Changes to grassland carbon pools and fluxes three years after organic matter amendment

Background/Question/Methods

Organic matter amendments to grassland soils have been proposed as a means to increase soil carbon (C) sequestration both directly by augmenting soil carbon storage and indirectly by stimulating plant growth.  However, few studies have explored the effects on organic matter additions on grassland C dynamics. We used replicated field experiments in two bioclimatic regions of California (a valley grassland (“Valley”) and a coastal grassland (“Coastal”)) to explore the effects of organic matter amendments on ecosystem C pools and fluxes. Organic amendments were applied at the start of the experiment as a 1.27 cm surface dressing (equivalent to 14 Mt C /ha). Soil respiration was measured weekly using a LI-8100 infrared gas analyzer during the first two years and monthly for the third year. Autotrophic and heterotrophic components of soil respiration were estimated using Keeling plots and bounded by literature values. Above- and belowground biomass were measured at the end of the growing seasons as an index of net primary productivity (NPP) in these annual plant dominated systems. Soil organic C content was measured at the end of each growing season, at four depth increments to 1 m. Net ecosystem productivity was calculated as the difference between C inputs and outputs. 

Results/Conclusions

During all three growing seasons, soil respiration rates were higher in the amended plots relative to controls. Carbon loss via soil respiration was stronger in the first year, with amended soils experiencing a 21 ± 1 % greater cumulative loss at the Valley site and 14 ± 3 % more at the Coastal site. The second and third years showed a similar but more variable trend. Aboveground NPP was significantly higher in the amended plots each year at both sites. We measured up to 50-60 % greater ANPP in amended relative to control plot. Belowground NPP in amended soil was 17 ± 7 % greater at the drier Valley site. Organic amendments increased soil C content at both sites relative to control soils (30 ± 22 % and 4 ± 7 % at the Valley and Coastal sites, respectively). Net ecosystem productivity was significantly positive at low to mid-range estimates of heterotrophic respiration (Rh), but zero or slightly negative with high estimates of Rh. These results suggest that compost addition can lead to an increase in net ecosystem C storage and effects are carried through time.