Nutrient Management Research Database

To better understand agricultural carbon fluxes in California, USA, we estimated changes in soil carbon and woody material between 1980 and 2000 on 3.63106 ha of farmland in California. Combining the CASA (Carnegie-Ames-Stanford Approach) model with data on harvest indices and yields, we calculated net primary production, woody production in orchard and vineyard crops, and soil carbon. Over the 21-yr period, two trends resulted in carbon sequestration. Yields increased an average of 20%, corresponding to greater plant biomass and more carbon returned to the soils. Also, orchards and vineyards increased in area from 0.7 x 10^-6 ha to 1.0 x 10^-6 ha, displacing field crops and sequestering woody carbon. Our model estimates that California’s agriculture sequestered an average of 19 g C
m^-2
yr^-1. Sequestration was lowest in non-rice annual cropland, which sequestered 9 g C
m^-2
yr^-1 of soil carbon, and highest on land that switched from annual cropland to perennial cropland. Land that switched from annual crops to vineyards sequestered 68 g C
m^-²
yr^-1, and land that switched from annual crops to orchards sequestered 85 g C
m^-2
yr^-1. Rice fields, because of a reduction in field burning, sequestered 55 g C
m^-2
yr^-1 in the 1990s. Over the 21 years, California’s 3.63106 ha of agricultural land sequestered 11.0 Tg C within soils and 3.5 Tg C in woody biomass, for a total of 14.5 Tg C statewide. This is equal to 0.7% of the state’s total fossil fuel emissions over the same time period. If California’s agriculture adopted conservation tillage, changed management of almond and walnut prunings, and used all of its orchard and vineyard waste wood in the biomass power plants in the state, California’s
agriculture could offset up to 1.6% of the fossil fuel emissions in the state.

This study uses the Carnegie-Ames-Stanford Approach (CASA) model to calculate changes in crop biomass carbon and soil C .  The authors modify the model so that net primary production (NPP) and amount of carbon in crop biomass returned to the soils could be added.  California's 15 largest agricultural counties were included and each was divided into 3 crop types.  

For annual crops NPP was calculated as NPP = Y x DM x CF/(HI x AF) and carbon returned to the soil was calculated as NPP x (1- HI x AF) where Y is yield, DM is the dry mass fraction, HI is the harvist index, AF is the plant aboveground fraction and CF is the carbon fraction of dry matter.  

For perennial crops NPP was set to be equal to the average NPP of the state's annual agriculture and adjusted to account for increase in yields over time.  The authors also assume that 20-30% of perennial NPP is in fine roots and that dry matter is made of 45% carbon.  

Several further adjustments were made to account for changes in management practices, the fate of woody biomass at the end of orchard lifespan, irrigation, temperature and crop area, and previous land use.  

Simulations were also ran to estimate effects of alternative management, including burning of rice fields, transition to conservation tillage, and incorporation of all orchard pruning to the soil.  

The model results estimate that 14.5 Tg C was sequestered between 1980 and 2000 for the 3.6 x 10⁶ ha of land under agriculture in California.  On average, perennial crops sequestered more carbon than annual crops. 

Yield increases for all crops and amount of area dedicated to orchard and vineyard crops resulted in carbon sequestration for California agriculture.  The authors suggest that conservation tillage, the allocation of more almond and walnut prunings to the soil, reducing burning of rice fields, and use of orchard waste wood in biomass power plants would increase carbon sequestration in California agriculture.