C uptake between 1860 and 2100 is 13% (particularly at high latitudes) and 16% (particularly at low latitudes) lower in simulations with N and P cycling, respectively, than in simulations without nutrient cycles. The combined effect of both nutrients reduces land C uptake by 25% compared to simulations without N or P cycling. However, the quantification of P limitation remains challenging as the poorly constrained processes of soil P sorption and biochemical mineralization strongly influence the strength of P limitation. After 2100, increased temperatures (+5 K) and high CO<sub>2</sub> (700 ppm) concentrations cause a shift from N to P limitation at high latitudes, while nutrient limitation in the tropics declines. The increase in P limitation at high-latitudes is induced by a strong increase in NPP and the low P sorption capacity of soils, while a decline in tropical NPP due to high autotrophic respiration rates alleviates N and P limitation. These findings indicate that global land C uptake in the 21st century is likely overestimated in models that neglect P and N limitation. In the long-term, insufficient P availability might become an important constraint on C cycling at high latitudes. Accordingly, we argue that the P cycle must be included in global models used for C cycle projections

Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe

“…This was mainly due to increases in P bound in soil carbon, which tends to have a lower C:P ratio than vegetation. This result is somewhat surprising given the paradigm that non-tropical systems are considered less phosphorus-limited than tropical systems 62 but consistent with more recent work suggesting equal nutrient limitation across biomes 63 and near future shifts from N to P limitations at high latitudes although nutrient limitations in the tropics decline 8 .…”

“…A high sensitivity to model assumptions was found in this study, indicating that the future mobilization of phosphorus stocks, in particular soil phosphorus and interactions with microbial processes is critical for sustaining changes in future carbon stocks. Unfortunately, current process understanding is lagging behind model needs 8,58 . Knowledge about the potential for ecosystems to tap into the labile-phosphorus pool and for nitrogen deposition to affect the processes by which plants may try to take up phosphorus from the labile pool appears crucial to determine the future carbon sequestration potential of terrestrial ecosystems and specially of tropical ecosystems.…”

“…The response of tropical ecosystems to nitrogen addition is more uncertain. Some models suggest that tropical forests will be unresponsive to nitrogen enrichment because it is already plentiful and phosphorus is the key limiting nutrient 8,51 . However, the diversity of conditions with nutrient limitation is very high in tropical forests 52,53 , and therefore uncertainty is large.…”

“…Recent progress in implementing mechanistic nitrogen and phosphorus schemes in terrestrial carbon cycle models underscores the importance of nutrient feedbacks, showing reductions in the twenty-first century productivity up to 50% (ref. 8). Despite these advances, however, there is large disagreement on the spatial pattern and the strength of future nutrient limitation 9 , and their interaction with the coupled climate-carbon cycle system remains highly uncertain.…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…The most common allocation approach assigns C to each plant component (usually leaf, stem, and root) via fixed ratios that vary with plant functional type (PFT), but not spatially or temporally [38,85,88,95,107,[113][114][115]. For models that include N (and less often P), N uptake plays a strong role in governing C assimilation and drives competition between plants and decomposers.…”