CARBON, NITROGEN, AND SULFUR POOLS IN PARTICLE-SIZE FRACTIONS AS INFLUENCED BY CLIMATE

The response of soil organic matter (SOM) dynamics to climate change m ay be deduced from changes in the distribution of SOM among different C pools. The distribution of soil organic carbon (SOC), total N, and t otal S in particle-size fractions were measured to assess the influenc es of climate. Clay (<2 mu m), silt (220 mu m), fine sand (20-250 mu m ), and coarse sand (250-2000 mu m) fractions were obtained from compos ite soil samples from the top 10 cm of 21 native grassland sites along temperature and precipitation transects from Saskatoon, Canada, to so uthern Texas, USA. The clay fraction contained about 43% of the total SOC, 56% of the total N, and 62% of the total S. The SOC and total-N c oncentrations in the clay fraction, relative to those in the bulk soil , increased significantly across sites with increasing annual temperat ure, decreasing annual precipitation, and decreasing clay content (mul tiple R-2 = 0.80** [significant at P = 0.001] for SOC and 0.83*** for N); the concentration of SOM in the fine sand fraction showed the opp osite trends. Principal axis component analyses confirmed that both cl ay and fine sand fractions comprised sensitive SOC and N pools related to climate, whereas S seemed to be controlled by factors other than t hose regulating the dynamics of SOC and N. These results suggest that SORI is preferably decayed from pools of the fine sand fractions with increasing temperature, resulting in a relative enrichment of SOM stab ilized on clay.