Carbon dynamics in biofuel cropping ecosystems after land use conversion: An application of time-series analysis and press-pulse model

Yahn-Jauh Su (1), Michael Abraha (2), Jiquan Chen (2), Stephen Hamilton(1) & G. Philips Robertson (1)
1 W.K. Kellogg Biological Station 2 Great Lake Bioenergy Research Center

Presented at the All Scientist Meeting and Investigators Field Tour (2021-09-23 to 2021-09-23 )

To develop sustainable biofuel cropping systems, the advanced understanding of the trajectories of carbon dynamics of cropping ecosystems after land use change (LUC) are crucial since the achievability of carbon neutrality has been long debated. The responses of the partitions (i.e., gross primary productivity (GPP) and ecosystem respiration (ER)) of ecosystem carbon fluxes to intrinsic properties and extrinsic forces may have very different patterns, strength and sensitivities to the forces under vary land use histories and major producers. The sum of ecosystem carbon fluxes (i.e., net exchange of CO2 (NEE)) of an ecosystem can be sink or source depending on the responses of the carbon partitions to its inherent attributes, and external climate regimes and human management.

A revised theoretical framework of press-pulse regimes was developed to better understand the trajectories of ecosystem properties determined by long-term press and abrupt pulses by Kominoski et al. (2018). To test the long-term press after LUC and stochastic climate pulses (i.e., drought, temperature), our experiment converted two former land uses: conventional corn farms (AGR) and conservated lands (CRP) into three biofuel crops: switchgrass (Sw), restored prairie (Pr), and continue corn ©, respectively, and one reference (Ref) which is CRP without agricultural practices in 2010. We monitored NEE, GPP and ER with environmental variables by eddy-covariance towers.

From the results of time series periodogram, we found that NEE, GPP and ER in all experiment farms have annual, half-year and 0.4-year cycles, while perennial cropping systems (switchgrass and restored prairie farms) have another cycle longer cycle more than 8 years. It reveals that perennial cropping system may need more than eight years to reach steady equilibrium after land use change. We also found that the flux patterns during growing seasons in annual and perennial crops were different. The length of growing season in annual crop farms are shorter but the peak was taller than those of perennial croplands. The Ref site has a peak of CO2 eflux in fall due to decomposition of litters while other harvested farms don’t have. In Ref and AGR-C, there is no long-term trends of carbon partitions. Those in Perennial crops (CRP-Sw, CRP-Pr, AGR-Sw and AGR-Pr) have saturated curves, implied the establishment of grassland structure and function require eight years or longer. The slightly decrease trend of CRP-C in NEE and ER revealed the slow release of stored carbon accumulated in CRP period.

The periodogram and time series decomposition can deconstruct complex trajectories of carbon fluxes in biofuel agricultural ecosystems that help us understand the responses of ecosystem to long-term press and pulses and forecast future ecosystem behavior underneath different scenarios.

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