Greenhouse gas (GHG) emissions result from short-term perturbations of agricultural systems such as precipitation and fertilization events. We hypothesized that those agricultural systems with contrasting management histories may respond differently to application events of water and N fertilizer with respect to GHG emissions. Studies with long-term management histories consisting of no-tillage (NT) and conventional tillage (CT) were coupled with high temporal resolution, automated chambers that monitored N 2 O and CO 2 emissions for 22 h following treatments. Treatments applied to NT and CT were (a) control (no water or N additions), (b) simulated precipitation to achieve approximately 80% water-filled pore space, and (c) precipitation plus fertilizer additions of 150 kg N ha −1 as ammonium nitrate. Emissions of CO 2 increased with increase in moisture and temperature and decreased under fertilizer application. Water and nitrogen treatments in CT at the sites with 2 and 12-year history produced N 2 O fluxes greater than NT by 142 and 68%, respectively. The site with 10-year history of NT produced similar amounts of N 2 O from CT and NT treatments. The same treatments at the site with 31 year-long NT history, despite being one of the lowest among all sites, demonstrated 380% higher N 2 O fluxes from the NT than CT, which was likely due to higher levels of labile organic matter present in NT treatments. GHG emissions data regressed on measured soil C and N properties, fractionation, and mineralization data showed that N 2 O flux increased with reduction of acid-hydrolyzable N and increase of NH 4-N in soil, which suggested that N 2 O production in the short-term water and water and N additions events is mostly produced via nitrification process. This indicates that neither the length of NT treatment nor the fertilizer application rate define the rate of N 2 O emissions, but the soil N availability controlled by organic matter mineralization rate. The current study demonstrates the need for further research on the effects of the early stages of NT adoption as well as long-term NT on N 2 O spikes associated with artificial or natural rainfall events immediately following extended dry periods.
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 SOM dynamics and storage characteristics. Future climate change projections through 2070 indicate significant shifts in MAT and MAP for the iPNW. We assessed surface (0-10 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.
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