When light is the limiting resource: quantifying inter- and intra-annual variability in carbon and nitrogen cycling in a lawn chronosequence

Background/Question/Methods

Lawn management, including irrigation, mowing and fertilizer use, removes or mitigates water and nutrients as limiting resources.  As a result, light – solar radiation – can be the key limiting resource to productivity.  Importantly, lawns exist in a mix of light environments due to shading from buildings and trees. This creates a novel ecosystem to address questions about interactions between carbon (C) and nitrogen (N) cycles. Here, we address two questions: how does light availability affect a) the magnitude of and b) the de/coupling of C and N cycles in lawns? We quantified C and N cycling in 14 lawns along an 80 year chronosequence that allows testing light limitation across a soil development gradient. Within each lawn, we used shading created from a mix of tree canopy and buildings to establish plots across a gradient of light availability. To address potential differences in timing and scale of analysis, we examined C and N cycling at 3 temporal scales – annual, seasonal and weekly. Carbon and N were measured across years as total soil C and N, seasonally as the within-year amplitude in aboveground plant C and N, and weekly as mowed C and N production plus soil inorganic N mineralization.

Results/Conclusions

Lawns accumulated soil C for the first 35 years before declining to zero net change in soil C. Soil C increases were coupled to soil N increases at approximately a 12:1 C/N ratio, but soil C and N levels were not affected by light conditions. Intra-annual C cycles were driven by light availability. Seasonally, aboveground plant C increased by less than 150 kg C∙ha^-1 in low light to approximately 800 kg C∙ha^-1 in high light conditions. Weekly, mowed C fluxes during peak productivity ranged from 10 kg C∙ha^-1∙wk^-1at low light to 80 kg C∙ha^-1∙wk^-1 in high light conditions. Intra-annual N cycles were positively correlated with lawn age, likely responding to soil N levels, but N fluxes exhibited independence from C cycling. Seasonally, plant N fluxes did not have the same magnitude of difference as C across the light gradient, indicating adjustments to plant C and N stoichiometry independent of N availability. Weekly, soil N mineralization rates increased with lawn age, while primary productivity did not change with age. Overall, intra-annual C and N fluxes exhibited decoupling because light was the dominant driver of C fluxes vs. age or soil development for N fluxes.