Carbon (C) sequestration in forests has been widely promoted as a practice to offset increasing atmospheric carbon dioxide (CO2) concentrations. Since the 1930's, shelterbelts or field windbreaks have been planted extensively in the Great Plains of the U.S. Shelterbelts are an agroforestry practice that consists of one to several rows of trees planted across crop fields or grazing lands to reduce wind speed and improve the local microclimate. Shelterbelts are most common in semiarid areas where they also protect the soil from wind erosion. Some measurements of the C storage potential of shelterbelt trees have been made but there have been no measurements of C sequestration in the leaf litter or soil under this agroforestry practice. Accurate assessment of the C stored in existing and potential shelterbelt plantings is needed to give full credit for its C sequestration potential.
|Establishing agroforestry practices, such as windbreaks, on formerly cropped or grazed land offers great potential to not only sequester carbon, but also improve soil quality.(Photo courtesy author)|
Planting trees on soils previously managed for crop or forage production has potential to change many soil properties by altering organic matter and nutrient cycling processes. In forest systems, litter-fall is the primary organic input, while in many cropping and grassland systems the primary organic input is the decomposition of roots or of incorporated residues and manures. Decomposition of leaf litter on the soil surface in forests tends to be reasonably rapid with significant losses of soluble organic forms of several nutrients by leaching. Forest soils therefore often have a thin, organic-rich O horizon over an deeper horizons that have relatively low concentrations of nutrients.
To assess the effect of shelterbelt planting on soil organic carbon (SOC), samples were collected within and adjacent to a 35 yr-old shelterbelt in eastern Nebraska to determine the amount of C stored in the litter and surface soil layers. The 2-row, north-south shelterbelt was composed of eastern red cedar and scotch pine. Adjacent fields had been cropped to wheat, grain sorghum, corn, and soybean. Four soil cores were collected within 10 inches of 118 grid points, divided into 0-3 and 3-6 inch depth increments, and composited by depth. Under the shelterbelt, all surface litter in a 20 x 20 inch square at 63 grid points was collected before soil sampling at those points. The litter was hand sorted into 8 categories (sticks, pine needles, etc.) with the remaining fine material divided into >2 mm (coarse) and <2 mm (fine) duff.
Soil organic carbon (SOC) concentration in the 0-3 inch layer in the uncultivated areas within the shelterbelt was 55% greater than in the cropped fields (3.04% versus 1.96%). The differences were less for the 3-6 inch layer (2.00% versus 1.78%) but SOC was still greater in the uncultivated areas. Significantly greater SOC in the surface soil layer within the shelterbelt is attributed to decomposition of tree litter, absence of tillage, erosion reduction, and deposition of wind-blown dust. Average litter mass was 0.95 lbs/sq ft with over 70% of the mass composed of coarse and fine duff. The duff fraction contained over 60% of the C in the litter layer. When added together, the C stored in the surface soil layers and leaf litter beneath the shelterbelt accounted for approximately 20% of the C stored in the shelterbelt. This estimate would likely increase by including SOC from deeper soil layers and C from roots.
Productive conservation is a term for land use practices that enhance or conserve soil, water, and air quality while maintaining a viable economic return. Agroforestry systems including shelterbelts can include diverse tree (e.g. nut, fruit, & veneer) and understory (e.g. forage, herbs, & medicinals) species that produce a wide variety of food, fiber, and specialty products. From a soil science perspective, agroforestry systems also offer great potential to improve soil quality by reducing soil disturbance and by providing permanent ground cover. In particular, the potential to increase SOC content of the surface layer is of great interest as SOC is a critical soil quality indicator through its role in C and nutrient cycling, enhancing soil fauna, and improving infiltration and soil water holding capacity.
Establishing agroforestry practices on formerly cropped or grazed land, especially if the soil is eroded or otherwise degraded, offers great potential to not only sequester significant C in the trees, litter, and soil, but also improve the quality of the soil. Better information on litter accumulation and changes in SOC and C in tree roots will allow better estimates of the full C sequestration potential of agroforestry systems. More research is also needed to identify which processes are responsible for the changes in SOC within agroforestry practices and management techniques to promote C storage.
By Tom Sauer and Cindy Cambardella (USDA-ARS National Soil Tilth Laboratory) and Jim Brandle (University of Nebraska-Lincoln)