Simulating long-term responses of rangeland soil carbon storage and greenhouse gas emissions to organic matter amendments

Background/Question/Methods

Organic matter application (generally manure or compost) to agricultural soils is a common land management strategy used to enhance crop or forage production, increase soil water holding capacity, and improve soil quality. Amending rangeland soils with organic matter has been proposed as a strategy to increase soil carbon (C) storage and offset greenhouse gas emissions from waste management. However, few biogeochemical studies have been conducted in amended rangeland soils. We used replicated field experiments in two bioclimatic regions of California (a valley grassland and a coast range grassland) to test the effects of composted organic matter amendments on ecosystem C storage. We measured soil respiration, above- and belowground plant production, soil C pools, and fluxes of soil nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) for three years following the application of 14 Mt C as composted organic material. We projected decadal changes to ecosystem C storage and soil greenhouse gas fluxes using the DayCent model.

 Results/Conclusions

During the three-year field experiment, we measured increases in plant production by 44 to 70 %, increasing aboveground C inputs by 161 to 436 g C m^-2. We also measured sustained and significant increases in CO₂ emissions from composted soils by 17 ± 4 % but no significant changes to soil CH₄ or N₂O emissions. Net annual C gains in amended plots over the three years were 67 ± 26, 99 ± 75, and 146 ± 65 g C m^-2 greater relative to controls. The DayCent ecosystem model was parameterized to each grassland site and then used to test the hypothesis that a one-time application of composted organic matter amendments has a long-term (decadal) impact on ecosystem C storage by acting as a slow release fertilizer. Model simulations resulted in similar trends and magnitude changes to plant production, soil respiration, and soil C. The model predicted greater soil N₂O emissions from amended soils compared to field data. However, results from model simulations project a steady increase in ecosystem C for at least thirty years, driven primarily by enhanced plant production and larger passive soil C pools following amendment. These results suggest that composted organic matter amendments can lead to an increase in net C storage in rangeland ecosystems and that effects carry over for several years.