Abstract
Soil organic matter (SOM) is a key indicator of agricultural productivity and overall soil health. Currently, dryland cropping systems of the inland Pacific Northwest (iPNW) span a large gradient in mean annual temperature (MAT) and precipitation (MAP). These climatic drivers are major determinants of surface soil organic matter dynamics and storage characteristics. Future climate change projections through 2070 indicate significant shifts in MAT and MAP for the iPNW. We assessed surface (0 to 10 cm) soil organic C and N as well as active and recalcitrant fractions of SOM within long-term experiments representing different tillage regimes and cropping intensities across the current climatic gradient of the iPNW. We discovered that current levels of soil C and N as well as various SOM fractions were positively correlated with MAP and negatively correlated with MAT. Furthermore, these climatic drivers were more influential than either tillage regime or cropping intensity in determining SOM levels and characteristics. Soil organic C and total N as well as the hydrolyzable and non-hydrolyzable fractions were negatively correlated with the current ratio of MAT to MAP, called the climate ratio. Future climate projections (2030 and 2070) forecast an increase of the climate ratio, thus predicting declines in surface SOM and associated soil health across the iPNW.
Highlights
Mean annual temperature (MAT) and precipitation (MAP) are major climatic drivers of Soil organic matter (SOM) levels and dynamics. Jenny (1941) classically compared SOM levels of grassland soils across the MAT and MAP gradients of the North American Great Plains
Climate plays a dominant role in determining soil C and N and soil N transformations within the inland Pacific Northwest (iPNW)
Many SOM characteristics and associated transformations are negatively correlated with the climate ratio, which is expected to significantly increase due to climate change in the coming century
Summary
Mean annual temperature (MAT) and precipitation (MAP) are major climatic drivers of SOM levels and dynamics. Jenny (1941) classically compared SOM levels of grassland soils across the MAT and MAP gradients of the North American Great Plains. Mean annual temperature (MAT) and precipitation (MAP) are major climatic drivers of SOM levels and dynamics. The greatest SOM levels occurred in northern regions and decreased as MAT increased to the south, while SOM increased from west-to-east in response to regional increases in MAP. MAP ranges from a low of 150 mm in south-central Washington and increases along a west-to-east gradient to a high of 600 mm in eastern Washington and northern Idaho (Schillinger et al, 2006). A MAT gradient runs inversely to the MAP gradient, ranging from 8.4◦C in the wetter regions of eastern Washington and northern Idaho to 10.9◦C in the drier regions of south-central Washington (Schillinger et al, 2006)
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